First placed on the Internet in February 2003.|
Essentially every part of an automobile engine, and in the entire vehicle, involves applications of Physics, generally described as being Engineering principles. The two fields are pretty close. Engineering generally has a more "practical" approach, of working to get results that will be useful information toward actual mechanisms, without necessarily actually understanding WHY some particular formula is as it is. Physics is less concerned with applications or usage of results, and more concerned about understanding the details about HOW and WHY some process proceeds as it does.
In my entire life, I have only ever seen very superficial presentations regarding the functioning of an automotive engine, usually just enough to tell various alternatives apart! Therefore, I have felt it appropriate to present a Physics perspective on the subject!
I am going to assume that you have at least a vague understanding of what goes on in an automotive engine, and that words like piston, crankshaft, connecting rod, and cylinder are understood.
In this drawing, we are looking at the end of the crankshaft, and the crankshaft is going to rotate counter-clockwise. Therefore, at this moment, the crankshaft is pushing the piston upward in the cylinder, and it is currently half way up. This is during the stage called Compression, where the gas-air mixture inside the volume above the piston is getting compressed by the upward movement of the piston.
I'm going to simplify some things to clarify some points, such as treating valves as being able to operate instantly, which they definitely do not do in real life. However, if the intake valve had closed when the piston was at is lowest point (90°F of crankshaft rotation before this drawing), the total amount of gas-air mixture in the COMPLETE cylinder (initially at approximately atmospheric pressure of 15 PSIA) is now already squeezed into just the volume of the cylinder above the piston. If you think about it, the initial gas-air mixture is here already squeezed into (about) HALF its original volume, and so it is already at about TWICE the initial pressure (or now 30 PSIA).
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This discussion is going to be about the so-called spark-ignition or Otto Cycle engine, the process that virtually all cars and small trucks operate on. There are a couple common alternatives: The compression-ignition or Diesel Cycle and the Brayton or Joule Cycle. The majority of this discussion actually applies to all three, but there are some differences. In Physics-talk, an Otto cycle has an isentropic compression, followed by a constant volume combustion explosion, followed by an isentropic expansion. In contrast, a Diesel cycle has an isentropic compression followed by a NON-explosive combustion at (relatively) constant pressure, followed by the isentropic expansion. Enough of that! They're much alike in many ways, and you can consult any College Engineering textbook regarding the differences.
If we discuss a very popular engine, the so-called small-block Chevy V-8 engine, we can put some numbers in here. The bore (diameter of the cylinder) is 4", and the stroke (twice the crankshaft throw radius) is 3.5". The volume of that cylindrical volume is therefore (PI) * R2 * H or 3.1416 * 2 * 2 * 3.5 or around 44 cubic inches. (Since that engine has eight cylinders that are each that volume, its total 'displacement' is 44 * 8 or around 350 cubic inches. This engine is generally called the Chevy 350 V-8.)
The area shown at the top of the drawing is an additional volume that remains even when the piston is at the very highest point, a location called TDC for Top Dead Center, which will mean more in our second drawing. The space above the piston at TDC is carefully designed. In this specific engine, it has a volume of around 6.3 cubic inches.
When the piston began its upward movement (at BDC, bottom dead center), there was then a volume of gas-air mixture above it of (44 + 6.3) or 50.3 cubic inches. When the piston has gotten to TDC, as in this drawing, all that gas-air mixture has now been compressed into the remaining 6.3 cubic inches. The ratio of these numbers, 50.3 / 6.3 is called the Compression Ratio of the engine. In this case, it is about 8.0.
As a side note, a popular modification to engines to improve their performance is to MILL the heads. This shaves a fraction of an inch of metal from the lower side of the heads. Do you see what the effect of this is? It REDUCES that remaining volume from 6.3 cubic inches down to maybe 5.3 cubic inches (depending on how much metal is milled off). In this particular engine, we would now have (44 + 5.3) or 49.3 cubic inches which gets squeezed into the remaining 5.3. If you do that division, you see that the compression ratio has now RISEN to 9.3 to 1. Technically, Milling the heads actually SLIGHTLY REDUCES the volume of fuel-air mixture that can fit in the cylinder, and it might seem that this might reduce performance. But the higher compression ratio increases the temperature inside the cylinder, which can enable the fuel-air mixture to burn faster. It also can permit using gasoline which has a higher octane rating, for various benefits which will be mentioned below.
Another popular modification is to change the crankshaft to use a similar one which has increased crankshaft throws. For example, if the standard 1.75" crankshaft throws are replaced with a different crankshaft which has 2.0" throws, then the STROKE of the engine is increased from 3.5" up to 4.0". This modification of STROKING this engine would increase the displacement from the standard 350 cid up to being a 400 cid engine. Usually, the connecting rods have to also be replaced, to keep the pistons from hitting the heads at TDC, but this modification can enable an engine to process a lot more fuel-air mixture, and therefore produce quite a lot more power. A side-effect can be bad. The structure of the engine, specifically the Main Bearings, was Designed to withstand the intended amount of power the engine could produce. When an engine is altered to produce a lot more power, then there are a LOT of mechanical stresses which are much greater, and such modified engines have greater chance to come apart.
There is a third popular engine modification which is commonly done. It is to have a Machine Shop BORE the engine out, where the cylinders (and therefore a new set of pistons) are then larger in diameter. Like the STROKING modification, this is done to increase the displacement of the engine, to be able to process greater amounts of fuel-air mixture and therefore to be able to produce more power.
Back to business now!
This drawing shows the moment when that gas-air mixture is most compressed. The 8.0 compression ratio means that the 15 PSIA beginning mixture, is now at about 8.0 times that pressure, or around 120 PSIA. (Technically, not precisely, because of some really technical characteristics of what happens when gases are compressed isentropically.) The cylinder compression is measured and is essentially this number. Except that that device is a gauge, so the reading would be 105 PSIG.
Most superficial descriptions of automotive engines then say that the gas-air mixture is ignited at that moment and that the even higher pressure of the exploding gas drives the piston down, turning the crankshaft. Reference is usually even made of 'advancing the timing' of the ignition spark, so it occurs maybe 10° or 20° BEFORE TDC, so the explosion has a moment to build up its full power by the time it gets to TDC. If you look at this drawing for a while, you should be able to see that that is impossible! If the explosion (and all its effects) occurred exactly at the moment shown in this drawing, at TDC, the crankshaft would not be given any rotation at all! Virtually the entire force of the explosion would initially act to try to drive the piston, connecting rod and crankshaft downward, out of the bottom of the engine, without giving it any rotation at all! (When this actually happens, VERY bad things tend to happen to the engine!)
If you are having trouble understanding this, get your bicycle out and put the pedal exactly straight up, and then put your weight on it. Your friends can then watch you fall over! But as a very small kid, you learned that you cannot put the pedal exactly straight up, and so you always put it some angle in front of being straight up. Your body weight then nicely gets the pedals turning and off you go!
All actual internal combustion engines rely on KEEPING that explosion pressure for as long as possible! In Calculus terms, the total effect regarding rotating the crankshaft is the Integral of the instantaneous net force (or actually torque) actually applied to the crankshaft by that connecting rod for as long as there is explosive pressure inside the cylinder. In an engine that is operating properly, contributions to this Integral begin at the instant of ignition and end when the exhaust valve begins to open. The instantaneous force applied as torque in rotating the crankshaft continuously changes during this "Power Stroke". It actually begins with a slight negative contribution since ignition is timed to occur before TDC, but not much pressure yet develops since the flame is still spreading inside the cylinder. The contribution becomes exactly zero at TDC, and then quickly rises (positively) as the internal burning and pressure continues and the leverage angle at the crankshaft also improves. As the piston goes downward, its expanding volume constantly reduces the remaining pressure, and engine cooling also does this, and good design times the exhaust valve to begin opening about when productive torque is no longer available. Quite a bit of Calculus is required to design a new engine to do this optimally.
So, from a truly accurate (Physics) perspective, a VERY complicated graph of resultant instantaneous torque would first need to be determined, and then that graph would be Integrated to determine actual total engine torque generated, and that quantity is divided by the angle range involved to calculate the AVERAGE torque, at that engine speed and under those conditions of spark advance and the rest. Such analysis is rarely actually done, and nearly always, simply experimental measurements of real engines are found by experiment to learn these things. (Not quite the way Physicists like to do things, but it has worked well for a hundred years!)
You might note that the pressure must be maintained within the cylinder throughout the entire power stroke for decent performance. This explains why an engine loses much of its power once the piston rings are worn (and therefore leaking pressure) or once the valve seats become worn or the valves distorted (and therefore leaking pressure). If an engine actually just relied on the instantaneous effects of the explosion, worn rings or valves would be of minimum importance, but the fact that the basic design of all ICEs relies on HOLDING the pressure while GRADUALLY actually using it, make those components extremely important.
It turns out to be sort of fortunate that the "flame-speed" of the explosion of the gasoline-air mixture is relatively slow! Under the conditions that generally exist inside a cylinder (during highway cruising), the flame front velocity is on the order of around 90 feet per second, or 60 mph. Mark's Standard Handbook for Mechanical Engineers, Section 9, Internal Combustion Engines, Flame Speed. Depending on exactly where the spark plug is located, that flame front must travel two to four inches in order to ignite all the gases in the cylinder. At 90 ft/sec, this then requires around 0.002 to 0.004 second for the combustion to complete. This might not sound like much, but engines spin amazingly fast, and these brief time durations of combustion always take many degrees of crankshaft rotation.
So even though the ignition occurred BEFORE TDC, and the very start of the combustion actually acts to try to make the engine run backwards, the ignition timing is carefully scheduled so that MOST of the combustion (and therefore combustion pressure on the piston head) occurs AFTER TDC. By the time that a maximum amount of the gas-air mixture is burning, the crankshaft has rotated a slight distance past TDC. This situation, and its consistency (due to consistency of the quality and burning characteristics of the gasoline), enables a modern engine to avoid seriously trying to spin backwards! The mathematics below shows that, for an engine speed around 1500 rpm (a normal driving situation) this is commonly around 10° AFTER TDC, when the greatest explosion pressure is present in the combustion chamber. Let's look at some preliminary calculations.
It is very well established that the explosion, and therefore the heat created, causes the gases in the combustion chamber to obey standard rules of Chemistry, such as the Ideal Gas Law. Because of the sudden heat, the gases try to expand immediately, but they cannot, so the pressure in those hot gases greatly and rapidly increases. Very consistently, the explosion pressure in an internal combustion engine rises to between 3.5 and 5 times the compression pressure. Since our example engine had a compression pressure of 120 PSIA, this results in a momentary explosion pressure that peaks at around 500 PSIA. (We are going to slightly cheat here and call it 515 PSIA to simplify the following math!)
Since the piston is 4" in diameter, the top surface of it is just PI * (4/2)2 or around 12.6 square inches. Each of those square inches experiences the 500 PSI(G) pressure (Pascal's Law), so the total force then instantaneously applied to the top of the piston is 12.6 * 500 or around 6300 pounds. (OK. It is ACTUALLY the 515 PSIA, but there is natural air pressure pressing against the UNDERSIDE of the piston as well, so the NET effect we are interested in is due to the GAUGE pressure. Not too different, but slightly!)
We might add a brief aside here, regarding an overview of a number of the things to be presented below. First, at this instant, the Overall Thermal Efficiency is amazingly high, more than 99%. That means that more than 99% of the chemical energy in the gasoline has now been converted into heat energy. Sounds impressive! Unfortunately, there is a (necessary) cooling system which will dispose of about 40% of that heat energy without it having done anything useful! And about another 40% of the heat energy will be sent out of the engine through the exhaust. Sadly, this only leaves around 20% of the actual chemical energy in the gasoline which is ACTUALLY USED to do anything useful! Put another way, when YOUR engine is producing 200 horsepower (150 kW) to drive your car, it actually had to produce about 1,000 horsepower (746 kW) worth of heat from gasoline, and then it DUMPS 400 horsepower (300 kW) worth by the operation of the cooling system and another 400 horsepower (300 kW) worth in the hot exhaust gases! Given that a lot of people have tried to work on improving internal combustion engines for more than a century, this is sort of disappointing news! It ALSO points out WHY people have thought up things like turbochargers (to try to capture some of the heat carried away in the exhaust) and WHY many small changes were made in and around engines to force them to run hotter, thereby slightly reducing the cooling system heat losses and slightly improving engine efficiency. Around 1975, they were happy to get 15% overall engine efficiency, which has now been raised to around 20% or even 21%. Yay!
Because of the geometry of the situation when the crankshaft has progressed 10° after TDC, the force diagram indicates that this downward force must be multiplied by (approximately) the sine of 10°, in order to determine the tangential force applied to the crankshaft. Approximately, because the connecting rod is no longer parallel with the axis of the cylinder bore, the actual angle being slightly higher, and an exact angle is easy to calculate with a thorough analysis. For now, 10° will give an approximate result for our purposes.
Therefore, the tangential (rotative) force actually transferred to the crankshaft is around 6300 * sin(10) or 6300 * 0.174 or around 1100 pounds. Since this force is applied to the throw of the crankshaft, at 1.75" radius from the centerline of the crankshaft, the torque transferred to the crankshaft is therefore 1100 * 1.75" or 1100 * 0.146 foot or 160 foot-pounds of torque. This calculation is in ball-park agreement with the published maximum torque curves for such engines, at 1500 rpm.
Notice that the radial force applied to the crankshaft (bearings) is around 6300 * cos(10) or around 6200 pounds! At that moment, the vast majority of the power of the explosion is trying to drive the crankshaft down out of the engine, without rotating it! And in seriously trying to abuse the bearings! Without engine oil, under pressure, in the bearings, they do not last long with 6200 pounds force against them!
In traditional automotive thinking, this sort of makes sense! As long as the piston rings do not leak too much and the valves do not leak too much, then those expanded gases inside the combustion chamber cannot escape. That means that, until the exhaust valve starts to open, all the pressure will act to push the piston downward. In order to get the most total power, it makes sense to keep that pressure acting as long as possible. This means that having the maximum pressure developed as soon as possible after TDC gives the most possible available degrees of productive crankshaft rotation. The benefit of this is seriously affected by the fact that, as the piston moves downward, the volume inside the combustion chamber keeps increasing, so the pressure drops (Ideal Gas Law). From a beginning combustion pressure of 500 PSIG in our example, at the later instant when the crankshaft had rotated 45° the volume has increased such that the pressure drops to around 200 PSIG (without any leakage) and by the time the crankshaft has advanced 90° the pressure is down to around 125 PSIG. The AVERAGE pressure during this 90° of rotation is referred to as Mean Effective Pressure (mep) and is commonly around 200 for common engines under power. (A V-8 engine that is a four-cycle requires each piston to provide the engine power for 90° of crankshaft rotation.) (This description is for best conditions, fairly high power and revs).
There are several important points to be made here.
The engine cooling system must be able to remove all that heat when the engine is under full load and power, so cooling systems are really designed to be pretty efficient. At 5,000 rpm, there is only about 0.003 seconds available to remove most of the extreme heat from the cylinder walls and head, and the gases inside start out at around 4000°F. As that heat is removed from the cylinder walls and head, the gases inside cool down (Ideal Gas Law again.). At 5,000 rpm, and in 0.003 second, the amount of cooling is limited. But now look at that same engine while idling at 500 rpm. That (cold) 200°F water flowing through the block and heads now has ten times as long to cool everything as the piston descends. In ten times as long, the gases inside the cylinder can get really cooled off. That is bad because lower temperature means lower pressure (Ideal Gas Law yet again) and so less pressure is left to push down on the piston.
Between the natural (Ideal Gas) pressure reduction due to the expansion as the piston goes down, and the forced cooling system cooling the gases and therefore also reducing the pressure, the momentary 500 PSIG that existed near TDC quickly dissipates, and there is a rather brief and somewhat weakened force/pressure pushing the piston down to create productive work. The mep drops way off and the full 90° of productive effect does not occur.
At even slower engine speeds, the engine is not able to reliably create the necessary amount of torque necessary to overcome friction and to drive the water pump, alternator and other systems, and to provide enough momentum to a flywheel to do the work of exhausting, intaking and compressing the gas-air mixture for the next explosion. This is why an automotive engine cannot run reliably at under an idling speed, often around 500 rpm, which is necessarily higher when the added load of an air conditioner is running (generally then at least 700 rpm) because it requires additional torque/horsepower.
In our example engine, at the situation shown here, our effective compression pressure was only 30 PSIG (the piston now being halfway down or a 2:1 compression ratio). Therefore the combustion pressure is only around 125 PSIG and the total force on the piston is around 1600 pounds. So even though the geometry is the best possible, with a sine(90°) = 1.0, the total torque transferred to the crankshaft is around 1600 * 1.0 * 0.146 or around 230 foot-pounds of torque. In real engines, it is usually actually less than this because the cooling system has already removed some heat from the gases. The exhaust valve usually begins to start to open about then, since there is relatively little benefit in staying closed due to the much lower pressure and force on the piston, which then releases the remaining pressure in the combustion chamber. There is also an advantage to giving the exhaust systems the longest possible available time to remove used gases to permit the most space for the incoming new gas-air mixture for the next time around. This is another area where optimal design is so mathematically complicated that the usual process is just to try various camshafts with different shaped exhaust valve lobes, and measure everything, to determine which performed best. Again, from a Physicist, Grrrrr!
Under the conditions of our engine running at 1500 rpm, the "flame-front speed" (essentially the rapidity of the burning) is around 90 feet/second inside the combustion chamber. A little geometry and algebra easily shows that the flame front has progressed across half of the combustion chamber (2") while the crankshaft has rotated around 18°. If the ignition spark was timed for around 15°BTDC, this would suggest that the maximum pressure in the combustion chamber would then occur around 18° later or 3° after TDC, as is commonly intended.
The actual reality is quite a bit more complicated than this, and we have simplified some things in the interest of clarity. As the flame-front progresses across a combustion chamber, the exploding gases act to additionally compress the gas-air mixture that has not yet ignited. The result is that the pressure created is not a symmetric smooth curve, but rather a curve that has generally greater pressures in the later portion of the actual combustion. Where we have considered the maximum combustion pressure to occur when the flame-front has progressed halfway (2") across the chamber, it generally occurs a little later than that due to these very complicated effects.
Actually, a reference in Mark's Standard Handbook for Mechanical Engineers, Section 9 states that the optimum spark advance is approximately 5/9 of the combustion time. This means that more time of combustion happens BEFORE TDC than after! This makes the point of the more powerful portion coming late in the combustion process, to not only overcome that 5/9 of combustion that acted to try to make the engine turn backwards, but also emphasizes the very small crankshaft angles involved that are the main point of this description! If the combustion had proceeded linearly (as we have implied in this simplified presentation) the engine would not even run (with standard spark advance), since over half of the combustion time occurs before TDC!
As with most everything else here, the situation is actually a little more complicated than that! The force applied to the top of the piston is proportional to the pressure of the gases applied to it, and THAT is proportional to the temperature of the gases inside the combustion chamber (all other things being equal!) The early part of the combustion process IS burning fuel and building up pressure, but the TOTAL pressure is somewhat cumulative. In ritzy math terms, it would be called the Calculus Integral of the pressure over time. So even though 5/9 of the TIME of burning may occur before TDC, the pressure is still not fully developed by then, and the cumulative pressure AFTER TDC is much higher. Which is why the maximum torque is developed when the spark is advanced around 5/9 of the total combustion time. But note that ALL of the combustion needs to be DONE before the piston is able to move very far down the cylinder, again meaning that the maximum force (pressure) is developed fairly close to when the crankshaft throw is nearly straight up, the worst possible mechanical (dis)advantage.
Between the additional benefit of this later development of maximum combustion pressure, and the negative value of the very early stages of combustion that occur before TDC, we have for simplicity treated the two as effectively canceling out each other, and that the entire combustion process occurred as if it happened instantaneously at a single 3° to 10° ATDC crankshaft angle. In reality, they seem to be relatively comparable effects, but there is no actual reason for insisting that they exactly cancel out. Spark advance, fuel-air ratio, octane rating of the fuel, temperature and many other effects affect each of them differently.
You can probably imagine why engine designers measure the performance of a new engine at every possible crankshaft angle (actually spark advance angle) and RPM speed, to determine the very best ignition advance for all situations. The math to try to predict that precisely enough from theoretical bases is really complex and it is far easier to simply build some engines and experimentally TRY many different combinations of all those parameters. When they find some combination that produces the most output torque and power (at a particular engine speed), they note that and then teach the computer to provide that advance and that fuel-air mixture!
They actually have the freedom to create an "advance curve" for most power or for best fuel economy. Usually, any vehicle you buy has an advance curve that is somewhere in between. But this issue explains why there are "performance chips" available for most computer controlled engines. Such chips simply replace the tame ignition advance curve with one that is better for maximum performance. Not much else is changed by such chips. They tend to cause poorer fuel economy and poorer environmental performance.
I am somewhat surprised that no manufacturer has yet offered a "switchable chip" capability! If three chips were installed, then normally the Middle (like previous chips) would be in effect. When a cruise control was engaged, a maximum economy chip would take over. When the driver hit a special GO button, for 30 seconds, a maximum performance chip would take over. The best of three worlds, I would think!
If an engine were intended to run at a constant speed with a constant load, it would be possible to fine-tune the exact best spark advance angle. However, vehicle engines must be able to go from idle to maximum performance rather quickly. There are many other engine conditions that also affect the amount of ideal spark advance, such as the relative richness of gas in the mixture and whether the air intake path is restricted or open. All these things are important for the following reason. When the ignition spark occurs substantially before TDC, a significant combustion pressure starts to build up even before TDC. If the engine was not already spinning, this could act to make it rotate backwards! Only the momentum of the crankshaft and flywheel makes it overcome this backward torque to get past TDC when good things start to happen.
Under some circumstances, too much of the mixture burns before TDC. Prior to no-lead gasolines, carbon deposits tended to develop inside the combustion chamber, and these deposits would sometimes become very hot. When fresh gas-air mixture would be introduced through the open intake valve, the hot carbon could spontaneously ignite it before the spark plug fired. This condition, while the intake valve was still open, would send a flame-front backwards up through the intake manifold and carburetor, causing what is called a backfire, flames actually coming up out of the carburetor. If the intake valve was nearly closed, then the explosion would try to rotate the engine backwards, which causes incredible stresses in almost everything and some internal part might break. Usually the head of the piston is what would lose, and a part of the top of a piston would get blown out (down into the crankcase). The engine would then make interesting wheezing sounds and it was essentially unusable due to massive shaking and rattling! Since unleaded gasolines have been used, carbon deposits are less common, and combined with computer controlled spark advance, backfiring is now very unusual.
However, if too low an octane gasoline is used in an engine, the flame-front can sometimes travel too rapidly across the combustion chamber. It might seem odd, but LOW octane gasoline has faster flame-front speeds than HIGH octane gasoline does! This causes too high a combustion pressure to develop during the 5/9 of the combustion period that occurs before TDC. That cylinder then does not contribute to the intended productive power but rather causes an effect that partially tries to make the engine run backwards. This causes tremendous stresses to occur in an engine, and the power of the explosion has no obvious method of release. The rotational momentum of the engine and flywheel permit the engine to continue past this event, but the instantaneous effect is usually a slight flexing of the top of the piston head, which makes a very unique metallic sound. This situation is called "engine knock" or "ping". It is quite undesirable. Regular engine knocking can cause a "blown piston" where a hole is blown through the weakest surface of the combustion chamber, the piston head.
In many engines, the radiator hose is around 1 1/2" in inside diameter, which gives around 2 square inches of cross sectional area, a situation that is true for most parts of a well designed cooling system. The water pump pushes that water at around 15 ft/sec (10 mph) through the passageways, when the automatic thermostat is fully opened. This means that about (15 * 12 * 2) 360 cubic inches of water per second can be circulated, which is about 12 pounds of water per second. It is common for the water to be heated by around 15°F in taking that wasted heat away from the cylinder walls and heads. It takes 1 Btu to raise one pound of water by 1°F, so we're talking about a MAXIMIUM of (12 * 15) 180 Btu/second of heat being removed. That might not sound like much, but it is! In an hour (3600 seconds), this COULD BE about 650,000 Btu! (More than ten times as much heat as most entire houses need in the dead of winter!) Down below, we will mention that 2544 Btu/hr is equal to one horsepower or 0.746 kW, so this MAXIMUM wasted heat represents around 250 horsepower (186 kW) or more of wasted energy from the gasoline (during hard acceleration, where the (stock) engine is creating its maximum productive horsepower).
During normal driving, the amount of heat removed from the engine is less than this, for several reasons. The modulating thermostat is generally only partially open, which only allows a partial flow of what was described above. The combustion chambers do not contain as much burning gasoline as during a drag strip run, so less fuel and therefore less energy is present that needs to be dealt with.
We can therefore see that the cooling system is necessarily designed so that it CAN remove an enormous fraction of all the energy/power that an internal combustion engine creates, which causes the "overall thermal efficiency" of any conventional automotive engine to have low thermal efficiency, even separate from all the mechanical losses related to the engine's operation. The calculations are extremely complex, and include variations depending on water flow rates and cooling system design, but they generally indicate that a conventional internal combustion engine cannot have an overall efficiency of greater than around the low 30% range. As noted below, there have been some experimental engines designed that have been measured at around 28%, but the most efficient production engines are around 25% and most vehicles on the highways now have engines which have around 21% overall efficiency.
We might as well add another analysis here! This is NOT an analysis that qualifies for an actual scientifically rigorous analysis, and is meant to simply provide some overall insights regarding what happens to the heat created inside a standard engine.
We will consider a normal driving situation, of a constant 60 mph trip for exactly one hour (covering 60 miles) on an Interstate highway. We will assume that the engine is this small-block Chevy 350 we have been using as an example. We will further assume that the fairly large vehicle will get exactly 20 MPG during this trip.
We can see that we will use up exactly three gallons of gasoline for this trip. Since each gallon of gasoline contains around 126,000 Btus of chemical energy in it, we will therefore use up 378,000 Btus of chemical energy.
That particular vehicle has a rear axle gearing ratio that causes the engine to turn around 1800 rpm to produce the power needed to maintain this constant 60 mph speed. Our engine therefore spins 1800 times every minute for 60 minutes or a total of 108,000 times during that hour trip. Four cylinders fire during each engine revolution, so we have a total of 432,000 cylinders fire during this trip.
Therefore, EACH cylinder burns an amount of gasoline (assuming proper air-fuel mixture, ignition timing, etc) which converts about 378,000 / 432,000 or 0.88 Btu of chemical energy into heat. So if we consider the firing of a single cylinder, we can say that about 0.88 Btu of heat is created during the combustion of the gasoline inside the cylinder. We know that energy cannot be created or destroyed, so this 0.88 Btu of heat energy must go somewhere!
We know that around 21% of that energy is able to be productively converted into moving the vehicle, so this accounts for about 0.18 Btu.
We know that we brought in a fuel-air mixture which was around 44 cubic inches (at original ambient temperature and pressure). Since we know that one pound of air takes up about 13 cubic feet at STP, the amount of mixture we put into the cylinder is about 1/500 pound of mixture. The engine will heat that up briefly to near the 4,000°F that gasoline burns at, but then due to the 5:1 expansion of those gases by the time the exhaust valve starts to open and due to the effect of the cooling system, the temperature of the gases leaving the engine as exhaust (under the conditions of this constant speed driving) can be around 700°F. (At drag strips, the exhaust gases can be far hotter than that, where they can cause the exhaust headers to glow reddish during nighttime runs. Since iron and steel begin to glow a dark red at around 800°F, we are assuming here that the exhaust is around the 700°F estimated here, due to not causing any (usual) obvious glowing of the exhaust manifolds.)
We also know that the Thermal Capacity of air is around 0.24 Btu/pound/°F. We can therefore calculate an approximate number for the amount of heat that the exhaust will carry away from our engine during our one-hour trip. Our one cylinder would therefore send away 1/500 pound * (700 - 70) temp rise * 0.24 or around 0.30 Btu of heat.
By implication, we can say that the remaining 0.39 Btu (0.87 - 0.18 - 0.30) of the heat must get carried away by the cooling system.
For THAT SPECIFIC SITUATION then, we can estimate that around 21% of the energy becomes useful power to move the vehicle; 35% of the energy gets lost in the exhaust gases; and 44% gets lost due to the cooling system and other radiative cooling effects.
An interesting side-note to this analysis is that around 1970, the productive efficiency of such engines was only around 15% and cooling system thermostats were designed to cause the cooling system to operate at around 20°F cooler than in today's engines, with the result being that the exhaust then carried away a much larger fraction of the energy, around 45%, with the cooling system and other radiation then accounting for around 40%.
During that constant-speed trip, in ONE HOUR the cylinders therefore created 378,000 Btus of heat energy from the chemical energy that was in the gasoline. Of that energy, only about 79,000 Btus of that energy was converted into moving the vehicle, while 132,000 Btus was lost in heat in the exhaust and 166,000 Btus was lost through the cooling system and by radiation.
We might also note that this analysis was for an engine speed of 1800 rpm and at minimum throttle opening. If that same engine was operating at 5400 rpm, it would clearly use around three times the amount of gasoline, and since maximum power would then be desired, the fuel-air mixture would likely be more rich. So we might easily then be looking at a situation which would be dealing with far over a million Btus per hour of chemical energy that was being converted into heat. Of that energy, the cooling system (and natural radiation) disposes of 166,000 Btu/hr and the exhaust gases dispose of an additional 113,000 Btu/hr of heat. All this to produce 79,000 Btu/hr of productive work in powering the vehicle! Separate from pointing out here the incredible wastefulness of the operation of all internal combustion engines, we mention these numbers because a COMPLETE medium-sized house in a cold climate generally only requires around 50,000 Btu/hr on the coldest February night! Vehicle engines THROW AWAY heat at several times the rate your house uses similar heat that you pay heating bills for!
It seems to me that a Doctoral Thesis might be available in Engineering for a gearhead student! The TEMPERATURE of the exhaust seems certain to have a direct relationship with the camshaft lobe shape of the exhaust valve cam. I am not aware that anyone has ever carefully researched that issue. But in a very tame engine, where the exhaust valve waited until later in the power stroke before starting to open, the Ideal Gas Law expansion of the gases would have dropped to a low temperature, while in a wilder engine where, in order to improve breathing of the engine, the exhaust valve opened far earlier, the gases should go into the exhaust header at both higher pressure and higher temperature. There may be a way to identify some aspects of a camshaft by simply measuring the (max) temperature of the exhaust! Just a thought!
At 5,000 rpm, this is good! Only around 0.5 Btu gets removed while the piston is still trying to do productive work, and in that very short period of time, the gases inside the cylinder cannot be overly chilled, and so the overall performance is good. (Normal automobile cooling systems are actually intended to start to overheat at high revs like this, for the lower speed efficiency concepts being considered here!) In our constant-speed example above, our engine was running at 1800 rpm, around 1/3 as fast, so the engine has around three times as long to get rid of our 0.39 Btu of heat. However, there is a new problem! The cooling system is still just as efficient as it was during the 5,000 rpm operation! So it COULD still be removing 0.5 Btu during each 0.003 second, or around a total of 1.5 Btus of heat removed from the cylinder. This would cause FAR too much cooling of the gases in the cylinder and the capability of producing horsepower or torque is greatly reduced! The gases could get cooled so quickly that the torque production curve could drop to near zero very rapidly!
I know that you are way ahead of me now! At a 500 rpm idling speed, the very effective cooling system has already had all sorts of time to BE ABLE TO remove virtually all the heat from those hot gases before the crankshaft has even rotated by 45°. It can never even get to having a beneficial mechanical leverage on the crankshaft before it has already gone fizzle!
See the situation? The cooling system MUST have adequate performance to be able to remove enough heat when the engine is wound out, but that results in it having too good a performance at all lower engine speeds. Such really good cooling performance makes engines last longer, so they have THAT going for them! But the basic performance of all internal combustion engines is tremendously reduced by how well the cooling system has to work!
The cooling system therefore also ALWAYS includes a "modulating thermostat" which partially blocks off the water flow when the water temperature is less than the maximum it was designed for. This minimizes the chance of the engine cooling system ever removing too much heat and keeps the engine at a relatively constant operating temperature. The effects of the thermostat (both its design temperature and its actual operation) have significant effects on calculations regarding the efficiency of an engine, and can cause some calculations to be off.
You might see why the cooling water pump is driven by the engine. At high speed, it runs very fast, to pump a lot of water to accomplish the full cooling described above. At slower engine speeds, the water is pushed more slowly so that it is able to capture less heat from the cylinder walls and heads. But these things do not eliminate the problem. The slower water speeds reduce some of the numbers described above, but it is still true that every running vehicle constantly discards more of the gasoline's energy as wasted heat than it uses to move the vehicle.
Older vehicles also had their very large radiators very exposed openly to the air at the front of the vehicle, because at that time COOLING was considered the central factor (related to engine survival!) As gasoline got more expensive and actual engine efficiencies have improved (from about 15% to about 21%), modern vehicles tend to have very small radiator openings in the front of a vehicle, along the theme of causing the engine to operate at a higher temperature. If the engine runs hotter, it consumes more of the undesirable NOx and other pollutions, enabling the vehicles to pass more rigid pollution testing. With that smaller radiator opening, less air can pass through the radiator and also less air passes alongside the engine itself, causing the now desirable higher engine temperatures! Now you know why! By the way, long ago, it was easy to work on nearly any engine, because it was in such an open area of the vehicle. Modern vehicles have many accessories right against the engine, so it is often hard to actually even see the engine when the hood is opened! That great difference is actually an intended difference!
In case you are curious, about 60% of the cylinder cooling is usually done through the cylinder walls and the remaining 40% through cooling the heads. This will probably NEVER come up in Trivial Pursuit!
Another related subject: Remember that I mentioned above that the standard cooling system design intentionally allows the engine to start to overheat when really revved up? (The expectation of the designers is that no standard driving would ever involve extended driving at such high revs.) If a vehicle is to be used for towing a heavy trailer, generally there is an extra-cost option of a "heavy duty" cooling system. When towing such a trailer, the engine can spend longer times at higher engine speeds and loading, where it would normally overheat. The extra cost "heavy-duty" cooling system rarely involves stronger water pumps or bigger hoses. Almost always, it only involves REDUCING the size of the water pump pulley (so it spins faster) and a thicker radiator (so there is more heat exchange surface to cool the water). From the above discussion, you probably realize that such a "heavy-duty" cooling system causes the engine to have WORSE efficiency and performance at low engine speeds, due to excessive cooling of the engine cylinders then! Less heat remains in the hot compressed gases in the cylinder pushing the piston downward, because the excessive cooling lowered that pressure due to the Ideal Gas Law! (The modulating thermostat mostly resolves this complication).
Prior to around 1980, cars and trucks had large radiators and very free airflow through them, and engines ran fairly cool. Even the standard thermostats were 180°F, again permitting cool engine operation, with the intention of enabling long engine life. When fuel efficiency and air pollution came to be politically important, the advantages described above, of intentionally reducing the effectiveness of the cooling system to reduce the cylinder heat losses to (slightly) increase efficiency, started appearing. Now, nearly all vehicles have rather small radiators and they have small grilles allowing air in to them! Modern radiators are actually too small to avoid overheating and so electric cooling fans are necessary to keep engines from boiling over. Similarly, modern thermostats are generally 195°F, which raises all the engine temperatures by 15°F. Look in any engine compartment today and you see a clutter of things surrounding the engine. That was not the case long ago, when free air flow around an engine was desired for engine durability. Now, the highest possible engine operating temperature is used, (reduced cooling performance described above) to improve engine efficiency and performance, which also reduces the amount of air pollution created in the process. Engine durability is less than it used to be, but people rarely seem to keep vehicles as long as they used to, so it is apparently not considered a problem. The technology of motor oils has greatly advanced, so that oil is able to last much longer in today's hotter engines than old oil would have lasted.
Finally on this tangent: Consider dragsters (rails) in a 1/4 mile drag race. They have no radiators or water pumps, but they are filled with (cold) water just before a race. That seems certainly necessary to keep the engine from blowing up. But an ideal situation would be that the water was ferociously boiling at the Finish Line when the engine was shut down, because that would indicate the highest possible engine (cylinder) temperatures during the race. I don't know if any research has ever been done on this, but I suspect that if two identical dragsters raced, the one that had had its engine running 30 seconds longer before the race should always win! (Unless the engine blows up!) The hotter engine cylinders should allow several percent additional power to remain to drive the pistons downward, particularly at the important start of the race. Engine durability would probably be severely reduced, but people who drag-race only think of winning! (Notice how Physics shows up in unexpected places and in unexpected ways?) (And, of course, that seriously overheated engine is more likely to dangerously blow itself apart, too!)
The buying public is fickle and by the late 1960s, a number of Muscle-cars were sold, then in the 1970s, Economy was again popular, and so forth. But an interesting detail was that the Political power of the giant auto manufacturers got the government to change the formulas regarding calculating horsepower. The engines did not really change, but engines that had been officially Rated at 200 horsepower suddenly became rated at maybe 270 horsepower! A Physicist does not like it when politicians do such things, but everyone in America now thought they were getting much more powerful engines in their cars! A change which WAS made was in lightening the vehicles, where cars which previously weighed around 4,000 pounds now often weighed around 3,500 pounds. The effect of that was to have the car have significantly higher acceleration from stop lights! NIT because of additional power, but instead because of Newton's Laws and F = m * a! Reduce the weight (mass) a lot and the same Force (Power) can create more Acceleration.
Later still, the government stopped doing any testing, and they decided to TRUST the vehicle manufacturers to determine horsepower ratings and gas mileage numbers, which then became called ESTIMATES in all advertising.
Granted that there HAVE been many incremental improvements in engine design in recent decades, but the Laws of Physics still apply!
Modern (2012) advertising tends to brag about Mileage Estimates of 30 mpg or even 40 mpg. But in extremely fine print, they note that the engine they are describing is 1.2 liters (72 cubic inches) or 1.6 liters (96 cubic inches) or 2.0 liters (120 cubic inches). The giant 350 cid or 427 cid engines sucked down the gasoline, while engines which are 1/6 as big in piston displacement CAN get much higher gas mileage. But there are unspoken details. The impressive numbers of 30 mpg or 40 mpg ARE possible, but only if you drive in a very restrained manner! Those new (tiny) engines ARE able to wind out to impressively high REVS, where they can sound like a Dentist's Drill, where they might create the horsepower claimed, but under those conditions, the gas mileage is far lower. No free lunch!
So the LANGUAGE is now rather different than it used to be. A giant V-8 engine running at 1600 rpm made a throaty growl and had impressive available torque for acceleration, even for a 4,000 pound car. A modern 2.0 liter (120 cubic inches) four cylinder engine running at 2,500 rpm makes a lawn-mower sound but also has extra torque for accelerating a 2,200 pound car. Are they the same? Not to me! But the point here is that evaluating big old V-8s and tiny new four-cylinder engines involves two rather different languages, even though the Physics is still the same. With the tiny modern engines, IF you run it at some specific rpm, you CAN get impressive mileage, but no advertising ever mentions that fact. The giant old V-8s had such large pistons that they were not very fussy regarding any precise engine speed, even without computer control. The point here is that in order that manufacturers not get sued too much regarding false advertising, they find the ideal situation to be able to achieve the very best fuel efficiency that particular engine could ever achieve. And they do other test runs where the engine produces the absolute maximum horsepower, again for advertising purposes.
It all makes me wish for the days when the government used to actually do the testing on cars and engines, where the numbers created were not BY the manufacturers themselves. But I guess those days are long gone, and manufacturers are now free to be 'optimistic' about the performace of their products.
A hemi head is actually a (somewhat) hemispherical head. Virtually all the other styles of overhead valve engine heads have relatively flat pistons and heads that have a relatively shallow recess in their heads, for the combustion to occur. Remember the roughly 6 cubic inches that must remain at TDC? With a 4" diameter cylinder, that equals roughly 1/2" in cylinder height, near the sides near zero and near the spark plug may be 3/4 inch. Now, a cylinder has to have both an intake valve and an exhaust valve, both in the head (in overhead valve engines, the most efficient designs). The flat shape of the usual combustion chamber limits the diameter of those valves, to well under HALF of the entire distance across the piston. An engine with 4" diameter pistons can therefore not have intake or exhaust valves which are larger than about 1.5" in diameter. By the way, the INTAKE valve is always larger in diameter than the exhaust valve. Do you know why? It is because the EXHAUST is DRIVEN OUT by the upward motion of the piston, while the INTAKE is SUCKED IN by the downward motion. It turns out that devices that suck air cause a lot more turbulence, and so it is less easy to do. The larger intake valves are therefore needed to provide the SAME necessary flow rates for the cylinder to be most efficient.
The hemi head uses a VERY deep combustion chamber, so that the distance across is about half the circumference of a circle (1.56 * diameter) rather than being only slightly more than the diameter. This allows a lot more available space for the two valves. The valves tend to therefore be at odd angles to benefit from this added size. The SINGLE actual advantage of a hemi head engine is that it has much larger diameter valves! This allows the fuel-air mixture to get in easier and the exhaust to get out easier. Bigger valves is a very good thing, and the hemi head design is the simplest way to provide the space for really large valves.
Since the hemispherical chamber is so tall, a flat-top piston would allow too much remaining volume for a good compression ratio, so all hemi head engines have to have dome-top pistons. So if you ever see a relatively flat-top piston, it is from a non-hemi, and a significantly domed piston is always from a hemi. (An engine can have flat-top pistons replaced with slightly domed pistons to increase compression ratio, but that is a very different effect.) Also, if you happen to see an unusually large valve, it is likely to have come from a hemi engine.
So, a hemi is not "magical" or anything, but merely is a design that permits bigger valves for better engine breathing. There is no other significant advantage of it. And, actually, the domed piston somewhat interferes with airflows and makes it less likely to get really uniform distribution of the gas-air mixture, and really good removal of all exhaust products, so some of the benefits of being a hemi are given up in exchange.
You may be aware that there are some newer engines that have four (smaller) valves per cylinder. This provides the improved breathing of the hemi while not having the disadvantages of domed pistons. But the engine is much more complex, and expensive.
It turns out that since the valves take that much time to open and to close, the "valve timing" meaning the shape and timing of the camshaft lobe shape and position OVERLAPS.
For example, the EXHAUST valve is ALWAYS designed to begin to open FAR BEFORE BDC (bottom dead center) so it necessarily RELEASES the productive pressure inside the cylinder during the POWER stroke. Even worse, the exhaust valve CONTINUES TO STAY OPEN beyond the end of the exhaust stroke and it is still open well into the INTAKE stroke! With BOTH valves then open, some of the fresh gas-air mixture being sent INTO the cylinder goes completely through and OUT THE EXHAUST! In fact, much of the rich sound of a high performance engine is due to this, where raw gas-air mixture goes through the engine and is then ignited by the extremely hot metal surfaces of the exhaust manifolds!
But this is clearly extremely wasteful of the precious gas-air mixture that drivers pay for at the gas pump! If you think about it, if an engine was IDEAL, there would be virtually NO exhaust sound at all, even without any muffler. The already completely burned up end products of the cylinder combustion would simply be squeezed out of the cylinder as the piston rose.
So an engine's overall efficiency is also affected by the valve timing and duration. This subject is very complex because both sides of the street are involved. IF an engine is to be designed to produce maximum power, then it is important to get rid of as much of the old exhaust gases in order to get more fresh gas-air mixture into the cylinder to burn. This can be done by greatly INCREASING the length of time that the valves are open. It is essentially conceded that a significant amount of unused fresh gas-air mixture goes through the engine unused, in order to be able to create the absolute maximum amount of power. That means that an engine that is set up for extra power is also WORSE on fuel efficiency. It might have seemed that the opposite should be true, but the REASON that the engine creates more power is because a LOT more fuel-air mixture goes through the cylinders, and the fact that a good deal of that is lost is ignored!
Manufacturers therefore design very conservative camshafts for their vehicles to be sold, but for their racecars that look the same, they have very different camshafts in them! Gearheads know that there are STOCK camshafts, STREET camshafts, and various levels of RACING camshafts. When the engine has a STOCK camshaft, it idles smoothly and starts easily. With the most extreme racing camshafts (such as used in Dragsters), as peculiar as it sounds, the valves virtually never close! Both valves are ONLY closed for a very brief time during the early part of the POWER stroke, in order to use the generated power to drive the piston downward. Beyond that, one or the other or both valves are at least partially open at all other times!
Anyone can instantly HEAR the effects of an engine with any exotic camshaft, because of that effect mentioned above regarding the rich sounds of exhaust when a lot of fuel is burning IN THE EXHAUST HEADERS! Such engines are also nearly impossible to cause to idle (with the valves rarely both being closed!) and so such engines tend to need to spin at 2,000 rpm or more to keep running (rather than the 550 rpm common in conventional cars with stock camshafts). Finally, a normal starter motor is only able to spin an engine a little faster than the needed 550 rpm for stable idling, so when you have an engine that cannot idle at below 2,000 rpm, starting it is a real problem. Around 40 years ago, some creative drag-racers discovered that they could cause a standard starter to spin fast enough if it was powered by two or three or even four batteries in series, instead of the standard one battery. (The starter motors do not last very long under this sort of abuse!)
In any case, the central point is that all camshafts have shapes which were developed by experimental results! Thousands of failed designs eventually narrowed it down to cam lobe shapes that are now used. Amazingly enough, there is virtually NO theoretical basis for almost anything about a camshaft! It was all Trial and Error! And after a hundred years or so, they have found cam lobe shapes that seem to be as good as they can be, whether for economy or for power or anywhere in between. A Physicist goes crazy when some technology advanced simply by an endless number of bad guesses! We prefer that there is actually some (theoretical) REASON and LOGIC behind trying new variants!
The thought that occurs to me is to get rid of the camshaft completely! Install VERY POWERFUL electrical solenoids. It seems certain that a 100-watt or 1000-watt solenoid should be able to OPEN a valve VIRTUALLY INSTANTLY! A second similar solenoid should be able to CLOSE that valve just as fast.
Rather than the existing situation where each valve GRADUALLY opens due to the leverage of the camshaft lobe, this concept would allow IMMEDIATE AND FULL FLOW. A standard camshaft lobe causes each valve to follow a (roughly) sinusoidal path regarding being opened. A mathematical Integration of that motion shows that the actual total airflow is only around HALF of what would theoretically be possible. So, it seems to me that if extremely strong solenoids forced the valves to SNAP open and closed, almost every aspect of engine performance should improve ENORMOUSLY!
It might be that some people are reading this page, or other people have realized the same (obvious) things mentioned just above, as there seem to be a number of companies that are trying solenoid-driven engine valves. I personally wonder if any of them are using any strict theoretical Physics in such efforts, or if they are relying on making a lot of assumptions which they see as common sense. I also realize that their primary reason for experimenting with this sort of idea is that they want to be able to sell engines which might have the fuel economy of 4-cylinder (or even 3-cylinder or 2-cylinder) operation, while still being able to advertise the horsepower of a 6-cylinder or even 8-cylinder engine. We shall see how they progress!
This geometrical mechanical advantage was a standard feature of the old steam engine locomotives, where the entire available steam force was always applied at the best possible mechanical advantage. In comparison, internal combustion engines are rather pitiful regarding mechanical efficiency! However, this hypothetical arrangement is not possible in a normal automotive engine. It is easy to see from geometrical analysis that the piston necessarily has dropped exactly halfway down the cylinder, with the loss of almost all compression advantages and there is no flexibility on this point.
In early 2004, I built a very strange engine out of mostly parts from Briggs and Stratton 3.5 horsepower lawn mower engines, which came fairly close to achieving this great improvement of mechanical leverage and advantage. It was around 9 cubic inches displacement and it ran on standard gasoline. At the standard speed of a lawn mower of 3600 rpm, it produced around 12 horsepower, entirely due to this mechanical improvement of usage of the available force and therefore production of torque. Unfortunately, my enthusiasm of the moment caused me to rev it up even more, and at around 6300 rpm it briefly created just over 43 horsepower when the components of the lawn mower engine decided to disintegrate. So it IS possible to greatly improve the performance of ICEs. It appears that except for rare instances when someone has explored some actual new approach (such as with the rotary engine or Stirling ideas), everyone seems locked into trying to find incremental improvements in existing technologies. Rather sad.
"Ground transportation vehicles are powered, by and large, exclusively
by internal-combustion engines. In passenger vehicles in particular,
the thermal efficiency of the [engine] cycle is of the order
of 10 to 15 percent."
from Mark's Standard Handbook for Mechanical Engineers, Tenth Edition (1995), page 9-29.
(That particular reference had been composed for an earlier Edition of Marks of the late 1970s, and the number had gotten somewhat outdated by the 1995 Edition.)
In the discussion above, we have seen WHY the overall efficiency is so dreadfully low for ICEs. The cooling system MUST get rid of around 40% of the fuel's energy, just to keep the engine from melting down or warping and failing. And the exhaust gases MUST carry away around another 40% of the energy from the fuel. That only leaves around 20% left which can be converted into useful mechanical energy. Yes, tweaking the exhaust system to reduce hot exhaust gas flow can help, but that also restricts the flow of air/oxygen INTO the cylinders and also creates more work for the pistons to do in pushing the gases out. Ditto, adding a turbocharger (a supercharger that increases the amount of oxygen/air pushed into the cylinders) which is powered by the exiting hot exhaust gases generally DOES have a positive benefit, but the improvement due to having more fuel-air to burn has to overcome the significant power required to force the exhaust out even harder in order to spin the turbine in the turbocharger. No free lunch!
By increasing the temperature of the thermostat, in other words, reducing the effectiveness of the cooling system and making the engine run hotter, a SLIGHT improvement in fuel economy is achieved. However, the hotter engine tends to heat incoming air up which REDUCES the air density and therefore reduces the power produced by the engine. Now you know WHY the engine seems to have more power if you replace the modern 195°F thermostat with a 165°F one, but the engine creates more pollution due to poorer burning and it also has worse gas mileage.
Engine manufacturers have come up with many dozens of different ideas to try to (incrementally) reduce the heat carried away by the cooling system and/or the heat carried away in the exhaust. But as just noted, all such changes tend to also have negative effects as well as the desired positive ones. And so the fact that most modern vehicles now on the road have around 21% overall thermal efficiency is NOT likely to significantly change, IF ICEs are used in the future.
Comment: You have certainly noticed that car manufacturers have been trying to explore hybrid cars, electric cars, fuel cell (hydrogen) cars, ethanol (E-85) (for a while) and many goofy ideas. Yes, they partly are doing that because the public is wound up over all the energy issues in the news. But doesn't it seem strange that they are spending billions of dollars on ideas which never seem to work out? There IS a reason that they never bother to tell us about! Around 2004, I discovered some PUBLISHED reports by the Oil Institute and other related organizations, which presented the data on consumption, usage and supplies of fossil fuel supplies. It scared the daylights out of me! Those (published) Reports were somewhat tricky in how they present the data, where it was difficult to compare the values of the data on consumption, usage and supplies (for each country and each year), but once the data is converted into the same units, the REMAINING EXISTING SUPPLIES are VERY low. That data indicated that the US (then) only had enough known petroleum to supply our current needs for just over FOUR YEARS (if no imports were made). The people who think natural gas is the answer for the future would see that only EIGHT years of supply of that was in the ground under America. That data (now updated with newer data from their June 2010 Report) is at Energy Supplies. SEE why the automotive engine manufacturers are trying to find some NEW ways to power the products they hope to sell in the future?
YOU can actually confirm the overall efficiency for yourself with your own car! I will use the example of one my Corvettes. At a constant 60 mph on a straight and level Interstate Highway, I get around 25 mph, which sounds GOOD for a Corvette! OK. According to GM information, the frontal area of the car is around 19 square feet, the aerodynamic coefficient of drag (due to the shape of the car, and which is fairly constant for different speeds) is 0.330 and the tire resistance drag is around 0.015 (depending on tire type, inflation pressure, temperature and speed). From this we can calculate that the Aerodynamic Drag at 60 mph (88 ft/sec) is 19 * 0.330 * (88)2/(13*32) pounds of force (the last factor being the air density in slugs per cubic foot), which gives 116.7 pounds of aerodynamic drag, at 60 mph. (at 70 mph, it is easy to calculate that it rises to 158.9 pounds.) Tire resistance drag is 0.015 * 3200 pounds (the vehicle weight) or 48 pounds at 60 mph (and around 60 pounds at 70 mph). This makes the Total Drag as 116.7 + 48 or 164.7 pounds at 60 mph (and 218.9 pounds at 70 mph) (and 51.9 + 32 or 83.9 pounds at 40 mph).
Clarification Note: Many articles and web-pages, and even many respected textbooks (including Marks), contain a serious error regarding the subject of the previous paragraph. They apparently see the V2 in the formula for Aerodynamic Drag, and they must believe that is therefore referring to some relationship to Kinetic Energy (which is 1/2 * M * V2), so they add in a 0.5 in their formulas! Nope! It only turns out that it is a fluke that there are two Vs in there and they happen to be identical! The relationship is actually one regarding the analysis of the Momentum (lb-ft) of the air colliding with the frontal area of the vehicle. FIRST, we are HITTING the air with a velocity of 88 ft/second. SECOND, the AMOUNT of air that we are hitting is given by the density of air times its cross-sectional area, times its "length" (per second). The coefficient of drag is essentially telling how quickly the air gets out of the way of the vehicle! So the correct formula is D = ρ * CD * S * V2, indicating the usual designations for the air density rho, the coefficient of drag, the frontal area of the vehicle and air velocity. The formula might be more clearly written as D = CD * V * ( S * V * ρ), where the contents of the parentheses are simply the mass-flow rate of the air, each second (or slugs / second). Multiply this by the velocity and end up with a Force!
At 60 mph, the total required horsepower to overcome this and maintain a constant speed is 164.7 * 88 / 550 or 26.4 horsepower (20 kW). (at 70 mph it is 40.9 HP [30.5 kW], a considerably higher drag load!) (the 550 is to convert feet-pounds per second into horsepower.) A horsepower is equivalent to 2544 Btu/hr (from above) so this is 67,200 Btu/hr (26.4 * 2544) of needed (or usable) output. In one hour of driving at that constant speed, we would therefore use up an amount of energy equal to 67,200 Btu. (at 70 mph, 104,000 Btu.)
A gallon of nearly any type of gasoline contains around 126,000 Btu of chemical energy. In the hour of driving, I would cover 60 miles and get the 25 mpg, which means that I would use 60/25 or 2.4 gallons of gasoline. That much gasoline has 126,000 * 2.4 or 302,000 Btus in it. Since the car used 67,200 Btu to maintain that 60 mph constant speed, the overall thermal efficiency is 67,200/302,000 or 22.2%.
At 70 mph, I tend to get around 21 mpg, and therefore would use up 3.3 gallons in traveling those 70 miles, or a gasoline energy content of 420,000 Btu. So we would have 104,000/420,000 or around 24.8% overall thermal efficiency. Interestingly, the thermal efficiency is actually higher at the higher speed, but it is more than overcome by the far greater total drag, which is why gasoline mileage goes down at high speeds.
A primary reason for this disappointing efficiency is this unfortunate mechanical arrangement where the majority of the force applied to the top of the pistons is NOT able to get transferred into torque in the crankshaft but instead attempts to drive the whole crankshaft down out of the engine. (Since pressure remains in the cylinder, it eventually gets to a point of having a better mechanical advantage, but by then the pressure in the cylinder has dropped quite a bit due to the piston lowering and the cooling system effectiveness.) A large amount of wasteful frictional and cooling system heating is the result of this inherent characteristic of automotive engines, and the engine bearings take a serious beating. The engine then needs a variety of systems (lubrication system, cooling system, etc) to then discard all this heat energy that is wasted.
We mentioned above that enormous amounts of heat must be removed (and discarded) from the cylinder walls and heads, an amount generally equal to 100% to 200% of the rated output of the engine. This should seem a shocking statement, that a 200 hp (150 kW) engine necessarily wastes 200 hp (150 kW) to 400 hp (300 kW) of energy through its cooling system! A lot of this has to be wasted because, when the explosion first created the maximum dynamic pressure in the cylinder, the piston had nowhere to go, being virtually at TDC. (This is essentially the definition of the Otto cycle engine, that of a constant volume combustion.) So those 4000°F gases are trapped above the piston, surrounded by a really efficient cooling system! Before the crankshaft has advanced enough degrees to start being able to transfer useful torque to the crankshaft, the cooling system has necessarily already greatly cooled off the hot gases! Does this seem like a poor design, or what? Enormous waste of energy is built into the design! ALL internal combustion engines face this situation!
There is another way to indicate this poor overall efficiency of automotive engines. Consider a small-sized, reasonably aerodynamic automobile, with an engine that is considered efficient, traveling at a constant 60 mph on a highway, with no significant wind. Because of the alleged efficiency, this vehicle gets 30 miles per gallon at that constant speed.
The total vehicle drag (F) can be shown to be around 140 pounds, 110 of which are due to aerodynamic drag and 30 of which are due to tire resistance frictional losses. The total actual power needed to overcome this drag is given by F * V (velocity). Our numbers are then 140 pounds * 88 feet/second or around 12,300 ft-lbs/sec. Dividing this by 550 converts it to horsepower, or around 22 actual horsepower (16 kW). (Very streamlined cars will have even lower aerodynamic drag and so this required power could be even less).
Since this vehicle has a 30 mpg gasoline consumption, it would use up exactly two gallons of gasoline to travel the 60 miles covered in one hour. Each gallon of gasoline contains about 126,000 Btu of available chemical energy. Therefore, two gallons contains 252,000 Btu, so the vehicle is using 252,000 Btu/hr. It is a fact that 2544 Btu/hr is equal to one horsepower, so this amount of energy in the gasoline represents around 100 horsepower.
The vehicle / engine efficiency would then be 22 hp / 100 hp, or around 22%, which confirms the earlier statement about the overall efficiency of this equipment.
Long ago, it occurred to me that NO ONE actually NEEDS or USES the 451 horsepower of a recently advertised car! That such great power is only ever used for less than 30 seconds at a time. That otherwise, most cars only need around 40 horsepower (30 kW) or less to cruise at constant speed on an expressway. Detroit never seemed to realize that, and they designed many vehicles with huge engines that were tremendous gas-guzzlers.
Around twenty years ago, in the late 1980s, I had a Oldsmobile Cutlass Ciera, which was a front-wheel drive car. I also had the carcass of an extremely old Volkswagen van from the 1960s. Something then occurred to me that has amazingly seemed to have never occurred to anyone in Detroit! If you have followed all this stuff up to here, this should make incredibly good sense to you! The rear axle of the Ciera didn't actually do much other than support the rear end of the car. So I dragged the Volkswagen "pancake" engine and transaxle across my yard and saw that it probably would have fit under the rear of the Ciera (if I removed the gas tank and put it somewhere else, as it was the ONLY apparent interference!).
So I was considering adding the second engine to the Ciera! A TINY engine! As near as I could tell, the pancake engine was flat enough that no actual changes should have been necessary to the Ciera, to allow it to remain at the same height. In other words, from an appearance point-of-view, the Ciera would have remained absolutely normal looking!
Have you caught on of why I thought this might be a good idea? At the time (late 1980s) I was one of very few people who seemed to really care about fuel efficiency. But I thought I had come up with a really good and really obvious solution. It seems to me that it still is just as valid today!
I was aware that my Ciera generally got around 17 or 18 MPG on the highway, but it was fun to drive because the 3.8 liter engine had a decent amount of power.
So, I intended to rig up the gas pedal where if it were pressed HALFWAY DOWN OR FARTHER, the (front) Ciera engine would start up, but otherwise it would NEVER actually be running! So during normal driving, the 1.2 liter Volkswagen engine would have powered the car. I was confident that it would have gotten at least 30 MPG, and reasonably likely around 35 MPG on the highway. The little engine had enough power to easily maintain the Ciera at 60 mph highway speed. So the vehicle would have gotten about the best gas mileage of any vehicle of that era (late 1980s)!
Now say that I wanted to do a hole-shot from a stoplight, or wanted to pass a car on a two-lane road. The Ciera engine would start up, and actually I would have had TWO engines both accelerating the Ciera! A rear-wheel-drive AND a front-wheel-drive! It likely would have had better acceleration than any other Ciera, due to the two engines!
So I would have wound up with a car that LOOKED absolutely normal, had acceleration at least as good as original and maybe better, and yet possibly TWICE the gas mileage! Cool?
It turned out that my life got extremely busy and I never got around to doing that interesting experiment! And since shortly after that I started driving my two Corvettes, and they are rear-wheel drive, I never considered any personal motivation to remain! Also, I am not sure that I would have wanted to maltreat a Corvette quite that badly!
I realize that adding an entire extra engine would add to the cost of manufacturing vehicles, when the manufacturers hire people exclusively to find ways to eliminate a tenth of a penny from the cost of the cigarette lighter! So maybe that is why they have never even thought of this concept. Seems pretty obvious to me though!
I must admit that I had earlier personally tried a truly stupid idea that was vaguely similar, and maybe those bad memories caused me to think of it during the 1980s. When I was in College, I drove two 1956 Ford convertibles. That year happened to have a very tall trunk. The motors in those cars were considered decently powerful, being V-8 292 cid engines. But as a young kid who liked to fool around with cars, well ...
I happened to have rebuilt a Mercury 383 engine, and I had toyed around with the idea of replacing the 292 with the 383 for, as Tim Allen would say "more power!". But I thought I came up with a better idea yet! I measured and measured and found a way to fit the large 383 engine inside the rear trunk of the '56 Ford! I decided to install it backwards, with the idea of having a normal (but much shorter) driveshaft. I got two spur gears (an especially dumb idea!!!), and put one on the snout of the 292's normal driveshaft and the other was rigidly mounted to the actual second driveshaft. The fact that the rear engine rotated backwards was then a good thing, because the spur gears rotated in opposite directions in order to mesh together, where either or both engines could then drive the car.
Well, it actually worked, for a few days! If I ONLY used the 292, and left the other transmission out of gear, only the second driveshaft rotated, and everything worked pretty normally. And I drove it a little with ONLY the 383 powering it. It was a little flaky but generally worked fine, although I never really pushed it hard. The car's handling was VERY strange, as the big old engine's 500 extra pounds so far in the rear made it somewhat spooky to drive.
I was still young, and my knowledge of Physics and Engineering was still limited. That's my story and I am sticking to it! But I had not realized that the big-bore 383 had a torque curve that was at much lower engine speeds than the smaller 292. On the single day when I fired up both engines and thought I was going to have really impressive acceleration from over 500 hp, that little detail very quickly sheared off all the teeth of the gears! As I was sitting just a foot away, I was hit by several of them as they exited the scene! I guess there was such a great torque difference between the very different engines that it happened at very low speed, which might have kept me from being killed, which might have happened if they had sheared off at high speed, when they were spinning very fast.
I always wondered after that what might have happened if I had been more conservative and put a second 292 in the trunk! But I suspect my knowledge of gears at the time was not sufficient even for that. So that car pretty much just sat after that until I eventually took the rear engine back out.
But maybe that experience caused me to even think that a second engine might make sense, many years later. By then, I had also seen in hot rod magazines where a few people had done what I had tried to do, but far more successfully!
See below, where a different variant of this idea now seems extremely interesting! Instead of a rear gasoline engine, two severely over-driven electric car starter motors! In that case, the actual engine (FWD) would be smaller, barely enough to maintain highway speed, maybe 50 hp (37 kW). The electric motors would be so severely over-driven (by feeding them 36 volts or 48 volts instead of 12) where they might provide a BRIEF BURST of around 400 hp (300 kW), but only for a maximum of 10 to 15 seconds (or else they would overheat and self-destruct). The premise would be an ECONOMICAL vehicle that LOOKED NORMAL but had a SMALLER than normal engine for really great gas mileage, but having the capability of a few seconds of spectacular acceleration, like gearheads dream about!
Until the 1940s, automobiles were TALL! The HOOD had to be tall because the vertical structure of inline engines required a lot of vertical space under the hood. But then stylists started wanting to sell cars that were sleeker and lower. A V-8 engine has its cylinders at a significant angle, which actually allows the engine to be designed with several inches less overall height. The extra pistons were also popular in providing more power, but there had been I-8 and even I-12 engines used earlier. An additional benefit for the stylists was that the V-8 engine was SHORTER (front to back) than even an I-6, for additional flexibility regarding styling.
There is no significant difference in overall efficiency between a V-8 and an I-6, and if they have the same number of cylinders and same displacement, the performance is very, very similar. STYLING was essentially the only real reason why V-8s took over for several decades!
In trucks, which are DESIGNED to be tall, there is no benefit of trying to save a couple vertical inches, so straight-line engines are nearly universal. The only other consideration is that a V-8 engine has more moving parts, which will eventually wear out and fail. Since trucks are intended to go as far as possible, another reason for considering an inline engine!
But then the design of the exhaust headers then comes into play. The cylinders fire in a different sequence in different engines. This all results in four separate surges of such unburned fuel-air mixture entering (each) exhaust manifold from four different cylinders. They do NOT fire equally spaced in time! FACTORY exhaust manifolds rarely considered any issues of interference of the surges interfering with each other, and different manufacturers' different firing sequences resulted in exhaust that therefore sound differently. The exhaust manifolds (and engine firing sequences) on the small block Chevy engines of the 1950s and 1960s were better designed than the others, which both created a unique sound pattern and also greatly reduced the needed power to force the exhaust out. You probably do NOT want to now get a more technical explanation of this, which is VERY complex.
For highest performance engines, custom-designed exhaust headers are used. Not all of them are designed really well, and some seem to have been designed to be decorative! But the really good ones were Engineered to permit each surge of exhaust gases to arrive at the joining point without any interference of any surge from any other cylinder. An interesting Engineering problem to solve, and many companies that sell custom exhaust manifolds did not seem to do the necessary calculations! (Personal opinion!) Finally, many dragstrip engines have SEPARATE exhaust headers for each cylinder, to completely eliminate any possible pressure conflicts that might use up some horsepower.
By the way, exhaust header design is SPEED DEPENDENT. Few of the designers seem to know that! The well-designed headers are designed so that at red-line engine speed, each pressure surge from a cylinder is able to clear before the next pressure surge arrives from a different cylinder.
Early cars had VERY heavy flywheels. Whether hand-cranked or with electric starters, that aided the starting of engines, as it permitted variations in how much gasoline had gotten into each cylinder, by allowing ANY cylinder which fired to increase the spinning speed so that the other cylinders could start behaving correctly.
Before around 1954, all cars had stick transmissions. That meant that when the clutch was pushed in, the engine could run with no external load. A very heavy flywheel had the added benefit of keeping the engine from blowing itself apart if the gas pedal was pushed all the way to the floor with the clutch released. The flywheel's Rotational Inertia was designed to be enough where the engine had to take many seconds of being floored without load, before the engine might rev up above its redline speed. At that time, engines were underpowered and also built like tanks, so they really rarely could rev up fast enough to do themselves damage anyway. Manufacturers LIKE if their new vehicles do not self-destruct!
In the 1950s and 1960s, muscle cars started being manufactured. In general, the manufacturers chose to install very heavy (thick) flywheels on their vehicles, such that the public would not be likely to over-rev any of their vehicles and get bad Public Relations. But they installed essentially identical but thinner flywheels in vehicles that were considered high-performance. Why? No FUNCTIONAL reason, actually. The thinner flywheels allowed the engines to run rougher, a disadvantage to the general public.
Finally getting to the point here! The thinner flywheel had less Rotational Inertia (I) which meant that TORQUE created by an engine which had the clutch disengaged WOULD REV UP FASTER! If a moderately noisy exhaust system/muffler like a glass-pack was used, the SOUND of the engine revving up unexpectedly fast SOUNDS like the engine is really powerful! It's quite an interesting change, and the sound effects are quite impressive!
Conveniently, both Ford and General Motors (and I assume Chrysler) used essentially identical flywheels in nearly all their vehicles for many years. Back then, when friends would bring their cars to me to improve them, they rarely had enough money to buy the big carburetors and improved intake manifolds and exhaust headers and camshafts to ACTUALLY make their cars hotter. I did not have to charge them too much to replace the stock (thick) flywheel with an identical one that was from a performance car (i.e., thinner) and also replace the stock muffler with a glasspack. When they would first sit in their car and rev it up, they were always amazed that it was still their car! Their IMPRESSION was that it sounded far more powerful! The glasspack muffler was so that when they were driving (in other words, the clutch was engaged and the engine was loaded), the fact that the engine was actually no more powerful would not be obvious, the louder exhaust distracting their attention.
Now, there IS a down-side to using a thinner flywheel, which I discovered one day back then. A VERY cute girl kept insisting on sitting in my (severely modified) car. She talked me into letting her start the engine, with the car inside my garage. I had installed a VERY heavy duty clutch, and I was pretty sure that she could never have pushed it down to do any shifting, so the car was not going to go anywhere. But the crazy girl pushed the gas pedal to the floor and kept it there! With the lighter flywheel and very powerful engine, it revved up very fast to speeds which seemed likely to destroy itself. Fortunately, I was sitting right there and I grabbed the ignition key and turned it off, which was certainly the only reason I did not have lots of expensive engine parts all over the garage! I never again allowed any girl to start the engine of any of my hotter cars!
So IF you did not provide the specified ring gap, then when the engine got hot, the expanding metal of the rings has nowhere to go! The softer metal of the piston generally loses this battle, and the engine either seizes up or comes apart, both making for VERY bad days!
The process IS quite interesting and it is definitely unique, and which happens to appeal to me personally! But I would note two facts for anyone who is ready to give somebody lots of money for something that allegedly uses the Stirling process:
The Stirling showed a significantly greater overall thermal efficiency than ICEs can have, but that is mostly due to the fact that extensive heat exchangers can capture and recover a lot of exhaust heat to be used again. Even though the pictures of Stirlings are very pretty, in order to operate efficiently, they MUST operate at extremely high air pressures (generally above 1,000 PSI) and at rather high temperatures (commonly above 1,200°F). These requirements represent some big complications in making useful products. One of the Stirlings that got a lot of press was around 450 pounds and it produced 30 hp at 39% overall thermal efficiency, and 40 hp at 33.3% efficiency. That was impressive efficiency numbers when ICEs were all around 15%, but it was a monstrous big engine for producing a disappointing amount of power!
Many Engineers have spent their lifetimes in trying to ensure more smooth progress of the Flame Front inside the cylinder. Honda (CVCC) and Ford both promoted methods of swirling the air inside the cylinder (in rather different ways) to try to improve this characteristic, and many other approaches have been tried. Back in the Stone Age, all engines were FLATHEADS. But the location of the valves in a Flathead greatly limited the airflows and performance and pretty much everything else. Overhead valve engines created a great improvement, better and faster airflows into and out of the cylinder, which was adopted by all engine manufacturers. Other improvements have generally been more subtle.
This practical (average) torque is also lower than the maximum numbers presented here. In a V-8 4-cycle engine, each piston is responsible for developing torque over a 90° range of crankshaft rotation, before the next piston can take over. We have generally been discussing maximum instantaneous torque for specific crankshaft positions. It should be clear that the measured torque of any engine will be less, because it represents the average of torque developed during that entire 90° of crankshaft rotation, because no other cylinder is yet firing.
The crankshaft angle torque curves vary greatly in shape for different engine speeds, being very narrow at low engine speeds and rather broad and fairly constant at high engine speeds. The very narrow angle range of productive power for an engine at idle combines with the earlier mentioned geometrical disadvantage to fully explain why automotive engines can stall at low idle speeds.
When EXPERTS mention Hydrogen in the future, they do NOT refer to BURNING it! They are talking about a technology that NASA developed in the 1960s for spacecraft, where hydrogen gas can DIRECTLY create electricity in a rather exotic device called a Fuel Cell. There is NO flame at all! Theoretically, a really good Fuel Cell might have nearly 100% efficiency. But even NASA with its unlimited budget never remotely came close to that. But multi-million-dollar fuel cells have been in many satellites, and they produced the few hundred watts of electricity (rarely even ONE horsepower-worth!) needed by the electronics onboard the satellite. The dream of a future hydrogen fuel source is based on enormous advances in the technology which may be hundreds of years away, of finding ways to make Fuel Cells that are very efficient, very high power, and very inexpensive. Don't hold your breath!
The thousands of people who see ways to become rich by selling ANYTHING that refers to hydrogen, simply see ways to take advantage of a public that does not know enough! So people send in their hundred dollars for some shiny device that has the word HYDROGEN on it, and they think that their vehicle will run faster, better, you know the pitch! Take the money and flush it down the toilet instead. It will save you some time and trouble!
Hydrogen has all sorts of DISADVANTAGES regarding being a motor fuel. Primarily, it DOES NOT EXIST NATURALLY and must be produced, by any of several processes that are all extremely expensive and high-tech to actually do on any decent scale. We have a presentation specifically about hydrogen, which presents the facts. An amusing detail is that to provide the same amount of chemical energy in a standard tankful of gasoline, you would need to tow TWO FILLED semi-trailers of hydrogen! But it happens to have another disadvantage which relates to the subject of this presentation.
Hydrogen CANNOT simply be MIXED with gasoline as a lot of people now seem to claim and think! GASEOUS hydrogen would simply create BUBBLES in the gasoline, and even if that is taken care of, it provides NO actual power boost benefits at all! And if you think you can afford the equipment to maintain LIQUID hydrogen at around minus 400°F, good luck! The advertisements for such products never mention these sorts of details!
Additionally, when YOUR engine was Designed and Engineered, it was to use known amounts of air to mix with the gasoline. To think that MASSIVE volumes of hydrogen gas could somehow be added in, even IF that were chemically valid, would require larger valves, larger passageways in intake manifolds, etc. Amusingly, there are highly promoted products being sold to the public which may even create as much hydrogen as YOU did in Chemistry class in High School, a tiny bubble the size of a marble, which they then claim gets somehow mixed in with the gasoline-air mixture. From a Lawyer's point-of-view, maybe that is not illegal, but it is certainly extremely unethical. Even IF such a product worked as claimed, an improvement of 0.0001 horsepower would probably never occur. But simply the WORD hydrogen makes some people think they should buy it. Rather sad.
(NOTE: There does not appear to be available any data regarding flame-front speed for Hydrogen gas when compressed as in a car engine. Therefore, we add the following discussion, which also shows the sort of far more comprehensive Physics research that is the basis for essentially all the statements made in this presentation.)
First, everyone is taught in school that Hydrogen "simply"
combines with Oxygen in the familiar (2) H2 + O2
↔ (2) H2O. That turns out to be an enormous
simplification! There are actually 19 different reactions that can
and do happen! Each releases different amounts of energy (with two of
them even REQUIRING energy to occur!). In general, two or more of
these reactions occur in rapid succession, with the end result being
the familiar reaction. Physicists and Chemists analyze ALL of those
19 unique reactions, in order to better understand exactly what
is going on and why. In fact, the overall reaction of Hydrogen
with Oxygen can occur in two VERY different ways! The DESIRED
one is by burning (conflagration) which has the flame-front speed
indicated, around 8 feet/second in the atmosphere. The UNDESIRED
one is by explosion (detonation) which has a flame-front speed
of 2,821 meters/second or 9,255 feet/second! That is around EIGHT TIMES
the speed of sound and many times faster than the fastest rifle
bullet travels! It is incredibly dangerous when Hydrogen decides
to detonate, and science does not yet have a very complete understanding
of why it sometimes does! Our discussion will be about the DESIRED
laminar flame-front process.|
Next, the velocity of the (laminar) flame-front is known to be very dependent on many different variables. Here is an equation that gives the flame-front velocity (speed):
(There are actually three different theories which exist to explain the motion of flame-front travel and this equation happens to be from the one that seems to be the best. Many of the equations involved are far more complex than this one. They were generally developed during the 1980s.)
If a number of reasonable assumptions are made, this can be greatly simplified into:
The exponents are different for each type of fuel gas, and for Hydrogen they have been experimentally determined (Milton and Keck 1984) to be α is 1.26 and β is 0.26
Note that all of this is based on ideal conditions; the perfect proportion of fuel and oxygen; perfect mixing; etc, and that real conditions are often not ideal.
If we assume that an engine has an (actual) compression ratio of 8:1, the pressure increase factor therefore would be 80.26 which is 1.717. The natural flame-front speed of 8 feet/second would therefore increase to 8 * 1.717 or 13.7 feet/second. We note that some 2004 research in Bergen, Norway shows a maximum atmospheric flame-front speed for Hydrogen as 2.8 meters/second, which is slightly higher than the 8 ft/sec cited above at 9.2 ft/second.
This is still far slower than the measured flame-front speeds inside gasoline-fired internal combustion engines (which is generally at least 90 feet/second during most driving). However, the dependence on temperature causes some improvement in this situation. Hydrogen burns at 2,755°C or 4,991°F. The heating of the gas occurs gradually during the process of the combustion, but if we assumed that the hydrogen got up to that temperature, the temperature dependence factor in the equation above would be around 18 to one. This implies that the COMBINATION of the higher pressure and the higher temperature MIGHT cause a flame-front speed which is comparable to that known to be in gasoline-fired internal combustion engines. But it does not appear that anyone has yet actually done such experiments to validate that statement.
If you have been following this reasoning, you now also know WHY engines NEED to have a COMPRESSION RATIO! Simply burning gasoline at atmospheric pressure would have far too slow of a flame front speed to be of any use in an engine! You never knew WHY before, did you? Now you do! It also indicates WHY compression ratios of 2:1 or 3:1 are never seen in engines.
Consider the inside of an engine cylinder in a normal car engine traveling down the highway. The engine may be rotating at 2,000 rpm, or 33 revolutions per second. The piston must therefore move upward and downward 33 times every second, and its (maximum) speed in the middle of its stroke is around 45 feet/second. If a fuel burning in the cylinder is to actually push down on the piston, in order to do actual work in propelling the vehicle, the fuel-air mixture needs to burn at a speed FASTER than the piston is moving! Otherwise, the slow-burning mixture would actually act to SLOW DOWN the piston! It would not only not do productive work, but it would require work FROM the piston.
The ACTUAL hydrogen flame-front speed inside an ICE might be sufficient for conventional burning as in current ICE engines, but someone needs to do the experiments to confirm that! But it suggests that yet another hurdle might lie in front of Hydrogen ever becoming a common motor fuel.
By the way, the INTENDED usage of Hydrogen in vehicles is quite different from this! The much-publicized Fuel Cell is a device which converts the energy in a fuel like Hydrogen DIRECTLY INTO ELECTRICITY. THERE IS NO BURNING INVOLVED! The premise for future vehicles is that they might use Fuel Cells to provide electricity for electric motor drive systems. Which means that Mortuary Services may be appropriate for the Internal Combustion Engine! But it may be another ten or twenty years before fuel-cell technology has developed to the point of that becoming realistic.
As an additional note here, when you see impressive demos on TV or in a video regarding Hydrogen being used as a fuel for a vehicle, try to check to see the source of that Hydrogen! In general, such demos use LIQUID Hydrogen (which is necessarily refrigerated to incredibly cold temperature, within a few degrees of Absolute Zero!) LIQUID Hydrogen does not have the problem of the huge volume of Hydrogen as a gas (where one pound takes up around 200 cubic feet) (one pound of liquid hydrogen takes up less than 1/4 cubic foot, almost 1,000 times smaller). Where we have discussed that one cubic foot of Hydrogen gas only contains around 360 Btus of chemical energy, one cubic foot of Liquid Hydrogen contains around 300,000 Btus of chemical energy in it, relatively comparable to the energy concentration of gasoline (about one-third of it). So, for demonstration purposes, a fairly small amount of LIQUID Hydrogen contains spectacular amounts of energy in it! Which then gives impressive performance by the demo vehicle. However, IF they used LIQUID Hydrogen, that (small) amount for the demo quite possibly cost them tens of thousands of dollars to buy!
But you might notice that even Liquid Hydrogen only actually contains around 1/3 of the chemical energy in it that gasoline does! A cubic foot of gasoline contains around 7.5 gallons, each of which contains around 126,000 Btus of chemical energy, for a total of around 945,000 Btus. The cubic foot of Liquid Hydrogen contains around 300,000 Btus. And as we noted, a cubic foot of gaseous Hydrogen only contains around 360 Btus. Another indication of WHY gasoline has been so popular - it is a very compact form of a lot of chemical energy! And also an indication that ALL the outrageous claims that people now make regarding Hydrogen (or variants of it) allegedly making enormous power, are simply deceptions.
OK. Finally, there are all kinds of hucksters who are trying to sell all manner of products that they claim will give you tremendous improvements in the gas mileage of your vehicle by somehow injecting Hydrogen into the engine. This is really sad regarding how deceptive their presentations are. Again, if you would inject LIQUID hydrogen into any engine, you COULD add a large amount of additional CHEMICAL ENERGY into the engine to be burned. However, what they try to sell are tiny devices which they claim are hydrogen generators. You should realize from this presentation that even if you could generate a cubic foot of hydrogen each minute (which is extremely difficult to do AND would require many horsepower from the engine to generate the needed electricity to do it), that would only be adding around 360 Btus of chemical energy in the hydrogen into the engine (remembering that a gallon of gasoline contains 126,000 Btus of chemical energy in it). A demo (for Reporters who do not know the right questions to ask) where LIQUID hydrogen was injected COULD show measurable improvement, but any device that tries to generate GASEOUS hydrogen to be injected is simply an expensive joke!
IF you have recognized the overall theme of this presentation, of recognizing and examining EVERY DETAIL regarding an engine's performance, I trust that you see that even YOU might now be able to assemble a similar analysis for any Diesel engine or a Wankel Rotary engine. Similarly, if YOU think you have come up with some enhancement in the design of internal combustion engines that none of the millions of Engineers have thought of in the past hundred years, you should now even have the framework to ANALYZE whether such an idea might either improve or degrade the operation of an engine, even BEFORE having to actually try to find investors to finance actually building the monstrosity you have thought up! Physics and Engineering are VERY handy to have around!
By the way, even though there are countless restrictions and rules controlling racing vehicles, no such rule is violated or even challenged by this concept.
They obviously would never offer me "millions of dollars" without knowing what it was for, but once they heard those three minutes, they would likely see that they then would no longer need me! From THEIR point-of-view, they would see it reasonable to say to me "Hey, Polack, here's a hundred bucks for your idea." Well, I may be Polish, but virtually no one has ever thought that I was stupid! At least, THAT stupid!
I don't really see any obvious way to resolve this, except that maybe a few hundred thousand could be put in Escrow (prior to hearing the brief description) and with some "performance payment" which would also then be paid to me (per mph increase, for example).
Sadly, it is a similar situation to where I have been seriously taken advantage of in the past and which I am currently feeling it necessary to be cautious about regarding several current inventions in other subjects. And I really see no logical way that I could feel safe regarding disclosing all the important information about! So it strikes me as simply an Interesting Situation!
Update on this after about seven years, in 2012, I may be about to prove that I really AM that stupid! I absolutely forbid any company or racing team to use this concept without first arranging a WRITTEN contract with me regarding permission from me. But here it is!
If you have paid attention to my web-pages, you know that the Vehicle Drag is nearly completely caused by Tire Drag and Frontal (Aerodynamic) Drag. The second of these is always the greater at any significant speed. The centrally important concept here is that Aerodynamic Drag is accurately proportional to the frontal area of a vehicle (along with air temperature and density and a shape factor (streamlining). You have seen front views of open-wheel racing vehicles, such as some Indy-style racing vehicles. Just a brief glimpse shows that roughly HALF of the total frontal area is represented by the tires, with the fuselage and small windshield being about the other half. The concept which I have long seen as so obvious is regarding replacing the tires with smaller diameter tires. If existing (front) tires are 25" in diameter and 12" wide, the two front tires now represent 600 square inches of frontal area, about 4.2 square feet. Consider replacing those tires with ones which are 20" in diameter and still 12" wide. The two tires would then present a frontal area of 480 square inches or about 3.4 square feet. If we add in maybe 5.0 square feet for the fuselage and all the rest, this would then mean that we might reduce the Aerodynamic Drag area from 9.2 square feet down to 8.4. This is more than an 8% reduction in frontal area, and since Aerodynamic Drag is proportional to that area, that might be an impressive reduction in the horsepower needed to get that racecar to travel along a racetrack. Precise analysis of this is greatly dependent on many other aspects of a specific vehicle, but the clear result of needing to consume about 8% less fuel during a race might eliminate one fuel-pit-stop during a race or of having a smaller and lighter fuel tank in the vehicle for better performance. As suggested above, this might also increase top cruising speed by around 13 mph, which seems to me to ensure victory! There are a number of other benefits which are less obvious (except to a Physicist). For example, in oval track racing on most racetracks that have moderate banking, the vehicle must slow down and then speed back up two or four times every lap. If the fuel tank only needed to contain 8% less fuel during a race, that might mean that less power and fuel might be needed during those hundreds of accelerations and braking might be less stressful on those components. Additionally, with tires which are 5" smaller in diameter, the chassis and engine of the vehicle would be 2.5" lower, for both better stability and another reduction in Aerodynamic Drag. Yes, the gearing would have to be changed to enable the smaller tires to spin 5/4 as fast at a given groundspeed, but that is now standard design. In other words, not much would actually be changed in the racecar except to use smaller diameter tires! There appears to be NO down-side to using this approach, as no changes to the engine or anything else is necessary so that no Regulations would ever be violated! Seems rather obvious to me!
But two-wheeled vehicles have an advantage of having fewer wheels which must flex each time the tires rotate, meaning less Tire Drag. In addition, motorcycles have smaller frontal areas than enclosed cars. These two effects make for far better fuel mileage than closed cars can offer.
But I thought of adding a vertical-axis gyroscope down near the ground. The reasoning is that a spinning gyroscope has a Physics effect where its spin axis tends to maintain its direction. You have seen this in children's tops and toy gyroscopes, where they can spin without falling over, as long as the rotor continued to spin. So I decided to do a preliminary experiment with an old bicycle and a flywheel from an antique refrigerator compressor. I bolted on a downward-pointing short axle below the pedal bearing, and I mounted the (heavy) flywheel onto that short axle. When the bike is standing vertical, the flywheel was exactly horizontal. I actually did not bother to install a drive motor to up-spin the flywheel as this eas an early experiment, and I used a standard fractional-horsepower electric motor and a belt and pulleys to upspin that flywheel.
It was pretty bizarre to be able to stop using the bike's kickstand, as the bike now would stand vertically erect even without a rider and without any motion. (A bike or a motorcycle normally needs the tires and wheels to be spinning to act as such gyroscopes to provide for the ability to not fall over. That is why you always had great trouble balancing a bike at really slow speed but it was easy to do at higher speed. The spinning wheels provided the gyroscopic effect which gave what is called a Metastable Equilibrium.
My intention was to provide that stability and equilibrium without needing to have the tires spinning. Due to Physics consequences, I knew that it was important to have it mounted with a vertical axle (for complicated reasons). My experiment certainly worked excellently.
I doubt if there would be any real use for this concept in bicycles or motorcycles, as the riders of such transportation already are familiar with the characteristics of their usage. But it seems to me that there might be a future for an even larger flywheel (gyroscope) under a four-passenger fiberglass body mounted on basically a motorcycle. I guess it would technically (and probably legally) still be a motorcycle, but if the entire vehicle weight was under 1,000 pounds and there were only two tires creating Tire Drag, this might have the stability and handling of a four-wheeled car but with far better fuel-mileage and far lower price. Such a design might replace current cars with ones that were far more popular and cheaper!
If a driver was alone, it would look rather peculiar, where the vehicle would look very unbalanced, with all the driver's weight being well off to the side. The spinning flywheel would need to have another major feature to counteract those effects, which is actually another field of Physics based on the Euler Equations.
Again, I only did the most primitive of an experiment regarding this idea, and it might not actually be of any value. I merely mention it here because the world's remaining supplies of petroleum are predicted to be entirely used up by the year 2043, as described in recent Annual Reports from the Oil and Gas Journal. Recent industrial development by China and India and dozens of other countries suggest it might be even earlier that we use it all up. Hydrogen Fuel-Cell vehicles will certainly not be available by then. So it might easily be that vehicles that are really stingy at using CNG (Compressed Natural Gas) or electric vehicles may need to be the popular motive forces after that. The current idea of taking two tons of steel, or more recently, a ton and a half of steel and plastic, in a modern car or SUV or truck, to get a 120-pound woman to a grocery store, figures soon to be history.
Personally, I have a suspicion that ALL modern vehicles may soon become obsolete, and something resembling the TRANS system which I tried to present around 1990, might replace all vehicles, including trains, (most) trucks and airplanes. It will likely depend on when anyone feels it really urgent enough to be desperate to consider other alternatives. In my view, politicians and Corporate Executives never like to rock the boat, as they want to maintain the proven profitability of whatever they know of as the status quo, so it may be some years before anyone seriously considers anything like TRANS. TRANS 200.0 mph passenger and freight national transportation system which uses no fossil fuels.
It represents a very unusual engine, which may not be very compatible with modern automotive manufacturing technology.
However, I later (late 2004) came up with a rather different concept of the same basic invention, which probably has massive application. It involves a retrofit modification of a conventional V-8 engine. Relatively few different parts are needed, generally using most of the original engine parts, including the block, heads, oil pump, water pump and all accessories. The heads need to have some machining done to them, and a different (and very strange) crankshaft and camshaft are needed, along with different connecting rods. It does NOT seem compatible with I-block or V-6 engines.
I am not interested in assisting giant corporations to make additional billions in profits; however I would be quite interested in advancing a retro-fit system and am open to the possibility of a mutual business effort regarding manufacturing and providing suitable kits.
The resulting (small block Chevy) is an engine that idles at around 60 rpm (instead of 600 rpm) so that it uses only 1/10 the fuel at stop signs and in rush hour traffic. It has greater torque output, on the order of 500 lb-ft, compared to the common 200 lb-ft that many V-8 engines produce. Also, where conventional engines produce that maximum torque only at around 1800 rpm, this engine had a relatively flat torque curve, even generating close to that 500 lb-ft near the 60 rpm idle speed! (Which is partly why it is able to idle at such a slow [actual slower than heartbeat-rate] speed.) The result of all these differences are that this engine has better gas mileage (by over 50% improvement) while also having acceleration performance that massively out-performs any conventional engine.
The specific levels of these improvements depend on some features of any specific engine design and construction. These figures are based on what is called the small-block Chevy (327 or 350 cid) engine.
I am NOT sure that the concept is compatible with V-6 engines, and I doubt that it is compatible with any four cylinder engines. The fact that V-8s have pretty much become dinosaurs may mean that this concept would have no possible future.
I do not intend to be providing Engineering assistance to individual people who only want to win trophies at a drag strip! Someone would have to convince me that there was a credible possibility that this improvement might actually advance to the stage of becoming a retro-fit kit, with credible marketing arrangements for millions of drivers to benefit from it.
This same general theme has resulted in yet another variant! In June 2009, I discovered a way to make an engine which is extremely different than either of the above, but which has the capability of even better performance and fuel economy, as well as several other surprising benefits. If this one works as calculated, it might represent an enormous advance in automotive design. I am currently working toward getting a prototype built.
I would be willing to help Detroit or Toyota or someone else to build this practical vehicle and probably economically priced vehicle, which has some vague similarities to some parts of the Tesla electric sports car!
Long ago, I realized that NO driver ever actually USES the huge horsepower of the over-powered cars that are sold, EXCEPT for a maximum of less than 30 seconds at a time. In all the time I have owned my Corvettes, and an Austin-Healey 3000 and other sports cars, there has NEVER been any time where I had my foot to the floor for more than 15 seconds, and that was during a quarter-mile drag where the vehicle went from zero to around 120 mph in around 13 seconds. So it occurred to me that it really is foolish for people to buy cars that have giant engines that are advertised as 470 horsepower or 505 horsepower! At all times other than those few seconds, the driver has to be paying for gasoline that is being burned for the CAPABILITY of that power and acceleration.
In an entire year of owning and driving a Corvette, I doubt that there are more than a twenty times when I really use massive power for more than maybe three seconds at a time. I realized that meant that I actually USED all the power that Corvettes are known for, for maybe ONE MINUTE TOTAL per year! And I am a relatively aggressive driver!
I had started assembling an experimental vehicle, based on a 1985 Oldsmobile Cutlass Ciera 3.0 liter V6 front-wheel drive car I then had. (It was later vandalized beyond possible repair, so I have not yet again pursued the project with any other car [yet]). The car was mid-sized, capable of holding five or six people, a pretty standard vehicle. Its moderate-sized engine permitted tolerable acceleration but never anything really interesting (to a Corvette owner!)
I noticed that the rear wheels (of the front-wheel-drive car) really did not do anything other than support the rear of the car!
I also knew that even a STANDARD car battery can contain around 80 ampere-hours of electric power in it, which, at 12 volts, is about 1 kWh (80 * 12 Wh, as discussed above). That meant that the one standard battery could provide about 1.5 horsepower for an hour, but that also meant that it contained enough power to provide 1.5 * 60 or 90 horsepower for one minute, or 180 horsepower for 30 seconds, or 360 horsepower for 15 seconds! (A deep-discharge battery has even more energy capacity.)
So my experiment was/is to be a car like the generic Cutlass Ciera, with its standard 120 hp economy engine, but where EACH of the rear wheels was replaced by an electric-motor-driven wheel, driven directly from TWO(*) batteries in series. (Total, two motors, resembling car starter motors, and four standard car batteries in the trunk, a rather minimal added expense beyond the modest cost of the standard Cutlass Ciera!
Maybe it would represent adding $1,000 to the cost of NEARLY ANY front-wheel-drive car. And what would be the result?
In the process of turning the engine to start a vehicle, it can briefly draw around 500 amperes of electricity from a (single) battery. At around 10 volts, that is around 5,000 watts. Since each horsepower is equal to 746 watts, a normal starter has the capability of producing around 7 horsepower or so (ball park, each Make and Model and engine size is different, and with modern vehicles with tiny motors, they need less horsepower during starting so most MODERN starters have less capability.
Just adding 7 (times two) horsepower would not be worth the trouble. But starter motors are designed to be durable enough to reliably start the vehicle for many years. So long ago, people learned that in order to start engines that had really exotic camshafts, a standard starter and battery just didn't cut it, it didn't turn the engine fast enough to start. So what was their solution? You guessed it! They used the SAME starter motor, but ran it on 24 volts instead of 12! Two batteries in series! In Electrical Engineering, a standard formula is that the POWER is proportional to the SQUARE of the voltage, if all other variables are kept the same. Instead of the starter producing around 7 horsepower to start the exotic engine, it produces around 28 horsepower. So at dragstrips, you often hear starter motors which sound like dentist's drills because they are spinning so fast. BUT AN IMPORTANT FACT IS THAT THEY STILL LAST FOR A DECENT TIME!.
My experiment was to use that (conservative) arrangement in the Ciera, four batteries. The experiment would therefore be expected to add around 56 horsepower (28 * 2) extra to the 120 horsepower of the conventional engine. Not spectacular, but the total of 176 horsepower would actually have greater benefit than that, because the 120 horsepower RATING of the standard engine actually got far less horsepower to the wheels! So I figured that my rather economical experiment should provide GREATER THAN 50% faster acceleration, likely close to double the acceleration. Given that using 24-volts to power race car starters has long shown that the starter survives pretty well, I consider that a very conservative experiment!
Of course, the next step would be to try THREE batteries for each of the starter motors. I am not aware of anyone who has done that before, so it is not clear how long the starter could operate before becoming toast. However, the simple fact that it is NEVER intended to be powered for more than 3 to 10 seconds at a time, figures to allow the starter windings plenty of time to cool back down!
In any case, using three batteries for each starter motor should produce as much as 7 * 32 or 63 horsepower at each rear wheel, or 126 additional horsepower. I am suspecting that an innocent looking Cutlass Ciera with a putt-putt engine should have impressive acceleration with 126 additional horsepower!
And of course, my sugar plum dreams would require at least TRYING four batteries for each! That would be 7 * 42 or 112 horsepower at each rear wheel, or 224 additional horsepower. Now keep in mind that these experiments would all use GENERIC STARTER MOTORS, and that the recent Tesla sports car uses a very exotic (and very expensive) motor and battery pack that has proven that even greater power could be had. Imagine if EACH of the rear wheels could provide 360 horsepower for 15 seconds, then that vehicle should have acceleration that would be beyond belief!
I intended to put an activating switch under the gas pedal, where when I would floor it, the Ciera engine might be producing its 120 hp, PLUS the horsepower from EACH rear wheel, or a total of a lot of horsepower (but for only 15 seconds max!)
Under all NORMAL driving, the Ciera would get the excellent gas mileage that its small engine could provide, and that engine could probably be even smaller, a four-cylinder instead. But for those few seconds when acceleration was desired, it could be spectacular!
Note that this vehicle was essentially ALREADY approved by the government safety testing and all the rest, so it would immediately be street-legal. The tire-grip might not permit it, but 0-60 in less than 3 seconds seems possible! FAR faster than ANY car on any road today!
And all from only maybe a $1,000 increase in the cost of the vehicle! Or the sky's the limit on cost for creative variants!
The giant vehicle manufacturers all design and build either under-powered tiny vehicles that get great gas mileage or they design and build vehicles with hyper-performing high-horsepower engines that perform great but which have lousy gas mileage. The approach I have described above is better than both, in that it combines the best of both general designs! And at a vehicle price that would not be much above their current under-powered offerings!
I guess that what I have described here is a sort of Hybrid vehicle, since the gasoline engine would drive several alternators that would recharge the batteries after a performance show. But it entirely different from what the vehicle manufacturers think is a Hybrid!
However, in my intent of modifying my Ciera, I was aware of two problems that seemed possibly hard to overcome. I knew that standard car starter motors only generate around 7 horsepower, where I wanted much more. The other problem is a result of that, in that a standard car battery is designed to have the energy drain rate of the standard starter.
I considered re-wiring a standard starter to have fewer windings of heavier wires, so that it drew a lot more current, and therefore generated more power. However, with my target of hundreds of horsepower, I was not really sure whether my modification of a starter motor would cut it! So I was quite excited when the Tesla came along and it has a single electric motor which they rate at 180 horsepower! And equally, their battery-pack is clearly capable of supplying the electricity very rapidly for such horsepower. So the Tesla apparently has the resolutions to BOTH of the issues that had concerned me! And where the Tesla needs to be able to withstand that level of energy flow continuously, all I would need would be a max of about 15 seconds worth. I suspect that would mean that less-expensive batteries might be sufficient and the motor could be designed to have an operating lifetime comparable to car starters, measured in minutes!
In any case, I believe my approach makes a lot more sense than what any of the giant vehicle manufacturers are now selling or designing, primarily since it can allow "nearly stock" vehicles, for both government safety approvals and for vehicle pricing that the public might be able to afford.
In a "don't do this at home" theme, there IS a possible safety issue. Say that one of the motors burned out or didn't start and the other one worked. Then ONE rear wheel would be producing a lot of torque and power, which seems likely to cause the vehicle to instantly go out of control. A bad deal! A related issue could be related to whip-lash injuries for occupants when all that extra power suddenly kicked in. If you have been in any high performance vehicle during a serious hole-shot, you know how you are thrown back into the seat! So this sort of concept would need a good deal of safety testing to make sure that unexpected things did not suddenly occur.
These are ball-park numbers used to simply show you how this all works. You could probably obtain the frontal area of your vehicle and the drag coefficient of it from the vehicle manufacturer.
We learned before that the Dynamic Pressure is related to the Momentum in the air and is simply the product of the mass-flow of the air times the speed. In the examples here, the one square foot cross-sectional area is air's density times volume (1/415 slug/cu ft * 88 f/s) times the velocity in feet per second (88 f/s) which is 18.6 pounds of Dynamic Pressure force.
A Large Sedan might have a frontal area of 22 square feet and a drag coefficient of around 0.43. Therefore, we would have an Aerodynamic Drag of 18.6 * 22 * 0.43 or 176 pounds. The Tire Drag for that vehicle weight would be about 45 pounds so the total Drag is about 220 pounds.
This drag is multiplied by the velocity (88) to get 19,500 ft-lb/second used to move the vehicle. We can convert this into horsepower (35.4) or watts ( 26,400 ) or Btus/hr ( 90,000 ). We know that a gallon of gasoline contains around 126,000 Btus of chemical energy in it, but also that automotive engines and equipment are not particularly efficient at around 21%, meaning that we then only get to use 26,500 Btus of that energy to move the vehicle.
So if we start with one gallon (26,500 Btus of available energy), and we know that we would need 90,000 Btus to drive an entire hour, we can see that our vehicle would travel 26.5/90 of that hour before running out of gasoline! This is just under 18 minutes, and since we are going 60 mph, we are going one mile per minute, and so we know that the car we just described would get around 18 mpg mileage. It ain't that complicated!
These are ball-park numbers used to simply show you how this all works. You could probably obtain the frontal area of your vehicle and the drag coefficient of it from the vehicle manufacturer.
We learned before that the Dynamic Pressure is related to the Momentum in the air and is simply the product of the mass-flow of the air times the speed. In the examples here, the one square foot cross-sectional area is density times volume (1/415 slug/cu ft * 88 f/s) times the velocity in feet per second (88 f/s) which is 18.6 pounds of Dynamic Pressure force.
A Compact might have a frontal area of 17 square feet and a drag coefficient of around 0.40. Therefore, we would have an Aerodynamic Drag of 18.6 * 17 * 0.4 or 125 pounds. The Tire Drag for that vehicle weight would be about 30 pounds so the total Drag is about 155 pounds.
This drag is multiplied by the velocity (88) to get 13,500 ft-lb/second used to move the vehicle. We can convert this into horsepower (24.8) or watts ( 18,500 ) or Btus/hr ( 63,000 ). We know that a gallon of gasoline contains around 126,000 Btus of chemical energy in it, but also that automotive engines and equipment are not particularly efficient at around 21%, meaning that we then only get to use 26,500 Btus of that energy to move the vehicle.
So if we start with one gallon (26,500 Btus of available energy), and we know that we would need 63,000 Btus to drive an entire hour, we can see that our vehicle would travel 26.5/63 of that hour before running out of gasoline! This is just under 25 minutes, and since we are going 60 mph, we are going one mile per minute, and so we know that the car we just described would get around 25 mpg mileage.
We learned before that the Dynamic Pressure is related to the Momentum in the air and is simply the product of the mass-flow of the air times the speed. In the examples here, the one square foot cross-sectional area is density times volume (1/415 slug/cu ft * 88 f/s) times the velocity in feet per second (88 f/s) which is 18.6 pounds of Dynamic Pressure force.
A medium-sized motorcycle might have a frontal area of 7 square feet and a drag coefficient of around 0.4. Therefore, we would have an Aerodynamic Drag of 18.6 * 7 * 0.4 or 52 pounds. The Tire Drag for that motorcycle weight would be about 5 pounds so the total Drag is about 57 pounds.
This drag is multiplied by the velocity (88) to get 5,000 ft-lb/second used to move the motorcycle, at that constant 60-mph speed. We can convert this into horsepower (9.2) or watts ( 6,800 ) or Btus/hr ( 23,000 ). We know that a gallon of gasoline contains around 126,000 Btus of chemical energy in it, but also that automotive engines and equipment are not particularly efficient at around 21%, meaning that we then only get to use 26,500 Btus of that energy to move the vehicle.
So if we start with one gallon (26,500 Btus of available energy), and we know that we would need 23,000 Btus to drive an entire hour, we can see that our motorcycle would travel 26.5/23 of that hour before running out of gasoline! This is just under 70 minutes, and since we are going 60 mph, we are going one mile per minute, and so we know that the motorcycle we just described would get around 70 mpg mileage (at that constant 60-mph cruising speed).
Obviously, a smaller and lighter motorcycle could get even higher mileage. But CARS have a maximum theoretical limit of around 65 or 70 mpg, due to their weight and frontal area, and very small motorcycles have a theoretical limit of around twice that, or around 140 mpg. These values are true for constant-speed traveling at 60 mph. At higher speed, the Aerodynamic Drag increases rapidly (essentially as the THIRD POWER of the vehicle speed, by the math shown just above where the number 88 shows up THREE TIMES in the final Drag figure). Of course, at slower speeds, the Aerodynamic Drag is less, also by that third power. So at 30 mph, the aerodynamic drag is only 1 / 23 or 1/8 that at 60 mph. Now you know WHY mileage is better at slower crusing speeds. Of course, stop and go driving tosses these gains out the window! But this theme brings up another interesting idea!
In any case, when you see some promoter on TV bragging about some car that gets 100 mpg or 150 mpg, he is being deceptive. NO car with the exception of a very miniature car could accomplish that by the laws of Physics, IF the situation was for cruising down the highway. But YOUR current car COULD demonstrate spectacular mileage if your intention is not normal driving but deception. See the following Footnote regarding how such deception can and is being done to us.
We learned before that the Dynamic Pressure is related to the Momentum in the air and is simply the product of the mass-flow of the air times the speed. In the examples here, the one square foot cross-sectional area is density times volume (1/415 slug/cu ft * 4.4 f/s) times the velocity in feet per second (4.4 f/s) which is 0.0465 pound of Dynamic Pressure force.
Let's again consider that gas-guzzler big sedan. We just said that a Large Sedan might have a frontal area of 22 square feet and a drag coefficient of around 0.43. Therefore, we would have an Aerodynamic Drag of 0.0465 * 22 * 0.43 or 0.44 pound. The Tire Drag for that vehicle weight would normally be about 45 pounds but in the spirit of deception, we might fill them to 90 PSI, without bothering to tell anyone that we did that! This effect and the very low speed would mean the tire sidewalls would hardly heat up at all from flexing, and the Tire Drag might be as low as 5 pounds. The total Drag (under these strange conditions) is therefore about 5.4 pounds.
This drag is multiplied by the velocity (4.4) to get 24 ft-lb/second used to move the vehicle. We can convert this into horsepower (0.04) or watts ( 32 ) or Btus/hr ( 110 ). We know that a gallon of gasoline contains around 126,000 Btus of chemical energy in it, but also that automotive engines and equipment are not particularly efficient at around 21%, meaning that we then only get to use 26,500 Btus of that energy to move the vehicle.
So if we start with one gallon (26,500 Btus of available energy), and we know that we would need 110 Btus to drive an entire hour, we can see that our vehicle would travel 26.5/0.11 hours before running out of gasoline! This is about 240 hours! On ONE gallon of gasoline! Since we are going a constant 3 mph, so we know that the big sedan car we just described would EXPERIMENTALLY DEMONSTRATE around 720 mpg mileage! In fact, if some advertiser thought it would cause some cars to be sold, they would certainly do such a ridiculous test, just to be able to keep themselves from being sued for claiming 720 miles per gallon! There actually were a variety of companies that did such things, in massively twisting the conditions to make their product look astoundingly good, but the government and the marketplace gradually caused them to fade.
Imagine what it would cost to HIRE a Test Driver to drive at a constant 3 miles per hour for ten days straight? But the good side is that drivers could probably switch by simply WALKING alongside the moving car, opening the door and calmly sitting down, with the previous driver simply stepping out on his side! Sounds like You-Tube material!
It actually turns out that the Drag Coefficient is probably even lower than the tiny amount we calculated above, because all the airflows would be laminar rather than turbulent. So the gasoline in this silly test might last even LONGER than just 10 constant days of driving at 3 MPH! And if the tire pressures were increased even more, or the tires filled with cool water, 1,000 miles-per-gallon might be a claim that could be made without being sued! Scary, huh?
But you may see the point in this silly discussion. Say that I was disreputable and I wanted you to buy "magic roses" which must be placed on top of the engine in your car, and I wanted to be able to put ads on TV that said that you would get 720 miles per gallon. A LOT of people would buy such things! Snake oil is what it used to be called! But see that such a disreputable operation could actually DO an incredibly slow speed test run, which could be documented by Observers to have been done, and they could then never get sued for those outrageous claims!
C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago