Compression Ratio and Engine Efficiency

The ideal gas law says that if a gas is squeezed into a smaller volume, you get more pressure.

During the compression cycle, the fuel is technically not a gas; it is atomized fuel droplets suspended in the air brought in during the intake cycle. But at the moment of combustion, the fuel-air mixture becomes a gas and the pressure it exerts downwards on the piston is going to be proportional to how small a volume the gas occupies, initially.

The ideal gas law also says that if you compress a gas, you increase pressure and temperature. So the compression cycle heats the air-fuel mixture ratio as the piston approaches top dead center. If the fuel charge has too much volatility, the increased temperature can cause pre-ignition: before the spark plug fires. That causes knocking.

Higher octane fuels have less volatility - they don't burn as easily. That's why you need higher octane for higher compression engines.

I am not familiar with the particular engines you refer to nor am I an automotive engineer. However, from a thermodynamic point of view, higher compression ratio should mean greater cycle efficiency-all other things being equal-which they are unlikely to be. By this I mean many factors affect engine efficiency such as combustion chamber design, efficiency of valve and cylinder head cooling etc.. Greater efficiencies should result from greater compression ratios as follows :

According to the gas laws, the temperature of a gas increases with pressure. According to the second law of thermodynamics (or at least as a corollory of it) , the efficiency of a gas power cycle is proportional to the difference in temperatures between when the gas fully heated and compressed (i.e. just after ignition in the case of an si gasoline engine) and the exhaust temperature . (Look up Otto air cycle, Carnot cycle efficiency etc.) Therefore, higher compression ratio means higher working fluid temperature (combustion gases in this case) means higher efficiencies. The main limitation is the fuel problem i.e. with gasolines things like pre-ignition occur if the fuel -air mixture is overcompressed and this destroys cycle efficiencies as well as physically damaging the combustion area. I hope this makes sense!

Thank you both for your very excellent and thoughtful answers to my question. I think you are both right; the answer is in the rather small, and unremarkable difference in engine thermal efficiency. The overall thermal efficiency of the otto cycle is in the range of 20%, with any increase above this being quite difficult to achieve. The compression of the cycle is, I'm sure, isentropic, or at least adiabatic. That being the case, the improvement in efficiency, due to an increase in the compression ratio, has to be quite minimal.

Thanks again,

Bob Clark

I'm not up on the thermodynamics enough to predict the effect of compression on engine thermal efficiency, but clearly you get a lot more power per unit mass out of the engine if you use higher compression. That means a lighter engine for the same power, which would certainly translate into better fuel efficiency.

The trade off is more expensive fuel - look at aircraft engines, using 100 octane av gas - you don't even want to know what that costs anymore. Aircraft use extremely high compression, to get the necessary horsepower at the lowest possible weight.

The trade off is the engine has to be running just right, and there is an effect on engine longevity. Aircraft engines are completely rebuilt every 2000 hours.

You wouldn't want to do that with your car, but on the other hand if you figure an average speed of 100 mph, that is 200,000 miles, which isn't bad.

Until you price the rebuild.

Aircraft use extremely high compression, to get the necessary horsepower at the lowest possible weight.

Actually that is not quite true. A Lycoming IO 540 (a typical general aviation engine) has a compression ratio of 8.7, lower than virtually any family sedan engine of today. This gives the engine a very low specific output for its displacement of .55 hp per cubic inch, whereas cars are now typically 1 hp per cubic inch. Although aircraft engines are overhauled every 1600 or 2000 hours, they are typically in very good condition at those times, because of the lower power output.

An IO 540 in a Cirrus cruises at about 200 miles per hour, so can go about 400,000 miles between overhauls.

You're right, the rebuild cost is staggering, as is the cost of a new single engine plane like the Cirrus -- about half a mill.

Ken

The low compression leads me to wonder... could you use standard regular gas in the aircraft rather than av-gas??

Bill

Hi Bill,

Funny you should ask...

About 25 years ago, I flew a 1946 J3 piper Cub from Pittsburgh PA, to Melbourne FL, to deliver it for its owner. Two of us would switch places and flying duties at each fuel stop (which were pretty frequent). It took two entire days, with not a moment to spare. So on the morning of the second day we needed to get going at the crack of dawn, before the airport opened up (and we'd arrived after closing the evening before). How to leave?? We called a towing service, had them pick us up at the hotel with a ten gallon can of car gas, and take us to the airport. He was very quiet all the way to the airport, apparently convinced we were axe murderers, because the idea of fueling a plane with car gas was foreign to him. He brightened up considerably when we poured the fuel into the plane, and said he really didn't know what to think until then.

The Piper ran fine on car gas, although we used premium -- but others have fueled Piper Cubs with regular. (On most, if not all, J-3's you can do this legally, because when they were built, you had the option.) On more recent engines, however avgas is usually a requirement from an engineering point of view (and a legal requirement, too.) For certain planes, you can get a Supplemental Type Certificate (I think that's the thing) that enables you to fuel up with car gas.

But I think that the reason for the high octane but low compression has to do with the very large cylinders and combustion chambers, with 360 Cu In engines having only 4 cylinders, and 540s having only 6. The flame front has a long way to travel, and may not reach the extreme edges of the cylinder before the mixture has been heated to detonation point (for lower octane). I suspect that part of the reason also has to do with the fact that mixture control on planes is manual -- the pilot can lean the mixture all the way to the point that the engine will not run (in fact that is the standard shutdown procedure -- you do not turn off the ignition). With very lean mixtures, there is less evaporative cooling, so detonation is more likely.

(Avgas has other characteristics too, such as lower vapor pressure, so it does not cause vapor lock at high altitudes, and consistent formulation throughout the country.)

What isn't discussed and is a significant variable to the compression pressure variables and fuel efficiencies aspect of the IC engine is fuel delivery, the metering of air and fuel and change of state. A phase change is adherent under the compression pressure almost regardless of the ratio. Keeping it at the normal levels of a gasoline engine comparable. In my experience and studies, the effective output power and waste heat and the work energy produced varies greatly just on the introduction of the properties of fuel alone. Omitting crank and valve timing, maintaining some basic constants with timing, plug rating, fuel type including what re-fueling source in which it was derived, compression ratio, lubricants, etc., do we determine what type of efficiency gains made regarding power output (usually on a rear wheel or engine dynometer) and efficiencies related to power output and fuel consumed.

We have found that higher static and or variable compression pressure ratio systems as with compressed air via turbo or supercharger applications, are ultimately more efficient regarding power out (transferable work energy) in relation to fuel consumption as apposed to lower static compression ratio IC engines. Quickly some will argue referencing forced air applications as consuming more fuel. Enter the variables of driving conditions and fuel delivery. Based on most common efi port or rail systems, ultimately given the same testing variable and driving conditions and methodology in driving, an engine that can produce more work energy per mole count of fuel will win out in overall efficiencies. (Many car manufactures have done similar testing to prove the same. Power output in relation to fuel expenditures per distance traveled etc.)

Thus bringing me back to how important introducing the fuel and the state in which the fuel is delivered with the air, air temperatures, fuel temperatures, controlling them and utilizing the IC conditions from operations to control cylinder temperatures and exhausting output temperatures from said ports. The (U)internal energy of the fuel is where the concentration is and maximizing the extraction there of where where all the formulas related to thermal dynamics will come in to play for ultimate quantification proving efficiencies.

For some light touching on the subject, the above with the provided links, may help with some details. There are tons more detail but you get the idea.

I gave it a GA, for the write up and for the popular hotrodding article.

There are a few points I'd like to make. first that Pre-ignition and pinking are not the same! Pre-ignition is the ignition of the fuel before the spark is fired, the fuel burns in the normal way; somethimes this will cause a knocking noise but is different to pinking which sounds like gravel runnibg through the engine. Pinking is fuel detonation as in dynamite, where the shockwave fires the fuel rather than a flame front burning it. The Octane rating is a measure of the fuels ability to avoid pinking, not preignition (preignition may lead to pinking however due to the increase in pressure). Compression ratio - or Pressure ratio in a Gas Turbine - has abig affect on a gasoline engines efficiency up to about 12-1 ratio. this is largely due to part throttle conditions where the high static ratio means better part throttle combustion. In a supercharged engine the engines ratio will normally be lowered to cope with the extra pressure and the extra heat in the charge. Fuel injection normally allows a slightly higher compression ratio as the fuel is atomised droplets and cools the charge in the cylinder as it vapourises, whereas carby engines the fuel vaporises in the manifold so there is no cooling effect during compression.

Hi Bob,

nice question, but really you'd be better off reading a book than asking us guys - unless of course you want the witty answers which haven't appeared yet (and I'll let you down on that too). Just about any text-ish book on IC engines will have a chapter on this.

Basically the thermodynamics says that the higher the compression ratio the higher the thermal efficiency of the cycle. Take it to extremes and logic tells you so anyway. However it is a "law of diminishing returns" - 80 years or so ago, largely because of fuel quality, typical compression ratios to avoid destructive detonation or pre-ignition, were about 5 or 6:1. Along came WWII and the need for high power and better efficiency, research into fuel quality, and the discovery of TEL. Fuel quality improve rapidly and CR's of 8-10 came along - together with 20-25% efficiency improvement.

Higher CRs than this will give some further improvement but you're only talking approx 10% max. Diesel engines have no detonation limit and are at their most efficient at about 14-16:1 - the extra pressure at higher CRs gives more friction and parasitic losses, which translates into a limit on the gain to be had. Obviously this depends upon the detail engine design.

Running lean also improves the theoretical thermodynamic efficiency, also helping the diesel engine's efficiency at part load, as well as the fact that nearly all diesel engines run unthrottled.....

For your info a leaned gasoline engine can get to a thermal efficiency of approx 36%, whereas a modern truck diesel will get around 43% - the area where diesels really win is at part-load, where the advantages mentioned above combine to give maybe a 2:1 improvement over a SI engine, depending on running conditions.

For info - yes SI aircraft engines are relatively lowly rated but that is because they run very high load factors - higher than just about any other power plant, other than shipping. Typically they will use max power for a few minutes to take-off and climb out, then use 80% power for cruise (maybe for several hours at a time) and only run at lower power for a few minutes descent. Compare that with a car or bike engine and you will see that 2000 hrs is a very tough challenge. Their training and glider tug use is even harder work, being basically repeat cycles of full power for a few minutes then back to idle - giving severe thermal cycle problems to all key engine parts.

I could go on, but you'd all be bored, so I won't....... DP

As the other guys have said, the engine will make more power and run more efficiently, however, there is a difference between the advertised, or if you'ld like to call it, static compression ratio. This is the volume of the cylinder with the piston at bottom dead center (BDC) compared to the volume of the cylinder with the piston at top dead center (TDC). This volume also includes the bowl in the cylinder head. There is also a dynamic compression ratio and this largely depends on the valve timing from the camshaft. There is a difference between the two compression ratios . The camshaft timing determines the cylinder pressures to a large extent. Camshafts determine the power ranges that the engine will produce the most horsepower and torque and they are a tradeoff of each other, i.e., more HP at higher RPM's will equate to less torque at lower RPM's. Most cars from the factory come with a camshaft that will produce the optimum HP/torque at the speed range most people drive at. I think the rest of what you were looking for is contained in the other answers.

Holy Mackeral!

I do believe, this is the very best thread I have ever read in CR-4. Your answers are very much appreciated. The web sites you have referenced for me are really great. It'll take a while for me to get them read and longer to get them understood. You have made me happy.

Thanks, Bob Clark

What's been ignored in previous discussions is that peak cylinder pressures only occur at full throttle. Thus it is possible to use a high compression engine with low octane gas, it you keep your foot out of it.

I was fortunate to have a 1967 Cougar with a 289 4bbl. Factory specs say that this is a 10.5:1 compression ratio engine and should run on high octane gasoline. I just ran mine on regular and never used more than 1/2 throttle. I kept up with the traffic, but didn't pass many. Initially it had a 3 speed automatic transmission which delivered about 17 miles per gallon. After I put in a 2.78 low 4 speed, my mileage jumped to 20.5 to 21 mpg using the same axle ratio as previously. I put more than 60,000 miles on the car before I sold it, and never detected any damage due to knocking or detonation.

There are engines which are designed along similar lines with a compression ration higher than that appropriate for the octane gas to be used. These engines typically have the intake valve opened a few degrees longer than normal to allow some of the incoming charge to escape. The expansion ratio is what you would expect from the higher nominal or "static" compression ratio. This provides the higher mileage without having the danger that someone will get in the car and destroy the engine by driving it with wide open throttle. I have heard this type of engine called a Miller cycle and have heard others say that this is not the correct name.

The previous guest was correct on the topic of octane. Higher octane fuels burn slower causing the force on the piston to be more of a "push" insted of a rapid "hit".

As far as temperature and pressure, the ideal gas law does NOT specifically state that an increase in pressure will cause an increase in temperature. Actually, the opposite occurs.

To clarify, I will use the terms vapor and liquid. When gasoline enters the combustion chamber some of it is in vapor form, the remainder in small liquid droplets. Ideally you would want all of the gasoline to be in vapor form. During the compression stroke, the piston attempts to compress the vapor / liquid mix, but liquid does not compress as easily as vapor does. The vapor then comes under increased pressure and an exothermic reaction occurs; the vapor cools down by giving off heat. The liquid droplets become the reciever for the heat (along with the surrounding engine components) bringing them closer to vaporization and "hopefully" flashpoint.

In an ideal 100% efficient state of combustion, the ratio between fuel and oxygen (not air) is 1:1. The resulting emissions would be pure water vapor. Greater pressure caused by compression could actually reduce engine efficiency as the entirely vapor with no liquid mix would cool and move toward the condensation point.

So the compression of an engine as established by it's designer, is directly related to how effectively the liquid gasoline can be converted to vapor as it enters the combustion chamber (among other factors). Increasing compression by itself does not neccessarily increase efficiency.

This thread has gotten a bit technical perhaps for some. It seems that some of the participants have gotten a bit off track in the area of thermodynamics. This can cause confusion for those who want a direct answer to the topic question without exploration of the underlying science.

For those who go deeper and haven't yet done so I'd like to suggest you all go dig out that thermodynamics book you suffered with some decades ago and review the following: The 2nd law, thermodynamic cycles, gas laws, control volumes, phase change of gases and liquids and the idea of partial pressures of gases.

Then maybe we can have a good discussion that leads to a simple layman's explanation for the "why" of the compression ratio effect that us old hot rodders have known and exploited ever since Ricardo first took advantage of it.

Ed Weldon

Then maybe we can have a good discussion that leads to a simple layman's explanation for the "why" of the compression ratio effect that us old hot rodders have known and exploited ever since Ricardo first took advantage of it.

Of course, there may not be particularly useful simple layman's explanation, beyond the observational facts that hot rodders have known for decades. As a high school student, I was getting heads milled, etc. because I knew that it worked, despite not really understanding the thermodynamics.*

Even then however, I knew that piston ring friction went up with compression pressure, I knew that rod bearing and main bearing friction also went up with compression pressure, and knew that it takes more energy to compress a gas to 1/11 its original volume than to compress it to 1/8. I also know that the rod is usually at an angle to the cylinder, so that the higher the pressure, the more the piston is forced against the side of the cylinder. I also knew (from having worked as a mechanic in high school -- compressor cylinder heads get hot) that gas compression caused raised temperatures, so knew that heat loss into the cylinder head, etc would increase with increased pressure. Therefore, one simple explanation would say that higher compression should lead to performance reductions. The energy contained in x amount of fuel is constant. Using more energy in compression and friction must therefore reduce performance, one could argue. (Unfortunately that argument is counter to observations.)

An oversimplified, but palatable explanation (for performance improvement instead of reduction) might center mainly around expansion ratio, rather than compression ratio. It is easy to see that a highly compressed mixture will attain a higher temperature (because of the compound effects of 1. increased reaction speed from reduced intermolecular distance, and 2. less available area at combustion peak time for conduction of heat.) It is also easy to see that the temperature difference between peak combustion temperature and exhaust gas temperature will be greater if the hot gas is allowed to expand more. It makes "sense" that more energy is extracted by the engine if the exhaust gas is relatively cool -- otherwise where did this energy go?

* I was a "poser" then re thermodynamics. I have just realized that I have been a "poser" re e=mc², which I thought I understood, at least regarding the obvious stuff. My view of the mass-energy equivalence has been that mass can be converted to energy, which is massless: in other words I would not expect something to change in weight as it cools. (I'd argue that "equivalence" does not necessarily mean "the same" -- just as F is not "the same" as MA... because we know we can apply a force to a wall, and the wall does not accelerate.) I thought I "knew" that photons are massless: 1. they have no rest mass, and 2. things with mass cannot move at light speed. A room does not change in mass when you turn on the lights, I had incorrectly assumed. Turns out a room does change in mass when you turn on the lights (assuming you get your electricity from a powerplant outside your room.) See Bob D's posts, which are a remarkably patient antidote to my dim-wittedness.

It is my understanding that when the compression goes up so does the NOx.

Brad

Correct, if measured at the exhaust valve.

On the other hand, at the tailpipe, for spark ignition engines, it effectively makes no difference. This is because the criteria emissions (CO, HC, and NOx) are all measured in grams per mile (actually parts of grams per mile) rather than in PPM, so very large engines (6-7 liter) with high compression (high enough that they are at the limit for detonation with premium gas) can fairly easily pass these, so small engines can pass even more easily, because they emit less total exhaust. (This is actually a bit of oversimplification, because small engines are often somewhat more highly loaded during the test, etc.)

Lean burning also causes NOx to increase (but causes CO and HC to decrease). (But again, this is at the exhaust valve. The catalytic converter has a very narrow range at which it can operate [=/- 1% from stoichiometric] which makes "lean burn" engines, which are many percent lean, a different proposition, with different controls, etc.)