[eschaider]

You are missing some things that are important here Anthony and confusing others.

Originally Posted by Anthony

Well, the compression ratio directly affects the efficiency of converting heat energy into mechanical energy.

Gasoline engines do not convert heat energy into mechanical energy. They convert chemical energy into mechanical energy by oxidizing the fuel and using the expanding products of combustion (oxidization) to push against the top of the piston.

The products of combustion take up more space than the fuel and air components prior to oxidization (ignition). The increased volume attributable to the combustion process at high pressure, push on the piston which is part of a crank slider mechanism that translates linear motion into rotary motion.

Originally Posted by Anthony

The higher the CR, the higher the efficiency, the greater the mechanical energy output for the same amount of air/fuel.

This is a yes and no statement. Higher compression ratios produce a faster burn rate. Remember the firecracker gunpowder on the ground vs wrapped tightly. Sooo, in that context higher compression ratios produce faster burn rates and allow for the oxidization of more fuel and air in the same period of time.

Intuitively the natural instinct is if a little is good more is better. That is true to a point. The point is once you have burnt all of the fuel and air as fast as possible without detonation. Further increasing compression at this point does not increase power output. What it does do is introduce detonation which saps power and destroys parts. Once you have burnt all the fuel /air in the chamber by 7 to 10 degrees ATDC you have done all that is possible to do.

Originally Posted by Anthony

This is why diesels are so much more fuel efficient at lower throttle positions as compared to gasoline engines.

Diesels are fuel efficient partly because compression ignition and spark ignition engines are different. The Diesel will have a much greater expansion ratio, what you are thinking of as compression. That is necessary for ignition because spark ignition of diesel fuel is not practical. The primary reason for the better fuel economy is the energy content of the fuel, it is much higher than gasoline.

This is why when you run any of the ethanol fuels while you can produce good power you can not produce the economy of the gasoline fueled equivalent engine. The ethanol fuel's energy content is just lower. All the other stuff is just interesting window dressing.

Originally Posted by Anthony

The diesel is converting more of the heat energy into mechanical energy.

Diesels do not convert heat energy into mechanical energy. They convert chemical energy into mechanical energy when the fuel is oxidized, just like a gasoline engine. The difference is the ignition process, diesels use compression ignition and gas engines use spark ignition. Once past the ignition event it is a chemical to mechanical conversion through oxidization of the fuel .

Originally Posted by Anthony

So you want to run as high of a CR as possible for the fuel you're using to get the most power output.

A basically correct answer for the wrong reasons. You want to run the highest compression ratio (in a spark ignition engine) as possible without incurring detonation. You are attempting to maximize the down force on the piston between 7 to 10 degreed ATDC to optimize torque.

Originally Posted by Anthony

Lower octane fuel actually has slightly higher energy potential than high octane fuel, but you will get more power from the higher octane fuel assuming you are running a higher CR, as the increased energy conversion efficiency of running a higher octane fuel with higher compression is greater than the higher heat content of the lower octane fuel with a lower CR.

Basically the right answer for the wrong reasons again.

Lower octane fuel can have a higher energy content. The problem is burning the entire volume of combustible fuel in time to maximize the pressure on the piston crown between 7 to 10 degrees ATDC. The fuel can not burn fast enough without detonating to produce the maximum down force on the piston between 7 and 10 degrees ATDC.

The burn rate of the high octane fuel is actually slower and more manageable than that of the lower octane fuel. By choosing the proper ignition point it is possible to optimize the down force on the piston between 7 and 10 degrees ATDC at a much higher level than with the lower octane fuel.

All the energy conversion, energy conversion efficiency, heat content etc. terminology sounds good but is being misused and is not informative nor helpful in describing or understanding the processes occurring in the combustion chamber.

Originally Posted by Anthony

Yes, the engine is an air pump, but I think there is alot more things at play in determining how much mechanical power an engine is able to put out still consuming the same air/fuel quantity.

Not really. It is as simple as using an acetylene torch. If you need more heat you use a bigger tip. Same thing with the engine. Think of the engine displacement as the oxygen control on the torch. If you need more power what do you do? You make the engine bigger or run it at a higher speed and add more fuel! If you want the torch to be hotter (more energy) you use bigger tip add oxygen and more acetylene. Same thing with the engine except now it is gasoline instead of acetylene and it is not a continuous burn it is staged incremental burns after each ignition event.

Originally Posted by Anthony

Stroke/rod ratio, combustion chamber dynamics, quench, etc, I'm sure a lot more than I know. Granted, you won't be doubling the power output, but these little things do add up to a degree.

This is not how you make power. This is how you fine tune the mechanism to minimize internal power losses.

If UHF TV's were around when you were growing up you will remember they had two tuning dials. Essentially a coarse and a fine adjustment. You would get the channel with the coarse adjustment and fine tune its video quality with the fine adjustment.

What you are talking about are the equivalent fine tuning controls on the engine and they should not be confused with the big dial that brings the horsepower.


Ed