[Anthony]

Originally Posted by eschaider

p.s. We have often heard the old saw that an engine is just an air pump. In fact that is why cams and head selections carry the significance they do. We are attempting to optimize the pumping efficiency of the engine. the short block is only about reliability.

As luck would have it it takes 10lbs of air to produce 100 horsepower. The interesting thing is it is fairly constant across fuel choices, i.e. gas, methanol, ethanol but not nitro — because nitro is a monopropellant.

Once we achieve a 100% Ve the only way (with out boosting) to increase power is ram tuning of the intake which tends to be rpm specific and not a uniform increase across the operating rpm band. So where does c/r fit into the picture?

If you run too much timing the in cylinder pressure rise is too steep (quick) and the mixture literally explodes at or very near TDC creating the engine killing knocking sound we all recognize and try to avoid. Increasing octane slows down the burn rate and allows a wider window in crank degrees for the burning mixture's pressure to work on the piston.

A Internal Combustion Engineer (ICE) tries to engineer his engine design metrics and ignition curve for the most commonly available fuels to produce the advertised horsepower target. He does this by manipulating the in chamber rate of burn (measured in feet per second).

Once you have fit the completition of the burn event into that 7 to 10 crank degree window ATDC you have not only done all you can for that charge but it has provided all it can for engine power production. If, for example, you were to increase c/r for this event in this engine there would be no increase in power.

Why, you say. Well you have already previously converted all the fuel and air through their normal oxidation (burning) process into pressure energy. There is no more fuel to convert therefore no more power to be made.


The 10 lbs of air per 100 horsepower is an excellent rule of thumb to tell you when you have gotten all there is to get for a particular mass flow of air. At that point fiddling with timing or c/r will bring no benefits to the table. If you want more power you need to process more air — it's really that simple. The rest of the dials will have no effect until you process more air. That means either a bigger engine, higher engine speed or a supercharger.

Ed

Well, the compression ratio directly affects the efficiency of converting heat energy into mechanical energy. The higher the CR, the higher the efficiency, the greater the mechanical energy output for the same amount of air/fuel. This is why diesels are so much more fuel efficient at lower throttle positions as compared to gasoline engines. The diesel is converting more of the heat energy into mechanical energy. So you want to run as high of a CR as possible for the fuel you're using to get the most power output. 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. 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. 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.