Quote:
Originally Posted by Isaac Zachary
The heat ratio is important.
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So the heat capacity ratio determines pressure and temperature change under adiabatic compression and expansion. Gasoline engines are knock limited (by charge temperature) and don't have any heat exchange steps, so using a lower heat capacity ratio working gas is okay, you just pair it with a higher compression ratio, and the forces seen by the piston are more or less the same.
Air is a desirable dilutant since it has oxygen to promote more complete combustion, but water can be useful too, e.g. at high load where you need to remove excess heat. Absolute heat capacity rather than heat capacity ratio can make a gas more useful for reducing temperatures and heat conduction losses, that's why cooled EGR is combined with lean burn in experimental high efficiency engines rather than just running even more lean.