Quote:
Originally Posted by Coheed
You are thinking of thermal efficiency. Volumetric efficiency is the ability of the engine to consume a specific volume of air based on the amount of displacement. Since raising compression ratio doesn't affect how much air the engine consumes, it does not affect volumetric efficiency.
A Rotary engine has great volumetric efficiency, but poor thermal efficiency, because of the low compression ratio in comparison to the otto cycle.
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To expand on this I will say that the reason that we are seeing high comp engines now is for fuel efficiency. Increasing static compression allows the engine to extract more mechanical energy from the fuel being burned. Note: Given two engines with everything equal except compression ratio they will have identical fuel consumption at a given rpm and MAP. The difference will be that the high compression engine will be producing more torque than the lower compression engine. Because fuel has a finite amount of energy which can be extracted through combustion, a higher compression engine will produce less waste heat (Coolant & Exhaust) and will actually result in an engine which takes slightly longer to spool a turbo. This is because you are getting the same volume of exhaust, but it is slightly cooler. A turbo relies on mass flow rate and heat to spool.
This explains why diesel engines are so much more efficient than gasoline engines. They have compression ratios which can be well over 20:1. Diesel fuel actually contains less energy than gasoline, but due to it's resistance to pre-ignition and detonation we can use higher compression ratios.