Slippery Pete,
At no stage have I referred to flame front speed or indeed to "better" burning in this thread, I have never referred solely to piston engines.
I have carefully not considered the kinetics of the hydrocarbon combustion processes as they follow and do not lead in this discussion on "why do we compress?" We can go into the kinetics if you wish presumably to bring some order into this discussion if you wish, the pun is intended.
I do however agree that rate (or flame front speed) will show a form of direct proportionality to gas pressure during combustion. It follows from simple rearrangement of the ideal Gas Equation pV = nRT. Pressure can be equated as concentration on rearrangement etc.
What I have stressed is that compression allows more combustion per unit time thereby releasing more energy to do useful work per unit time.
We compress primarily to make the engine more powerful. You can be as efficient as you like in your isolated system engine but if you lack the power to reach Vr what's the point! I believe Oggers said something similar some time ago albeit in a different way.
Your efficiency argument is predicated on thermodynamically closed systems.
Remember that piston or gas turbine engines are more or less fully open systems thermodynamically.
The heating effect due to increased combustion per unit time is orders of magnitude greater than that relating to heating from compression alone.
Beyond a defined threshold, the need to add less combustion energy to an already compressed fluid is relatively trivial in an open system.
Hope this helps!
CW
Last edited by chris weston; 16th Jan 2012 at 16:00.