Chris,
I'll say it again REALLY SLOWLY so you can understand.
Your theory of more compression = more air molecules only applies in a turbine with a fixed volume combustion chamber. It doesn't apply in a fixed volume piston engine (because the swept volume determines the number of molecules you can get in there). And yet a higher compression piston engine is also more efficienct. If it was (as you argue) all about the number of oxygen molecules, a high compression piston engine would be no more thermodynamically efficienct than a low compression one.
As they say on mythbusters, myth BUSTED.
In fact, thermodynamic efficiency has nothing to do with how many oxygen molecules are there - the only time that becomes important is if there are not enough oxygen molecules to combust all the fuel.
When will all the lies and myths end? Every new page on this thread brings more people with more false arguments. So far we've had volumetric efficiency (consequence of operating a piston engine inefficiently), faster burning fuel, incomplete combustion, valve timing, ignition timing... it goes on and on and on.
These are all important things for tuning or selecting and engine for an application, because they make it practical and useable over a large RPM range (rather than super-efficienct at one particular RPM), and because we have to manage inadequacies of the fuel, and the temperature of the components.
But the fact remains, the only thermodynamic reason a higher compression or pressure ratio engine is more efficient (THE QUESTION IN THE OP) - is because by the end of the cycle, less waste heat has been transferred to the air. IT'S SIMLPE FN PHYSICS.
You put in an amount of energy (X) into an engine in the form of fuel, you want to get some of that energy out in the form of useful work (Y), but unfortunately you get a lot of waste heat (W). Maximum thermodynamic efficiency is the concept of ... for a given amount of fuel energy (X), you get the maximum useful work (Y). High compression or pressure ratio engines do this better because less of the original fuel energy (X) is converted to waste heat (W).
Conservation of energy - I would expect most high school students to know this.
Anyway, let the next round of myths begin on thermodynamic efficiency at higher compression. I wonder what we'll get next...
Last edited by Slippery_Pete; 26th Nov 2011 at 01:59.
Reason: Typo