I'll take a stab at answering the original question regarding engine efficiency at different RPM's. I believe any given piston engine will have "sweet spot" for maximum fuel efficiency. I've heard that for car engines (gas, normally aspirated) that spot is typically at about 40 - 45% of redline RPM.

Lower than that speed, efficiency is reduced by pumping losses due to the low intake manifold pressure. At the higher end, frictional losses and volumetric inefficiency take their toll.

Other factors that would affect the optimum RPM would be altitude and grade, since both those influence the throttle position and thereby pumping losses. Diesel and/or turbocharging also change things...

...and that's all I know about that.

Angus

On Mar 25, 2012, at 5:41 PM, David Beierl <dbeierl@ATTGLOBAL.NET> wrote:

>> the energy lost by compressing the fuel/air charge and operating
>> the valve train, ... - these parasitic losses all increase
>> more-or-less linearly with increasing rpm.
> 
> I believe I misspoke here.  I think the total energy to actually
> compress equal masses of charge will not increase linearly with rpm,
> but remain at least somewhat the same.  I'm hedging a bit
> ("somewhat") because the energy aspects of compressing gases in the
> real world is a more hairy subject than I know how to tackle.
> 
> And apparently it works differently if you get a diabatic to do it for you. ;-)
> 
> Yours,
> David