Originally posted by yebo
kunin natin sa basic physics.

there will be higher loses at higher rpm for a reciprocating piston engine. remember that momentum is 1/2 x M x V x V. the higher the engine rpm the higher will be the momentum of the piston because the crankshaft rotates faster, and this momentum of the piston is loss when the piston reverses direction. a piston travelling at say 6 meters per second and then reverses direction will have 36 times more momentum loss than a piston travelling at 1 meter per second that reverses direction. this is due to the momentum equation, the value of velocity being raised to the 2nd power (squared). 1 x 1 = 1, 6 x 6 = 36. even though the total number of revolutions is the same (6000 x 1 minute = 1000 x 6 minutes) the mechanical losses are higher on an engine at 6000 rpm due to this higher energy losses from the piston momentum. thus the consumption of fuel will be higher.

the thermodynamic losses (heat, volumetric efficiency, flue gas) will also be more on the engine running at 6000 rpm (assuming they are both at the same temperature) due to the higher energy input.

this applies for an engine irregardless of load, whether zero load or any value.
Overly simplistic... you're equating an engine to a reciprocating piston, when it's much more than that. Yes the piston will decelerate to zero just as it changes direction, but because you have the conrod and the crankshaft the energy is not wasted. The expansion stroke of a piston actually helps the compression of the next one, and all of this momentum is transferred via the crankshaft onto the flywheel. In fact, the higher the RPM the more momentum you store in the flywheel, which is where all of the engine's energy is really stored. In this regard, simple physics tells us that the best way to maintain maximum power is to rev as high as possible, because the stored potential energy and inertial kinetics in the flywheel is maximized...

The drawback to high rpm is not in inertial loss, but in frictional loss and material strength. The piston has to be made out of something, and sooner or later that material is going to reach a stress level when it can not take anymore. Frictional loss factors in as combustion energy is converted into more heat and less flywheel momentum, but flywheel speed (and hence energy) is still directly related to high RPM.