Why does CI engine performance change with load and speed?

[kannans]

why the mechanical efficiency and brake thermal efficiency curve increases as load increases and reaches a maximum value and then again decreases( in contand speed test)

 

[Kozy]

Because with more load at a constant speed you have a wider throttle opening, an engine is at best efficiency when completely un-throttled.

 

[pnorm91]

Because with more load at a constant speed you have a wider throttle opening, an engine is at best efficiency when completely un-throttled.

Are you saying that across the board? It seems to me that you would be more or less looking a bell curve for this efficiency. It seems that the thermal efficiency is going to decrease at a near exponetial rate as the power increases.

 

[Kozy]

Volumetric efficiency tends to fall as speed rises, if that is what you mean?