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