What is the main reason the chemical energy of a fuel is never entirely converted to mechanical energy in an engine?

A) Some of the energy is lost as heat.
B) Some of the energy is converted to mass.
C) Some of the energy gets used for cooling the engine.
D) Some of the energy flows into shaft to increase its rotational speed.

Option A)Some of the energy is lost as heat

Explanation:

Energy conversion is not always 100% accurate. In engines, due to the friction generated as machine parts roll over another some of the chemical energy of the fuel is lost as heat. That is why machine parts are lubricated to reduce friction, and then energy loss.

Thus, 100% chemical energy does not yield 100% mechanical energy, as some are lost as heat.