Okay, no other transfer of energy exists. So let's see what heat engines are. Heat engines are basically systems or mechanisms that convert one form of energy into a second form of energy, namely heat into work. And this occurs under constant temperature. So let's see what the layout of a heat engine is. A heat engine is composed of a long cylindrical tube that contains molecules inside this area and that contains a movable piston controlled by an outside force, maybe your hand that's moving it up or down.