Operating the hot-air engine P as a thermal engine, heat pump and a refrigerator

The experiment P2.6.1.5 qualitatively investigates the operation of the hot-air engine P as a thermal engine. The hot-air engine P is a  transparent model to show the basics of energy transformation. The hot-air engine P is driven by an alcohol burner and transforms via a small generator  the mechanical energy into a voltage which can be measured.
In addition, the operation of the hot-air engine P as a heat pump and a refrigerating machine is shown. By variing the speed of the electric motor, i.e. the mechanical power supplied to the hot-air engine P, the impact on heating or refrigerating power is observed.