A very common question is how to compare the thrust number of an airplane engine with the mechanical power of a piston engine (the kind of engine in cars and in many airplanes with propellers). It is hard to compare these two. This is because they are not measuring the same exact thing. A piston engine does not move the plane. It just turns the propeller, which moves the plane. Because of this, piston engines are rated by how much power they give to the propeller.