Generalized efficiency and its application to microscopic engines

We generalize the definition of the energy output of an engine as the minim um energy input required to accomplish the same task as the engine. Applyin g this new concept to molecular motors makes it possible to measure their e fficiency even without attaching any external load to them. This way we can compare and characterize the operation of molecular motors in various situ ations. We also investigate how the thermally driven motors differ from oth er motors.