On optimizing maximum-power heat engines

For a general class of heat engines operating at maximum power, in which the generic sources of irreversibility are finite-rate heat transfer and friction only, we investigate (1) the time-dependent driving functions that maximize power when heat input and heat rejection are constrained to be nonisothermal, as is the case in many conventional heat engines, and (2) the specific impact of friction on the nature of the engine cycle that maximizes power, and on the engine's power-efficiency characteristic. The extent to which maximum power is affected by the constraints on the driving function is evaluated, as well as the time divisions on the different branches of the optimal cycle. The fundamental differences in engine performance that arise from frictional losses being internally dissipative, as opposed to externally dissipative, are derived, and illustrative examples are presented.