(1) Optimal Operation of Finite-time Tricycles with Heat Conduction Losses

We consider the issue of in-principle limits to the finite-time operation of a cycling working fluid acting as an agent in the transfer of heat among three heat reservoirs. The set of feasible operations of such a heat engine is explicitly described and its boundary is chara cterized as the set of operations which allocate time optimally among the heat conduction br anches for a wide variety of cost functions. One point on this boundary represents the oper ation that maximizes the heat output at the high-temperature heat reservoir. There is a natura l notion of efficiency for such an operation and the result in this case generalizes the well-kn own result of Curzon and Ahlborn for the efficiency of a Carnot-like heat engine at maximu m power