A FINITE-TIME HEAT ENGINE WITH FIXED-CAPACITY HEAT TRANSFERS

We consider a finite-time heat engine in which the temperature T(t) of the working fluid during heat exchanges with the hot and cold reservo irs is determined passively by the heat capacity of the fluid. The set of feasible operations of this engine, whose irreversibility arises s olely from the temperature difference between the reservoir(s) and the working fluid, is described in terms of an inequality which sets limi ts on the rate P of work output and the rate D of entropy production a ssociated with a cycle that takes place in time tau. Those operations that lie on the boundary of this set are the ones that achieve a speci fied work output and entropy production in minimum time; this leads na turally to a notion of time efficiency for any operation within the se t. The results obtained here extend our previous framework for Camot-l ike processes to examine the time efficiency, as well as the power eff iciency, of the corresponding Otto- and Brayton-cycle based engines. T he present results and the earlier ones can be seen as two extremes of a continuum in which the external control on the temperature of the w orking fluid varies between (i) passively allowing the T(t) correspond ing to a constant heat capacity response and (ii) actively achieving a ny desired T(t).