Power optimized work limit for internally irreversible reciprocating engines

The theory of irreversible thermodynamics for reciprocating externally irre versible cycles gives rise to an optimum efficiency at maximum power output of eta = 1 - (T-L/T-H)(0.5) for internally reversible Carnot cycles, in co ntrast to the upper limit for Carnot cycles of eta = 1 - (T-L/T-H) obtained from classical thermodynamics. It is shown here in addition, for the inter nally irreversible reciprocating Carnot cycle using linear heat transfer mo des, that the optimum work output at maximum power (W-opt) is less than lan d in the limit of no internal irreversibility is equal to) exactly one-half of the work potential of the externally reversible cycle operating at maxi mum thermal efficiency (Carnot work, W-rev) between the same temperature li mits (i.e., W-opt less than or equal to 1/2W(rev)). To accomplish this the analysis goes one step further than earlier works to make use of time symme try to minimize overall cycle time and thus better optimize overall cycle p ower. Because this novel procedure implies the concurrent use of first and second laws of thermodynamics, it automatically ensures optimal allocation of thermal conductances at the hot and cold ends while simultaneously achie ving both minimization of internal entropy generation and maximization of s pecific cycle work. Based on linear heat transfer laws, this expression for optimum work is shown to be independent of heat conductances. (C) 2000 Els evier Science Ltd. All rights reserved.