Thermodynamics in finite time: extremals for imperfect heat engines

Abstract

A general description is developed for processes involving work and two heat reservoirs or three heat reservoirs in terms of rates for continuous processes or of cycle averages for periodic processes. The description is applied to heat engines having friction, thermal resistance, and heat losses in order to determine the maximum power and maximum efficiency of such engines. By use of a geometric representation the reversible and irreversible parts of a process are separated as the components of a vector. This leads to the definition of a dimensionless quantity that measures irreversibility and is related in a complementary way to the traditional concept of efficiency. The new quantity appears to be useful in cases where efficiency has no well‐defined meaning.