Performance of Curzon-Ahlborn engine along with its engine speed and compression ratio

Abstract

We evaluated the ideal limits of performance (power and efficiency) of heat engines operated with external heat sources along with their engine speed and compression ratio, using the method of adjoint equations based on variational principle. It is known that the power and efficiency of heat engines are maximum when the finite-time heat-transfer from/to the heat sources occurs isothermally, and such an engine is called Curzon–Ahlborn (CA) engine, so we derived a formula to express the temperature in the isothermal process of the CA engine as functions of the rate constant (or time constant) of either expansion or compression of the volume of the working fluid during that process. Using this formula, we found that the CA engine has the maximum of compression ratio and the slowest limit of engine speed for each compression ratio while at each compression ratio the thermal efficiency becomes greater with increasing engine speed and at each engine speed the efficiency increases with increasing compression ratio. These characteristics indicate the ideal performance envelope of the heat engines operated with external heat sources.