  The derivation of the efficiency of Carnot cycle is usually done by
calculating the heats involved in two isothermal processes and making use of
the associated adiabatic relation for a given working substance's equation of
state, usually the ideal gas. We present a derivation of Carnot efficiency
using the same procedure with Redlich-Kwong gas as working substance to answer
the calculation difficulties raised by Agrawal and Menon. We also show that
using the same procedure, the Carnot efficiency may be derived regardless of
the functional form of the gas equation of state.
