A perfectly reversible engine using an ideal gas MUST use wh

A perfectly reversible engine using an ideal gas MUST use which of the following types of processes:

(a) Frictionless adiabatic process

(b) Frictionless isochoric process

(c) Frictionless isobaric process

(d) Frictionless isothermal process

Explain in a few sentences your choices(There can be more than one answer)

Solution

(d) Frictionless isothermal process

A heat engine involves heat transfer between the source and the sink. For the engine to be totally reversible, the heat transfer should occur between infinitesimally small temperature differences. Essentially, these processes should be isothermal. Friction is irreversible and thus all processes should be frictionless.

You may argue that since the cycle involves two temperatures, processes between these temperatures should be adiabatic, in order to satisfy reversible heat transfer condition. But, Stirling and Ericsson cycles achieve Carnot efficiency without any adiabatic processes by using regeneration.