A Carnot cycle is reversible because it is quasistatic and frictionless, meaning it can be traversed in either direction without net energy loss; run in reverse, the same engine becomes a Carnot refrigerator, absorbing heat Q2 from a cold reservoir at T2 and, with work input W, rejecting Q1 = Q2 + W to a hot reservoir at T1. This reverse operation does not violate the second law of thermodynamics because the external work source contributes more entropy to the universe than the refrigeration process removes. The efficiency formula eta = 1 - Q2/Q1 = 1 - T2/T1 holds specifically for the Carnot cycle, while eta = W/Q1 (equivalently (Q1-Q2)/Q1) is the general definition of efficiency applicable to any heat engine; this distinction and the reversibility property together form the basis, within thermodynamics, for later establishing the Carnot engine as the theoretical upper bound on heat-engine efficiency.
A Carnot cycle is reversible because it is quasistatic and frictionless, meaning it can be traversed in either direction without net energy loss; run in reverse, the same engine becomes a Carnot refr…