A Simple Approach to Heat Engine Efficiency

Temperature-entropy ( TS ) diagrams are introduced as a way to simplify the calculation of efficiency for reversible heat engines. The equation for the efficiency of the Stirling cycle is derived from its TS diagram. The conflict over the efficiency of the Stirling cycle is thereby resolved; the standard Stirling cycle does not have an efficiency equal to the Carnot cycle efficiency. The historical confusion about the efficiency of the Stirling cycle is shown to arise from two sources: a careless definition of heat input into the cycle, and the unclear role of the regenerator when used with a Stirling cycle. The Stirling cycle with a reversible regenerator is shown to be a Carnot cycle.