A Stirling-Cycle Engine the Stirling cycle is similar to the Otto cycle, except that the compression and expansion of the gas are done at constant tem- perature, not adiabatically as in c the Otto cycle. The Stirling cycle is used in external com bustion engines (in fact, burning fuel is not necessary; any way of producing a temperature difference will do solar, geo thermal, ocean temperature gradient, etc.), which means that
The gas inside the cylinder is not used in the combustion process. Heat is supplied by burning fuel steadily outside the cylinder, instead of explosively inside the cylinder as in the Otto cycle. For this reason Stirling cycle engines are quieter than Otto-cycle engines, since there are no intake and exhaust valves (a major source of engine noise). While small Stirling engines are used for a variety of purposes, Stirling engines for automobiles have not been successful because they are larger, heavier, and more expensive than conventional automobile engines. In the cycle, the working fluid goes through the following sequence of steps (Fig. 20.30): (i) Compressed isothermally at temperature T1 from the initial state a to state b, with a compression ratio r. (ii) Heated at constant volume to state c at temperature T2• (iii) Expanded isothermally at T2 to state d. (iv) Cooled at constant volume back to the initial state a. Assume that the working fluid is n moles of an ideal gas (for which Cv is independent of temperature).