A computer model of a stirling engine using a two-phase two-component working fluid

Abstract

Stirling engines are potentially the most efficient converters of thermal energy to mechanical work that are manufacturable. In addition to its high efficiency capability, it also operates relatively cleanly, quietly, and with multiple fuel capability. Its one deficiency is the low specific power (power output/engine size). In the past few decades, attempts have been made to overcome this problem by increasing the mean pressure of the engine. This did achieve high enough specific power to be competitive with the diesel engine but caused problems in sealing. These sealing problems have prevented the engine from being reliable and easily manufacturable. It is proposed that there is another method of increasing specific power. This is by utilizing a two-phase two-component (TPTC) working fluid or one having a carrier gas and a component which changes phase during the cycle, such as a mixture of water and air. In order to study the effect of adding water to the working fluid of a Stirling engine on its operation, a computer model of the system was developed. The TPTC model is an extension of the one devised by Urieli to model a Stirling engine with an ideal gas working fluid and includes provisions for accounting for differences in heat transfer and flow losses for a TPTC working fluid. When most of the water is removed from the working fluid of the TPTC model, its results approach those of Urieli for an ideal gas engine. When water was added to the working fluid, the power output was dramatically increased by 150%. This is consistent with predictions made by Walker with a more idealized model. This model can then be used to optimize the configuration of a TPTC engine to produce a powerful yet efficient Stirling engine operating at a relatively low mean engine pressure.