Stirling engines operate on closed regenerative thermodynamic cycles with compression and expansion of the working fluids at different temperature levels. They may be used as prime movers, refrigerating machines, heat pumps, or pressure generators. Conventional machines use a gaseous working fluid, but substantial improvement in specific output may be gained with a partially reactive, condensing working fluid. The working fluid then consists of an inert gaseous carrier with a chemically reactive, condensing working fluid such as nitrogen tetroxide (N2O4). This may be liquid in the cold compression space and then evaporates and dissociates in the regenerative process to be in the elemental gaseous phase in the hot expansion space. The change of state of one component reduces the required compression work and has the effect of increasing the engine volume compression ratio with consequent benefit to the specific output. The results obtained using idealized theory show that an improvement may be gained in net cycle work of twice the output with a simple gaseous working fluid with no penalties in size, weight, or cost of the engine. The degree of improvement depends on the design and operating conditions of the engine. The effects of variation of some of these parameters are explored.

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