A Review and thermodynamic analysis of an external combustion, reciprocating engine
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The open cycle reciprocating Brayton engine concept uses separate and different cylinders for compression and expansion, and combustion occurs in an external burner at approximately constant pressure. Although this general engine concept dates back to Brayton in 1876, no known engine has been constructed or tested. Because of the nature of the combustion process (continuous) and the separation of the compression and expansion processes, the open cycle reciprocating Brayton engine has the potential to produce lower emissions and to possess higher efficiencies than conventional reciprocating engines. In contrast to conventional engines, the Brayton engine may use intercooling, recuperation and reduced heat loss technologies. This report reviews previous efforts concerning this concept, and describes the development and use of a thermodynamic based simulation for the open cycle reciprocating Brayton engine. Using this simulation, the indicated thermal efficiency of the base case open cycle reciprocating Brayton engine was 45.6%. In addition, the indicated thermal efficiency for high expander wall temperatures was as high as 50% for the highest wall temperatures. For a continuous flow combustor such as proposed for the open cycle reciprocating Brayton engine, the hydrocarbon and carbon monoxide emissions will be negligible for well designed combustors. Regarding nitric oxides, similar gas turbine combustors have been designed to minimize the nitric oxide emissions. Today's gas turbines easily may produce less than 25 ppm of nitric oxide, and some are even as low as 10 ppm. For the more typical emission of 25 ppm, the open cycle reciprocating Brayton engine is expected to emit less than one-half of the Federal regulation for nitric oxides without any catalyst system.
