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This paper presents a theoretical study of different strategies of waste heat recovery in an internal combustion engine, operating in a hybrid vehicle (spark ignition engine and electric motor). Many of the previous studies of energy recovery from waste heat focused on running thermodynamic cycles with the objective of supplying air-conditioning loads. There are two elements of this study that are different from previous studies: first, the end use of the recovered waste heat is the generation of electric power, and, second, the implementation of these heat recovery strategies takes place in a hybrid vehicle. The constant load conditions for the SI-engine in the hybrid vehicle are a potential advantage for the implementation of a heat recovery system. Three configurations of Rankine cycles were considered: a cycle running with the exhaust gases, a cycle with the engine coolant system, and a combined exhaust-engine coolant system.

A modified Brayton cycle is incorporated into a continuous combustion engine system. This 6-stroke engine system is described and illustrated with pressure-volume diagrams. Potential advantages over the traditional 4-stroke Otto cycle are reviewed in the areas of emissions, flexible-fuel use, energy conversion efficiency, and noise. A detailed 1-D air standard thermodynamic model of the K6 cycle is generated and used to investigate the potential efficiency of this cycle and analyzed from partial throttle to wide-open throttle power output. The affects of compression ratio and expansion variations on efficiency are evaluated. The power output and power density are estimated. Key assumptions in the analysis of the thermodynamic model are discussed. Comparisons are made to a similar level of analysis of an Otto cycle 4-stroke engine. A utility engine simulating this cycle operating on compressed air is described.