The effectiveness or exergy efficiency of the Zeldovich-von Neumann-Doering (ZND) cycle for a detonation engine is compared to the effectiveness of the Brayton, Otto, and Diesel cycles relative to the thermal efficiency of the ideal Carnot cycle for methane-air powered heat engines. Quantifying effectiveness through combined first and second law thermodynamic cycle analysis allows for determination of not only superior cycle power output, but also in determining which processes can performance gains be realized. The ZND cycle is used to model the detonation engine because it incorporates the supersonic flow processes of shock compression, and Rayleigh heat addition instead of the Humphrey cycle which assumes constant volume heat addition. A simplified pulse detonation turbine engine generator is modeled by the ZND cycle. A novel thermodynamic cycle referred to as the detonation engine piston (DEP) cycle is proposed for a pulse detonation engine that utilizes a piston for power extraction. The DEP cycle is similar to ZND cycle except the heat rejection process is at constant volume in contrast to the constant pressure heat rejection of the ZND cycle. Methane-air combustion is examined for the engine cycles for its practicality stemming from methane's widespread commercial availability. Effectiveness is calculated both by subsystem component analysis, and by indirect cycle thermal efficiency analysis. The reactant and product gas properties were calculated using the complete thermodynamic equilibrium. Comparison of the cycles show that for equivalent pressure ratios, the ZND and DEP cycles for pulse detonation engines produced superior thermal efficiency and effectiveness than the Brayton, Otto, and Diesel cycle heat engines.
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