A Thermodynamic Analysis for Detonation-Free Engine Performance

A method of thermodynamic analysis of power cycles incorporating more complete expansion and turbocharging with after-cooling is presented; it predicts the performance of constant volume combustion engines under detonation-free conditions. The analysis involves expressing the detonation limit in terms of the firing pressure and a theoretical adiabatic end-gas temperature. Expressing the detonation limit in this form, the performance map for any constant volume combustion engine can be predicted. The map yields detonation-free operation over a wide range of bmep and bsfc by identifying the required combinations of fuel-air ratio, compression ratio, expansion ratio, and blower pressure ratio. The analysis requires information on the combustion characteristics of the specific engine, namely, the deviations of the thermal efficiency and the firing pressure from those derived from the equivalent fuel-air cycle. Analysis showed that a significant gain of engine output can be realized without detonation occurring by employing higher blower pressure ratio and firing pressure. The lower limit of fuel consumption is determined directly by the performance of the turbocharger and indirectly by the heating of the inlet air and the detonation limit. Tests performed on a 17-in-bore single cylinder gas engine at two different expansion ratios verified the analytical prediction of detonation-free operation over a wide range of bmep, from 135 to 346 psi. The type of performance map resulting from this analysis is useful in the selection of engine performance and engine parameters for the development of new engines.