A New Technique for Thermodynamic Engine Modeling

A recently developed set of governing equations for piston engine performance shows that it is a function of four fundamental engine efficiencies (combustion, thermodynamic cycle or indicated thermal, volumetric, and mechanical), as well as engine operational and design parameters. This set of governing equations suggests a different technique for engine modeling: model each efficiency individually and combine these efficiency submodels to obtain an overall engine model. In this context, a simple method for predicting the combustion efficiency of piston engines is desired. Various methods are proposed and compared with experimental results. These combustion efficiency models are then combined with various models for the volumetric, mechanical, and indicated thermal efficiencies to produce three different engine models of varying degrees of sophisticatio n. Predictions of the resulting engine models are then compared to experimental data. It is shown that combustion efficiency is almost independent of load, speed, and compression ratio and is not strongly dependent on fuel type of fuels similar to gasoline. It is concluded that combustion efficiency may be accurately modeled using equilibrium considerations, at least for spark-ignition engines with near-optimum ignition timing. Also, although the new approach to engine performance modeling does not promise to be more accurate than currently available codes, it does present the performance characteristics in a more straightforward and easily interpreted manner.