Understanding HCCI Combustion in a Free Piston Engine with a Multi-Zone, Control-Mass Model with Thermal Stratification and Chemical Kinetics 2019-01-0958

Homogeneous Charge Compression Ignition (HCCI) is a combustion concept with the potential for future clean and efficient automotive powertrains. In HCCI, the thermal stratification has been proven to play an important role in dictating the combustion process, mainly caused by heat transfer to the wall during compression. In this study, a multi-zone, control-mass model with thermal stratification and chemical kinetics was developed to simulate HCCI combustion. In this kind of model, the initial conditions and the zonal heat transfer fraction distribution are critical for the modeling accuracy and usually require case-by-case tuning. Instead, in this study, the Thermal Stratification Analysis (TSA) methodology is used to generate the zonal heat transfer fraction distribution from experimental HCCI data collected on a fired, metal engine. It is found that this normalized distribution determined by the TSA methodology from one experimental operating condition is wide-ranging enough to be applied to all the simulations of different engines, engine speeds, compression ratios, fuels, and/or equivalence ratios by comparing the simulation results with corresponding experimental data. Therefore, no case-by-case tuning is needed for the individual zones in this multi-zone model, with the only tuning being the initial temperature (intake valve closing temperature).

Then, this validated multi-zone model is used to study HCCI combustion of crank-slider engines (CSEs) and free piston engines (FPEs). An FPE is an engine without a crankshaft. Thus, the piston can move freely which results in some unique features. The piston acceleration near the top dead center (TDC) is much higher than CSEs; hence, the compression and expansion processes are faster than CSEs. The results show that the burn duration for isooctane, which can be treated as a single-stage fuel, is independent of the engine speed on both CSEs and FPEs, proving that HCCI is mainly kinetically controlled. The burn duration of the FPEs is found to be shorter than CSEs since a higher IVC temperature is required and weaker thermal stratification is developed due to the rapid compression process. In addition, HCCI FPE does not mitigate the pressure rise rate, compared to the spark- or compression-ignition FPEs.