Assessment of Empirical Heat Transfer Models for a CFR Engine Operated in HCCI Mode

Homogeneous charge compression ignition (HCCI) engines are a promising alternative to traditional spark- and compression-ignition engines, due to their high thermal efficiency and near-zero emissions of NO_x and soot. Simulation software is an essential tool in the development and optimization of these engines. The heat transfer submodel used in simulation software has a large influence on the accuracy of the simulation results, due to its significant effect on the combustion. In this work several empirical heat transfer models are assessed on their ability to accurately predict the heat flux in a CFR engine during HCCI operation. Models are investigated that are developed for traditional spark- and compression-ignition engines such as those from Annand [1], Woschni [2] and Hohenberg [3] and also models developed for HCCI engines such as those from Chang et al. [4] and Hensel et al. [5]. The heat flux is measured in a CFR engine operated in both motored and HCCI mode and compared to the predicted heat flux by the aforementioned models. It is shown that these models are unable to accurately predict the heat flux during HCCI operation if the model coefficients are not properly calibrated. The models from Annand, Hohenberg and Woschni overestimate the heat flux, whereas the models from Chang et al. and Hensel et al. underestimate it during the entire engine cycle if the original model coefficients are used. If the model coefficients are properly calibrated, the models from Annand, Hohenberg and Hensel et al. are able to predict the heat flux during HCCI operation for one engine operating point. However, if the same model coefficients are used for another operating point, the models are unable to satisfactorily predict the heat flux.