Investigating the significance of octane number in controlling HCCI engines

The role of octane number in controlling homogeneous charge compression ignition (HCCI) combustion and emissions, in a multi-cylinder engine was studied. An improved probability density function based stochastic reactor model coupled with a commercial engine cycle simulator was presented. A convective heat loss sub-model (based on a stochastic jump process) and a coalescence-dispersion approach was employed to account for inhomogeneities due to turbulent mixing, fluid-wall interactions, and fluctuations. The base case comparison of the model predictions with the measurements demonstrated the robustness of the integrated full cycle model in reliably predicting the auto-ignition timing, peak pressure as well as CO, HC, and NOx emissions. The improved PDF-based engine cycle simulator outperformed the widely used single-zone based cycle models and provided a significant insight into CO emissions prediction. The auto-ignition timing and the in-cylinder pressure and emissions were sensitive to octane number variation. The magnitudes as well as the trends for the combustion parameters and the emissions (CO, HC, and NOx) were predicted reasonably well by the model. This is an abstract of a paper presented at the 30th International Symposium on Combustion (Chicago, IL 7/25-30/2004).