A 1-D thermo-fluid dynamic simulation code, including a quasi-D combustion model coupled with a detailed kinetic scheme, is used to analyze the combustion process in HCCI engines. The chemical mechanism has previously been validated in comparison with experimental data over a wide range of operating conditions. To explore the impact on model predictions, the cylinder was divided into multiple zones to characterize the conditions of the in-cylinder charge. Particular attention is devoted to the numerical algorithm in order to ensure the robustness and efficiency of the large system solution. This numerical model allows study of the autoignition of the air fuel mixture and determines the chemical evolution of the system.
The proposed model was compared with in-cylinder temperature and chemical species profiles. The experimental activity was carried out in the combustion chamber of a single cylinder air cooled engine operating in HCCI mode. A customized cylinder head allows easy sampling access, and a fast acting sampling valve was used to collect in cylinder gas samples for subsequent chromatographic analysis.
Different criteria for the definition of the multizone approach were considered and discussed focusing the attention on the possible types of stratification inside the combustion chamber. The number of zone volumes was varied and the effect of different initial temperatures and mixing was analyzed. The inner zones were considered as adiabatic and heat exchange was limited to the peripheral zones. The comparisons between model results and experimental data support the reliability of the approach when applied to the analysis of reaction intermediates, while a parametric analysis provides information about the sensitivity of the system to temperature and composition stratification inside the combustion chamber.
Finally, the model was used to investigate the effect of NOx and non-homogeneous distribution of the charge on the auto-ignition timing.