The formation of exothermic centers was modeled with a Stochastic Reactor Model (SRM) to investigate their impact on HCCI combustion. By varying the exhaust valve temperature, and thus assigning more realistic wall temperatures, the formation of exothermic centers and the ignition timing was shifted in time.
To be able to study the exothermic centers, their formation and their distribution, Scatter plots, standard deviation plots and Probability Density Function (PDF) plots were constructed on the basis of the data the SRM calculations provided. The standard deviation for the particle temperatures was found to be an useful indicator of the degree of homogeneity within the combustion chamber, and thus of how efficient the combustion process was. It was observed that when the standard deviation of the temperature was higher, the emissions of CO and of hydrocarbons present at the end of the closed cycle were higher. It was thus concluded that the standard deviation of the temperature, provided some indication of such emissions as those of hydrocarbons and CO. Since no NOx model was used for the calculations, no conclusions can be made on the relation of NOx formation and the standard deviation of temperature.
The standard deviation does not provide any absolute levels concerning the parameter in question. PDF plots do just this, while at the same time providing a detailed picture of the spread of the parameters being studied. According to the PDF results obtained, higher absolute temperature implies more NOx to be present in the “hot” case. Still, the temperatures were so low that the typically very low levels of NOx in HCCI engines could be expected to be maintained. It was shown that promoting exothermic centers could be one way of counteracting emissions of hydrocarbons and CO which are a problem in HCCI engines.