| The ignition delay (ID) affects many aspects of the diesel engine operation including its fuel economy, the emission of species such as hydrocarbons, nitrogen oxides, and smoke particulates. The goal of this research work is to develop a method to compute ID in engines, taking into consideration the heterogeneity of the charge. Two ID models have been developed to compute the ID in direct injection diesel engines under actual running conditions. The first ID model takes into consideration the heterogeneity of the charge, and a global reaction rate. The second ID model takes into consideration the heterogeneity of the charge, and the multi-step autoignition reactions. In the both ID models, the region where ignition starts in the fuel spray is divided into elements which successively mix, form an ignitable mixture, and start the formation of chain carriers. In the first ID model, a Global Autoignition Function β(T,P) is formulated and data on autoignition of fuels in constant volume vessels is utilized to compute the function. The first ID model indicates that autoignition occurs when the normalized cumulative concentration of the chain carriers, Ψ(t) reaches unity. The computed values from the first ID model were found to be in good agreement with experimental data obtained in an actual engine running under different ambient temperatures. In the second ID model, the global rates of the autoignition of the first ID model is replaced by a thirty-two steps autoignition reactions which is proposed by Sahetchian et al in 1995. The reaction site temperature, concentration and mass fraction of chemical species during ID period are calculated from the second ID model. Maps are developed for the ID and equivalence ratio, different ambient temperatures. In the maps, there is a range in which autoignition takes place under the same ambient temperature. ID data obtained from engine experiments occur where found close to the minimum value of ID in the range. The model developed in this research can be utilized to determine ID in engines from data obtained in constant volume vessels. The detailed kinetic model can be used to investigate the effect of dilution with gases on autoignition processes. Extend the detail chemical reaction model included the variation of equivalence ratio, one element to another within the reaction site. |
