A Preliminary Study on the Effect of Low Temperature Kinetics on Engine Modeling

Modeling autoignition in diesel engines is a challenging task because of the wide range of equivalence ratios over which it takes place. A variety of detailed autoignition models has been proposed in literature for different fuels. Since these models include about one thousand chemical reactions and more than one hundred species, their application to CFD engines simulations requires a very high computational time, so that they are of no practical interest. In order to lower the computational time, a number of reduced models has been developed including the shell model, which is one of the most used. This model does not take into account low temperature kinetics and consists of seven reactions and three radicals. The use of this model in engine simulations shows its limits when applied to delayed injections because of the predominant influence of the low temperature kinetics. A modified version of the shell model is proposed in the present study. It includes the effect of low temperature kinetics by the addition of two more radicals and three new kinetics reactions. The model has been implemented in a modified version of the KIVA3V code. The performance of the new kinetic scheme has been investigated by computing the ignition delay at different operating conditions in bomb like simulations. The investigated operating conditions were obtained by changing gas temperature and pressure, air to fuel ratio (i.e., oxygen concentration). The effect of the pre-exponential factor in the rate of production of the intermediate agent has also been investigated. The model has also been applied to predict autoignition of a commercial small bore direct injection diesel engine for different injection timings. Results showed that, for delayed injections, the new model was able to better predict the heat released and the pressure traces. The influence of the modified shell model on engine emissions has also been analyzed.