Modeling and Experiments of NOx Formation in DI-PCCI Combustion

Formation of nitrogen oxides (NOx) in direct-injection premixed charge compression ignition (DI-PCCI) combustion simulated in a constant volume vessel was investigated using an ignition-combustion model that combines a stochastic mixing model with a reduced chemical reaction scheme. Several improvements were made to the model in order to predict the combustion processes in DI-PCCI. Calculations were carried out for the injection and ambient conditions equivalent to the measurements using the constant volume vessel. Analysis of the calculated results clarified the effects of mixture heterogeneity on NO concentrations and the mechanisms are discussed. The results show that the model successfully represents the experimental tendency for NO concentration when the injection conditions and ambient oxygen mole fraction are varied. NO reduction, due to enhanced mixing with smaller nozzle orifice, is explained based on the analysis of probability density functions (PDFs) for the equivalence ratio ϕ and the ϕ-NO production rate relations. In addition, the reason for the tendency observed for the measurement is clarified; that increasing the fuel injection quantity weakens the effect of NO reduction by enhanced mixing.