A Physics and Tabulated Chemistry Based Compression Ignition Combustion Model: from Chemistry Limited to Mixing Limited Combustion Modes

Abstract

This paper presents a new 0D phenomenological approach to predict the combustion process in multi injection Diesel engines operated under a large range of running conditions. The aim of this work is to develop a physical approach in order to improve the prediction of in-cylinder pressure and heat release. Main contributions of this study are the modeling of the premixed part of the Diesel combustion with a further extension of the model for multi-injection strategies. In the present model, the rate of heat release due to the combustion for the premixed phase is related to the mean reaction rate of fuel which is evaluated by an approach based on tabulated local reaction rate of fuel and on the determination of the Probability Density Function (PDF) of the mixture fraction (Z), in order to take into consideration the local variations of the fuel-air ratio. The shape of the PDF is presumed as a standardized <i>β<i/>-function. Mixture fraction fluctuations are described by using a transport equation for the variance of Z. The standard mixture fraction concept established in the case of diffusion flames is here adapted to premixed combustion to describe inhomogeneity of the fuel-air ratio in the control volume. The detailed chemistry is described using a tabulated database for reaction rates and cool flame ignition delay as a function of the progress variable c. The mixing-controlled combustion model is based on the calculation of a characteristic mixing frequency which is a function of the turbulence density, and on the evolution of the available fuel vapor mass in the control volume. The developed combustion model is one sub-model of a thermodynamic model based on the mathematical formulation of the conventional two-zone approach. In addition, an extended sub-model for multi injection is developed to take into account interactions between each spray by describing their impact on the mixture formation. Numerical results from simulations are compared with experimental measurements carried out on a 2 liter Renault Diesel engine and good agreements are found.