Abstract
This paper presents a new 0D phenomenological approach to predict the combustion process in Diesel engines operated under various 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 for developing a tool for engine pre-mapping. The main contribution of this study is the modeling of the premixed part of the Diesel combustion.
In phenomenological Diesel combustion models, the premixed combustion phase is sometimes modeled as the propagation of a turbulent flame front. However, experimental studies have shown that this phase of Diesel combustion is actually a rapid combustion of part of the fuel injected and mixed with the surrounding gas. This mixture ignites quasi instantaneously when favorable thermodynamic conditions are locally reached. A chemical process then controls this combustion.
In the present model, the rate of heat release by combustion for the premixed phase is related to the mean reaction rate of fuel. The latter 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 with a standardized β-function. Mixture fraction fluctuations are described by using a differential 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 the 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.
Premixed zone volume and total entrained ambient gas mass flow rate are calculated using a detailed spray model. 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. This zero-dimensional model incorporates several sub-models describing turbulence, vaporization, and fuel introduction rate. The purpose of this approach is to directly relate physical model parameters to operating conditions and engine parameters.
Numerical results from simulations are compared with experimental measurements carried out on a 2 liter Renault Diesel engine. For all investigated operating conditions, simulated cylinder pressure and heat release rate traces show a good agreement with experimental data.
Keywords: 0D Model, Diesel, Combustion, Probability Density Function, Tabulated Chemistry