Computer simulation for combustion and exhaust emissions in spark ignition engine

Abstract

In order to predict the exhaust emission concentration without any measured pressure data or empirical burned mass fraction, a new method of combustion simulation was developed. It consists of two major parts: A flame propagation model, and an emission formation model. The flame propagation model, which is based on the burning velocity of the gaseous mixture, gives the pressure and temperature history in the cylinder and in the burned mass fraction for various parameters. In this model, the flame was assumed to propagate across the combustion cylinder as a spherical front centered around a spark plug; a heterogenous temperature field and uniform pressure in the cylinder were assumed. The cylinder consists, of unburned gas and numerous elements of burned gas which burned during successive small increments, of the crank angle. Using various values obtained with the flame propagation model, emission concentration were calculated in individual elements of burned gas, assuming partial equilibrium. Eleven chemical species were taken into consideration. Nine species, other than N and NO, were assumed to be equal to the equilibrium concentration throughout the time of interest. The formation of NO was assumed to be a nonequilibrium process controlled by chemical kinetics and to take place only in the post flame. Computer simulation developed here gives results consistent with the experimental data and it may be utilized for the practical design of a spark ignition engine.