Combustion and NO Formation in a Stratified-Charge Engine: a Two-Turbulent Equations Model

Abstract

A theoretical model of turbulent flame propagation in reciprocating stratified-charge engines is developed and compared to a previous model. A spacial variation of mixture ratio is considered such that there is a fuel-rich region in the center of the cylinder and a fuel lean-region near the cylinder walls. The spark in the fuel-rich region results in a premixed type of flame propagating toward the walls. A diffusion-type flame results in the wake of the other flame due to the mixing of the excess air and fuel.

The first model which was presented in a previous paper is based upon a prescribed time variation of the turbulent diffusivity depending upon the piston velocity. In the present work, a two-turbulent-equations model is developed and the turbulent diffusivity varies in space and time as well. Without experimental data it is difficult to ascertain that the second model is superior to the first one. However, because of the coupling of the turbulence phenomenon to the combustion itself, the second model is believed to be better than the first one. In both models calculations of the temperature and species mass fractions as functions of time and space are made. Also, the pressure is calculated as a function of time.

A parametric study is performed with the second model. Parameters concerning both the engine and the mixture are varied. In particular a comparison with an engine operating with a premixed gas mixture is made. Trends indicate methods to “optimize” the combustion process so that a “satisfactory” reduction in NO could be achieved with a “small” drop in the pressure level.