Three Dimensional Modelling of Combustion in a Prechamber Diesel Engine 890666

Diesel engines remain very attractive for automotive applications especially for urban driving, owing to a lower fuel consumption when compared to spark-ignition engines. Because they allow us to achieve a better compromise between efficiency, noise and emissions, most of the european diesel passenger cars are equipped with prechamber diesel engines rather than direct injection engines. However, the probability of new anti-pollution regulations (especially those concerning particulates emissions) will require a better understanding of combustion in this kind of engines.

Three dimensional modelling appears to be a very good way to analyse the influence of the major parameters involved in the diesel combustion process. The computer code KIVA was modified and improved in order to be applicable to very complex combustion chamber shapes. For this purpose, a new mesh generator was used and coupled to KIVA.

Modelling the diesel combustion also required the implementation of new sub-models in the code. k-ε turbulence model was used and the boundary conditions were described by the appropriate law-of-the-wall. Heat transfer rates to the wall were computed using a model based on a k-ε formulation. In order to describe the auto-ignition at the beginning of the combustion, a simplified four step kinetics mechanism was developped and implemented. It is based on the assumption that the turbulent mixing rate is much faster than the chemical reaction rate of pre-reactions occuring during the auto-ignition delay. On the contrary, combustion itself was supposed to be controlled by species and heat diffusion only. This assumption allowed the use of the Magnussen Eddy Break Up combustion model.

The complete model was then applied to the simulation of flow and combustion in a Ricardo COMET prechamber diesel engine. We studied the fluid dynamics and the evolution of combustion in both the pre and main combustion chambers. The influence of the position of the glow plug and of the piston shape were investigated. The studies of the flow showed that addition of the glow plug in the prechamber reduces the swirl level to less than half its level without the plug.

Comparisons between computational results and experimental flame visualizations performed elsewhere revealed a good qualitative agreement. The computed global heat release rate was also found to follow the experimental trends.