3D simulation: a powerful tool to shorten the combustion chamber design process in SI engines

摘要

The complexity of SI engines concepts has emphasized the need for comprehensive tools to understand both combustion and mixture preparation: 3D Computational Fluid Dynamics is one of them. Significant improvements have been achieved during the recent past years in 3D CFD techniques. With the improved computational speeds, those techniques have now reached a level where they can be used for the development of combustion systems. Up to now, 3D CFD have been used in assistance to test bench, to analyze the in-cylinder flow fields and combustion phenomena. But the ultimate purpose of simulation is to be able to evaluate and discriminate different technical definitions in short loop. The purpose of this paper is to present a study in which 3D CFD calculation was used to reduce the amount of tests in the development of a gasoline engine. In full load conditions, knock is a major limitation to the increased volumetric efficiency, linked to the downsizing strategy, especially at low speed. Thus, 3D CFD has to be predictive on the sensitivity of a combustion chamber to the knock phenomenon, in order to compare several configurations in terms of full load performance. Last year, a new methodology has been developed allowing to compare technical definitions in full load conditions and to produce simulation results comparable to engine test bench results. This methodology was applied to a multi ignition engine: 5 spark plugs were set up in a single cylinder head to evaluate the potential of the multi ignition on the combustion process, in particular on the knock sensitivity. At the same time, 3D CFD were performed with KIVA ECFM to understand the knock localization and to investigate on spark plugs location. The 3D CFD simulations allowed answering some questions regarding full load results, in particular: To analyze the atypical shape of the heat release rate in dual spark ignitions; To confirm and explain the hierarchy between several configurations; To show the strong dependence of the results on the combustion chamber by correlating the heat release rate with both flame/wall, and flame/flame interactions.