An investigation of combustion chamber shapes for small automotive direct injection diesel engines employing spray impaction

摘要

In modern compact combustion chambers of direct injection diesel engines with high-speed injection spray, the impaction of the spray on the wall is almost inevitable. This may result in high levels of unburnt hydrocarbons. However, a number ofcombustion systems have been developed which deliberately employ wall impaction as a means of breaking up the fuel spray and/or directing it in a desired direction. These include chambers which employ impaction sites attached to the cylinder head or acut-off pip in the piston bowl centre. A further type of geometry, which has been the subject of an earlier investigation by the authors, is presented here, in which cut-off pips are provided for a number of sprays directed not only straight down into the combustion chamber centre but also across and down into the bowl in the normal radial direction. For the current analysis, the sizes of the pips and their positions are based on the previous work. All these cases are investigated using a computation fluid dynamics code employing non-orthogonal grid systems, theκ-εturbulence model, the implicit PISO (pressure implicit by splitting of operators) solution scheme, and the discrete droplet model for the sprays.Comparisons of simulations with relevant experimental data show that the code returns good qualitative and quantitative results. Apparently good qualitative results are also returned for a number of the combustion chambers employing impaction sitesattached to the combustion head and in the centre of the piston bowl. Simulations of combustion chambers employing a number of impaction sites around the piston bowl indicate the likely advantages of such a system over a conventional highly swirled one.