AN INVESTIGATION OF COMBUSTION-CHAMBER SHAPES FOR SMALL AUTOMOTIVE DIRECT-INJECTION DIESEL-ENGINES EMPLOYING SPRAY IMPACTION

In modern compact combustion chambers of direct injection diesel engin es with high-speed injection spray, the impaction of the spray on the wall is almost inevitable. This may result in high levels of unburnt h ydrocarbons. However, a number of combustion systems have been develop ed which deliberately employ wall impaction as a means of breaking up the fuel spray and/or directing it in a desired direction. These inclu de chambers which employ impaction sites attached to the cylinder head or a cut-off pip in the piston bowl centre. A further type of geometr y, which has been the subject of an earlier investigation by the autho rs, 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 di rection. For the current analysis, the sizes of the pips and their pos itions are based on the previous work. All these eases are investigate d using a computation fluid dynamics code employing non-orthogonal gri d systems, the k-epsilon turbulence model, the implicit PISO (pressure implicit by splitting of operators) solution scheme, and the discrete droplet model for the sprays. Comparisons of simulations with relevan t experimental data show that the code returns good qualitative and qu antitative results. Apparently good qualitative results are also retur ned for a number of the combustion chambers employing impaction sites attached to the combustion head and in the centre of the piston bowl. Simulations of combustion chambers employing a number of impaction sit es around the piston bowl indicate the likely advantages of such a sys tem over a conventional highly swirled one.