Heat Loss Analysis for Various Piston Geometries in a Heavy-Duty Methanol PPC Engine 2018-01-1726
Partially premixed combustion (PPC) in internal combustion engine as a low temperature combustion strategy has shown great potential to achieve high thermodynamic efficiency. Methanol due to its unique properties is considered as a preferable PPC engine fuel. The injection timing to achieve methanol PPC conditions should be set very close to TDC, allowing to utilize spray-bowl interaction to further improve combustion process in terms of emissions and heat losses. In this study CFD simulations are performed to investigate spray-bowl interaction for a number of different piston designs and its impact on the heat transfer and the overall piston performance. The validation case is based on a single cylinder heavy-duty Scania D13 engine with a compression ratio 15. The operation point is set to low load 5.42 IMEPg bar with SOI -3 aTDC. After satisfactory agreement with experiments in terms of combustion phasing, in-cylinder pressure and heat release rate, the effect of piston bowl geometry is investigated by performing several CFD simulations with modified piston bowl geometry while keeping the compression ratio, CA50 and injection conditions the same as the baseline case. The influence of the wall temperature gradient, the near wall effective conductivity and the piston bowl area on the heat transfer is studied. It was observed that the flow structures that re-direct the hot vapor away from the in-cylinder walls will reduce the wall area that actively transfer the heat. The final piston performance comparison showed that piston bowl designs with a reduced area to volume ratio does not guarantee lower heat loss. Therefore, the mixing process as the result of the spray-bowl interaction and the resulting fuel distribution are considered as the main mechanisms to minimize the total heat losses.