Numerical studies of chemically enhanced combustion in internal combustion engines using advanced piston geometry

The flow field and combustion in a SI engine with a novel piston are examined using a numerical modeling approach. This special piston geometry consists of a central bowl (main chamber) and a small cavity (micro-chamber) built into the piston head; two chambers are connected by a narrow channel. The small cavity traps a part of charge and produces chemically active species which subsequently enhance ignition and combustion. With similar piston designs, SI engines run stably without spark and produce less pollutant emissions, including NOx, without reducing fuel economy.; The calculations are carried out in three parts. In the first part, the flow field and flame propagation are simulated using a multi-dimensional code KIVA-II. Emphases are placed on the thermal quench of the flame by cold connecting channel walls. The effects of the channel size on flame quenching are investigated. Inflow and outflow in the micro-chamber are simulated. In the second part, low temperature, partial oxidation of fuel occurring in the micro-chamber is calculated using a zero-dimensional, detailed kinetic model. Hydrogen and methanol are studied to establish the calculation procedure to be applied in the future to more complex fuels. The dominant intermediate species formed in the micro-chamber are found to be H{dollar}rmsb2Osb2 and CHsb2O.{dollar} These species flow out of the micro-chamber, seed the succeeding fresh charge and enhance the autoignition of the charge by changing the dominant chain initiation pathways and reducing ignition delay time. With the presence of these intermediate species in the fresh charge, the low temperature, chemically enhanced autoignition process in the main chamber is demonstrated numerically. Finally, the effect of exhaust gas recirculation (EGR) on NOx formation is investigated. It is found that EGR will not likely provide the sources of key intermediate species as the micro-chamber does.; This study provides a theoretical basis and demonstrated principle of the enhanced combustion phenomena caused by the special design of piston geometry. This technology, as already proven in real engine tests, can be applied to direct injected systems and achieve significant reduction of ignition delay and pollutant emissions.