The role of piston bowl shape in controlling soot emissions from heavy-duty Diesel engines

Abstract: The role of piston bowl shape in controlling soot emissions from heavy-duty Diesel engines. Diesel combustion is often subject to a trade-off between emissions of soot and nitrogen oxides (NOx). This trade-off limits the scope for restricting the CO2 footprint of diesel engines by reducing fuel consumption while simultaneously reducing emissions of soot and NOx.  This thesis examines a new piston bowl shape for diesel engines that was found to significantly reduce soot emissions, enabling the typical soot-NOx trade-off to be limited. Experimental and simulation studies showed that the improvements originated from enhancement of soot oxidation late in the combustion cycle, after the end of the fuel injection period. The late-cycle oxidation rate increased when the supply of oxygen to a radially developing turbulent mixing zone (RMZ) was improved.  The mixing features of the RMZ depended on the interaction between the high velocity, high momentum fuel jet and the piston bowl shape. A clear coupling was identified between the piston bowl shape and the transport of oxygen to the trailing edge of the RMZ. Specifically, it was shown that the presence of a wave-shaped protrusion caused a smoother flow in the flame-flame area that improved oxygen transport to the trailing edge. This improved oxygen supply increased late-cycle mixing and thus increased the soot oxidation rate, reducing net soot emissions by up to 80% and also improving fuel consumption. Experiments with different fuel types revealed that this strategy also improves late cycle mixing for renewable fuels. The findings were used to develop a qualitative model describing late-cycle heavy-duty low-swirl combustion. Together with previously introduced qualitative models of free diesel jets, this model could help clarify events that have important effects on various flame interactions. In addition, a new experimental setup was developed to enable detailed studies of RMZ behaviour in a spray-chamber. The setup consists of a two-hole fuel nozzle combined with different wall geometries. It is suggested that new insights into RMZ behaviour and late-cycle oxidation will be obtained by combining experiments using this setup with simulation studies.  The work was partly funded by the Swedish Energy Agency (project 30754-1&2).