The main objective of this study is to investigate the effect of pilot injection mass and timing on main combustion and engine emissions. The experiments have been conducted on a single-cylinder diesel engine at fixed engine speed with various loads. In the computational fluid dynamics (CFD) simulations of the combustion, only a segment of the cylinder was considered. A numerical multiphase simulation of the internal nozzle flow delivered the required initial conditions for the spray primary breakup model. For the combustion the ECFM-3Z model was employed with a two-stage autoignition model.
The measurements show that a pilot injection can reduce the nitrogen oxides (NOx) emissions at low engine load. With higher engine loads an increase in the NOx emission values was observed. The numerical investigation exhibited that the thermal nitrogen monoxide (NO) formation is a mixing-controlled process. The NO is formed generally in two different zones. The first one is located near the cylinder head. The interaction between the cylinder head and the bowl-guided spray leads to the formation of eddies, which enhance the jet-air mixing. The second region is in the piston bowl. The numerical simulation showed that a tumble is generated due to the spray-piston interaction. This tumble mixes the fuel in the piston bowl with fresh cylinder charge. The combustion at stoichiometric conditions and high temperature in these two zones induces the thermal formation of the NO.
Pilot injection affects the thermal NO by changing the ignition position of the main injection. At low loads, the pilot injection influences the ignition position of the main injection strongly, leading to different fuel-air-distribution in the combustion chamber. At these conditions, a small amount of the fuel burns under stoichiometric conditions, and thus a smaller amount of NO is generated. At higher loads, the influence of the pilot injection on the ignition position is small because the spray development is slower at higher gas densities. The equivalence ratio at the geometrical NO formation regions does not change with the pilot injection. The increase in the NOx could be explained by the high gas temperature caused by the combustion of the pilot injection.
