The Effect of Charge Air and Fuel Injection Parameters on Combustion with High Levels of EGR in a HDDI Single Cylinder Diesel Engine

When increasing EGR from low levels to levels corresponding to low temperature combustion, soot emissions first start to increase (due to reductions in soot oxidation), before decreasing to almost zero (due to very low rates of soot formation). At the EGR level where soot emissions start to increase, the NO_x emissions are still low, but not low enough to comply with future emission standards. The purpose of this study was therefore to investigate the possibilities for moving the so-called “soot bump” (increase in soot) to higher EGR levels or reducing the magnitude of the soot bump. This involved an experimental investigation of parameters affecting the combustion and thus the engine-out emissions. The parameters investigated were: charge air pressure, injection pressure, EGR temperature and post injection (with different dwell times) for a wide range of EGR rates. The goal was to find a suitable combination of operational parameters at which both NO_x and soot engine-out emissions are minimized, with a minimal fuel consumption penalty. A single cylinder heavy duty direct injection diesel engine, operating at 50% load, with an electronically controlled unit injector with variable needle opening pressure was used in the experiments. Its geometry is based on the Volvo D12C engine in which the compression ratio has been reduced from 18.5 to 14.

The results show that high charge air pressure, low EGR temperature and high injection pressure are important parameters for achieving low emissions. Post injection was found to have limited effects on the emissions, at least under the investigated conditions. Very low soot emissions could be obtained, but the NO_x emissions remain too high and need to be further reduced in order to comply with future emission standards.