An Investigation of Low Temperature Combustion Using high Levels of EGR

Licentiatavhandling, 2007

The diesel engine offers good fuel economy and generates low carbon dioxide (CO2) emissions, but it is also a source of undesirably high particulate matter (PM) and nitrogen oxide (NOx) emissions, which are subject to increasingly strict legislative limits because of their adverse effects on the environment and human health. Thus, in recent years, several combustion concepts have been developed to meet future legislation requirements, one of which is Low Temperature Combustion (LTC) using high levels of EGR (Exhaust Gas Recycling). In order to investigate the possibilities for using low temperature combustion to reduce undesirable emissions, investigations were performed using a single-cylinder research engine. The goal was to find a suitable combination of operational parameters at which both NOx and soot engine-out emissions are minimized, with a minimal fuel consumption penalty. When increasing EGR from low levels to levels corresponding to LTC, 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 NOx emissions are low, but not low enough to comply with future emission standards. However, emissions of both soot and NOx can be reduced to low levels with LTC obtained using high levels of EGR (approximately 60 %). Unfortunately, low temperature combustion is accompanied by increases in fuel consumption and emissions of hydrocarbons (HCs) and CO, but these drawbacks can be reduced by advancing the injection timing. The results also show that high charge air pressure, low EGR temperature and high injection pressure are important parameters for achieving low engine-out emissions. Very low soot emissions can be obtained at intermediate EGR levels without increased CO and HC emissions, but the NOx emissions remain too high and need to be further reduced in order to comply with future emission standards. Using high EGR levels, NOx and soot emissions can meet future emission standards, but the fuel consumption penalty associated with the high CO and HC emissions needs to be reduced.