When increasing EGR from low levels to a level that corresponds to low temperature combustion, soot emissions initially increase due to lower soot oxidation before decreasing to almost zero due to very low soot formation. At the EGR level where soot emissions start to increase, the NOx emissions are low, but not sufficiently low to comply with future emission standards and at the EGR level where low temperature combustion occurs CO and HC emissions are too high.
The purpose of this study was to investigate the possibilities for shifting the so-called soot bump (where soot levels are increased) to higher EGR levels, or to reduce the magnitude of the soot bump using very high injection pressures (up to 240 MPa) while reducing the NOx emissions using EGR. The possibility of reducing the CO and HC emissions at high EGR levels due to the increased mixing caused by higher injection pressure was also investigated and the flame was visualized using an endoscope at chosen EGR values. In addition, flame temperatures and soot volume fractions were determined using two-color pyrometry.
A single-cylinder heavy duty direct injection Diesel engine equipped with an electronically controlled unit injector with variable needle opening pressure was used in the investigations, with geometry based on the Volvo D12C production engine but with the compression ratio reduced from 18.5 to 14.
The results from the studied cases showed that increasing EGR from 0 to 30% reduces both soot formation and oxidation resulting in higher engine-out soot emissions. Also, an increase in NOP for a case without EGR was found to give both high soot formation and high soot oxidation, resulting in reduced engine-out soot emissions. When EGR was added, increased NOP resulted in increased soot oxidation and thus in lower engine out soot emissions. An increase in NOP was also found to reduce HC and CO emissions at high EGR levels.
