The effects of fuel-injection schedules on emissions of NO _x and smoke in a diesel engine during partial-premix combustion 2007-24-0011

Our aim was to quantify, through engine calibration, benefits in emissions of NOx and smoke, and effects on Specific Fuel Consumption (SFC) and noise, whilst operating with partial-premix combustion. Tests were conducted using a current passenger car diesel engine in series production; thus highlighting limitations in contemporary engine hardware. By scheduling three fuel injections - hereafter designated early (90-62^obTDC), middle (49-31^obTDC) and late (39-15^obTDC) - we sought to quantify the effect of injection timing (with constant fuel-demand) on engine performance. These results were then compared to those obtained with the same engine running under conventional diesel combustion. All tests were conducted at a desired steady-state operating point of 2000 rpm and 3 bar BMEP.

The timing of the three injection events had different effects on combustion, as follows. With timing of the early injection, emissions of NOx and smoke were least affected. With middle injection, fuel mass delivered and emissions of NOx were more sensitive to the chosen timing than with the early and late injections. This plausibly relates to interactions between rail-pressure fluctuations and the timing of the injection event. With the timing of the late injection, measured fuel mass was unaffected (due to lack of rail-pressure interactions), but emissions of smoke were modified significantly. This could be explained by the interaction of the late-arriving liquid “slug” of fuel in the main combustion event i.e., insufficient opportunity was furnished for much evaporation or premixing to occur.

Optimum partial premix (at the above operating point) reduced emissions of NOx, compared with the conventional-combustion baseline, by 60%, for similar levels of smoke emissions. However, an increase of 27% in SFC was observed, together with a noticeable increase in combustion noise (60% based on cylinder-pressure rise rate), plausibly due to the higher percentage of premix burn.