The effects of fuel-injection schedules on emissions of NO<sub>x</sub> and smoke in a diesel engine during partial-premix combustion

Abstract

<div class="htmlview paragraph">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 <i>early</i> (90-62<sup>o</sup>bTDC), <i>middle</i> (49-31<sup>o</sup>bTDC) and <i>late</i> (39-15<sup>o</sup>bTDC) - 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.</div> <div class="htmlview paragraph">The timing of the three injection events had different effects on combustion, as follows. With timing of the <i><u>early</u></i> injection, emissions of NOx and smoke were least affected. With <i><u>middle</u></i> 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 <i><u>late</u></i> 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>i.e.</i>, insufficient opportunity was furnished for much evaporation or premixing to occur.</div> <div class="htmlview paragraph">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.</div>