Soot Emission Reduction from Post Injection Strategies in a High Pressure Direct-Injection Natural Gas Engine

Compression ignition engines, including those that use natural gas as the major fuel, produce emissions of NOx and particulate matter (PM). Westport Inc. has developed the pilot-ignited high-pressure direct-injection (HPDI) natural gas engine system. Although HPDI engines produce less soot than comparable conventional diesel engines, further reductions in engine-out soot emissions is desired. In diesel engines, multiple injections can help reduce both NOx and PM. The effect of post injections on HPDI engines was not studied previously. The present research shows that late injection of a second gas pulse can significantly reduce PM and CO from HPDI engines without significantly increasing NOx or fuel consumption. In-cylinder pressure measurements were used to characterize the heat release resulting from the multiple injections. Experiments showed that most close-coupled split injection strategies provided no significant emissions benefit and less stable operation. However, post injection of 15-20% of the fuel 1.5-2.5 ms after the end of the first injection can reduce PM and CO by over 80%. Using this strategy has only a small effect on other emissions and fuel consumption. Methane emission is reduced about 25%, NOx changes is almost within the variability of results, maximum pressure of cylinder increases within 5bar and fuel consumption will increase about 1%. Based on the literature for diesel engines, we expect that enhanced mixing due to the second injection and increased local temperature within the cylinder may be major contributors to the soot reductions.