Influence of pilot injection on combustion characteristics and emissions in a di diesel engine fueled with diesel and DME

This work experimentally investigates how the dwell time between pilot injection and main injection influences combustion characteristics and emissions (NOx, CO, THC and Smoke) in a single-cylinder DI diesel engine. Additionally, results from diesel injection are compared with those shown in dimethyl ether (DME) injection under the identical injection strategy to demonstrate the sensitivity of the combustion characteristics and emissions to changes of the fuel type. Two fuel injection systems are applied for this experiment due to the differences of fuel characteristic with regard to physical and chemical properties. The injection strategy is accomplished by varying the dwell time (10°CA, 16°CA and 22°CA) between injections at five main injection timings (-4°CA aTDC, -2°CA aTDC, TDC, 2°CA aTDC and 4°CA aTDC). It was found that pilot injection offers good potential to lower the heat-release rate with reduced pressure traces regardless of the dwell time between injections and fuel type. Pilot injection also allows stable combustion even for extensive combustion phasing retard, which reduces the NOx emissions. This means that it is indeed possible, and higher BMEP could be achieved for pilot-main injection operation. Contrary to the expectations, pilot injection are found to play a lesser role in reducing the NOx emissions, but NOx formation is mainly suppressed by the combustion phasing retard effect. A comparison of the emissions between diesel injection and DME injection under identical injection strategy shows that even DME injection exhibits higher NOx emissions than diesel injection, pilot injection is found to be more effective on DME for reducing the amount of NOx with combustion phasing retard, which indicates a level of NOx emissions similar to that of diesel injection. The CO and THC emission levels vary significantly with injection strategies in diesel injection, but they are typically found to be lower than 100 ppm in DME injections. Although the diesel pilot-main injection conditions show higher PM emissions than single-injection condition, DME has little PM regardless of injection strategy.