Effect of injector configurations on combustion and emissions in a gasoline direct-injection compression ignition engine under low-load conditions

The effect of injector configurations on combustion and emissions is investigated at low-load conditions in a gasoline direct-injection compression ignition engine. A total of five injectors with higher number of nozzle holes (10, 14) and narrow injection angles (70 degrees, 100 degrees) were tested while comparing with the baseline injector with eight holes and injection angle of 146 degrees. Diesel combustion was also performed at similar operating conditions for the comparison with gasoline combustion. First, at the idle operation (engine speed=800r/min, engine load=10%), combustion stability was very low with baseline injector due to the excessive mixing process. The narrow injection angle was helpful to enhance combustion stability, attributed to suppressing the formation of over-lean mixture by the fuel stratification within narrow area. However, higher number of nozzle holes were unfavorable with frequent misfires by excessive fuel/air mixing. Second, at the low-load operation (engine speed=1200r/min, engine load=25%), combustion and emissions were investigated according to the injection timing. For all injectors, partially premixed compression ignition combustion was realized with early injection which exhibited low nitrogen oxide emissions resulted from lean premixed combustion. However, high hydrocarbon and carbon monoxide emissions were produced with wide injection angle due to the over-mixing and trapped fuel in squish and crevice volume. Hydrocarbon and carbon monoxide emissions were significantly reduced with narrow injection angle, attributed to the fuel stratification inside piston bowl. On the other hand, hydrocarbon and smoke emissions were aggravated in diesel combustion with narrow injection angle, attributed to fuel-rich mixture resulted from the significant fuel impingement on piston bowl. Fuel impingement occurred for both gasoline and diesel; however, evaporation process was much slower for diesel fuel due to the lower volatility. Significant pool fire and rich combustion were observed for diesel combustion, while blue chemiluminescence was dominated in gasoline combustion which meant well-premixed combustion.