20. Study on Effects of Multiple Injection at Intake Stroke on Combustion and Emissions of GDI Engine Under Warm Up Conditions

An experimental investigation on multiple injection at intake stroke under warm up conditions was performed in a GDI engine. With a constant excess air ratio, the effects of multiple injection strategy at intake stroke on combustion and emissions were studied. The results showed that, compared to single injection, with the increase of injection times and coolant water temperature, cylinder pressure increased and its corresponding crank angle was advanced for multiple injection at intake stroke, while the peak transient heat released rate increased, but its occurrence timing was slightly advanced. Meanwhile, THC emissions decreased obviously for multiple injection, but NO_x emissions increased. The particle number concentration under warm up conditions of multiple injection at intake stroke reduced, which also decreased with the increase of coolant temperature. However, when at low coolant temperature, the size of particle number concentration showed bimodal distribution of nucleation mode and accumulation mode. With the increase of coolant temperature, the peak size of particle number concentration behaved as a unimodal distribution. Comparing to the third injection timing at three different injection modes, it showed that, with the advance in injection timing, the peak cylinder pressure and the maximum transient heat released rate increased, while their corresponding crank angle were advanced. Meanwhile, NO_x emissions increased, THC emissions and the particle number concentration decreased.