Comparative study on the effects of inlet heating, inlet boosting, and double-injection strategy on partially premixed combustion

Partially premixed combustion (PPC) is a low temperature combustion (LTC) concept which can relieve soot-NO _x trade-off without sacrificing efficiency. However, at low load operating range, PPC with low reactivity fuel generally undergoes long ignition delay, which gives rise to high pressure rise rate, fast heat release and even misfires. To solve these problems and maintain high efficiency simultaneously, inlet heating, inlet boosting and double-injection strategy are experimentally investigated in a heavy-duty engine. BH80 (80vol% n-butanol and 20vol% n-heptane) are blended and tested at 8 bar gIMEP in PPC mode. Inlet heating (from 40 ^o C to 100 ^o C), inlet boosting (from 1.4 bar to 2.5 bar) and a double-injection strategy (pilot/main injection) are attempted to reduce the maximum pressure rise rate (PRRmax). The results show that all three methods can achieve negligible soot emissions. Moreover, a correlation between global temperature at TDC and ignition delay is noticed. In other words, high global temperature after compression stroke makes BH80 easier to ignite. As a consequence, the ignition delay shortens and the maximum pressure rise rate decreases. Compared to inlet heating and inlet boosting, the double-injection strategy shows more advantages in reducing pressure rise rates and obtaining high gross indicated efficiency (GIE). Specifically, with a well-tuned double-injection strategy, 3.6 bar/ ^o CA PRRmax and 49.5% GIE are achieved. In addition, when more fuel is injected in the pilot injection pulse, NO _x emissions are significantly decreased. However, a longer pilot pulse also produces more CO and HC emissions and leads to lower combustion efficiency.