Triple Injection Strategies for Gasoline Compression Ignition (GCI) Combustion in a Single-Cylinder Small-Bore Common-Rail Diesel Engine

Implementing triple injection strategies in partially premixed charge-based gasoline compression ignition (GCI) engines has shown to achieve improved engine efficiency and reduced NO_x and smoke emissions in many previous studies. While the impact of the triple injections on engine performance and engine-out emissions are well known, their role in controlling the mixture homogeneity and charge premixedness is currently poorly understood. The present study shows correspondence between the triple injection strategies and mixture homogeneity/premixedness through the experimental tests of second/third injection proportion and their timing variations with an aim to explain the observed GCI engine performance and emission trends. The experiments were conducted in a single cylinder, small-bore common-rail diesel engine fuelled with a commercial gasoline fuel of 95 research octane number (RON) and running at 2000 rpm and 830 kPa indicated mean effective pressure conditions. While the first injection proportion and timing were fixed at 40% and 170 °CA bTDC, the second injection proportion was varied between 5 and 20% (i.e. third injection of 40~55%) and the timing was varied from 20 to 80 °CA bTDC. The third injection timing was also swept from 2 to 11 °CA bTDC. The results show that increased second injection proportion causes higher peak in-cylinder pressure and apparent heat release rate (aHRR) due to increased charge premixing. This leads to lower smoke emissions but increased combustion-induced noise and NO_x emissions. Despite higher peak in-cylinder pressure, the engine efficiency shows a decreasing trend with increased second-injection proportion because of the lower late-cycle pressure at the expansion stroke and increased wall wetting. Advanced second injection timing leads to lower peak in-cylinder pressure and aHRR and thereby decreasing engine efficiency, which is associated with the increased mixture homogeneity. Consequently, the smoke/NO_x and combustion-induced noise emissions are reduced while the uHC emissions become higher, similar to homogeneous charge compression ignition (HCCI) combustion. In comparison, the advanced third injection timing leads to higher peak in-cylinder pressure and aHRR as well as lower uHC/CO emissions suggesting increased charge premixing with no wall wetting concerns. However, the engine efficiency shows a decreasing trend because of the lower expansion pressure. A typical smoke-NO_x trade-off is found with decreasing smoke and increasing NO_x emissions which once again indicates increased charge premixing.