Experimental assessment of lean-burn characteristics for a modified diesel engine operated in methanol direct injection spark ignition (DISI) mode at full throttle condition

In this study, the lean-burn characteristics of a high-compression direct injection spark ignition (DISI) methanol engine were investigated. The tests were conducted on a modified diesel engine in methanol DISI mode at three excess air ratios (1.2, 1.3, and 1.4) and injection timings from −60° to −300° crank angle (CA) after top dead center (ATDC). The engine was run at 1500 rpm, 100% throttle opening, and spark timing at maximum brake torque condition. The results demonstrated that the test engine under lean-burn condition can obtain the higher indicated thermal efficiency (ITE), but the lower indicated mean effective pressure (IMEP). The THC, CO, CO₂ and NO_X emissions are all reduced with the increased λ. Injection timing had a large influence on the combustion and emissions and these influences became largely in the case of retarded injection conditions. There existed an optimum injection timing of −240 °CA ATDC where the IMEP and ITE got their highest values along with the lowest coefficient of variation in indicated mean effective pressure (COV_IMEP). Meanwhile, the THC and CO emissions are lowest, but the CO₂ and NO_X emissions highest. The methanol engine can obtain the maximum toque of 27.6 N·m, which is 25.3% more than the original diesel engine at the same speed. Meanwhile, there was no knocking throughout the entire experiment, and the maximum pressure rise rate and COV_IMEP were found to be 0.4 MPa and 4.1%, and those are much less than the critical points of 1.0 MPa and 5%, respectively.