Effects of fuel injection timings and methanol split ratio in M/D/M strategy on a diesel/methanol dual-fuel direct injection engine

•Effects of fuel injection timings and split ratio in M/D/M strategy are investigated.•Too advanced methanol injection leads to wall wetting and too delayed leads to knock.•Delayed diesel injection and large dwell could reduce knock intensity effectively.•Excessive split ratios at high load should be avoided to prevent knock. To improve fuel economy and emissions of a diesel/methanol dual-fuel direct injection engine at high load, a methanol/diesel/methanol (M/D/M) strategy is proposed and investigated. This M/D/M strategy consists of injecting a portion of methanol in the compression stroke to form a certain concentration of premixed charge, then injecting diesel near the TDC to ignite the premixed methanol/air mixture, and finally injecting the rest methanol into the flame to achieve diffusion combustion. In this work, a numerical study is conducted to investigate the effects of fuel injection timings and methanol split ratio (SR) in the M/D/M strategy. The results show that too advanced methanol injection timing (MSOI) leads to wall wetting while too delayed MSOI easily leads to knock; MSOI-30 is the most suitable setting to avoid these undesirable outcomes. Delayed diesel injection timing (DSOI) and large dwell can control knock intensity effectively. Moreover, increasing SR properly helps to improve fuel economy, while an excessive SR is prone to knocking. Accounting for engine performance and emissions, a SR of 50:50 paired with a MSOI-30/DSOI-5/MSOI-3 setup is the best overall configuration.