Investigation of injection strategy for a diesel engine with directly injected methanol and pilot diesel at medium load

•Advanced fuel injection contributes to low EISFC and high RI in D/M mode.•Too small dwell leads to misfire or knocking in D/M mode.•A larger ratio of the first methanol injection achieves better fuel economy in M/D/M mode.•M/D mode is found to produce the best results, followed by M/D/M mode and then D/M mode. A 3D simulation model is conducted to investigate the injection strategy for a diesel engine with directly injected methanol and pilot diesel at medium load. Three injection strategies are proposed: D/M and M/D modes are where methanol injection occurs after and before diesel injection respectively, and M/D/M mode is where methanol is injected once before and once after diesel injection. Optimal settings for each strategy are obtained and compared, yielding the following results: in D/M mode, earlier fuel injection contributes to lower equivalent indicated specific fuel consumption (EISFC) but higher ringing intensity (RI). Moreover, too small dwell leads to misfire or knocking, and too large dwell is detrimental to fuel economy. Secondly, in M/D mode, methanol injection should not be too delayed to avoid knocking and dwell should not be too small to prevent misfire. Thirdly, in M/D/M mode, a larger methanol ratio in the first injection achieves lower EISFC, soot, total unburned hydrocarbon (THC) and carbon monoxide (CO) emissions, but higher nitrogen oxides (NOx) emission. Lastly, M/D mode is found to produce the best results, followed by M/D/M mode and then D/M mode.