Study on the performance of diesel-methanol diffusion combustion with dual-direct injection system on a high-speed light-duty engine

[Display omitted] •The approximate constant pressure combustion process was found in DMDC mode.•The ITE of the DMDC mode increased by 10% compared to the CDC mode.•The NOx and CO2 emissions of the DMDC mode decreased by 57% and 19%, respectively.•Both the heat transfer loss and exhaust loss decreased by above 10%. To investigate the high methanol substitution ratio (MSR) combustion performance of diesel-methanol diffusion combustion (DMDC) mode, the engine experiments were conducted based on a high-speed light-duty engine with a dual-direct injection system in this study. Effects of MSR on the combustion, emission and energy distribution characteristics, as well as the difference between the DMDC mode and the conventional diesel combustion (CDC) mode, were investigated in detail under four engine loads at a constant engine speed of 3000 rpm and a similar CA50 of 7.5 °CA ATDC. The experimental results show that the MSR can reach 87% at BMEP = 0.55 MPa. Under all operating conditions, the NOx and soot emissions of the DMDC mode are significantly reduced simultaneously compared to those of the CDC mode, and the reduction is enlarged as both MSR and loads increase. At BMEP = 0.14 MPa, the indicated thermal efficiency (ITE), THC and CO emissions of the DMDC mode are inferior to that of the CDC mode. However, the combustion and emission performances are gradually improved when the BMEP exceeds 0.27 MPa. Especially at BMEP = 0.55 MPa, the THC and CO emissions of the DMDC mode are lower and the ITE increases by 10% compared to the CDC mode, and the ITE is more sensitive to the MSR. According to the energy distribution analysis, it can be found that the heat transfer loss and exhaust loss of the DMDC mode have been dramatically reduced, resulting in the increased ITE compared to the CDC mode.