Experimental investigation on methanol auto-ignition in a compression ignition engine under DMDF mode

Abstract

At high loads, Diesel-Methanol Dual Fuel (DMDF) engines strongly rely on high methanol substitution for diesel proportion (MSP) to achieve high efficiency combustion and reduce nitrogen oxides (NOx) and soot emissions. However, high MSP is limited by the probability of methanol auto-ignition. The experimental study was conducted to investigate into the influence factors of methanol auto-ignition under diesel engine conditions at 50% of full load. Meanwhile the testing results concerning combustion characteristics and emissions have been analyzed. Experimental results showed that if there was apparent heat release before TDC, which was earlier than diesel fuel injection, it indicated that methanol auto-ignition occurred. In order to avoid methanol auto-ignition which is very easily to cause knock or rapid pressure rise, the in-cylinder mean temperature should be less than 990 K (at this case, the intake temperature was 338 K) before diesel injection timing. As intake temperature increased, methanol heat release timing was advanced and the heat release rate (HRR) was gradually accelerated. If the injection timing of diesel was later than that of methanol auto-ignition, almost all diesel fuel would be burned in the form of diffusion combustion in the methanol flame, leading to the increase of soot emissions. Methanol auto-ignition would be inhibited by EGR gas. The EGR and MSP had a great impact on both the ignition timing and the peak HRR of methanol auto-ignition. As EGR ratio increased, so did the incomplete combustion loss of methanol. As long as methanol auto-ignition occurred, the increase of MSP would result in a decrease in NOx emissions, but soot emissions were increased instead.