Experimental investigations of the effects of pilot injection on combustion and gaseous emission characteristics of diesel/methanol dual fuel engine

•Effect of pilot injection on DMDF combustion characteristic and gaseous emissions were analyzed.•Regulated and unregulated gaseous emissions from the engine with DMDF system were examined.•Compared with single injection, pilot injection strategy could reduce most of gaseous emissions.•Increasing pilot quantity could reduce HC, CO and most of unregulated emissions of DMDF engine.•Advancing pilot injection timing caused an increase in some unregulated emissions on M50 mode.

A diesel engine with 6-cylinder, turbocharged intercooling and common rail system was modified to operate on diesel/methanol dual fuel (DMDF). To optimize combustion process at high methanol substitution ratio (MSR) and further decrease exhaust emissions of DMDF engine at low load condition, pilot injection strategy was applied on it and the combustion, regulated and unregulated gaseous emission (included NOx, CO, THC, carbon dioxide (CO2), nitrous oxide (N2O), unburned methanol (CH3OH), formaldehyde (HCHO), formic acid (HCOOH), 1,3-butadiene (1,3-C4H6), benzene (C6H6) and toluene (C7H8)) characteristics were experimentally investigated. Experimental results reveal that the application of pilot injection could improve combustion stability and fuel economy at high MSR, and it can also reduce regulated emissions CO, THC except NOx, and unregulated emissions tested in this study except CO2 on M0 and M10 mode and toluene on M50 mode when compared with single injection cases. Increase pilot injection quantity and advance pilot injection timing both cause an increase in in-cylinder temperature and pressure before main combustion. The variations of pilot injection quantity and timing have less effect on regulated and unregulated gaseous emissions except NOx on M0 and M10 mode than higher MSR mode. With the rise of pilot quantity and the advance of pilot injection timing, HC and CO emissions decrease gradually but NOx emissions ascend; almost all unregulated emissions tested in this study on M30 mode reduce gradually; but when the MSR continues to rise to 50%, increasing emissions in unburned methanol, 1,3-butadiene and benzene are observed.