The diesel methanol dual fuel (DMDF) engine is confronting with the problem of high unregulated emissions of methanol and formaldehyde, although it has better fuel consumption and lower regulated emissions. The previous studies on unregulated emissions of DMDF engine have focused on the measuring methanol and formaldehyde concentrations in the exhaust without distinguishing whether emissions come from leakage during valve overlap or incomplete combustion in the cylinder. In this study, a multi-dimensional computation fluid dynamics (CFD) model coupled with detailed chemical kinetic mechanism was developed to investigate the emissions of methanol and formaldehyde in DMDF engine by using CONVERGE. The model was validated against at 50% load of 1340r/min in a six-cylinder DMDF engine. The results show that the unburned methanol and formaldehyde in the cylinder are the main component of the total methanol and formaldehyde emissions, while the leakage of methanol can be negligible. This is due to the high air fuel ratio in the intake manifold and the low air inflow during the valve overlap period. Then the effects of different intake valve opening (IVO) and exhaust valve closing (EVC) on the unregulated emissions were investigated. It was found that increased valve overlap period can affected the leakage of methanol significantly. However, the increased leaking methanol still was negligible due to the very small mass ratio. Based on the above results, the total methanol and formaldehyde emissions of DMDF engine are hardly affected by the leakage of methanol, but are almost entirely derived from the unburned methanol and formaldehyde in the cylinder.
Keywords diesel methanol dual fuel engine, methanol emission, valve overlaps, numerical study, formaldehyde emission