Numerical investigation and experimental validation on the leakage of methanol and formaldehyde in diesel methanol dual fuel engine with different valve overlap

The diesel methanol dual fuel (DMDF) engine is confronting with the problem of high unregulated emissions of methanol and formaldehyde. The previous studies on unregulated emissions of DMDF engine 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 computation fluid dynamics (CFD) model coupled with a detailed chemical kinetic mechanism was developed to investigate the methanol and formaldehyde emissions of DMDF engine. The results showed that the unburned methanol and formaldehyde in the cylinder were the main part of the total methanol and formaldehyde emissions based on the 41 °CA valve overlap of simulation model. Then the effects of different intake valve opening (IVO) and exhaust valve closing (EVC) on methanol emissions were investigated. The results showed that only increasing IVO or EVC had very little effect on the leakage of methanol during scavenging. However, when the IVO and EVC changed simultaneously, the valve lift would significantly increase during the valve overlap of scavenging, which had a great impact on methanol leakage. The simulation results were validated by experiments in a 6170 marine engine. The experimental results were consistent with the simulation results and the increase of effective flow area caused by valve lift during large valve overlap was the most important reason for the increase of methanol emission. Therefore, all engines have almost no leakage in small valve overlap. However, with the increase of valve overlap, the actual results were different due to the different valve lift and mixture concentration of different engines.