Abstract

The auto-ignition of cylinder oil droplets could trigger pre-ignition, the most threatening abnormal combustion in low-speed two-stroke Otto cycle dual-fuel engines. In this study, both the experiment of cylinder oil stripping and numerical simulation of in-cylinder charge motion were conducted to reveal the motion state of cylinder oil near scavenging ports and its spatial distribution after entering combustion chamber. Further, a measure to suppress the occurrence of abnormal combustion was also investigated by numerical study. Experimental results illuminate that cylinder oil both in the form of film and single droplet would strip under the flow velocity of engine’s scavenging ports, which demonstrated the oil stripping near scavenging ports was one of its pathways into cylinder. Numerical results indicate that at high-load condition, the gathered region of oil-vapor mainly located near cylinder wall. In contrast, at low-load condition, gathered region of oil-vapor distributed near cylinder wall and the dead-region under exhaust valve. Auto-ignition of oil and pre-ignition of natural-gas were more likely to occur in these gathered regions of oil-vapor. Decreasing engine load reduced scavenging pressure, which raised the residual mass of in-cylinder oil. And the reduction of the diameter of stripped cylinder oil droplets promoted oil evaporation, which increased the oil concentration in the gathered region of oil-vapor. These two factors could further intensify the occurrence tendency of auto-ignition of cylinder oil. Moreover, numerical results also show that the combination of proper increase of downward angle of natural-gas injector and the advance of injection timing could significantly improve the mixture homogeneity and decrease the escaping ratio of natural-gas. The rich-fuel region near cylinder wall and under exhaust valve reduced apparently as well, which meant fuel concentration in gathered regions of oil-vapor decreased. The reduction of pre-mixture concentration in gathered region of oil-vapor could decrease the occurrence possibility of auto-ignition of cylinder oil. Thus the occurrence tendency of pre-ignition of natural-gas and subsequent severe abnormal combustion could also be inhibited.

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