Numerical And Experimental Investigations On Lubricating Oil Droplet Evaporation And Ignition Under High-temperature And High-pressure Conditions

High-power-density Spark Ignition?SI?engine could reach a relatively high thermal efficiency and a relatively low CO₂ emission,which is equipped with a supercharger and a Gasoline Direct Injection system.However,under the working condition with a low speed and a high load of high-power-density SI engine,super-knock appears.As a destructive abnormal combustion phenomenon,super-knock sets a strong limitation on the power density and the thermal efficiency of SI engine.It is reported by the previous researches that low-speed pre-ignition?LSPI?is a necessary condition for super-knock,and the lubricating oil droplet flying into the cylinder is an essential source for pre-ignition in SI engine.However,the influence of the lubricating oil droplet on the ignition process of the ambient gaseous mixture needs to be investigated further in detail.Focusing on these issues,the evaporation of a single lubricating oil droplet under high-temperature and high-pressure conditions,the ignition of the oil droplet in combustible ambient gaseous mixture,and the explosion of the oil droplet under extreme conditions are discussed in this thesis.Numerical simulation of the evaporation of a single lubricating oil droplet under high-temperature and high-pressure conditions is performed based on a one-dimension transient mathematical model of the evaporation of a multi-component droplet.The results show that during the evaporation process of the lubricating oil droplet,it would first absorb heat from the environment and its temperature would rise to a relatively high level.Then the more volatile component would start gasify firstly at the droplet surface.After that,a balance between the gasification of the more volatile component and the diffusion of it within the oil droplet would be constructed.Finally,the less volatile component would concentrate in the droplet.Under the conditions mentioned in this paper,the increase of environmental pressure will restrain the evaporation of the oil droplet.The increase of environmental temperature will make the evaporation pattern of the oil droplet get closer to onion-skin mode of gasification rather than batch distillation mode.Actually,the evaporation pattern of the oil droplet is always between these two modes.The evaporation processes under different initial droplet radius conditions are similar to each other.Due to the higher attainable maximum of temperature and the longer droplet life time than the iso-octane droplet,the lubricating oil droplet is likely to become a local hot spot in the cylinder and induce pre-ignition in the ambient gaseous mixture.The ignition of a single lubricating oil droplet in combustible ambient gaseous mixture is investigated based on a rapid compression machine?RCM?with visualization devices.The results show that the introduction of the lubricating oil droplet would induce flame propagation firstly from it and decrease the ignition delay of the ambient gaseous mixture.Furthermore,within the negative temperature coefficient?NTC?range the lubricating oil droplet would also induce strong pressure oscillation.The ignition of the lubricating oil droplet would be earlier and the induction of pre-ignition in the ambient gaseous mixture by it would be more obvious when the initial droplet volume is smaller.In the NTC range,the increase of environmental temperature would promote the pre-ignition induction in the ambient gaseous mixture by the lubricating oil droplet.However,it works inversely in the high-temperature region out of NTC.The different pressure-dependence mechanisms of pre-ignition induction by the oil droplet under different temperature conditions could be attributed to the trade-off effect of environmental pressure on droplet evaporation and chemical reaction in the gas phase.Combining the RCM with photomicrography method,the explosion of lubricating oil droplet after homogeneous charge compression ignition of the ambient gaseous mixture is investigated under the conditions when the initial droplet volume is relatively big and the ignition delay of the gas phase is relatively long.The results show that the explosion of oil droplet consists of many sub procedures such as breaking-up,evaporation,pyrolysis and the production of soot.The pressure wave during combustion is the main factor accounting for the explosion of oil droplet.Under these conditions,the introduction of oil droplet makes little influence on the ignition delay of the ambient gaseous mixture.Based on the above results of study,a map for the interaction between the lubricating oil droplet and the ambient gaseous mixture has been designed and made.The results show that the interaction pattern depends on the competition between two time scales,which are the physical delay of droplet evaporation and the chemical delay of gas phase reaction.