Investigation on Ignition of a Single Lubricating Oil Droplet in Premixed Combustible Mixture at Engine-Relevant Conditions

The ignition of lubricating oil droplet has been proved to be the main factor for pre-ignition and the following super-knock in turbocharged gasoline direct injection engine. In this paper, the ignition process of lubricating oil droplet in combustible ambient gaseous mixture was investigated in a rapid compression machine (RCM). The pre-ignition induction by oil droplet of the ambient gaseous mixture was analyzed under different initial droplet volume and effective temperature conditions. The oil droplet was suspended on a tungsten fiber in the combustion chamber and the ignition process was recorded by a high-speed camera through the quartz window mounted at the end of the combustion chamber. The pressure traces were also obtained by a sensor in order to get the ignition delay and analyze the combustion process in detail.

The results show that with the decrease of initial droplet volume, the pre-ignition induction by oil droplet in the ambient gaseous mixture would be more remarkable. The effect of effective temperature on droplet/gas ignition is quite complex. The negative temperature coefficient (NTC) behavior makes the pre-ignition induction and pressure oscillation triggering by the oil droplet more remarkable within the moderate temperature range. The droplet would evaporate faster under higher temperature conditions, which is also benefit for pre-ignition induction by oil droplet. However, when the effective temperature is very high (at 910 K), the oil droplet would not make influence on ignition delay because the ambient gaseous mixture ignites very fast. Essentially, the pre-ignition induction by lubricating oil droplet on the ambient gaseous mixture depends on the time scale competition between physical delay of droplet evaporation and chemical delay of the ambient gaseous mixture.