Characteristics of Auto-Ignition for Lubricants and Lubricant/Gasoline Based on an Innovative Single Droplet Test System 2020-01-1428

Due to the advantages of low weight, low emissions and good fuel economy, downsized turbocharged gasoline direct injection (GDI) engines are widely-applied nowadays. However, Low-Speed Pre-Ignition (LSPI) phenomenon observed in these engines restricts their improvement of performance. Some researchers have shown that auto-ignition of lubricant in the combustion chamber has a great effect on the LSPI frequency. To study the auto-ignition characteristics of lubricant, an innovative single droplet auto-ignition test system for lubricant and its mixture is designed and developed, with better accuracy and effectiveness. The experiments are carried out by hanging lubricant droplets on the thermocouple node under active thermo-atmosphere provided by a small “Dibble burner”. The auto-ignition process of lubricant droplets is recorded by a high-speed camera. Influences of different base oil types, viscosities, calcium contents, initial droplet diameters, co-flow speeds, new oil, used oil and blending ratios of lubricant and gasoline on the ignition delay time of droplets are investigated at different droplet temperatures. The background co-flow field temperature varies from 823K to 1323K. Equivalent diameters of droplets, 0.99mm, 1.24mm and 1.63mm, generated by micro-syringes are compared for their characteristics of auto-ignition. The results show that the ignition delay time of all the droplets is significantly shortened with the increase of droplet temperature. When the droplet temperature is lower than 1073K, the ignition delay of droplets from Type III base oil is shorter than that of droplets from Type IV base oil. Besides, lubricant droplets with higher viscosities or larger initial diameters have a longer ignition delay. Furthermore, increasing both the calcium content and co-flow speed can obviously promote the auto-ignition process of droplets. Moreover, there is a critical blending ratio for gasoline to lubricant, whose value is between 20% and 30%. When it is lower than the critical blending ratio, the ignition delay of lubricant droplets increases with the rising of blending ratio. When it is higher than the critical blending ratio, the rule is contrary.