Hu, T., Teng, H., Luo, X., Lu, C. et al., "Influence of Fuel Dilution of Crankcase Oil on Ignitability of Oil Particles in a Highly Boosted Gasoline Direct Injection Engine," SAE Technical Paper 2015-01-2811, 2015, doi:10.4271/2015-01-2811.
The relationship between fuel dilution of the crankcase oil and low-speed pre-ignition (LSPI) was studied experimentally with a highly-boosted 1.8L turbocharged gasoline direct injection (TGDI) engine fueled with RON93 gasoline. It was found that properties of oil particles entered the engine cylinder were affected significantly by fuel dilution. The gasoline content in the oil represents those with long carbon chain or heavy species in gasoline, with much lower boiling points and auto ignition temperatures than those for the undiluted engine oil. Thus, dilution of the engine oil by these gasoline species lowers the volatility and the minimum auto ignition temperature of the engine oil. With 15% fuel content in the oil, the flash point and the fire point of the SAE 5W30 oil dropped from 245 °C to 90 °C and from 265 °C to 150 °C, respectively. The initial boiling point for the diluted oil could be lower than the wall temperatures for some locations on the combustion chamber roof or on the cylinder wall above the top piston ring reverse location. Once attached onto these hot areas, the oil particles entering the engine cylinder could form self-ignitable gaseous mixture easily, and become self ignited in late of the compression stroke under high loads, triggering pre-ignition. It was demonstrated that frequency of LSPI is linked strongly to the minimum auto ignition temperature of the oil particles.