Kassai, M., Torii, K., Shiraishi, T., Noda, T. et al., "Research on the Effect of Lubricant Oil and Fuel Properties on LSPI Occurrence in Boosted S. I. Engines," SAE Technical Paper 2016-01-2292, 2016, doi:10.4271/2016-01-2292.
The effects of lubricant oil and fuel properties on low speed pre-ignition (LSPI) occurrence in boosted S.I. engines were experimentally evaluated with multi-cylinder engine and de-correlated oil and fuel matrices. Further, the auto-ignitability of fuel spray droplets and evaporated homogeneous fuel/oil mixtures were evaluated in a combustion bomb and pressure differential scanning calorimetry (PDSC) tests to analyze the fundamental ignition process.The work investigated the effect of engine conditions, fuel volatility and various lubricant additives on LSPI occurrence. The results support the validity of aspects of the LSPI mechanism hypothesis based on the phenomenon of droplets of lubricant oil/fuel mixture (caused by adhesion of fuel spray on the liner wall) flying into the chamber and autoigniting before spark ignition.Combustion bomb experiments confirmed that lubricant oil sprays have higher auto-ignitability than gasoline fuel components, and no particular effects of lubricant additives on ignitability were observed. However, under the conditions of the PDSC test, it was shown that the oxidative stability of fuel/oil mixtures is lower as the fuel/oil ratio is increased, and higher levels of calcium lubricant additives can bestow enhanced oxidation stability on some mixtures. These results indicate that calcium may promote autoignition during combustion under preheated and premixed mixture conditions, even though it plays an active role in preventing liquid phase oxidation.From this analysis, it is hypothesized that in real engines, certain lubricant additives, initially preheated by the autoignition of oil derived droplets, may give a greater propensity for subsequent flame propagation and abnormal combustion phenomena.