The performance and efficiency of spark ignited gasoline engines is often limited by end-gas knock. In particular, when operating the engine at high loads, combustion phasing is retarded to prevent knock, resulting in a significant reduction of engine efficiency. Since the invention of the spark ignition (SI) engine, much work has been devoted to improve and regulate fuel characteristics, such as octane number, to suppress engine knock. The auto-ignition tendency of the engine lubricant however, as described by cetane number (CN), has received little attention, as it has been assumed that engine lubricant effects on knock are insignificant, primarily due to low levels of average oil consumption. However, with modern SI engines being developed to operate at higher loads and closer to knock limits, the reactivity of engine lubricants can impact the knock behavior.
Unconventional oil formulations and additives have been found to lower the lubricant reactivity while maintaining necessary lubricating properties. Lubricants with low combustion reactivity allowed the engine to be operated with improved combustion phasing (spark timing) or higher geometric compression ratio, both of which enable the engine to operate at higher efficiency. Additionally, higher operating loads at a given combustion phasing (e.g. MBT timing) and compression ratio were feasible using lubricants with low combustion reactivity.
