Engine Oil Additive Impacts on Low Speed Pre-Ignition

Low speed pre-ignition (LSPI) is an undesirable combustion phenomenon that limits the fuel economy, drivability, emissions and durability performance of modern turbocharged engines. Because of the potential to catastrophically damage an engine after only a single pre-ignition event, the ability to reduce LSPI frequency has grown in importance over the last several years. This is evident in the significant increase in industry publications. It became apparent that certain engine oil components impact the frequency of LSPI events when evaluated in engine tests, notably calcium detergent, molybdenum and phosphorus. However, a close examination of the impact of other formulation additives is lacking. A systematic evaluation of the impact of the detergent package, including single-metal and bimetal detergent systems, ashless and ash-containing additives has been undertaken using a GM 2.0L Ecotec engine installed on a conventional engine dynamometer test stand. Consistent with previous reports, the detergent system was found to have the largest impact on LSPI frequency. Furthermore, once a balanced detergent system was identified and its LSPI impact was minimized, the effect of other additives, ash-containing as well as ashless, became apparent. In order to develop a robust additive package that maintains performance in critical bench and engine tests while offering optimal LSPI protection, consideration must be given to the impact of all lubricant additives not simply the detergent type and treat rate.