Low Speed Pre-Ignition (LSPI) manifests at the juncture of lubricants, fuels and engine hardware. Much of the industry focus has been on lubricants, resulting in LSPI performance requirements in both API SN PlUS and GM dexos® lubricant specifications. Fuels have received less attention but potentially present an even bigger challenge, being less easy to control and more widely varied, especially since fuel properties that have shown to amplify LSPI are not necessarily bounded by current fuel specifications.
Engine design and calibration solutions to the LSPI problem have been the near-exclusive domain of Original Equipment Manufacturers (OEMs) and their suppliers and fall broadly into two categories. Firstly, LSPI mitigation, which includes hardware designs that are robust against damaging knock that often follows LSPI, or reactive calibration adjustments based on historical LSPI events. The second approach is to prevent LSPI by pre-emptive calibration in LSPI-prone regions of the engine map, as well as hardware designs with reduced LSPI tendencies. Related to engine hardware, there is a growing body of evidence about the impact of engine age on LSPI, which has important implications for in-service LSPI performance. There are still many contradictions about engine effects on LSPI, for example, that different engine designs respond differently to changes in boundary conditions such as oil and coolant temperatures.
These observations reveal that a true fundamental understanding of the interaction between all the factors that play a role in LSPI is still beyond our grasp. Recent research at Southwest Research Institute (SwRI) made some headway towards this goal by removing the LSPI experiment from the highly variable conditions found in an engine and replicating engine-like environment in more controlled test surroundings. Combined with powerful analytical tools such as in-situ sampling and optical studies, we are starting to fill in the picture of the interaction between engine, fuels and lubricants.
Christopher Chadwell is an Assistant Director in the Department of Automotive Propulsion Systems at Southwest Research Institute where he has worked for more than 18 years. He has responsibility for Spark Ignition (SI) engine research, transmission testing, drivetrain testing and electrified powertrain development and testing including batteries, battery management systems and electric powertrains. Chris and his team have been working to study the fundamental causes of LSPI and mitigation techniques since 2010. The work started rather modestly with two internally funded projects just trying to replicate the phenomenon, and was followed by a research consortium called the Pre-ignition Prevention Program or P3, that Chris was responsible for starting and managing. The P3 program budget exceeded USD7 million and succeeded in determining the sensitivity of LSPI to various factors including the formulation of fuels and lubricants.