LSPI is a direct result of the industry trend to downsized engines and the use of turbocharging to maintain power levels. LSPI has many names including stochastic pre-ignition, but it is frequently referred to as mega-knock or deto-knock. It occurs at low speeds when the engine is heavily loaded during wide open throttle events. Research has demonstrated that tiny droplets in the combustion chamber comprising a mixture of fuel and oil prematurely ignite prior to the spark plug firing. The result is an abnormal, uncontrolled combustion that hammers the piston with massive spikes in cylinder pressure.
The role of lubricants was not clearly understood during early investigation of the problem. But as LSPI problems became more widespread, efforts to control them quickly advanced beyond the simpler explanation of hot spots or surface ignition points within the cylinder. LSPI was found to incorporate random pre-ignition events most likely initiated by the auto-ignition of minute oil droplets or deposit particles in the mixture. Damage from LSPI was found to be more severe than first recognized and it became a serious challenge to automakers seeking to maximized performance and fuel economy.
It was discovered that high pressure direct injection caued a dilution of of the surface tension and viscosity of the lubricating oil. The resulting droplets accumulated in the crevice volume above the top ring where they were forced out by the mechanical action of the rising piston on the compression stroke. The chemical makeup of these droplets causes them to vaporize, creating an auto-ignition scenario or LSPI. Addressing the chemistry provided a major clue to solving the challenge.
Smaller, boosted gasoline engines operating at low speeds and high torque have offered a great solution to performance and fuel efficiency requirements, but LSPI stands in the way of reliable progress. Downsized and boosted engines spend much more time in a low-speed, high-torque environment that invites LSPI. Investigating the specific chemical causality finally penetrated the fog to pinpoint the need for lubricant-specific modifications that resist LSPI. Industry research indicated that the detergent chemistry in modern motor oils increased the tendency for LSPI. Driven Racing Oil was invited to participate in a research program that looked deeply at the effects of each additive typically used in a motor oil formula. This allowed the research team to determine which additives in motor oil increased LSPI tendency, decreased LSPI tendency or was neutral towards LSPI. The research team was able to determine that both Calcium and Sodium based detergent additives contributed to increased levels of LSPI.