Many modern engine platforms use turbochargers to meet higher fuel economy performance, which is often combined with downsizing the engine displacement. Operating downsized, turbocharged, direct injection engines at low speeds and high loads has led to an abnormal combustion phenomenon known as Low Speed Pre-Ignition (LSPI), wherein the fuel-air mixture ignites before the spark occurs. LSPI can lead to extremely high pressures in the combustion chamber, which can damage hardware such as pistons, piston rings, and spark plugs.
Lubricants, fuels, and engine operating conditions have been shown to impact LSPI. Any of these can be modified to improve LSPI performance. One solution which has been used widely in the industry is reformulating the lubricant additive package. In particular, calcium-based detergents have been shown to promote LSPI, while magnesium detergents appear neutral to LSPI. Reducing the usage of calcium detergents can impact other performance areas such as deposit control and fuel economy, and limits formulating flexibility. Finding an additive solution to LSPI which does not require a reduction in calcium allows for more component options in formulating for the numerous performance areas required of modern lubricants.
This paper will discuss formulating solutions for LSPI that do not require using magnesium detergents in place of calcium. In some cases, this can involve using alternative detergent chemistry, such as potassium and lithium based detergents. Additionally, several options have been found which allow formulating an oil with full calcium while suppressing LSPI by using zirconium, zinc (other than ZDDP), or cobalt based boosters.