Low-speed pre-ignition (LSPI), also known as stochastic pre-ignition (SPI), is a pre-ignition event that occurs in gasoline vehicle engines when there is a premature ignition of the main fuel charge. LSPI is most common in certain turbocharged direct-injection vehicles operating in low-speed and high-load driving conditions.
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LSPI events are random and infrequent, and their effects on impacted vehicles can include very high-pressure spikes, loud knocking noises and sometimes catastrophic engine damage.
Impact on engine design
Automakers use engine downsizing to help improve vehicles’ fuel efficiency and reduce emissions, and use turbocharger technology to recover power lost in the downsizing process. The presence of LSPI limits automakers’ ability to capture the full potential of turbocharged engines to meet increasing fuel-efficiency requirements and to further reduce carbon-dioxide emissions.
Updated oil-performance standards are taking shape to address LSPI. General Motors’ next-generation dexos1 specification, scheduled for release in 2015, will include an engine test based on a GM 2.0-liter four-cylinder Ecotec that will test for LSPI. The proposed ILSAC GF-6 standard will also include a test for oil-related LSPI events in gasoline direct-injection engines based on a Ford 2.0 L four-cylinder Ecoboost engine.
Potential solutions
Researchers have been unable to pinpoint a single root cause for all LSPI instances. However, tests involving the use of engine oils have shown engine oils can be formulated to prevent LSPI while maintaining the oil’s basic performances.
The Southwest Research Institute (SwRI) launched the two-year Preignition Prevention Program (P3) consortium in 2011 aimed at understanding the source of LSPI and working towards developing a standardized test for lubes / fuels The consortium's objectives included examining the interaction between fuel and oil in LSPI events, understanding how hardware design could be used to help mitigate LSPI, and identifying fluids that can help reduce LSPI occurrences.
The oil additive industry is also looking at using engine oil additives to suppress LSPI while retaining the fuel-saving benefits of existing engine technologies.