Low Speed Pre-Ignition (LSPI) Durability – A Study of LSPI in Fresh and Aged Engine Oils 2018-01-0934

Downsized gasoline engines, coupled with gasoline direct injection (GDI) and turbocharging, have provided an effective means to meet both emissions standards and customers’ drivability expectations. As a result, these engines have become more and more common in the passenger vehicle marketplace over the past 10 years. To maximize fuel economy, these engines are commonly calibrated to operate at low speeds and high engine loads – well into the traditional ‘knock-limited’ region. Advanced engine controls and GDI have effectively suppressed knock and allowed the engines to operate in this high efficiency region more often than was historically possible. Unfortunately, many of these downsized, boosted engines have experienced a different type of uncontrolled combustion. This combustion occurs when the engine is operating under high load and low speed conditions and has been named Low Speed Pre-Ignition (LSPI).

LSPI has shown to be very damaging to engine hardware. Although LSPI is a random occurrence, engine oils, specifically engine oil additives, have been shown to directly impact LSPI propensity. This is now well-known within the industry and several engine tests have been developed and adopted to measure the ability of an oil to prevent LSPI propensity of an engine oil. Two examples are the proposed GF-6 LSPI test and General Motor’s Stochastic Pre-Ignition (SPI) test. Although these tests effectively quantify the LSPI propensity of a fresh oil, they do not quantify the LSPI propensity of an aged oil. This is an important distinction as many of the chemical additives in engine oils change and degrade over time. Additionally, oil contaminants such as wear metals may also contribute to changes in LSPI propensity.

In this paper, the concept of LSPI durability is introduced. LSPI durability is defined as the ability of an engine oil to minimize or eliminate LSPI, not only while the oil is fresh, but also over a relevant oil drain interval (ODI). To quantify LSPI durability, an engine test methodology that evaluates LSPI on fresh oil, then ages the oil using a relevant duty cycle, then finally re-evaluates the aged oil for LSPI propensity was developed. Using this three-part testing protocol, four different oil formulations were evaluated. Results show that some formulating strategies work well for LSPI prevention only with fresh oil, while one prevents LSPI when fresh and aged.