Turbocharged, direct-injected (DI) gasoline engines are rapidly growing in use because their design provides improved fuel efficiency when compared with other gasoline engine designs. In particular, there can be significant efficiency benefits achieved when a smaller, turbocharged DI engine is used to replace a larger, conventional engine of the same power. However, the higher power density of these engines makes them prone to a phenomenon called low speed pre-ignition (LSPI). LSPI is an abnormal combustion event in which the fuel-air mixture ignites before intended, causing excessive pressures inside the engine's cylinders. In mild cases, this can cause engine noise, but when severe enough, LSPI can cause engine damage. There are several factors that contribute to LSPI, of which lubricating oil has been observed to be one. For information about the latest product specifications, which are designed to mitigate LSPI, visit ILSAC GF-6.

the emergence of the LSPI phenomenon

Demand for improved fuel economy and reduced emissions has driven passenger car manufacturers to design downsized, turbocharged engines. While these engines provide higher power density and improved efficiency, they are also prone to LSPI. Original equipment manufacturers (OEMs) can adjust the operating conditions in the engines they design to address this problem, but this can reduce the engine's efficiency, which these engines were intended to improve. In fact, LSPI tends to become more prominent in the operating regime that is most beneficial to attaining improved fuel economy.

Many OEMs in the United States, Europe and Japan – working in conjunction with certain additive companies and oil marketers – have conducted extensive research on the LSPI phenomena. From this work, there is a growing body of knowledge that is demonstrating how hardware design, fuel composition and lubricant compositions can impact LSPI. Unlike conventional knock, an LSPI event cannot be predicted and corrected by adjusting spark timing. As such, mitigating pre-ignition involves altering the design of the engine or lubricant to help avoid this emerging issue.

LSPI test being conducted on a turbocharged gasoline direct injection (GDI) engine at an independent laboratory. Turbocharged, direct-injected gasoline engines are rapidly growing in use because of their design to provide improved fuel efficiency when compared with other gasoline engine designs.

causes of LSPI

Low speed pre-ignition (LSPI) is a premature combustion event, occurring prior to spark ignition in turbocharged, downsized gasoline vehicles. As the name implies, it occurs when engines operate at low speeds and high loads. It can result in extremely high cylinder-pressures and can lead to heavy knock. At a minimum, LSPI can generate an audible knocking noise noticeable to the driver. Repeated exposure to these conditions can cause engine hardware failure, including broken spark plugs and cracked pistons. A characteristic of LSPI is that one event often leads to subsequent events. The events will frequently occur in an alternating pattern between pre-ignition and regular combustion.

LSPI chart: pressure and time

Peak pressure values for a span of combustion cycles. LSPI occurrences are observed by the large spikes in peak pressure.

Several theories exist to explain the mechanism of LSPI. One theory is centered on an oil droplet entering the combustion chamber from a crevice between the piston and cylinder wall. In this mechanism, the oil droplet mixes with fuel and auto-ignites. A second theory focuses on deposits as the ignition source for LSPI. Evidence has been shown for both mechanisms, and they are not necessarily mutually exclusive.

Chevron Oronite is conducting extensive research in this area to help advance the understanding of LSPI, particularly on the impact of lubricant additives. From these studies, solutions are beginning to emerge to help mitigate the effects of LSPI.

Damaged piston from severe LSPI events.

the role of the lubricant in LSPI

Many factors have been demonstrated to impact low speed pre-ignition (LSPI), including: engine designs, fuel composition and lubricant composition. On the lubricant side, the most noticeable impact has been from the detergent chemistry. Oils with higher concentrations of calcium, which is found in many detergent systems, have been shown to increase the frequency of LSPI. The exact chemistry of the detergent is less important to LSPI than the calcium content. Conversely, magnesium-based detergents do not seem to promote LSPI. Although reducing calcium may seem like a solution to control LSPI, there may be other performance tradeoffs to consider. In addition, there are other additives that can also help reduce LSPI events. This provides an opportunity to formulate for robust LSPI performance, while maintaining the level of detergency needed to help keep engines clean and neutralize acids generated during combustion.

Aside from the detergent system, there are many other additive and lubricant compositions that can influence LSPI. Molybdenum compounds, for example, not only provide frictional benefits, but also have been shown to decrease LSPI when used at high levels. Base oils also affect LSPI events. Both the quality of the base stock (i.e., Group II vs. Group III) and the viscosity can have secondary effects on LSPI. The effect on LSPI from these other lubricant aspects are not as significant as the detergent system, but can shift the LSPI frequency in oils that are more prone to LSPI.

LSPI chart: high or low calcium

Additive effects on the frequency of LSPI in the Ford test. The "High Ca" oil has a calcium concentration typical of those in the market today. The "Low Ca" has been reformulated to resist LSPI, and is representative of how many next generation oils will behave.

providing additive solutions to help mitigate LSPI

New and upcoming engine oil specifications include low speed pre-ignition (LSPI) prevention. ILSAC GF-6 includes a Ford engine test to discriminate oils based on LSPI event prevention (reduction). Thus, all oils that make ILSAC GF-6 claims are formulated to address LSPI. Additionally, many OEMs are developing in-house LSPI tests for their own engine designs. For example, GM's dexos1® specification now includes a GM stochastic pre-ignition test. This test is similar to the Ford test used in GF-6, albeit at different operating conditions.

test tomparison table

When comparing the many and varied tests for LSPI impact, it is important to understand the characterization and quantification of an LSPI event can have a bigger impact than different OEM hardware. It is known that LSPI can lead to high pressures. Thus, one obvious way to quantify LSPI is to monitor the in-cylinder pressure for abnormal spikes. Another approach is to simply monitor for any cycle where combustion starts before the spark, as that is undoubtedly pre-ignition. While these differences in details may seem trivial, they can significantly impact the interpretation of the test results and, in turn, formulations.

Instrumented gasoline direct-injection (GDI) engine on LSPI stand.

It is important to ensure that any new engine oil specifications be based on performance (such as in the newly available engine tests) rather than on chemical limits. Although lowering calcium was one of the initial levers identified for reducing LSPI, it is not the only lever, and calcium detergents have benefits in other performance areas. Oronite has established a firm understanding of formulating for future specifications with LSPI requirements, while still being committed to delivering performance in piston deposit control and neutralizing acids from combustion processes.

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