Several theories exist to explain the mechanism of LSPI. One is centered on an oil droplet entering the combustion chamber from a crevice between the piston and cylinder wall, where it mixes with the fuel and 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.

Many OEMs in the United States, Europe and Japan-working alongside additive companies and oil marketers-have conducted extensive research on LSPI. This work has generated a growing body of knowledge that demonstrates how hardware design, fuel and lubricant compositions can impact LSPI.

Playing Catch-up

LSPI is being addressed in the upcoming ILSAC GF-6 passenger car engine oil specification, but the introduction of GF-6 has been severely delayed due to the need to develop several new engine tests. According to Martin Birze, Lubrizols regional business manager for passenger car motor oils, the slow pace of development for ILSAC GF-6 could have a detrimental effect on lubricant quality.

LSPI is the number one problem in gasoline direct injected and turbocharged engines, he noted at a meeting of the Petroleum Quality Institute of America last fall, urging that action be taken quickly to address the issue.

LSPI has become so severe that automakers requested a supplemental category that meets current API SN requirements and includes a test to determine the LSPI protection potential of engine oils. The interim standard, API SN Plus, adopted the 2012 Ford 2-liter, four-cylinder EcoBoost engine test to determine oil performance against LSPI. This test will also be part of the ILSAC GF-6 specification as the ASTM Sequence IX test.

After some back and forth, SN Plus licensing began this month, but GF-6 is not expected until the first half of 2020.

Even with this action, Birze cautioned, While SN Plus will help initially, much work still needs to be done to determine how the potential for LSPI changes over the drain interval and over the lifetime of the vehicle.

Detergent Effect

One characteristic of LSPI is that one pre-ignition event often leads to subsequent events. The events frequently occur in an alternating pattern between pre-ignition and regular combustion. Unlike conventional knock, LSPI cannot be predicted and corrected by adjusting spark timing. Mitigating pre-ignition requires altering the design of the engine or the formulation of the lubricant to help avoid the problem.

While OEMs can adjust engine operating conditions to address the issue, this approach can adversely affect efficiency, precisely the factor these engines were intended to improve. LSPI tends to become more prominent in the operating regime that can benefit most from improved fuel economy, a number of companies have found.

Detergents are the primary additives impacting LSPI, explained Steve Haffner, president of SGH Consulting, at the ICIS Pan-American Base Oils & Lubricants Conference in Jersey City, New Jersey, in November. Calcium-based detergents, which are widely used in the automotive market, have been found to promote pre-ignition, while magnesium chemistries do not appear to cause LSPI. As a result, he said, the choice of detergent likely overwhelms any response to other factors.

Oils with higher concentrations of calcium, which is found in many detergent systems, have been shown to increase the frequency of LSPI. In fact, research from additive companies Infineum, Chevron Oronite and Lubrizol indicates that the exact chemistry of the detergent is less important to LSPI than the calcium content. On the other hand, magnesium-based detergents do not seem to promote LSPI.

Although reducing calcium may seem like a solution, there may be other performance trade-offs to consider. In addition, other additives can help reduce LSPI events. This provides an opportunity to formulate for robust LSPI performance, while maintaining the level of detergency needed to keep engines clean and neutralize acids generated during combustion.

The effect on LSPI from other lubricant components is not as significant as those from the detergent system, but can shift the LSPI frequency in oils that are more prone to the problem. For example, Oronite noted that molybdenum compounds not only provide frictional benefits, but have been shown to decrease LSPI when used at high levels.

Additive Solutions

In 2016, Infineum ran a research and development program to analyze the LSPI phenomenon. Initial assessments focused on developing a better statistical approach to measure and quantify LSPI activity. The company applied data analysis techniques to understand the effects of calcium and magnesium detergents on LSPI, as well as the responses of other key components and parameters in modern lubricants.

The study used the General Motors 2-liter Ecotec LHU engine, with all LSPI or knock suppression strategies disabled to prevent them from impacting the data. All studies used industry standard fuel, enabling researchers to focus on measuring lubricant-related effects without introducing fuel-related variables.

The first study examined the impact of calcium level, a known LSPI promoter. The average number of LSPI events was recorded for different calcium concentrations, corresponding to the range of calcium sulfonate in current commercial lubricant formulations. The study showed a direct relationship between calcium concentration and the average number of LSPI events, confirming calcium as an LSPI promoter.

The additive makers second study focused on the impact of magnesium-based detergents. In contrast to calcium, varying the concentration of magnesium had little effect on the occurrence of LSPI, which means magnesium could be either a neutral additive or a quencher. Subsequent tests investigated the influence of mixtures of calcium and magnesium on LSPI and found that magnesium did not quench calcium-induced LSPI, confirming it as LSPI neutral.

The next test assessed a formulation containing a mixture of 0.159 percent calcium and 0.042 percent sodium. Here, the mixture almost doubled the LSPI activity when compared to the effect of calcium alone. This implies that sodium is a more aggressive LSPI promoter in the presence of calcium.

The most widely reported LSPI quencher is zinc dialkyldithiophosphate. However, the phosphorus constraints in lubricant specifications to protect three-way catalysts make it unrealistic to use high levels of ZDDP to reduce LSPI.

These observations open up the debate for increasing the maximum phosphorus levels in the next generation of industry and OEM lubricant specifications. However, this would rely on the lubricants ability to maintain phosphorus retention characteristics to ensure continued catalyst protection or on OEMs being open to increasing the phosphorus content in future lubricants to enable better LSPI control.

What the Future Holds

New and developing engine oil specifications include LSPI prevention. ILSAC GF-6 is expected to include the Ford-sponsored Sequence IX engine test to measure oils LSPI prevention, meaning that all oils that claim to meet the GF-6 standard will need to be formulated to address LSPI.

In addition, many OEMs are developing in-house LSPI tests for their own engine designs. For example, GMs Dexos1 specification now includes a GM stochastic pre-ignition test. This test is similar to the Sequence IX in GF-6, albeit at different operating conditions.

In comparing the many and varied tests for LSPI impact, it is important to understand that the characterization and quantification of an LSPI event can have a bigger impact than OEM hardware. LSPI can lead to high pressures, so one way to quantify LSPI is to monitor in-cylinder pressure for abnormal spikes.

Another approach is to simply monitor for any cycle where combustion starts before the spark, because that is undoubtedly pre-ignition. While these differences in detail may seem trivial, they can significantly impact the interpretation of the test results and, in turn, formulations.

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.

The concern about the negative effect of calcium on LSPI could result in new chemical limits being added to future OEM and industry specifications. This might even include a limit on total sulfated ash or individual sodium, calcium and magnesium levels. However, applying an upper limit on sulfated ash could restrict the level of beneficial chemistries, such as magnesium detergent, with no clear technical benefit.

Richard Beercheck was managing editor of LubesnGreases-Europe-Middle East-Africa for five years. Prior to that, he worked as communications manager at Lubrizol for 15 years. Contact him at dbeercheck@gmail.com.