The problem will be addressed in the ILSAC GF-6 specification. Unfortunately, the introduction of GF-6 has been severely delayed due to the need to develop several new engine tests. However, the problem has become so severe that automakers have requested a supplemental category that meets current API SN requirements, and that includes a new LSPI test. Industry has requested a first use date of January 1, 2018, for this category.

Although API has not agreed to the January 1 first use date, lubricant suppliers were advised to be prepared for an introduction of the supplemental category in the first quarter of next year. This view was supported by Martin Birze, Lubrizols regional business manager for PCMO, at a meeting of the Petroleum Quality Institute of America last month. There is no reason why industry cant move quickly on GF-5+, he told attendees.

What Is LSPI?

According to Cecile Pera, senior engineer at Infineum International Ltd., LSPI is an abnormal engine cycle observed at high loads and low engine speeds in turbocharged spark-ignited engines, most of the time with direct injection. Speaking at the UNITI Mineral Oil Technology Congress in Stuttgart in April, Pera explained that LSPI can lead to destructive super-knock in downsized engines and is a significant concern for several OEMs. Unfortunately, the cause of LSPI is still unknown, she said, and although OEMs can adjust engine operating conditions to address it, they cannot do so without compromising engine efficiency.

Despite the fact that several complex interactions have been identified, including engine technologies, operating conditions and fuel and lubricant properties, Pera noted that there is still no consensus on the mechanisms of LSPI. Paradoxically, while LSPI cannot be eliminated in an engine, its unpredictable nature makes it extremely difficult to reproduce in a controlled test engine.

To understand the role of the lubricant in LSPI – and the potential link between it and combustion – Infineum employed a dedicated ignition quality tester to measure the auto-ignition propensity of lubricants.

Pera explained that LSPI involves three steps, all of which are based on a specific combustion process.

Step 1: Preignition is caused by hot spots in the combustion chamber, but the source of preignition is not totally known. Some research groups implicated liquid fuel/lubricant droplets while others highlighted solid flaking deposits. This preignition happens before the spark timing but, on its own, does not damage the engine.

Step 2: Subsequent flame propagation. This stage is similar to the flame propagation following a normal spark event in normal combustion and does not damage the engine. The issue here is that its timing is uncontrolled, which leads to extreme in-cylinder pressure and high temperature.

Step 3: Induced super-knock in the unburned fuel/air mixture. Auto-ignition is very similar to well-known knock, but it is much more severe because of the earlier, uncontrolled combustion timing and the fact that it involves a greater mass of unburned mixture.

Pera said that the transition between safe flame propagation and destructive detonation in an engine can be characterized by following combustion theory. She showed a diagram of the process that mixes hot spot characteristics (depending on fuel/lubricant auto-ignition propensity), fuel combustion properties and engine response through acoustic wave coupling.

Ignition Quality Tester

Infineums research emphasized the need to characterize the auto-ignition propensity of fuel/oil mixtures. To this end, the company adapted an ignition quality tester (IQT), a device that measures diesel and aviation fuel auto-ignition, to undertake this work.

Pera explained that the IQT was used to measure absolute ignition delay time under LSPI-engine-relevant conditions, and to capture part of the physical-chemical coupling hypotheses responsible for LSPI. Step one targeted hot-spots created by the auto-ignition of lubricant/fuel mixtures. The IQT-measurement, which is based on ASTM method D6890, required several repeat tests.

To obtain relevant results independent of fuel variability, Infineum defined a gasoline surrogate based on the emulation of auto-ignition and several key fuel properties. Pera related that the IQT showed that adding lubricant to the fuel promotes auto-ignition significantly.

Detergent Effects

The effect of calcium- and magnesium-based detergents on LSPI has been the subject of past studies at Infineum. These tests showed that calcium-based detergents strongly promote LSPI while magnesium-based detergents appeared to have nearly no effect on LSPI. However, Pera noted, various calcium and magnesium concentrations have been observed to have no statistically measurable difference in the IQT for gaseous fuel/oil/air auto-ignition.

Researchers mixed peroxide with oil to focus the IQT auto-ignition study on the branching and propagation chain of the combustion process. However, while peroxide increased reactivity, the process again showed no sensitivity to calcium or magnesium content, Pera said. These results seem to indicate that calcium does not play a role either in the initiation or the branching chemistry reactions of oil auto-ignition in the gaseous phase.

Base Stock Selection

Pera presented data showing that the largest impact on auto-ignition measured with the IQT was related to the chemical composition of the base stock. In the test, different quality base stocks were added to the fuel samples, and results of the IQT tests highlighted that the propensity for auto-ignition increased from API Group I to Group IV. We also noted that some Group V formulations are very resistant to auto-ignition (no auto-ignition promotion effect compared to pure fuel), which may be interesting for LSPI-resistant formulations, she said.

Conclusions

Pera concluded by saying LSPI is an extremely complex phenomenon that involves multiple intricate couplings, and combustion is only one of the elements. Many questions remain unanswered, in particular why calcium has an LSPI-promoting effect while magnesium does not. Perhaps further modification of the IQT to allow solid-phase auto-ignition (auto-ignition catalyzed by a solid deposit) may help to answer this question.

Meanwhile, this experiment has enabled Infineum to better understand the role of the base stock in LSPI mechanisms. Depending on the specific engine operating conditions and the type of LSPI, base stock selection could be a significant factor in the formulation of LSPI-resistant lubricants, she said.