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The new passenger car engine oil specification called ILSAC GF-6 (and maybe API SN Resource Conserving) is scheduled to go into effect in 2016, with many new wrinkles we havent seen before. In addition to tougher performance targets for some tests and major modifications for others, GF-6 oils will feature a subcategory for very low viscosity oils that boost fuel economy; a new timing-chain wear test; and possibly an engine test for low speed pre-ignition, or LSPI.

Ive covered the low-vis, fuel-saving subcategory in earlier columns, and timing chain wear seems a relatively straightforward addition. But LSPI is an entirely new tack for the auto industry. So before reviewing how the new test is coming along, Id like to get into what LSPI is and why it matters.

First, some definitions: Pre-ignition is the ignition of the fuel/air mixture prior to the spark plug firing. Anytime something causes the mixture in the combustion chamber to ignite prior to the spark plug event, it is classified as pre-ignition. Contrast that with detonation which is defined as unburned end gas which, under increasing pressure and heat (from the normal progressive burning process and hot combustion chamber metals), spontaneously combusts – ignited solely by the intense heat and pressure present in the engine.

While it sounds like both phenomena are the same, there are differences. The result is the same however, and includes loss of power, worsened fuel economy, increased emissions and engine damage.

If you go back into the history of the automobile, youll find that early engines had such low compression ratios (Fords original Model T engines was about 4-to-1) that relatively poor quality gasoline was OK, since combustion was managed nicely by engine timing. It didnt take long however for the automobile manufacturers to realize that higher compression ratios offered performance advantages in terms of horsepower and torque. The race was on!

By the early 1920s, General Motors Research had discovered that tetraethyl lead at very low treat rates in gasoline raised the octane number of fuels, which controlled engine ping or pre-ignition and allowed the development of higher compression ratio engines. Engines became larger and had higher compression ratios, up to 9.5 to 1. And with that, the power available to move ever larger vehicles was assured. (At this moment the Beach Boys lyric Shes real fine, my 409 is running through my head.)

That horsepower/cubic inch expansion continued through the middle of the last century up until the early 1970s, when it became clear that lead emissions were hazardous to health and to catalytic converters. Legislation began at that point to phase out lead and with that, compression ratios fell back a bit. Engine design was changing rapidly as well; displacements were dropping, and ignition and fuel delivery systems were being modified to improve fuel economy and emissions performance. As Ive noted before, the advent of the on-board computer made these massive changes in engine design possible.

Were at a point now where compression ratios are again climbing, and engine size – but not power output – is dropping. Its amazing to me that in the space of about 40 years, engines have gone from the behemoth 400+ cubic inch (6.5-liter) V-8 producing about 0.7 hp/cu.in., to todays 122 cubic inch, four-cylinder engines that spit out about 1.64 hp/cu.in. Double the horsepower in less than half the displacement! A tipping point for sure.

In fact, GF-6 is the tipping point for a fresh evaluation, if not control, of pre-ignition. And this time, surprisingly, the focus is not on fuel composition but rather on the properties of the engine oil.

Youre probably saying to yourself, Huh, what does engine oil composition have to do with pre-ignition? According to an SAE paper presented by Toyota, one strategy to improve engine fuel economy while maintaining drivability is to combine turbocharging and direct fuel injection with engine downsizing (a trend which is gaining favor in the automobile market).

The fuel economy benefit, Toyota indicated, results from reductions in mechanical friction that are due to the smaller engine displacement and down-speeding by higher transmission gear ratios. The result can be high engine torque at low engine speed. However, Toyota has seen an abnormal combustion phenomenon – low-speed pre-ignition – to which this configuration seems particularly prone.

In another SAE paper from Toyota, researchers reported the auto-ignition of an engine oil droplet from the piston crevice in the combustion chamber – a case of unexpected and random LSPI. This study, the authors went on to say, shows that engine oil formulations have significant effects on LSPI.

Southwest Research Institute, the automotive testing laboratory based in San Antonio, Texas, weighed in on this topic in its own SAE paper. It noted that since the invention of the spark-ignited engine, a lot of work has been devoted to improving and regulating fuel characteristics such as octane number, to suppress engine knock. However, the role of the engine lubricant received little attention. Thats because the auto industry had always assumed that engine lubricant effects on knock are insignificant, and mostly attributable to low levels of oil consumption.

However, with modern spark-ignited engines being developed to operate at higher loads and closer to knock limits, the reactivity of engine lubricants can impact the knock behavior, Southwest Research suggested. This is where the combination of small turbocharged engines with higher transmission gear ratios rears its head.

Many automotive OEMs now believe that LSPI is related to both fuel and lubricant properties. The number of direct-injection turbocharged engines will grow in the next few years, so many in the automotive industry believe that the GF-6 specification has to include a meaningful test to screen for lubricant-and fuel-related LSPI events.

This raises the question of what is it about oil composition thats important for LSPI, and what should we do about it. There are several properties which might impact the LSPI performance of an engine lubricant; one is the oils oxidation stability.

Using a prototype turbocharged direct-injection, spark-ignited engine, Toyota has found that an engine oils spontaneous ignition temperature (determined by high-pressure differential scanning calorimetry) correlates with LSPI frequency. It believes this could be a very important factor in the LSPI puzzle. Southwest Research also believes that the auto-ignition tendency of the engine lubricant may be a factor. Who would have thought that the cetane number of engine oil would be critical?

An interesting aside: Although it isnt directly related, the importance of a lubricants compression ignition can be seen in another application, in rock drill oils. Rock drills are air powered and generate some very high pressures, which can cause the oil to spontaneously ignite if its not correctly formulated.

Turning back to automotive engines, it is possible that a chemical test might be used to define this problem. However, the engine oil and additive industries are leery of any chemical test since it could severely restrict formulation choices. Which then brings us to engine testing.

The Low Speed Pre-Ignition Test for GF-6 is being co-developed with Ford, additive companies Lubrizol and Infineum, and the independent test laboratories Intertek and Southwest Research Institute. They are using a Ford 2.0-liter EcoBoost engine as the test platform, but since the test is still in the early development phase, operating conditions and test length are still being determined, sources familiar with the work told me.

The goal is to create an engine sequence test that will be able to discriminate between engine lubricants with respect to the inhibition of LSPI. Both Intertek and Southwest Research have installed EcoBoost test stands, and a preliminary procedure has been identified. The test employs an advanced combustion analysis system to measure peak cylinder pressure and 2 percent mass-fraction burn locations around the engine, which allows LSPI events to be identified using statistically valid methods.

A final procedure is a ways off, and various test parameters are being studied. In addition, Ford is looking for both good and bad oils to use as reference oils, to verify the test can discriminate between them.

This will be a tough challenge for the industry as it is a departure from the basic oxidation, wear and deposit-type tests which have been the norm in the past. As one member of the test development effort told me, LSPI events are random, infrequent occurrences that happen at low engine speed and high torque in turbocharged gasoline direct-injection engines. Under low speed, high torque operating conditions, a preignition event produces heavy knock, which can cause catastrophic damage.

It boils down to this: Does engine oil cause LSPI? I sure hope we find out before 2016.

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