How to identify the effects of fuel property on low-speed pre-ignition (LSPI)?

28 October 2019

Gradual improvements in engine design and tooling have endowed modern engines with greater power; as a result, engines can be substantially downsized. In addition to delivering cost and weight savings, it turns out that smaller engines are more fuel-efficient and produce fewer emissions; however, a major hurdle to their introduction is LSPI (low-speed pre-ignition) of the fuel. To combat LSPI fuel is being painstakingly redesigned.

What causes LSPI?

Determining the causes is a much researched and debated problem. A key factor is that engines with a smaller displacement need to increase the fuel injection pressure. For some reason, or many, this increases the likelihood of the fuel igniting before the spark, which throws out the timing and causes high pressures, temperatures and shock waves. This is not good news for pistons, engine linings, valves, injectors or emissions.

LSPI, or more colloquially ‘super knock’, first became a problem for engine designers more than 10 years ago and has proven a hard nut to crack. Firstly, the phenomenon is unpredictable. The problem is known to be worse at low speeds with a heavy load, but laboratory tests have proven unreliable in predicting how the engines behave in the field when subjected to varying loads, roads and conditions.

Even the maths are hard

Collecting and analysing test results, especially in the field, is itself difficult. LSPI fuel tests must monitor both the frequency and severity of the events without the sensors interfering with the engine. Complex statistics are then needed to determine which factors correlate with the knocking detected.

Those factors include fuel ingredients and additives, such as detergents, antioxidants and lubricants, and the combustion process – temperatures, pressures, timing and so forth. There is also the effect of engine designs on fluid flows and the load the engine is carrying under different driving conditions and manoeuvres. It is not surprising that conclusive answers have been elusive.

New maths

Ford and Infineum recently collaborated in the development of a new statistical toolset to identify LSPI events accurately enough for the new ILSAC GF-6 oil standards. ILSAC GF-6 was required because older motor oils (ILSAC GF-5) are no longer adequate for the latest commercial engines.

Previous analysis methods had been shown to make inaccurate assumptions about the symmetry of engine responses to lubricants, producing both false positives and negatives when counting LSPI incidents. They also failed to eliminate false data due to transducer failures in the detectors.

The new methodologies are being hailed as an exciting statistical development that can improve the accurate plotting of any parameter that departs from a normal curve in skewness or kurtosis. Some believe the new methods will make it much easier to hone future fuels, oils and engines to eliminate LSPI; as a result, we will all benefit from a new generation of lighter, cleaner motor vehicles.


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