The push for cleaner engines has led manufacturers to downsize these machines; therefore, vehicles with smaller engines now use technical advances to provide the same power as previously provided by larger engines. The smaller engines produce fewer emissions; however, they can be subject to ‘super knock’, which can damage the engine. The technical name for this is a low speed pre-ignition event (LSPI), which has fuel additive and lubricant developers working hard to figure out how to combat it.
Ten years of research – little agreement on the cause
Downsized engines have a high power density, which seems to encourage LSPI; however, it is an unpredictable phenomenon. This is the most difficult kind to study, as it is hard to replicate predictably in the test environment.
LSPI seems to be caused by an interaction between the fuel, the combustion process, and one or more additives or lubricants. These variables complicate the search for the underlying cause. Research has been undertaken into the action of fuel additives such as detergents and antioxidants; however, this has not yet produced a conclusive answer.
There has also been research into the process of combustion. This has looked at the role that lubricants are playing in LSPI events, especially the likelihood of auto-ignition. LSPI tends to occur when the engine is heavily loaded but being used at low speed. It is most common in turbocharged engines that are spark-ignited with direct fuel injection, whereby the fuel and air mixture ignites prematurely before the spark.
What causes pre-ignition?
In the past, it was assumed that this was caused by hot surfaces in the engine igniting the fuel, such as spark plugs or exhaust valves that were overheated; now, researchers are looking at fuel and lubricant droplets or flaking deposits in the combustion chamber. The pre-ignition itself does not damage the engine. This is followed by flame propagation, as it would if started normally by a spark. Again, this does not in itself damage the engine; however, as this happens at the wrong time, it can lead to very high pressure in the engine cylinder and a very high engine temperature. There is also a lot of unburned fuel and air mixture available. What can happen is not so much propagation as detonation, which can seriously damage the engine.
What part do gasoline, lubricants and detergents play?
It is important to look at the fuels and lubricants being used, as there is the possibility of developing a specific LSPI fuel that would prevent these events – even in downsized engines. Detergents based on calcium and magnesium have been investigated to see whether they are the culprits. Researchers continue to experiment with a wide range of additives in an attempt to discover which encourage LSPI. Calcium seems to promote these events, while magnesium appears to prevent them.
The base petrol mixture matters
Researchers have tried adding different qualities of base stock to the fuel being tested. The type of base stock and the range of specific chemical components present in the finished fuel seems to have an effect on LSPI, with some formulations proving very resistant to LSPI, and others appearing to promote it. This could be a way forward for the engine manufacturers looking to develop solutions to the phenomenon or to approve lubricants or additive packages that help control LSPI.
It is clear that this complex and multifactorial phenomenon has not yet been thoroughly decoded, with plenty of research still to be done.