LSPI: what you should know

What is LSPI?

LSPI or Low Speed Pre-Ignition is an undesirable event that happens during combustion. Specifically, the fuel-air mixture ignites too early, resulting in excessive pressures inside the cylinders.

LSPI can in many cases be heard as ‘engine noise’. But what’s worse, LSPI can cause severe damage. This makes it one of the most important challenges in engine technology today.

What causes LSPI?

A bit of background: to satisfy the global demand for better fuel economy and reduced emissions, car manufacturers have developed downsized, turbocharged (also called supercharged) gasoline engines. These engines can operate at low speeds and high torque. Their higher power density and better efficiency achieve the goals set by the market and, in many countries, legislation.

For example, Ford and GM have launched a 3-cylinder, 1.0 L boosted engines with a reported similar output to their 1.6 L four-cylinder engines, while delivering a 20% better fuel economy and lower CO2 emissions. The best indication of their success is the fact that millions of these engines are being produced each year.

But a problem with these engines has arisen. Known as LSPI or low-speed pre-ignition, an early combustion before normal spark initiated combustion can occur. This can be identified by a heavy knocking sound audible by the driver.

We know that LSPI occurs in downsized engines, but the exact mechanical cause for LSPI is still unclear. One theory says an oil droplet enters the combustion chamber through an opening between the piston and the cylinder wall, where it mixes with fuel and ignites. Another theory assumes deposits are the ignition source. Both situations have been proved to occur, and they are not mutually exclusive.

What are the effects?

Besides the sound, LSPI can result in catastrophic damage in just a few engine cycles. Because pistons and connecting rods are rising when LSPI occurs, these are especially prone to be damaged. In extreme cases, piston rings are damaged or even broken, connecting rods are bent, and spark plugs are harmed.

For automakers, the effect of LSPI is that it limits their ability to use the full potential of turbocharged engines to run more efficiently. In other words, LSPI is a bottleneck for innovation.

What is being done about it

So what can prevent LSPI? Research into the process has been very challenging due to the destructive nature of LSPI and its random occurrences. But there are strong indications that lubricant composition can play a key role in suppressing LSPI. Additive suppliers have performed groundbreaking research in this matter.

For example: oils with higher concentrations of calcium seem to increase the chance of LSPI occurring. On the other hand, magnesium-based detergents and molybdenum compounds do not seem to promote LSPI and have shown even to decrease it. 

But fixing LSPI isn’t as simple as taking out one component and adding another — each component performs multiple functions, and changing the compositions always means a lubricant’s performance is altered on different levels.

How will LSPI prevention be reflected in specifications?

The lubricants industry continues their efforts to develop a meaningful engine test to measure the effect of the composition of lubricants on LSPI. Our goal is to design next generation lubricant technologies that put an end to LSPI and enable OEMs to continue their development of fuel-saving, low-emission engines.

Upcoming oil specifications already include LSPI prevention, and ILSAC GF-6 is expected to include a Ford engine test to indicate reduced chances of LSPI occurring.

Summary:

• LSPI or Low Speed Pre-Ignition is an undesirable event that happens during combustion.
• We know that LSPI occurs in downsized engines, but the exact mechanical cause for LSPI is still unclear.
• Besides engines noise, LSPI can result in catastrophic damage in just a few engine cycles.
• There are strong indications that lubricant composition can play a key role in suppressing LSPI.
• Our goal is to design next generation lubricant technologies that put an end to LSPI and enable OEMs to continue their development of fuel-saving, low-emission engines.