Engine damage looms: Understanding the threat of low‑speed pre‑ignition

This phenomenon primarily affects modern engines, especially those equipped with direct injection. Although it has been known for decades, it has intensified with increasingly stringent emission standards. It gradually but effectively causes engine damage. Why does it occur and can it be prevented?

Knock, or premature combustion of the fuel-air mixture, has been known for years and was initially eliminated by electronic ignition control. However, with the evolution of gasoline engines and their increased load due to factors like turbocharging, direct gasoline injection, or high combustion temperatures, knocking has returned in a new, more dangerous form.

A recently recognized phenomenon has been termed low-speed pre-ignition (LSPI), also known as super-knock or super-knocking.

The official response from the industry (mainly oil and additive manufacturers) to this question is that LSPI is an uncontrolled, accidental detonation caused by the ignition of oil particles that enter the cylinders, carbon deposits, soot, etc. Anything that can ignite in the cylinder can cause LSPI. It mainly occurs at low RPM and high load when the cylinder is filled with a fuel-air mixture and the ignition moment is near. At higher RPMs, this phenomenon is called SPI.

This industry response has led to a campaign promoting superior (and more expensive) oils with additional additives. Anti-LSPI oils meet the API SN Plus standard and should be used instead of others. The problem is that even when using these oils, the LSPI phenomenon still occurs.

The main causes of this phenomenon are oil vapour entering the combustion chambers from the breather and excess fuel in the combustion chamber that the engine uses to compensate for losses caused by resistance.

Excess fuel gets into the oil, causing it to degrade faster and deteriorate its lubricating properties. When it mixes with gasoline, it becomes an even more flammable substance. The vapours then re-enter the engine through the hiatus, perpetuating the cycle.

The second reason, related to the first, is excessive engine load resulting from insufficient lubrication and other stresses in the drive system. This is most noticeable in units equipped with direct injection, small displacement, or lubricated with very low-viscosity oil. When the engine can't produce the required torque, it adds fuel, which gets into the oil in its unburnt form. As mentioned earlier, these causes occur simultaneously and fully depend on each other.

Another point to consider is that a large amount of soot is produced in the engine due to these phenomena, forming deposits that are also responsible for LSPI, mainly according to the official version prepared by the oil industry. If we compare it to a bleeding wound, the oil industry claims that the cause of blood leaking through the bandage is the poor quality of the bandage. This could be partly true, but the cause could be also the wound that needs stitching.

We propose using a system to separate vapours from the crankcase and apply them to the test car. Without effective separation, many unnecessary substances would have gotten into the engine. Although LSPI doesn’t occur in this engine, it used to happen frequently.

What about excessive load and insufficient lubrication?

Engines should be lubricated with oil that meets their and users’ needs, not with universal oil that often meets standards only on paper. Everyone drives differently, and the engine oil should be adapted accordingly. Equally important is the oil in the drive system, which also generates significant losses and strains the engine, forcing it to deliver excessive amounts of fuel to counteract this load.

LSPI and SPI are intensifying and increasingly appearing on the list of problems due to the rising demands for CO2 emissions and Euro standards. Modern engines must ensure low fuel consumption and appropriate dynamics, which involves operating at low RPM with high loads.

Engineers strive for engines to generate high torque at low RPM. However, it is at these speeds that the load on engines is the highest. Without reducing losses, there is no chance to avoid LSPI, which cannot be achieved with a universal oil intended for all engines and without eliminating the problem of contaminants entering the cylinders.

The other side of the coin is exhaust systems, which are equipped with increasing filters to capture contaminants from exhaust gases. The more filters there are, the greater the load on the engine, leading to a rise in contaminants. Until recently, there was no problem with GPF filters because, theoretically, they should burn out during normal driving. Unfortunately, this, too, has changed.

Furthermore, complex exhaust gas recirculation systems and breather systems without separators to filter oil vapours from the mixture entering the engine complicate efforts to combat LSPI. However, these systems pose more of a problem for the users, as their engines will not withstand the intended mileage.