Modern engines battle with low‑speed pre‑ignition resurgence

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

Knock, or pre-ignition of the fuel-air mixture, has been known for years and was eliminated through electronic ignition control. However, with the evolution of petrol engines and the increase in their load resulting from, among other things, turbocharging, direct petrol injection, or high combustion temperatures, the knocking phenomenon has returned in a new, more dangerous form.

The recently discovered phenomenon is termed low-speed pre-ignition (LSPI), also known as super knock.

The official response from the industry (mainly oil companies and additive manufacturers) to this question is roughly that LSPI is uncontrolled, accidental detonations resulting from the ignition of oil particles entering the cylinders, carbon deposits, etc. Anything that can ignite and end up in the cylinder is the cause of LSPI. It occurs mainly at low RPMs and high load when the cylinder is filled with the fuel-air mixture and the moment of ignition is approaching. At higher RPMs, the same phenomenon is called SPI.

This response follows a campaign to use better (read: more expensive) oils with even more additives. So-called anti-LSPI oils meet the API SN Plus standard, and these should be used, not others. The problem is that the LSPI phenomenon still occurs even when they are used.

According to Robert Halicki, a mechanic who has been studying engines for LSPI in his workshop for over a decade, the main cause of this phenomenon is oil vapours entering the combustion chambers from the crankcase ventilation system and excess fuel in the combustion chamber, which the engine uses to compensate for losses caused by resistance.

Excess fuel gets into the oil, causing it to degrade faster (worsening its lubricating properties). When it mixes with petrol, it becomes an even more flammable substance. These increasingly flammable vapours enter the engine through the crankcase ventilation system, and the cycle continues.

The second cause, interconnected with the first, is the excessive engine load due to inadequate lubrication and loads in other areas of the drivetrain. This is most noticeable in units equipped with direct injection, small displacement, or those lubricated with very low-viscosity oil. When the engine cannot generate the required torque, it adds fuel, which, in its unburned form, ends up in the oil. And this was already mentioned earlier because both causes occur simultaneously and are fully dependent on each other.

Another significant issue arises from the previously described phenomena, which generates a lot of soot in the engine, and deposits form, contributing to LSPI, mainly according to the oil industry's official version. To use a comparison, it is like a bleeding wound; the oil industry suggests that the cause of blood leakage through the bandage is the poor quality of the bandage. And they may be somewhat correct. However, Robert Halicki points out that the real issue is the wound, which needs stitching.

"I developed a full vapour separation system from the crankcase, which was applied in my test car. After 60,000 kilometres (rounding to 37,000 miles), the engine was clean inside with no carbon deposits. After 10,000 kilometres (rounding to 6,200 miles), I removed about 2 litres (rounding to half a gallon), mainly of water with a strong smell of petrol and oil from the separator. All these unnecessary substances would have entered the engine without effective separation. This engine does not exhibit LSPI, though it suffered greatly from it before. I perform the same process, along with other actions, on my clients' cars," says Robert Halicki.

And what about excessive load and inadequate lubrication?

Engines should be lubricated with oil that suits their needs and the needs of the users, not a universal one that often meets standards only on paper. Everyone drives differently, and the engine oil must be matched accordingly. However, the oil in the drivetrain is equally important, as it generates significant losses and burdens the engine, forcing it to add excessive fuel.

LSPI and SPI phenomena are intensifying and appearing more frequently on the problem list alongside the increasing CO2 emission requirements and Euro standards. Modern engines must ensure low fuel consumption and adequate dynamics by operating at low RPMs with high loads.

Designers aim for engines to produce high torque at low RPMs. However, engine load is highest at such speeds. Without reducing losses, there is no chance of avoiding LSPI, and this cannot be achieved with a universal oil intended for all engine types and without eliminating the problem of contaminants entering the cylinders.

The other side of the coin is the exhaust systems, which are equipped with an increasing number of filters designed to capture exhaust contaminants. The more filters, the higher the load on the engine, leading to increased contamination. Until recently, GPF filters had no issues because they should theoretically burn off in all situations during normal driving. Unfortunately, this has changed too.

Let's add to this list complex exhaust gas recirculation systems and crankcase ventilation systems without separators to separate oil vapours from the mixture received by the engine. All this stands in the way of designers battling the LSPI phenomenon. However, it is less of a problem for the designers themselves and more for the users because their engines will not last the intended mileage.