Low Speed Pre-Ignition: Background, Causes, Solutions

Reading time: ca. 6 min. | An article from Florian Wagner | TUNAP Blog

No doubt about it: for some time, sentiment in the automotive industry has been at rock bottom, among other things, due to the continuing discussions about emissions. But to keep things in perspective, development of the internal combustion engine over the last 20 years has also produced some positive results: after all, carmakers have managed to construct smaller, very efficient engines with much lower fuel consumption and emission values. In other words, over the years, internal combustion engines have become smaller and more economical, but also more powerful.

Despite the trend to electric drives, this potential of the petrol engine has by no means yet been fully utilised. According to a recent study, in 2040, two thirds of vehicles in Germany will have an internal combustion engine.

For developers the question is: will this continuing slimming trend for engines reach a bottom limit at some time? The answer is: yes, mainly for two reasons: components such as injection nozzles, valves and camshaft need space after all. Besides, the LSPI (Low-Speed Pre-Ignition) phenomenon sets limits for increasingly extreme downsizing of internal combustion engines.

What does LSPI mean and how does it happen? To put it in a nutshell, the fuel-air mixture ignites prematurely and uncontrollably – in other words, before being ignited by the ignition plug. In the worst-case scenario, this can completely destroy the engine. There are various solution approaches, but LSPI still poses a particular challenge for engine developers.

To gain a better understanding of the connections, it is worth taking a look at the basic sequences of a four-stroke engine. In the first step, the piston sucks the petrol-air mixture into the cylinder. Secondly, the piston compresses the mixture and, just before it reaches the top dead centre, the ignition plug triggers ignition. In the third stage, the fuel mixture combusts and pushes the piston down again while it performs mechanical work. In the fourth and last stage, the piston rushes upward again to the top dead point and presses the combustion gases out of the cylinder through the outlet valve, which is now open.

So much for the theory. But how does LSPI actually occur in practice? This irregular combustion happens in the transition from stage two to stage three. The combustion gas ignites before it is ignited by the ignition plug. This causes an undesired counterpressure against the rising piston, which can often be as high as 200 to 300 bar. The problem is that most engines are not designed for such high pressures. Generally, the intended constant load is around 90 bar with peak loads of approximately 120 bar.

There are many different causes for LSPI. There is no single reason: for example, it is clear that fuels and their chemical compositions have different effects. For instance, it has been shown that, as the ethanol concentration in the fuel increases, the tendency to LSPI decreases. Other investigations suggest that self-ignition of the fuel is connected with the pressure and temperature in the combustion chamber.

Nevertheless, two main causes of LSPI can be identified. On the one hand, fuel-oil droplets: these can form when the injected fuel covers the cylinder wall, which is lubricated with oil. This fuel-oil compound has a higher combustibility than pure petrol.

The second main cause of LSPI is red-hot particles. Normally, soot and oil particles accumulate on the cylinder walls, pistons and valves. Normal ignition can cause these to become red-hot. These extremely hot particles can ignite the fuel-air mixture prematurely in the transition from stage two to stage three.

Comprehensive solutions are needed to come to grips with the problem of LSPI. Developments are necessary in engine design, oil formulation and fuel quality. Fortunately, motorists themselves can effectively prevent the main causes of LSPI – fuel-oil droplets and red-hot particles: the latest solutions, such as the additive microflex^® 978 Combustion Chamber Cleaner, clean the combustion chamber and the piston head of the cylinder. It has been proved that this considerably reduces the risk of LSPI and, consequently, engine damage.