The Causes and Solutions Of Low Speed Premature Ignition (LSPI)

Low-speed early combustion is most commonly seen in rapid acceleration conditions at low engine speed. Premature ignition in the combustion chamber before ignition of the spark plug can lead to abnormal combustion and increased cylinder pressure. Low-speed premature ignition events can cause the pressure in the cylinder to be out of control, resulting in a loud knock sound. In some cases, a single low-speed early ignition event is enough to cause severe engine damage.

There are two potential drivers of low-speed preignition

1. Oil droplets induce premature ignition

2. Combustion chamber sediments lead to early ignition

How is premature combustion formed?

Low-speed early ignition is an uncontrolled combustion that occurs before the spark plug ignits.

During the intake stroke, the nozzle sprays oil mist into the cylinder at an angle facing the cylinder wall, and some of the fuel reacts directly with the residual film of the oil, which prevents wear on the honing lines. When the fuel oil is mixed with the residual oil film, the mixture will be liquefied, resulting in a portion of the honing grain not protected in the piston ring. Droplets form as the compression stroke is scraped on the rising piston. As the piston rises, the combustion chamber will reach the conditions that cause the fuel and oil mixture to burn, causing an unexpected combustion pressure shock wave, that is, low-speed early ignition phenomenon.

Simply put, the piston in the compression stroke has not yet reached the top dead center, and the oil thrown from the cylinder wall has ignited the gasoline in advance.

How to solve low speed early ignition?

Engine manufacturers can reduce the risk of low-speed premature combustion by strengthening pistons or over injection, but this can lead to reduced engine efficiency, which negatively affects fuel consumption and emissions.

The reasons for LSPI are more complex, one of which is that the oil formula is not suitable for TGDI engines. Choosing a suitable engine oil can effectively reduce the incidence of low-speed premature combustion.