What is knocking combustion and what are its symptoms?

Knocking combustion phenomena can lead to serious car failures. Until recently, this risk was mainly present in older car models because, from 2022, every new car should be fitted with a knock sensor. However, the problem has returned in the latest petrol engines, where knocking combustion phenomena can occur at low engine speeds LSPI.


Knock combustion process

Principle of operation of the knock sensor

The engine consists of many components whose operation is controlled by various types of sensors. The engine temperature sensor, intake air pressure sensor or coolant temperature sensor are just a few examples. Each is responsible for different components and has an individual task. The failure of one of them (e.g. the crankshaft sensor) can cause a lot of trouble for any vehicle owner.

In the engine, knock sensors are located outside the engine block. Their function is to register the knocking sound in all operating states of the engine in order to prevent damage. The knock sensor 'hears' the vibrations of the engine block caused by structure-borne noise and converts them into a voltage signal. These are then filtered and analysed by the corresponding controller. A faulty knock sensor can produce characteristic symptoms.

LSPI knocking phenomenon

This phenomenon is a relatively new concept in the automotive industry. It is a derivative of knocking combustion, which the automotive industry dealt with with the help of technological developments with the introduction of the spark-ignition internal combustion engine. Paradoxically, these technological developments, particularly miniaturisation, have led to the revival of the knocking combustion problem in the form of a very dangerous phenomenon called LSPI (Low Speed Pre-ignition), roughly translated as premature ignition at low engine speeds.

Consequences of knocking combustion

The consequences of uncontrolled ignition of the above-mentioned mixture for the engine can be disastrous. The flame during detonation combustion spreads faster and the mixture usually ignites evenly, providing a lot of energy. This temporarily loads the pistons, connecting rods and crankshaft and increases the temperature in the combustion chamber.

The consequence of LSPI knocking combustion is primarily accelerated wear of the elements of the crank-piston system (mechanically moving parts of the engine), which are subject to enormous forces. Moreover, the LSPI phenomenon may lead to complete damage to the drive unit.

Symptoms of knock sensor failure

Symptoms of a damaged combustion sensor can be detected by the controller, which will identify the fault and switch to emergency operation mode. It is worth remembering that it is best to check the combustion sensor in a professional mechanical workshop.

Here are some symptoms that may indicate a sensor failure:

  • illuminated indicator,
  • saved error code,
  • low engine power,
  • increased fuel consumption.

Causes of damage to the knock sensor

The causes of knock sensor damage include:

  • internal failure,
  • short circuit,
  • mechanical trauma,
  • corrosion.
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