Knock Control

Courtesy of VOLVO CARS CORPORATION

Knock occurs in the combustion chamber when the fuel and air mixture self ignites. This can occur either before or after the spark plug has produced an ignition spark. In both cases the gas in two or more places ignites in the combustion chamber.

This results in an extremely fast combustion process with flames from several directions. When these flames collide, the pressure in the cylinder increases rapidly and there is a mechanical knocking sound.

If any cylinder knocks, then certain types of vibrations in the engine block. These vibrations are transmitted to the knock sensor (7/23), which is bolted in the engine block.

The mechanical stress generated in the knock sensor's piezo-electric materials result in generation of a voltage. The Engine Control Module (ECM) (4/46) can then, using the impulse sensor (7/25), decide which cylinder is knocking.

The knock sensor also interprets a proportion of normal engine sound. The control module is able to recognize the vibrations which correspond to knocking by filtering, amplifying and using software to compare the signal using a model knock signal.

If the knock sensor detects knocking in the engine above a certain threshold value, the ignition timing is first retarded and then the fuel / air mixture is enriched by a separate function to eliminate knocking.