Knock sensor

from Wikipedia, the free encyclopedia

The knock sensor is part of the electronic knock control in an internal combustion engine . It is a structure-borne sound sensor whose signal is examined for the high-frequency vibration components typical of knocking with the help of electronic filters and a digital signal processor . One or more of these sensors should detect detonations in each cylinder . In some cases, this acoustic method can lead to false signals due to other mechanical influences (stone impact, chain rattle, etc.).

Basics

The knock sensor can be seen as the ear of an electronic assembly that picks up engine oscillations or vibrations from internal combustion engines and delivers them as input signals in the form of electrical voltage to the engine control unit, which in turn compares them with specified values. If the knock sensor registers a knocking of the engine through knock measurement using a diagnostic device , the ignition point is automatically adjusted in the "retarded" direction until no more knocking noises are detected. The ignition point is then gradually adjusted again in the "early" direction until knocking is registered again. An early ignition point leads to better utilization of the combustion energy and thus to increased performance and better engine efficiency. This control system keeps the engine close to what is known as the knock limit . In this way, the highest possible power output and thus optimal operation of the engine in terms of efficiency are achieved. It also compensates for fluctuations in fuel quality and prevents engine damage.

Electrically, the knock sensor is a piezoelectric sensor. To be precise, it is a longitudinal element.

Causes of the engine knock

The cause of the so-called engine knock is self-ignition of the mixture in areas that are far away from the spark plug. Auto-ignition occurs most of all when using low octane fuel . During compression and after the onset of combustion, the temperature and pressure in the cylinder rise sharply. In areas that are recorded very late by regular combustion, the so-called end gas area, there are pre-reactions in the remaining fresh mixture, which lead to this mixture being spontaneously converted when a critical temperature threshold is exceeded before the actual flame reaches it. In contrast to "normal" deflagrative combustion, one speaks of a detonation.

Modern gasoline engines tend to knock partly because a high compression ratio is desirable from a thermodynamic point of view . Common values ​​here are 8.5: 1 for turbocharged engines, 10.5: 1 for naturally aspirated engines with intake manifold injection and up to 12: 1 for naturally aspirated engines with direct injection. The high compression also results in high peak pressures and temperatures, which promote the auto-ignition described above.

literature

  • Peter Gerigk, Detlef Bruhn and others: Motor vehicle mechatronics-car technology, 1st edition, Westermann-Schulbuchverlag GmbH, Braunschweig, 2013, ISBN 978-3-14-231810-3