The knock sensor responds to spark knock caused by over advanced timing. The knock sensor is mounted directly on to the side of the cylinder block, they “listen” for engine pinking and send an oscillating voltage signal to the ECU. The ECU uses this information to control the ignition timing.
Pinking is a phenomenon caused by the explosion in the combustion chamber taking place too early. When this occurs the flame front within the combustion chamber collides with the still rising piston causing the characteristic pinking or pinging noise which sounds like many marbles dropping onto a steel plate. In mild cases it results in decreased engine power, in severe cases it results in major engine damage.
The more advanced the ignition timing is set on an engine the higher the theoretical power, up to a limit. The limit is generally just before the point of pinking. It is the job of the Knock sensor, in conjunction with the ECU, to keep the ignition timing at this peak setting.
The ECU will advance the ignition timing until pinking is detected then retard it by, say 10 degrees, this process is repeated many times per second. Some ECUs have the capability of advancing and retarding the timing individually per cylinder, that is, a four cylinder engine may have four different ignition advance settings. The timing setting achieved is a variation on a timing “map” within the ECU, which takes into consideration parameters such as engine speed, load and temperature.
The Knock sensor can also have a secondary role. The point at which pinking occurs is also the point at which peak NOx emissions are generated. By accurately controlling the timing, the emissions of an engine can be reduced.
How they work:
The sensing element consists of a piezo-ceramic element and a seismic mass which is clamped into place by the locating bolt. The sensor has a specific exciting frequency which is matched to the frequency vibration band within which pinking occurs
