The knock sensor has decisively changed the ignition system of the petrol engine. Some voices exaggerated this change, speaking pompously of the intelligent engine, where in fact they mean that the engine can automatically adapt itself almost to every kind of fuel. Since then engines which are designed for premium petrol can be operated also with regular fuel. For premium plus engines, premium is also possible. And all that, without any changes in the hardware. Another advantage is an improved mileage, less polluting exhaust gases and higher performance/torque. The improved self-adaptation with ageing of the engine is important, also.
For shorter engines mostly one knock sensor is sufficient, with more than four cylinders or V- or opposing cylinders two are necessary. The knock sensor is simply screwed with the necessary torque (!) onto a suitable surface of the cylinder block and is connected with the engine control module electrically by wires 1 and 2.
A most often ring-shaped mass (blue), clamped with a certain preliminary tension, counteracts inside the knock sensor oscillations of the engine. Thereby it transfers forces to the Piezo-crystal layer (red) at which already low forces produce different charges at both surfaces (black). The control unit interprets them, distinguishing very exactly between the engine shutdown, the normal run, and engine knocking. It is important that the sensor is positioned such that no knocking of any cylinder is ignored by the control unit.
It is interesting that you can get the knock sensor to behave as if it would be inserted in an engine by using a lot of artificial signals. It shows this by mostly flawless reactions and no error messages. If, however, you try to copy the signals of the knock sensor and to get the control unit to react, e.g., by taking back the ignition, it becomes difficult. This may succeed for a short time, but then, nevertheless, the control unit notes either a missing normal run or engine knocking.
While mounting the knock sensors do not use additional washers.