Knock Sensor: Description and Operation

KNOCK SENSORS





The V8 EMS has two knock sensors located in the V of the engine, one per cylinder bank. The sensors are connected to the ECM (engine control module) via a twisted pair.

The knock sensors produce a voltage signal in proportion to the amount of mechanical vibration generated at each ignition point. Each sensor monitors the related cylinder bank.

The knock sensors incorporate a piezo-ceramic crystal. This crystal produces a voltage whenever an outside force tries to deflect it, (i.e. exerts a mechanical load on it). When the engine is running, the compression waves in the material of the cylinder block, caused by the combustion of the fuel/air mixture within the cylinders, deflect the crystal and produce an output voltage signal. The signals are supplied to the ECM (engine control module), which compares them with `mapped' signals stored in memory. From this, the ECM (engine control module) can determine when detonation occurs on individual cylinders. When detonation is detected, the ECM (engine control module) retards the ignition timing on that cylinder for a number of engine cycles, then gradually returns it to the original setting.

Care must be taken at all times to avoid damaging the knock sensors, but particularly during removal and fitting procedures. The recommendations regarding torque and surface preparation must be adhered to. The torque applied to the sensor and the quality of the surface preparation both have an influence over the transfer of mechanical noise from the cylinder block to the crystal.

The ECM (engine control module) uses the signals supplied by the knock sensors, in conjunction with the signal it receives from the camshaft sensor, to determine the optimum ignition point for each cylinder. The ignition point is set according to preprogrammed ignition maps stored within the ECM (engine control module). The ECM (engine control module) is programmed to use ignition maps for 98 RON premium specification fuel. It will also function on 91 RON regular specification fuel and learn new adaptions. If the only fuel available is of poor quality, or the customer switches to a lower grade of fuel after using a high grade for a period of time, the engine may suffer slight pre-ignition for a short period. This amount of pre-ignition will not damage the engine. This situation will be evident while the ECM (engine control module) learns and then modifies its internal mapping to compensate for the variation in fuel quality. This feature is called adaption. The ECM (engine control module) has the capability of adapting its fuel and ignition control outputs in response to several sensor inputs.

The ECM (engine control module) will cancel closed loop control of the ignition system if the signal received from either knock sensor becomes implausible. In these circumstances the ECM (engine control module) will default to a safe ignition map. This measure ensures the engine will not become damaged if low quality fuel is used. The MIL lamp will not illuminate, although the driver may notice that the engine 'pinks' in some driving conditions and displays a drop in performance and smoothness.

When a knock sensor fault is stored, the ECM (engine control module) will also store details of the engine speed, engine load and the coolant temperature.