Knock Sensor: Description and Operation

Knock Sensors
Knock Sensors are required to prevent detonation (pinging) from damaging the engine. The EMS2000 will retard the ignition timing (cylinder selective) based on the input of this sensor. Detonation occurs due to:

- High Compression Ratio
- Poor Quality Fuel
- High Level of Cylinder Filling
- Maximum Timing Advance Curve
- High Intake and Engine Temperatures
- Combustion Chamber Carbon Build-up

The knock sensor is a "Piezo-electric accelerometer" producing an output voltage proportional to mechanical vibration (knock) produced by the engine. Knock is caused by high-pressure waves of uncontrolled spontaneous combustion of gasses in the cylinder.

The EMS2000 receives a signal from the knock sensor, filters out any noise and calculates if the engine is knocking. During knock the crystal oscillation increases, the EMS2000 compares the signal received to known signal profiles in the memory.

From the camshaft and crankshaft signals supplying information regarding the position of the engine in it's cycle, the EMS2000 is able to determine which cylinder is knocking and will retard the ignition, on that particular cylinder until the knock is eliminated. It then gradually advances the ignition again towards the original setting until knock occurs again.


Function
The knock sensor is a piezo-electric acceleration sensor. The sensor produces an output voltage proportional to the mechanical vibrations generated by the engine. The engine control module receives a signal, filters out the noise and then calculates whether the engine is knocking. The camshaft and the crankshaft provide the engine control module with position information. The engine control module can use this to calculate which cylinder is knocking. The ignition for this cylinder is set to late as far as is necessary to stop the knocking. This only takes place within the limit values. The engine control module then adjusts the ignition to early until the optimum firing point for this cylinder under these conditions is reached.

Installation location
The knock sensor is located on the intake end of the engine block.

Failure
If the knock sensor fails, the engine control module is unable to detect and correct knocking. The engine then knocks. The performance of the engine is reduced and the fuel consumption is influenced. Standard values are used for the ignition timing. A fault code is written to the engine control module.
l module.