Knock Sensor: Description and Operation
Knock ControlTasks of the knock control system
Operation of an engine with knocking combustion over a prolonged period can lead to serious damage. Knocking tendency is increased by:
Increased compression ratio
High cylinder charge
Poor fuel quality (RON/MON)
High intake air and engine temperatures
The compression ratio can also reach excessively high values due to deposits or production-related scatter.
On engines without knock control, these unfavorable influences must be taken into consideration in the ignition design by providing a safety distance to the knock limit. However, this results in unavoidable losses in efficiency in the upper load range.
The knock control can prevent knocking engine operation. For this purpose it retards the ignition timing of the affected cylinder(s) (cylinder-selective) only as far as necessary and only if there is an actual risk of knocking. In this way, the ignition characteristic map can be adapted to the optimum consumption values without having to take the knock limit into consideration. A safety distance is no longer necessary.
The knock control system carries out all knock-related corrections to the ignition timing and enables perfect operation also with regular grade fuel (minimum RON 91).
The knock control provides:
Protection against knocking damage also under unfavorable conditions
High efficiency due to optimum utilization of the fuel quality and consideration of the relevant engine status
Logistics advantages with regard to fuel availability
Lower consumption and higher torque over the entire upper load range (corresponding to the fuel quality used).
Design of knock control system
The M73 is equipped with a cylinder-selective, adaptive knock control system. Two knock sensors per row of cylinders detect knocking combustion. The sensor signals are evaluated in the DME control modules.
Increased compression ratio
High cylinder charge
Poor fuel quality (RON/MON)
High intake air and engine temperatures
The compression ratio can also reach excessively high values due to deposits or production-related scatter.
On engines without knock control, these unfavorable influences must be taken into consideration in the ignition design by providing a safety distance to the knock limit. However, this results in unavoidable losses in efficiency in the upper load range.
The knock control can prevent knocking engine operation. For this purpose it retards the ignition timing of the affected cylinder(s) (cylinder-selective) only as far as necessary and only if there is an actual risk of knocking. In this way, the ignition characteristic map can be adapted to the optimum consumption values without having to take the knock limit into consideration. A safety distance is no longer necessary.
The knock control system carries out all knock-related corrections to the ignition timing and enables perfect operation also with regular grade fuel (minimum RON 91).
The knock control provides:
Protection against knocking damage also under unfavorable conditions
High efficiency due to optimum utilization of the fuel quality and consideration of the relevant engine status
Logistics advantages with regard to fuel availability
Lower consumption and higher torque over the entire upper load range (corresponding to the fuel quality used).
The knock sensor is a piezo-electric structure-borne noise microphone. It picks up the structure-borne noise and converts it into voltage signals.
Function of the knock control system
If knocking occurs, the ignition is retarded for a certain number of working cycles and then gradually approaches the original value.
A defect is stored in the defect code memory of the DME control module in the event of a knock sensor failing. In this case, both rows of cylinders are always protected by constant retard setting of the ignition timing (knock protection function in DME control module I and II) .
Installation Locations/Conditions
The 4 knock sensors are secured with 8 mm screws on the cylinder heads of the engine block between the two rows of cylinders. They are arranged in such a way that one sensor monitors three adjacent cylinders.
Only screw locking compound may be used to lock the screws. Washers, spring washers or serrated lock washers must under no circumstances be used.
Self-diagnosis and emergency operation of the knock control system
Self-diagnosis of the knock control system includes following checks:
Check for sensor signal interference/line break, plug connector defective etc.
Self-test of entire evaluation circuit
Check of the basic engine noise level recorded by the knock sensors
The knock control system is switched off if a fault is found during the course of one of these checks. The emergency program adopts the task of controlling the ignition timing. At the same time, a defect code is stored in the defect code memory. The emergency program ensures damage-free operation as from minimum RON 91. It depends on the engine load, speed and temperature.
The diagnosis procedure cannot detect whether the plug connectors of the sensors have been interchanged. The engine can be damaged if the sensors are interchanged. Particular care must therefore be taken during service work to ensure that the sensors are connected correctly (see repair instructions).