Knock Control System
Tasks 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 cylinder(s) concerned (cylinder-selective) as far as necessary only when there is an actual danger of engine knocking. In this way the ignition characteristic map can be laid out to combustion-optimum 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 M52 and M43 are equipped with a cylinder bank-selective, adaptive knock
signals are evaluated in the DME control unit.
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. The retard setting can be controlled individually for each cylinder bank (cylinder bank selective).
In the event of the knock sensor failing, a fault code is entered in the fault code memory of the DME control unit. In the case of fault the engine is protected by constant retard setting of the ignition timing.
Installation Locations/Conditions
The double knock sensor is secured by means of an 8 mm screw on the water jacket of the engine block between both cylinder banks. It is arranged such that each sensor monitors one cylinder bank.
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 basic engine noise level detected by the knock sensor
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.
endeavors to provide the ideal air-fuel mixture ratio (Lambda = 1) for combustion.
Stereo lambda control is used on the M52 with MS42. The oxygen sensors are mounted in the exhaust manifold.
There is only one bank on the M43 with BMS46. The oxygen sensors are mounted in the Y-pipe.
They measure the residual oxygen in the exhaust gas and send corresponding voltage values to the control unit. Here, if necessary, the mixture composition is corrected accordingly in that the injection timing is varied. In the event of the oxygen sensor failing, the engine control unit assumes emission control with a program substitute value (approx. 0.45 V). This location allows the dead times for the individual exhaust paths to be reduced and more precisely monitored.
On the M52, each oxygen sensor registers three cylinders with corresponding exhaust section (cylinders 1-3 and 4-6). Test example: If the injector opening time of the first line of cylinders (cylinders 1-3) is changed, a reaction should be observed on the oxygen sensor of the first line. If this is not the case, renew the probes.
On the M43, one oxygen sensor senses all 4 cylinders.
The operation of the control oxygen sensor is similarly monitored. Malfunctions of the oxygen sensor, e.g. caused by using leaded fuel, are detected in the engine control unit by a change in the lambda control frequency.
The oxygen sensors are heated as a temperature of approx. 300 °C is required for effective operation of the oxygen sensors. The heater is actuated by the engine control unit.
monitoring the efficiency of the catalytic converter and monitoring the function of the oxygen sensors before the catalytic converter.
The oxygen sensors are heated as a temperature of approx. 300 °C is required for affective operation of the oxygen sensors. The heater is actuated by the engine control unit.