Knock control

The engine is equipped with a cylinder-specific, adaptive knock control system. Knocking tendency is increased by:

• Increased compression ratio
• High cylinder filling
• Low fuel grade/quality (research octane number/motor octane number)
• High intake-air and engine temperatures.

The value of the compression ratio can also become too high due to spread due to deposits or the manufacturing process. For engines without knock control, these unfavorable effects must be taken into consideration when designing the ignition system by applying a safety margin to the anti-knock limit. However, this results in unavoidable losses in efficiency in the upper load range. The knock control prevents knock. It retards the ignition point of the affected cylinder(s) (cylinder-specific) only as far as necessary and only if there is an actual risk of knocking. In this way, the ignition timing 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 adopts all of the corrections to ignition timing required to prevent knock and also allows completely satisfactory operation on regular petrol (minimum research octane number of 91). The knock control provides:

• Protection against damage resulting from engine knock (also under unfavorable conditions)
• adaptive functionality that allows the system to respond immediately to knock hazards by instantaneously dialing in the correct ignition timing, even under rapidly changing conditions
• Lower fuel consumption and higher torque throughout the entire upper end of the load range (variable according to the fuel grade being used)
• High levels of economy thanks to optimal exploitation of the available fuel grade (octane) and the ability to compensate for variations in the engine's condition.

Self-diagnosis of the knock control system includes the following checks:

• Checks for signal-related malfunctions, such as those stemming from open wires and plug connections
• Self-test of evaluation circuit
• Check of the level of engine-generated noise registered by the knock sensors.

The knock control system is switched off if a fault is found during the course of one of these checks. An emergency program assumes control of the ignition timing while simultaneously imposing a limit on torque generation. At the same time, the relevant fault is entered in the DME fault memory. The emergency program ensures damage-free operation on fuels with a Research Octane Number of 91 or above. The emergency program's response varies according to load factor, engine speed and coolant temperature.

Notice! Knock sensors in correct installation positions! 

If multiple knock sensors are installed (e.g. with engine N55), the diagnostic function cannot detect and inadvertent mix-up of the two knock sensors. Installation of the knock sensors in the correct respective positions is of decisive importance for their correct operation. The length of the connecting cable and the distance to the cable connection identify the correct positions for the individual knock sensors. Ensure that the knock sensors are in the correct positions during installation!