Knock Control
Knock Control
For a turbocharged engine, the N14 is comparatively highly compressed (compression ratio of 10.5 : 1.
For this reason, knock control monitors the combustion process. The functions of the knock control have been extended. The DME control module also detects super-knocking (special form of spontaneous ignition).
Brief description of components
The following components are described for the knock control:
Knock sensor
The knock sensor detects the combustion knock. Spontaneous ignition is an unwanted ignition caused by glowing parts in the cylinder. Spontaneous ignition can occur on engine with direct fuel injection. The knocking is detected on a cylinder-selective basis by the DME control module.
System functions
The following system functions are described for the knock control:
Knock Control
The engine is equipped with a cylinder-selective, adaptive knock control system. The knock sensor detects the combustion knock. The sensor signal is evaluated in the DME control module. Longer operation of an engine with knocking combustion can lead to severe damage.
Knocking is favoured by:
- Increased compression ratio
- High cylinder charge
- Poor fuel quality (RON/MON)
- High intake air and engine temperature
The compression ratio can also reach excessively high values due to deposits or production-related scatter. On engines without knock control, these unfavourable 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. Knock control prevents knocking. 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 angle 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 damage resulting from engine knock (also under unfavourable conditions)
- Lower consumption and higher torque over the entire upper load range (corresponding to the fuel quality used).
- High efficiency due to optimum utilization of the fuel quality and consideration of the relevant engine status
Self-diagnosis of the knock control system includes the following checks:
- Check for disrupted signal, e.g. break in wiring or plug-in connection defective
- Self-test of evaluation circuit
- Check of the noise level picked up by the knock sensor for the engine
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, the relevant fault is entered in the DME fault memory. The emergency program ensures damage-free operation as from minimum RON 91. It depends on the engine load, speed and engine temperature.
Super-knocking
Super-knocking is irregular combustion that occurs on highly turbocharged engines. Here, the maximum combustion pressure of approx. 100 bar rises to up to 200 bar. The causes of this are impurities in the combustion chamber (oil, residual gas, carbon particles) that trigger inflammation of the mixture before the actual point of ignition. This is why super-knocking cannot be eliminated by means of ignition interventions.
If the DME detects super-knocking, the power output is reduced to protect against engine damage. Brief super-knocking caused by individual dirt particles does not lead to a fault memory entry.
Notes for Service department
General information
NOTE: 1 knock sensor for all 4 cylinders.
The knock sensor detects knock of a cylinder-selective basis.
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