Knock Control
Knock Control
Knock control
At 10.5:1 this powerplant's compression ratio is relatively high for an engine with forced induction. For this reason a knock-control system monitors the combustion process. The knock control features extended functionality. The DME control unit also recognizes super-knocking (a special kind of auto-ignition).
Brief component description
Descriptions are provided for the following knock-control system components:
Knock sensor
The knock sensor detects knock during combustion. Auto-ignition is undesirable ignition caused by glowing surfaces within the cylinder. Auto-ignition events can occur on engines with direct fuel injection. The DME control unit has the ability to recognize knock in individual cylinders.
System functions
This section describes the following system functions related to knock control:
Knock control
The engine is equipped with a cylinder-specific, adaptive knock control system. The knock sensor detects knock during combustion. The sensor signal is processed and evaluated in the DME control unit. 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 filling
- Poor fuel grade (RON/MON)
- High intake air and engine temperature
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 performs all the necessary corrections to the ignition point due to knock and also makes trouble-free driving with regular grade petrol (minimum RM 91) possible. The knock control provides:
- Protection against damage resulting from engine knock (also under unfavorable conditions)
- Lower consumption and higher torque over the entire upper load range (corresponding to the fuel grade used).
- High levels of economy thanks to optimal exploitation of the available fuel grade (octane level) and the ability to compensate for variations in the engine's condition
Self-diagnosis of the knock control system includes the following checks:
- Check for signal failure or interference, for example, from an open wire or defective plug connection
- 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 angle. 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 designates irregular combustion that occurs in engines operating with high-boost forced induction. When this condition occurs the combustion pressure rises from roughly 100 bar up to levels as high as 200 bar. This condition stems from contaminants in the combustion chamber (oil, residual gases, carbon particles) that ignite the mixture before the actual firing point. It is thus impossible to combat super-knocking by intervening in the ignition timing.
When the DME digital engine electronics system detects super-knock it responds by reducing output to protect the engine. Transient super-knocking can be caused by isolated contaminant particles. Super-knocking results in deactivation of the injection (for 3 to 6 cycles) on the affected cylinder. A fault (DTC) is also entered in the fault memory.
Notes for Service department
General notes
NOTICE: 1 Knock sensor for all four cylinders.
The knock sensor can selectively identify knock at specific individual cylinders.
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