BASIC KNOCK CONTROL OPERATION
Engines with high compression ratios cannot normally be operated with optimal spark advance as they would otherwise be damaged by detonation during combustion. As a result, the spark advance in a conventional ignition systems is set with a corresponding safety margin to the detonation limit. The use of knock control dispenses with the need for this safety margin and the need to have an octane number plug. This means that the engine is always operated with optimal spark advance and provides the following advantages:
§ high performance
§ good torque values
§ low fuel consumption
§ automatic adjustment to fuel quality
§ no engine damage due to knocking combustion Function Diagram of Knock Control
FUNCTION DIAGRAM OF KNOCK CONTROL
ENGINE MANAGEMENT
The knock sensor supplies a structure‑bome signal in which all secondary noises are also contained (e.g. engine vibrations). Because the knocking frequency of the C 20 LET engine has been determined in trials in the region of 15 kHz, only this frequency is used for further evaluation.
This frequency is conveyed to the integrator only within the measuring window (10° ‑ 60° ATDC), where the integrator aligns the signal within the measuring window. The signal so formed, is allotted to the appropriate cylinder by the A‑D converter.
The actual value of this cylinder is now compared with is reference level (average value of the last 16 phases). If the actual value exceeds the reference level by a certain amount, the combustion is recognised as knocking.
If the actual value lies below a certain level related to engine speed, then the actual value is used as a new reference level for knocking recognition. Thus the knock control reacts to even minimal engine noise.
If the knock control has recognised knocking combustion for any one cylinder, the CPU will adjust the dwell angle by 3° in a retard direction for the next phase. The dwell angle of the other cylinders is not affected by this measure (cylinder‑selective control). The dwell angle adjustment in a 'retard' direction is repeated for every combustion which is recognised as knocking and for each cylinder selectively (individually). If no more detonation is sensed, the ignition is adjusted by 0.75° in direction 'advance' after 20 to 120 knock‑free combustions (approx. 2 seconds). This is repeated until the pilot control value is reached again or until knocking combustion is registered. The knock control only affects the dwell angle in an engine speed dependent load range in which knocking combustion is to be expected.
As the knock limit varies from one cylinder to another in an engine and can change drastically within the operating range, every cylinder has its own ignition point for operation at the knock limit. This type of 'cylinder‑selective' knock recognition and control is an essential advantage in Motronic M 2.7 because it allows the optimisation of engine performance and fuel economy.
Figure 6C‑22 shows the individual cylinder knock control for a 4 cylinder engine, such as the C 20 LET fitted to the Calibra 04 Turbo.
ENGINE MANAGEMENT
Automatic Octane Number Adjustment
The knock control makes automatic octane number adjustment possible.
Two ignition characteristic maps are programmed in the control unit.
The knock control computer selects the appropriate ignition characteristic map for the fuel quantity according to the following criteria:
§ After 50 knocking combustions the control unit switches to the map with the more retarded dwell angle (low octane number).
§ After approx. 8.5 minutes of knock‑free operation, the control unit switches back to the map with the more advanced dwell angle (higher octane number).
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