Control Unit Software
The control unit's task is to retard the firing angle when knock is recognized, in order to avoid further knock. The signal provided by the knock module is filtered by control unit software before knock can be recognized.
The signals are filtered by
· a duration window which is only open in the period in which ignition knock can occur (pulses which occur outside this period are not further processed).
· a signal filter which eliminates all pulses whose signal width is below a minimum value based on the engine speed and load. The signal window and filter increase the certainty of knock recognition.
If a pulse has passed these stages, the control unit allocates it to a cylinder. That means that the knock pulse is credited to the cylinder that fired immediately previously. A camshaft sensor is used for this sort of allocation.
On the next power stroke in this cylinder, the firing angle is retarded. The amount of retardation depends on the engine speed. The firing angles for the remaining cylinders remain unaffected by this measure. This is what is meant by the expression "selective knock control".
The retardation of the firing angle is repeated each time knocking combustion is recognized. If knock no longer occurs, the ignition is advanced again in steps of 0.3 °CA, at intervals depending on the engine speed. This is repeated until either the value prior to retardation is attained, or knocking combustion is registered again. This process is called fast or engine-synchronized firing angle correction.
Cylinder-selective adaptation of the retardation values occurs in time loops of half a second (assuming that a calibrated engine speed limit has been exceeded). If, at this point in time, the fast correction retardation is greater than a set limit, a part of this correction is adopted as an adaptation value.The adaptation values are stored in 15 (per cylinder) engine speed and load dependent learn cells. This procedure with slower and faster adaptation is comparable with the method used by integrators and block learn memories.
If the fast adaptation sinks below the limit, no further adaptation occurs. Thereafter, the adapted value decreases by 0.3 °CA in programmable intervals of at least 0.5 s. Adaptation is performed only in the learn cells allocated to this particular engine operating condition.
The program is also equipped with three characteristic maps for 91, 95 and 98 RON. Starting from the 98 RON characteristic map, the system can only switch to the next lower octane field if the adaptive knock control's ignition advance is large enough.
If the next lower characteristic map is selected, the previous adaptation values are reset to zero at the same time.