Knock Control System
The knock control system is made up of three parts:
1. Knock sensor (signal detection)
2. Knock module (signal processing)
3. Control unit (signal evaluation)
Knock Sensor
The sensor is located on the engine block below the intake manifold between cylinders 2 and 3. The resonance frequency of the Piezo sensor is 8 kHz, which corresponds to the engine knock frequency. Mechanical vibrations on the engine block are continually converted by the sensor into an electric signal (alternating voltage). If there is a considerably higher output voltage level, the sensor signals engine knock to the connected knock module.
For diagnosis purposes, the control unit applies approx. 2.5 volts direct voltage to the knock sensor. If the direct voltage level changes due to a lead short circuit or interruption, the control unit switches to the 91 RON characteristic map and, in addition, the firing angle is retarded for safety reasons by 5° CA. This prevents any damage to the engine.
| Fehler! Datei kann nicht geöffnet werden! | |||
|---|---|---|---|
| .P. D8157.tif | 9 | 7,5 | tiff;; |
| Engine | Part Number | Knock frequency | |
| C 16 XE | 10 456 144 | 8000 Hz |
Knock Module
The task of this module is to continually monitor the knock sensor signal and, in case of recognition of engine knock, to produce a square pulse which can be evaluated by the control unit.
For the first time, the knock module (SNEF module) is positioned dierctly in the control unit.
If engine knock occurs, the output of the knock module drops from 9 V to 0 V. The duration of the pulse produced by the module is proportional to the intensity of the engine knock.
If the module output signal remains at 0 V (low) for longer than 4 s, the control unit assumes malfunction and switches to the 91 RON characteristic map and, in addition, the firing angle is retarded for safety reasons by 5° CA.
Control Unit
The task of the control unit is to retard the firing angle in the event of engine knock to prevent further knock. The pulse from the knock module is filtered by the control unit software before engine knock is recognized.
The signals are filtered, firstly by a pulse window which is only opened for the period in which ignition knock can occur (pulses occurring outside this period are not processed further). They are also filtered by a pulse filter which eliminates all pulses whose width is below an engine speed/load-dependent minimum value. The pulse window and filter increase the certainty of knock recognition.
If a pulse has passed this stage, the control unit allocates it to a cylinder. This means, the knock pulse is attributed to the cylinder that fired most recently. This type of allocation uses a cylinder counter (software).
In the next work cycle for this cylinder, its firing angle is retarded. The amount of retardation is programmable, dependent on engine speed. The firing angles of the other cylinders remain unaffected by this measure. Hence the expression "selective knock control".
Control Unit (continued)
The retardation of the firing angle is repeated each time knocking combustion is recognized.
If knock does not occur again, the ignition is advanced again in 0.3 °CA steps over a period of time dependent on engine speed. This is repeated until the pre-regulation firing angle is attained or knocking combustion is registered again. This process is called fast or engine-synchronized firing angle correction.
Cylinder-selective adaptation of retardation values takes place in a half-second time loop (provided a calibrated engine speed limit is exceeded). If the fast correction retardation at this point is larger than a limiting value, a part of this correction is taken over as an adaptation value. The adaptation values are stored in 15
(per cylinder) engine speed/load-dependent learn cells. This procedure with slow and fast adaptation is comparable with the method used in integrators and block learn memories.
If the fast adjustment is below the limiting value, no further adaptation takes place. The adapted value then decreases by 0.3 °CA in programmable time gaps of at least 0.5 s. Adaptation/adjustment occurs only on the learn cells allocated to this particular engine operating condition.
In addition, the program has three characteristic maps for 91, 95 and 98 RON. Starting at the characteristic map for 98 RON, the control unit will switch to the next lower octane field if the knock control retardation adaptation is large enough. If the next lower characteristic map is selected, the previous adaptation values are simultaneously reset to zero.