Ionic current control unit: S85B50

To optimise the engine control performance in terms of exhaust and output, the combustion rate must be known in every operating situation. Ionic current measurement is used to achieve this.

Installation location

Both ionic current control units are attached to the cylinder head cover at the front on the cylinder heads.

Item Description Item Description
1 Cylinder bank for cylinders 1-5 2 Ionic current control unit
3 Ionic current control unit 2 4 Cylinder bank for cylinders 6-10


How it works

The ionic current control unit carries out the ionic current measurement. The ionic current measurement is used for the knock control and misfire detection.

The ignition is triggered by the engine control unit via the ionic current control unit.
During combustion, the thermal energy in the combustion chamber produces ions (negatively-charged molecules). The number of ions is dependent on the combustion quality.
The ions in the gas mixture enable it to conduct electricity.

The ionic current control unit applies a constant voltage at the spark plug following completion of the ignition spark. The spark plug measures the current which is produced (ionic current). This means that the spark plug also performs the function of a sensor.
The ionic current control unit receives and amplifies the signals from the spark plugs. The DME control module evaluates the ionic current measurement. If required, the ignition timing could then be modified, for example.

Item Description Item Description
A Ignition B Ionic current measurement
1 Spark plug 2 Selector switch
3 Ionic current control unit 4 Switch for measurement in the ionic current control unit
5 DME control module    


The ion current curve is a direct function of the combustion pressure in the cylinder:

  • Efficient combustion -> higher combustion pressure
  • Poor combustion -> lower combustion pressure
Item Description Item Description
1 Combustion pressure in bar 2 Ionic current in mA
3 Build-up of ignition spark 4 Extinguishing of ignition spark, decay of residual energy
5 Start of combustion, pressure build-up in cylinder 6 Combustion, maximum combustion pressure in cylinder
7 Crankshaft angle (degrees)    













Fault recognition

Knocking creates pressure peaks in the combustion chamber. This causes additional free ions to be released which changes the measured ionic current.

Item Description Item Description
A Combustion knock B Misfiring
1 Firing point 2 End of ignition spark
3 Extinguishing of ignition spark, decay of residual energy 4 Start of combustion, pressure build-up in cylinder
5 Knock signal 6 Missing signal during combustion misfiring
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