Uneven running data and misfire detection

The uneven running data from the individual cylinders is used by the digital engine module (DME) to detect misfiring.

Function

Analysis of uneven running data is only possible when the engine is idling. The engine temperature and load affect the data. Therefore, thresholds only ever apply under defined general conditions:

• Engine at operating temperature
  

• All auxiliary consumer units (air conditioning, rear window heater, ...) off
  

The crankshaft sensor is used to obtain uneven running data. This sensor continuously measures crankshaft acceleration. Analysis of the signal by the DME allows conclusions to be drawn about the quality of combustion in individual cylinders. Individual cylinders with poor combustion can be identified very well in this way.

Theoretically, when combustion is perfectly even, the values for all cylinders are zero.

There are various factors which can be the cause of uneven running:

• Differences in mixture between cylinders
  

• Misfiring
  

• Extraneous air
  

• Problems with compression in the combustion chamber
  

• Inadequate fuel supply
  

• Poor fuel quality
  

Because of this wide variety of possible causes, precise thresholds for intervention cannot be defined.

For the purposes of detecting misfires, the crankshaft-sensor incremental gear is divided by the control unit into:

• 2 segments in the case of 4-cylinder engines (2 ignitions per crankshaft revolution)
  

• 3 segments in the case of 6-cylinder engines (3 ignitions per crankshaft revolution)
  

• 4 segments in the case of 8-cylinder engines (4 ignitions per crankshaft revolution)
  

• 6 segments in the case of 12-cylinder engines (6 ignitions per crankshaft revolution)
  

The DME measures and continuously analyses the periodic interval of each individual segment of the incremental gear. For each point on the characteristic map, the maximum permissible uneven running values are stored on the DME as a function of engine speed, load and engine temperature.

Diagnosis

If those values are exceeded for a certain number of combustion cycles, the cylinders on which faults are detected are stored in the fault memory. Depending on the severity of the misfires detected, fuel injection for the cylinder concerned may be shut off.

The OBD lamp (MIL) on the instrument cluster is always switched on whenever misfires are detected.