1.3.1 Monitoring function

The method of engine misfire detection is based on evaluating the engine speed fluctuations. The engine torque is a function of engine speed, engine load and the moment of inertia.

In order to detect misfiring at any cylinder, the torque of each cylinder is evaluated by metering the time between two ignitions, which is a measure for the mean value of the speed of this angular segment. A change of the engine torque results in a change of the engine speed.

It is also an influence of the load torque, such as the influences of different road surface, e.g. pavement, potholes etc. If the mean engine speed is measured, influences caused by road surfaces have to be eliminated.

This method consists of following main parts:

Data acquisition: 

The duration of the crankshaft segments is measured continuously for every combustion cycle.

Segment time adaptation (P1396): 

Within a defined engine speed range and during fuel cut-off, the segment time adaptation, instead of the misfire detection, is carried out. If the segment time adaptation is out of the maximum adaptation range, failure P1396 is stored.

With progressing adaptation the sensitivity of the misfire detection is increasing. The adaptation values are stored and taken into consideration for the calculation of the engine roughness.

Calculation of the engine roughness: 

The engine roughness is derived from the differences of the segment durations.

Different statistical methods are used to distinguish between normal changes of the segment duration and the changes due to misfiring.

Determination of misfiring: 

Misfire detection is performed by comparing the engine roughness threshold value with the engine roughness value. If the threshold is exceeded, single misfire is detected. The decision, whether the threshold shortfall of the irregular running is evaluated, depends on the monitoring conditions.