1.3.1 Monitoring Function

The method to detect engine misfires is based on an evaluation of the engine speed fluctuations. The engine torque is a function of engine speed, engine load, and the moment of inertia.

In order to detect a misfire for any cylinder, the torque of each cylinder is evaluated by metering the time between two ignitions. This time is a measure for the mean value of the angular segment's speed. 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. When the mean engine speed is measured, influences caused by road surfaces have to be eliminated.

This method consists of the following main elements:

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 is executed, instead of misfire detection. This is done in three engine speed ranges. When the segment time adaptation is out of the maximum adaptation range, DTC P1396 is stored.

The sensitivity of the misfire detection increases when the adaptation progresses. The adaptation values are stored and taken into consideration for the calculation of the engine roughness.

Engine Roughness Calculation: 

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

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

Engine Roughness Value Correction: 

The adaptation during firing operation (fuel-on adaptation) learns systematic differences of the calculated engine roughness between the individual cylinders, depending on the current operating point of the engine.

The learned fuel-on adaptation values are used to correct the engine roughness values to improve the overall signal-to-noise range.

Misfiring Determination: 

Misfire detection is performed by comparing the engine roughness threshold value to the engine roughness value. When the threshold is exceeded, single misfire is detected. Whether the monitoring conditions are met determines if the threshold shortfall of the irregular running is evaluated.