1.3.1 Monitoring Function

Engine misfires are detected using a method 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 a misfire for any of the cylinders, each cylinder's torque is evaluated by metering the time between two ignitions. This value is a measurement for the mean value of the angular segment's speed. A change in the engine torque results in a change in 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): 

Instead of misfire detection, the segment time adaptation is executed within a defined engine speed range and during fuel cut-off. This process is performed in three engine speed ranges. When the segment time adaptation exceeds the maximum adaptation range, DTC P1396 is stored.

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

Engine Roughness Calculation: 

The engine roughness is derived from the segment duration differences.

Different statistical methods are utilized 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 executed 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 whether the threshold shortfall of the irregular running is evaluated.