Misfire Detection
Misfire monitoring structure
Statistics: Fault Processing
GENERAL DESCRIPTION
The method of engine misfire detection is based on evaluating the engine speed fluctuations.
In order to detect misfiring at any cylinder the torque of each cylinder is evaluated by metering the time between two ignition events, which is a measure for the mean value of the speed of this angular segment. This means, a change of the engine torque results in a change of the engine speed.
Additionally the influence of the load torque will be determined. This means the influences of different road surfaces, e.g. pavement, pot holes etc.,
If the mean engine speed is measured, influences caused by road surfaces have to be eliminated.
This method consists of the following main parts;
- data acquisition, adaptation of sensor wheel is included
- calculation of engine roughness
- comparison with a threshold depending on operating points
- some extreme conditions, during which misfire detections should be disabled for a short time
- fault processing, counting procedure of single misfire events
1. Data acquisition
The duration of the crankshaft segments is measured continuously for every combustion cycle.
2. Sensor wheel adaptation
Within a defined engine speed range and during fuel cut-off, the adaptation of the sensor wheel tolerances, instead of the misfire detection, is carried out.
With progressing adaptation the sensitivity of the misfire detection is increasing.
The adaptation values are stored in a non-volatile memory and taken into consideration for the calculation of the engine roughness.
3. Misfire detection
The following operating steps are performed for each measured segment corrected by the sensor wheel adaptation.
3.1 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.
3.2 Detecting of multiple misfiring
If several cylinders are misfiring (e.g. alternating one combustion/one misfire event) the calculated engine roughness values may be that low, so that the threshold is not exceeded during misfiring and therefore misfiring would not be detected. Based on this fact, the periodicity of the engine roughness value is used as additional information during multiple misfiring. The engine roughness value is filtered and a new multiple filter value is created. If this filter value increases due to multiple misfiring, the roughness threshold is decreased. By applying this strategy, multiple misfiring is detected reliably.
3.3 Calculation of the engine roughness threshold value
The engine roughness threshold value consists of the base value which is determined by a load/speed dependent map.
During warm-up a coolant temperature dependent correction value is added. In case of multiple misfiring the threshold is reduced by an adjustable factor.
Without sufficient sensor wheel adaptation the engine roughness threshold is limited to a speed dependent minimum value.
A change of the threshold towards a smaller value is limited by a variation constant.
4.0 Determination of misfiring
Misfire detection is performed by comparing the engine roughness threshold value with the engine roughness value.
4.1 Statistics, fault processing
Within an interval of 1000 crankshaft revolutions the detected misfiring events are added for each cylinder. If the sum of all cylinder misfire incidents exceeds a predetermined value, the fault code for emission relevant misfiring is preliminary stored. If only one cylinder is misfiring, a cylinder selective fault code is stored. If more than one cylinder is misfiring, the fault code for multiple misfiring is also stored.
Within an interval of 200 crankshaft revolutions the detected number of misfiring events is weighted and calculated for each cylinder. The weighting factor is determined by a load/speed dependent map. If the sum of cylinder misfire incidents exceeds a predetermined value the fault code for indicating catalyst damage relevant misfiring is stored and the MIL is illuminated at once (blinking).
If the cylinder selective count exceeds the predetermined threshold the following measures take place:
- the lambda closed loop system is switched to open-loop
- the cylinder selective fault code is stored. If more than one cylinder is misfiring, the fault code for multiple misfire is also stored
- the fuel supply to the respective cylinder is cut-oft
All misfire counters are reset after each interval.