Misfire Detection
Statistics: Fault Processing
Tables of OBD Requirements for Misfire Monitoring
GENERAL DESCRIPTION
The method of engine misfire detection is based on monitoring the crankshaft acceleration.
The engine roughness is derived from the differences from the segment periods (120° crank angle) durations which are corrected and compared to a load and engine-speed dependent thresholds. Different statistical methods are used to distinguish between normal changes of the segment duration and changes due to misfire.
Segment Period measurement
The segment periods are measured through an angular range of 120° crank angle. The segment starts 78° before TDC. The beginning and end of the segments are located at the same angle. The duration of the crankshaft segments is measured continuously.
Sensor wheel adaptation
To eliminate manufacturing tolerances and off-center installation the adaptation of the sensor wheel tolerances is carried out during fuel cut-off.
The segments periods are corrected by the adaptation values. With progressing adaptation the sensitivity of the misfire detection is increasing.
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 the base value is multiplied by a coolant temperature dependant correction value.
Without sufficient sensor wheel adaptation the engine roughness threshold is limited depending on the wheel tolerances expected.
MISFIRE MONITORING STRUCTURE
Disablement of the misfire detection
The misfire detection is deactivated during the following extreme operating conditions:
- engine speed < 512rpm
- engine load < minimum load = positive torque line, including disablement area between 3000 and 6240 rpm
- load during 120° crank angle is changing rapidly - disablement for 0.5 Sec.
- throttle angle is changing rapidly - disablement for 0.7 sec.
- retarded ignition timing
various functions or CAN requests (e.g. gearshift signal, torque reduction) cause retarded ignition. This results in jumps of segment periods and crankshaft vibration which cannot be distinguished from misfire disablement: during 12 x 120° (4 crankshaft revolutions), if retarded timing ignition exceeds 10.1° crank angle.
- during engine start and up to 5 sec. after start
- when air conditioning is switched on for 0.5 sec.
- during fuel cut-off for one or more cylinder
- rough road conditions
wheel speed fluctuation > 6 rev./sec*2......42 rev./sec*2
dependent on vehicle speed > 3km/h.....210 km/h
disablement for 0.25.....1.0 seconds
Misfire monitoring, fault processing
Error window
Within an interval of 200 and 1000 crankshaft revolutions "error windows" to check for similar engine conditions are determined. Upon detection of misfire the window is extended if the current operating point is not within the window.
Engine operating point window
The engine operating window is updated with each segment without misfire.
Misfire detection
Within an interval of 1000 crankshaft revolutions (3000 segments) the detected misfire events are added for each cylinder. If the sum of all cylinder misfire incidents exceeds a predetermined value a fault code is stored.
If more than one cylinder is misfiring, all misfiring cylinders will be specified and the individual fault codes for all misfiring cylinders will be 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 a fault code is stored and the MIL is illuminated at once. 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 codes for all individual cylinders will be stored
- the fuel supply to the respective cylinder is cut-off
All misfire counters are reset after each interval.