2002 2BMXV04.4LEV, 2BMXT04.4E53 & 2BMXT04.6XHP

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General Description - Method of engine misfire detection is based on evaluating engine speed fluctuations. To detect misfiring at any cylinder, torque of each cylinder is evaluated by metering time between 2 ignition events, which is a measure for mean value of speed of this angular segment. This means that a change of engine torque results in a change of engine speed. In addition , influence of load torque will be determined, such as influences of different road surfaces. If mean engine speed is to be measured, influences caused by road surfaces have to be eliminated. See Fig 1. This method consists of 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.
Fig 1: Misfire Monitoring Structure
Courtesy of BMW OF NORTH AMERICA, INC.
Monitoring Cycle - Monitoring cycle is represented by the following:
- Data Acquisition - Duration of crankshaft segments is measured continuously for every combustion cycle.
- Sensor Wheel Adaptation - Within a defined engine speed range and during fuel cut off, adaptation of sensor wheel tolerances is carried out instead of misfire detection. For test group 2BMXVO4.4LEV (models 745i and 745Li), after this the total operation range of engine is corrected to equalize engine roughness across all cylinders. Thereafter, maximum sensitivity to engine roughness at a given load/speed site is achieved. For test groups 2BMXVO4.4LEV (all models), 2BMXTO4.4E53 and 2BMXTO4.6XHP, with progressing adaptation, sensitivity of misfire detection is increasing. Adaptation values are stored in a non-volatile memory and taken into consideration for calculation of engine roughness.
- Misfire Detection - Following operating steps are performed for each measured segment corrected by sensor wheel adaptation.
  1. Calculation Of Engine Roughness - Engine roughness is derived from differences of segment durations. Different statistical methods are used to distinguish between normal changes of segment duration and changes due to misfiring.
  2. Detecting Of Multiple Misfiring - If several cylinders are misfiring, calculated engine roughness values may be so low that threshold is not exceeded during misfiring, and therefore misfiring would not be detected. Based on this, periodicity of engine roughness value is used as additional information during multiple misfiring. Engine roughness value is filtered and a new multiple filter value is created. If this filter value increases due to multiple misfiring, roughness threshold is decreased. By applying this strategy, multiple misfiring is detected reliably.
  3. Calculation Of Engine Roughness Threshold Value - Engine roughness threshold value consists of 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, threshold is reduced by an adjustable factor. Without sufficient sensor wheel adaptation, engine roughness threshold is limited to a speed dependent minimum value. A change of threshold toward a smaller value is limited by a variation constant.
Determination Of Misfiring - Misfire detection is performed by comparing engine roughness threshold value with engine roughness value. If a misfire event is detected in a cylinder, misfire detection of next cylinder in firing order is deactivated to prevent a faulty diagnosis.
- Fault Processing Statistics - Within an interval of 1000 crankshaft revolutions, detected misfiring events are added for each cylinder. If sum of all cylinder misfire incidents exceeds a predetermined value, fault code for emission relevant misfiring is preliminarily stored. If only one cylinder is misfiring, a cylinder selective fault code is stored. If more than one cylinder is misfiring, fault code for multiple misfiring is also stored. Within an interval of 200 crankshaft revolutions, detected number of misfiring events is weighted and calculated for each cylinder. Weighting factor is determined by a load/speed dependent map. If sum of cylinder misfire incidents exceeds a predetermined value, fault code for indicating catalyst damage relevant misfiring is stored and MIL is illuminated at once. If cylinder selective count exceeds predetermined threshold, following measures take place:
  1. Lambda closed loop system is switched to open-loop.
  2. Cylinder selective fault code is stored. If more than one cylinder is misfiring, fault code for multiple misfire is also stored.
  3. Fuel supply to respective cylinder is cut off.
  Counters are reset after each interval. See Fig 2.
  
  Fig 2: Fault Processing Statistics
  Courtesy of BMW OF NORTH AMERICA, INC.