911 Carrera 4 Cabriolet (996) F6-3.4L (1999)
Ignition Failure Sensor: Description and Operation
MISFIRE MONITORING
GENERAL DESCRIPTION
Engine misfire detection is based on evaluating engine speed fluctuations. In order to detect misfiring in any cylinder, the torque of each cylinder is
evaluated by measuring the time between two ignition events. This is also a measure for the mean value of the speed of this angular segment. This means,
a change of engine torque results in a change of engine speed. Additionally, the influence of the load torque is be evaluated, such as the effects of
different road conditions (pavement, pot holes etc.). If the mean engine speed is measured the influences caused by road surfaces are eliminated. This
method consists of the following strategies:
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data acquisition, including adaptation of sensor wheel
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calculation of engine roughness
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comparison with threshold limits dependent on engine operating range
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extreme conditions, during which misfire detection is disabled for a short time
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fault processing, counting procedure of single misfire events
Misfire monitoring structure
1. Data acquisition
The duration of the crankshaft segment intervals are measured continuously for each combustion cycle.
2. Sensor wheel adaptation
Within a defined engine speed range and during fuel cut-off, the adaptation of the sensor wheel tolerance, instead of the misfire detection, is
performed. With progressing adaptation the sensitivity of the misfire detection is increased.
The adaptation values are stored in nonvolatile memory and utilized 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 determined from the differences of the segment duration. Several statistical methods are used to distinguish between
normal changes of the segment duration and changes due to misfiring.