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西门子系统的失火监测
Legal requirements
The diagnosis must monitor the engine for misfiring. If misfiring is detected, the cylinder (s) that has (have) misfired must be identified. An error condition exists, if misfire rates occur in the monitoring ranges which result in damage to the catalyst or increase exhaust emissions to values above the EOBD thresholds
General Description
The system checks for misfiring by monitoring the crankshaft acceleration. Misfiring causes the angular velocity of the crankshaft to drop in an angular range specific to the cylinder in question. The misfiring detection is based on the observation of this variation of segment period (720°/N CRK) for N cylinder engines.
To compute an engine roughness value, 7 continuous valid segments are required for a 6 cylinder engine.
Main causes of misfire: injector shut off, fuel pressure problems, fuel combustion problems, ignition cut off.
Strategy description
Segment period correction
The prerequisite for reliable misfire detection is accurate segment period measurement. However, the period between two TDC, at constant speed, is subject to variations due to manufacturing tolerances and off centre installation.
These inaccuracies are systematic, so they can be « learned » during fuel cut off periods and used for compensation. By this way, the systematic error introduced by the tolerances of the target wheel is largely eliminated.
Misfire Algorithm
OBD statistic flowchart
Misfiring threshold
The irregular ( misfiring ) engine roughness ER Values computed from the segment periods measured and corrected are compared to load and engine speed dependent thresholds THD_ER. If |RATE_MIS| › |THD_ER| =› a single misfire event is detected.
Threshold determination THD_ER: the threshold is influenced mainly by the engine speed and load. Other influencing parameters are the engine temperature and the target wheel learning degree, thus the threshold is corrected in cold coolant temperature conditions, and until a first flywheel learning is complete.
Fade out conditions
The misfire is monitored continuously, except in low engine speed and load:
Variation in the engine torque caused by other phenomenon than misfiring is recognised in order to inhibit misfire monitoring (fade out conditions) temporary:
- maximum air mass gradient
- maximum throttle gradient
- ignition retardation without change limitation. e.g. Gear shift
- air conditioning compressor activation
- rough road detection
- traction Control activation.
Furthermore, the detection has to be suppressed in case of:
- failure on Manifold Air Pressure sensor,
- failure on Electronic Throttle,
- failure on Crank signal sensor,
- failure on Cam signal sensor
- failure on Main Relay
- failure on Battery Voltage
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