氧传感器诊断
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3.3.2 Oxygen sensors
Directive 98/69/EC only uses the term “oxygen sensor” when referring to this generic type of sensor. However, the colloquial trade term within the UK is λ (lambda) sensor, although some manufacturers refer to them as HEGO or UEGO sensors. In this report we use solely the term oxygen sensor.
This sensor (which is based on a zirconia oxygen cell) is used to monitor the oxygen content of the exhaust. It produces a voltage dependent on the oxygen concentration. The ECU uses this voltage as one of the parameters to control fuelling. During operation the ECU compares the voltage produced by the oxygen sensor with stored upper and lower voltage limits (rich and lean switch points). The ECU control strategy will reduce the amount of fuel delivered to the engine by the fuel injectors (forming a lean mixture) until the lean switch point is reached.
It will then increase the fuelling (forming a rich mixture) until the rich switch point is achieved, when the cycle is repeated. The voltage output of the oxygen sensor varies cyclically and this can be used for fault diagnosis. The oscillation frequency is dependent on engine speed and air mass flow (load).
Further details about this important sensor are given in Appendix 3.
EOBD systems have two oxygen sensors. The pre-catalyst sensor provides fuelling control inputs during closed loop operation and the post-catalyst sensor provides data from which catalyst buffering capabilities can be derived. The output from this sensor is also used to monitor any drift of the pre-catalyst sensor. This is achieved by the EOBD system dynamically “tuning” the voltage/oxygen concentration calibration curve for the pre-catalyst oxygen sensor as its ages, thereby compensating, to some degree, for ageing effects. Both sensors are monitored by the EOBD system for open circuit and for deterioration using diagnostic strategies summarised in Appendix 3.
Appedix 2
FURTHER TECHNICAL DETAILS ABOUT ZIRCONIA OXYGEN SENSORS
Zirconia oxygen sensors require temperatures of at least 350°C to operate. To achieve this temperature more quickly (and allow fuelling control to be under closed loop feedback control from the oxygen sensor) most sensors have heaters built into them. Another advantage of the sensors having an in-built heater is that they can be located further down exhaust stream. This improves their durability by making them less susceptible to thermal excursions within the vehicle’s exhaust. Oxygen sensor light-off times are typically about 20 seconds. There are some planar sensors in development that can light off in 10–15 sec.
Oxygen sensors control AFR to within ± 2% at steady state. Transients are not so good due to fuel film build up in the inlet manifold, which also occur when the engine is cold. Their response time is in the order of 30–50 ms.
Diagnostic strategies for the detection of oxygen sensor deterioration The ECU will monitor the oxygen sensor voltage output (and consequently the oxygen concentration in the exhaust before the catalyst) against stored values to derive diagnostic information. Some of the parameters monitored are
- Rich to lean threshold voltage
- Lean to rich threshold voltage
- Low sensor voltage for switch time calculation
- High sensor voltage for switch time calculation
- Rich to lean sensor switch time
- Lean to rich sensor switch time
- Minimum sensor voltage for test cycle
- Maximum sensor voltage for test cycle
- Time between sensor transitions
The result is averaged over a manufacturer specified number of cycles and compared with a threshold value. If this value is exceeded, a fault code is stored. If the threshold is exceeded in the next set of averaged cycles, the MIL will be illuminated.
Oxygen sensor failure and error modes.
The vehicle’s exhaust can in some fault conditions become hot enough to melt the oxygen sensor, thereby causing it to fail.
Circuit Failure.
Electrical connectors can fail in the harsh environments of the sensor and the vehicle’s wiring loom.
Sensor heater element failure.
Inadequate heating reduces the signal amplitude of the oxygen sensor and changes the voltage/oxygen concentration calibration characteristics. Consequently for a “cool” oxygen sensor the start of closed loop operation will be delayed and emissions will increase during operation. Control circuits are monitored by the ECU and compared against stored values. Test sequence values outside threshold limits will results in a fault code being recorded.
Poisoning.
Contaminated air on the reference side of the electrode, leaded fuel, and rich mixture can contaminate the Oxygen sensor. Using silicon spray during sensor installation can also cause problems. Contamination may be temporary resulting in many returned sensors performing normally when tested.
— An In-Service Emissions Test for Spark Ignition (SI) Petrol Engines – PPAD 9/107/09 Phase 2a Report Evaluation of the significance of OBD/OBM.
Lambda probe
Voltage curve shift diagnosis and adaption of the probe before the catalyst Ageing or poisoning can cause a shift in the voltage curve of the probe before the catalyst. This shift is detected by the engine control unit and can be compensated (adapted) within defined bounds.The diagnosis sequence is basically the same despite the new broadband lambda probe.
Lamdba probe heater diagnosis
By measuring the probe heating resistance, the engine control unit checks the heat output of the lamdba probe heater for correctness.
Reaction time diagnosis of probe before catalyst
The reaction time of the probe before the catalyst can also deteriorate due to ageing or poisoning.
The procedure for diagnosis of these faults was previously explained in Self-Study Programme 175. However, the signals from the probe before the catalyst have changed due to the use of broadband lambda probes. Hence, the description of this diagnosis routine with the current signals from probe before the catalyst.
Modulation of the fuel/air mixture by the engine control unit is prerequisite for reaction time diagnosis. This modulation takes the form of slight fluctuation between lean and rich mixture. It is induced artificially by the engine control unit, because the lambda value can be controlled by using the broadband lambda probe to such as high degree of accuracy that it is possible to maintain a constant value of l=1. For optimal operation, however, the catalyst requires the mixture composition to fluctuate slightly. Therefore, the engine control unit modulates this mixture when a broadband lambda probe is being used.
- The signal from the probe before the catalyst follows modulation of the fuel/air mixture by the engine control unit
- The signal from the probe before the catalyst can no longer follow modulation of the fuel/air mixture.
Diagnostic routines
Control limit diagnosis of probe after catalyst
When the fuel/air mixture is of optimal composition, the voltage of the probe after the catalyst will be in the region of l=1. If the probe after the catalyst produces a higher or lower average voltage, this indicates that the fuel/air mixture is too rich or too lean. The engine control unit therefore changes its lambda control value (this affects the fuel/air-mixture composition) until the probe after the catalyst again signals l=1. This lambda control value has defined control limits. If these control limits are exceeded, EOBD assumes that there is a fault in the probe after the catalyst or in the exhaust system (secondary air).
Lean fuel/air mixture and correct control
The probe after the catalyst signals a rise in oxygen concentration in the exhaust gas to the engine
control unit through a voltage reduction. The engine control unit then increases the lambda control value,
and the fuel/air mixture is enriched. The voltage of the probe after the catalyst rises and the engine
control unit is again able to reduce the lambda control value. This control loop extends over a
lengthy vehicle operating period.
Lean fuel/air mixture and reaching of control limit value
In this case, too, the probe after the catalyst signals a rise in oxygen concentration in the exhaust gas to the engine control unit through a voltage reduction. The engine control unit then increases the lambda
control value, and the fuel/air mixture is enriched. Despite this enrichment of the fuel/air mixture, the
probe voltage remains low (due to the fault) and the engine control unit continues to increase the lambda
control value until the control limit is reached and the fault is detected.
Motion diagnosis of probe after catalyst
The operating performance of the probe after the catalyst is monitored also. To this end, the engine control unit checks the signals from the probe in acceleration and overrun modes. When the vehicle is accelerating, the fuel/air mixture is rich, the oxygen concentration in the exhaust gas decreases and the probe voltage must rise. In overrun mode, the exact opposite applies: fuel feed is off, the oxygen concentration in the exhaust gas increases and the probe voltage must drop. If the probe after the catalyst does not react as expected, the engine control unit assumes that the probe after the catalyst is defective.
— “Euro On-Board Diagnostic System For petrol engines Self-Study Programme 231″, © VOLKSWAGEN AG, Wolfsburg
