Component description for a O2 sensor - GF07.04-P-6100SM
ENGINE 113.986
ENGINE 113.987
ENGINE 113.988
ENGINE 113.989
Task
The O2 sensors detect levels of residual oxygen in the exhaust and transmit corresponding voltage signals to the ME control unit (N3/10).
O 2 sensors upstream of the catalytic converter (control sensors):
- Lambda control
- Self adaptation of the mixture formation
- Function chain test
O 2 sensors downstream of the catalytic converter (master sensors):
- Two-sensor control
- Monitoring three way catalytic converter effect
Design
The sensors used are potential-free insulated O2 planar sensors.
The active sensor ceramic consists of a gas-permeable ceramic body manufactured from zirconium dioxide. A protective tube with several slots protects the ceramic body from mechanical stresses and temperature jumps.
They are connected electrically by means of a 4-pin connector. The ground cable for the sensor signal is routed individually up to the ME control unit for each O2 sensor.
Functional principle
Function
The sensor ceramic is conductive for oxygen ions from approx. 300°C.
If the oxygen concentration on both sides of the sensor ceramic differs, a voltage is produced at the boundary surfaces as a result of the particular properties of the sensor ceramic (Nernst voltage). This Nernst voltage produces the O2 sensor signal which is a measure of the residual oxygen in the exhaust gas.
The analysis circuit for the O2 sensor signal (in the ME control unit) produces a so-called sensor back-voltage of about 450 mV on the O2 sensor.
If the O2 sensor is cold, the sensor internal resistance is so high that the O2 sensor voltage is initially the same as the back voltage irrespective of the mixture composition.
The back-voltage at the ME control unit can be measured to the ground for the O2 sensor signal when the O2 sensor is disconnected.
O 2 sensor heater
The O2 sensors are heated in order to rapidly warm up the sensor ceramic to operating temperature. The sensor heater is actuated by the ME control unit over a ground signal. The current in the cold state is increased approximately by a factor of 4.
The sensor heater is switched off at coolant temperatures below approx. 20°C and at high engine speeds in order to avoid overheating (thermal shock).
O 2 sensor voltage
The O2 sensor signal experiences a steep voltage jump at the transition from a rich to a lean mixture (λ=1). This property is utilized for the lambda closed-loop control.
Curve of the O 2 sensor voltage (shown schematically)
Faults on the O2 sensor can be detected by means of the O2 sensor signal.
Defined limit values must be maintained for the O2 sensor voltage, the cycle duration and the sensor status change.