Component description of the O2 sensor - GF07.04-P-6100KE
ENGINE 271.921 /940 /941 /944 /946 /948 /955 /956
Arrangement
The O2 sensor upstream of the catalytic converter is located upstream of the firewall catalytic converter and the O2 sensor downstream of the catalytic converter is located downstream of the firewall catalytic converter.
Task
The O2 detect the residual oxygen share in the exhaust and transmit corresponding voltage signals to the ME control unit (N3/10).
O 2 sensor upstream of TWC (slave sensor):
- Closed-loop mixture control
- Self-adaptation of mixture formation
- Function chain test
O 2 sensor downstream of TWC (master sensor):
- Two-sensor control
- Monitoring of the catalytic converter effect
Design
The oxygen sensor are voltage free insulated planar (flat) oxygen sensors.
The active sensor ceramic consists of a gas permeable ceramic body made of zirconium dioxide. A protective tube with several slots protects the ceramic body from mechanical stresses and from temperature jumps.
Function of the wideband oxygen sensor (schematic)
The sensor element consists of a combination of a Nernst concentration cell (sensor cell) and an oxygen pump cell which transports (pumps) the oxygen ions.
The pump action is a purely physical process.
When operating in combination with the control electronics which are housed in the ME control unit, the wideband oxygen sensor can be measured exactly in a further lambda range (0.7< λ <4.0).
Sensor heater for a wideband oxygen sensor
An integrated sensor heater ensures that it remains at the required operating temperature of approx. 750°C. The O2 sensor is heated constantly when the engine is operated in order to guarantee sensor functions when the engine is operated. The sensor heater is actuated by the ME control unit through a ground signal.
Functional principle (signal voltage) of the discrete oxygen sensor
The signal voltage hat has a steep voltage jump (=1) at the transition from a rich to a lean mixture (λ =1). This property is utilized for the lambda closed-loop control.
Function of the discreet oxygen sensor (schematic)
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 voltage (signal voltage) is the measure for the residual oxygen content in the exhaust.
The electronics in the ME control unit give a so-called sensor back voltage of about ca. 450 mV at the oxygen sensor. When the oxygen sensor the internal resistance of the sensor is so high that the sensor voltage is initially equal to the back voltage irrespective of the mixture composition (λ).
The sensor back voltage at the ME control unit can be measured to the sensor ground if the oxygen sensor is disconnected.
Sensor heater for a discrete oxygen sensor
The oxygen sensor is heated in order to bring the ceramic probe body rapidly up to operating temperature. The sensor heater is actuated by the ME control unit through a ground signal. The heater current in the cold state is increased approximately by the factor 4.
The sensor heater is switched off at coolant temperatures below approx. 20°C and at high engine speeds in order to avoid overheating (thermo shock).