O2 sensors, component description engine 272 - GF07.04-P-6100V
ENGINE 272.942 in MODEL 171
ENGINE 272.963 in MODEL 171
Shown on engine 272.963
O 2 sensors upstream TWC, location
The O2 sensors upstream TWC are bolted into the exhaust pipe upstream of the catalytic convertors.
Task of the O 2 sensors upstream TWC
The O2 sensors upstream TWC (control sensors) are designed as wideband oxygen sensors and detect the residual oxygen content in the exhaust gas for the following tasks:
- Lambda control
- Self-adaptation of mixture formation
- Function chain tests.
O 2 sensors upstream TWC, task
The balance resistor (6) is dependent on the overall length (L) and on the type of sensor.
O 2 sensors upstream TWC, function
While changing the wideband oxygen sensor, the balance resistor is already fitted on the new sensor and must not be changed (resistance depends on the sensor).
The broadband oxygen sensor is a planar two-cell limit current sensor which as a result of its modular design integrates several functions.
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.
The sensor is able to measure accurately not only at λ = 1, but also in the lean and in the rich range.
In combination with the control electronics integrated in the ME-SFI control unit, it supplies within a wide lambda range an unequivocal signal within a wide lambda range (0.7 < λ < 4.0).
In the case of the oxygen pump cell oxygen ions are "pumped" from the cathode to the anode when an electric voltage is applied to the zirconium dioxide ceramic.
The oxygen pump cell and the Nernst concentration cell are arranged in such a way as to produce a diffusion gap of about 10 -50 μm between them.
Two porous platinum electrodes, a pump electrode and a Nernst measuring electrode are located within this gap.
The diffusion gap is connected to the exhaust gas by means of a gas inlet hole. The porous diffusion barrier in this case limits the extent to which the oxygen molecules are able to continue flowing.
As a result of this, provided the pump voltage is sufficient, a limit current is produced which is proportional to the oxygen concentration in the exhaust gas. The reference electrode of the Nernst concentration cell is exposed to the ambient air by means of an opening in the reference air passage. The components of the exhaust gas diffuse through the diffusion gap to the electrodes of the oxygen pump cell and of the Nernst concentration cell where they are brought into a thermodynamic equilibrium.
Sensor signal of pump flow
The electronic circuit in the ME-SFI control unit controls the pump flow through the oxygen pump cell in such a way as to maintain a constant composition of the gas in the diffusion gap λ = 1.
This corresponds to a voltage at the Nernst concentration cell of U N = 450 mV.
The current required to do this (pump flow), is converted by the ME-SFI control unit into a lambda value.
If the exhaust gas is "lean", U N < 450 mV and the pump cell is actuated in such a way that oxygen is pumped out of the diffusion gap.
if the exhaust gas is rich, U N > 450 mV and the direction of flow is reversed in such a way that the pump cell pumps oxygen into the diffusion gap.
Based on the law of diffusion the pump flow in this case is proportional to the oxygen concentration in the exhaust.
An integrated sensor heating system ensures that it remains at the required operating temperature of approx. 750 °C. To ensure the sensor function during engine operation, the broadband sensor is heated constantly.
The temperature control and temperature measurement (by measuring the internal resistance) is performed by the ME-SFI control unit with an internal control electronics.
As a result of its wide control range and its constant sensor characteristic, the wideband oxygen sensor can be used for lambda control of < 0.7 and λ < 4. It therefore also influences leaner and richer air-fuel mixture compositions, for example during deceleration fuel shutoff or when the engine is warming up.
With these characteristics, these sensors are also suitable for lambda control of gasoline engine lean-burn concepts, diesel engines and gas engines.