Operation

OPERATION

The O2 sensor is a planar zirconium dioxide (ZrO2) dual cell limiting current probe with a integral heater. The term wide ban, refers to the ability of the O2 sensor to generate a clear signal over a wide air-fuel ratio measuring range. As a dual sensor, it incorporates a second O2 chamber (oxygen pump cell), which requires a separate voltage supply.

The sensor element combines a sensor cell (8) and an oxygen pump cell (9). Both cells are made of zirconium-dioxide (ZrO2) and are coated with porous platinum electrodes. The sensor cell operates just like a typical O2 sensor. The oxygen pump cell transport oxygen ions when voltage is applied.

A gas sample chamber (5) is sandwiched between the oxygen pump cell and the sensor cell. A pump electrode and sensor cell electrode are located in the sample chamber. A sample passage (10) connects the sample chamber to the surrounding exhaust gas. A sensor cell electrode is located in the reference air channel (6), which connects to the outside air.





At high temperatures, certain ceramic materials, such as zirconium-dioxide (ZrO2) become oxygen ion conductors. In a typical O2 sensor, the ZrO2 is used as a solid electrolyte, which conducts oxygen ions. The solid electrolyte is sandwiched between two platinum electrodes. The sensor generates a small voltage when oxygen moves from the high concentration side to the low concentration side.

The same hold true if the process is reversed. If voltage is applied to the platinum electrodes, oxygen can be pumped from one side of the solid electrolyte to the other (from cathode to anode), becoming an oxygen pump. The amount of current flow is directly proportional to the amount of oxygen pumped by the sensor. When the oxygen level on the supply side reaches zero, the current stops.

The ECM activates the integral heater element to raise the temperature of the sensor to 700C (192°F) for the ZrO2 to become conductive. The heater element is designed to reach this temperature within 8 seconds and maintaining it at this level. In cold temperatures, this can be delayed up to 5 minutes to prevent damaging the ceramic coating of the sensor from water condensation. Once the sensor is heated, the exhaust gas components diffuse through the gas sample chamber. Upon reaching the electrodes on the oxygen pump and concentration cells they reach state of thermodynamic balance.





The sensor cell measures the difference between the oxygen concentration in the gas sample chamber and the oxygen concentration in the outside air from the reference air channel. A small voltage is generated across the sensor, which is proportional to the air-fuel ratio in the sample chamber. At stoichiometric ratio (14.7 lbs. of dry air to 1 part fuel), the corresponding open circuit voltage at the sensor cell is 450 mV. If the stoichiometric ratio in the sampler chamber is higher than 1 (excess air) a lower voltage is produced. If the stoichiometric ratio is lower than 1 (insufficient air) a higher voltage is produced.

The ECM uses this voltage signal to determine how and when to run the oxygen pump cell. The goal of the ECM is to modulate the pumping current through the pump cell to always maintain stoichiometric air-fuel ratio (14.7 to 1) in the gas sample chamber. When stoichiometry is reached, there is no current flowing to the oxygen pump.

High Excess Air Mode
When the exhaust gas is too lean, the oxygen concentration in the gas sample chamber is high. The sensor cell measures the difference between the oxygen concentrations in the sampler chamber and the reference air channel. A voltage lower than 450mV is generated across the sensor cell, which is proportional to the air-fuel ratio in the sample chamber. The ECM compares the sensor cell voltage with the reference voltage (V Ref), which corresponds to the stoichiometric point voltage. Since sensor cell voltage is lower than V Ref, the ECM determines a lean condition exists. Am amplifier applies an appropriate voltage to the pump cell to transfer oxygen from the gas sample chamber

Low Excess Air Mode
With low excess air mode, the oxygen concentration in the gas sample chamber is low. The sensor cell measures the difference between oxygen concentrations in the gas sample chamber and the reference air channel. A voltage higher than 450 mV is generated across the sensor cell, which is proportional to the air-fuel ratio in the sample chamber. The ECM determines a low excess air condition exists. The polarity of the pump cell is reversed and so is the direction of the current flow.