Oxygen and Air-to-Fuel Ratio Sensors for Automobiles
1) Potentiometric Oxygen Sensor(want more detailed information?)
When a Pt/YSZ/Pt cell with a U-shape lies between the two different oxygen partial pressures, one can get the electromotive forces(Vmax) across the YSZ as follows: where R, T, F, Po2(air), and Po2(exh.) are gas constant, absolute temperature, Faraday constant, oxygen partial pressures at the air and exhaust gas, respectively. The e.m.f. of this sensor changes abruptly at the stoichiometric point. Therefore, this sensor has been widely used for the stoichiometric control of an automobile engine.

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2) Limiting Current-Type Oxygen Sensor(want more detailed information?)
The oxygen concentration of a gas can be determined by the limiting current of a electrochemical pumping cell witha diffusion barrier. This sensor is called as 'Limiting current-type oxygen sensor'. The figure shows the principle of this amperometric sensor. In the low voltage region, the oxygen pumping amount is less than the gas diffusion from the ambient toward the cathode.(region 1) However, the oxygen pumping amount is limited by the gas diffusion above the certain voltage.(region 2) This current is called as 'limiting current'(Ilim) and can be expressed as follows assuming the Knudsen type gas diffusion: where Do2, s, P, l, and Xo2 are the diffusion coefficient of oxygen, the cross-sectional area of the pore, the pressure of the gas, the length of the pore and molar fraction of ambient oxygen, respectively. Therefore, one can get the limiting currents proportional to the oxygen concentrations.

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3) Wide Range Air/Fuel Ratio Sensor

The wide range air/fuel ratio sensor can be set up by combining the potentiometric and limting current sensors. The current of the pumping cell was controlled for getting the constant e.m.f.(generally about 450mV, that is, Po2 at cavity is about 10-10) at the sensing cell by feedback algorithm. In the fuel-lean region, the oxygen concentration at the ambient and cavity is relatively high. Therefore, the oxygen should be pumped from the cavity to the ambient to get Po2(cavity)=10-10. And the oxygen pumping amount increases with A/F ratio because the oxygen concentration is higher at the more lean A/F region. On the other hand, in the fuel region, the oxygen should be pumped toward the cavity. And the Po2=10-10 is attained by the oxidation reaction between the reducing gases at the cavity and oxygen pumped from 1 to 2. So the oxygen pumping amount increases with decreasing the A/F ratio because the more oxygen is needed in the oxidation reaction with the reducing gases(CO, H2, and CmHn) at the richer region. Therefore, the A/F ratio can be measures by the amount and direction of the oxygen pumping to get the constant e.m.f. across the sensing cell.

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