Fig. The schematic structure of potentiometric oxygen sensor.
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: Vmax=(RT/4F)ln[Po2(air)/Po2(exh.)]
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.
The other names of potentiometric oxygen sensor for gasoline engine
1) Zirconia EGO(Exhaust Gas Oxygen) sensor
2) Zirconia HEGO(Heated Exhaust Gas Oxygen) sensor
3) Lambda Sensor
4) Oxygen sensor
The purpose of installing oxygen sensor
The fuel-rich and fuel-lean conditions are desirable in the view of engine power and fuel consumption, respectively. However, the gasoline engine with 3-way catalyst operates engine at stoichiometric point because it is the optimum point for maximum catalytic converting efficiency, that is, for minimizing pollutant emmission including Hydrocarbon, CO, and NOx. Hydrocarbon and CO are emitted at rich condition, whereas NOx is at lean one. The former two should be oxidized into H2O and CO2, whereas the other should be reduced into N2. That's why the engine should be operated at stoichiometric point. In conclusion, for reducing pollutant emissions.
Fig. The close loop control of gasoline engine using oxygen lambda sensor
What's difference between heated and non-heated sensors?
The operation temperature regime for zirconia solid electrolyte is about 400-900C. When one use the non-heated oxygen sensor, the sensor reaches operation temperatures by the heat of exhaust gas. It takes usually at least 2-3min for the operationthe from the idling. Unfortunately, about 80% of the pollutant emmsion is concentrated at this starting period when one considered 0.5-1 hour driving. It indicates that the faster operation at stoichiometric point is desirable for the less pollutant emission. The heater in oxygen sensor reduced the warm-up time for sensor operation from 2-3min to only about 30sec.
The origin for the name 'Lambda sensor'
When one plot the pollutant concentrations as a function of air/fuel ratio, The CO and HC concentrations decreased rapidly around stoichiometric point, whereas NOx did increase. This line makes the shape of Greek character lambda. That's why we call the oxygen sensor for gasoline engine as 'lambda sensor'.
Do the GDI engine and lean-burn engine also employ this potentiometric oxygen sensor?
No. The engines above operates usually at lean condition although there is also the driving at the stoichiometric point for getting engine power. These engine employ the wide range air/fuel sensor.
All the gasoline engine except the above two use this sensor?
No, it depends on country. The most of advanced country regulates the use of 3-way catalyst and lambda sensor for reducing air pollutions. However, in some countries, it is not the obligation. The 3-way catalyst deteriorates quickly by the exposure of lead component in gas. So, the unleaded fuel should be used for 3-way catalyst and lambda sensor.
Fig. The sensing principle of limiting current-type oxygen sensor
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: Ilim=(4FDo2sP/RTl)Xo2
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.
What is the advantage of this sensor in comparison to potentiometric one?
The sensing signal of this sensor is proportional to oxygen concentration, whereas that of potentiometric one is proportional to lnXo2. Therefore, the sensing of high concentration of oxygen is advantageous. On the contrary, the low Xo2 region is impossible.
The comparison between normal and Knudsen diffusion
The gas diffusion barrier is essential for constructing limiting current-type oxygen sensor. The gas diffusion mechanism changes from normal to Knudsen type when the pore size decreased. When the pore diameter is very larger than the mean free path of gas molecules, the gas diffusion does not influenced by pore. However, the collision between gas molecule and pore wall becomes frequent at very small pores. When the pore size becomes almost same order with mean free path or less than that, the Knudsen diffusion is predominant. The following table summarizes the main difference in the sensors by the normal and Knudsen diffusion. As shown in the table, the Knudsen diffusion is desirable for getting the linearity till high Xo2 and less temperature dependency. On the other hands, the normal diffusion can nullify the pressure dependency of sensing signal. The two diffusion mechanisms can be mixed in order to remove temperature dependency of signal.
Fig. The sensing principle of wide range air/fuel 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.