Electronic oxygen sensor

EGR can seriously degrade engine performance, especially at idle, under load at low speed, and during cold start. Control of the amount of EGR during these phases can be accompHshed by the same electronic computer controller used in the closed loop oxygen sensor TWC system. Thus the desired NO reduction is achieved while at the same time retaining good driveabiUty. 

Tends to burn visually cleaner at the exliaust tailpipe, since it operates in a closed-loop electronic mode (oxygen sensors interacting with the powertrain control module) to maintain an ideal air/fucl ratio of 14.7 1. 

Such tight mixture control is beyond the capability of the traditional carburetor. Consequently, after sorting through a number of alternatives, industry has settled on closed-loop-controlled port-fuel injection. Typically, an electronically controlled fuel injector is mounted in the intake port to each cylinder. A sensor in the air intake system tells an onboard computer what the airdow rate is, and the computer tells the fuel injectors how much fuel to inject for a stoichiometric ratio. An oxygen sensor checks the oxygen content in the exliaust stream and tells the computer to make a correction if the air/fuel ratio has drifted outside the desired range. This closed-loop control avoids unnecessary use ot an inefficient rich mixture during vehicle cruise. 

When a solid electrolyte component is interfaced with two electronically conducting (e.g. metal) films (electrodes) a solid electrolyte galvanic cell is formed (Fig. 3.3). Cells of this type with YSZ solid electrolyte are used as oxygen sensors.8 The potential difference U R that develops spontaneously between the two electrodes (W and R designate working and reference electrode, respectively) is given by ... 

Electrolytes, oxygen sensors, fuel cells, electronic conduction coatings, and furnace elements. 

In recent years further concepts have been developed for the construction of polymer-based diodes, requiring either two conjugated polymers (PA and poly(A-methyl-pyrrole) 2 > or poly(A-methylpyrrole in a p-type silicon wafer solid-state field-effect transistor By modifying the transistor switching, these electronic devices can also be employed as pH-sensitive chemical sensors or as hydrogen or oxygen sensors 221) in aqueous solutions. Recently a PPy alcohol sensor has also been reported 222). 

In general Zr02 oxygen sensors consist of a tube-like solid-state Zr02 electrolyte where the electronic conductivity is based on oxygen ion charge carrier transport. The inner and outer surface of the yttrium-doped and stabilized zirconia tube is covered by porous platinum electrodes. 

While a number of designs have been used, most oxygen sensors for automotive applications consist of a hollow, closed end tube, a schematic of which is shown in Figure 2. As shown, the interior of the closed end tube is open to the atmosphere which serves as a constant or reference oxygen partial pressure while the exterior is exposed to the exhaust gas. The voltage signal produced by the electrolyte is sensed by electrodes on the inner and outer surface of the sensor. These, in turn, are connected to the electronics package of the closed loop system. 

Figure 6 is a schematic of a closed loop system. It consists basically of an oxygen sensor to monitor the exhaust air-fuel ratio, a "black box" electronic control system, a carburetor or fuel injector which is controlled and adjusted by the "black box" and, finally, a three-way or dual bed converter. The signal from the oxygen sensor is monitored continuously by the electronics package which then adjusts the carburetor or fuel injector to control the air-fuel ratio at stoichiometric. 

As mentioned earlier, the oxidation of carbon monoxide and hydrocarbons should be achieved simultaneously with the reduction of nitrogen oxides. However, the first reaction needs oxygen in excess, whereas the second one needs a mixture (fuel-oxygen) rich in fuel. The solution was found with the development of an oxygen sensor placed at exhaust emissions, which would set the air-to-fuel ratio at the desired value in real time. So, the combination of electronics and catalysis and the progress in these fields led to better control of the exhaust emissions from automotive vehicles. 

Fig. 5. Electron micrograph of a microcontainment oxygen sensor (source ICB)...

Sridhar, K.R. and Blanchard, J.A. (1999) Electronic conduction in low oxygen partial pressure measurements using an amperometric zirconia oxygen sensor. Sens. Actuators B, 59, 60-7. 

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