Exhaust sensors

Figure 6.16 Car exhaust sensor (schematic) fitted with a stabilized zirconia ceramic tube as electrolyte.

Fig. 4.37 Form of a lambda automobile exhaust sensor (the forms may vary from model to model but the essentials are the same).

Application time and storage time are important for the organization of the replacement of exhausted sensors to handle the supply. 

The range of sample characteristics and manner of their detection, is much larger than can be realistically addressed in the space of a single chapter. We will confine this chapter mainly to the chemical sensor research areas discussed in other chapters in this volume, dividing them into electrical, optical, and mass and thermal measurements. Our focus will furthermore be on the generic chemical and physical phenomena upon which such measurements can be based, as opposed to the alternative organization that would address chemical sensors in the context of their application (i.e, auto exhaust sensor, clinical diagnostic sensor, environmental sensor) or of the kinds of samples detected (i.e, CO sensors, humidity sensor, biosensor, etc.), as used in a previous ACS Symposium Series volume on Chemical Sensors (D. Schuetzle, R. Hammerle, Eds., ACS Sympos. Ser. 309, 1986). 

Fig. 2.34 Reductant dosing control scheme for transient systems with high NO -conversion including exhaust sensors and catalyst model...

Exhaust gas temperature sensor Catalytic substrate Heat insulator... 

In 1957, Ethyl Corp. announced anew antiknock compound, methylcyclopentadienyknanganese tricarbonyl [12108-13-3] (MMT). MMT is almost as effective as lead on a per gram of metal basis, but because manganese was more expensive than lead, MMT was not widely used until limits were placed on the lead content of gasoline. MMT was used in unleaded fuel between 1975 and 1978. After a large fleet test suggested that MMT could increase exhaust emissions because it interfered with catalysts and oxygen sensors, EPA banned its use in unleaded fuel in 1978. MMT is used in Canada in unleaded fuel. 

Electrochemical Microsensors. The most successful chemical microsensor in use as of the mid-1990s is the oxygen sensor found in the exhaust system of almost all modem automobiles (see Exhaust control, automotive). It is an electrochemical sensor that uses a soHd electrolyte, often doped Zr02, as an oxygen ion conductor. The sensor exemplifies many of the properties considered desirable for all chemical microsensors. It works in a process-control situation and has very fast (- 100 ms) response time for feedback control. It is relatively inexpensive because it is designed specifically for one task and is mass-produced. It is relatively immune to other chemical species found in exhaust that could act as interferants. It performs in a very hostile environment and is reHable over a long period of time (36). 

The success of the O2 sensor has made the auto manufacturers, regulators, and environmentalists anxious to extend chemical sensing to a variety of tailpipe gases, notably CO, NO, and short-chain hydrocarbons. Considerable research and development is needed for these molecules to be monitored in the hostile exhaust system environment (36). 

The doped Zr02 stmctures are used as electrochemical sensors, as, for example, when used to detect oxygen in automotive exhaust (see Exhaust CONTROL, automotive). The sensor voltage is governed by the Nemst equation (eq. 17) where the activities are replaced by oxygen partial pressures and the air inside the chamber is used as reference. 

In an actual exhaust system controlled by the signal of the oxygen sensor, stoichiometry is never maintained, rather, it cycles periodically rich and lean one to three times per second, ie, one-half of the time there is too much oxygen and one-half of the time there is too Httle. Incorporation of cerium oxide or other oxygen storage components solves this problem. The ceria adsorbs O2 that would otherwise escape during the lean half cycle, and during the rich half cycle the CO reacts with the adsorbed O2 (32,44,59—63). The TWC catalyst effectiveness is dependent on the use of Rh to reduce NO and... 

The function of the oxygen sensor and the closed loop fuel metering system is to maintain the air and fuel mixture at the stoichiometric condition as it passes into the engine for combustion ie, there should be no excess air or excess fuel. The main purpose is to permit the TWC catalyst to operate effectively to control HC, CO, and NO emissions. The oxygen sensor is located in the exhaust system ahead of the catalyst so that it is exposed to the exhaust of aU cylinders (see Fig. 4). The sensor analyzes the combustion event after it happens. Therefore, the system is sometimes caUed a closed loop feedback system. There is an inherent time delay in such a system and thus the system is constandy correcting the air/fuel mixture cycles around the stoichiometric control point rather than maintaining a desired air/fuel mixture. 

Fig. 12. Schematic of lambda sensor (a) in exhaust pipe and (b) internal components.

J. W. Butier and co-workers, FastKesponse Zirconia Sensor-Fased Instrument for Measurement of the Air (Fuel Katio of Combustion Exhaust, SAE 840061, Society of Automotive Engineers, Warrendale, Pa., 1984. 

It is important to have the most accurate measurement of exhaust temperature attainable. Note that Fig. 8-55 shows the sensor inserted into the diyer upstream of the rotating seal, because leakage there could cause the temperature in the exhaust duc t to read low—even lower than the wet-bulb temperature, an impossibility without leakage of either heat or outside air. 

The paper describes the different chemical sensors and mathematical methods applied and presents the review of electronic tongue application for quantitative analysis (heavy metals and other impurities in river water, uranium in former mines, metal impurities in exhaust gases, ets) and for classification and taste determination of some beverages (coffee, bear, juice, wines), vegetable oil, milk, etc. [1]. 

Electrochemistry plays an important role in the large domain of. sensors, especially for gas analysis, that turn the chemical concentration of a gas component into an electrical signal. The longest-established sensors of this kind depend on superionic conductors, notably stabilised zirconia. The most important is probably the oxygen sensor used for analysing automobile exhaust gases (Figure 11.10). The space on one side of a solid-oxide electrolyte is filled with the gas to be analysed, the other side... 

Figure 11.10. Gas sensor to monitor oxygen content of exhaust gases from automobile engines...

Sensors TS 1-2-4 regulate the batteries for heating and cooling in a se quence to achieve the required temperatures (Fig. 9.56). Regulating valves for heat recovery are controlled by a frequency converter RCl for the pump motor. When a greater output is required from the heating battery, the pump motor speed increases before the valve MV2 opens. If the extract temperature is lower than the outdoor temperature, the speed of the pump motor increases before valve MVl opens. To avoid ice formation at low outdoor temperatures, the sensor TS7 operates on a lower limit, depending on the demands of the battery in the exhaust. 

Variable Air Volume Fume Cupboards This type of cupboard incorporates a variable air volume (VAV) controller that regulates the amount of air exhausted from the cupboard such that the face velocity remains essentially constant irrespective of the sash position. A sensor detects either the sash position, the pressure differential l>etween the fume cupboard interior and the room, or the vekxity at some point in the cupboard. This information is used to control either the exhaust fan speed or the position of a control damper. The supply air volume flow rate into the laboratory or workspace should also be regulated. It should be remembered that with the sash in the closed position the amount of air to dilute contaminants in both the fume cupboard and the laboratory is reduced and that there could, for example, be difficulty in reducing contaminant levels below the lower exphasive level. 

Type B1 cabinets must be hard-ducted, preferably to their own dedicated exhaust system, or to a properly designed laboratory building exhaust. Blowers on laboratory exhaust systems should be located at the terminal end of the duct work. A failure in the building exhaust system may not be apparent to the user, as the supply blowers in the cabinet will continue to operate. A pressure-dependent monitor should be installed to sound an alarm and shut off the BSC supply fan, should failure in exhaust airflow occur. Since this feature is not supplied by all cabinet manufacturers, it is prudent to install a sensor in the exhaust system as necessary. To maintain critical operations, laboratories using Type B1 BSCs should connect the exhaust blow er to the emergency power supply. 

Air is normally the reference gas used in the exhaust gas sensor. If the oxygen partial pressure in the engine exhaust gas is known as a function of the engine air/fuel ratio, the theoretical galvanic potential of the sensor is easily determined by the Nernst equation. 

Application The zirconia oxygen sensor is widely used for combustion control processes and for air/fuel ratio regulation in internal combustion engines. The closed-end portion of the electrode tube is inserted into the exhaust gas stream. In the control of industrial combustion processes, no out stack sampling system is required. 

Eddy, David S. Physical Principles of the Zircoiiia Exhaust Gas Sensor, IEEE Trans.Vehicniar Technol. VT-23 (1974), pp. 125-128. 

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