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IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 50, NO. 6, DECEMBER 2001 1543

Automotive Exhaust Gas Sensing SystemsJ. H. Visser, Member, IEEE,and R. E. Soltis

Abstract—Gas sensors have become an integral component ofcontrol systems for internal combustion engines to provide infor-mation for feedback control of air-to-fuel ratio (A/F) to achieveimproved vehicle performance and fuel economy as well as de-creased levels of emission. Increasingly stringent limits on evap-orative emissions as well as the requirement of having on-boarddiagnostics (OBD), which includes catalyst monitoring, necessitatethe monitoring of exhaust gas constituents [i.e., carbon monoxide(CO), hydrocarbons (HCs), and oxides of nitrogen (NO)]. Thedifferent sensing requirements, testing procedures, environmentalparameters, and need for microsystem-based realizations are dis-cussed.

Index Terms—Automotive, gas sensors, sensing systems.

I. INTRODUCTION

M OTOR vehicles have two main types of emissions: 1) ex-haust gas emissions from the tailpipe and 2) evaporative

emissions from the fuel system (the latter are not discussed inthis paper). Worldwide, legislation for both types of emissionshas become increasingly stringent. This trend to lower emis-sions is expected to continue in the foreseeable future. In recentyears, starting in California, vehicles are also required to haveon-board diagnostic (OBD) systems. These OBD systems mustmonitor the performance of a number of emissions-related ve-hicle components and systems.

Since the first emissions standards for motor vehicleswere introduced (in the mid-1960s in California), the tailpipeemissions of gasoline-powered vehicles sold in the U.S. marketbetween 1983 and 1992 have dropped 96% for hydrocarbons(HCs) and carbon monoxide (CO) and 76% for oxides ofnitrogen (NO ), as compared to the uncontrolled levels. Thesereductions are made possible through the application of severaladvanced technologies including electronic fuel injectionsystems, on-board computers, catalytic converters, exhaustgas recirculation (EGR), and feedback control systems (basedon the oxygen sensor) for metering air and fuel mixture [1].Further reductions in emissions, being phased in over a numberof years, have been legislated in the U.S., Europe, and otherparts of the world.

Currently, the only exhaust gas sensor used on vehicles is theoxygen sensor, which has been used universally in vehicles formore than 15 years. The primary function of this sensor is forfeedback control of the air-to-fuel ratio (A/F) to maintain thegasoline/air mixture close to stoichiometry in order to minimizeemissions. This control is necessary due to the fact that the ef-ficiency of the three-way-catalyst (TWC) on the vehicle is verysharply peaked near the stoichiometric A/F.

Manuscript received March 1, 2001; revised September 4, 2001.The authors are with the Ford Research Laboratory, Dearborn, MI 48121

USA.Publisher Item Identifier S 0018-9456(01)10928-9.

II. EXHAUST GAS ENVIRONMENT

The tailpipe emissions are present only when the vehicle isoperated (as opposed to the evaporative emissions) and the un-desirable emissions can be divided into four main categories:

1) well over a hundred different species of HCs (includingoxygenated “HC,” such as aldehydes and ketones);

2) carbon monoxide (CO);3) oxides of nitrogen (NO);4) particulates (mainly from diesel engines).

In most parts of the world, emissions standards for these cat-egories exist (and are becoming increasingly stringent) or areexpected in the near future. In addition, carbon dioxide as a“greenhouse gas” has come under scrutiny as well. Improve-ments in fuel economy will reduce carbon dioxide emissions.Typical exhaust gas temperatures can attain values as high as1000 C during high speed and load operations. Many corro-sive compounds also can be found in the exhaust gas stream in-cluding those containing phosphorus, sulfur, silicates, and leaddepending upon the particular fuel and oil used in the vehicle.Water vapor is present in the exhaust gas in very high concen-trations as well.

Exhaust gas sensors must be able to tolerate extreme coldtemperatures approaching40 C. Ideally, these sensorsshould have infinitely short start-up times so that they areready to function as soon as the ignition switch is turnedon. The typical minimum operating temperature of currentproduction-type zirconia-(ZrO)-based oxygen sensors isapproximately 350–400 C. In order to rapidly achieve thesetemperatures, the sensors must be capable of withstanding thestresses associated with large temperature gradients. Exhaustgas sensors are expected to operate in this harsh temperatureand chemical environment for up to 150 000 miles without anymaintenance or calibration.

III. T HE FEDERAL TEST PROCEDURE(FTP)

The emissions standards in the U.S. are expressed in g/mileas measured over the federal test procedure (FTP). The FTP isa drive cycle that simulates an average trip in a city area and in-cludes cold start, several accelerations and decelerations, idling,and cruises. It is carried out on a chassis-dynamometer underwell-controlled, standardized conditions. A speed trace of anactual FTP is shown in Fig. 1 and shows that the FTP consistsof three (driving) parts: the cold phase (0–505 s), the stabilizedphase (505–1372 s), a 10-min break with the engine off, andthe hot phase (1972–2477 s, a repeat of the 505 s cold phasewith a warmed-up engine). Other countries have either adaptedthis U.S. test cycle or devised their own. As exhaust gas emis-sions have come down considerably over the years, the relativecontribution of the “cold start” (approximately the first minute)

0018–9456/01$10.00 © 2001 IEEE

1544 IEEE TRANSACTIONS ON INSTRUMENTATION AND MEASUREMENT, VOL. 50, NO. 6, DECEMBER 2001

Fig. 1. Measured speed in miles per hour (mi/h) during an FTP.

has increased. In this portion of the test, the catalyst is not yetfunctional, because it has not heated up to its ideal operatingtemperature of approximately 400–800C.

The exhaust gases produced during the FTP test are collectedin three bags using the so-called constant-volume samplingmethod [1]. In this procedure, the exhaust gas is greatlydiluted with air to prevent condensation of water vapor (andwater-soluble emissions) and inhibit reactions between emis-sions. Subsequent analysis is carried out to determine the totalemissions mass in each three bags and a weighted averageemissions mass (over the entire test) in g/mile is obtained.

It is also possible to selectively obtain second-by-secondvolume concentrations of (total) HCs [by using a flame ioniza-tion detector (FID)], CO (by using an infrared detector), andNO (by using a detector based on chemiluminescence). TheFID measures (in good approximation) the concentration ofcarbon atoms from HCs. In this way a mix of more than 100different HCs can be represented by one number, usually ex-pressed in ppmCor ppmC . As an example, a concentration of30 ppm pentane would result in a reading of 150 ppmCor 50ppmC . All legislation with respect to exhaust gas HCs is basedon measuring HCs with an FID. Most exhaust gas HC sensorswill not have an output proportional to the concentration ofcarbon atoms. Therefore, an additional task is to relate thesensor output with the FID output. The presence of oxygenated“HCs” will reduce the accuracy of the measurement.

IV. ENGINE CONTROL SYSTEM OVERVIEW

The electronic engine control system utilizes input from sev-eral sensors to obtain improved vehicle performance and fueleconomy, as well as decreased levels of harmful emissions. Theprimary engine parameters, which are managed by the elec-tronic engine control system, include the amount of fuel, timingof the spark (ignition), and the use of EGR. In modern auto-mobiles, electronic fuel injectors precisely meter-out the appro-priate amount of fuel while a mass-air flow meter is used togauge the air flow into the combustion chamber. Unfortunately,variability in the fuel injectors and mass-air flow sensor, as wellas difficulty to ensure complete vaporization and even mixing ofthe fuel itself do not allow for precise control of the air-to-fuelmixture without some type of feedback control mechanism. Thetypical emission control system on current production vehiclespowered by gasoline-fueled internal combustion engines uti-lizes a ZrO-based electrochemical cell as an oxygen sensor to

provide the sole source of feedback. Most emissions control sys-tems currently employ two heated exhaust gas oxygen (HEGO)sensors along with a TWC to minimize emissions. EGR is alsoused to lower the temperature during the combustion process,thereby reducing the levels of NOproduced.

The most critical element of the emission control system ar-guably may be the catalyst. The early catalysts which were in-troduced on vehicles in the mid-1970s were simple oxidationcatalysts consisting of precious metal coated beads or mono-liths. These catalysts were designed to lower the amount of COand HCs coming out of the tailpipe by simply combusting thesespecies with secondary air injected into the catalyst. When gov-ernment regulators began to place limits on the amount of NOthat automobiles could emit, EGR was initially used to controlNO levels. As stricter legislation was drafted to reduce NOemissions levels to 2.0 g/mile, new technology in the form ofthe TWC was developed to meet this demand. The TWC has thecapability for both the oxidation of combustible species, suchas CO and HCs, as well as dissociation of the smog-formingNO . It utilizes precious metal catalytic material such as plat-inum and/or palladium along with an oxygen storage medium toreact the combustibles. Rhodium is typically used to dissociateNO into nitrogen and oxygen molecules. Although the TWCis highly effective at removing these pollutants from the exhauststream, the efficiency of this type of catalyst falls off rapidly asthe A/F deviates from the stoichiometric value. Fig. 2 shows theefficiency of a typical catalyst for the various emission compo-nents as a function of A/F.

In order to maintain the A/F near stoichiometry, an oxygensensor mounted in the exhaust gas is used to provide informa-tion for feedback control. The ZrO-based electrochemical cellused for oxygen sensing relies on the fact that, at thermody-namic equilibrium and for a given temperature, there is a directrelationship between the oxygen partial pressure in the exhaustgas and the A/F. Fig. 3 shows the relation between the oxygenpartial pressure in the exhaust gas and the A/F. It is evident thatthere is a large and abrupt change in the equilibrium oxygenpartial pressure at the stoichiometric A/F which can be used todetect and control the A/F at its stoichiometric value.

Most engine control systems utilize a limit-cycle type of feed-back control, given the switching nature of the ZrOoxygensensor. A typical control system will switch the A/F both richand lean around the stoichiometric value, attempting to mini-mize the magnitude of the deviations. During the lean excur-sions, the oxygen storage component of the catalyst, ceria, forexample, takes up oxygen from the exhaust gas. Conversely,

VISSER AND SOLTIS: AUTOMOTIVE EXHAUST GAS SENSING SYSTEMS 1545

Fig. 2. Efficiency for a typical TWC for various exhaust gas constituents as afunction of A/F.

Fig. 3. Dependence of the oxygen partial pressure in the exhaust gas on A/F.

during the rich excursions, the ceria gives up its stored oxygen toreact with the combustibles utilizing the oxidation catalyst. Aspreviously mentioned, rhodium in the TWC is used to dissociateNO into oxygen and nitrogen. In order to minimize the ampli-tude of these rich and lean excursions, the time period for theseswitches is maintained as short as possible. Typical limit-cycleswitching frequencies vary in the range from less than 0.5 Hzup to approaching 2 Hz depending on the engine operating con-ditions.

Over the course of the past 20 years, two differentoxygen-sensor technologies for automotive applications havebeen developed. The first is a resistive-type sensor-based ontitanium dioxide. This material is a semiconductor whoseconductivity is sensitive to oxygen defects (vacancies) in thelattice. As the oxygen deficiency increases, the concentrationof electrons in the conduction band increases due to theincreased number of oxygen vacancies, and the conductivityof the material increases. The incorporation of platinum intothis material facilitates the exchange of oxygen between the

titanium dioxide lattice and the surrounding atmosphere atelevated temperatures. The conductivity of the material thenvaries with the partial pressure of oxygen in the ambientenvironment, e.g., the exhaust gas, and therefore can be used todetermine the amount of oxygen present.

The second type of oxygen sensor is an electrochemical cellbased on oxygen-ion conducting ZrO. This type of sensor con-sists of a single electrochemical cell which has one electrodeexposed to a reference atmosphere, (typically air), and the otherelectrode exposed to the measurement (exhaust) gas. The opencircuit voltage of this ZrO electrochemical cell changes withvariations in the oxygen partial pressures existing adjacent to itstwo electrodes. Since one electrode is exposed to a known ref-erence atmosphere, the emf generated by this cell gives a mea-surement of the concentration of oxygen in the exhaust gas. Thismeasurement can be used by the engine control unit to adjustthe A/F so as to maintain it close to stoichiometry, thereby min-imizing the unwanted emissions. In the past, as well as for thenear-term future, the ZrO-based oxygen sensor technology hasdominated the automotive applications.

V. OBD HC CATALYST CONVERSIONEFFICIENCYMONITORING

One of the more recent applications for ZrO-based oxygensensors has been in the area of diagnostics and monitoring. Inparticular, many automobile manufacturers currently employadditional oxygen sensors as catalyst monitoring sensors(CMS). For this application, a second oxygen sensor is placeddownstream of the TWC, in addition to the sensor which islocated upstream of the catalyst for feedback control of theA/F. The function of this sensor is to detect deterioration inthe conversion efficiency of the TWC as the catalyst ages orbecomes contaminated. Current federal regulations requiremonitoring of the effectiveness of the TWC to remove HCsfrom the exhaust gas stream. Ideally, a total HC detector,e.g., an FID which effectively counts the total number ofcarbon atoms in the gas stream, would be used for this CMS.Although there has been much effort world-wide devoted to thedevelopment of a selective HC sensor, which is also compatiblewith the exhaust gas environment, unfortunately these effortshave only produced limited success.

The ZrO -based CMS relies on the fact that there is a cor-relation, although admittedly a weak one, between the oxygenstorage capability of the TWC and its ability to remove HC con-stituents from the exhaust stream [2]. As was previously dis-cussed, the TWC is composed of media which is capable ofstoring oxygen during the “lean” duration of the limit-cycle con-trol period which is then used to react with the combustiblesduring the “rich” part of the limit cycle. The most commonoxygen storage media used in automotive catalysts is ceria. Asthe catalyst begins to deteriorate, it can no longer store oxygenas efficiently. As a result, not all of the oxygen present during the“lean” segment of the limit cycle can be stored by the degradedcatalyst. The CMS detects this “excess” oxygen by generating a“low” emf output. Conversely, during the “rich” portion of thelimit cycle, there is insufficient stored oxygen in the catalyst tofully react with all of the combustibles in the exhaust, resultingin a “high” emf output generated by the CMS. By counting the


number of “switches” that the CMS undergoes relative to thenumber of “switches” that the feedback control HEGO experi-ences, a crude estimate of the catalyst efficiency can be deter-mined. More sophisticated detection algorithms have been de-veloped, however they all rely on this weak correlation betweenoxygen storage capability of the TWC and HC conversion ef-ficiency. The need still exists for a selective HC sensor whichhas the sensitivity and durability to function in the exhaust gasenvironment.

VI. PROPORTIONALA/F SENSORS

Although successfully used for stoichiometric A/F control formany years, Nernst-type oxygen sensors are not useful for appli-cations away from stoichiometry because of their low sensitivity(i.e., weak logarithmic dependence of the emf on oxygen partialpressure). However, single- and double-ZrO-cell sensors havebeen developed [3], [4] which have much higher sensitivity andare therefore applicable to A/F measurement and control over awide range, from very rich to very lean A/F mixtures. These sen-sors are based on oxygen pumping, which involves the transferof oxygen from one side of a ZrOcell to the other by passingan electric current through the device.

VII. SYSTEMS USING OTHER EXHAUST GAS SENSORS

The accuracy of the air-to-fuel feedback control system(under static and dynamic driving conditions) is of paramountimportance for low tailpipe emissions, because the efficiency ofthe catalyst is sharply peaked as a function of A/F. One idea [5]to further optimize the A/F for lowest emissions, is to add anadditional feedback loop to the air-to-fuel control system. Thiscan be realized by measuring HCs (and/or CO) and NObehindthe catalytic converter. The air-to-fuel feedback system wouldthen minimize the (normalized) difference of the two sensorsignals to achieve the lowest possible emissions. This worksbecause the level of combustible gas concentrations substan-tially increase under fuel “rich” conditions, while substantiallyincreased levels of NOindicate fuel “lean” conditions. Byconstantly measuring the exhaust gas concentrations as well asother readily available engine operating parameters, such astemperature, speed and load, and mass air flow, an actual massof emissions can be obtained [6]. This information could beused to monitor the tailpipe emissions of the entire combustionsystem.

A method to do a strict HC conversion efficiency measure-ment with a calorimetric sensor (by directly measuring the com-bustible gases upstream and downstream of the catalyst) wasdisclosed in [7]. By placing the sensor in a bypass configura-tion, as shown in Fig. 4, the same sensor can be used to measurethe upstream and downstream emissions, thus reducing the errordue to limits in the sensor accuracy.

Also, if two sensors were used, upstream and downstreamof the catalytic converter, the measurement accuracy could fur-ther deteriorate over time due to different rates of sensor aging.The bypass configuration allows for a limited sensor exposureto the extreme temperatures and harsh chemical environment

Fig. 4. Exhaust gas HC sensor in a bypass configuration.

Fig. 5. Ratio of downstream and upstream calorimetric sensor measurementsas function of conversion efficiency.

found in the exhaust gas. Other advantages include a better con-trol of temperature and gas flow as well as the potential to addair to ensure proper operation of the calorimetric sensor. Be-cause the flow in the bypass to the sensor is small, it can be fedinto the engine intake manifold once it has passed the sensor.This patent also discloses results obtained in a steady-state en-gine-dynamometer experiment with three differently aged cat-alytic converters (see Fig. 5). Exhaust gas samples were takenupstream and downstream of the catalytic converter and mea-sured with an FID (plotted along the-axis in Fig. 5) and with acommercially available [8] differential calorimetric sensor op-erated at 400 C ( -axis). Although the output of this partic-ular calorimetric sensor depends on the flow rate, the flow ratedependency disappears to a large extent once the ratio of down-stream (tailpipe) and upstream (feedgas) measurements is taken.The relation between sensor output and catalyst HC conversionefficiency is much stronger than the relation between oxygenstorage capacity and conversion efficiency as measured in thesteady-state laboratory experiment [2]. It should be noted thatthe older engine used in this experiment had feedgas HC con-centrations almost equal to the CO concentrations. Newer en-gines have considerable less feedgas HC concentrations. The ex-tent to which the results above are valid for these newer enginesand the impact of dynamic (“FTP”-like) measurement condi-tions are under investigation.

It is clear that the system response time will decrease in abypass configuration. The additional cost and complexity arealso severe drawbacks. Other methods to perform OBD catalystmonitoring using one or more exhaust gas sensors are beinginvestigated as well.


Fig. 6. Exhaust gas HC sensor consisting of a ZrOpumping cell and a Nernstsensing cell.

VIII. E XHAUST GAS HC SENSORS

A. General

None of the systems discussed above is possible without oneor more exhaust gas sensors other than the oxygen sensor. Un-fortunately, gas sensors for HCs or CO suitable for automotiveexhaust gas applications do not currently exist. On the otherhand, the NO sensor based on ZrOshows much promise forexhaust gas applications in the near future.

B. Sensor Specifications

The exact specifications for exhaust gas HC sensors in var-ious applications are not yet exactly determined. However, theymust have a response time in the order of 1 s (for the OBD cata-lyst monitoring application) or faster (for an air-to-fuel controlapplication) and a sensitivity preferably in the order of 1 ppm.Exhaust gas HC sensors must be able to withstand high temper-atures, soot/particulates, water/water vapor, and be capable ofoperating in conditions of variable temperature, flow and pres-sure, and gas composition. Some selectivity, especially againstCO may be required (CO is present in the exhaust gas in muchhigher concentrations than HCs are). Other requirements for anautomotive application are low cost, durability, and stability (noneed for recalibration). For some applications a fast warm-uptime may also be required. For a HC sensor an additional chal-lenge is to be able to measure a mix of over 100 HC species thatare present in the automotive exhaust as pointed out earlier.

The major challenges for several exhaust gas HC sensingtechnologies will be discussed below. This is not a complete re-view of all potential exhaust gas sensing technologies. Not in-cluded in this review are optical techniques, SiC-based sensors,certain electrochemical techniques, and others.

C. ZrO -Based Exhaust Gas HC Sensors

Because the ZrO-based oxygen sensor has proven to be areliable automotive sensor, the question arises whether this orrelated sensor configurations can be used to measure HCs. Oneidea being pursued is to have two electrodes with different cat-alytic properties exposed to the exhaust gas [9], [10]. The gasmixture deviates differently from thermodynamic equilibriumat each electrode, resulting in an output emf related to the ex-haust gas constituents. The stability of the electrode materials orits microstructure is one of the major challenges in this research.

Another ZrO -based gas sensor incorporates an additionalZrO element for oxygen pumping, as shown schematically inFig. 6. By passing a current through one of the ZrOcells,

Fig. 7. Semiconductor-type sensor S in a restricted volume V with oxygenprovided by a ZrO pumping cell.

oxygen is transferred from one side of the cell to the other (i.e.,“oxygen pumping”). The other ZrOcell acts as an ordinaryNernst cell. Our research shows such sensors can (to some ex-tent even selectively) be used for measuring combustible gasesin air [11], [40]. The selectivity in this case is not obtained ina chemical way, but rather is based on differences in diffusiv-ities of the combustible gases. This type of combustible gassensor does not have the sensitivity to accurately measure com-bustible gas concentrations below approximately 0.1%. In addi-tion, changes in oxygen concentration in the gas mixture pose aproblem for this sensor.

A sensor structure essentially identical to the ZrO-basedNO sensor (discussed below) might also be applicable tothe exhaust gas HC sensing problem [12]. In this sensor, afirst ZrO pumping cell removes all oxygen without oxidizingthe combustible gases. The combustible gases further diffuseinto the sensor structure where a second pumping cell is usedto “titrate” the amount of oxygen needed for stoichiometriccombustion. One of the important issues here is the requirementfor stable electrodes with little or no catalytic activity for theoxidation of combustible gases.

D. Semiconductor-Type Exhaust Gas HC Sensors

The use of a semiconductor-type gas sensor (e.g., SnO,many other materials are possible as well) as a HC sensorrequires generally a gas mixture with a constant oxygen con-tent. By placing the sensor in a restricted volume, as shown inFig. 7, oxygen can be provided or removed with the ZrOcell.In this particular structure with an air reference, it is possibleto measure HCs even when no oxygen is present in the gasmixture. A second ZrOcell can be added to accurately controlthe amount of oxygen inside the restricted volume. Laboratoryresults obtained with experimental devices of this type haveshown this principle [13].

Although semiconducting-type gas sensors have an excellentsensitivity to combustible gases, the selectivity and stability aregenerally poor and not well understood. The results of a world-wide research effort on metal oxide materials for gas sensor ap-plications has not yet produced a sensor suitable for automo-tive applications. A rather unique approach is the fabricationand operation of metal oxide films using arrays of micro-hot-plates made by silicon micromachining [14]. This fabricationtechnique allows integration of an array of gas sensors of var-ious films with separate temperature control (during both fabri-cation and operation) for each element in the array.


E. Microcalorimetric Exhaust Gas HC Sensors

Catalytic calorimetric gas sensors measure the rise in tem-perature of a (low thermal conductivity) substrate caused by theoxidation of combustible gases on a catalytic layer depositedon the substrate. The temperature rise can be measured witha platinum resistance thermometer (thermistors or thermocou-ples/thermopiles could also be used). To improve the detectionlimit of the sensor the temperature rise is usually measured dif-ferentially by adding a second element (e.g., a Pt resistor) withthermal characteristics identical to those of the sensing element,but without a catalytic layer.

Catalytic calorimetric gas sensors typically operate in the300–550 C temperature range, making them in principleapplicable for automotive applications. Although generallyof lower sensitivity than semiconducting-type gas sensors,catalytic calorimetric sensors appear to be considerably morestable, faster responding, and less susceptible to varyingamounts of water vapor and to varying amounts of oxygen(provided enough oxygen is present). Existing calorimetric sen-sors [8], [15]–[17], based on different fabrication technologies,have been characterized, and found not to be compatible withthe automotive exhaust gas environment. Most of our researchhas been concentrated on silicon micromachined calorimetricsensors [18], [19]. The use of a silicon micromachined structureas platform for chemical sensing has been studied extensivelyin the last decade [14], [20]–[25]. Use of these structuresfor silicon microcalorimeters has also been widely reported[26]–[31]. Microfilament-based calorimetric sensors can befound in [32]–[35].

For automotive exhaust gas applications, one of the first ques-tions to ask must be whether a properly packaged silicon mi-crostructure can survive in the harsh exhaust gas environment(either in the direct exhaust or in a bypass configuration). Al-though no definitive answer can be given at this point, our ex-perience and a recent publication show promising results [36].

Reducing the size of a calorimetric sensor by using siliconmicromachining as the method of fabrication results in a fasterresponse time by reducing the thermal mass (a 20 ms thermal re-sponse time was measured for the Ford microcalorimeter) and apotentially lower detection limit by better temperature compen-sation between reference and catalytic element while still min-imizing crosstalk between the two elements (an approximately5 ppmC detection limit under laboratory conditions was mea-sured). Other reasons to study silicon micromachined gas sen-sors are lower power consumption for operation, a potentiallylower manufacturing cost, and the possibility to easily manu-facture sensor arrays.

IX. SENSORS FORMEASURING OXYGEN CONTAINING

MOLECULES

A. General

A single-cell or a double-cell oxygen-pumping device canbe used to measure the concentration of oxygen containingmolecules in the exhaust gas, e.g., HO, CO , SO , andNO . For example, to measure the HO concentration with asingle-cell oxygen-pumping device, one can apply to the cell avoltage which is greater than the dissociation voltage for HO;

(a)

(b)

Fig. 8. Schematic of ZrO double-cell sensing devices for measuringoxygen-containing molecules in the exhaust gas. (b) has the addition ofdiffusion barrier between the two cells to reduce interference from other gases.

measurement of the current through the cell could, in principle,provide a measure of the oxygen generated from the decompo-sition of water and thus a measure of the HO concentration inthe gas. However, in the exhaust gas this measurement is com-plicated by the fact that oxygen and other molecules are alsodissociated. If the concentrations of these other molecules wereconstant, then the concentration of HO could still be deter-mined unambiguously by subtracting the contribution of theseother molecules. However, the concentrations of the differentoxygen-containing molecules in the exhaust gas vary withengine operating conditions. This problem can be overcome byusing a device shown schematically in Fig. 8(a), consisting oftwo ZrO cells in a structure that defines a restricted volumewhich can be accessed by the exhaust gas through an aperture[37]. To measure the concentration of an oxygen containingmolecule in the presence of variable concentrations of oxygenand other oxygen-containing molecules, a voltage with properpolarity and with magnitude (slightly smaller than the dissocia-tion voltage for measurement molecule of interest) is applied tothe first cell. This voltage dissociates and removes oxygen, aswell as all molecules having dissociation potentials less than thedissociation voltage of the measurement gas, which diffuse intothe restricted volume through the aperture. A second voltagelarger than the dissociation potential of the measurement gasis applied to the second cell to decompose this molecule. Thecurrent through the second cell, then provides a measure ofthe concentration of the measurement molecule in the gas.The functionality of the device of Fig. 8(a) can be improvedby adding more components including additional sensing andpumping ZrO cells. As an example, Fig. 8(b) demonstratesthat the two pumping cells of Fig. 8(a) can be separated bya second diffusion barrier (e.g., a wall with an aperture ora porous diffusion barrier) to eliminate the possibility that


some oxygen molecules reach the second pumping cell. In thisrealization, some response time of the sensor will undoubtedlybe sacrificed for a gain in accuracy. Also, a third ZrOcell, usedas a sensing cell, can be added in the compartment of the firstpumping cell to monitor the pumping action of this pumpingcell for increased accuracy or measuring low concentrationlevels. A similar addition can be made to the second pumpingcell. Other modifications include the elimination of the secondZrO pumping cell, and instead, incorporating some othersensor for the molecule of interest. It should be also noted, thatthe current through the first cell can provide information on theconcentration of the oxygen-containing molecules removed bythis cell. For example, the first cell can be used as a proportionaloxygen sensor, or even as a double-cell sensor (UEGO) byappropriate modifications in the device structure.

B. Water Vapor Sensor

An automotive water vapor sensor can be useful in a numberof applications. For example, when placed in the exhaust gas ofa vehicle which can utilize gasoline/methanol as a fuel, it canbe used to measure the fuel composition. This measurement offuel composition is made possible because the concentration ofwater in the exhaust gas of such a vehicle operated at the stoi-chiometric A/F ratio is a unique function of the methanol con-tent in the methanol/gasoline fuel mixture. The limitation of thissensing approach is that the fuel composition measurement ismade after the engine has been started. Nevertheless, this ap-proach might be still useful in combination with a less accuratecapacitive-type fuel sensor which measures the fuel composi-tion directly in the fuel line before the engine is started. A watervapor sensor can also be used to measure the amount of EGRby measuring the water content in the intake-air/EGR mixture.As was the case with the aforementioned methanol/gasoline fuelsensor, this type of measurement is possible because of the factthat the concentration of water vapor in this mixture is directlyproportional to the amount of EGR. Variable humidity in theambient air decreases the accuracy of both of these measure-ments.

C. NO Sensor

The general method for measuring oxygen containingmolecules in the exhaust gas described above can also be usedto measure the NO and NOin the exhaust gas. Currently,there is much interest in the development of NOsensors forvehicles with diesel engines. Since NOis thermodynamicallyunstable, this structure and method can be employed by using,in the first pumping cell, an electrode, which is not catalytic forthe decomposition of NOto remove only oxygen. The secondcell employs an electrode, which catalyzes the decompositionof NO . The electric current through the second cell providesa measure of the NOconcentration. Prototype NOsensorsof this type have been recently developed by several suppliers.The most advanced of these appears to be the one made byNGK Locke [38], [39].

ACKNOWLEDGMENT

The authors wish to especially acknowledge the contributionsby E. M. Logothetis, M. Zanini, J. R. McBride, K. E. Nietering,A. Kovalchuk, T. S. Morley, and L. Rimai.

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J. H. Visser (S’85–M’90) received the M.S. (cum laude) and Ph.D. (StevenHoogendijk Award) degrees in electrical engineering in 1985 and 1989, respec-tively, both from Delft University of Technology, Delft, the Netherlands.

He joined the Ford Research Laboratory, Dearborn, MI, in 1990, where heis currently working as a Senior Technical Specialist. His research interests in-clude exhaust gas constituents sensors, fuel vapor sensors, and gas sensor systemapplications. He received 16 U.S. patents for his work.

R. E. Soltis received the B.S. and M.S. degrees in physics from John CarrollUniversity, Cleveland, OH, and the Ph.D. degree in physics from Wayne StateUniversity, Detroit, MI.

He joined the Research Staff of Ford Motor Company, Dearborn, MI, in1984, where he currently works as a Senior Technical Specialist. His main re-search interests include electronic materials for gas sensors as, well as multi-phase nanocomposite materials. He has co-authored over 50 publications andreceived 31 U.S. patents during his career.

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