WO1999026064A1

WO1999026064A1 - Exhaust gas sensor with flow control without a diffusion barrier

Info

Publication number: WO1999026064A1
Application number: PCT/US1998/024235
Authority: WO
Inventors: Jeffrey Naber; Daniel Young; Edward Balko; Patrick Blosser
Original assignee: Motorola, Inc.; Engelhard Corporation
Priority date: 1997-11-14
Filing date: 1998-11-13
Publication date: 1999-05-27
Also published as: AU1405999A

Abstract

The present invention provides a gas sensor (10) for detecting levels of combustible species in a gas stream. Preferably, the sensor (10) includes a substrate (24) having a sensing element (16) disposed on a first surface (40) thereof for sensing exothermic reactions corresponding to a catalyzing of the oxidation of the combustible species. The concentration of combustible species in the gas stream is determined by determining the rate of gas flow over the sensor, determining an effective mass transfer coefficient to the sensor based on the rate of gas flow over the sensor and the gas temperature, determining a proportionality of combustion heat sensed by the sensor to the concentration of the combustible species, and converting the proportionality into a true concentration of the combustible species in the gas stream.

Description

EXHAUST GAS SENSOR WITH FLOW CONTROL WITHOUT A DIFFUSION BARRIER

Field of the Invention The present invention generally relates to gas component sensors, and more particularly, to a gas sensor disposed within a housing eliminating the need for a diffusion barrier while enhancing the detection of combustible species in a gas stream.

Background of the Invention

Sensors for the detection of particular compounds present in a high temperature gas stream find numerous applications in many different mechanical systems. For example, detection of certain compounds in a high temperature gas stream is important in industrial emission monitoring for detection of gas pollutants, such as sulfur dioxide (SO₂), in residential heating systems for detection of carbon monoxide (CO), and in automobile exhaust systems for various combustible compounds including hydrocarbons.

In automotive applications, gas sensors can be placed at various locations within the vehicle including the exhaust system, the fuel vapor recovery system and the induction system. Exhaust gas from an internal combustion engine, as an example, typically contains hydrogen (H₂), carbon monoxide (CO), methane (CH₄), carbon dioxide (CO₂), oxides of nitrogen (NOx), water (H₂O), and non-methane hydrocarbons

where n is an integer larger than 1 and m is an integer whose value depends upon the kind of hydrocarbon compound, for example, alkane, alkene, or aromatics. Important environmental pollution concerns dictate that the emission of pollutants be minimized.

To minimize pollutants in the engine exhaust, sensors can be placed before and after the catalytic converter to monitor the performance of the converter. Also, the emission of hydrocarbons can be controlled, in part, by an engine exhaust control system that receives a feedback signal from an exhaust gas sensor capable of selectively detecting the presence of hydrocarbons in the engine exhaust. One method for monitoring the performance of a catalytic converter includes the use of oxygen sensors within the exhaust gas system. By measuring the amount of oxygen in the exhaust gas entering and exiting a catalytic converter, an estimate of the oxygen storage capacity of the catalytic converter can be made. As the converter ages, the oxygen storing materials within the converter sinter and lose the ability to effectively store oxygen. It was commonly believed that the catalytic materials age at about the same rate as the oxygen storing materials. As the catalytic materials age the efficiency of the converter declines. Accordingly, in theory, estimating the amount of oxygen storage capacity of the catalytic converter provides an indirect measurement of the catalytic converter efficiency. It has been more recently shown that this method provides a rather imprecise measure of converter performance.

A sensor that directly estimates the hydrocarbon concentration in an exhaust gas stream can be used to provide a more precise determination of catalytic converter efficiency. For example, several types of sensing elements have been developed for detecting various chemical species within an exhaust gas stream. These sensing elements include calorimetric sensors having a catalyst coating, semiconductor metal oxide based sensors, and the like. Calorimetric hydrocarbon gas sensors measure the amount of heat released by the catalytic oxidation of hydrocarbons contained within the exhaust gas.

Typically, a diffusion barrier is added to the sensor to remove dependencies of flow on the flux of the reactant to the catalyst. This makes the measurement more independent of the external gas flow rate. The diffusion barrier also provides a degree of thermal isolation for the catalyst and protects the catalyst from scouring by the gas stream. In sensors incorporating an oxygen source, the diffusion barrier also helps to maintain a corridor of oxygen in close proximity to the catalyst surface.

The diffusion barrier is commonly a solid material with a well controlled porosity bonded or otherwise secured to the sensor. Since its characteristics must be well controlled, a diffusion barrier increases the complexity and cost of manufacturing the sensor element. Furthermore, the addition of the porous barrier reduces the sensitivity of the sensor by restricting the flux of the reactant to the catalyst. Moreover, the diffusion barrier reduces the time response of the sensor by retarding the movement of the reactant to the catalyst.

Therefore, it would be desirable to provide a gas sensor without a diffusion barrier for increasing the sensitivity of the sensor and lowering manufacturing costs while maintaining the independence of the sensor from fluctuations in external gas flow rates.

Brief Description of the Drawings In order to appreciate the manner in which the advantages and objects of the invention are obtained, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. Understanding that these drawings only depict preferred embodiments of the present invention and are not therefore to be considered limiting in scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings in which:

Figure 1 illustrates, in cross-section, a gas sensor disposed within a housing in accordance with one embodiment of the present invention;

Figure 2 illustrates a top view of the gas sensor shown in Figure 1 ; Figure 3 is an exploded perspective view of the sensor housing of

Figure 1 arranged in accordance with the invention; Figure 4 is a cross sectional view of the sensor housing of Figure 1 taken along line 4-4;

Figure 5 illustrates the flow field plots for a gas sensor disposed within a housing and arranged parallel to an exhaust duct in accordance with the invention;

Figure 6 illustrates the gas sensor of Figure 1 and an electrical circuit analogy corresponding thereto;

Figure 7 depicts a plurality of mass transport coefficient measurements over a range of flows over a range of expected velocities in the sensor housing of Figure 1. Figure 8 depicts a partial cross-sectional view of a catalytic converter typically used in motor vehicles and into which a gas sensor is integrated in accordance with a preferred embodiment of the present invention; and

Figure 9 further depicts, in cross-sectional view, the catalytic converter and sensor arrangement shown in Figure 8; and

Figure 10 depicts a gas sensor adapted to a smoke stack application in accordance with still another preferred embodiment of the present invention.

It will be appreciated that for simplicity and clarity of illustration, elements shown in the Figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other. Further, where considered appropriate, reference numerals have been repeated among the Figures to indicate corresponding elements.

Detailed Description of the Preferred Embodiments The present invention is directed to a gas sensor without a diffusion barrier for the detection of combustible gases in a gas stream. The gas concentration measurements made by the gas sensor can be converted to electrical signals and relayed to a display device or a control unit and operated upon appropriately. In a particularly preferred embodiment of the present invention a gas sensor without a diffusion barrier is adapted for use in the exhaust gas system of an automobile. Electronic circuitry within the engine control unit can analyze the electrical signals from the gas sensor and determine the efficiency of the automobile's catalytic converter at converting combustible gas species, and particularly, may be adapted for controlling operation of the engine. The following discussion relates to an exemplary sensor of the differential calorimetric type for use in automotive applications. No limitation of the true scope of the present invention should be drawn from the following detailed description of a preferred embodiment of a sensor for such applications.

The gas sensor of the invention operates by selectively oxidizing combustible gas species such as hydrocarbons and carbon monoxide within a gas stream at a catalyst surface located within the gas sensor. Advantageously, the sensor is disposed within a housing wherein the gas flow rate is well controlled. As such, a calibration can be made to adjust for changes in the flux of the reactant to the sensor.

Figure 1 illustrates, in cross-section, a preferred embodiment of the present invention in the form of a gas sensor 10 disposed within a housing 12. A sensing element 14 includes a catalyst 16 and an electrochemical oxygen source 18. The oxygen source 18 is arranged in spaced relationship with sensing element 14. Electrochemical oxygen source 18 includes an outer electrode 20 and an inner electrode 22. The electrodes are separated from sensing element 14 by a multi-layer substrate 24. Substrate 24 includes a plurality of overlying insulative layers on which electrical circuitry and resistive heating elements are arranged. Each of the plurality of overlying insulative layers are ceramic layers laminated together to form the multi-layered ceramic substrate 24. With the exception of the top layer, each ceramic layer supports screen-printed metalization defined in different patterns to form the various functional elements necessary to measure and control temperature within gas sensor 10. Additionally, substrate 24 includes a plurality of vias 26. Vias 26 provide communication between the sensing element 14 and an oxygen storage region 28. While shown as a plurality of apertures, vias 26 may also be formed as a slotted portion of substrate 24 or a channel dividing regions 16a and 16b.

Electrochemical oxygen source 18 is separated from substrate 24 by first and second ceramic layers 30 and 32. Additionally, an electrolyte 34 separates outer and inner electrodes 20 and 22. Preferably, electrolyte 34 is yttrium stabilized zirconia, and outer and inner electrodes 20 and 22 are constructed of porous platinum metal. Placing a voltage across electrodes 20 and 22 generates oxygen by breaking down water, oxygen and carbon dioxide in the exhaust gas at outer electrode 20 and conducting oxygen ions across electrolyte 34 to inner electrode 22. A porous protective layer 36 overlies outer electrode 20 and extends from first ceramic layer 30 to a sensor end wall 38. Porous protective layer 36 functions to protect outer electrode 20 from scouring by the exhaust gas, while permitting water, oxygen and carbon dioxide to diffuse to outer electrode 20. Preferably, porous protective layer 36 has high-porosity and is constructed from a material, such as spinel, alumina, corierite, mullite, steatite, stabilized zirconia or other porous ceramic.

Oxygen is desorbed from inner electrode 22 and contained within oxygen storage region 28. Oxygen storage region 28 extends from inner ceramic layer 32 to end wall 38. Oxygen within oxygen storage region 28 is transported through vias 26 to active surface 40. In addition to providing an oxygen supply source for catalytic oxidation of combustibles at catalyst 16, oxygen source 18 can be operated in reverse to remove oxygen from active surface 40 and oxygen storage region 28. By removing oxygen, the combustion of combustible gases within gas sensor 10 can be effectively terminated. By terminating the catalytic oxidation at active surface 40, gas sensor 10 can be calibrated after installation into an automotive exhaust gas system.

Referring momentarily to Figure 2, a top view of gas sensor 10 is illustrated depicting the arrangement of catalyst 16 and vias 26. Catalyst 16 is partitioned into an active region 16a and a reference region 16b. Active region 16a includes a catalyst composition specifically formulated to catalyze the oxidation of hydrocarbons at active surface 40a. However, the catalyst in reference region 16b lacks the specific chemical formulation necessary to catalyze the oxidation of hydrocarbons at active surface 40b. Accordingly, the exothermic reaction heat measured by temperature sensing circuitry for reactions taking place at active surface 40b can be compared with the exothermic reaction heat measured at active surface 40a. The difference in the amount of heat produced between active region 16a and reference region 16b is attributed to the oxidation of hydrocarbon species within the exhaust gas.

Referring again to Figure 1, the housing 12 includes an outer shroud 42 consisting of a cylindrical side wall 44 terminating in an integrally formed frustoconical cap 46. The cylindrical side wall 44 includes at least one radial inlet port 48 formed therethrough interconnecting an interior volume of the outer shroud 42 with the external gas stream. The frustoconical cap 46 includes an axial outlet port 50 formed therethrough enabling gas flow from the interior of the outer shroud 42. The outer shroud 42 encompasses a discrete inner shroud 52 which is disposed within the cylindrical side wall 44 and defines an inner channel 54 of the housing 12. the inner shroud 52 may be fixed to the outer shroud 42 by tack welding, press fitting, or other conventional means. The inner shroud 52 includes a plurality of longitudinal flutes 56 formed about an outer periphery thereof which fluidly interconnect the inlet ports 48 and the channel 54.

The outer shroud 42 is fixedly secured within an inner radial surface of a threaded fitting 58 which includes a nut 60 adjacent a ring 62 and a threaded portion 64. This is preferably accomplished by crimping a flange 59 of the threaded fitting 58 over a radially projecting ring 61 of the outer shroud 42 to form a roll flange 63. The ring 62 is secured to a cylindrical member 66 substantially encompassing the remainder of the sensor 10 by a second roll flange 67 or other conventional means. As can be seen, the sensor 10 is held within the housing 12 through cooperation of a ceramic insert 69, ferrule 71, and the ring 62 and nut 60. The threaded portion 64 and nut 60 cooperate to secure the housing 12 within an exhaust duct (not shown) of an exhaust system of a motor vehicle (also not shown). According to this configuration, the housing 12 functions as a venturi.

Turning now to Figures 3 and 4, the housing 12 is illustrated in greater detail. As can be seen, the outer shroud 42 includes a plurality of openings 68 therein forming the inlet ports 48 of the cylindrical side wall 44. Furthermore, the longitudinal flutes 56 have a radius of curvature selected to compliment the diameter of the openings 68 for directing the flow of gas therealong. For example, the flutes 56 may terminate in a beveled end (not shown) proximate the openings 68 rather than extending along the entire length of the inner shroud 52. As will be described in greater detail below, the length of the inner shroud 52 is well-controlled relative to the length of the cylindrical side wall 44 so as to maintain pre-selected flow conditions within the housing channel 54 around the sensor 10. Referring now to Figure 5, a cross sectional view of the gas sensor 10 is shown. The sensor 10 includes a width dimension oriented parallel to an exhaust duct 70. Figure 5 also illustrates exemplary flow field plots 72 of exhaust gas flow around and through the housing 12 that are representative of flows to which gas sensor 10 may be subjected. It should be noted that the flow plots depicted herein represent an exemplary flow scenario in which one of the inlet ports 48 is positioned in-line with the direction of the gas flow (i.e., front facing). One would expect the flow plot to change to some extent if the inlet port 48 is positioned differently. Therefore, the illustrated orientation is preferred.

As the gas stream flows through the duct 70 it passes over the outlet port 50 and creates a pressure difference between the channel 54 within the inner shroud 52 and the duct 70 itself. This causes the gas within the housing 12 to be forced out of the outlet port 50 and into the external gas stream. Additionally, a portion of the gas flowing through the duct 70 passes into the inlet ports 48. This portion enters through the inlet ports 48 and impinges upon an inner radial surface of the longitudinal flutes 56, turns, and slows down. The longitudinal flutes 56 direct the gas along their length away from the inlet ports 48 towards the end of the channel 54. After the gas travels the length of the inner shroud 52, it turns and enters into the channel 54, again slowing down. At this point, the gas is forced towards the outlet port 50 by the pressure difference between the channel 54 and duct 70. As such, the gas overflows the sensor 10 and catalyst 14 (FIG. 1). Advantageously, the outer shroud 42 and the inner shroud 52 cooperate to control the gas flow rate inside of the housing 12. Preferably, the gas flow rate within the housing 12 over the sensor 10 is reduced to approximately 0 - 10 m/s. Of course, the gas flow rate will fluctuate somewhat with changes in the external flow rate. However, as compared to the prior art which does not implement the housing, the changes in internal flow rate are minimal.

It should also be noted that varying the length of the inner shroud 52 relative to the cylindrical side wall 44 effects the internal flow characteristics of the gas within the channel 54 of the housing 12. For instance, the gap 74 between the bottom edge 76 of the inner shroud 52 and the end 78 of the cylindrical insert 69 can be manipulated to control the velocity of the gas flow through the channel 54. Additionally, the tendency of the gas to swirl within the channel 54 can be reduced by controlling the gap 74 between the edge 76 of the inner shroud 52 and the end 78 of the cylindrical insert 69. It is presently preferred to select the length of the inner shroud 52 based on the dimensions of the desired gap 74 rather than to manipulate the position of the inner shroud 52 relative to the cylindrical side wall 44 to control the size of the gap 74 so that proper positioning of the inner shroud 52 relative to the frustoconical cap 46 is ensured. The skilled practitioner will recognize that the exact distance the inner shroud 52 should terminate relative to the end 78 of the cylindrical insert 69 depends on a number of factors including but not limited to, the orientation of the sensor 10 relative to the external gas flow, the velocity of the external gas flow at the inlet ports 48, the size of the inlet ports 48, and the configuration of the flutes 56. Furthermore, the design of the housing 12 creates laminar flow conditions for all external gas velocities which the housed sensor 10 is likely to experience, e.g. 0-200 m/s. If desired a conventional flow straightening device 80 (FIG. 1) may be incorporated into the housing 12 to provide an additional means by which laminar flow may be imposed across the sensor 10. Referring again generally to Figure 1 , those skilled in the art will also recognize that the concentration of combustibles at active surface 40 is not equal to the concentration of combustibles in the exhaust gas at some distance from active surface 40. The diffusive flux of combustibles from the exhaust gas through the housing 12 to active surface 40 is a function of the difference in the concentration of combustibles at active surface 40 and the concentration of combustibles elsewhere in the exhaust gas. In a preferred embodiment, the concentration of combustibles at active surface 40 is substantially zero. Since all of the combustibles must be completely oxidized to maintain a near-zero combustible concentration at active surface 40, the heat released by the exothermic oxidation reaction will also be proportional to the combustible concentration in the exhaust gas. Detection of the exothermic reaction heat at the active surface 40 yields a numeric value indicative of the combustible concentration. However, this numeric value must be manipulated to compensate for the flux to yield the true combustible concentration. This manipulation may be performed in an application specific integrated circuit or a microprocessor 82. Engine speed, load and gas temperature upon which the calculation is based are typically available within a conventional vehicle engine control unit and/or can be read by sensor electronics 86. From the engine rotational speed, load and gas temperature the gas flow rate across the outlet port 50 through the duct 70 may be calculated. This in turn permits the laminar flow velocity across the sensor 10 within the housing 12 to be calculated or obtained via a calibration table contained within the application specific integrated circuit or microprocessor 82. Alternatively, flow data may be received from a vehicle mass flow sensor. Still a further alternative method of determining the gas velocity within the housing 12 is to incorporate a conventional anemometer 88 into the housing 12 to make a direct determination of the gas velocity.

With the internal gas velocity and gas temperature known and with the flow geometry across the sensor 10 fixed by the housing 12, the effective mass transfer coefficient to the sensing element 14 can be calculated. From this, the proportionality between the combustion heat sensed by the sensor 10 and analyte concentration may be defined for any external flow rate and the true analyte concentration can be made by a calculation.

Figure 6 illustrates the sensor 10 and an electrical circuit 90 analogy corresponding thereto. The flux of combustibles to the sensor 10 can be represented mathematically by equation 1 :

c ^^" ([HC]_o- [HC]_C = k_Ca [HC]_Cat ή_HC I A 1

^■ _.HC\_o

Ilk + l/k, Cat

Wherein:

A = Surface area ke_g, = Catalyst reaction rate coefficient k_M = Bulk gas mass transfer coefficient.

The HCs in square brackets are the concentrations of combustibles, typically hydrocarbons, at the respective positions,

Cat = At the surface of the catalyst,

O = Outer (free-stream).

In the electrical circuit analogy 90 for the sensor 10, the combustible concentrations are the potentials (voltages) 92, 94, the inverse of the transport parameters are resistors 96, 98, and the species flux (n_HC/A) is the current 100. For this configuration, the transport resistances act in series. The catalyst 16 is designed such that its resistance is much less than the mass transfer resistance (\lk_Cat< < l/k_M). In other words, the sensor 10 operates in a mass transfer limited mode and not in a reaction rate limited mode. Also, the bulk gas mass transfer resistance

(l/k_M) is dependent upon the velocity profile 102 above the sensor 10. To correct the sensor signal based upon an estimated velocity and the dependence of the bulk gas mass transport resistance on the velocity and still operating under the mass transfer limit we have,

Sensor Signal = Ap = φή_HC= A-k_m(V)- HC]

or

Figure 7 shows a plurality of mass transport coefficient measurements 104, 106, 108, and 110 over a range of flows 112 over a range of expected velocities 114 in the sensor housing 12. The measurements were made in a simulated exhaust with carbon monoxide (CO) levels of 500 and 1000 ppm. The mass transfer coefficient k_M was determined from the known concentration of carbon monoxide, catalyst 16 surface area and measured delta power on the two element sensor 10 with a total combustible catalyst (TC) on one element 16a and no catalyst on the second (reference) 16b. As used herein, measured delta power refers to the differential power applied to each of the heating elements incorporated into the substrate layers and utilized to substantially maintain the catalyst regions 16a and 16b at equal temperatures. As Figure 7 shows, there is an expected increase in mass transfer k_M with velocity.

Using the mass transfer coefficient measurement, the effect of flow on the reactant concentration can be calibrated out in the application specific integrated circuit or microprocessor 82 and a determination of the reactant concentration can be made knowing the delta power and velocity. Advantageously, the velocity in the housing 12 across the face of the sensor 10 is controlled by the design of the inner 52 and outer shrouds 42 of the housing 12. Further, this velocity is a function of the velocity in the exhaust duct 70 which can be determined from engine speed, load and gas temperature in the engine control unit 84 or sensor electronics 86. As such, the mass transfer coefficient can be estimated from this calculated velocity and the combustible concentration of the exhaust gas can be determined given the mass transfer coefficient at that flow condition.

Recognizing that a catalytic converter typically incorporated into the exhaust gas system of a motor vehicle is a laminar flow device, its properties may be advantageously employed to eliminate the use of housing 12. Referring to Figures 8 and 9 a gas sensor 120 is integrated directly into a catalytic converter 122 as may typically be found in an automotive exhaust system. Converter 122 includes an outer housing 124 into which a catalyst substrate 126 is secured. Catalyst substrate 126 includes a cell structure 128 as is known in the art but includes sensor region 130 formed by removing a portion of cell structure 128. Gas sensor 120 is suitably mechanically secured or cemented into region 130 with electrical connections being made using mineral insulated cable. Exhaust flow enters converter 120 at entrance 132. Cell structure 128 acts essentially as a laminar flow device by channeling exhaust gas through the plurality of openings 134 of cell structure 128 and for directing exhaust gas over sensor 120. In this application the flow properties over sensor 120 will be fairly well known. The sensor output may therefore be compensated in accordance with the present invention for providing a calibrated output value indicative of hydrocarbon concentration in the exhaust gas. In applications apart from motor vehicle type applications, the sensor may also be exposed to flow conditions which are either laminar or turbulent, but in any event well known or predictable. Referring to Figure 10, a portion of a smoke stack 150 is shown to include stack flue 152. Secured or suspended within flue 152 by any suitable mechanical attachment (not shown) is a gas sensor 154. The flow 156 within flue 152 will likely be laminar but may be turbulent. In any event, it will be generally well known. Sensor 154 is aligned parallel to flow for several important reasons. First, since the catalyst regions of sensor 154 are arranged parallel, each region will experience substantially the same flow at substantially the same moment in time. In addition, the flow will be substantially undisturbed as it passes sensor 154. Further shown in Figure 10 is a simple shroud member 158 into which sensor 154 is positioned. Shroud acts to restrict and straighten the flow 160 past sensor 154 where sensor 154 is installed within a stack. In an open environment, shroud 158 will assist in creating a natural convective flow 160 over sensor 154. In either case, a pumping device is not required to create a flow of gas over sensor 154. Thus it is apparent that there has been provided, in accordance with the invention, a gas sensor that fully meets the advantages set forth above. Although the invention has been described and illustrated with reference to specific illustrative embodiments thereof, it is not intended that the invention be limited to those illustrated embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the spirit of the invention.

For instance, a porous protective coating may be applied over the sensing element for added resistance to scouring by gas particulates. It is therefore intended to include within the invention all such variations and modifications as fall within the scope of the appended claims and equivalents thereof.

Claims

CL╬öIMSWhat is Claimed is:

1. A method of determining the concentration of combustible species in a gas stream comprising: disposing a gas sensor (10) within the gas stream; determining a rate of gas flow over the sensor (10); determining an effective mass transfer coefficient to the sensor (10) based on the rate of gas flow over the sensor (10) and the gas temperature; determining a proportionality of energy of combustible species sensed by the sensor (10); and converting the proportionality associated with the combustible species into a concentration of the combustible species in the exhaust gas.

2. The method of claim 1, comprising disposing the gas sensor (10) within a housing (12) and determining a rate of gas flow over the sensor (10) within the housing (12).

3. The method of claim 2, wherein the housing (12) comprises a preselected configuration for fixing the flow geometry of exhaust gasses over the sensor (10).

4. The method of claim 1 , wherein the step of determining a rate of gas flow comprise providing a flow measuring device in the gas stream.

5. The method of claim 1 , wherein the sensor is disposed within a housing (12) and wherein the step of determining a rate of gas flow comprises disposing a flow measuring device within the housing.

6. The method of claim 1, wherein the step of converting the proportionality into the concentration of combustibles species further comprises: calibrating out the effect of flux on the reactant concentration based on the rate of gas flow over the sensor (10) and a change in power as sensed by the sensor.

7. The method of claim 1, wherein the step of disposing a gas sensor within the gas stream comprises disposing the gas sensor within an exhaust system (70) of a motor vehicle.

8. The method of claim 7, comprising: calculating an effective mass transfer coefficient to the sensing element (14); calibrating out the effect of flux on a reactant concentration using the mass transfer coefficient and based on a change in power sensed by the sensor (10) and a velocity of the exhaust gas over the sensor (10).

9. The method of claim 1, wherein the step of disposing a gas sensor within the gas stream comprising disposing the gas sensor within a catalytic converter portion (122) of an exhaust system (70) of a motor vehicle.

10. The method of claim 1, wherein the step of determining a rate of gas flow over the sensor (10) comprises determining engine speed, load and exhaust gas temperature. ll . A gas sensor (10) for detecting levels of combustible species in engine exhaust systems of a motor vehicle comprising: a substrate (24); a sensing element (16) disposed on a first surface (40) of the substrate (16) for sensing energy corresponding to a catalytic oxidation reaction of the combustible species; a sensor controller (82) coupled to receive an output of the sensing element, the controller including a processor and a data structure, the data structure comprising a calibration algorithm and calibration data, the processor operable upon the sensing element output and the cahbration data in accordance with the cahbration algorithm to provide a sensor output signal indicative of levels of combustible species in the engine exhaust system.