WO1999025966A1

WO1999025966A1 - Apparatus and method for determining catalytic converter operation

Info

Publication number: WO1999025966A1
Application number: PCT/US1998/024351
Authority: WO
Inventors: Jeffrey Naber; 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: AU1410599A

Abstract

An apparatus and method for processing signals obtained from a hydrocarbon gas sensor (216) adapted within automobile exhaust system (204) for determining operation of a catalytic converter (208) that is part of the exhaust system (204). In this arrangement the apparatus functions in accordance with a preferred method to detect failure of the catalytic converter (208) as mandated by legislation pertaining to motor vehicle On-Board Diagnostics (OBD). Signals from the hydrocarbon sensor (216) are collected, qualified and sorted according to the specific driving conditions under which they are generated. Elements within the engine control unit (264) analyze the signals and determine the efficiency of various emission control systems on the vehicle, such as the catalytic converter's efficiency at converting hydrocarbons in the exhaust gas to non-polluting gas species.

Description

APPARATUS AND METHOD FOR DETERMINING CATALYTIC CONVERTER OPERATION

FIELD OF THE INVENTION This invention relates, in general, to emission control systems, and more particularly, to a method and apparatus utilizing a differential calorimetric gas sensor for determining operation of a catalytic converter.

BACKGROUND OF THE INVENTION In automotive applications, exhaust gas from an internal combustion engine typically contains hydrogen (H₂), carbon monoxide (CO), methane (CH₄), carbon dioxide (CO₂), oxides of nitrogen (NO , water (H₂O), and non-methane hydrocarbons (C_nl , where n is an integer larger than 1 and m is an integer whose value depends upon the specific hydrocarbon compound, for example, alkane, alkene or aromatics. Important environmental pollution concerns dictate that the emission of hydrocarbons be minimized. To this end, catalytic converters, which convert polluting gas species such as hydrocarbon to non-polluting gas species such as carbon dioxide and water, have been incorporated into the exhaust systems of automotive vehicles to minimize pollutants from the engine exhaust. Since these converters have a finite length, legislation has been recently proposed that would require system diagnostics that evaluate the efficiency of such converters. In this regard, sensors employed in the exhaust system and coupled to the engine control unit monitor the performance of the converter. A sensor capable of selectively detecting the presence of certain chemical compounds in the engine exhaust might also be utilized in a feedback arrangement to control the emission of pollutants by an engine by providing a basis for adjusting operating parameters.

One method for monitoring the performance of a catalytic converter (referred to herein as converter, catalyst, three-way catalyst or TWC) includes the use of oxygen sensors within the exhaust gas system. By measuring the amount of oxygen in the exhaust gas entering and exiting the catalytic converter, an estimate of the amount of oxygen stored in the catalytic converter can be made. The dual oxygen sensor arrangement is utilized by nearly all automobile manufacturers for evaluating the performance of the catalytic converter. The system compares time dependent signals from the pre-catalyst exhaust gas oxygen (EGO) sensor and the post- (or mid-bed) catalyst EGO sensor to evaluate the oxygen storage capacity (OSC) of the converter. 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 within the converter 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, by estimating the oxygen storage capacity of the catalytic converter, an indirect measurement of the catalytic converter efficiency can be obtained. The main disadvantage of the system is that OSC is shown to poorly correlate to hydrocarbon conversion efficiency of the converter. Another disadvantage is that when applied to low emission vehicles (LEVs) and ultra low emission vehicles (ULEVs), it will be necessary to monitor increasingly smaller portions of the converter leading to less reliable correlations to total 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 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 such as described in United States patent 5,476,001 and co-pending United States patent application filed of even date herewith and entitled "Exhaust Gas Sensor" of which the present inventors are joint-inventors and the disclosure of which is hereby expressly incorporated herein by reference. A system disclosed in United States patent 5,265,417 utilizes pre-catalyst and post-catalyst HC sensors to calculate hydrocarbon conversion efficiency of the catalytic converter. There are several disadvantages to this system. The pre-catalyst and post-catalyst sensors must make measurements in significantly different gas mixtures (inferring the same exothermic reactions will not occur on sensors in the two locations) and then compare signals for evaluating performance. Another disadvantage derives from having to take a difference between a large signal and small signal that are measured at different times owing to the physical separation of the sensors. The time intervals for signal comparison must also change in a manner that corresponds to the mass air flow through the exhaust system. Still another disadvantage lies with the fact that the two sensors are not expected to age at equal rates because of the different environment each experiences during operation. Another approach described in United States patent 5,265,417 uses a sampling system to bring samples of pre-catalyst and post-catalyst exhaust gases to a single sensor for comparison of concentrations and determinations of HC conversion efficiencies. This is a high cost, low reliability system adding valves and switches for sampling. This system also has a relatively slow response time. Thus there is a need for an accurate, low cost and high reliability apparatus and method for determining catalytic converter operation and efficiency.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 illustrates, in cross-section, a differential calorimetric hydrocarbon gas sensor arranged for use in accordance with one embodiment of the invention;

Figure 2 illustrates a cross-sectional plan view of the differential calorimetric hydrocarbon gas sensor taken along line 2-2 in Figure 1 ;

Figure 3 is an exploded perspective view of a multi-layered substrate arranged in accordance with the invention;

Figure 4 illustrates, in cross-section, a differential calorimetric hydrocarbon gas sensor arranged in accordance with an alternative embodiment of the invention;

Figure 5 illustrates a cross-sectional plan view of the differential calorimetric hydrocarbon gas sensor taken along line 5-5 in Figure 4;

Figure 6 is an schematic illustration of a typical exhaust system for an automotive vehicle;

Figures 7a - 7c are schematic illustrations of alternate exhaust gas system configurations and the associated preferred sensor locations for such systems; Figure 8 is a flow chart illustrating in graphic form operation of a differential calorimetric hydrocarbon sensor in accordance with a preferred embodiment of the present invention;

Figure 9 is a flow chart illustrating in graphic form a method of determining catalytic converter operation in accordance with a preferred embodiment of the present invention;

Figures 10a and 10b are graphs demonstrating characteristics of the present invention in several preferred implementations;

Figure 11 is a partial cross-sectional view taken along line 11-11 of Figure 6 and illustrating an alternate sensor location; and

Figure 12 is a partial cross-sectional view taken along line 12-12 of Figure 6 further illustrating an alternate sensor location.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In an exemplary embodiment, the present invention provides an apparatus and method for processing signals obtained from a hydrocarbon gas sensor adapted within automobile exhaust system for determining operation of a catalytic converter that is part of the exhaust system. In this arrangement the apparatus functions in accordance with a preferred method to detect failure of the catalytic converter as mandated by legislation pertaining to motor vehicle On-Board Diagnostics

(OBD). Signals from the hydrocarbon sensor are collected, qualified and sorted according to the specific driving conditions under which they are generated. Elements within the engine control unit analyze the signals and determine the efficiency of various emission control systems on the vehicle, such as the catalytic converter's efficiency at converting hydrocarbons in the exhaust gas to non-polluting gas species.

The invention is not, of course, so limited in its application. In a preferred implementation described below the hydrocarbon sensor utilized is a differential calorimetric hydrocarbon sensor. It will be appreciated that the present invention has application to systems incorporating other sensor types. To facilitate understanding of the present invention, a hydrocarbon sensor suitable for use with the present invention in the exemplary application is described below. Hydrocarbon Gas Sensor Structure

Figure 1 illustrates, in cross-section, a differential calorimetric hydrocarbon gas sensor 10 arranged in accordance with one embodiment of the invention. A sensing element 12 includes a catalytic layer 14, defined by catalytic layers 14a, 14b, and a transport layer 16. An electrochemical oxygen source 18 is arranged in spaced relationship with sensing element 12. Electrochemical oxygen source 18 includes an outer electrode 20 and an inner electrode 22. The electrodes are separated by an electrolyte layer 24. Sensing element 12 is separated from electrochemical oxygen source 18 by a multi-layer substrate 26. As will subsequently be described, substrate 26 includes a plurality of overlying insulative layers on which electrical circuitry are arranged and define the temperature compensation circuitry and temperature reference circuitry. Additionally, substrate 26 includes a plurality of passageways or vias 28 extending through substrate 26. Vias 28 provide communication between transport layer 16 and an oxygen storage region 30. As will subsequently be described in greater detail, electrochemical oxygen source 18 and sensing element 12 are operatively disposed on opposite sides of substrate

26 in a stacked relationship such that substrate 26 regulates the temperature of both sensing element 12 and electrochemical oxygen source 18.

A diffusion barrier 32 overlies transport layer 16. Diffusion barrier 32 limits the rate at which exhaust gases diffuse to transport layer 16. Once in transport layer 16, the exhaust gases diffuse through transport layer 16 and are subsequently oxidized at an active surface 34, defined by active surface 34a and active surface 34b, of catalytic layer 14. Oxygen is transported from oxygen storage region 30 to transport layer 16 through vias 28 in sufficient quantities to permit complete oxidization of the relevant chemical compounds diffusing through diffusion barrier 32. By controlling the rate of diffusion of hydrocarbon species arriving at active surface 34, the rate of heat released by the oxidation reaction can be directly correlated with the concentration of hydrocarbons in transport layer 16.

Those skilled in the art will recognize that the concentration of hydrocarbons at active surface 34 is not equal to the concentration of hydrocarbons in the exhaust gas at some distance therefrom. The diffusive flux of hydrocarbons from the exhaust gas to active surface 34 is a function of the difference in the concentration of hydrocarbons at active surface 34 and the concentration of hydrocarbons elsewhere in the exhaust gas. In a preferred embodiment, the concentration of hydrocarbons at active surface 34 is substantially zero. Diffusion barrier 32 so limits the diffusive flux of hydrocarbon species to active surface 34 that substantially all of the hydrocarbons are oxidized upon arrival at active surface 34. Accordingly, the diffusive flux of hydrocarbons across diffusion barrier 32 will be proportional to the hydrocarbon concentration in the exhaust gas. Moreover, the diffusion of hydrocarbon species through diffusion barrier 32 is the rate determining step governing the transport of hydrocarbon species from the exhaust gas to active surface 34. Thus, the present invention permits a differential calorimetric measurement which accurately determines the hydrocarbon concentration in the exhaust gas by means of measuring the amount of heat released by exothermic catalytic reactions. Since all of the hydrocarbons must be completely oxidized to maintain a near-zero hydrocarbon concentration at active surface 34, the heat released by the exothermic oxidation reaction will also be proportional to the hydrocarbon concentration in the exhaust gas. Furthermore, with diffusive transport through diffusion barrier 32 largely determining the flux of hydrocarbon species arriving at active surface 34, this flux is less sensitive to other elements of the hydrocarbon transport process such as diffusion through the exhaust gas or convective transport by the exhaust gas. As a result, the proportionality constant between the heat released by exothermic oxidation and the hydrocarbon concentration in the exhaust gas is largely unaltered by variations in other transport processes.

In addition to limiting the hydrocarbon diffusive flux to active surface 34, diffusion barrier 32 also functions to protect catalytic layer 14 from scouring by particles entrained in the flowing exhaust gas. Further, by reducing heat exchange between the sensor and the environment, diffusion barrier 32 functions as a thermal barrier to limit temperature fluctuations in the heat measuring devices located in heating element 26. In the absence of diffusion barrier 32, temperature fluctuations in the exhaust gas may be transmitted to the heat measuring devices without attenuation, causing signal noise in the sensor output. The ability of diffusion barrier 32 to maintain a stable diffusion rate and to provide a suitable thermal barrier is enhanced by thermally coupling diffusion barrier

32 directly to substrate 26, and indirectly (through interface region 16) to catalytic layer 14. Preferably, diffusion barrier 32 has a low-porosity and is constructed from a material, such as spinel, alumina, cordierite, mullite, steatite, stabilized zirconia or other porous ceramic. Preferably, transport layer 16 has a high-porosity and is constructed from a material, such as spinel, alumina, cordierite, mullite, steatite, stabilized zirconia or other porous ceramic. Alternatively, transport layer 16 can be a gas cavity within calorimetric hydrocarbon gas sensor 10.

As presently preferred, electrochemical oxygen source 18 is a multi-layered element operatively disposed beneath substrate 26. More specifically, outer and inner ceramic layers 36 and 38 are respectively disposed on a distal end of electrolyte 24. Outer and inner electrodes 20 and 22 are respectively disposed on a proximal end of electrolyte

24. Preferably, electrolyte 24 is yttrium stabilized zirconia, and outer and inner electrodes 20 and 22 are constructed of porous platinum metal. Placing a DC voltage across electrodes 20 and 22 generates oxygen ions by breaking down water and carbon dioxide in the exhaust gas at outer electrode 20 and conducting oxygen ions through electrolyte 24 to inner electrode 22. A porous protective layer 40 overlies outer electrode 20 and extends from outer ceramic layer 36 to a sensor end wall 42. Porous protective layer 40 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, protective layer 40 has a high porosity and is constructed from a material, such as spinel, alumina, cordierite, mullite, steatite, stabilized zirconia or other porous ceramic.

Oxygen is desorbed from inner electrode 22 and contained within oxygen storage region 30. Oxygen storage region 30 extends from inner ceramic layer 38 to end wall 42. Oxygen within oxygen storage region 30 is transported through vias 28 to transport layer 16. In addition to providing an oxygen supply source for catalytic oxidation of combustible species at active surface 34, oxygen source 18 can be operated in reverse by reversing the polarity of the DC voltage applied across electrodes 20 and 22 to remove oxygen from transport layer 16 and oxygen storage region 30. Furthermore, oxygen source 18 can be used to break down water, oxygen and carbon dioxide at inner electrode 22. Hydrogen and carbon monoxide produced in this process are desorbed from inner electrode 22 into oxygen storage region 30 and diffuse through vias 28 into interface region 16. Oxygen ions are conducted across electrolyte 24 to outer electrode 20 and desorbed into the exhaust gas. Oxygen entering interface region 16 through diffusion barrier 32 is preferentially consumed by catalyzed combustion with hydrogen and carbon monoxide, limiting hydrocarbon combustion with oxygen from that source. By removing the available oxygen, the oxidized combustion of hydrocarbons within calorimetric hydrocarbon gas sensor 10 can be effectively terminated so that differential calorimetric hydrocarbon gas sensor 10 can be calibrated after installation into an automotive exhaust gas system,

Figure 2 shows a cross-sectional plan view of differential calorimetric hydrocarbon gas sensor 10 illustrating the arrangement of catalytic layer 34 and vias 28. Catalytic layer 34 is partitioned into an active region 14a and a reference region 14b.

Active region 14a includes a catalyst composition specifically formulated to catalyze the oxidation of all relevant combustible gas species at active surface 34a. More specifically, the catalyst in active region 14a accelerates the oxidation of substantially all non-methane hydrocarbons, carbon monoxide and hydrogen. Reference region 14b includes a catalyst composition specifically formulated to catalyze the oxidation of selective gas species at reference surface 34b. More specifically, the catalyst in reference region 14b accelerates the oxidation of carbon monoxide (CO) and hydrogen (H₂) but not hydrocarbons. By utilizing active region 14a and reference region 14b in catalyst layer 34 which differ in their catalytic activity for hydrocarbon oxidation, it is possible to implement a differential calorimetric measurement of the differences in the heat released by the oxidation of reference region 14b and by active region 14a. More specifically, the exothermic reaction heat measured by heat sensing circuitry (such as circuitry within engine control unit 264, Figure 6) for reactions taking place at active surface 34a can be compared with the exothermic reaction heat measured by the heat sensing circuitry for reactions taking place at reference surface 34b. The difference in the amount of heat produced between active region 14a and reference region 14b can be attributable to the oxidation of specific hydrocarbon species within the exhaust gas.

With reference to Figure 3, an exploded perspective view of substrate 26 is illustrated showing a plurality of overlying insulative layers that are laminated together to form a multi-layered substrate 26. In a preferred embodiment, each of the plurality of overlying insulative layers are ceramic layers laminated together to form a multi-layered ceramic substrate. Multi-layer green tape technology provides a capable and economic, and thus preferred, process for production of substrate 26. With the exception of top layer 44, each ceramic layer supports screen-printed metalization arranged in different patterns to define the various functional elements necessary to measure the heat and control the temperature within differential calorimetric hydrocarbon gas sensor 10.

More specifically, temperature reference circuitry includes temperature-sensitive elements 46a, 46b such as resistive temperature detectors (RTDs) or thermocouples which overlie intermediate layer 48. Temperature-sensitive elements 46a, 46b reside directly below active region 14a and reference region 14b, respectively, and function to measure the temperature, for use by temperature compensation circuitry in maintaining a substantially isothermal condition at active and reference regions 14a, 14b disposed on the surface of top layer 44. The temperature compensation circuitry includes heat-generating elements 50a, 50b such as resistive compensation heaters disposed on the surface of intermediate layer 52 directly below temperature-sensitive elements 46a, 46b, respectively, to maintain substrate 26 and catalytic layer 14 at a substantially constant operating temperature. Thus, gas sensor 10 can be conceptually thought of including a pair of calorimeters — the active calorimeter or calorimeter A including active surface 34a, temperature-sensitive element 46a and compensation heater 50a and the reference calorimeter or calorimeter B including reference surface 34b, temperature-sensitive element 46b and compensation heater 50b.

Multi-layered substrate 26 further includes a first primary heater 58 located on intermediate layer 60, and a second primary heater 62 located on bottom layer 64 below first primary heater 58. Primary heaters 58, 62 are intended to supply the highly variable and often large quantities of heat needed to maintain calorimeter A and calorimeter B within a specified temperature range. Electrical ground plane 54 defined by a metalized layer overlies intermediate layer 56 and electrically isolates compensation heaters 50a, 50b from a first primary heater 58 and second primary heater 60. Importantly, electrical ground plane 54 is located in intermediate layer 56 and overlies intermediate layer 60 and bottom layer 64. In addition to the various metalized ceramic layers shown in Figure 3, substrate 26 may include additional layers to promote the adhesive bonding of the various layers constituting substrate 26. In addition to the various ceramic layers shown in FIG. 3, substrate 26 can include additional layers to promote the mechanical strength and optimize the thermal conductance of substrate 26. Importantly, the metalization overlying intermediate layers 56 and 60, and bottom layer 64 includes a plurality of slots 66 which function to promote adhesive bonding within substrate 26.

In general, substrate 26 is fabricated by first forming electrical vias 68 in each layer of green tape and filling the vias with metal, followed by screen-printing metalization onto individual layers of ceramic green tape. Vias 68 provide access to the respective metalized layers to establish electrical communication between the various layers of substrate 26, while slots 66 function to promote bonding between ceramic layers of substrate 26. Additionally, vias 68 provide access to the respective metalized layers to attach lead wires to substrate 26 for communication between differential calorimetric hydrocarbon gas sensor 10 and control circuitry (not shown), such as may be implemented in the engine control unit 264 or as part of sensor 10, which control sensor 10. After the individual ceramic green tape layers have been impregnated with the appropriate metalizations, multi-layer substrate 26 is assembled and fired. After cooling, catalytic layer 14 including active region 14a and reference region 14b is deposited on top layer 44 and fired to promote adhesion of catalytic layer 14 thereto.

An alternative embodiment of the invention is illustrated, in cross-section, in Figures 4-5. Reference numerals incremented by a factor of one hundred (100) have been used to indicate element in Figures 4-5 which most closely corresponds to elements illustrated in Figures 1-2. Differential calorimetric hydrocarbon gas sensor 110 includes a catalytic layer 114 defined by catalytic layers 114a, 114b and having catalytic surfaces 134c, 134b formed thereon, encased within a low-porosity diffusion barrier 132 and separated therefrom by a high-porosity transport layer 116. Catalytic layer 114 overlies a multi-layer substrate 126. An oxygen source 118 resides below substrate 126 generally opposite from catalytic layer 114. Oxygen source 118 includes a zirconia electrolyte layer 124 and porous platinum electrodes 120, 122. Oxygen source 118 is separated from substrate 126 by an oxygen storage region 130. Areas of oxygen storage region 130 not in contact with platinum electrode 122 are covered by an impermeable layer 142. Oxygen source 118 and oxygen storage region 130 are encased within a porous protective layer 140. In comparison with the previous embodiment, the oxygen source of the embodiment illustrated in Figure 14 includes a less extensive zirconia electrolyte layer. By restricting the longitudinal extent of the oxygen source, more precise thermal management is possible within the differential calorimetric hydrocarbon gas sensor. A cross-sectional plan view of calorimetric hydrocarbon gas sensor 110 is shown in Figure 5. A plurality of vias 128 extend through substrate 126 and connect transport layer 116 and oxygen storage region 130. In a manner analogous to the previous embodiment, vias 128 provide a diffusion pathway for oxygen produced by oxygen source 118 to diffuse through substrate 126. In contrast with the previous embodiment, vias 128 are aligned adjacent to low-porosity diffusion barrier 132. By comparing Figures 2 and 5 it becomes apparent that the invention contemplates the formation of vias in the substrate in a variety of geometric configurations. Additionally, it is contemplated that the vias be open, or alternatively that they be filled with a porous material. Catalytic oxidation of total combustibles of the exhaust gas, including hydrocarbons, is carried out in active region 114a, while oxidation of selective combustibles of the exhaust gas, excluding hydrocarbons, occurs at reference region 114b. A plurality of vias 168 provide electrical connection to the various layers within substrate 126, as well as providing access to the respective metalized layers to attach lead wires to substrate 126 for communication between differential calorimetric hydrocarbon gas sensor

110 and external electronic circuitry (not shown).

Catalytic Compositions

As described above, the present invention employs two different catalysts within catalytic layer 14 ~ a total combustible catalyst or active region 14a and a CO selective catalyst or reference region 14b. Active region 14a is a stable, high-activity catalyst composition which, at suitable oxidizing conditions and temperatures, will completely convert all residual combustible gases such as non-methane hydrocarbons, carbon monoxide, and hydrogen to carbon dioxide and water. The composition of active region 14a is designed to provide complete combustion of substantially all residual combustible gases, at temperatures greater than approximately 450°C when a sufficient concentration of oxidizing agent, such as oxygen or air or other oxidizing material, is present. The chemical and physical properties of active region 14a are able to withstand the operational environments of the automotive exhaust gas.

The catalyst composition for active region 14a comprises active metal components such as one or more of the following elements: platinum, rhodium, palladium, iridium, and ruthenium. Generally, platinum, rhodium and palladium are preferred. Combinations of platinum and rhodium are even more preferred. These active metals are preferably supported on a stable refractory support such as alumina, zirconia, titania, silica, silica alumina or other similar ceramic materials. High surface area materials such as gamma alumina are preferred. Optionally, an oxygen storage material such as ceria may be added to the catalyst formulation. However, this material is not essential for utility in a sensor where there is sufficient oxidizing agent present in the environment or where an oxidizing agent, such as air or oxygen, is provided by external means. Thus, even more preferred are refractory materials that are especially stabilized by thermal, geothermal or chemical means, such as precalcined alumina and ceria stabilized zirconia, a disclosure of which is provided in U.S. Patent No. 5,057,483 entitled "Catalyst composition containing segrated platinum and rhodium components" issued October 15, 1991 to C.Z. Wan and assigned to Engelhard, the disclosure of which is expressly incorporated by reference herein. The particle size of the catalyst should be such that a binder can be used to adhere the catalyst formulation to substrate 26 of sensor 10. In addition, the particle size and uniformity of the catalyst should be such that the processes for catalyst deposition, such as screen printing, are feasible. In general, the mean particle size of the catalyst material should be less than ten microns in diameter with a more or less normal distribution about that mean. More preferred is a mean particle size of approximately 5 microns in diameter. Specific examples of presently preferred catalyst compositions are disclosed and described in co-pending United States Patent Application filed of even date herewith and entitled "Exhaust Gas Sensor Incorporating an Oxygen Source" the present inventors being joint inventors thereof and the disclosure of which is hereby expressly incorporated herein by reference. Hydrocarbon Gas Sensor Placement

As previously discussed, a use for the differential calorimetric hydrocarbon sensor is providing a signal corresponding to hydrocarbon concentrations in an exhaust gas stream of an internal combustion engine powered vehicle. In the preferred implementation the differential calorimetric hydrocarbon gas sensor provides a signal that correlates to non-methane hydrocarbon concentrations in post-converter exhaust gas under a wide range of operating conditions, e.g., exhaust gas flow rate, exhaust gas temperature and air-to-fuel ratio (AFR). The device is preferably designed to be independent of operating conditions; however, complete independence cannot be achieved and thus sensor placement is significant. In this regard, and as discussed more fully in the aforementioned United States patent application, appropriate sensor placement is dictated by the exhaust gas flow rate, the exhaust gas temperature, the air-to-fuel ratio, the catalyst temperature, and condensation prevention.

With reference now to Figure 6, automotive vehicle 200 includes internal combustion engine 202 having exhaust system 204 operatively connected thereto. Engine control unit 264 includes control circuitry and heat sensing circuitry for operating sensor assembly 216. Engine control unit 264 may contain dedicated hardware circuitry, but more typically, engine control unit 264 contains at least a processor and memory having a data structure for storing control programs, operation parameters and operating data as is well known in the art. Exhaust system 204 includes drop down pipe 206 connected at a first end to engine 202 at an exhaust manifold (not shown) and connected at a second end to the inlet of catalytic converter 208. Intermediate pipe 210 is connected at a first end to the outlet of catalytic converter 208 and extends toward the rear of vehicle 200 where it is connected with the inlet of muffler 212. Tail pipe 214 has a first end which is connected to the outlet of muffler 212 and a second end which is open to the atmosphere.

As presently preferred, exhaust gas sensor assembly 216 of the present invention may be located at any point rearward of catalytic converter 208 in a substantially straight portion of the exhaust system. For example, exhaust gas sensor assembly 216 could be located in the straight portion 210a of intermediate pipe 210 directly aft of catalytic converter 208, in the straight portion 210b of intermediate pipe 210 directly forward of muffler 212, or alternately in the straight portion 214a of tail pipe 214 directly aft of muffler 212. Hence a great amount of flexibility is offered to the vehicle designer in sensor placement. Placement of exhaust gas sensor assembly 216 in a substantially straight portion of the exhaust system minimizes the effects of flow turbulence and permits the sensor to be inserted substantially peφendicular into the exhaust gas flow. By providing an orientation substantially peφendicular to the exhaust gas flow results in more reproducible results and minimizes application-to-application variations in sampling and sensor response times, thereby providing accurate evaluation of the catalytic converter. In addition to sensor 216, sensor 218 of the same or similar construction as sensor 216, is placed in a pre-catalyst position. For example, as shown in Figure 6, sensor 218 is located in drop down pipe 206. During vehicle operation, sensors 216 and 218 operate as described in the aforementioned United States patent application.

Converter Efficiency Determination

In accordance with a preferred embodiment of the present invention, signals are collected by engine control unit 264 from sensor 216 concurrently with one or more of engine revolutions per minute (RPM), manifold vacuum, throttle position, fuel flow, air/fuel (A/F) ratio (both pre-converter and post-converter), transmission position, vehicle speed, exhaust gas oxygen (EGO) sensor data and coolant temperature and are stored within a data structure of the memory of engine control unit 264. Signals may be collected at a suitable rate based upon sensor response time, and one signal per second data collection is contemplated. It should be further appreciated that to reduce memory requirements for the collected data, mean and/or average data representing the sensor signals and the corresponding operating conditions may be calculated and stored. The sensor signals are sorted according to a specific vehicle operating parameter, for example a directly measured operating condition such as RPM or a calculated operating condition such as mass flow rate. From the sorted signals a relationship is established between the sensor signals and the operating parameter. In a preferred embodiment, a line is best fit to the data. A slope is calculated for the best fit line, and the value of the slope is used as a measure of the catalyst performance. Of course, a higher order correlation or other statistical relationship may be used without departing from the fair scope of the preset invention. Comparison to a predetermined threshold or a threshold that is engine out emissions or vehicle mileage dependent, enables catalyst evaluation. It is noted that as the converter ages, the slope of the best fit line of sensor output versus engine RPM increases, and is indicative of converter efficiency. A certain amount of decrease in the converter is allowable, and hence the slope threshold adjustment is provided, but when the slope of the best fit line exceeds the adjusted threshold, there is an indication that the converter efficiency has fallen below an acceptable level.

The above process for evaluating converter efficiency is illustrated in Figure 8. At engine start 800, vehicle mileage is checked and a slope threshold is determined 802 based upon the mileage. One will appreciate that vehicle systems differ as do emission control requirements (e.g., California Air Resources Board (CARB) emission control requirements as compared to Federal emission control requirements). The slope threshold indicating system malfunction and the threshold adjustment criteria are determined and calibrated for each vehicle system in order for emission control requirements to be met. Threshold adjustment criteria may be in the form of look-up calibration tables, or as calibration algorithms retained in engine control unit 264 memory.

As will be discussed below, for catalytic converter diagnosis, the same process is performed to evaluate engine out emissions. Both tailpipe emissions and engine out emissions are considered, although not directly compared, before illuminating either the catalytic converter or engine emission malfunction indicator lights as will be described. Evaluation of the catalyst before it is fully operational is difficult, and because of the number of variables that can influence converter light-off, hydrocarbon emissions before the converter heats up can vary significantly. It is possible, and may be useful, to correlate converter light-off to engine coolant temperature and to use this correlation as an indication as to when to turn on sensor 216 to begin making measurements. It may also be useful to wait until the engine has gone into closed loop control, i.e., the engine system has come up to normal operating temperature and has exited open loop cold-start mode or to evaluate EGO sensor output to identify lean/rich operation and exclude data accordingly. What should be appreciated is that it is contemplated to selectively collect the data. For example, a simple approach for selectively collecting data that has proven effective in the present invention is to introduce a delay, step 804, prior to taking measurements. It is contemplated to delay approximately about 100 - 120 seconds before beginning sensor measurements. After the delay, the sensor is turned on, step 806, and after a suitable period to allow the sensor to come to full operation, approximately about 20 seconds, sensor signal data concurrent with vehicle operation data is collected, step 808. Next the data is sorted and the slope calculated, step 810. The slope calculated at step 810 is compared with the slope threshold established at step 802, step 812. If the calculated slope is below the threshold, the data collection, correlation process, steps 806 - 810, continues. If the slope exceeds the threshold, a failure signal is sent for catalytic converter diagnosis, step 814.

Catalytic converter diagnosis is illustrated in Figure 9. As noted above, if tailpipe emissions, i.e., post-converter emissions, exceed acceptable levels the reason may be converter failure. However, the high tailpipe emissions may also be the result of high engine out emissions or a combination of both high engine out emissions and converter failure. The diagnostic cycle begins at engine start, step 900. An acceptable emissions level, based upon the vehicle mileage is determined, step 902 and retained for later evaluation. As discussed above, a delay of approximately 120 seconds is introduced to provide reasonable assurance the engine and converter system have attained sufficient operating temperature, step 904. At step 906, the tailpipe emissions are evaluated in accordance with the process illustrated in Figure 8. At step 908 the engine out emissions are evaluated also in accordance with the process illustrated in Figure 8. That is, a hydrocarbon sensor 218 in its pre-catalyst location measures in correlation to various operating parameters the engine out emissions. The data are sorted and evaluated as described to determine if the engine out emissions meet acceptable parameters. In accordance with the present invention, if engine out emissions are acceptable, but the converter evaluation returns a failure signal, then the converter failure malfunction indicator light (MEL) is set, step 910. Otherwise, the engine emissions MEL is set, step

912. If no error is detected, the test is passed, step 914.

Second generation On-Board Diagnostics (OBD-II) regulations require that an indication be made when tailpipe emissions exceed certain levels. Sensor assembly

216 will provide a signal which correlates to exhaust gas non-methane hydrocarbon concentrations which may be used to evaluate converter efficiency. One of ordinary skill in the art will appreciate that where sensor assembly 216 is packaged so as to make it mass transfer limited in operation, it is necessary to multiply the sensor signal by a mass air flow signal from the vehicle. If as detected by signals from sensor 216 the tailpipe emissions are lower than the regulated value, no problem exists, and it is not necessary to determine conversion efficiency of the converter or to examine engine out emissions. When the tailpipe emissions are too high, the problem may be either converter failure, engine out emissions at a level which may not be properly compensated for by the converter or a combination of both. However, when engine out emissions for HC, CO and NOx are abnormally high, it would likely not be possible to accurately diagnose catalyst performance owing to the unusually high exothermic and/or incorrect air-to-fuel ratios that would prevent proper operation of the converter. Therefore, for evaluating the efficiency of the converter, it is only necessary to determine if engine out emissions are normal when tailpipe emissions are excessive. Reducing the collective signals to a single value (or several values) affords the additional advantages of being able to better compare results to the Federal Testing Protocol (FTP) driving cycle (which can be used to establish acceptable threshold adjustment criteria for indicating catalyst failure); allows for driver-to-driver variability and driving cycle-to-driving cycle variability that will change actual run-to-run emission levels from a vehicle; and provides threshold adjustment and threshold adjustment criteria based upon predictable sensor aging and/or changes in regulated emissions at different vehicle mileage. To limit drive cycle variation effects on catalyst evaluation, selective collection, or windowing the data to preferred vehicle-operating conditions, of the data may be necessary. Preferably data that is atypical of operation is eliminated to help smooth the plot and eliminate extraneous data points that may induce error in the slope calculation. For example, high and low engine speeds may cause scattering of sensor signals. Therefore, a nominal engine speed operating range may be selected for data collection.

Alternative approaches may utilize information from the pre-catalyst and post-catalyst EGO sensors. Data indicating, for example, that the air/fuel (A/F) ratio exceeds 1.00 suggests operating conditions are sufficiently lean to ensure reliable hydrocarbon sensor signals. Similarly, data following sudden throttle changes may be eliminated. Sudden throttle position changes, such as sudden throttle closing, may result in loss of stoichiometric engine control introducing noise into the hydrocarbon sensor signals. Furthermore, A/F data may be collected concurrently with sensor signals and used to determine if full combustion is possible. Sensor signals for rich conditions may be eliminated or qualified. Driving conditions, such as trailer towing or steep hill climbing, may cause variation in the sensor signals. By observing engine RPM, gear ratios, manifold air pressure (MAP), mass air flow (MAF) and time under condition provides an indication that an unusual driving condition exists and that sensor data should be eliminated. Additionally, data relating to environmental and operating temperature may be utilized to anticipate catalyst light off. For example, data for very cold days may be eliminated, or qualified, to prevent premature signaling of converter failure. Additionally, pre-catalyst and post-catalyst exhaust gas temperatures may be utilized to qualify the sensor signal and reduce signal noise due to engine transients and to reduce premature converter malfunction indications. An additional use of the signal value evaluation of catalyst operation is to determine engine and catalyst aging trends. Slopes calculated from the evaluation algorithms can be stored in a separate data structure and charted with time. The trends in the values can be studied to determine if aging is progressive, and may provide a diagnostic indication of engine or catalyst system problems. For example, sudden changes in the sensor signal without a concomitant change in engine out emissions may be an indication of sensor failure. Sudden changes in engine out emissions may be indicative of misfire.

With reference to Figures 7a - 7c, it will be appreciated that there are many configurations for exhaust gas systems in motor vehicles. These alternate exhaust gas system configurations may suggest differing strategies for monitoring converter performance. For simplicity, the same reference numerals are repeated to represent like elements within the exhaust systems. Shown in Figure 7a, a dual-bank exhaust system 204a is shown coupled to engine 202. Each bank, includes a drop down pipe 206a and 206b respectively and catalytic converters 208a and 208b. The remainder of each bank is shown simply as pipe 210a and 210b, while one of ordinary skill in the art will appreciate that the system will include suitable mufflers and/or resonators and tail pipes. Positioned within each bank are hydrocarbon sensors 216a, 216b and 218a and 218b, respectively. Sensors 216a and 216b are positioned in the post converter location while sensors 218a and 218b in the pre-converter locations. Since each bank operates as a separate exhaust gas system, each may be independently monitored in accordance with the present invention to determine converter performance.

Referring to Figure 7b, a dual bank system 204b is shown to include close-coupled converters 208a and 208b in each bank, respectively. Sensors 216a, 216b and 218a, 218b are positioned in post-converter and pre-converter locations in each exhaust bank, respectively. The close-couple converters 208a and 208b, positioned substantially closer to the engine exhaust gas manifolds, will light-off appreciably faster than a converter positioned further downstream in the exhaust system. Therefore, it may not be necessary to exclude the first several minutes of vehicle operation data. Because of the location of the close-coupled converters 208a and 208b near the exhaust gas manifold, these converters will also experience substantially higher operating temperature and flow fluctuations. In this regard, one of skill in the art will appreciate the necessity of excluding, for example, low or high φm data, rapidly changing φm data, or other data collected during periods suggesting rapidly changing operating conditions at converters 208a and 208b.

In Figure 7c, exhaust gas system 204c includes close-coupled light-off converters 220a and 220b. Downstream, the banks are joined into a single intermediate pipe 210 which includes converter 208. Sensor 216 is shown positioned in a post-converter location in pipe 210. Sensors 218a and 218b are positioned upstream of the close-coupled light-off converters. One will again appreciate that the fast light-off nature of the close-coupled converters 220a and 220b may eliminate the need to exclude certain early operation data. In addition, the downstream location of converter 208 and sensor 216 may eliminate certain flow and temperature variation noise allowing sampling over a wider range of operating conditions.

What is readily appreciated from the foregoing discussion is that many possible configurations for exhaust gas systems and corresponding sensor location and converter evaluation strategy. Several examples of such systems have been shown, but it will be appreciated that the present invention has application to all such exhaust gas systems. Additionally, the present invention may be adapted for partial system monitoring. For example, the system may be adapted, through sensor location and parameter selection, to monitor a close-coupled converter independent of overall system performance. Moreover, selection of the evaluation criteria will reflect the exhaust gas system and expected operation conditions. One should immediately appreciate that the present invention is not limited to a particular exhaust gas system configuration, data selection criteria or evaluation strategy.

Referring to Figures 11 and 12 still another configuration is shown in which a hydrocarbon sensor 250 is integrated directly into a catalytic converter 252. Converter 252 includes an outer housing 262 into which a catalyst substrate 254 is secured. Catalyst substrate 254 includes a cell structure 258 as is known in the art, but it also includes sensor region 256 formed by removing a portion of cell structure 258. Hydrocarbon sensor 250 may be either mechanically secured or cemented into region 256 with electrical connections being made using mineral insulated cable. Exhaust flow enters converter 252 at entrance 260. Cell structure 258 acts essentially as a laminar flow device for directing exhaust gas over sensor 250 thus limiting sensor variation due to flow disturbances.

The graph shown in Figure 10a demonstrates for several sets of data collected from vehicle testing the effectiveness of the present invention for evaluating converter performance. The data shown in the graph illustrate the correlation of slope values versus grams of hydrocarbons per mile ( g(HC)/mile) emissions for a series of catalytic converters during FTP-75 runs. The filtering for this data contemplated excluding all data points for which gas temperature was less than 400° C. The graph demonstrates that increases in slope value, as calculated in accordance with preferred embodiments of the present invention, indicates a likely increase in hydrocarbon emissions which may require further evaluation of vehicle systems performance. In Figure 10b, another graph presents correlation data for several different vehicle types having different exhaust systems and catalytic converters. The filter for this data contemplated excluding all data points within the first 120 seconds of vehicle operation. The graphs of Figures 10a and 10b illustrate that different vehicle types will have different slope/hydrocarbon emission correlation characteristics, but retain a consistent relationship usable for converter performance evaluation. What is important to consider and the data demonstrate is that the slope will change in a predictable manner with normal aging of the converter. Should the vehicle malfunction in a manner not generally associated with aging, dramatic changes in slope values will provide an indication of such malfunctions. Thus the present invention provides both a means of determining system failure due to aging as well as due to premature malfunction.

Several preferred embodiments of the present invention provide a means and process for diagnosing tailpipe emissions and engine out emissions. More specifically, the present invention is adaptable to determine whether either of the tailpipe emissions or engine out emissions exceed standards utilizing, for example, differential calorimetric hydrocarbon sensors. 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 illustrative embodiments. Those skilled in the art will recognize that variations and modifications can be made without departing from the spirit of the invention. 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

CLAIMS WE CLAIM:

1. A method of determining catalytic converter operation in an internal combustion engine powered vehicle (200) having an emission control system (204) including a catalytic converter (208), the method comprising: sensing a characteristic of an exhaust gas stream within the emission control (204) system, sensing at least one vehicle operation parameter; forming a relationship between the sensed characteristic and the sensed vehicle operation parameter; and using the relationship for determining the catalytic converter operation.

2. The method of claim 1 comprising the step of correlating the sensed characteristic and the at least one vehicle operation parameter.

3. The method of claim 2 wherein the at least one other signal comprises at least one of: engine revolutions per minute, coolant temperature, exhaust gas oxygen sensor data, air-to-fuel ratio, throttle position, manifold vacuum, gear ratio, vehicle speed and fuel flow.

4. The method of claim 2 wherein the relationship comprises one of a slope of a best fit line of the sensed characteristic versus the at least one vehicle operation parameters.

5. The method of claim 1 wherein the characteristic comprises one of total hydrocarbon concentration, non-methane hydrocarbon concentration, carbon monoxide concentration, hydrogen concentration and oxides of nitrogen concentration.

6. The method of claim 1 wherein the step of sensing a characteristic comprises sensing the characteristic at one of a position adjacent a close-coupled converter, within the catalytic converter and down stream of the catalytic converter.

7. The method of claim 1 further comprising sensing an pre-catalyst characteristic of the exhaust gas stream up stream of the catalytic converter forming a relationship between the sensed pre-catalyst characteristic and the sensed vehicle operation parameter and using the relationship for determining engine operation.

8. The method of claim 1 wherein further comprising comparing catalytic converter operation and engine operation for determining catalytic converter operation.

9. In an internal combustion engine powered vehicle (200) having an emission control system (204)including a catalytic converter (208), an apparatus for determining catalytic converter operation comprising: a sensor (216) located within the emission control system (209), the sensor having an output indicative of a characteristic of exhaust gas flowing in the emission control system (204); a processor (264) coupled to receive the sensor signal and at least one other signal indicative of vehicle operation, a memory coupled to the processor, the memory including a data structure for storing the sensor signal and at least one other signal and the processor operable upon the sensor signal and the at least one other signal for forming a correlation therebetween and for evaluating a characteristic of the correlation to determine catalytic converter operation.

10. The apparatus of claim 9 wherein the at least one other signal comprises at least one of: engine revolutions per minute, coolant temperature, exhaust gas oxygen sensor data, air-to-fuel ratio, throttle position, manifold vacuum, gear ratio, vehicle speed, fuel flow and a computed operating parameter.