Abstract:
The present invention is a method and apparatus for measuring the total NO x  concentration in a gas stream utilizing the principles of a NO x  sensor, i.e., mixed potential sensor. The exhaust gas is first conditioned by a catalyst assembly that converts the various species of nitrogen oxide gases present to a fixed steady state concentration ratio of NO 2 /NO, where NO 2  is approximately 0–10% of the total NO x  concentration present in the gas exhaust, thereby enabling the NO x  sensor to generate a meaningful and reproducible determination of the concentration of total NO x  present in the gas being measured. The catalyst assembly also functions to oxidize any unburned combustibles such as CH 4 , CO, etc., and remove potential contaminants such as SO 2 .

Description:
RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Patent Application No. 60/574,622, filed May 26, 2004, and is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The present invention relates in general to the measurement of NO x  gases in exhaust streams generated from the combustion of hydrocarbons and particularly the combustion of diesel fuels in cars and trucks. 
     BACKGROUND OF THE INVENTION 
     One known NO x  sensor is configured as a flat plate multilayer ceramic package design that includes two or more chambers. In the first chamber there are electrodes attached to an oxygen ion conducting electrolyte membrane, thereby forming an oxygen pump to remove the oxygen. In addition, NO 2  is decomposed to NO and one-half O 2 . The free oxygen is removed in the first chamber so that theoretically the only gas that enters the second chamber is NO. Another oxygen pump is in the second chamber and is a NO decomposing element that removes the oxygen from the NO. The electrical current produced from the decomposition of NO and the transport of oxygen is correlated to the NO concentration. 
     There are a number of concerns that affect the commercial application of this known NO x  sensor. For example, when the NO x  concentration to be detected is low, there is significant interference from the residual oxygen. In addition, the signal current is very small, thus making it susceptible to electronic noise commonly found in an automobile. Also, the exhaust gas typically has pulsations in the flow rate caused by cylinder firings that influence the ability of the oxygen pump to effectively remove all of the free oxygen and may result in measurement error. This device may also contain a small diffusion hole that limits the passage of gas into the measurement chambers and is prone to clogging. 
     Another known NO x  sensor utilizes a similar flat plate multilayer ceramic package design. There are a few significant differences in the operation principle for this sensor; namely, the sensor is a mixed potential type rather than amperometric, and the use of the first chamber is for converting NO to NO 2  and vice versa. It is a well established phenomenon of mixed potential NO x  sensors that the voltage signal generated from the gas species NO and NO 2  are of opposite sign, thereby making it difficult to distinguish a meaningful voltage signal in the presence of both gases. Some sensors have attempted to overcome this problem by utilizing the flat plate multilayer package type design with two separate chambers built into the design. Attempts have also been made to convert all of the NO x  gas species into a single species with the use of an electrochemical oxygen pump that pumps oxygen into the first chamber—thereby converting all of the gas to NO 2 —or conversely by removing oxygen from the chamber and reducing all of the NO 2  to NO. This conditioned gas then passes into the second chamber where the NO x  concentration is measured by the voltage signal generated from a mixed potential type sensor. 
     There are a number of limitations to this approach that have hampered the commercialization of this configuration. One significant concern is the reproducibility of the conversion system to completely convert all the NO x  gases into a single species under varying gas concentration conditions. In addition, the oxygen pump conversion cell tends to degrade with time, further contributing to the issue of reproducibility. Because the effects of these concerns are magnified in the low concentration range, this measurement approach is not well suited for detecting low concentrations of NO x  gases. 
     Additional drawbacks common to both of the sensor mechanisms disclosed above stem from the fundamental design of the flat plate ceramic multilayer system. Response times tend to be slow because of the complexity of the device where gas first enters a diffusion port, is conditioned in a first chamber, and then diffuses into a second chamber. Achieving rapid gas exchange that can keep up with the dynamic environment of the engine exhaust is difficult to achieve in these configurations. Also, the corrosiveness of the gas—along with fine particulates—may result in the clogging of the diffusion controlling port, or at the very least, changes in the gas flow dynamics with time. Finally, the pulsations in the gas flow rates due to cylinder firings and the accompanying electrical noise typical of automobiles make it difficult to control and monitor the low voltage and current circuits associated with these devices. 
     Another known NO x  sensor utilizes a zeolite catalyst to condition the gas prior to being measured by the sensor. Although this catalyst has been demonstrated to be effective in controlled gas environments, no data has been reported wherein the catalyst has suitably performed in H 2 O containing gases. Exhaust gases from combustion processes such as diesel exhaust always contain some H 2 O vapor as this is one of the major chemical byproducts of combustion of hydrocarbon fuels along with CO 2 . As such, the utilization of the NO x  sensor incorporating a zeolite catalyst in such applications is limited because of the catalyst&#39;s well known instability in the presence of H 2 O. 
     The present invention is provided to address these and other considerations. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method and apparatus for determining NO x  concentration of an exhaust gas. The apparatus comprises an input assembly capable of receiving the exhaust gas and producing a conditioned gas output. The input assembly includes at least three of the following stages: a stage including a catalyst structure for converting NH 3  in the exhaust gas to N 2  and H 2 O; a stage including a catalyst structure for absorbing SO 2  or H 2 S from the exhaust gas; a stage including a catalyst structure for oxidizing unburned hydrocarbons and gases to higher oxidation states; and a stage including a catalyst structure to establish a steady state equilibrium concentration ratio between NO and NO 2 . A NO x  sensor is operably connected to the input assembly and receives the conditioned gas output of the input assembly wherein the concentration of the total NO x  present can be determined. 
     A further aspect of the present invention includes the NO x  sensor including a mixed potential sensor receiving the conditioned gas output and generating a voltage signal being a function of the concentration of the total NO x  present. 
     Another aspect of the present invention includes the NO x  sensor including a porous semi-conductive layer capable of absorbing NO x  gases wherein a physical property is monitored to determine the concentration of NO x  present. 
     A still further aspect of the present invention includes an oxygen senor. The oxygen sensor and the NO x  sensor cooperate to determine the NO x  concentration in the exhaust gas. 
     Yet another further aspect of the present invention includes an electronic system utilizing a formula and capable of calculating the NO x  concentration of the exhaust gas based on a measured oxygen concentration. The electronic system can include a database and a data table, wherein the electronic system, database, or data table cooperate to determine the NO x  concentration of the exhaust gas as a function of oxygen concentration 
     An object of the present invention is to overcome the problems commonly associated with mixed potential NO x  sensors and to provide a sensor useful for measuring total NO x  concentration in an exhaust gas stream. 
     Another object of the present invention is to provide a catalyst assembly that conditions the exhaust gas prior to entering the sensor(s) whereby the ratio of NO 2 /NO is in the range of 0.01–0.10. 
     A further object of the invention is to provide an accurate and reproducible voltage signal that correlates to the total NO x  concentration in the exhaust gas. 
     A still further object of the present invention is to oxidize any unburned combustibles, e.g., C 3 H 6 , CH 4 , CO, etc; that are typical of an exhaust gas stream, and to remove or reduce the concentration of gases such as SO 2  or H 2 S that may interfere with the lifetime performance of the electrode(s) and/or sensor. 
     Another further object of the present invention is to provide a sensor that is capable of measuring NO x  concentration as low as 1 ppm. 
     Yet another object of the present invention is to incorporate an oxygen sensor within the body of the NO x  sensor so that oxygen and NO x  concentrations can be measured simultaneously; thereby enabling the accurate determination of the total NO x  concentration that is a function of the oxygen concentration. 
     A still further object of the present invention is to provide a voltage output signal that is not influenced by other gas constituents in the exhaust gas, e.g., hydrocarbons, CO, CO 2 , SO 2 , H 2 , NH 3 , and H 2 O. 
     Yet a still further object of the present invention is to provide a NO x  sensor having a voltage output signal that is not significantly affected by the presence of SO 2  concentrations up to 100 ppm, and preferably below 15 ppm. 
     And yet another object of the present invention is to provide a NO x  sensor capable of measuring total NO x  concentration in the range of 0.1–1500 ppm, and preferably from 1–1500 ppm. 
     Other advantages and aspects of the present invention will become apparent upon reading the following description of the drawings and detailed description of the invention. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic representation of one embodiment of the input assembly of the present invention; 
         FIG. 2  is a schematic representation of one embodiment of the present invention; 
         FIG. 3  is a graph of data obtained using the embodiment shown in  FIG. 2  that demonstrates the relationship between NO x  concentration and the voltage signal generated by the sensor; 
         FIG. 4  is a plot of the voltage signal generated with varying concentrations of NO x  gas in the low concentration range of 1–20 ppm; 
         FIG. 5  is a graph showing the response time signal of a NO x  sensor when the NO x  concentration is varied from 470 ppm to 940 ppm; and, 
         FIG. 6  is schematic diagram of one embodiment of the present invention depicting an integrated sensor including a single electrolyte tube with two sensing electrodes on the outside of the tube, namely, a NO x  sensing electrode and an O 2  sensing electrode, along with a single reference electrode on the inside of the tube—included within a housing is the input assembly and heater(s), i.e., an internal dual-zone heating rod; 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     While the present invention is capable of embodiment in many different forms, there is shown in the drawings and will herein be described in detail preferred embodiments of the invention with the understanding that the present disclosure is to be considered as an exemplification of the principles of the invention and is not intended to limit the broad aspect of the invention to the embodiments illustrated. 
     One embodiment of the present invention is directed to a method and apparatus for determining NO x  concentration of an exhaust gas. An apparatus  10  comprises an input assembly  12  (shown in  FIG. 1 ) capable of receiving the exhaust gas and producing a conditioned output gas. The input assembly  12  includes at least three of the following four stages: a first stage  14  including a first catalyst structure for converting NH 3  in the exhaust gas to N 2  and H 2 O (to prevent cross sensitivity); a second stage  16  including a second catalyst structure having an absorbent material for absorbing SO 2  from the exhaust gas; a third stage  18  including a third catalyst structure for oxidizing unburned hydrocarbons (and ammonia) and gases to higher oxidation states; and, a fourth stage  20  including a fourth catalyst structure for establishing a steady state equilibrium concentration ratio between NO and NO 2 . In one embodiment, the first catalyst structure of the first stage  14  comprises a catalyst material from the group consisting of Cu, Ag, NiAl 2 O 4 , MnO 2 , V 2 O 5 , and any mixture thereof. It is to be understood that the sequence of stages within the input assembly  12  is not limited to any specific order. 
       FIG. 2  depicts a preferred embodiment of the present invention to achieve an accurate measurement of total NO x  concentration in a gas stream. A NO x  sensor  22  is operably connected to the input assembly  12  and receives the conditioned output gas from the input assembly wherein the concentration of the total NO x  present can be determined. In this embodiment, the exhaust gas passes through a three-stage input assembly  12 . The initial stage  16  shown in  FIG. 2  includes a catalyst structure including an absorbent material such as CaO, MgO, or a compound from the spinel or perovskite group of materials that serve the function of removing SO 2  from the exhaust gas stream. The absorbent material can be in the form of a packed pellet or infiltrated support that may be periodically replaced during servicing without disassembling the rest of the apparatus  10 . 
     The catalyst structure of the next stage  18  of the input assembly  12  shown in  FIG. 2  includes an oxidation catalyst, e.g., RuO 2  or CoO 2 , which functions to oxidize unburned hydrocarbons and convert CO to CO 2 . The final stage  20  of the input assembly  12  shown in  FIG. 2  a catalyst structure including a silver metal configured as a mesh or a coating on a ceramic substrate that acts to establish a steady state concentration ratio between NO and NO 2  wherein the NO 2  percentage of the total NO x  gas present is in the range of 0–5% optimally, and at least within the range of 0–10%. 
     After the exhaust gas has been conditioned by the input assembly  12 , it passes to a NO x  Sensor cavity, i.e., a mixed potential sensor  22 , wherein a mixed potential voltage signal is generated. In one embodiment, the mixed potential sensor includes a metallic sensing electrode. The metallic sensing electrode includes an electrolyte material having a range of approximately 10–40 vol. %. In one embodiment, the electrolyte material is approximately 15–25 vol. %. The electrolyte material may comprise a doped zirconia material, or one of the following: ceria, gadolinia, hafnia, thoria, bismuth oxide, or any mixture thereof. The mixed potential voltage signal is a function of the concentration of the total NO x  present.  FIGS. 3 and 4  depict typical graphs of voltage with respect to the logarithm of the total NO x  concentration—in the range of 10–1000 ppm ( FIG. 3 ) and 1–20 ppm ( FIG. 4 )  13  and is independent of the NO x  gas species that enter the apparatus  10 . 
     In some modifications of the present invention, the voltage signal will be proportional to the logarithm of the NO x  concentration; while it may also be possible to construct the apparatus such that in the low NO x  concentration range, e.g., 1–30 ppm, the voltage output signal will be directly proportional to the NO x  concentration, i.e., linear dependence. 
     In another embodiment of the present invention, an oxygen sensor  26  is incorporated with the apparatus  10 . Referring to  FIG. 6 , the oxygen sensor  26  is configured within the housing  24 . More specifically,  FIG. 6  depict an integrated sensor including a single electrolyte tube having two sensing electrodes on the outside of the tube—namely, a NO x  sensing electrode  22  and an O 2  sensing electrode  26 —along with a single reference electrode  30  inside of the tube. Included within the same housing  24  are the input assembly  12  and a heating device, e.g., an internal dual-zone heating rod  28  shown in  FIG. 6 . Such a configuration is capable of performing in gas environments with rapidly changing oxygen concentrations. 
     An oxygen ion conducting electrolyte membrane may be used for both the oxygen sensor  26  and the NO x  sensor  22 . To improve performance, the oxygen sensor  26  may be located within an environment having a different temperature than the environment wherein the NO x  sensor  22  resides. The different heating areas may be accomplished by inserting a heating rod  28  inside of a ceramic electrolyte tube, wherein the heating rod shown in  FIG. 6  is constructed with two separate heating zones. Alternatively, a single temperature heating rod can be utilized and the design of the insulation can be modified to control the heat loss to create two different temperature zones; or, a heater external to the sensing element can be implemented to produce the desired temperature zones. Preferred performance of the present invention is achieved when the temperature proximate the NO x  sensor  22  is accurately controlled to 450–550° C. and the temperature proximate the oxygen sensor  26  and the input assembly  12  are maintained at 700–800° C. This results in a rapid response of the oxygen sensor  26  and maximum efficiency of the input assembly  12 . 
     An additional aspect of the NO x  sensor  22  design may include the sensor tip protruding approximately one inch into the exhaust gas stream—thereby adhering to the design principles utilized in the widely used lambda oxygen sensor. This configuration facilitates maintaining two distinct temperature zones between the NO x  sensor  22  portion of the ceramic tube outside of the exhaust manifold and within the sensor body housing—thereby creating enough distance from the oxygen sensor  26  so that the two different temperature zones can be effectively achieved. 
     Located near the NO x  sensor  22  electrode is a gas exit port comprising a small diameter stainless steel tube that when connected to some type of suction device (not shown), will draw the exhaust gas stream through the porous input assembly  12 , past the oxygen sensor electrode  26 , past the NO x  sensor  22  electrode, and exiting the housing  24 . The suction device can be a small air pump, or the gas suction can be accomplished using the vacuum lines commonly implemented in internal combustion engines. It is also contemplated that that the gas suction can be connected to the exhaust gas recirculation system found in newer types of automobiles. Alternatively, the housing  24  can be designed so that a portion of the exhaust gas stream is diverted into the sensor housing thereby passing through the input assembly  12  to the sensing electrode  22 . This variation may be achieved by various hole patterns in the tubular sheathing that is part of the metal housing  24 . 
     It is to be understood that although the preferred embodiments shown here are based on a tubular geometry design, the concepts that enable the apparatus to perform accurately can also be extended to other design components such as a flat plate ceramic multilayer package design, a single electrolyte disk type design, and so forth. 
     To further facilitate the understanding of the present invention, several exemplifications of the present invention are provided. It is to be understood that the present invention is not limited to these exemplifications. 
     EXAMPLE 1 
     A NO x  sensor  22  having a structure of the kind shown in  FIG. 2  was constructed of a tubular electrolyte body fabricated by the addition of a binder to a commercially available 8 mole % Y 2 O 3  doped zirconia powder. The binder/powder mixture was dispensed into a tooling followed by isostatic pressing at 25,000 psi. The ceramic portion was machined to final dimensions and then sintered at 1475° C. for two (2) hours. Next, the ceramic electrolyte was coated with electrodes. The inside of the tube along with a stripe on the outside of the tube (current collector) were coated with a platinum paste electrode material followed by firing at 1000° C. for one (1) hour. Then, the tip of the tube was coated with a tungsten oxide/zirconia mixture that contacted the platinum stripe current collector so that electrical contact was made. The electrode coating was dried and fired at high temperature to promote good adhesion. 
     The input assembly  12  was fabricated by using a ⅜″ diameter stainless steel tube as the housing  24 . On the gas exit end of the tube, a silver mesh plug was installed by press fitting the plug into the end of the tube. On the upstream gas flow side of the silver plug, 0.5 grams of ruthenium oxide powder was inserted into the stainless steel tube. This powder was lightly compacted by using a rod to press the powder against the surface of the silver mesh plug. Next, 1.0 gram of CaO powder was inserted into the tube and again a rod was used to lightly compact this powder against the ruthenium oxide powder. Finally, a piece of nickel mesh screen was pressed into the tube and compacted against the CaO powder to keep the powders in place. 
     The apparatus was tested wherein a gas stream would flow first through the input assembly  12  and then to the NO x  sensor electrode. Gases were mixed together using a four-channel mass flow controller system that enabled changing the NO x  concentration in the gas stream and measuring the sensor voltage signal. A typical voltage response curve generated by varying the NO x  concentration between 50–1000 ppm total NO x  is shown in  FIG. 3 . 
     EXAMPLE 2 
     A NO x  sensor fabricated as described in Example 1 was tested at low concentrations of NO x  gases to demonstrate the low range capability of the present invention. Gases were mixed together using a four-channel mass flow controller system that enabled changing the NO x  concentration in the gas stream and measuring the sensor voltage signal. A certified gas cylinder with a concentration of 20 ppm NO/balance nitrogen was used for this test. The concentration was varied by mixing this gas cylinder with gases from a nitrogen and oxygen cylinder. The concentration was varied in increments of 1 ppm from 1–20 ppm. A graph showing the voltage output signal as a function of NO x  concentration is shown in  FIG. 4 . 
     EXAMPLE 3 
     The NO x  sensor fabricated as described in Example 1 was also tested for sensor response time to demonstrate the apparatus&#39; ability to function as part of a control system in a NO x  removal device. Gases were mixed together using a four-channel mass flow controller system that enabled changing the NO x  concentration in the gas stream and measuring the sensor voltage signal. The gas concentration was switched between 470 ppm and 940 ppm NO x  at a flow rate of 500 cc/min. The voltage signal was monitored continuously using a data acquisition system with a sampling rate of three readings per second. The sensor response time is defined as a 90% step change of the total voltage signal when the concentration of the NO x  gas is changed. A sensor response time curve is shown in  FIG. 5  that indicates a sensor response time of 2.7 seconds when the NO x  gas concentration is changed from 470 ppm to 940 ppm. 
     EXAMPLE 4 
     A combined NO x  and oxygen sensor was fabricated as shown in  FIG. 6 . A tubular electrolyte body was fabricated by addition of binder to a commercially available 8 mole % Y 2 O 3  doped zirconia powder. The binder/powder mixture was dispensed into a tooling followed by isostatic pressing at 25,000 psi. The ceramic part was machined to its final dimensions and then sintered at 1475° C. for two (2) hours. Next, the ceramic electrolyte was coated with electrodes. The inside of the tube—along with two stripes on the outside of the tube (current collectors) and the oxygen sensing electrode on the tip—were coated with a platinum paste electrode material followed by firing at 1000° C. for one (1) hour. Then, a 1 cm by 1 cm patch on the side of the tube was coated with a tungsten oxide/zirconia mixture that slightly overlapped the platinum stripe current collector so that electrical contact was made. The electrode coating was dried at 80° C. followed by firing at high temperature to promote adhesion. 
     The input assembly was fabricated by using a ⅜″ diameter stainless steel tube as the housing. On the gas exit end of the tube, a silver mesh plug was installed by press-fitting the plug into the end of the tube. The silver mesh plug was fabricated by cutting twenty-five 0.30″ diameter pieces of eighty (80) mesh silver screen and spot welding them together to form a compact plug. On the upstream gas flow side of the silver plug, 0.5 grams of ruthenium oxide powder was inserted into the stainless steel tube. This powder was lightly compacted by using a rod to press the powder against the surface of the silver mesh plug. Finally, a piece of nickel mesh screen was pressed into the tube and compacted against the RuO 2  powder to keep the powder in place. 
     While specific embodiments of the present invention have been illustrated and described, numerous modifications come to mind without significantly departing from the spirit of the invention, and the scope of protection is only limited by the scope of the accompanying claims.