Abstract:
To determine the oxidizable portion of exhaust gases in the presence of the reducible portion with the legally required precision, a method and a sensor are disclosed for analyzing a flow of exhaust gas components. The sensor includes a limit current measurer, one limit current pump for reducible gases and, downstream from this pump in the direction of diffusion, another limit pump for oxidizable gases. The electrodes of the limit current pump for reducible gases are made of a material that does not catalyze the reaction between oxidizable and reducible gases.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to a sensor for analyzing a stream of exhaust gas components, the sensor including a limit current measurer, and a method for determining exhaust gas components, in particular, by using the sensor.  
         BACKGROUND INFORMATION  
         [0002]    A plurality of instruments for the analysis of exhaust gases of motor vehicles are described, for example, in the book “Automotive Electronics Handbook” (1995), McGraw Hill Inc., Section 6 “Exhaust Gas Sensors.” Such instruments include, for example, the λ=1 probe, which is an equilibrium sensor that checks, by measuring the Nernst voltage, whether the air-fuel mixture injected into a gasoline engine has a λ value of approximately one. The UEGO sensor (also known as the universal sensor) is also an equilibrium sensor, which is operated as a combination of a sensor based on the Nernst principle and a limit value probe, which are immersed in the exhaust gas of the internal combustion engine and whose measuring current, which depends on the λ value, is used to regulate the λ value. The operation of mixed-potential sensors, which are disequilibrium sensor type instruments, is based on the fact that reduced catalytic activity prevents a gas equilibrium from being established on the electrode of a ZrO 2  galvanic cell. As a result, no state of oxidation/reduction equilibrium can be established in oxygen and a mixed potential is formed, which is determined, among other things, by electrode activity, temperature, and gas composition. By “passively” measuring a signal dependent on the state of the electrode, mixed-potential sensors allow conclusions to be drawn concerning the gases in question. Their use in practice, however, is problematic, since they only operate properly in a very narrow temperature range, and their signal is often dependent on their history—their properties change as they age. A NO x  pump sensor is also a disequilibrium sensor. It is used for determining NO x  in the presence of oxygen. It operates as follows: oxygen is pumped out of a first cathodic limit current cell, the electrode being made of platinum-gold, which prevents NO x  from also being pumped out. Therefore, a limit current, which is proportional to the NO x  level in the exhaust gas, can be measured in a second cathodic limit current cell.  
           [0003]    To date, there is no known field-usable method of determining the oxidizable components of exhaust gases in the presence of the reducible components with the accuracy required by law.  
         SUMMARY OF THE INVENTION  
         [0004]    An object of the present invention is to provide such a method.  
           [0005]    The sensor according to the present invention is a disequilibrium sensor having a simple design. Reducible and oxidizable gases can be analyzed by this single sensor, i.e., no separate sensors are needed for the two analyses. Analysis is performed using current limit probes. Current limit probes “actively” measure the diffusion characteristics. Their electrodes only have to pump and prevent catalysis or only allow anodic oxidation. Aging may necessitate a slight increase in the pump voltage to reach the limit current. The measured signal, however, is actually the diffusion resistance. The sensor according to the present invention contains no closed-circuit control, so no expensive electronic circuitry is needed. The sensor is well suited for measuring exhaust gases both in engines operated in the lean range, such as diesel engines, and in engines, such as gasoline engines, operated in the λ=1 range. The sensor according to the present invention can determine the sum of both reducible and oxidizable gases with considerable accuracy, so that it can often replace expensive, complex, and bulky analyzers. The sensor can also be used for on-board diagnostics (OBD). The operation of the sensor is based on the fact that, by suitably selecting the electrode material in the cathode cell, its catalytic activity is so low that, despite the very high temperatures, a reaction between reducible and oxidizable gases is almost impossible even if there is an excess of reducible exhaust gas components such as oxygen. The lowest possible catalytic activity of the electrode material can also be supported by appropriate material morphology, the most favorable morphology being determined by simple tests.  
           [0006]    The electrode material of the cathode cell is advantageously made of platinum-gold.  
           [0007]    The two limit current pumps can be advantageously mounted on a single substrate. This not only makes a very compact sensor arrangement possible, but also results in a sensor so similar to the UEGO sensor, that it can be manufactured on the UEGO sensor assembly line without considerable adjustments being required.  
           [0008]    The limit current pump for oxidizable gases and the limit current pump for reducible gases can be advantageously operated at a constant pump voltage. The limit current pump for reducible gases can, however, also be operated at a pump voltage that is independent of the limit current to avoid decomposition of H 2 O and CO 2  in the exhaust gas.  
           [0009]    In an advantageous embodiment of the sensor according to the present invention, at least two selective pump cells are provided for oxidizable gases, the electrode materials being selected so that in the cells upstream from the last cell in the direction of diffusion only the reaction of one oxidizable exhaust gas component is allowed. The materials are selected for their composition, with spinel advantageously added, for example, and for their morphology.  
           [0010]    The oxidizable components of a lean exhaust gas and the reducible and oxidizable components of an exhaust gas with λ=1 can be advantageously determined using the method according to the present invention. This application is particularly advantageous, since it allows the efficiency of a three-way catalyst to be determined in the range from 0% to 100%. The λ probes used so far only measure in the approximate range of 80% to 100%.  
           [0011]    With the embodiment of the sensor according to the present invention having at least two pump cells for oxidizable gases, the carbon monoxide and ammonia or carbon monoxide and hydrocarbon levels can be advantageously determined side by side. Selectivity, for example, in the SCR method (see below), and detection of NH 3  in the presence of CO in the exhaust gas can be achieved by suitable actions on the system such as an upstream oxidation catalyst to oxidize CO prior to introducing NH 3  or its precursor.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 schematically shows a cross section of an embodiment of a sensor according to the present invention.  
         [0013]    [0013]FIG. 2 shows a detail of the embodiment illustrated in FIG. 1 and, in an associated diagram, the concentrations of the oxidizable exhaust gas components and the reducible exhaust gas components diffusing with the exhaust gas from the sensor inlet through the sensor plotted against the location coordinates.  
         [0014]    [0014]FIG. 3 a  schematically shows the path of a fuel mixture injected into a gasoline engine, through the engine, a first Nernst cell, the catalyst and then either through a second Nernst cell or through a sensor according to the present invention.  
         [0015]    [0015]FIG. 3 b  shows, in a diagram, the Nernst voltage measured at the first Nernst cell of FIG. 3 a  and the Nernst voltage, measured at the second Nernst cell in different operating states, plotted against time.  
         [0016]    [0016]FIG. 3 c  shows, in a diagram, the limit currents of reducible and oxidizable exhaust components measured in three different operating states in the sensor of FIG. 3 a , plotted against time.  
         [0017]    [0017]FIG. 4 shows, in a diagram, the measured limit current of an O 2 -containing gas plotted against O 2  concentration, taking into consideration both when the gas contains no H 2 O or CO 2  and when the gas contains H 2 O and CO 2 , the limit current being given for two pump voltages in the latter case.  
         [0018]    [0018]FIG. 5 schematically shows another embodiment of the sensor according to the present invention.  
         [0019]    [0019]FIG. 6 schematically shows a cross section through a detail of another embodiment, suitable for selective determination of two oxidizable exhaust gas components, of the sensor according to the present invention and, in an associated diagram, the concentrations measured using an exhaust gas probe, of the two oxidizable exhaust gas components and the reducible exhaust gas components diffusing from the sensor inlet through the sensor plotted against the location coordinates. 
     
    
     DETAILED DESCRIPTION  
       [0020]    The embodiments of a sensor according to the present invention, described in the following, are particularly advantageous, but it should be pointed out that they are only named as examples, and a plurality of versions are possible within the framework of the present invention.  
         [0021]    Sensor  1  illustrated in FIG. 1 has four consecutive ceramic layers  2 ,  3 ,  4 , and  5 . An electric heater  6  is arranged at the boundary between layers  2  and  3 . Layer  4  has a preferably circular through orifice  7 , and layer  5  has a relatively small through orifice  8 , which is concentric to orifice  7 . An annular electrode  9 , whose outer diameter is approximately equal to the diameter of orifice  7  and whose inner diameter is somewhat greater than the diameter of opening  8 , is mounted on the outside of layer  5 . Two annular electrodes  10  and  11 , which do not touch, are mounted on the inside of layer  5  concentrically to one another and to electrode  9 , the outer diameter of electrode  11  being approximately equal to the outer diameter of opening  7  and the inner diameter of electrode  10  being approximately equal to the inner diameter of electrode  9 . Electrodes  9 ,  10 , and  9 ,  11  each form a limit current cell  12 ,  13 . Diffusion resistors  14  and  15 , forming annular barriers in orifice  7 , are preferably mounted between orifice  8  and electrode  10  and between electrodes  10  and  11 . Pump voltages U p1  and U p2  are applied between electrodes  9 ,  10  and  9 ,  11 , both of which voltages are constant. As an alternative, U p1  is dependent on limit current I gr1  generated (U p1 =a+b·I gr1 ). The dependence is achieved using a conventional electronic circuit. Electrodes  10  and  11  are contacted by conductors  16  and  17 , which conduct away limit currents I gr1  and I gr2  , whose intensities are measured by measuring resistors  18  and  19 , respectively. The arrows perpendicular to the electrodes indicate the directions of the O −  ion flows in the limit current cells.  
         [0022]    Electrode  10  is made of a material that virtually does not catalyze the oxidation of oxidizable exhaust gas components in the presence of oxidizing agents. The catalytic action, or rather non-action, of the electrode material can also be influenced by its morphology. Such a material is platinum-gold, for example. An advantageous material for electrode  11  is platinum-rhodium.  
         [0023]    Initially we shall elucidate the use of sensor  1  for the detection of oxidizable gases in lean exhaust gases.  
         [0024]    As FIG. 2 shows, exhaust gas  20  to be analyzed enters sensor  1  through orifice  8  and diffuses past electrodes  10  and  11 . All reducible gases of the exhaust gas are removed by suction in limit current probe  12  by a cathodic limit current. This is illustrated by curve  21  in the diagram of FIG. 2, where concentration is plotted against the path traveled by the gas in the sensor. The reducible gases (in particular, oxygen) and oxidizable gases should not react with one another. To prevent hydrocarbons (HC), for example, from reacting with oxygen, high temperatures and catalytic reactions must be avoided. On the other hand, limit current cells require certain minimum temperatures, which are the higher the higher the limit current. A compromise must therefore be found. When lean mixtures are analyzed, the sensor will operate between approximately 700° and approximately 800° C. The temperature effect is partially eliminated by the very high spatial velocity of the exhaust gas in orifice  7 , which prevents the establishment of thermodynamic equilibrium. In particular, however, a reaction is avoided when electrode  10  (cathode) is made of a material that has little or no catalytic action. Such a material is platinum-gold, for example. A catalytic effect can also be reduced by suitably configuring the diffusion path (gas phase diffusion, Knudsen diffusion).  
         [0025]    The cathodic limit current can be measured in measuring resistor  18 . Its value, however, is of little importance if the exhaust gas is lean, since the reducible gas is made up mainly of oxygen. Pump cell  12  can be operated at a constant pump voltage. If H 2 O and/or CO 2  are reduced, then it causes no problem if the H 2  or CO obtained immediately react with O 2 . With very inactive electrodes  10 , it may be advantageous, however, to work with a current-dependent pump voltage (U p =a+bI gr ) (see above) to avoid the reduction of H 2 O and CO 2 .  
         [0026]    HC, carbon monoxide (CO) and ammonia (NH 3 ) are particularly important as oxidizable gases. NH 3  gets into the exhaust gas in the selective catalytic reduction (SCR) process, if more NH 3  is added to decompose NO x , for example, in the form of a precursor such as urea, than is needed for reacting the NO x  present. As curve  22  in FIG. 2 shows, the oxidizable gases are anodically oxidized in limit current cell  13 . The sum of concentrations of oxidizable gases is determined from the intensity of the anodic limit current. Since the concentrations of the oxidizable gases are low and the anodic limit current is also low, heater interference may occur. However, since it also occurs in the cathodic limit current, current peaks that occur simultaneously in both limit currents can be filtered out.  
         [0027]    Engines that run in the lean range are diesel engines and, occasionally, BDE engines.  
         [0028]    Sensor  1  can be used at temperatures between approximately 600° and approximately 700° C., preferably also to detect reducible and oxidizable gases in mixtures in the λ=1 range, in particular, for catalyst monitoring. In this area also the concentration of reducible gases are relevant; therefore, the anodic and cathodic limit currents are measured and used for determining concentration.  
         [0029]    Exhaust gases of engines operating near λ=1 only contain a small amount of oxygen. Reducible components such as NO x  and oxidizable components such as CO, H 2 , and HC may occur in the exhaust as harmful gases and are brought to very low values by catalysts. Today&#39;s technology makes it possible to control engines very accurately. However, engines operated with a three-way catalyst should not be regulated at λ=1. They should rather be operated on either side of λ=1 (for example, λ=0.975 to 1.025) using a precisely balanced “pendulum mode” to allow the harmful gases to be eliminated in the exhaust gas. FIG. 3 a  schematically shows an arrangement composed of a gasoline engine and a catalyst, including an injection pump, various sensors, and a controller. The fuel mixture having the predefined composition is injected from injection pump  30  via line  31  into engine  32 , where it undergoes combustion. The exhaust gas is removed from the engine through line  33  and passed through a Nernst probe  34 . Due to the “pendulum mode,” signal  40  of Nernst probe  34 , a λ=1 probe, has the shape shown in FIG. 3 b,  which is symmetric to the voltage corresponding to λ=1 (450 mV). If the signal deviates from that shape, the composition of the fuel-air mixture is modified using controller  35  as a function of signal  40 . After having passed through Nernst probe  34 , the exhaust gas passes through 3-way catalyst  36  and may pass through an on-board diagnosis I (OBD I) probe  37 , which is another λ=1 probe. Depending on the operating state, Nernst probe  37  returns signals  41 ,  43 , and  44  shown in the diagram of FIG. 3 b.  Signal  41 , which emulates the pendulum waves, shows that the catalyst is not working in an optimum manner. Curve  41 , however, only shows that the efficiency of the catalyst is about 80% or less. This conclusion is imprecise, since a catalyst is still usable even with an efficiency of 50%. Signals  43  and  44  form straight lines, which means that the catalyst is working (efficiency&gt;80%), i.e., it succeeded in equalizing the pendulum movements. Signal  43  is situated within the range between approximately 300 and approximately 600 mV, delimited by straight lines  42 . This leads to the conclusion that the engine is controlled by the means to work at λ=1. Signal  44  is less than 300 mV. This leads to the conclusion that the fuel-air mixture is too lean, and the engine control is not working in an optimum manner.  
         [0030]    If Nernst probe  37  is replaced with sensor  1  (reference No.  38 ), two signals  47 ,  48 ,  49  are obtained in each of the three different operating states, as shown in the diagram of FIG. 3 c.  Signals  47  are approximately symmetrical to the zero line. This means that the engine control operates properly. However, signals  47  are too far apart, which means that the exhaust gas still contains considerable amounts of reducible and oxidizable components, which indicates that the catalyst has a limited efficiency. Signal  47  not only allows one to draw a quantitative conclusion on the proportion of reducible and oxidizable exhaust gas components, but also on the degree of remaining catalyst efficiency, not only in the range between 80% and 100%, but also between 0% and 100%. This is very important in practice, since it allows catalysts to be used for a considerably longer time than is possible today. Signals  48  are symmetric to the zero line, which again shows proper engine control, but they are so close to the zero line that they are within the area delimited by lines  46 , which define the upper limits of the legally allowed level of reducible and oxidizable contaminants. Proximity to the zero line proves that the catalyst is working in an optimum manner. Signals  49  are obtained when the exhaust gas composition is too close to the lean range, which means that the engine control is not working properly. The signals allow conclusions to be drawn on the catalyst quality and the proportion of reducible and oxidizable exhaust gas components as in the cases characterized by signals  47  and  48 . To draw a quantitative conclusion on pollutants in the exhaust gas, the related art would have required analysis using expensive and bulky instruments such as an IR spectrometer. Sensor  1  meets the basic requirements for use in OBD II.  
         [0031]    The proportions of reducible and oxidizable gases measured using sensor  1  are proportional to the measured limit currents I gr . The reducible gas present in the greatest proportion is oxygen. If a pump voltage of about 800 mV is used, the proportionality between limit current and O 2  level is linear for higher O 2  levels. For O 2  levels &lt;2%, which is particularly interesting for the λ=1 range, if the exhaust gas contains H 2 O and CO 2,  there is deviation from linearity, caused by the reduction of CO 2  and H 2 O with liberation of O 2 . The diagram of FIG. 4, where the limit current is plotted against the O 2  level in air (21% oxygen) (curve  50 ) and against a gas mixture, which contains CO 2  and H 2 O in addition to oxygen, shows this relationship with curves  51  and  52 . Curves  51  and  52  differ in that the underlying pump voltage is 800 mV for curve  51  and 600 mV for curve  52 . It can be seen that for the lower voltage, there is less deviation from linearity. From at least this fact the inventor derived the idea to make pump voltage dependent on the limit current according to the equation U p =a+b·I gr  (see above). As an alternative, he integrated a Nernst cell into sensor  1 . The sensor thus modified is illustrated in FIG. 5, where a ceramic plate  60  next to plate  3  and a ceramic plate  61  next to ceramic plate  60  are inserted between ceramic plates  3  and  4 . Plate  61  has a through orifice  62 , which can have the same bottom face as, and can be arranged concentrically to, orifice  7 . Orifice  62  is connected to outside air and is used as air reference. The surface of plate  4  adjacent to orifice  62  is covered with a metal electrode  63 , and in the cathode chamber of orifice  7 , an annular electrode  64 , which is concentric with orifice  7 , is mounted on the surface of the plate adjacent to orifice  62 . The Nernst cell is controlled in the traditional manner. The cathodic current is controlled so that the voltage of the Nernst cell is 450 mV, for example. This results in that only the amount of gas required for keeping the anodic part in the λ range is removed by suction in the cathode part, thus guaranteeing that H 2 O and CO 2  are not decomposed.  
         [0032]    [0032]FIG. 6 shows a cross sectional detail of a sensor, which has a cathode cell  67  with electrodes  70  and  71  and two anode cells  68  and  69  with electrodes  70 ,  73  and  70 ,  75  in opening  7 . The sensor of FIG. 6 differs from those shown in FIGS. 1 through 3 by the additional anode cell  68 . It is used to selectively and separately determine two oxidizable gases in the exhaust gas. To achieve such selectivity, electrode  73  must be made of a material that only catalyzes the oxidation of one of the oxidizable gases. Such materials have been found during the development of mixed-potential sensors. It is spinel containing cobalt and chromium. The composition that is suitable for the special case and the suitable morphology of the electrode to selectively oxidize a gas can be determined by simple tests. For example, to determine CO and HC or CO and NH 3  separately in an exhaust gas in the λ=1 range, CO is oxidized in the first anodic cell  68  after the reducible gases have been removed in the cathodic cell (electrodes  70  and  71 ), and HC and NH 3  are oxidized in the second anodic cell  69 . The procedures are illustrated in the diagrams of FIG. 6, where the gas concentrations are plotted against the distance traveled by gas diffusion, curve  80  being that of the reducible gases, curve  81  of CO, and curve  82  of HC and NH 3 . Electrode  71  is made of Pt—Au; electrode  73  contains CoCrMnO 4 , and electrode  75  is made of Pt—Rh.