Sensor and method for monitoring and controlling catalysts, especially motor vehicle catalysts

Abstract

A sensor and method for monitoring and controlling catalytic converters, in particular motor vehicle catalytic converters, is designed as a temperature probe which is provided with a catalytic coating. A catalytically coated temperature probe is used for control purposes, with the quantity and/or type of reagents fed to the catalytic converter being at least partially controlled by the sensor.

Description

[0001] This application claims the priority of German patent document 101 24 550.5, filed May 19, 2001 (PCT International Application No.: PCT/EP02/05452), the disclosure of which is expressly incorporated by reference herein.

BACKGROUND AND SUMMARY OF THE INVENTION

[0002] The invention relates to a sensor for monitoring and/or controlling a catalytic converter in a vehicle.

[0003] The main emitters of nitrogen oxides (NOx) in industrialized countries are traffic, fossil-fired power plants and industrial plants. While emissions from power plants and industry are steadily decreasing, the proportion contributed by the automobile is becoming ever more prominent. Ever more stringent exhaust emissions regulations combined with the pressure to reduce fuel consumption therefore require new concepts both for the internal combustion engine and for exhaust-gas purification. This also requires new concepts for the monitoring of exhaust-gas purification systems.

[0004] German Laid-Open Specification DE 26 43 739 A1 discloses a method for monitoring the activity of catalytic converters for exhaust-gas purification, in which two temperature probes are arranged close together in an exhaust system, one of which may have a catalytic coating. The catalytic activity of the catalytic converter arranged in the exhaust system can be determined by comparing the temperature signals from the temperature probes.

[0005] A number of problems arise in connection with catalytic converters which are used in exhaust-gas purification systems. In a stoichiometrically operated spark-ignition engine (known as the “&lgr;=1 engine”), the air/fuel ratio &lgr; of the untreated exhaust gas is detected with the aid of a first &lgr; sensor. Then, the air/fuel ratio is adjusted slightly as a function of the control deviation from the ideal state &lgr;=1. In practice, what is obtained in this way is a &lgr; oscillation in the untreated exhaust gas about the stoichiometric point (&lgr;=1). However, &lgr;=1 must be maintained on average over the course of time. The oscillation frequency is in the range from a few tenths of a second to a few seconds. The ability of the so-called “three-way catalytic converter” arranged downstream of the first &lgr; sensor to store oxygen, assures that optimum conversion is always implemented, provided that the catalytic converter is still in good condition. As the quality of the catalytic converter decreases, however, the conversion rate drops and the light-off temperature rises. In parallel, the ability to store oxygen also decreases. A second probes &lgr; sensor, arranged downstream of the catalytic converter, will then be able to detect the control oscillation.

[0006] It is possible to ascertain the quality of the catalytic converter by evaluating the differences in the signals between the first and second &lgr; sensors, as described, for example, in German patent documents DE 41 12 478 and DE 42 09 136. In particular, it is recommended for the amplitude ratio of the two sensors to be used as a quality criterion. However, this method is reaching its limits for new types of high-efficiency catalytic converters, such as are needed, for example, for ULEV or SULEV requirements.

[0007] In principle, high levels of nitrogen oxides are formed in a combustion engine operating with excess air (for example what is known as a “lean-burn engine” or a diesel engine). Therefore, in one possible exhaust-gas purification concept, a catalytic converter that is able to store nitrogen oxides for a certain length of time is introduced into the exhaust section of a motor vehicle. After a “storage phase”, in which the catalytic converter is “filled” or “laden” with the exhaust-gas component which is to be stored, there follows a desorption phase, in which the catalytic converter is “emptied” (also referred to as “regeneration”). Gas sensors are used in conventional techniques to detect the catalytic converter filling level and for subsequent adjustment of the fuel/air ratio. These gas sensors may be heatable oxygen sensors (HEGO), as proposed, for example, in German patent document DE 197 44 738, or a combination of NOx sensor and oxygen sensor.

[0008] Temperature probes (currently generally thermocouples) are useful both for the exhaust-gas aftertreatment concepts of the “lean-burn engine” and for those of the “&lgr;=1 engine”. In the “&lgr;=1 engine”, such temperature probes are used in particular for improved detection of the catalytic converter quality, known as On-Board Diagnosis (OBD). On account of the heat of reaction which is evolved (and, of course, decreases in an aged catalytic converter on account of the lower conversion rates which it achieves), it is possible to increase considerably the reliability of detection of the catalytic converter quality by using a temperature probe. One example of this is to be found in German patent document DE 42 01 136 and in the documents taken into consideration for the assessment of this patent.

[0009] The use of a temperature probe is also expedient for the control and OBD of “lean-burn engines” as described in the specialist literature.

[0010] In addition to conventional thermocouples, temperature-dependent electrical resistors in planar form, such as described in the specialist literature, are also being used in the exhaust section of an automobile.

[0011] All such temperature probes are introduced into the exhaust gas as shown in FIG. 1, which represents prior art. The temperature probe Tf is installed either in position A (i.e., in the exhaust pipe upstream of the catalytic converter 2), in which case the gas stream flows over it upstream of the catalytic converter, or in position C (i e., in the exhaust pipe downstream of the catalytic converter 6), so that the gas stream flows across it downstream of the catalytic converter. Alternatively, the temperature probe is introduced directly into the catalytic converter body 8 in position B through a bore in the catalytic converter housing 4, and the gas flows over it inside the catalytic converter.

[0012] The sensor measures the temperature of the gas at the location where it is installed. To determine the heat of reaction, however, it would be preferable for the temperature probe to measure the temperature of the catalytically active layer itself. (For example, a temperature probe installed in position C measures only the proportion of the temperature increase brought about by heat of reaction which is transferred to the gas phase.) Since prior art temperature probes have a considerable heat capacity, while the gas has a very low heat capacity, the temperature increases which are to be measured are extremely slight. Moreover, temperature probes of this type are very slow, since the entire temperature probe must first be heated to the gas temperature.

[0013] More recent approaches therefore seek to reduce the thermal mass of the temperature probe, for example by using ultra thin thermocouples with a diameter of only 0.5 mm. However, designs of this type are not stable in the long term. The drawback that only the gas temperature is measured continues.

[0014] Since temperature probes of this type measure only the gas temperature and are relatively slow, they can be used to detect the quality of a catalytic converter, as demonstrated by the use examples mentioned above in the literature, only if a complex analysis module is added.

[0015] A typical catalytic converter structure is as described in FIG. 2, which diagrammatically depicts a section through a catalytic converter monolith 10. A special catalytically active coating 12, which determines the function of the catalytic converter, has been applied to the inner walls (known as webs) 16 of the catalytic converter 10. However, there are also what are known as unsupported catalytic converters, in which the webs are made from catalytically active material. The catalytically active coating can then be dispensed with.

[0016] Unfortunately, none of the prior art known to the inventors is able to measure accurately (or at least reasonably approximately) the temperature of the catalytically active coating which has been applied to the monolith.

[0017] Accordingly, it is an object of the present invention to provide a sensor and monitoring method that makes it possible to measure the layer temperature and to obtain rapid signal changes which even allow thermal state management of the catalytic converter.

[0018] This and other objects and advantages are achieved by the sensor and monitoring method according to the invention, which utilizes a temperature probe with a catalytically active coating. In one embodiment, the catalytically active coating has the same or similar physical properties as a catalytic converter which is also present in the exhaust section.

[0019] In a further special embodiment, the coating of the temperature probe and the catalytically active coating or the catalytically active material of the catalytic converter are identical.

[0020] In a further embodiment, the catalytically active coating at least partially comprises the material of the catalytic converter, if the latter is designed as an unsupported catalytic converter.

[0021] In a further embodiment of the invention, the temperature of the coating of the catalytic converter or the temperature of the catalytic converter is measured directly by integrating a temperature probe of this type into the catalytic converter itself.

[0022] Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIG. 1 shows the arrangement of temperature probes in an exhaust gas line according to the prior art;

[0024] FIG. 2 shows a typical catalytic converter structure;

[0025] FIG. 3 shows an embodiment of a coated temperature probe according to the invention;

[0026] FIG. 4 shows an exemplary embodiment of the catalytically active coating on the body of the measuring arrangement;

[0027] FIG. 5 shows an advantageous integration of the temperature measuring apparatus according to the invention into a catalytic converter;

[0028] FIG. 6 is a graphic depiction of measurement results for temperature versus time;

[0029] FIG. 7 shows an arrangement of three temperature probes according to the invention along a catalytic converter;

[0030] FIG. 8 is a graphic depiction of measured temperature versus time for the three sensors of FIG. 7;

[0031] FIG. 9 shows another embodiment of the invention;

[0032] FIG. 10 shows a temperature probe according to the invention integrated into a catalytic converter, in the form of a film or foil structure;

[0033] FIG. 11 shows a plurality of foil sensor arrangements wound to form a catalytic converter monolith; and

[0034] FIG. 12 shows a partial cross section through a catalytic converter monolith.

DETAILED DESCRIPTION OF THE DRAWINGS

[0035] FIG. 3 shows an embodiment of a coated temperature probe 40 of the type according to the invention. Electrical contact pads 44 have been applied to one side, known as the “connection side”, of a substrate 42. The supply conductors 46 lead to the resistance-measuring structure 48 on the other side, known as the “sensor side”, of the temperature probe 40. Contact pads 44, supply conductors 46 and resistance-measuring structure 48 may, for example, be applied using thick-film technology.

[0036] A typical material for this embodiment is platinum. However, combinations (for example platinum as material for the resistance-measuring structure 48 and for the supply conductors 46 and gold or other materials), with which it is simple to make contact for the contact pads 44, are also conceivable. Suitable substrates are made, for example from commercially available Al2O3 (e.g. Rubalit 708S, CeramTec). Porous Al2O3 substrates have a lower thermal conductivity, leading to reduced dissipation of heat and therefore to a greater and faster signal change. In principle, all electrically insulating, thermally stable materials are suitable for use as the substrate material.

[0037] As an alternative to thick-film technology, the temperature probe 40 may also be produced using thin-film technology, for example from platinum.

[0038] The resistance-measuring structure 48 is coated with the catalytically active coating 50. The coating may be formed on one side or alternatively on two sides. One simple option for applying the catalytically active coating may be a dipping process followed by a firing process. Since the coated temperature probe is positioned in the exhaust section together with an exhaust-gas catalytic converter, it is recommended that the temperature probe be coated with exactly the same solution as the catalytic converter.

[0039] In one specific embodiment, the layer thickness of the catalytically active coating of the temperature probe is precisely equal to the layer thickness of the catalytically active coating of the catalytic converter. Both active coatings should as far as possible have an identical morphology.

[0040] As an alternative to a temperature-measuring resistor structure, it is also possible for a structure comprising a pair of thermocouples to be applied to the substrate. In this case, the area in which the transition between the two thermocouple materials takes place also has to be coated by the catalytically active material. Of course, it is also possible for a thin, for example electrically insulating interlayer to be applied between temperature probe and catalytically active material.

[0041] Providing a thermocouple comprising two thermocouple wires with the catalytically active coating directly is also to be considered as falling within the scope of the invention. Alternatively, the thermocouple wires are provided with a material to which the catalytically active coating is then applied.

[0042] In all these embodiments, it is important that optimum thermal contact (as short a distance as possible and as good a thermally conductive connection as possible) be maintained between the temperature probe (e.g., the pair of thermocouples or resistance thermometer) and catalytically active coating. Optimally, the catalytically active coating is in contact with the temperature probe to the greatest extent possible.

[0043] An exemplary embodiment is shown in FIG. 4. The entire measuring arrangement 60 comprises a body 62, which ideally has a low thermal mass. At the same time, it is attempted to keep its surface area to which the catalytically active coating 64 is applied as large as possible. The gas stream 68 flows onto the catalytically active coating 64. The body 62 is provided with a thermocouple 66 which is in the maximum possible thermal contact with the body 62.

[0044] The actual manner of attachment of measuring arrangements of the above type directly in the exhaust pipe is not a primary subject of the present invention.

[0045] FIG. 5 shows a particularly advantageous embodiment of the invention integrated in a catalytic converter 70. The catalytic converter monolith has webs 16, which are coated with the catalytically active coating 12, and passages 14. A portion of a web 16 has been removed from the catalytic converter monolith, and a coated temperature probe, consisting of the components described in FIG. 3 (of which only the substrate 72 and the catalytically active coating 74 are shown) has been integrated in the catalytic converter 10 in place of the removed portion of the web 16. The figure also indicates the electrical connection lines 76.

[0046] The advantage of this embodiment is that the gas can continue to flow through the passages in the catalytic converter. Since the substrate 72 is of approximately the same thickness the web 74, the temperature probe according to the invention does not impede the gas flow. And because the same chemical reactions take place at the catalytically active coating 74 of the temperature probe as at the coated catalytic converter, the temperature probe according to the invention can measure the temperature of the catalytically active coating itself which changes as a result of the reaction, rather than measuring simply the gas temperature.

[0047] A typical measurement result illustrates this point. In FIG. 6, a temperature probe according to the invention provided with a catalytically active coating was integrated in a catalytic converter monolith. The sensor was arranged at the gas inlet of the catalytic converter. The catalytic converter—and therefore also the coated temperature probe—was first exposed to lean exhaust gas (exhaust-gas temperature approx. 510° C., &lgr;=1.1) for a prolonged period of time. The measured-value recording was started at t=0 sec. After approx. t=10 sec, the catalytic converter and coated temperature probe were each exposed for 3 sec to rich exhaust gas (&lgr;=0.96) and then for 3 sec to lean (&lgr;=1.03) exhaust gas. This &lgr; change was then repeated continuously. The measured-value recording ended at t=100 sec. The same test was carried out with a sensor integrated in the catalytic converter at the same location but coated only with a catalytically inactive covering layer.

[0048] The figure clearly reveals that the catalytically coated temperature probe is far more dynamic. The temperature shift revealed by the temperature probe according to the invention is also significantly greater than in the case of the temperature probe provided with a non-catalytic coating. A temperature probe arranged downstream of the catalytic converter does not detect the increases in temperature which occur as a result of the exothermic reaction in the event of a &lgr; change on the layer. Rather, it merely indicates a slight increase in temperature.

[0049] The invention also relates to an arrangement in which a plurality of temperature probes according to the invention of this type are installed in a catalytic converter monolith. FIG. 7, for example, shows three temperature probes arranged along the catalytic converter in the direction of flow of the exhaust gas. An offset with respect to the passages is also shown as a particularly advantageous embodiment.

[0050] This arrangement was used to carry out the measurements whose results are given in FIG. 8. An NOx storage catalytic converter with three sensors arranged as described in FIG. 7 was installed in a measuring apparatus, in which synthetic exhaust gas flowed through it. The synthetic exhaust gas was preheated to a gas temperature of 310° C. The space velocity was 28 000 1/h. The catalytic converter was laden for 3 min with a lean synthetic exhaust gas containing nitrogen oxides (&lgr;≈2). Then, it was switched over to rich exhaust gas (&lgr;≈0.75) for approx. 5 sec, in order to regenerate the NOx storage catalytic converter. In FIG. 8, this step change takes place at approximately t=12.5 sec. It is apparent that an extremely rapid first temperature peak with a temperature increase of approx. 65° C. is formed at the sensor 1, i.e., at the entry to the catalytic converter. When the exhaust gas is switched back to a lean gas, a further brief temperature peak occurs, amounting to a rise of around 80° C. compared to the steady state. A temperature peak of this type no longer occurs over the further path along the catalytic converter. The sensors 2 and 3 show a slow increase by temperatures of approx. 60° C. and approx. 50° C., respectively. Comparative measurements using a conventional thermocouple downstream of the catalytic converter revealed a temperature increase of only a few degrees.

[0051] This figure once again clearly illustrates the advantages of the invention. It is possible to detect temperature increases which occur locally and very quickly in the layer and which have effects on the functionality of the layer and therefore on the system as a whole. On account of the sensor properties, the sensor signals are particularly suitable for process control. In particular, the type and quantity of the reagents fed to the catalytic converter can be controlled at least in part by the sensor. Of course, it is also possible to construct a closed-loop control circuit, which can be used for closed-loop control of the type and quantity of reagents fed to the catalytic converter on the basis of the sensor signal.

[0052] A number of possible production methods are suitable in terms of manufacturing technology: on the one hand, it is possible first to install the substrate provided with the uncoated temperature probe in the uncoated catalytic converter monolith, and then to coat the temperature probe together with the catalytic converter monolith. On the other hand, however, it is also possible to integrate a temperature probe which has already been coated, into a monolith which has already been coated. The latter option is more complex, but can be used to record even broader measured variables, as shown below, for example by providing the two sides of the substrate with different coatings.

[0053] Of course, the integration of the temperature probe according to the invention in the catalytic converter does not necessarily have to take place exactly as outlined in FIG. 5. For example, it is also possible to select a substrate thickness which is greater than the thickness of a web. In this case, it should be ensured that at least the coated side of the temperature probe according to the invention is exposed to incoming gas flow in exactly the same way as the remaining catalytically active layer. For example, as shown in FIG. 9, it is possible for the substrate 72 to be made thicker than a passage. In this case (which is recommended in particular for catalytic converters with a high passage density—e.g., 600 cpsi or above—or for thick substrates), it is merely the case that one row of adjacent passages will be “blocked”. An embodiment as shown in FIG. 9 increases the mechanical stability. Details of the specific mechanical design do not form the subject of the present invention.

[0054] FIG. 9 illustrates yet a further embodiment of the present invention. The temperature probe(s) are coated on both sides. The additional coating 78 (with the indicated supply conductors 80) can be designed in the same way as the coating 74, which leads to an increased reliability of the measurement signal. However, it is also possible to use a catalytically inactive coating or a coating with a different catalytic activity, so that the exothermic nature and therefore the quality of the catalytic converter can be determined directly from the difference between the two temperature signals.

[0055] In all these embodiments, it is important to ensure that the flow in the catalytic converter is not significantly altered by the introduction of the device according to the invention. The incoming flow onto the temperature probe according to the invention should correspond to the incoming flow onto the catalytically active coating of the catalytic converter monolith.

[0056] A further embodiment results if the embodiment according to the invention is combined with sensors as proposed, for example, in German patent documents DE 198 05 928 and DE 100 64 499, and analysis can be carried out as described in German patent document DE 100 64 499.

[0057] One example will be given here. An IDK structure as proposed in German patent document DE 100 64 499 (FIG. 2) is applied to the underside of the temperature probe according to the invention which, for example, is installed as indicated in FIG. 9. Unlike in German patent document DE 100 64 499, the temperature probe is provided with the catalytically active coating. With this method, it is then possible to determine not only the quality of the catalytic converter but also its state. Moreover, the catalytically active coating of the temperature probe according to the invention offers the option of more accurately determining the layer temperature, and thereby more accurately determining the temperature-dependent electrical properties of the catalytically active coating, since more accurate temperature correction can be performed.

[0058] According to a further configuration of the invention, the IDK structure or another electrode arrangement is applied to the same side of the substrate together with the coated temperature probe. This means that space for a further sensor element (e.g., a gas sensor or a state sensor) is available on the rear side. An example of a recommended electrode arrangement which already includes a temperature probe is what is known as the IDKT arrangement as described in German patent document DE 100 41 921. In this arrangement, a thin conductor track, which serves as a counterelectrode as well as a temperature probe, snakes between the two IDK electrodes, which in this case are connected in parallel. This IDKT arrangement is then followed by the catalytically active coating. By way of example, a temperature probe with a catalytically inactive coating may be applied to the other side of the substrate, or alternatively a gas sensor may be applied thereto.

[0059] In a further simple arrangement, the temperature probe and electrode arrangement (e.g., IDK) are arranged next to one another, which keeps the rear side clear.

[0060] A further simple arrangement consists in integrating the temperature probe according to the invention (if appropriate with a state sensor, as mentioned above) in the catalytic converter by using a film/foil structure (in particular a metal foil structure) as a catalyst support, this foil structure subsequently being wound to form a catalytic converter monolith. This is shown in FIG. 10. Temperature probe and measuring electrodes (measuring electrodes for example in an IDK arrangement) were applied to a foil with a length L and a width B, L corresponding to what will subsequently be the length of the catalytic converter monolith.

[0061] The temperature probe and measuring electrodes together have been referred to in FIG. 10 as the sensor element. The sensor element may comprise just a temperature probe or may comprise a combination of temperature probe and electrode arrangement. They may either be arranged next to one another or may form an integrated structure (e.g., in the form of an IDKT structure as described above). In FIG. 10, the sensor elements are connected to supply conductors leading to a contact-making arrangement for the purpose of transmitting signals to the engine electronics. The supply conductors between sensor and contact-making arrangement are only diagrammatically indicated in FIG. 10.

[0062] A structure of this type as described in FIG. 10 has the advantage that a desired number of sensors and temperature probes can be applied to the special foil. The temperature probes may, for example, be conventional thin thermocouples or temperature-dependent resistors applied using layer technology. The abovementioned suitable contact-making option is retained on the foil.

[0063] The next process step then involves winding the foil to form a catalytic converter monolith. In this case, the foils are wound and pushed into a casing, which then forms the outer wall of the catalytic converter monolith, as illustrated in FIG. 11. It should then be ensured that the contact surface (contact-making feature) remains accessible for subsequent further processing. Then, the monolith can be coated, with the sensors of course also acquiring their coating at the same time, this coating then being identical to the coating of the catalytic converter.

[0064] FIG. 12 is intended to assist understanding of a “wound catalytic converter”. The excerpt from the cross section through a catalytic converter monolith reveals alternating corrugated foils and uncorrugated foils (also known as intermediate foils), which are arranged in such a way as to form gas-carrying passages. One of the intermediate foils may be designed as a sensor foil, as outlined in FIG. 10. FIG. 12 also illustrates the outer wall of the catalytic converter monolith, which may be designed in such a way that contact can be made with the electrical lines so that the signals can be passed to the engine electronics.

[0065] By using a suitable arrangement, the sensors can monitor and/or detect the cross section of the catalytic converter support at a suitable location (e.g., by a plurality of sensors being arranged in plane “e”, e.g., in each case one sensor in regions “a”, “b” or “c”). However, they may also provide an indication as to the performance of the catalytic converter in the direction of flow, for example as a result of in each case one sensor being arranged in the regions “e”, “f” or “g”. A combination of a plurality of sensors over the cross section and over the catalytic converter length allows detection of the entire volume of the catalytic converter, so that influences such as “uneven distribution” of the flow in terms of gas composition, temperature, flow velocity, pulsation, etc., could also be measured. From these measurements, it is possible to derive engine measures, so that in turn an improved conversion performance can be achieved.

[0066] Finally, it should be pointed out that the device according to the invention of a catalytically actively coated temperature probe integrated in the catalytic converter is possible for virtually all catalytic converters which are to be found in the exhaust system (for example both for an NOx storage catalytic converter and for a “three-way catalytic converter” or NH3-formation catalytic converter or a NH3-SCR catalytic converter or an HC adsorber or alternatively for a particulate filter). This also applies to the possibility of combining additional state sensors, as described in FIG. 9 to 12, with the temperature probe according to the invention.

[0067] The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.

Claims

1. A sensor for monitoring or controlling a catalytic converter, said sensor comprising:

a first sensor element, which is provided with a catalytic coating, is designed as a first temperature probe, and is integrated into the catalytic converter;

at least one second sensor element;

wherein the first and second sensor elements are arranged on a common substrate, on which the sensor is constructed.

2. The sensor according to claim 1, wherein:

the second sensor element has a catalytic coating; and

the catalytic coating of the first temperature probe and the coating of the second sensor element comprise at least part of a catalytic material which has been applied to the catalytic converter or a catalytic material of which the catalytic converter consists.

3. The sensor according to claim 1, wherein the second sensor element comprises a second temperature probe.

4. The sensor according to claim 2, wherein the second sensor element has an electrode structure is suitable for recording an electrical property of its coating.

5. The sensor according to claim 1, wherein at least one sensor element is arranged on each of a top side and an underside of the substrate.

6. The sensor according to claim 1, wherein the sensor is arranged in an exhaust system of a motor vehicle.

7. A method for the open-loop or closed-loop control of a catalytic exhaust-gas purification process with the aid of a temperature signal from a sensor according to claim 1, wherein a signal from the second sensor, which represents an electrical property of the catalytic coating, is additionally used for the open-loop or closed-loop control of the catalytic exhaust-gas purification process.

8. (Cancelled)