Method for diagnosing an exhaust gas catalytic converter, diagnosis device and motor vehicle having such a device

Abstract

A method for diagnosing an exhaust-gas catalytic converter (28) arranged in an exhaust-gas system (20) of an internal combustion engine (12) for the catalytic conversion of at least one exhaust-gas component from the internal combustion engine (12), which has an exhaust-gas recirculation means with which a partial stream of the exhaust gas can be drawn off downstream of the catalytic converter (14) and fed into the combustion air of the internal combustion engine (12), involves determining the current raw emission (NO.sub.xraw) of the internal combustion engine (12) in of the exhaust-gas component; measuring the current concentration of the exhaust-gas component (NO.sub.xmeas) in the exhaust gas upstream of the catalytic converter (28); and determining a diagnostic value for the catalytic converter (28) in terms of the conversion of the exhaust-gas component as a function of the modelled raw emission (NO.sub.xraw) from the internal combustion engine (12) and the measured concentration of the exhaust-gas component (NO.sub.xmeas). The invention further relates to a diagnostic device configured to perform the method and a motor vehicle having such a device.

Claims

1. A method for diagnosing an exhaust-gas catalytic converter configured to catalytically convert at least one exhaust-gas component, the catalytic converter being arranged in an exhaust-gas system of an internal combustion engine having an engine control unit and an exhaust-gas recirculation system configured to withdraw a partial stream of exhaust gas downstream from the exhaust-gas catalytic converter and to feed the partial stream into combustion air of the internal combustion engine, said engine control unit configured to implement the method, the method comprising: determining a momentary raw emission (NO.sub.xraw) of the internal combustion engine in terms of an exhaust-gas component; measuring a momentary concentration of the exhaust-gas component (NO.sub.xmeas) in the exhaust gas upstream from the exhaust-gas catalytic converter; and determining a diagnostic value of the exhaust-gas catalytic converter in terms of the conversion of the exhaust-gas component as a function of a modeled raw emission (NO.sub.xraw) of the internal combustion engine and of the measured concentration of the exhaust-gas component (NO.sub.xmeas).

2. The method according to claim 1, further comprising determining an efficiency () of the exhaust-gas catalytic converter as a diagnostic value as a function of the ratio of the modeled raw emission (NO.sub.xraw) to the measured concentration (NO.sub.xmeas).

3. The method according to claim 2, wherein the efficiency () is determined according to the equation below, wherein _EGR stands for the exhaust-gas recirculation rate: $=1-\frac{{}_{}{\mathrm{EGR}}^{-1}\left({\mathrm{NO}}_x\mathrm{meas}-{\mathrm{NO}}_x\mathrm{raw}\right)}{{\mathrm{NO}}_x\mathrm{raw}}.$

4. The method according to claim 3 further comprising: comparing the efficiency () to a target value (target) or to a threshold value (_tv), and, in the case of a minimum deviation from the target value (_target) or if the threshold value (_tv) has been exceeded, ascertaining a fault in the exhaust-gas catalytic converter, and outputting the fault to the engine control unit.

5. The method according to claim 4, further comprising modeling the target value (_target) and/or the threshold value (_tv) as a function of a momentary operating point of the internal combustion engine using a characteristic map.

6. The method according to claim 5 further comprising modeling the raw emission (NO.sub.xraw) of the internal combustion engine as a function of the momentary operating point of the internal combustion engine using a characteristic map.

7. A diagnostic device for diagnosing an exhaust-gas catalytic converter, wherein the diagnostic device comprises an engine control unit configured to: determine a momentary raw emission (NO.sub.xraw) of an internal combustion engine in terms of an exhaust-gas component; measure a momentary concentration of the exhaust-gas component (NO.sub.xmeas) in an exhaust gas upstream from an exhaust-gas catalytic converter; and determine a diagnostic value of the exhaust-gas catalytic converter in terms of the conversion of the exhaust-gas component as a function of a modeled raw emission (NO.sub.xraw) of the internal combustion engine and of the measured concentration of the exhaust-gas component (NO.sub.xmeas).

8. A motor vehicle comprising: an internal combustion engine, an exhaust-gas system connected to the internal combustion engine, an exhaust-gas catalytic converter arranged in the exhaust-gas system, said catalytic converter configured to catalytically convert at least one exhaust-gas component of the internal combustion engine, an exhaust-gas sensor located upstream from the exhaust-gas catalytic converter and configured to measure a momentary concentration of the at least one exhaust-gas component (NO.sub.xmeas) in exhaust gas, an exhaust-gas recirculation system configured to withdraw a partial stream of exhaust gas downstream from the exhaust-gas catalytic converter and to feed the withdrawn partial stream of exhaust gas into combustion air of the internal combustion engine; and the diagnostic device configured to: determine a momentary raw emission (NO.sub.xraw) of the internal combustion engine in terms of the at least one exhaust-gas component; measure, via the exhaust-gas sensor, the momentary concentration of the at least one exhaust-gas component (NO.sub.xmeas) in the exhaust gas upstream from the exhaust-gas catalytic converter; and determine a diagnostic value of the exhaust-gas catalytic converter in terms of the conversion of the exhaust-gas component as a function of a modeled raw emission (NO.sub.xraw) of the internal combustion engine and of the measured concentration of the at least one exhaust-gas component (NO.sub.xmeas).

9. The motor vehicle according to claim 8, wherein the exhaust-gas catalytic converter is configured to reduce nitrogen oxides.

10. The motor vehicle according to claim 8, wherein the at least one exhaust-gas component comprises nitrogen oxides (NO.sub.x) and the exhaust-gas sensor is a NO.sub.x sensor.

11. The motor vehicle according to claim 9, wherein the catalytic converter is a SCR catalytic converter.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will be explained in greater detail below on the basis of embodiments making reference to the accompanying drawings. The following is shown:

(2) FIG. 1 a schematic view of an exhaust-gas system according to an advantageous embodiment of the invention; and

(3) FIG. 2 a flow chart for carrying out a diagnosis of an SCR catalytic converter according to an advantageous embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) The invention will be presented below on the basis of an example of an SCR catalytic converter. However, it goes without saying that the invention can also be employed for other exhaust-gas catalytic converters.

(5) FIG. 1 shows a drawing of a motor vehicle which is designated in its entirety by the reference numeral 10 and which is driven by an internal combustion engine 12 that is lean-running, at least at times, especially by a diesel engine that serves as the source of traction. The internal combustion engine 12 here has, for instance, four cylinders, whereby any number of cylinders diverging from this is likewise possible.

(6) The internal combustion engine 12 is supplied with combustion air via an air line 14 and via an air manifold 16 that conveys the drawn-in air to the cylinders. The combustion air drawn in from the ambient air is cleaned of particulate constituents by means of an air filter 18.

(7) The motor vehicle 10 also has an exhaust-gas system which is designated in its entirety by the reference numeral 20 and which serves for the catalytic after-treatment of exhaust gas from the internal combustion engine 12. The exhaust-gas system 20 comprises an exhaust-gas manifold 22 that connects the individual cylinder outlets of the internal combustion engine 12 to an exhaust-gas conduit 24. The exhaust-gas conduit 24 has a section (shown here) close to the engine as well as an underbody section (not shown here) that ends in the exhaust pipe. The exhaust-gas conduit 24 houses various components for the exhaust-gas after-treatment.

(8) In the example shown, the exhaust gas first reaches an oxidation catalytic converter 26. This catalytic converter has a substrate that is coated with a catalytic coating that catalyzes the oxidation of exhaust-gas components. In particular, it is well-suited for converting unburned hydrocarbons HC and carbon monoxide CO into CO.sub.2 and H.sub.2O. Moreover, the catalytic coating of the oxidation catalytic converter 26 is configured to oxidize No and N.sub.2O to form NO.sub.2 in order to increase the ratio of NO.sub.2 to NO. The catalytic coating of the oxidation catalytic converter 26 contains as the catalytic component particularly at least one element from the group of platinum metals Pt, Pd, Rh, Ru, Os or Ir, or else a combination thereof, especially Pt and/or Pd. The catalytic coating of the oxidation catalytic converter 26 also contains a washcoat comprising a porous ceramic matrix having a large specific surface area, for example, on the basis of zeolite, which is doped with the catalytic component. The substrate of the oxidation catalytic converter 26 can be a metallic substrate or a ceramic monolith, especially a honeycomb-like structure having a plurality of continuous, parallel flow channels. Suitable ceramic materials include aluminum oxide, cordierite, mullite and silicon carbide. Suitable material substrates are made out of stainless steel or iron-chromium alloys.

(9) Downstream from the oxidation catalytic converter 26, there is another exhaust-gas catalytic converter, here an SCR catalytic converter 28 in the exhaust-gas conduit 24. The SCR catalytic converter 28, like the oxidation catalytic converter 26, comprises a catalytic substrate on a metallic or ceramic basis, preferably on a ceramic basis. Suitable ceramic or metallic materials correspond to those mentioned in conjunction with the oxidation catalytic converter. The inner walls of the parallel and continuous flow channels of the substrate are coated with an SCR catalytic coating that brings about the reduction of nitrogen oxides to form nitrogen under selective consumption of a reductant. The coating, in turn, comprises a washcoat consisting of a porous ceramic matrix having a large specific surface area (e.g. a zeolite on an aluminum silicate basis), with catalytic substances distributed thereupon. Suitable SCR catalytic substances encompass especially non-noble metals such as Fe, Cu, Va, Cr, Mo, W as well as combinations thereof. These substances are deposited onto the zeolite and/or the zeolite metals are partially replaced by the corresponding non-noble metals through the modality of ion exchange. The SCR catalytic converter 28 is preferably arranged in a place that is close to the engine. In particular, the distance (path of the exhaust gas) between the cylinder outlet and an inlet face of the SCR catalytic converter 28 amounts to 120 cm at the maximum.

(10) The exhaust-gas system 20 also has a reductant metering unit 30 with which the reductant or a precursor compound thereof is metered into the exhaust gas. For instance, the reductant is introduced into the exhaust-gas stream by means of a nozzle located upstream from the SCR catalytic converter 28. The reductant can typically be ammonia NH.sub.3 that is metered in in the form of a precursor compound, especially in the form of urea. Preferably, the urea in the form of an aqueous solution is conveyed and metered in from a reservoir (not shown here). In a mixer 32 installed downstream from the metering unit 30, the urea is mixed with the hot exhaust gas and decomposed to form NH.sub.3 and CO.sub.2 through the modality of thermolysis and hydrolysis. The NH.sub.3 is stored in the coating of the SCR catalytic converter 28, where it is used for the reduction of nitrogen oxides. The reductant is usually metered in via the metering unit 30 by means of a control system (not shown here) which regulates the unit 30 as a function of a given operating point of the engine 12, especially as a function of the momentary NO.sub.x concentration in the exhaust gas.

(11) The vehicle 10 also comprises an exhaust-gas turbocharger that has a turbine 34 arranged in the exhaust-gas conduit 24, said turbine being joined, for example, by means of a shaft to a compressor 36 situated in the air line 14. The turbine 34 withdraws kinetic energy from the exhaust gas in order to drive the compressor 36 and in order to compress the drawn-in combustion air. Normally, downstream from the compressor 36, there is an intercooler (not shown here) by means of which heat that was generated by the compression is withdrawn from the combustion air.

(12) The motor vehicle 10 also has a low-pressure exhaust-gas recirculation system (LP-EGR) 38. It has an exhaust-gas recirculation line 40 that, on the low-pressure side of the turbine 34 downstream from the SCR catalytic converter 28, withdraws a partial stream of the exhaust gas from the exhaust-gas conduit 24 and feeds it into the air line 14 on the low-pressure side of the compressor 36. An EGR cooler 42 situated in the EGR line 40 cools the hot, recirculated exhaust gas. The EGR rate, that is to say, the recirculated portion of exhaust gas in the combustion air of the internal combustion engine 12, is regulated by means of an EGR valve 44 likewise situated in the EGR line 40. Normally, the EGR valve 44 is regulated as a function of a given operating point of the internal combustion engine 12, whereby the valve 44 can be continuously varied between a completely closed position (EGR rates of zero, complete deactivation of the EGR) and a completely open position.

(13) Like all exhaust-gas catalytic converters, the SCR catalytic converter 28 is also subject to an age-related worsening of its catalytic activity. For this reason, there is a need for an ongoing diagnosis of the SCR catalytic converter 28 in order to detect an unacceptable weakening of its catalytic activity. According to the invention, the SCR catalytic converter 28 is diagnosed by means of a NO.sub.x sensor 46 situated upstream from it. Preferably, the sensor 46 is installed upstream from the reductant metering unit 30 and especially preferably upstream from the oxidation catalytic converter 26. Since the NO.sub.x sensor 46 is arranged very close to the engine 12, it can quickly reach operational readiness after a cold start of the engine 12. An output signal NO.sub.xmeas of the NO.sub.x sensor 46 is entered as an input quantity into a diagnostic device 48. Moreover, the diagnostic device 48 receives information about the momentary EGR rate _EGR and the momentary operating point of the internal combustion engine 12, especially in the form of the engine load L and the engine speed n. As a function of these and, if applicable, other quantities, the diagnostics device performs a diagnosis of the SCR catalytic converter 28 by means of the method according to the invention, as will be elaborated upon in greater detail below with reference to FIG. 2.

(14) By way of an example, FIG. 2 shows the sequence of the method according to the invention for the diagnosis of an SCR catalytic converter, in the form of a flow chart that is executed at regular intervals by the diagnostic device 48.

(15) The method is initialized in step S1 and then proceeds to the query S2, which checks whether the NO.sub.x sensor 46 is active. If this is not the case, for instance, after a cold start, the diagnosis cannot be carried out and the method returns to the starting point. If, on the other hand, the NO.sub.x sensor 46 is active, that is to say, if its output signal has been activated, the method proceeds to a second query S3, which checks whether the exhaust-gas recirculation system is active, that is to say, whether the EGR valve 44 is at least partially open. If the exhaust-gas recirculation system is not active, the diagnosis cannot be carried out and the method returns to its starting point. If, in contrast, the exhaust-gas recirculation system is active, that is to say, the answer to the query in S3 is yes, then the diagnosis of the SCR catalytic converter 28 is carried out.

(16) For this purpose, in step S4, the diagnostic device 48 reads in the output signal of the NO.sub.x sensor 46 and, as a function of the sensor signal, determines the momentary concentration of nitrogen oxides NO.sub.xmeas in the exhaust gas. Typically, for this purpose, a stored sensor characteristic line is employed which represents the NO.sub.x concentration as a function of the sensor signal, for instance, a sensor voltage. It goes without saying that the term concentration refers to any information about the content of the exhaust-gas component in the exhaust gas, irrespective of the unit used.

(17) Subsequently, in step S5, the diagnostic device 48 reads in several input quantities. In particular, these include the engine load L, which is determined, for instance, on the basis of the gas pedal actuation by the driver, the engine speed n as well as the momentary EGR rate _EGR. The momentary NO.sub.x raw emission NO.sub.xraw is modeled as a function of the engine load L and of the engine speed n. This can either be calculated by means of a mathematical model or else it can make use of stored characteristic lines or characteristic maps. In particular, a characteristic map is used that depicts the NO.sub.x raw emission NO.sub.xraw as a function of the engine load L and of the engine speed n. The determination of such characteristic maps, for example, on an engine test bench, is a known procedure and will not be elaborated upon here.

(18) Subsequently, in step S7, a diagnostic value is determined for the SCR catalytic converter 28 in terms of its conversion of NO.sub.x. In particular, an efficiency is calculated here, for which purpose the modeled raw emission NO.sub.xraw is related to the measured NO.sub.x concentration NO.sub.xmeas. For example, the efficiency can be determined according to the following equation:

(19) $=1-\frac{{}_{}{\mathrm{EGR}}^{-1}\left({\mathrm{NO}}_x\mathrm{meas}-{\mathrm{NO}}_x\mathrm{raw}\right)}{{\mathrm{NO}}_x\mathrm{raw}}$

(20) Subsequently, in step S8, the determined efficiency is compared to an efficiency threshold value _sw. Preferably, the threshold value _sw is predetermined by the diagnostic device 48 as a function of the engine operating point (L,n). If the momentarily determined efficiency is equal to or greater than _sw, the answer to the query is no, meaning that the SCR catalytic converter 28 is intact, and the method returns to the starting point. Optionally, the momentary diagnostic value can be stored for documentation purposes. However, if the answer to the query in step S8 is yes, that is to say, if the efficiency has fallen below the threshold value then, in step S9, a fault in the catalytic converter 28 is determined. The fault can be output as a visual and/or acoustic signal to the driver of the vehicle, and/or it can be relayed to the engine control unit, from where it can be read out at the time of the next servicing.

LIST OF REFERENCE NUMERALS

(21) 10 motor vehicle 12 internal combustion engine 14 air line 16 air manifold 18 air filter 20 exhaust-gas system 22 exhaust-gas manifold 24 exhaust-gas conduit 26 oxidation catalytic converter 28 exhaust-gas catalytic converter/SCR catalytic converter 30 reductant metering unit 32 mixer 34 turbine 36 compressor 38 low-pressure exhaust-gas recirculation system 40 exhaust-gas recirculation line 42 EGR cooler 44 EGR valve 46 NO.sub.x sensor 48 diagnostic device _EGR EGR rate (portion of exhaust gas in the combustion air) L engine load n engine speed NO.sub.xmeas measured concentration of the exhaust-gas component in the exhaust gas upstream from the exhaust-gas catalytic converter NO.sub.xraw modeled raw emission of the exhaust-gas component in the internal combustion engine