Method for diagnosing an exhaust gas catalytic converter, and motor vehicle

Abstract

The invention relates to a method and to a motor vehicle for diagnosing an exhaust gas catalytic converter (28), which is arranged in an exhaust gas tract (20) of an internal combustion engine (12) and is suitable for converting at least one exhaust gas component, wherein the exhaust gas tract (20) has an exhaust gas sensor (48) arranged upstream of the exhaust gas catalytic converter (28) and has an exhaust gas recirculation system (38), which is designed to remove at least part of the exhaust gas from an exhaust gas duct (24) of the exhaust gas tract (20) downstream of the exhaust gas catalytic converter (28) and to feed the removed exhaust gas to the internal combustion engine (12). The method comprises the following steps: measuring a first concentration (NOX1) of the exhaust gas component upstream of the exhaust gas catalytic converter (28) by means of the exhaust gas sensor (48) during fueled operation of the internal combustion engine (12), recirculating at least part of the exhaust gas by means of the exhaust gas recirculation system (38) during unfueled overrun of the internal combustion engine (12), measuring a second concentration (NOX2) of the exhaust gas component by means of the exhaust gas sensor (48) during the unfueled overrun of the internal combustion engine (12), and determining a state of the exhaust gas catalytic converter (28) in dependence on the first and second concentrations (NOX1, NOX2) of the exhaust gas component.

Claims

1. A method for diagnosing an exhaust gas catalytic converter (28) that is arranged in the exhaust gas tract (20) of the internal combustion engine (12) of a motor vehicle (10) and that is suitable for the conversion of at least one exhaust gas component, whereby an exhaust gas sensor (48) is installed in the exhaust gas tract (20) upstream from the exhaust gas catalytic converter (28), the motor vehicle (10) having an exhaust gas recirculation system (38) that is designed so that at least some exhaust gas can be removed from an exhaust gas pipe (24) of the exhaust gas tract (20) downstream from the exhaust gas catalytic converter (28) and can be fed to the internal combustion engine (12), comprising the following steps: measuring a first concentration (NOX1) of the at least one exhaust gas component upstream from the exhaust gas catalytic converter (28) by means of the exhaust gas sensor (48) during operation of the internal combustion engine (12) with the throttle open; recirculating at least some exhaust gas via the exhaust gas recirculation system (38) during an overrun mode of operation of the internal combustion engine (12) with the throttle closed; measuring a second concentration (NOX2) of the exhaust gas component by means of the exhaust gas sensor (48) during the overrun mode of operation of the internal combustion engine (12) with the throttle closed; and determining the status of the exhaust gas catalytic converter (28) as a function of the first and second concentrations (NOX1, NOX2) of the at least one exhaust gas component.

2. The method according to claim 1, wherein the at least one exhaust gas component comprises nitrogen oxides (NOx), and the exhaust gas catalytic converter (28) is an NOx catalytic converter or an SCR catalytic converter, and the exhaust gas sensor (48) is an NOx sensor.

3. The method according to claim 1, wherein the first concentration (NOX1) of the at least one exhaust gas component is measured after an overrun phase of the vehicle (10) has been ascertained, before the start of the overrun mode of the internal combustion engine (12) with the throttle closed.

4. The method according to claim 1, wherein the second concentration (NOX2) of the at least one exhaust gas component is measured during the overrun mode of operation of the internal combustion engine (12) with the throttle closed, after a prescribed waiting time (T) has passed.

5. The method according to claim 1, wherein the waiting time (T) is prescribed so as to correspond at least to a circulation time of the exhaust gas since the measurement of the first concentration (NOX1) of the at least one exhaust gas component.

6. The method according to claim 1, wherein, during the overrun mode of operation of the internal combustion engine (12) with the throttle closed, all of the exhaust gas is recirculated via the exhaust gas recirculation system (38).

7. The method according to claim 1, wherein the exhaust gas is recirculated via the exhaust gas recirculation system (38) by closing an exhaust gas blocking element (46) arranged downstream from a branching site of an exhaust gas recirculation line (40) of the exhaust gas recirculation system (38).

8. A motor vehicle (10), comprising: an internal combustion engine (12); an exhaust gas tract (20) with an exhaust gas pipe (24), an exhaust gas catalytic converter (28) that is arranged in the exhaust gas pipe (24) and that is suitable for the conversion of at least one exhaust gas component, and an exhaust gas sensor (48) that is arranged upstream from the exhaust gas catalytic converter (28) and that serves to detect the at least one exhaust gas component; an exhaust gas recirculation system (38) that is configured to remove at least some exhaust gas from the exhaust gas pipe (24) downstream from the exhaust gas catalytic converter (28) and to feed it to the internal combustion engine (12); and a diagnostic device (50) designed to carry out the method for diagnosing the exhaust gas catalytic converter (28) by: measuring a first concentration (NOX1) of the at least one exhaust gas component upstream from the exhaust gas catalytic converter (28) by means of the exhaust gas sensor (48) during operation of the internal combustion engine (12) with the throttle open; recirculating the at least some exhaust gas via the exhaust gas recirculation system (38) during an overrun mode of operation of the internal combustion engine (12) with the throttle closed; measuring a second concentration (NOX2) of the at least one exhaust gas component by means of the exhaust gas sensor (48) during the overrun mode of operation of the internal combustion engine (12) with the throttle closed; and determining the status of the exhaust gas catalytic converter (28) as a function of the first and second concentrations (NOX1, NOX2) of the at least one exhaust gas component.

9. A motor vehicle according to claim 8, also comprising an exhaust gas blocking element (46) arranged downstream from the branching site of an exhaust gas recirculation line (40) of the exhaust gas recirculation system (38).

10. The motor vehicle according to claim 8, also comprising an exhaust gas turbocharger (34, 36) with a turbine (34) arranged in the exhaust gas pipe (24), whereby the exhaust gas recirculation system (38) is configured as a low-pressure exhaust gas recirculation system.

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 of a motor vehicle 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 the diagnosis 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 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 has, for instance, four cylinders here, 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 as well as 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 tract 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 tract 20 comprises an exhaust gas manifold 22 that connects the individual outlets of the cylinders of the internal combustion engine 12 to a shared exhaust gas pipe 24. The exhaust gas pipe 24 ends into an exhaust tailpipe (shown on the right in the drawing) and it holds several components for the after-treatment of the exhaust gas.

(8) In the example shown, the exhaust gas first reaches an oxidation catalytic converter 26. This catalytic converter has a catalyst 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 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 pipe 24. The SCR catalytic converter 28, like the oxidation catalytic converter 26, comprises a catalyst 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.

(10) The exhaust gas tract 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 in the exhaust gas. The reductant is usually metered in via the metering unit 30 by means of a control system (not shown here) which regulates the metering 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) It goes without saying that the exhaust gas tract 20 can have exhaust gas after-treatment components that differ from those shown in FIG. 1 or that are in addition to those, especially a particulate filter to remove particulate exhaust gas components.

(12) The vehicle 10 also comprises an exhaust gas turbocharger that has a turbine 34 arranged in the exhaust gas pipe 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.

(13) 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 or a complete stream of the exhaust gas from the exhaust gas pipe 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.

(14) Downstream from the branching site of the EGR line 40 from the exhaust gas pipe 24, there is an exhaust gas blocking element 46 that can be adjusted, preferably continuously, between an open and a closed position. The exhaust gas blocking element 46 can be configured, for instance, as an exhaust gas flap. In its closed position, the exhaust gas blocking element 46 blocks the exhaust gas line 24 essentially completely.

(15) Like all exhaust gas catalytic converters, the SCR catalytic converter 28 is also prone to an age-related deterioration 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 reduction of its catalytic activity and in order to ensure a precise metering of the reductant. According to the invention, the SCR catalytic converter 28 is diagnosed by means of an NO.sub.x sensor 48 situated upstream from it. Preferably, the sensor 48 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 48 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 NOX1, NOX2 of the NO.sub.x sensor 48 is entered as an input quantity into a diagnostic device 50. Moreover, the diagnostic device 50 can receive information about the momentary operating point of the vehicle 10 and of the internal combustion engine 12, especially the vehicle speed v_Fzg and a desired torque M_w requested by the driver by actuating the gas pedal. 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. For this purpose, the diagnostics device emits control signals (indicated by solid arrows in FIG. 1), by means of which the EGR valve 44, the exhaust gas flap 46 and the fuel feed to the internal combustion engine 12 can all be actuated.

(16) A preferred sequence of the method according to the invention for diagnosing the SCR catalytic converter 28 will be explained in greater detail below making reference to FIG. 2.

(17) The method starts in step S1 and then proceeds to the step S2, in which the operating point of the vehicle 10 is checked as to whether an overrun phase is present, that is to say, whether the momentary kinetic energy of the vehicle is greater than the torque M_w currently being requested by the driver. For purposes of ascertaining the presence of an overrun phase of the vehicle, the diagnostics unit 50 can evaluate, for instance, the torque M_w being requested by the driver as well as the momentary vehicle speed V_Fzg. As an alternative, the diagnostics unit 50 can also directly receive an appertaining overrun phase signal from a general engine control unit. If the query S2 did not ascertain an overrun phase of the vehicle 10, the method is interrupted and returns to step S1.

(18) If, on the other hand, an overrun phase of the vehicle is detected, the method proceeds to step S3, in which a first concentration of nitrogen oxides NOX1 in the exhaust gas is measured by means of the NO.sub.x sensor 48. Therefore, the first NO.sub.x concentration NOX1 corresponds to the NO.sub.x raw emissions of the internal combustion engine 12 during operation with the throttle open, said NO.sub.x raw emissions being present at the inlet of the SCR catalytic converter 28.

(19) Then the method proceeds to step S4, in which the fuel feed to the internal combustion engine 12 is switched off, that is to say, the fuel volume KS that is to be injected is set to zero. At the same time, a throttle valve (not shown in FIG. 1) in the air line 14 of the internal combustion engine 12 remains open.

(20) In the subsequent step S5, the exhaust gas recirculation via the LP exhaust gas recirculation system 38 is initiated. Towards this end, the exhaust gas flap (AK) 46 is closed and the EGR valve 44 of the EGR line 40 is opened to its maximum. The exhaust gas recirculated via the EGR line 40 is conveyed by the internal combustion engine 10 that is being turned over by the vehicle 10 in the overrun phase.

(21) The subsequent steps S6 and S7 correspond to a timer function. For this purpose, a time meter t is raised by a prescribed increment A in step S6. In the query S7 that follows, it is checked whether the time meter t has reached a prescribed waiting time T. In this context, the prescribed waiting time T is predefined in such a way that it corresponds at least to the exhaust gas circulation time that the exhaust gas needs from the moment when the first NO.sub.x concentration NOX1 is measured in step S3 until it has once again reached the NO.sub.x sensor 48. As along as the time meter t is smaller than the waiting time T, in other words, as long as the response to the query in S7 is no, the method returns to step S6 in order for the time meter t to count up once again.

(22) Once the time meter t has reached the waiting time T, in other words, once the response to the query in S7 is yes, the method proceeds to step S8, in which the NO.sub.x concentration of the exhaust gas is measured once again with the NO.sub.x sensor 48. Owing to the preceding waiting loop in the form of steps S6 and S7, it is ensured that the concentration measurement made by the sensor 48 at the point in time of the measurement in step S8 is carried out in an exhaust gas that has already passed through the SCR catalytic converter 28. Ideally, the measurement in step S8 is carried out in the same exhaust gas volume that also served as the basis for the first concentration measurement in step S3. Therefore, the NO.sub.x concentration NOX2 measured in step S8 corresponds to the concentration present downstream from the SCR catalytic converter 28.

(23) In step S9, the status of the SCR catalytic converter 28 is ascertained as a function of the first NO.sub.x concentration NOX1 and second NO.sub.x concentration NOX2 measured in steps S3 and S8. For example, the status of the catalytic converter in the form of its efficiency can be ascertained in accordance with the following equation:

(24) $=1-\frac{\mathrm{NOX}2}{\mathrm{NOX}1}$

(25) When it comes to an ideally intact SCR system, the NO.sub.x concentration NOX2 during the overrun phase is close to zero and the efficiency is close to 1. In a completely inactive SCR system, in contrast, the NOX2 is at the level of the NO.sub.x raw emissions NOX1. In this case, the efficiency is close to zero.

(26) In the subsequent query in step S10, a comparison is made between the ascertained status of the catalytic converter and a suitable threshold value .sub.SW. In the case of a damaged SCR catalytic converter 28, the query S10 ascertains whether the efficiency has fallen below the threshold and, in step S11, an error signal is output. In contrast, if the ascertained efficiency is greater than the prescribed threshold, the SCR system is working satisfactorily and the method returns to its starting point.

LIST OF REFERENCE NUMERALS

(27) 10 motor vehicle 12 internal combustion engine 14 air line 16 air manifold 18 air filter 20 exhaust gas tract 22 exhaust gas manifold 24 exhaust gas pipe 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 exhaust gas blocking element 48 NO.sub.x sensor 50 diagnostic device NOX1 first concentration of the exhaust gas component during operation when the throttle is open NOX2 second concentration of the exhaust gas component during operation when the throttle is closed M_w requested driving torque V_Fzg vehicle speed catalytic converter status/catalytic converter efficiency .sub.SW threshold value t time meter time increment