EP1098694B1 - METHOD FOR REGENERATING AN NOx STORAGE CATALYST - Google Patents

Description

The invention relates to a method for the regeneration of a NOx storage catalytic converter according to the preamble of the main claim.

In order to further reduce the fuel consumption of Otto engines, lean-burn engines are being used more and more. In order to meet the required exhaust emission limit values, special exhaust gas aftertreatment is necessary in such internal combustion engines. NOx storage catalytic converters are used for this. Due to their coating, these NOx storage catalytic converters are able to absorb NOx compounds from the exhaust gas that arise during lean combustion during a storage phase. During a regeneration phase, the absorbed or stored NOx compounds are converted into harmless compounds with the addition of a reducing agent. CO, H ₂ and HC (hydrocarbons) can be used as reducing agents for lean-burn gasoline internal combustion engines. These are generated by briefly operating the internal combustion engine with a rich mixture and made available to the NOx storage catalytic converter as exhaust gas components, as a result of which the stored NOx compounds in the catalytic converter are broken down.

The efficiency of such a NOx storage catalytic converter essentially depends on optimal regeneration. If the amount of regeneration agent is too small, the stored NOx is not broken down sufficiently, as a result of which the efficiency with which NOx is absorbed from the exhaust gas deteriorates. If the amount of regenerant is too high, optimal NOx conversion rates are achieved, but an inadmissibly high emission of reducing agent occurs. The optimal amount of regenerant fluctuates over the life of a vehicle. The possible cause for this can be the change in the NOx mass flow emitted by the internal combustion engine. Another reason is the change in the storage capacity of the catalyst, which decreases, for example due to the storage of sulfate, since sulfur present in the fuel is burned to SO ₂ , oxidized to sulfate by the catalyst when there is an excess of air, and is stored by the coating in a manner similar to NO ₂ , However, the binding of sulfate in storage is much stronger. However, sulfate is not converted during a regeneration phase, but remains bound in the NOx storage catalytic converter. With increasing sulphate storage, the capacity of the NOx storage catalytic converter decreases.

In German patent application DE 197 05 335 by the same applicant is a process for triggering sulfate regeneration described for a NOx storage catalyst in which a sulfate regeneration phase at predetermined times is carried out. When triggering sulfate regeneration in addition to the amount of sulfate stored, the thermal aging of the NOx storage catalytic converter is taken into account.

EP 0 597 106 A1 describes a method for regeneration a NOx storage catalytic converter is known, in which the NOx storage catalytic converter amount of NOx compounds absorbed in Dependency on operating data of the internal combustion engine is calculated becomes. When a predetermined limit is exceeded of NOx stored in the NOx storage catalytic converter becomes one Regeneration phase initiated. This way, however a reliable compliance with the exhaust emission limit values not guaranteed.

DE 195 11 548 A1 describes a method and a device for nitrogen oxide reduction in the exhaust gas of an internal combustion engine known. The internal combustion engine can alternate in lean operation and in stoichiometric or enrichment operation be driven. The nitrogen oxides are in the Lean operating phases recorded by an adsorber, which in regenerates the stoichiometric or enrichment phases becomes. The content of hydrocarbons in the exhaust gas, Carbon monoxides or nitrogen oxides downstream of the nitrogen oxide adsorber is measured and then in each case to a stoichiometric or enrichment operating phase switched if the measured hydrocarbon and carbon monoxide content exceeds the specified level or if the measured nitrogen oxide content falls below a predetermined level. This is a Corresponding hydrocarbon, carbon monoxide or nitrogen oxide sensor arranged in the exhaust pipe downstream of the adsorber, wherein the sensor information is a device for Controlling the fuel / air ratio.

US 5,554,269 describes the construction of a NOx sensor described for internal combustion engine exhaust. A Evaluation of the signal supplied by this NOx sensor for setting an optimal amount of regeneration agent for a NOx nitrogen oxide adsorber is not addressed.

It is usual to check the NOx storage catalytic converter a NOx sensor is arranged downstream of the catalyst. Such a sensor is for example from N. Kato et al., "Performance of Thick Film NOx Sensor on Diesel and Gasoline Engines ", Society of Automotive Engineers, publ. No.970858 known.

The invention has for its object to provide a method with the regeneration of a NOx storage catalytic converter so that it operates with optimal efficiency becomes.

This object is achieved by the invention defined in claim 1 solved.

In the regeneration phase, one is connected to a NOx sensor tapped signal evaluated to determine whether the The amount of regenerant was optimal. The signal used for this is tapped on an amperometric NOx sensor. This signal gives the lambda value or the oxygen concentration in the exhaust gas again and exhibits two-point behavior on, i.e. the signal changes in the range from lambda = 1 strong with small lambda changes.

In a preferred embodiment of the method, the amount of regenerant to be supplied to the NOx storage catalyst adapted to the optimal value. As a starches reduced Reducing agent requirement from a reduced storage capacity of the NOx storage catalytic converter, can Storage capacity preferably a sulfate regeneration be performed.

The advantage that can be achieved with the invention thus exists in particular in that over the entire life of the vehicle the optimal amount of regenerant is supplied.

Advantageous embodiments of the invention are in the subclaims characterized.

Fig. 1

eine schematische Darstellung einer Brennkraftmaschine mit einem NOx-Speicherkatalysator,

Fig. 2

ein Diagramm mit dem zeitlichen Verlauf des Ausgangssignals während der Regeneration des NOx-Speicherkatalysators, das am NOx-Meßaufnehmer abgegriffen wird,

Fig. 3

einen Ablaufplan zum Durchführen des Verfahrens und

Fig. 4

eine schematisierte Schnittdarstellung durch einen NOx-Meßaufnehmer.

The invention is explained in more detail below with reference to the drawing. The drawing shows:

Fig. 1

1 shows a schematic illustration of an internal combustion engine with a NOx storage catalytic converter,

Fig. 2

1 shows a diagram with the time course of the output signal during the regeneration of the NOx storage catalytic converter, which is tapped at the NOx sensor,

Fig. 3

a flowchart for performing the method and

Fig. 4

a schematic sectional view through a NOx sensor.

1 shows an internal combustion engine in the form of a block diagram with exhaust gas aftertreatment system in which the process is applied. Only the parts and components are included shown that are necessary for understanding the invention.

An internal combustion engine 10 has an intake tract 11 and one Exhaust tract 12 on. In the intake tract 11 there is a fuel metering device present, of which only one injector 13 is shown schematically. In the exhaust tract 12 there is one Precat lambda sensor 14, a NOx storage catalytic converter 15 and a NOx sensor 16 is provided downstream thereof. With help the pre-cat lambda sensor 14 becomes the air / fuel ratio determined in the exhaust gas upstream of the NOx storage catalytic converter 15. The NOx sensor 16 is used, among other things, for checking of the NOx storage catalyst 15. Operation of the internal combustion engine 10 is regulated by an operating control device 17, which has a memory 18 in which, among other things A plurality of threshold values are stored. The operating control unit 17 is via a schematically represented data and control line 19 with further sensors and actuators connected.

Depending on the operating mode of the internal combustion engine 10, come here in particular Lambda-1 controlled operation, homogeneously lean operation and stratified-lean operation in question, the NOx storage catalytic converter 15 close to air / fuel ratios Lambda = 1 also have three-way properties, or instead a NOx storage catalytic converter 15 also a device from two catalysts, a NOx storage catalyst and a three-way catalyst.

The NOx sensor 16 present downstream of the NOx storage catalytic converter 15 is an amperometric sensor. It is shown in more detail in a schematic sectional illustration in FIG. 4 under reference number 34. It consists of a solid electrolyte 26, for example ZrO _2, and contains the exhaust gas to be measured via a diffusion barrier 33. The exhaust gas diffuses through the diffusion barrier 33 into a first measuring cell 20. The oxygen content in the measuring cell 20 is measured by means of a first Nernst voltage V0 between a first electrode 21 and a reference electrode 29 exposed to ambient air. The first electrode 21 can also be made in several parts or with multiple taps. Both electrodes 21, 29 are conventional platinum electrodes. The reference electrode 29 is arranged in an air duct 28, into which ambient air passes through an opening 27.

The measured value of the first Nernst voltage V0 is used to to set a control voltage Vp0. The control voltage Vp0 drives a first oxygen ion pump current Ip0 through the Solid state electrolytes 26 between the first electrode 21 and an outer electrode 22. The one with a broken line shown control intervention of the first Nernst voltage V0 to the control voltage Vp0 has the consequence that the oxygen-ion pumping current Ip0 is set so that in the first Measuring cell 20 a certain oxygen concentration or certain oxygen partial pressure is present.

The first measuring cell 20 is over a further diffusion barrier 23 connected to a second measuring cell 24. Through this Diffusion barrier 23 diffuses this in the first measuring cell 20 existing gas. Due to the diffusion, the second measuring cell 24 a correspondingly lower, second oxygen concentration or oxygen partial pressure. This second oxygen concentration is again via a Nernst voltage V1 between a second electrode 25, the is also a conventional platinum electrode, and the reference electrode 29 measured, and to regulate a second Oxygen ion pumping current Ip1 used. The second oxygen-ion pumping current Ip1 out of the first measuring cell 20 flows from the second electrode 25 through the solid electrolyte 26 through to the outer electrode 22. With the help of second Nernst voltage V1 becomes the second oxygen-ion pumping current Ip1 adjusted so that in the second measuring cell 24th a certain, low, second oxygen concentration is present.

That of the previous processes in the measuring cells 20 and 24 NOx not affected is now at the measuring electrode 30, which is catalytic is effectively designed by applying the voltage V2 decomposes and the oxygen released as a measure of the NOx concentration at the measuring electrode 30 and thus in measuring exhaust gas in a measuring current Ip2 to the reference electrode 29 pumped out.

The following voltage is generated in the first measuring cell 20: U first measuring cell = RT / (4F). (Ln P O2, first measuring cell - ln P 02, exhaust gas ) + R0.Ip0 where P _{01, first measuring cell / exhaust gas is} the oxygen partial pressure in the first measuring cell or the exhaust gas, R the gas constant, T the absolute gas temperature, F the Faraday constant, R0 a contact resistance between the first electrode 21 and the solid electrolyte 26 and Ip0 the first Oxygen ion pump current is.

The following voltage results in the second measuring cell: U second measuring cell = RT / (4F). (Ln P O2, ambient air - ln P 02, second measuring cell ) where P _{02, ambient air / second measuring cell is} the oxygen partial pressure in the ambient air or the second measuring cell.

By tapping the differential voltage between the outer electrode 22 and the reference electrode 29, the two measuring cells 20 and 24 are connected in series, so that in a first approximation at a sufficiently homogeneous temperature of the NOx sensor 34, a sufficiently low current Ip0 and a sufficiently equal oxygen partial pressure at the taps the inner electrode 21 has the following relationship: U two point = RT / (4F). (ln P O2, ambient air - ln P 02, second measuring cell + ln P 02, first measuring cell - ln P 02, exhaust gas ) = RT / (4F). (ln P O2, ambient air - ln P 02, exhaust gas )

This relationship describes the two-point behavior of one Lambda probe. This differential voltage between the outer electrode 22 and the reference electrode 29 is used as the output signal US for the process for the regeneration of a NOx storage catalytic converter used.

That caused by the contact resistance R0 in equation (I) Measurement error in the voltage in the first measuring cell 20 can advantageously be corrected. For this, a certain one Resistance value assumed and an Ip0-dependent Compensation carried out. Furthermore, advantageously Correction of the output signal US with regard to the temperature of the sensor 34 take place.

2 shows the time course of the output signal US of the NOx sensor 16 during the regeneration phase of the NOx storage catalytic converter 15. Further in this illustration the course of the pre-cat lambda setpoint LAMSOLL is drawn. The pre-cat lambda setpoint LAMSOLL jumps at the beginning of Regeneration phase of the NOx storage catalytic converter 15 from one Value in the lean range (Lambda = 1.4) to a value for rich mixture (lambda = 0.85). At the end of the regeneration phase the internal combustion engine 10 is operated lean again.

At the end of the storage phase preceding the regeneration phase the output signal US is approximately 0.03 V. At the beginning During the regeneration phase, this voltage rises continuously on. The lambda value drops towards the end of the regeneration phase UL on the NOx sensor 16 downstream of the NOx storage catalytic converter 15 under 1 and the output signal US rises steeply. UL later rises back to lean values Mix and US drops again.

To determine whether the NOx storage catalyst 15 in a Regeneration phase supplied amount of regeneration agent is optimal, the procedure is now as follows:

Two total values are calculated. A first sum value FL1 is made with a certain frequency (e.g. 100 Hz) sampled output signal US from the beginning of the regeneration phase until a threshold value SW is exceeded (e.g. 0.25 V). This total value corresponds to that with the Reference number FL1 in Fig. 3 marked area. On second sum value FL2 is sampled from the one with the same frequency Output signal US from exceeding the threshold SW until the threshold falls below again SW calculated. This total value corresponds to that with the area marked with the reference symbol FL2 in FIG. 3. Naturally can use faces FL1 and FL2 instead of summation also be formed through continuous integration.

The optimal amount of regenerant was the NOx storage catalyst 15 then supplied when the total value FL2 is greater than a threshold value SW1 and the total value FL2 between a lower threshold USW2 and an upper one Threshold value OSW2 is.

3 is a flow chart for determining the optimal amount of regenerant shown. First, the sum values or areas FL1 and FL2 calculated and buffered (Step S1). The memory 18 is then of the operating control device 17, the threshold value SW1 for the Sum value FL1 and the threshold values USW2 and OSW2 for the Total value FL2 read out (step S2).

Now it is checked whether the amount of regenerant supplied is optimal (step S3). This is the case if the Sum value FL1 lies above the threshold value SW1 and the sum value FL2 from the lower threshold USW2 and from the upper Threshold OSW2 limited range. Are these two Conditions are met (step S4), there is no intervention necessary, the amount of regenerant used was optimal and the process is ended (step S11).

It turns out that these two conditions are not met are (step S3), it was in the regeneration phase the NOx storage catalyst 15 a non-optimal amount of regenerant fed. Depending on the total values FL1, FL2 can now be determined whether the amount of regenerant must be enlarged or reduced in order to achieve an optimal one To achieve regeneration of the NOx storage catalytic converter 15. To do this, it is first checked whether the total value FL1 is above the threshold value SW1 and the total value FL2 below the lower one Threshold value USW2 lies (step S5). Is that the case, the amount of regenerant is too small and must be increased (step S11, case A). The increase in the amount of regenerant can by changing the air ratio during the regeneration phase towards fat. alternative the regeneration phase can also be carried out longer be what is usually preferable since the variation of the lambda value in the regeneration phase only in narrow Limits (e.g. between 0.75 and 0.85) are possible. Was for following regeneration phases a larger amount of regeneration agent set, the procedure is finished (step S11).

It turns out in step 5 that the total value FL1 below the Threshold value SW2 and the total value FL2 above the lower one Threshold USW2, it is checked whether the total value FL1 above the threshold SW2 and the total value FL2 above the upper threshold value OSW2 (step S7). Then the amount of regeneration agent is too large and must be reduced (Step S8, case B). The reduction in the amount of regenerant can be done analogously to the enlargement in case A. Was a smaller amount of regenerant for future use Regeneration phases of the NOx storage catalytic converter 15 are stored, the method is ended (step S11).

It was found in step S7 that the total value FL1 not above the threshold value SW1 and the total value FL2 is above the upper threshold OSW2, is checked first whether the special case FL1 = SW1 is present (step S9). Is this the case, no rule intervention is necessary and the procedure is ended (step S11). If this is not the case, the total value must be FL1 are below the threshold SW1 (step S10). As a result, the storage capacity of the NOx storage catalytic converter 15 has dropped (case C). For optimal To achieve conversion behavior of the exhaust system must consequently the storage phase can be shortened. For example by reducing that in a computational catalyst model used storage capacity. Likewise the threshold value SW1 must be lowered. falls below the threshold value SW1 during the useful life of the Internal combustion engine 10 a lower limit, this means that the catalyst capacity has reached a minimum value what e.g. can be caused by sulfate storage. In In this case, a sulfate regeneration is preferably requested and carried out, such as in the German Patent application 197 05 335 is described. After done Sulfate regeneration can be the threshold SW1 again be set to the initial value.

The mentioned threshold values SW, SW1, USW2, OSW2 are on determined on a test bench.

Claims (10)

1. Method for regenerating a NOx storage catalytic converter (15),

   which is arranged in the exhaust gas tract (12) of an internal combustion engine (10) operated with an excess of air,

   downstream from which a NOx sensor (16) is arranged and

   which catalytically converts stored NOx in a regeneration phase with the addition of a reducing agent, whereby the reducing agent is generated by brief operation of the internal combustion engine (10) with a rich air/fuel mixture (lambda < 1),

   characterised in that
   an amperometric sensor (34) comprising a solid-state electrolyte (26) is used as the NOx sensor (16), which

   has a first measuring cell (20), in which the oxygen concentration is measured via a first Nernst voltage (V0) between a first electrode (21) and a reference electrode (29) exposed to ambient air and regulated by means of a first oxygen ion pump flow (Ip0) between the first electrode (21) and an external electrode (22), and

   has a second measuring cell (24), which is connected to the first measuring cell (20) and in which the oxygen concentration is measured via a second Nemst voltage (V1) between a second electrode (25) and the reference electrode (29), and that

   when the two measuring cells (20, 24) are connected in series, the voltage between the external electrode (22) and the reference electrode (29) is picked up and this output signal (US) demonstrating two-step action and based on the oxygen concentration is detected during the regeneration phase and that

   a criterion is derived from the pattern over time of the output signal (US) to determine whether the quantity of regeneration agent must be changed to achieve optimum regeneration of the NOx storage catalytic converter (15).
2. Method according to Claim 1, characterised in that two cumulative values (FL1, FL2) are formed as criteria, whereby

   the first cumulative value (FL1) is calculated from the output signal (US) scanned with a specific frequency from the start of regeneration until a predefined threshold value (SW1) is exceeded

   the second cumulative value (FL2) is calculated from the output signal (US) scanned with the same frequency from the point when this threshold value (SW) is exceeded until the measured value is below the threshold value (SW),

   the cumulative values (FL1, FL2) are compared with associated threshold values (SW1, USW2, OSW2) and

   depending on the result of the comparison the quantity of regeneration agent is kept constant, increased or reduced.
3. Method according to Claim 2, characterised in that the quantity of reducing agent is kept constant, if the first cumulative value (FL1) is greater than the threshold value (SW1) and the second cumulative value (SW2) is within a range bounded by the lower threshold value (USW2) and the upper threshold value (OSW2).
4. Method according to Claim 2, characterised in that the quantity of reducing agent is increased, if the first cumulative value (FL1) is greater than the threshold value (SW1) and the second cumulative value (SW2) is smaller than the lower threshold value (USW2).
5. Method according to Claim 2, characterised in that the quantity of regeneration agent is reduced, if the first cumulative value (FL1) is greater than the threshold value (SW1) and the second cumulative value (FL2) is greater than the upper threshold value (OSW2).
6. Method according to Claim 4, characterised in that the quantity of reducing agent is increased by lengthening the regeneration phase.
7. Method according to Claim 5, characterised in that the quantity of reducing agent is reduced by shortening the regeneration phase.
8. Method according to one of the preceding claims, characterised in that the duration of a storage phase of the NOx storage catalytic converter (15), during which the internal combustion engine (14) is operated with an excess of air, is shortened and sulphate regeneration is carried out for the storage catalytic converter (15), if the cumulative value (FL1) is smaller than the threshold value (SW1).
9. Method according to one of the preceding claims, characterised in that the output signal (US) is corrected on the basis of the first oxygen ion pump flow (Ip0) in order to compensate for an error voltage due to a transition impedance (R0) crossed by the first oxygen ion pump flow (Ip0).
10. Method according to one of the preceding claims, characterised in that the output signal (US) is corrected on the basis of the temperature of the NOx sensor (16, 34).

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