EP1124050B1 - Method and device for desulphating a nox accumulator catalyst - Google Patents

Description

The invention relates to a method and a device according to the preamble of the patent claim 1 mentioned.

One way to reduce the fuel consumption of internal combustion engines is the lean engine operation. This means that the internal combustion engine is not operated with a stoichiometric ratio of air and fuel, but with excess air. In this operating mode, the nitrogen oxide (NO _x ) emissions can not be reduced when using a current three-way catalytic converter, so that the statutory exhaust limits are not sustainable. For this reason, nitrogen oxide storage catalysts are used for exhaust aftertreatment in lean engine operation. These store between the emitted by the internal combustion engine NO _x raw emissions during engine storage operation between. After some time, the engine is operated in fat to release the stored nitrogen oxides again and chemically convert it into harmless substances. As a rich engine operation, an engine operation is called with excess fuel.

Fuels and engine lubricants commonly used today also contain undesirable components in addition to the desired hydrocarbon chains. These include, among others, the sulfur and its chemical compounds. During engine operation, sulfur in the form of sulfur dioxide (SO ₂ ) is expelled from the engine. A problem for the new coatings of NO _x storage catalysts is their susceptibility to sulfur poisoning by sulfate formation in and on the catalyst material. As a result of sulfur poisoning, the storage capacity of the NO _x nitrogen oxide storage catalyst decreases until it becomes ineffective.

The invention is based on EP-A-0 915 244, in which a method of operation an internal combustion engine for desulfating one in an emission control system arranged NOx storage catalyst by cyclically changing the air ratio is described, wherein the NOx emission in the combustion exhaust gas after a NOx storage catalyst continuously or intermittently measured and at the same time another value for a NOx concentration upstream of the NOx storage catalyst is determined and the quotient of the value of the NOx concentration before the NOx storage catalyst to the NOx measurement after the NOx storage catalyst is formed and this continuously with a first setpoint is compared, wherein falls below a first setpoint desulfation is initiated.

A disadvantage of the cited prior art is that the duration of desulfation is predetermined by a fixed time window. Due to the inexact termination criterion, d. H. Termination of desulfation after a predetermined period, desulphation usually takes longer than necessary for desulphation is. This means that even after the NOx storage catalyst already completely desulfated, the "fat" operation, d. H. Operation of the internal combustion engine with a fuel surplus, continues. This leads to considerable emission disadvantages, the fuel consumption of the internal combustion engine is increased and hydrocarbons leave the exhaust system unburned and pollute the environment. But also during the desulfurization occur hydrocarbon breakthroughs, which also leave the exhaust system unburned and pollute the environment unnecessarily. Furthermore, a subsequent to a Desulfatisierungsphase Hydrogen sulfide emission can not be prevented.

Object of our invention is the hydrocarbon emission already during and to minimize after desulfating the NOx storage catalyst.

This object is achieved by the features in the characterizing part of the claim 1 achieved in that during the desulfation in each lean phase the NOx concentration in the combustion exhaust gas after the NOx storage catalyst measured and at the same time another value for the NOx concentration in the combustion exhaust gas is determined before the NOx storage catalyst, the quotient of the Value of the NOx concentration upstream of the NOx storage catalyst to the NOx measurement formed after the NOx storage catalyst and this with a second Setpoint is continuously compared, and that when exceeding the second setpoint the desulphation is stopped.

By the inventive method according to claim 1 and the inventive Device according to claim 6 all o. G. disadvantage avoided.

The basic idea of the invention is based on measurements which confirm that the sulfur dioxide concentration (SO ₂ ) emitted by an NO _x storage catalyst during desulfurization is indirectly detected with an NO _x sensor. This effect is exploited to determine the degree of desulfation of a NO _x storage catalyst. This determination is independent of the sulfur content in the fuel and of the aging of the NO _x storage catalytic converter.

By measuring the NO _x emission in the flow direction after the NO _x storage catalyst, a demolition time for these is set by falling below a defined second setpoint representative of the SO ₂ concentration in the combustion exhaust gas after the NO _x storage catalyst during desulfation. Advantageous for determining the degree of desulfurization of the NO _x storage catalytic converter is the performance of a NO _x measurement instead of a complex SO ₂ concentration measurement in the combustion exhaust gas. The method set forth herein allows for demand desulfation regardless of the sulfur content of the fuel derived directly from the NO _x concentration in the combustion exhaust gas after the NO _x storage catalyst and the NO _x concentration in the combustion exhaust gas upstream of the NO _x storage catalyst. This leads, in addition to the engine storage operation, to further fuel savings, since the desulfation is only as long as it is necessary to restore the necessary NO _x catalyst efficiency, is performed. This in turn is advantageously derived from a longer catalyst life, since both excessive sulfurization and too long operation with temperature increase in the NO _x storage catalyst is avoided. The increase in temperature is due inter alia by an exothermic reaction due to the rich engine operating phases in the NO _x storage catalytic converter. It additionally supports the sulfur degradation. In addition, a consistently high NO _x conversion rate of the storage catalytic converter is realized over its entire service life.

Advantageous according to claim 7, the constant availability of the current NO _x concentration in the combustion exhaust gas before the NO _x storage by a simple NO _x measurement with a second NO _x sensor, as by quotient NO _x concentration in the combustion exhaust gas before the NO _x- storage catalyst for NO _x emission after the NO _x storage catalyst, the catalyst efficiency is determined.

The advantage of claim 8 is the elimination of a component when using an existing controller instead of a second NO _x sensor to provide the NO _x concentration in the combustion exhaust gas before the NO _x storage catalytic converter. In a memory, a map with the NO _x concentration in the combustion exhaust gas is stored in front of the NO _x storage catalytic converter for each operating point of the internal combustion engine. The map values are determined by measurements on an internal combustion engine or calculated by means of combustion models in a computing unit in the control unit.

Advantageous according to claim 9 is the adaptation of changes in Internal combustion engine combustion process by simple software changes in the Control unit.

Positive according to claim 10 is the use of a combination sensor, which also measures O _{2 in} addition to NO _x . This means that for lean-burn operation, the currently customary O ₂ sensors (lambda probe for λ measurement) for three-way catalysts can be replaced by a combination sensor.

Fig. 1: Prinzipaufbau zum Betrieb einer Brennkraftmaschine zur Desulfatisierung eines Stickoxidspeicherkatalysators

Fig. 2: weitere Ausbildungsvariante von Fig. 1

Fig. 3: weitere Ausbildungsvariante von Fig. 1

Fig. 4: Zeitlicher Verlauf von Schwefeldioxid- und Schwefelwasserstoffemission während einer kontinuierlichen Desulfatisierung

Fig. 5: Zeitlicher Verlauf von Schwefeldioxid- und Stickoxid-Emission während einer Wechseldesulfatisierung

Fig. 6: Flußdiagramm zur Erkennung einer Schwefelvergiftung

Fig. 7: Flußdiagramm zur bedarfsgerechten Beendung einer Wechseldesulfatisierung

Further details of a preferred embodiment can be taken from the seven accompanying drawings. They show in detail:

Fig. 1: Principle structure for operating an internal combustion engine for desulfating a nitrogen oxide storage catalyst

2: further training variant of Fig. 1st

3: further training variant of FIG. 1

Fig. 4: Time course of sulfur dioxide and hydrogen sulfide emission during a continuous desulfation

Fig. 5: Time course of sulfur dioxide and nitrogen oxide emission during a Wechseldesulfatierung

Fig. 6: Flow chart for the detection of sulfur poisoning

Fig. 7: Flow chart for demand-ended termination of a Wechseleldesulfatisierung

Fig. 1 shows a controlled by an electronic control unit 1 multi-cylinder internal combustion engine 2, which is designed for lean engine operation. The combustion exhaust gases are discharged through an exhaust system 3. An arranged in the exhaust system 3 NO _x storage catalyst 4 cleans the combustion exhaust gases of nitrogen oxides. A first NO _x sensor 5, arranged downstream of the NO _x storage catalytic converter 4, measures the NO _x emission downstream of the NO _x storage catalytic converter 4. In front of the NO _x storage catalytic converter 4, a second NO _x sensor 6 is mounted in the Exhaust system 3 is arranged, which measures the NO _x concentration in the combustion exhaust gas in front of the NO _x storage catalytic converter 4. Both NO _x sensors 5 and 6 are connected to the control unit 1. By quotient of the NO _x concentration in the combustion exhaust gas before the NO _x storage 4 for NO _x emission after the NO _x storage 4, the catalyst efficiency, or its loading with sulfur compounds are determined by the control unit 1. The control unit 1 has a memory with setpoint values 27. By comparing the quotient with a first setpoint value, the internal combustion engine 2 is controlled such that the NO _x storage catalytic converter 4 is protected from an inadmissibly high degree of sulfurization and desulfated as required. The first setpoint value is dependent on the load currently requested by the internal combustion engine 2, corresponding to the current operating state.

Fig. 2 shows an embodiment variant of the invention. The representation essentially corresponds to the arrangement shown in FIG. The only difference is that the second NO _x sensor 6 is replaced by a memory with map 28. In this map, the NO _x concentrations in the combustion exhaust gas of the internal combustion engine 2 are stored in front of the NO _x storage catalytic converter 4. The NO _x concentrations stored in the map before the NO _x storage catalytic converter 4 are obtained either by measurements or by numerical simulations with combustion models. Instead of the measurement with the second NO _x sensor 6, the NO _x concentrations stored in the map, corresponding to the current operating state, are polled continuously before the NO _x storage catalytic converter 4 of the internal combustion engine 2. The quotient formation and the reference value comparison with the first reference value are carried out as in the first-mentioned basic structure.

Fig. 3 shows a further embodiment variant of the invention. The representation essentially corresponds to the arrangement shown in FIG. The difference is that the second NO _x sensor 6 is replaced by the arithmetic unit with a combustion model in the control unit 1. This calculates the current NO _x concentration in front of the NO _x storage catalytic converter 4 in accordance with the operating state of the internal combustion engine 2. The quotient formation and the setpoint comparison with the first desired value are carried out as in the first-mentioned basic structure.

4 shows a time profile of sulfur dioxide emissions 7, hydrogen sulfide emissions 8, air ratio λ 9, nitrogen oxide emissions 10 measured according to the NO _x storage catalytic converter 4, temperatures of the combustion exhaust gas 11 measured in the NO _x storage catalytic converter 4, the nitrogen oxide Concentrations in the combustion exhaust gas 12 and temperatures of the combustion exhaust gas 13, measured in front of the NO _x storage catalyst 4, during a continuous desulfation. The values measured on a suction tube-injecting 6-cylinder internal combustion engine at 3250 rpm. The desulfation is carried out exclusively by rich engine operation - no alternating desulfation. The X-axis shows the time course in seconds, the Y-axis shows the relative amplitude of the measured values. The desulphation begins at about 55 seconds, the air ratio λ 9 decreases. The course of the temperature of the combustion exhaust gas 13 upstream of the NO _x storage catalyst 4 increases faster than the course of the temperature of the combustion exhaust gas 11 in the NO _x storage catalyst 4. The sulfur dioxide emission 7 reaches a maximum after about 15 seconds, and falls then back to a low level. A few seconds later, the hydrogen sulfide emission 8 increases sharply. As shown in the problem, this is the time when desulfation must be stopped if no hydrogen sulphide is allowed to be emitted.

Fig. 5 shows a simultaneous measurement during a change desulfation. about the time axis in seconds are shown in FIG. 2, all previously measured values determined at the same measuring points. The Emissions are offset by measurement system-related signal propagation times the air ratio λ 9 delayed by about 10 seconds.

Alternating desulfation is realized by alternately rich and lean engine operation, clearly recognizable by the nitrogen oxide emissions 10 and the air ratio λ 9. The beginning of the alternating desulfation is about 50 seconds. It can be clearly seen in each rich phase, the increase in the sulfur dioxide emission 7, which indicates a sulfur discharge due to desulfation. In each lean phase an increased nitric oxide emission 10 can be seen. Part of the nitrogen oxide concentration 12 present in front of the NO _x storage catalyst 4 passes through it. The temperature profile of the combustion exhaust gas 11 in the NO _x storage catalyst 4 shows higher values than the temperature curve 13 before about 120 seconds after the beginning of the measurement. When looking at the envelopes 14 and 14 'of the courses of sulfur dioxide 7 and nitrogen oxide emission 10, the common drop in sulfur dioxide 7 and nitrogen oxide emission 10 is noticeable from about 100 seconds. It can be seen that the measurement of the nitrogen oxide emission 10 is representative of the sulfur dioxide emissions 7. When the nitrogen oxide emission 10 becomes small, it can be concluded that the sulfur is largely decomposed from the NO _x storage catalyst 4. If the nitrogen oxide emission 10 falls below a second desired value during desulfation, desulfation is stopped.

Fig. 6 shows a flow diagram for detection of a sulfur poisoning of the NO _x storage catalytic converter 4. The start 15 of the internal combustion engine 2 starts the measurement of the NO _x emission 16 after the NO _x storage catalytic converter 4 by the first NO _x sensor 5 and simultaneously the determination of the NO _x concentration 17 in the combustion exhaust gas before the NO _x storage catalyst 4. The control unit 1 performs a quotient 18 of the NO _x concentration in the combustion exhaust gas before the NO _x storage catalyst 4 to the NO _x emission. Subsequently, a setpoint comparison 19 between the quotient and a first setpoint value, which is read from the memory with setpoint values 19, is carried out. The first setpoint value is dependent on the instantaneous operating point of the internal combustion engine 2. If the first setpoint value is not undershot, steps 16, 17 and 18 continue unchanged, if the first setpoint value is undershot the control unit 1 checks the possibility of desulfation 20. If this is not feasible, the controller 1 continues to perform the setpoint comparison 19 with current quotients until the possibility of desulfation 20 is given. If the operating state of the internal combustion engine 2 permits the desulfation 21 to be carried out, it is started.

The determination of the NO _x concentration 17 in the combustion exhaust gas upstream of the NO _x storage catalytic converter 4 takes place according to the embodiment in FIG. 1 either from the sensor output signal of the second NO _x sensor 6, or according to FIG. 2 from the characteristic map of the memory with characteristic map 28 , or corresponding to FIG. 3 from a NO _x calculation of the arithmetic unit 29 with a combustion model in the control unit 1.

FIG. 7 shows a flowchart for the demand-ended termination of a change desulfation. The internal combustion engine 2 is in the operating state change dysulfatation 21. The first NO _x sensor 5 measures the NO _x emission 22 after the NO _x storage catalytic converter 4 during a lean engine operation. The lean engine operation lasts between one and thirty seconds, depending on the catalytic coating; The rich engine operation also lasts between one and thirty seconds, depending on the catalytic coating, and is terminated before the formation of hydrogen sulfide. At the same time, the actual NO _x concentration 23 in the combustion exhaust gas before the NO _x storage catalyst 4 is determined.

The determination of the NO _x concentration 23 is carried out according to the embodiment in FIG. 1 either from the sensor output signal of the second NO _x sensor 6, or according to FIG. 2 from the characteristic map of the memory with map 28, or according to FIG. 3 from a NO _x calculation of the arithmetic unit 29 with a combustion model in the control unit 1.

Both NO _x values (before and after the NO _x storage catalytic converter 4) are available to the control unit 1. The control unit 1 carries out a quotient formation 24 of the NO _x concentration in the combustion exhaust gas upstream of the NO _x storage catalytic converter 4 for the NO _x emission and subsequently carries out a further setpoint comparison 25 with a second setpoint value from the memory with setpoint values 27. The second set value likewise depends on the instantaneous operating point of the internal combustion engine 2. If the quotient does not exceed the second set value, the alternating desulfation 21 is continued. If the second setpoint value is exceeded, the control unit 1 ends the alternating desulfation 26. The internal combustion engine can again be operated in lean engine operation.

LIST OF REFERENCE NUMBERS

1.

control unit

Second

Internal combustion engine

Third

exhaust system

4th

NO _x storage catalyst

5th

first NO _x sensor

6th

second NO _x sensor

7th

Sulfur dioxide emissions

8th.

Hydrogen sulfide emissions

9th

Air ratio λ

10th

Nitrogen oxide emissions

11th

Temperature of the combustion exhaust gas in the NO _x storage catalytic converter

12th

Nitrogen oxide concentration in the combustion exhaust gas before NO _x storage catalyst

13th

Temperature of the combustion exhaust gas before NO _x storage catalyst

14, 14 '

Envelopes of emissions of sulfur dioxide and nitrogen oxides

15th

Starting the internal combustion engine

16th

Measurement of NO _x emissions

17th

Determination of the NO _x concentration in the combustion exhaust gas upstream of the NO storage catalytic converter

18

quotient

19th

Setpoint comparison

20th

Possibility of desulfation

21st

Execution of desulfation

22nd

Measurement of NO _x emissions

23rd

Determination of the NO _x concentration in the combustion exhaust gas upstream of the NO _x storage catalytic converter

24th

quotient

25th

Setpoint comparison

26th

Terminating desulfation

27th

Memory with setpoints

28th

Memory with map

29th

computer unit

Claims (10)

1. A method of operating an internal combustion engine for desulphation, by cyclic change of the air ratio, of an NO_x storage catalytic converter disposed in an exhaust gas purification system, characterised in that

   the NO_x emission in the exhaust gas after leaving an NO_x storage catalytic converter is continuously or intermittently measured (16) and an additional value is simultaneously determined for an NO_x concentration in front of the NO_x storage catalytic converter (17),

   the quotient of the NO_x concentration in front of the NO_x storage catalytic converter and the NO_x measured after the NO_x storage catalytic converter is obtained (18) and is continuously compared (19) with a first set value dependent on the load on the engine, and

   if the quotient is below the first set value (20), desulphation is initiated (21),

   during the desulphation process (21) the NO_x concentration in the exhaust gas after the NO_x storage catalytic converter is measured (22) in each lean phase and simultaneously an additional value is determined for the NO_x concentration in the exhaust gas in front of the NO_x storage catalytic converter (23),

   the quotient of the NO_x concentration in front of the NO_x storage catalytic converter and the NO_x measured after the NO_x storage catalytic converter is obtained (24) and is continuously compared with a second set value (25), and

   if the second set value is exceeded, the desulphation process is stopped (26).
2. A method according to claim 1, characterised in that the set values are read out of a set-value memory (27).
3. A method according to claim 1, characterised in that the NO_x concentration in the exhaust gas is measured in front of the NO_x storage catalytic converter (4).
4. A method according to claim 1, characterised in that the NO_x concentration in the exhaust gas in front of the NO_x storage catalytic converter (4) is read out of a performance-graph memory (28).
5. A method according to claim 1, characterised in that the NO_x concentration in the exhaust gas in front of the NO_x storage catalytic converter (4) is calculated by an arithmetic unit (29), using a combustion model.
6. A device for working the method according to any of the preceding claims, characterised in that an NO_x sensor (5) is disposed behind an NO_x storage catalytic converter (4) in the direction of flow and is connected to a control unit (1) for desulphating the NO_x storage catalytic converter (4) in dependence on the output signal of the NO_x sensor and an additional value for the NO_x concentration in the exhaust gas in front of the NO_x storage catalytic converter (4).
7. A device according to claim 6, characterised in that an additional NO_x sensor (6) is disposed in front of the NO_x storage catalytic converter (4) and connected to the control unit (1).
8. A device according to claim 6, characterised in that the NO_x concentration in the exhaust gas in front of the NO_x storage catalytic converter (4) is filed in a performance-graph memory (28) and can be read out by the control unit (1).
9. A device according to claim 6, characterised in that the NO_x concentration in the exhaust gas in front of the NO_x storage catalytic converter (4) can be calculated by a computer unit (29) in the control unit (1), using a combustion model.
10. A device according to any of the previous claims, characterised in that the NO_x sensor (5) can be combined with an oxygen sensor in a common casing.

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