EP1049861B1 - LEAN REGENERATION OF NOx STORAGE UNITS - Google Patents

Description

The invention relates to a method for regeneration of a NOx storage catalytic converter in lean-burn internal combustion engines. An internal combustion engine can be operated lean if it is used for at least a subset of all conceivable speed-load combinations (in particular 1.5 x idle speed up to 0.25 x nominal speed, 0.05 to 0.15 x P _{me, max} ) with lambda> 1.1 and particularly advantageously lambda> 1.3 for periods of> 10 seconds and particularly advantageously> 30 seconds.

The air-fuel ratio in the exhaust gas of lambda-controlled gasoline engines is usually by one or more in the exhaust line before and / or after the Catalytic converter (s) arranged Lambda probe (s) monitors. Conventional jump probes show a pronounced voltage jump at lambda = 1 Engine control is used to direct the injection quantity at high voltage "Lean" and shift towards "fat" at low voltage. With the The control frequency of the probe thus becomes slightly rich and slightly lean exhaust gas in the engine generated and pushed into the exhaust system. In addition, all cylinders run at real Engine operation neither without deviation from one another nor without deviation from Lambda desired signal. Because the gas columns on the way through the exhaust gas aftertreatment are mixed, stoichiometric control on the catalyst does not come into focus separate exhaust gas qualities, but clouds with rich and lean exhaust gas.

All of the current technical and patent literature, particularly that of Toyota, gives a rich as a prerequisite for the regeneration of NOx storage catalysts or stoichiometric air-fuel ratio. For the regeneration one oxygen-free environment and the presence of a reducing agent (HC or CO) required. With a homogeneous flow, e.g. on synthesis gas test benches this boundary condition over the entire catalyst cross-section for any length of time Represent periods; this is not possible on the motor.

US-A-5,577,382 describes an exhaust gas purification system with a NOx storage device, in which the condition or the aging (degree of wear) of the NOx storage device is determined. An O ₂ sensor, which measures the air / fuel ratio, is arranged behind the NOx accumulator in the exhaust gas flow direction. In order to determine the aging of the NOx accumulator, the λ value of the fuel / air mixture is briefly changed from lean to rich or vice versa. The aging of the NOx accumulator can be determined from the peak value of the output signal of the O ₂ sensor during the change in the λ value. The regeneration of the NOx storage is described in column 7, lines 37-44, where, by selecting an appropriate correction factor, the exhaust gas is made rich (λ <1) for a predetermined time in order to regenerate the NOx storage.

US-A-5,622,047 describes a method for the detection of the saturation or Filling condition of an exhaust gas catalytic converter, i.e. a NOx storage catalytic converter, with an oxygen sensor arranged behind it. The publication also describes a Method for detecting the aging or the condition of such a catalyst. To determine the condition of the catalyst, the change in Oxygen signal as a function of the fuel / air ratio supplied to the NOx accumulator evaluated, this ratio being varied accordingly. On Such a measurement program for determining the state of the catalyst is carried out after a predetermined operating time measured in operating kilometers, so for example all 2,000 km, carried out automatically. The regeneration of a full NOx storage takes place in a known manner by loading the store with rich exhaust gas the memory is regenerated.

EP-A-719923 describes a device for controlling a lean operable Engine, due to the deviations in the air-fuel ratio of individual cylinders among themselves is avoided.

The invention is therefore based on the object of a method for the operation and regeneration of a To create NOx storage of an internal combustion engine, the regeneration also in one oxygen-laden exhaust gas atmosphere is performed.

The object is solved by the subject matter of claim 1. preferred Embodiments of the invention are the subject of the dependent claims.

Through tests with NOx storage catalytic converters on a lean-out engine with λ-control depending on the operating state it could be demonstrated that at stoichiometric exhaust gas and relatively high residual oxygen concentrations safe NOx regeneration is possible. Each element of the NOx storage catalytic converter alternating with fat and lean depending on time and location Exhaust gas applied. The NOx regeneration takes place in the time portions with richer Acted upon instead. This effect is in accordance with the invention Process used for NOx storage catalytic converters even with lean exhaust gas to be able to clean. The regeneration time depends on the time shares of the fat ones admission; compared to globally rich exhaust gas in the invention Procedure to expect an extension of the regeneration period.

Advantageously, the λ value oscillates in the direction of the time axis around a mean value λ _m , the mean value λ _{m being} greater than or equal to one, in particular 1,0 1.05. The oscillation of the λ value around the mean value λ _{m can} be a sinusoidal oscillation or, for example, a triangular oscillation, such as a sawtooth.

In order to obtain a specific regeneration behavior of the NOx storage, advantageously changes the amplitude of the vibration. Furthermore, the Frequency of the vibration can be made variable. In other words, it can Amplitude or frequency modulation of the λ function take place. In the corresponding If necessary, amplitude and frequency modulation can be combined.

The mean λ value can advantageously be controlled by cylinder-selective regulation of the Internal combustion engine are generated. That is, some of the cylinders have a rich λ value operated while the other part of the cylinders with a lean λ value is operated. The individual λ values can both the rich cylinder and those of the lean cylinders are different from each other and from each other and are adapted to the respective requirement. Furthermore, the λ value of the individual cylinders can be changed from cycle to cycle.

Furthermore, the regulation of the lean exhaust gas can advantageously by a Change in the rate of emaciation or by changing the dead times the injection quantity change are generated.

The control frequencies of the λ oscillation are currently in the order of 0.1 up to 20 Hz and are ultimately a function of the reaction times of the λ probes used. With the development of probes with faster response times, it will be possible to increase the control frequency, with very high control frequencies because of the decreasing "cloud formation" negatively on the regeneration times at im Can affect medium lean exhaust gas.

Die Figuren 1 - 5

zeigen jeweils im oberen Teil schematisch einen Speicherkatalysator und im unteren Teil die entsprechende λ-Funktion;

Fig. 6

zeigt eine Amplitudenmodulation des λ-Signals,

Fig. 7

zeigt eine Frequenzmodulation des λ-Signals,

Fig. 8

zeigt Verlaufsformungen des λ-Signals in der Form eines linksseitigen und rechtsseitigen Sägezahns,

Fig. 9

zeigt den Verlauf des λ-Signals bei einer Regelung des mageren Abgases durch Änderung der Totzeiten der Einspritzmengenänderung,

Fig. 10

zeigt den Verlauf des λ-Signals bei einer Regelung des mageren Abgases durch Änderung der Ausmagerungsgeschwindigkeit,

Fig. 11

zeigt die Einregelung von λ > 1 durch zylinderselektive Einspritzmengen-Beeinflussung bei Breitband-Lambdasonden, und

Fig. 12

zeigt die Einregelung von λ > 1 durch zylinderselektive Einspritzmengen-Beeinflussung bei Sprungantwort-Lambdasonden.

Preferred embodiments of the invention are explained below with reference to the drawings.

Figures 1-5

each schematically show a storage catalytic converter in the upper part and the corresponding λ function in the lower part;

Fig. 6

shows an amplitude modulation of the λ signal,

Fig. 7

shows a frequency modulation of the λ signal,

Fig. 8

shows waveforms of the λ signal in the form of a left-sided and right-sided sawtooth,

Fig. 9

shows the course of the λ signal when regulating the lean exhaust gas by changing the dead times of the injection quantity change,

Fig. 10

shows the course of the λ signal in a control of the lean exhaust gas by changing the leaning rate,

Fig. 11

shows the regulation of λ> 1 by cylinder-selective injection quantity influencing in broadband lambda sensors, and

Fig. 12

shows the regulation of λ> 1 by cylinder-selective injection quantity influencing in step response lambda sensors.

Figures 1-5 graphically show the underlying mechanisms of lean regeneration of NOx stores. An exhaust system 1, which has a NOx storage catalytic converter 2, is shown in the respective upper part of FIGS. 1-5. An idealized catalytic converter element 3 is considered in the storage catalytic converter, the flow through the catalytic converter element 3 being shown with the different exhaust gas qualities. The corresponding λ values are plotted against time t in the lower part of FIGS. 1-5. The origin of the time axis is located at λ value one. The λ values greater than one (lean exhaust gas) are shown above and the λ values less than one (rich exhaust gas) are shown below. Furthermore, the mean value λ _{m is shown} as a dashed line.

1 shows an exclusively rich exhaust gas 4, represented by the hatching of the entire exhaust system 1. With this rich regeneration, the shortest regeneration times are possible thanks to time and location-resolved almost 100% rich flow. Because of the complete flow through the storage catalytic converter 2 with rich exhaust gas 4, any catalytic converter element 3 is always flowed through with rich exhaust gas 4 in both temporal and spatial resolution. In the lower part of FIG. 1, the time course of λ on the catalyst element 4 is shown. Due to the control frequency of the λ probe, the λ value oscillates around an average value λ _m , the amplitude of the λ oscillation always being in the rich range, ie λ <1. In other words, as is shown schematically in the upper part of FIG. 1, the exhaust gas always flows through the catalytic element 3, the exhaust gas being periodically more or less rich.

FIG. 2 shows the situation in an exhaust system 1 with a NOx storage catalytic converter, through which rich exhaust gas 4 and lean exhaust gas 5 flow. This is represented schematically by rich exhaust gas clouds 4 (hatched areas) which are surrounded by lean exhaust gas woikens 5 (shown as white areas). It is therefore clear that any catalytic converter element 3 is flowed through in a temporally and spatially resolving manner, alternately with rich exhaust gas 4 and lean exhaust gas 5. That is, there are already small, lean portions in the exhaust gas, so that time and location-resolved flow through all catalyst zones is not always rich. Shown in the lower part of FIG. 2, this means that the mean value λ _{m is} closer to λ = 1 and the amplitudes of the λ value exceed the stoichiometric value of λ = 1. Regeneration therefore takes place in the hatched areas I in which the λ value falls below the value 1, while no regeneration takes place in the areas II in which the λ value is exceeded by 1. On average, the NOx storage is regenerated based on the average value λ _m close to 1.

Fig. 3 shows a similar situation as Fig. 2 with the difference that the mean value λ _m = 1. The time components of rich and lean exhaust gas 4, 5 are thus approximately the same, which leads to approximately the same areas I of regeneration and areas II of no regeneration in the lower part of FIG. 3. The NOx storage is also regenerated here, however, the regeneration duration continues to increase.

Fig. 4 shows the situation with globally leaner exhaust gas, represented schematically by the fact that the number of clouds of rich exhaust gas 4 is less than the lean exhaust gas 5. The time and location components of the rich exhaust gas 4 continue to decrease and the regeneration duration increases increasingly , In the lower representation of FIG. 4, this means that the amplitudes of λ are largely above the value 1 and only a small part of the values of λ are below the value 1. The mean value λ _m is above 1. The regeneration areas I are smaller than the areas II in which no regeneration takes place. However, the NOx storage is still regenerated here.

Fig. 5 shows the situation with very lean exhaust gas 5. In the illustration, there are no more rich exhaust gas clouds, or the time and place components of the rich exhaust gas are too small for a practical regeneration period. A net storage discharge is only possible if the NOx mass flow converted by the time and location-resolved regeneration is greater than the lean NOx storage. In the lower λ representation, the course of λ and the mean value λ _{m are} now completely above 1, ie there is only area II without regeneration.

6 shows the increase in the amount of reducing agent at a predetermined mean value λm by increasing the λ amplitude. With increasing pollutant content tend to expect a shorter regeneration period. It can be clearly seen that the amplitude of λ increases in the direction of the arrow and therefore also the Regeneration areas I are larger in area, i. H. the one for regeneration necessary pollutant content increases in the direction of the arrow. Likewise, the Regeneration speed in the direction of the arrow. The increase in amplitude of λ can also be called amplitude modulation.

FIG. 7 shows an optimization of the regeneration speed in NOx stores with oxygen storage capability by changing the control frequency, represented by a λ oscillation in which the control frequency is varied. In the present example, the control frequency is reduced, with λ fluctuating around an average value λ _m . The frequency of the change between rich and lean flow (wobble frequency) influences the regeneration time. As the oxygen storage capacity of the NOx storage increases, a decreasing wobble frequency also causes a decrease in the regeneration times. Regeneration areas I, non-regeneration areas II, and areas III can be seen in the areas defined by the oscillation of λ, regeneration also not taking place in areas III below the stoichiometric λ value, since stored O ₂ consumes in the case of an oxygen-storing NOx store becomes. In the direction of the arrow there is therefore a decrease in the control frequency, an increase in the usable amount of reducing agent and an increase in the regeneration rate. The control frequencies are, as already mentioned, in the order of 0.1 to 20 Hz.

8 shows the variation of the pollutant reduction properties by Shape shaping of λ. It has been shown that a formation of the fat-lean jumps affects the regeneration behavior. The left graph of FIG. 8 shows a rapid greasing and subsequent slow emaciation. This results in the Representation of the sawtooth curve falling rapidly after fat falling, also shown referred to as the right-hand sawtooth, which shortens the NOx regeneration by the NOx conversion starts quickly, but there is a risk of HC and CO breakthroughs. The sawtooth shown on the right represents a rapid thinning and subsequently slow greasing (left-hand saw tooth). Such a quick one after a fat-decreasing course, the NOx conversion starts more slowly with better control of HC and CO penetrations and causes a longer one Regeneration time compared to the left sawtooth course of the quick greasing. The thinning and greasing must deviate from the idealized image representation not necessarily linear. Furthermore, areas I are again the Regeneration and without regeneration II shown.

The corresponding exhaust gas quality is regulated by a Step response or broadband lambda probe.

9 shows the control of a lean exhaust gas by changing the dead times of the injection quantity change in the case of a step response probe. With such a step response probe, the setting of the globally lean exhaust gas takes place through different dead times between the detection of lean exhaust gas and the command to enrich the mixture and the detection of rich exhaust gas and the command to lean the mixture. Plotted in FIG. 9 is the probe voltage V _{S of} a step response probe versus time t in the upper graph and the corresponding injection quantity signal E versus time in the lower part of FIG. 9, once for an average value λ _m = 1 and in the right Part for λ _m > 1. T1 means the dead time until the detection of "lambda lean", T2 the dead time until the readjustment of the injection after rich, T3 the dead time until the detection of "lambda rich" and T4 the dead time until the readjustment of the injection after lean. In this way, the proposals of FIGS. 4 and 6 can be implemented. The arrow shown in the right part of FIG. 9 for λm> 1 shows a longer dead time T4 with lean exhaust gas in comparison to T4 of the left part of FIG. 9 for λ _m = 1.

FIG. 10 shows a control of a lean exhaust gas with a step response probe by changing the leaning rate, that is to say by means of different rates of change during enriching and leaning, whereby the ancestor of FIG. 8 is realized. The meaning of the reference symbols T1, T2, T3 and T4 corresponds to that of FIG. 9. The two arrows in the right part of FIG. 10 for λ _m > 1 indicate a faster and stronger emaciation with lean exhaust gas.

In the case of broadband probes, the target signal of the average lambda λ _{m is set} to the desired value (lambda> 1) and the time components of rich exhaust gas are monitored via frequency and amplitude.

A: Steuerung der fetten Zylinder 1 und 4, angepaßte Regelung der Zylinder 2 und 3 (Idealfall).

B: Nachregelung des Zylinders 2, falls Zylinder 4 vom vorgegebenen Sollwert abweicht.

C: Eine bewußt unterschiedliche Ansteuerung der fetten Zylinder 1 und 4 wird durch eine geänderte Regelung der Zylinder 2 und 3 ausgeglichen.

11 shows the regulation of λ _m > 1 by influencing the cylinder-selective injection quantity in broadband lambda sensors, the mean value λ _{m being represented} by the dashed line. In addition to the application of the same injection signal to all cylinders, it is also conceivable that with an n-cylinder engine 1 to (n-1) cylinders (here cylinders 1, 2, 3 and 4) receive an injection signal that differs from the other cylinders, whereby more than two different injection signals can also be specified (e.g. 1x very rich, 1x slightly rich, 2x lean). In the case of broadband lambda probes, the lambda signal continues to be used for global control to λ _m > 1. When controlling the rich and / or stoichiometrically running cylinders, lambda is regulated by regulating at least one lean-running cylinder. 11 is divided into areas A, B and C, which have the following meanings:

A: Control of rich cylinders 1 and 4, adjusted regulation of cylinders 2 and 3 (ideal case).

B: Readjustment of cylinder 2 if cylinder 4 deviates from the specified setpoint.

C: A deliberately different control of the rich cylinders 1 and 4 is compensated for by a modified regulation of the cylinders 2 and 3.

Finally, FIG. 12 shows a cylinder-selective injection quantity influencing Lean exhaust gas generation with a step response probe. The means thick line the λ of the individual cylinders, the dashed line an averaged λ over one cycle and the double solid line the mean λm over several Cycles. Here too, the origin of the time axis is λ = 1. For step response probes the bold and / or stoichiometric cylinders are also controlled and the lean-running cylinder regulated. Since with the step response probe no information to get the degree of emaciation is initially a leaner than that Average desired lambda value λm (1 + 2 * Δλ, i.e. lambda 1.06 if desired Lambda 1.03) started in a controlled manner by operating cylinders 2 and 3 very lean become. S denotes the step response of the probe.

The leaning out of cylinders 2 and 3 will be successive over the following work cycles withdrawn until the step response probe detects globally rich exhaust gas. Then it will be the highest lean value of the lean-running cylinders was set again. Fig. 12 principally illustrates this procedure, although here too different cylinder-specific leanings / enrichments are conceivable.

For lean gasoline and diesel engines, this procedure reduces in addition to that Consumption also HC and CO breakthroughs during regeneration; at Diesel engines also have particle emissions.

If all or part of the reduction in consumption is converted into a more frequent regeneration, additional advantages in sulfur poisoning are to be expected, since a part of the deposited sulfates is also removed again with each NOx regeneration. The released sulfur is increasingly emitted in the form of SO ₂ ; the odor-causing H ₂ S formation is largely suppressed. Likewise, only minimal liquid NH ₃ formation is to be expected.

LIST OF REFERENCE NUMBERS

1

exhaust system

2

NOx storage catalytic converter

3

catalyst element

4

Fat exhaust gas

5

Lean exhaust

I

regeneration area

II

Non regeneration area

III

Non regeneration area

T1

dead

T2

dead

T3

dead

T4

dead

.lambda..sub.m

mean λ value

V _p

probe voltage

e

Injection quantity signal

S

Step response signal

Claims (12)

1. Method for the operation and regeneration of an NOx-storage catalytic converter (2) of a lean-mix operable, λ-controlled internal combustion engine having at least one λ-probe, wherein a lean operation with λ > 1.1 is performed for more than 10 seconds, characterised in that during regeneration each volume element of the NOx-storage catalytic converter (2) is influenced alternately with rich and lean exhaust gas in a time and position-dependent manner, in that the λ-value performs an oscillation in the direction of the time axis about a mean value λ_m, wherein the minimum value of the oscillation is less than the value λ = 1 and the maximum value of the oscillation exceeds the value λ = 1.
2. Method as claimed in claim 1, characterised in that the mean value λ_m is greater than or equal to 1.
3. Method as claimed in any one of claims 1 or 2, characterised in that the frequency of the λ-oscillation is greater than or equal to 0.1 Hz.
4. Method as claimed in any one of the preceding claims, characterised in that the λ-oscillation is a sinusoidal oscillation.
5. Method as claimed in any one of claims 1 to 3, characterised in that the λ-oscillation is a triangular oscillation.
6. Method as claimed in any one of claims 1 to 3, characterised in that the amplitude of the λ-oscillation is changed.
7. Method as claimed in any one of the preceding claims, characterised in that the frequency of the λ-oscillation is variable.
8. Method as claimed in any one of the preceding claims, characterised in that the mean value λ_m is produced by a cylinder-selective control of the internal combustion engine.
9. Method as claimed in claim 8, characterised in that some of the cylinders are operated with a rich λ-value, whereas others are operated with a lean λ-value.
10. Method as claimed in claim 9, characterised in that both the λ-values of the rich-operated cylinders and also those of the lean-operated cylinders are different in each case.
11. Method as claimed in any one of claims 1 to 7, characterised in that the control of the lean exhaust gas is produced by changing the rate at which the operation becomes leaner.
12. Method as claimed in any one of claims 1 - 7, characterised in that the control of the lean exhaust gas is produced by a change in the dead times of the change in injection quantity.

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