DE19801815A1 - Lean-burn i.c. engine exhaust gas cleaning process - Google Patents

Abstract

A lean-burn internal combustion engine exhaust gas cleaning process comprises alternate exposure of catalyst portions to rich and lean exhaust gas in a relatively oxygen-rich exhaust gas. An exhaust gas cleaning process, for a lean-burn, lambda regulated internal combustion (i.c) engine with a NOx storage catalyst and a lambda probe, comprises exposing each volume element of the catalyst at alternating times and locations to rich and lean exhaust gas when the exhaust gas is a stoichiometric or lean exhaust gas with relatively high oxygen concentration. The lambda value preferably oscillates about a mean value lambda m of \-1, the oscillation being a sinusoidal or triangular oscillation of variable amplitude and/or frequency and the frequency being \-0.1 Hz. The mean value lambda m is generated by cylinder-selective regulation of the engine, in which some of the cylinders are operated with different rich lambda values and the others are operated with different lean lambda values. Regulation of the lean exhaust gas is achieved by changing the fuel reduction rate or by varying the dead times of the injected quantity changes.

Description

The invention relates to a method for the regeneration of a NO _x 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 × idle speed up to 0.25 × nominal speed, 0.05 to 0.15 × P _{me, max} ) with lambda <1.1 and particularly advantageously lambda <1.3 over 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, which in the engine control is used to the injection quantity at high voltage in the direction of "lean" and at low voltage to shift towards "fat". With the control frequency of the probe Thus, slightly rich and slightly lean exhaust gas is generated in the engine and into the exhaust system pushed out. Moreover, all cylinders do not run without real engine operation Deviation from each other without any deviation from the lambda target signal. Since the Gas columns are mixed on the way through the exhaust gas aftertreatment stoichiometric control on the catalytic converter no sharply separated exhaust gas qualities, but clouds with rich and lean exhaust gas.

All of the current technical and patent literature, in particular that of Toyota, specifies a rich or stoichiometric air-fuel ratio as a prerequisite for the regeneration of NO _x storage catalytic converters. For the regeneration, an oxygen-free environment and the presence of a reducing agent (HC or CO) are required. With a homogeneous flow, such as. B. on synthesis gas test benches, this boundary condition can be represented over the entire catalyst cross-section for any length of time; this is not possible on the motor.

The invention is therefore based on the object of providing a regeneration method for the NO _x store of an internal combustion engine which also works in an oxygen-containing exhaust gas atmosphere.

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

In the method according to the invention for cleaning the exhaust gas of a lean-operated, λ-controlled internal combustion engine with a NO _x storage catalytic converter and a λ probe, each volume element of the NO _x storage catalytic converter becomes time-dependent and location-dependent in the case of stoichiometric or lean exhaust gas with a relatively high oxygen concentration in the exhaust gas alternately with rich and lean exhaust gas.

Experiments with NO _x storage catalytic converters on a lean-out engine with operating state-dependent λ control have shown that safe NO _x regeneration is indeed possible with stoichiometric exhaust gas and relatively high residual oxygen concentrations. Each element of the NO _x storage catalytic converter is alternately charged with rich and lean exhaust gas depending on the time and location. The NO _x regeneration takes place in the time portions with a rich load. This effect is used in accordance with the method according to the invention in order to be able to clean NO _x storage catalytic converters even in the case of lean exhaust gas. The duration of regeneration depends on the proportion of time the fat is applied; compared to globally rich exhaust gas, an increase in the regeneration duration can be expected in the method according to the invention.

The λ value advantageously oscillates in the direction of the time axis around an average value λ _m , the average value λ _{m being} greater than or equal to one, in particular ≧ 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 NO _x store, the amplitude of the oscillation is advantageously changed. Furthermore, the frequency of the vibration can also be made variable. In other words, the λ function can be amplitude or frequency modulated. In the corresponding application, 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 cylinder is operated with a lean λ value. The individual λ values of both the rich cylinder and those of the lean cylinders are and will be different from each other and from each other adapted to the respective requirement. Furthermore, the λ value of the individual cylinder can be from Cycle to cycle can be changed.

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

The control frequencies of the λ oscillation are currently in the order of 0.1 to 20 Hz and are ultimately a function of the response times of the λ probes used. With the Development of probes with faster response times will allow that Control frequency increase, with very high control frequencies because of decreasing "cloud formation" negatively on the regeneration times with on average lean Exhaust gas can impact.

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

Figs. 1-5 show, respectively, in the upper part schematically illustrates a storage catalyst and at the bottom of the corresponding function λ

Fig. 6 shows an amplitude modulation of the λ signal,

Fig. 7 shows a frequency modulation of the λ signal,

Fig. 8 shows the course of the formations λ signal in the form of a left-side and right-side sawtooth,

Fig. 9 shows the course of the λ signal at a control of the lean exhaust gas by changing the dead times of the injection quantity change,

Fig. 10 shows the course of the λ signal at a control of the lean exhaust gas by changing the Ausmagerungsgeschwindigkeit,

Fig. 11 shows When adjusting of λ <1 by cylinder-specific injection quantity interference in broadband lambda probe, and

Fig. 12. When adjusting of λ <1 indicates by cylinder-specific injection quantity influencing at step response lambda probe.

FIGS. 1-5 graphically show the underlying mechanisms of the lean NO _x regeneration of -Save. An exhaust system 1 , which has a NO _x 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.

Fig. 1 shows an exclusively rich exhaust gas 4, represented by the hatching filling 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 NO _x storage catalyst, which is traversed by rich exhaust gas 4 and lean exhaust gas 5 . This is shown schematically by rich exhaust gas clouds 4 (hatched areas) which are surrounded by lean exhaust gas clouds 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 NO _x store 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. A regeneration of the NO _x storage also takes place here, however the regeneration period continues to increase.

FIG. 4 shows the situation with globally leaner exhaust gas, represented schematically in that the number of clouds of rich exhaust gas 4 is less than the lean exhaust gas 5 . The time and place components of the rich exhaust gas 4 continue to decrease and the regeneration time increases. 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 1 are smaller than the areas II in which no regeneration takes place. However, regeneration of the NO _x storage takes place here as well.

Fig. 5 shows the situation at very lean exhaust 5. In the illustration, there are no longer any 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 NO _x mass flow converted by the time and location-resolved regeneration is greater than the lean NO _x 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.

Fig. 6 shows the increase in the amount of reducing agent is for a given mean value λ _m by increasing the λ amplitude. As the proportion of pollutants rises, the regeneration duration tends to decrease. It can be clearly seen that the amplitude of λ increases in the direction of the arrow and therefore the regeneration areas also increase in area, ie the proportion of pollutants required for regeneration increases in the direction of the arrow. The regeneration speed also increases in the direction of the arrow. The increase in amplitude of λ can also be referred to as amplitude modulation.

FIG. 7 shows an optimization of the regeneration speed in NO _x stores with the ability to store oxygen 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 NO _x 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, no regeneration taking place in areas III below the stoichiometric λ value, since stored O ₂ in the case of an oxygen-storing NO _x store is consumed. 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.

Fig. 8 shows the variation of the pollutant reduction properties by a shape of λ. It has been shown that shaping the fat-lean jumps influences the regeneration behavior. The left graph in FIG. 8 shows a rapid enrichment and subsequent slow leaning. This results in the display shown on the left to quickly fat falling sawtooth, also referred to as a right-side sawtooth, which shortens the NO _x regeneration by the _NOx conversion starts quickly, but there is a risk of HC and CO carbon copies. The sawtooth shown on the right represents a rapid leaning and then slow enrichment (left-sided sawtooth). Such a course falling rapidly after fat causes a slower start of the NO _x conversion with a better control of the HC and CO breakthroughs and causes a longer regeneration time in comparison to the left saw tooth course of the rapid enrichment. In contrast to the idealized image representation, the emaciation and enrichment need not necessarily be linear. Furthermore, areas I of regeneration and those without regeneration II are shown again.

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

Fig. 9 shows the scheme of a lean exhaust gas by changing the dead times of the injection rate change in a step response probe. With such a step response sensor, the global lean exhaust gas is set by 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 in relation to the time t in the upper graph and the corresponding injection quantity signal E in relation to the time in the lower part of FIG. 9, specifically 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 after the dead time until the readjustment of the injection skinny. In this way, the proposals of FIGS. 4 and 6 can be implemented. The arrow shown in the right part of FIG. 9 for λ _m <1 points to 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 mean 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.

Fig. 11 shows When adjusting by λ _m <1 by cylinder-selective Einspritzmengen- interference in broadband Lambsdasonden, wherein the mean value λ _m is 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. 1 × very rich, 1 × slightly rich, 2 × lean). In the case of broadband lambda probes, the lambda signal is still 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. Figs. 11 is divided into the regions 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 for the generation of a lean exhaust gas with a step response probe. The thick line means the λ of the individual cylinders, the dashed line means an averaged λ over one cycle and the double solid line means the average λ _m over several cycles. Here, too, the origin of the time axis is λ = 1. In the case of step response probes, the bold and / or stoichiometric cylinders are also controlled and the lean cylinders are regulated. Since no information about the degree of leanness can be obtained with the step response probe, a leaner than the average desired lambda value λ _m (1 + 2 * Δλ, ie lambda 1.06 with desired lambda 1.03) is first approached by the Cylinders 2 and 3 are operated very lean. S denotes the step response of the probe.

The leaning of cylinders 2 and 3 is gradually reduced over the following work cycles until the step response probe detects globally rich exhaust gas. Then the highest lean value of the lean-running cylinders is set again. Fig. 12 illustrates this approach, in principle, different also for individual cylinders Ausmagerungen / enrichments are conceivable.

For lean gasoline and diesel engines, this procedure reduces in addition to that Consumption also HC and CO breakthroughs during regeneration; in diesel engines moreover, the 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 part of the attached sulfates is also removed again with each NO _x 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.

Reference list

1

Exhaust system

2nd

NO _x

Storage catalytic converter

3rd

Catalyst element

4th

Fat exhaust gas

5

Lean exhaust
I regeneration area
II Non-regeneration area III Non-regeneration area
T1 dead time
T2 dead time
T3 dead time
T4 dead time
λ _m

mean λ value
V _p

Probe voltage
E injection quantity signal
S step response signal

Claims (12)

1. A method for cleaning the exhaust gas of a lean, λ-controlled internal combustion engine with a NO _x storage catalyst ( 2 ) and a λ probe, characterized in that each volume element of NO _x with stoichiometric or lean exhaust gas with a relatively high oxygen concentration in the exhaust gas - Storage catalytic converter is alternately charged with rich and lean exhaust gas depending on the time and location. 2. The method according to claim 1, characterized in that the λ value oscillates in the direction of the time axis around an average value λ _m , the average value λ _{m being} greater than or equal to one. 3. The method according to claim 1 or 2, characterized in that the vibration of λ is a sinusoidal oscillation. 4. The method according to claim 1 or 2, characterized in that the vibration of λ is a triangular wave. 5. The method according to any one of claims 3 or 4, characterized in that the Amplitude of the λ vibration is changed. 6. The method according to any one of claims 2-5, characterized in that the frequency the λ vibration is variable.   7. The method according to any one of the preceding claims, characterized in that the average value λ _{m is} generated by a cylinder-selective control of the internal combustion engine. 8. The method according to claim 7, characterized in that a part of the cylinder with be operated with a rich λ value, while the other part of the cylinder with a lean λ value are operated. 9. The method according to claim 8, characterized in that both the λ values of the rich cylinders as well as those of the lean cylinders differ from each other are. 10. The method according to any one of claims 1-6, characterized in that the Regulation of the lean exhaust gas by changing the Lean rate is generated. 11. The method according to any one of claims 1-6, characterized in that the Regulation of the lean exhaust gas by changing the dead times of the Injection quantity change is generated. 12. The method according to any one of the preceding claims, characterized in that that the frequency of the vibration is greater than or equal to 0.1 Hz.