EP1163431B1 - METHOD FOR OPERATION OF A NOx STORAGE CATALYST IN INTERNAL COMBUSTION ENGINES - Google Patents

Description

State of the art

The invention relates to the operation of a NOx storage catalytic converter in internal combustion engines with a lean fuel / air mixture are operated. Examples such internal combustion engines are operated lean Gasoline engines or diesel engines.

When burning lean fuel / air mixtures come for exhaust gas aftertreatment NOx storage catalytic converters for Use that the emitted in lean engine operation Store nitrogen oxides in a first operating phase Length is on the order of minutes. In a second shorter operating phase, the length of which in Seconds, the memory is emptied by supplying exhaust gas with reducing agent to the Storage catalyst.

The storage capacity of the NOx catalyst is Depends on the load and decreases continuously. Takes that first phase too long, undesirable results Nitrogen oxide emissions. Too long a second phase results in increased HC and CO emissions.

So there is the problem of switching between the two Make phases so that neither increased NOx nor HC and CO emissions occur.

In this context, it is known from DE 197 398 48 the respective degree of filling of the NOx storage with NOx model. In doing so, the NOx entry is made in the memory Operating modes of the engine such as intake air mass flow and Mixture composition determined. Is a certain one Degree of filling is reached from the first phase to the second phase changed. The declining in the second phase The degree of filling is also modeled or a Completion of the second phase when an exhaust gas probe a complete behind the storage catalytic converter Regeneration signals.

Modeling in one or both phases requires one very high computational effort and thus represents high Motor control requirements. In addition, the Catalyst due to aging processes in its storage and Conversion behavior changed.

In some vehicle applications, especially in Diesel vehicles with NOx catalytic converters will regenerate preferably by injecting fuel into the Exhaust system in front of the catalytic converter reached. In this variant is the transition from lean to rich and the whole Regeneration phase is particularly critical since the varying Flow conditions the right dosage for generation a homogeneous distribution of the reducing agent in the exhaust gas before Complicate storage catalytic converter. Among the changing This appears in the flow conditions in the exhaust gas Transition phase cannot be modeled with reasonable effort his.

Against this background, the invention addresses the problem to control the change between the two phases.

This problem is caused by the combination of features of the Claim solved.

benefits

An advantage of the invention lies in the essential more uniform conditions of the regeneration phase.

Another advantage is a significantly reduced Computational effort in the control of loading and Regeneration of the catalyst ..

Another advantage is a simple possibility to check the exhaust gas treatment system and in the procedural adjustment of the tax strategy to a for example, catalyst behavior caused by aging.

The following is an embodiment of the invention explained in more detail.

Figure 1 shows the technical environment in which the invention their effect unfolds. 2 shows temporal courses different signals. Fig3 shows a modified Structure for realizing the invention and Fig. 4 shows a Embodiment in the form of a flow chart.

1 shows an internal combustion engine 1 a NOx storage catalytic converter 2, exhaust gas probes 3 and 4, one Control unit 5, a fuel metering means 6, and various sensors 7, 8, 9 for load L and speed n and possibly further operating parameters of the internal combustion engine such as temperatures, throttle valve position etc.

Forms from the mentioned and possibly other input signals the control unit etc. Fuel metering signals with which the Fuel metering means 6 is controlled. The Fuel metering means 6 can be used for a so-called Intake manifold injection as well Direct gasoline injection into the combustion chambers 1a of the individual Be designed cylinder. The variation of Mixture composition can change the Injection pulse widths take place with which the Fuel metering is controlled. The core of the In this environment, the method according to the invention relates to primarily the control unit 5 and the behind the Exhaust gas probe 4 arranged as a catalyst.

Fig. 2 represents the change in Fig. 2a Mixture composition lambda upstream of the catalytic converter (line 2a) in connection with the signal US of the rear exhaust gas probe 4 (Line 2b) and the NOx concentration (line 2c) behind that The rear exhaust probe can, for example as an oxygen sensor, as a hydrocarbon sensor (HC sensor, as a carbon dioxide sensor (CO sensor) or as Nitrogen oxide sensor can be realized. The signal is shown an oxygen sensor that detects a lack of oxygen high signal level and one with excess oxygen delivers low signal level.

In a first phase Phl from t = 0 to t = 60 the Engine with lambda greater than one, i.e. with excess air operated. The low signal level of the rear probe (Line 2b) indicates that air or there is an excess of oxygen. At time t = 60 the mixture composition of lambda is greater than one Lambda less than one, i.e. reversed lack of oxygen. Short afterwards, at about time t = 62, the rear one reacts Sensor 4 on the lack of oxygen with an increase in its Signals from low to high level.

As can be seen from FIG. 2, the time t = 62 For example, by exceeding the threshold of the signal of the rear probe can be determined.

The change in mixture composition shown leads to the fact that the internal combustion engine hydrocarbons and Carbon monoxide emitted as a reducing agent. As an alternative to Emission of reducing exhaust components can do that Reducing agent also from a storage tank 11 via a Control unit 5 controlled valve 12 the exhaust gas before Catalyst are supplied. The engine can then be operated continuously with a lean mixture.

A corresponding modification of the structure in FIG. 1 is shown in FIG Fig. 3 shown.

In the method according to the invention, the Regeneration phase not modeled mathematically and therefore kept variable. Instead, each becomes Regeneration a predetermined, constant mass Fuel in the exhaust system in front of the catalytic converter brought in. The storage phase is then in duration adapted to the regeneration phase. Will be mismatches by an exhaust gas probe arranged behind the catalytic converter determined and by influencing the length of the Injection phase reduced. This is the injection phase shortened if an exhaust gas probe towards the end of the Regeneration phase does not change sufficiently Concentration of an exhaust gas component signals. Kick one however, such a change too soon will Injection phase extended.

The advantage of the much more uniform conditions of Regeneration phase then results from the fact that only the Mass flow of the fuel to be injected to the Exhaust gas mass flow must be adjusted to a rich exhaust gas to generate with a certain desired lambda value.

The advantage of a significantly reduced computing effort results from the possible waiver of one Modeling one for the complete regeneration of the NOx storage necessary total fuel mass.

Another advantage of an easy way to Examination of the exhaust gas treatment system shows how follows: Give way to the injection times, which change with the Stop performing the method according to the invention very much from plausible given values, one lies Malfunction.

The further advantage of the procedural adjustment of the Control strategy to a caused, for example, by aging The catalyst behavior is as follows: exceeds the to be stored in the NOx storage in the storage phase Amount of aging capacity decreasing due to aging, this is caused by a reaction of the exhaust gas probe in the subsequent regeneration phase noticed and at Tax strategy considered.

An embodiment of a method according to the invention is shown in Fig. 4.

Step 1 represents lean engine operation Mixture. That emitted by the engine in this operating phase NOx is absorbed by the storage catalytic converter.

The degree of filling of the storage catalytic converter is in the Step 2 calculated from engine operating parameters as it is known for example from DE 1 97 398 48.

If the degree of filling reaches a threshold value SW-NOx, the control unit triggers a regeneration of the Storage catalytic converter. This is in steps 3 and 4 shown.

Is essential to the invention that this with a predetermined Reductant mass happens. The predetermined one Reducing agent mass can in the exemplary embodiment of FIG. 3 from the tank 11 via the controllable valve 12 into the exhaust gas be metered in front of the storage catalytic converter. in the Embodiment of FIG. 2 is the predetermined Reductant mass in the exhaust gas by a rich Engine operation generated. For example, all for the normal engine operation with a lean mixture Fuel metering signals in a predetermined manner as long be enlarged until the sum of the enlargements of the Fuel metering signals of the desired, for regeneration predetermined fuel mass corresponds.

If this metered fuel or reducing agent mass is lean again. Towards the end of the Regeneration phase is the reaction of the rear probe 4 evaluated for regeneration. If the rear probe is an oxygen sensor, your signal can be used with a Threshold are compared (step 5).

If the signal does not reach the threshold, it means at the end of the regeneration there is still no lack of oxygen behind the catalyst. The The amount of reducing agent is then not complete Regeneration of the NOx storage catalytic converter was sufficient. As In contrast to the state of the art, this does not result The amount of reducing agent increases, but the injection phase shortened. The example shown is shortened by reducing the threshold value SW-NOx in Step 6 strongly from what, for example, by exceeding the Threshold can be determined in step 5, step 7 extends the storage phase by increasing the threshold value SW-NOx.

Claims (1)

1. Method for the regeneration of an NOx storage catalytic converter (2) in the exhaust gas from internal combustion engines (1) which, when there is an excess of oxygen in the exhaust gas in a storage phase, takes up NOx from the exhaust gas and which, in a regeneration phase when there is a deficit of oxygen in the exhaust gas, is regenerated by releasing nitrogen, which method involves switching alternately between storage phases and regeneration phases and in which method an exhaust-gas sensor (4) arranged downstream of the NOx storage catalytic converter (2) signals complete regeneration of the NOx storage catalytic converter (2), characterized in that the deficit of oxygen in the exhaust gas for regeneration of the storage catalytic converter (2) is generated by a defined mass of fuel, which is to be kept constant, in the exhaust gas upstream of the NOx storage catalytic converter (2), and in that the exhaust-gas sensor (4) influences the length of the NOx storage phase in such a manner that the storage phase is shortened if the exhaust-gas sensor (4), towards the end of the regeneration phase, does not signal a sufficient change in the concentration of an exhaust-gas component or in such a manner that the storage phase is lengthened if the exhaust-gas sensor (4) signals the change in the concentration of the exhaust-gas component prematurely.

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