DE10005954A1 - Desulfurization of a storage catalytic converter by heating - Google Patents

Abstract

A method for heating a main catalyst arranged behind at least one pre-catalyst is disclosed, whereby unburnt fuel generated by the engine is introduced into the exhaust gas, before the main catalyst with the simultaneous presence of an excess of air before the main catalyst. The generation of unburnt fuel by the engine is temporally and/or spatially coupled to the generation of an excess of air in the exhaust gas by the engine.

Description

State of the art

In internal combustion engines in motor vehicles with lean engine operation (λ <1), a so-called storage catalytic converter for the nitrogen oxides NO _x produced during combustion is required to meet legally prescribed exhaust gas limit values. Sulfur oxides continue to be generated during combustion. Due to the high affinity of the active centers, ie the storage locations of the NO _x storage catalytic converter with respect to the sulfur oxides (SO _x ) formed during the combustion of the fuel, the active centers are primarily occupied by the SO _x . The resulting sulfates are so thermally stable that they are not released during normal driving. As a result, the storage capacity of the catalyst for the nitrogen oxides decreases with increasing sulfur loading. At an elevated temperature in the catalyst (T <600 ° C), with simultaneously reducing conditions (λ <1), the sulfates are no longer thermodynamically stable and are released as hydrogen sulfide (H ₂ S) and sulfur dioxide (SO ₂ ). In order to maintain or restore the storage capacity, the storage catalytic converter must be operated briefly in bold at elevated temperatures at certain intervals. This is known for example from EP 580 389.

For heating to the required temperature for Desulphurization of the storage catalyst can be done later Ignition angle can be set, what about a Deterioration of the efficiency of the internal combustion engine leads to an increased exhaust gas temperature, which leads to heating of the storage catalytic converter. The heating effect can be caused by generating combustible mixture in front of the catalyst be reinforced. The generation of preferably takes place combustible mixture by increasing the unburned HC Raw engine emissions in front of the catalytic converters with an excess of oxygen in the exhaust gas. That before The resulting combustible mixture reacts in the Catalyst exothermic and heats it up.

Modern exhaust gas cleaning systems have in addition to that Storage catalyst other catalysts, in particular a pre-catalytic converter arranged close to the engine.

Due to the conventionally used heating of the Storage catalytic converter, the pre-catalyst can be thermally strong be charged. This can be premature Deactivate the catalyst.

The object of the invention is to heat the To enable storage catalytic converter, which is an impermissible Avoid heating the pre-catalyst.

This object is achieved with the features of claim 1.

The essence of the invention is unburned mixture through motor measures and at the same time to ensure that this is not or only to a small extent can react exothermically in the pre-catalyst.  

This is achieved through a temporal or spatial decoupling the emission of excess air in the exhaust gas and excess HC in Exhaust gas reached.

This can be relatively independent of the temperatures in the Pre-catalysts and the current load point of the engine, the required temperature can be set in the main catalyst. The one that occurs with the conventionally used methods high thermal load on the pre-catalyst, which leads to a Deactivation can be avoided.

Embodiments of the invention are in the figures are shown and are described below.

Fig. 1 represents the prior art.

Fig. 2 shows a first embodiment of the invention and Fig. 2 shows a second embodiment.

1 in FIG. 1 represents an internal combustion engine which is supplied with air from an intake pipe 2 and with fuel from an injection nozzle arrangement 3 . The injection nozzle arrangement is controlled by a control unit 4 with injection peak widths. The injection pulse widths are calculated on the basis of detected operating parameters of the internal combustion engine. Examples of such operating parameters are the amount of air drawn in which is detected by a sensor 5 , the speed of the engine which is detected by a sensor 6 and the composition of the exhaust gas, for example its oxygen content, which is detected by a sensor 7 . In addition to the injection valve arrangement, the control unit controls the ignition device 8 in order to ignite the fuel / air mixture in the individual engine cylinders 9-12 at the right time in each case.

The number 13 denotes a pre-catalytic converter and the number 14 a storage catalytic converter.

In the arrangement shown, the problem of heating the storage catalytic converter through engine measures becomes clear: the increase in the exhaust gas temperature through spark ignition and the generation of a combustible mixture with a mixture composition of lambda 1 a directly behind the internal combustion engine not only has an effect in the storage catalytic converter, but also undesirably in the pre-catalyst.

A remedy is shown in FIG. 2: As an essential difference from FIG. 1, FIG. 2 shows two separate precatalysts, one precatalyst being assigned to a specific cylinder group. The assignment is achieved by separating the exhaust pipes.

For reasons of clarity, the periphery of the sensors and the fuel and air supply from FIG. 1 are not shown here, as in FIG. 3 below. This periphery is present both in the subject of FIG. 2 and in the subject of FIG. 3, so these two figures are to be considered in connection with FIG. 1.

The separation of the exhaust gas routing of different cylinder groups enables a merger of a lean and a rich exhaust gas flow before Storage catalytic converter.

For this purpose, the cylinder group, for example, whose exhaust gas flows through the pre-catalytic converter 1 (V. cat. 1) is operated with a rich mixture without excess air (lambda 1 b). As a result, the exhaust gas from this group of cylinders contains unburned fuel with a lack of oxygen. Due to the lack of oxygen, the excess fuel in the pre-catalytic converter 1 cannot react exothermically. Pre-catalyst 1 is therefore not heated.

Furthermore, if exhaust gas with excess fuel flows through the pre-catalyst 1 , the other cylinder group, the exhaust gas of which flows through the pre-catalyst 2 (V.Kat. 2), is operated with a lack of fuel and thus with an excess of oxygen (Lambda 2 b). This results in an excess of oxygen in the pre-catalyst 2 , for which no fuel is available as a reactant in the pre-catalyst 2 . So that there is no exothermic reaction in the pre-catalyst 2 , so that the pre-catalyst 2 is not heated.

A merging of the excess air of the second cylinder group with the excess fuel of the first cylinder group to form an exhaust gas that corresponds to a lambda value Lambda 3 b takes place only behind the two pre-catalysts. The exhaust gas with lambda 3b thus contains both unburned fuel and the necessary reaction partner oxygen. Both components only react exothermically in the storage catalytic converter and thus heat it up as desired.

In other words: for those in this embodiment shown invention are at least two pre-catalysts necessary for realization. In the heating phase, the two pre-catalysts with different lambdas (λ1b and λ2b) acted upon. A realization is through the Bank way (bank 1 / bank 2) different Injection quantities of the fuel are possible without any problems. On Lambda value must be greater than 1 (lean), the other be less than 1 (bold). The mixture lambda (λ3b) should  resulting from the individual lambdas (λ1b and λ2b) and the Set exhaust gas mass flows to a value around λ3b = 1. in the Storage catalytic converter is then implemented in the exhaust gas present calorific value, mainly from the fat Exhaust gas according to V. Cat. comes with the available oxygen, which is mainly the lean exhaust gas according to V.Kat. originated. The temperature increase in the main catalyst results from the implementation of the not fully oxidized components with the oxygen.

Variants are also conceivable in which more than two Pre-catalysts are used. The collecting lambda before Sp. Kat. Is then replaced by the single lambdas of the V.Kat. again set to about 1. An application is also with other numbers of cylinders conceivable, but always according to the invention there must be at least one two-cylinder arrangement.

In the second exemplary embodiment, which is shown in FIG. 3, rich and lean exhaust gas packs likewise only mix after the pre-catalytic converter.

In this embodiment, the is to be heated Main catalyst a mixing element (static mixer) upstream. This consists, for example, of a Cavity that is arranged with angled to each other Flow baffles is provided. The flow baffles direct the individual flow volumes into each other and slow down the flow. This creates a thorough mixing individual portions of exhaust gas reached.

The core of the second embodiment lies in Mixing the lean and rich portions of exhaust gas before Storage catalytic converter, the lean and fat  Exhaust gas portions here separated from each other by motor be generated.

For this purpose, the engine always becomes alternately rich in the heating phase (λ <1) and lean (λ <1) operated. Alternatively, you can individual cylinders due to a different injection quantity be operated fat and lean. The resulting fat and lean exhaust gas packages are only partially in the Pre-catalyst backmixed so that even after Pre-catalytic converter lean and rich exhaust gas packs are present.

In the static downstream of the pre-catalyst Mixer then takes place through the one taking place there Backmixing a homogenization of the exhaust gas (the Behavior of the static mixer is similar due to the Smear the residence time with an ideal stirred tank small volume).

The conversion of the im takes place in the storage catalytic converter Exhaust gas calorific value, mainly from the fat exhaust packs comes with the available oxygen, which mainly comes from the lean exhaust gas packages. The Temperature increase in the main catalyst results from the Implementation of the not fully oxidized components the oxygen.

The period of the fat and lean cycles is determined according to the ratios of the expected exhaust gas volume flow and the volume of the static mixer, or the required quality of backmixing. The lowest possible Oxygen storage capacity of the pre-catalyst is to strive to avoid unnecessary dead time regarding the rich and lean exhaust gas packages introduced into the system becomes.  

As an alternative to heating a storage catalytic converter the invention also for heating a three-way catalyst be used behind at least one pre-catalyst is arranged. The term main catalyst in claim 1 should cover these two alternatives.

Claims (1)

Method for heating a main catalytic converter arranged behind at least one pre-catalytic converter by supplying motor-generated unburned fuel in the exhaust gas upstream of the main catalytic converter with the simultaneous presence of excess air in front of the main catalytic converter, characterized in that the motor-driven production of unburned fuel is temporally and / or spatially dependent on the motor production is decoupled from excess air in the exhaust gas.