EP1257736A1

EP1257736A1 - Desulphurisation of a storage catalyst by heating

Info

Publication number: EP1257736A1
Application number: EP01911381A
Authority: EP
Inventors: Eberhard Schnaibel; Oliver Wiemers; Matthias Dall; Andreas Kufferath
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2000-02-09
Filing date: 2001-01-24
Publication date: 2002-11-20
Also published as: JP2003522874A; US20030131588A1; KR20020076289A; DE10005954A1; WO2001059271A1; BR0108148A

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

Desulfurization of a storage catalytic converter by heating

State of the art

In the case of internal combustion engines in motor vehicles, when engine operation is lean (λ> l), legally prescribed requirements are met

Exhaust gas limit values require a so-called storage catalytic converter for the nitrogen oxides NO produced during combustion. Sulfur oxides continue to be generated during combustion. Due to the high affinity of the active centers, ie the storage locations of the NCv storage catalytic converter with respect to the sulfur oxides (SO) generated during the combustion of the fuel, the active centers are primarily occupied by the S0 _X. The resulting sulfates are so thermally stable that they are not released again 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 (S0 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. To heat up to the required temperature for desulfurizing the storage catalyst, a late ignition angle can be set, which leads to an increase in the efficiency of the internal combustion engine to an increased exhaust gas temperature, which leads to heating of the storage catalyst. The heating effect can be increased by generating a combustible mixture in front of the catalytic converter. The fuel mixture is preferably produced by increasing the unburned raw HC emissions of the engine upstream of the catalytic converters in conjunction with an excess of oxygen in the exhaust gas. The combustible mixture thus formed in front of the catalyst reacts exothermically in the catalyst and heats it up.

In addition to the storage catalytic converter, modern exhaust gas cleaning systems have further catalytic converters, 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 heavily loaded. This can lead to an early deactivation of the catalyst.

The object of the invention is to enable heating of the storage catalytic converter, which avoids inadmissible heating of the precatalyst.

This object is achieved with the features of claim 1.

The essence of the invention is to provide unburned mixture by means of engine measures and at the same time to ensure that this cannot react or can only exotherm to a small extent in the pre-catalyst. This is achieved by temporally or spatially decoupling the emission of excess air in the exhaust gas and excess HC in the exhaust gas.

This can be relatively independent of the temperatures in the

Pre-catalytic converters and the current load point of the engine, the required temperature can be set in the main catalytic converter. The high thermal load on the pre-catalytic converter which occurs in the conventionally used methods and which can lead to deactivation is avoided.

Exemplary embodiments of the invention are shown in the figures and are described below.

Fig. 1 represents the prior art.

FIG. 2 shows a first exemplary embodiment of the invention and FIG. 2 shows a second exemplary embodiment.

1 in FIG. 1 represents an internal combustion engine, which consists of an intake pipe 2 with air and one

Injector arrangement 3 is supplied with fuel. 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 device controls the ignition device 8 in order to ignite the fuel / air mixture in the individual engine cylinders 9-12 at the right time. 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 by motor

Measures clear: The increase in exhaust gas temperature due to spark ignition and the generation of a combustible mixture with a mixture composition of Lambda la directly behind the internal combustion engine not only affects the storage catalytic converter, but undesirably already in the pre-catalytic converter.

A remedy is shown in FIG. 2: As an essential difference from FIG. 1, FIG. 2 shows two separate pre-catalysts, each with a pre-catalyst of a specific one

Cylinder group is assigned. 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 the following FIG. 3. 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.

According to the invention, the separation of the exhaust gas routing of different cylinder groups enables a lean and a rich exhaust gas flow to be brought together in front of the storage catalytic converter.

For this purpose, for example, the cylinder group, the exhaust gas of which flows through the pre-catalytic converter 1 (V.Kat.l), is operated with a rich mixture without excess air (Lambdalb). As a result, the exhaust gas from this cylinder group contains unburned fuel simultaneous lack of oxygen. Because of the lack of oxygen, the excess fuel in the pre-catalyst 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 (Lambda2b). 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 in the second cylinder group with the excess fuel in the first cylinder group to form an exhaust gas that corresponds to a lambda value Lambda3b takes place only behind the two

Pre-catalysts instead. The exhaust gas with lambda 3 b 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 the invention shown in this exemplary embodiment, at least two pre-catalysts are necessary for implementation. In the heating phase, the two pre-catalysts are acted on with different lambdas (λlb and λ2b). Realization is easily possible through the bank method (bank 1 / bank 2) of different injection quantities of the fuel. One lambda value must be greater than 1 (lean), the other less than 1 (rich). The mixture lambda (λ3b) should resulting from the individual lambdas (λlb and λ2b) and the exhaust gas mass flows, ό.uf set a value around λ3b = 1. The storage catalytic converter then converts the calorific value present in the exhaust gas, which mainly comes from the rich exhaust gas according to V. Kat., With the existing oxygen, which mainly comes from the lean exhaust gas according to V. Kat. The temperature increase in the main catalyst results from the reaction 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 in front of Sp. Cat. Is then reset to approximately 1 by the individual lambdas of the 5th Cat. Use with other numbers of cylinders is also conceivable, whereby according to the invention there must always 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 exemplary embodiment, a mixing element (static mixer) is connected upstream of the main catalyst to be heated. This consists, for example, of a cavity which is arranged at an angle to one another

Flow baffles is provided. The flow baffles direct the individual flow volumes into each other and slow down the flow. Mixing of individual portions of exhaust gas is thereby achieved.

The core of the second exemplary embodiment is the mixing of the lean and rich portions of exhaust gas upstream of the storage catalytic converter, the lean and rich ones Exhaust gas portions are generated here separately from one another by engine.

For this purpose, the engine is always operated alternately rich (λ <1) and lean (λ> 1) in the heating phase. Alternatively, individual cylinders can be operated rich and lean due to a different injection quantity. The resulting rich and lean exhaust gas packets are only partially mixed back in the pre-catalyst so that lean and rich exhaust gas packets are still present after the pre-catalyst.

In the static mixer connected downstream of the pre-catalyst, the exhaust gas is then homogenized by the back mixing there (the behavior of the static mixer is similar to that of

Smear the dwell time in an ideal Ruhr boiler with a small volume).

In the storage catalytic converter, the calorific value present in the exhaust gas, which mainly comes from the rich exhaust gas packets, is then converted with the available oxygen, which mainly comes from the lean exhaust gas packets. The temperature increase in the main catalyst results from the reaction of the not fully oxidized components with the oxygen.

The period of the rich and lean cycles depends on the conditions of the expected exhaust gas volume flow and the volume of the static mixer, or the required goodness of the jerk mixing. The lowest possible oxygen storage capacity of the pre-catalytic converter should be aimed at, so that unnecessary dead time regarding the rich and lean exhaust gas packets is not introduced into the system. As an alternative to heating a storage catalytic converter, the invention can also be used to heat a three-way catalytic converter which is arranged behind at least one pre-catalytic converter. The term main catalyst in claim 1 is intended to cover these two alternatives.

Claims

Expectations

1. Method for heating one behind at least one. Pre-catalytic converter arranged main catalytic converter by supplying motor-generated unburned fuel in the exhaust gas upstream of the main catalytic converter in the presence of excess air in front of the main catalytic converter, characterized in that the motor-driven generation of unburned fuel is decoupled in time and / or space from the motor-generated generation of excess air in the exhaust gas.