EP1058578B1 - Regeneration of a nox storage catalytic converter of an internal combustion engine - Google Patents

Description

The invention relates to the regeneration of a NOx storage catalyst Internal combustion engine, for example a lean mix engine, a DI engine or a diesel engine, according to the preamble of claims 1 and 6.

For cleaning the exhaust gas of an engine with an internal combustion of a Fuel, the nitrogen oxide produced during combustion must be reduced. at common internal combustion engines, which are regulated to an average λ of 1 this can be achieved with good success using 3-way catalysts. at Combustion engines, however, that partially or entirely with λ values greater than 1 operated, such as lean-mix gasoline engines, DI gasoline engines and Diesel engines, there is currently no such established exhaust gas purification process. at Such engine types are currently in processes for exhaust gas treatment zeolite catalysts (hereinafter also referred to as "lean mix catalysts") and NOx storage catalysts in use. The zeolite catalysts become thermal deactivated, which is why they apply to engines for vehicles that are subject to the approval process Durability must prove, can not be used. Can also only the hydrocarbons in this catalyst for the reduction of nitrogen oxides Exhaust gas can be used so that only relatively low nitrogen oxide conversions are achieved become. These are often only 15% when you consider the partial reduction of nitrogen oxides to nitrous oxide is not taken into account. Furthermore, these catalysts insufficient CO and HC conversion disadvantageous if they do not contain any precious metal. More promising than the zeolite catalysts just discussed are NOx storage catalysts, since these are both the hydrocarbons in the exhaust gas as well as using hydrogen and CO as reducing agents. Basically taken are 3-way catalysts with one component Storage of NOx. However, the NOx trap is particularly inferior longer lean phases of the engine with NOx and is therefore no longer effective. That is why it is necessary with NOx storage catalytic converters to store the stored NOx periodically remove from memory, d. H. reduce the stored NOx.

EP 0 540 280 describes exhaust gas treatment using a Exhaust gas treatment system, which is a means of storing and releasing the NOx contains, the nitrogen oxides are temporarily stored in the lean operation of the engine and thermally released again by heating the introduced exhaust gas become. The catalyst is then released by means of a NOx-decomposing catalyst Nitrogen oxides decomposed again under oxidizing conditions. In particular, the NOx-decomposing catalyst from a 3-way catalyst and / or a zeolite catalyst exist, which is operated at λ less than or equal to one. The disadvantage is in particular that this catalyst is not sufficiently thermally stable, and that to avoid damage, as is typical in such catalysts occur under high loads and exhaust gas temperatures at λ = 1, an exhaust gas circuit is required, with appropriate actuating and control units for the Operation are necessary. Furthermore, these parts have the problem of durability unresolved. Furthermore, the parts of the exhaust system that are in during the phases where the engine is operated stoichiometrically, the exhaust gas does not flow through it, or, conversely, for parts that are not on the lean during operation Exhaust gas to be flowed through, the question of the operating temperature unsolved. It is particularly problematic is the temperature range in which the 3-way catalysts start because in this phase by partial reduction of the motor nitrogen oxides nitrous oxide is increasingly formed. Should be done by periodically cooling the 3-way catalytic converter this area has to be crossed again and again with a excessive production of nitrous oxide, which is due to the Greenhouse gas relevance of this gas is undesirable.

EP 0 562 805 describes an exhaust gas aftertreatment of an internal combustion engine in which the exhaust system two alternately flowed, arranged parallel to each other Lean NOx catalysts. Furthermore, the known device comprises a Device for changing the space velocity of the exhaust gas to an optimal To be able to adjust the space velocity of the exhaust gas. In addition, the Exhaust system is a means of injecting HC directly into the exhaust duct. Here too Long-term suitability is questionable, since zeolite catalysts are not thermal are stable, especially not tolerate a rich or stoichiometric exhaust gas. Also the durability of the facility for changing the Exhaust gas circuit space velocity problematic. Even if the NOx conversion by better adapting the space velocities of the exhaust gas are higher than for typical zeolite catalysts, the catalyst reaches according to EP 0 562 805 does not meet the requirements required to comply with the newer exhaust gas limit values Magnitude of over 90%. Furthermore, the problem of nitrous oxide formation and the insufficient HC and CO conversion of these catalysts unsolved.

EP 0 580 389 discloses a method for exhaust gas aftertreatment of lean-burn engines, which is equipped with a NOx absorber based on alkali, alkaline earth or rare earth metals, a 3-way catalytic converter arranged downstream, and with sensors for detecting the load and the Exhaust gas temperature are equipped. The information from the sensors limits the area within which the NOx absorber is able to store nitrogen oxides. Regeneration of the catalyst is accomplished by greasing for a predetermined period. A disadvantage of this known device is the separation between the absorber and the 3-way catalytic converter, since the predominantly engine-generated nitrogen oxides must first be oxidized to NO ₂ in order to be stored in the absorber.

EP 0 560 991 describes a system for treating an exhaust gas Internal combustion engine, in which the absorber and the catalyst in one housing are included. The nitrogen oxides are stored when the engine is operated lean will, d. H. the exhaust gas is lean and released when the oxygen concentration in the Exhaust gas is reduced to rich or stoichiometric λ values, so that the released NOx reduced with the unburned hydrocarbons and the CO of the exhaust gas becomes. Typically, the switch from lean to fat or stoichiometric operation accompanied by torque jumps, which the driver of the Vehicle only feels desirable if it is during a Acceleration phase occur. These torque jumps occur during one Constant operating phase, so they are perceived as extremely undesirable. Since in Normally, the NOx storage is emptied during constant operating phases is attempted, this torque jumps by shifting to reduce the ignition timing simultaneously with the enrichment.

Furthermore, the currently known NOx storage catalytic converters are deactivated by sulfur-containing fuel. The NOx-absorbing material of the NOx storage catalyst, in particular BaO or BaCO _3, forms thermally stable sulfates with the SO ₂ present in the exhaust gas, which oxidizes to SO ₃ on the platinum present in the catalyst, which can be decomposed at a temperature. which is above the temperature at which the nitrates formed from the storage material and the NO ₂ are decomposed. For the decomposition of these sulfates, depending on the sulfur content of the fuel used, a sulfate regeneration program is therefore carried out from time to time, with a temperature increase to approx. 600-700 ° C. by enriching the exhaust gas, causing the sulfates to decompose. A disadvantage of the enrichment, however, is that this is normally correlated with an increase in engine power, so that ultimately the vehicle accelerates unintentionally when the desulfation is carried out.

EP-A-0 625 633 shows a method and a device for cleaning the exhaust gas of a lean-burn internal combustion engine according to the preamble of claims 1 and 6.

The invention has for its object a method and an apparatus for Exhaust gas treatment of an internal combustion engine to create a reduction in the NOx content of the exhaust gas and possibly the sulfate content of the NOx storage without the occurrence of a torque jump or an increased power output causes.

This object is achieved by a method according to claim 1 and an apparatus Claim 6 solved. Preferred embodiments of the invention are the subject of Dependent claims.

The invention is used in the detoxification of exhaust gases from an internal combustion engine, the detoxification being in particular a reduction of the nitrogen oxides. The invention can also be used for the intermediate storage of SOx, as can be the case depending on the sulfur content of the fuel in exhaust gas cleaning devices of lean-burn internal combustion engines. NOx is usually stored by storing NO ₂ (for example as nitrate), alkaline earth oxides and / or carbonates (for example BaO) being particularly suitable. Such substances are also able to store SOx, in particular in the form of SO ₃ as sulfates. Since desulfation requires higher temperatures than denitrification, a SOx storage can be placed in front of a NOx storage if desired, which means that the SOx storage has higher temperatures than the NOx storage and the NOx storage is not poisoned by SOx. During the regeneration, ie the operation of the storage with an exhaust gas close to λ = 1 or rich, SOx and NOx are released again, the NOx being converted catalytically with the HC and / or CO present. The SOx, on the other hand, is released either as such or after reaction with CO and / or HC in a wide variety of compounds, the SOx not being stored in the subsequent NOx storage under the present regeneration conditions.

In the following, only NOx storage is used if nothing else is noted, these statements apply equally to a SOx memory or a combination of these memories.

The exhaust gas purification device preferably includes an internal combustion engine more than one cylinder has a NOx storage catalytic converter arranged downstream of the internal combustion engine, wherein the exhaust gas continuously into the NOx storage of the Catalyst flows such that the NOx is absorbed in the NOx storage as soon as the machine is operated lean, and the NOx is released as soon as the Oxygen concentration of the exhaust gas is lowered, during which Release phase of the NOx the machine with a gross λ value slightly above the stoichiometric ratio of λ = 1 is operated.

The λ value is preferably ≥ 1.01 during the release phase of the NOx. You can choose between lean and slightly above the stoichiometric exhaust gas value lying machine operation can be switched back and forth, the times of the Switching depends on the duration of the lean operation.

Is preferably used to generate the almost stoichiometric exhaust gas flow a gross value of λ ≥ 1 selectively greased part of the cylinders individually, while the other part of the cylinders continues to be operated lean. This is done selectively cylinder-specific λ detuning preferably during a constant Operating phase without load changes. The cylinder-specific enrichment takes place advantageously at half or a number of cylinders close to half. Also be the enriched cylinders in particular to λ ≤ 0.9 and particularly advantageously λ ≤ 0.85 enriched. This extreme enrichment is compared to a company near λ = 1 achieved that the torque fluctuation is less because the Power output with a strong enrichment is lower than near λ = 1. This requires less correction across the throttle and / or over the entire Injection quantity of the fuel. The enrichment up to λ = 0.7 can be advantageous here and less done. It also follows that the invention is particularly useful found in gasoline engines, particularly advantageous in direct injection engines. By the invention it is also achieved that no cylinder-specific throttling is provided must, so that the proportion of mechanical control elements does not increase.

With a 4-cylinder engine, for example, 2 cylinders can be greased and two Cylinders remain in lean operation. The same applies to motors with one of them deviating number of cylinders. The division of the cylinders in half is not mandatory, but other ratios can be selected according to the requirements, the Conditions can also be changed during operation.

The selective cylinder-specific λ detuning can be done with a control unit be made.

In summary, the exhaust gas treatment method described here causes one good conversion not only of nitrogen oxides but of all exhaust gas components by during unsteady operating phases of the engine due to driving behavior is enriched or operated with a λ = 1, causing the NOx storage again is emptied, and during long stationary operation between lean and almost stoichiometric operation with a λ slightly larger than 1 switches back and forth. This is effected by motor by generating almost stoichiometric exhaust the engine is selectively enriched with part of the cylinders and the other part of the cylinders continues to run lean. There are only a few Torque fluctuations and the engine power is not increased. Therefore one takes further measures such as an excessive adjustment of the ignition timing dispense. Advantageously, the production of nitrous oxide lies with that The inventive method not over that of known 3-way catalysts. Furthermore, the device according to the invention brings about the corresponding selective cylinder-specific λ detuning a desulfation of the NOx storage.

The following table shows measured values of the gross nitrogen oxide conversion η _{NOx of} an engine with lean and regeneration operation in a continuous sequence of lean operation and subsequent regeneration operation once with a rich λ value of 0.85 and once with a slightly above the stoichiometric value λ value of 1.01. It can be seen from the table that even in the regeneration process according to the invention, which operates somewhat above the stoichiometric λ value, a gross conversion is achieved which is even somewhat higher than that of the richer exhaust gas of the known processes. η _NOx [%] Regeneration λ 87 0.85 88 1.01

A preferred embodiment is explained below with reference to FIG schematically represents the arrangement of the exhaust system on an internal combustion engine.

In FIG. 1, reference numeral 1 denotes a machine with more than one Cylinders, such as a lean-mix gasoline engine, a di-gasoline engine or a Diesel engine, shown, which is arranged below an exhaust gas cleaning system 2, which has a NOx storage catalytic converter 3. Through selective cylinder-specific Detuning of the λ values of some of the cylinders (not shown) and the others Lean operation of the other part of the cylinders becomes a gross λ value, i.e. H. on averaged λ value of all cylinders, slightly above the stoichiometric value of the Exhaust gas set for regeneration of the NOx storage 3 and thus regeneration and desulfation of the NOx storage causes.

Claims (8)

1. Method for the NO_x and if appropriate SO_x regeneration of an NO_x storage catalytic converter which is arranged in an exhaust-gas purification system of an internal combustion engine with more than one cylinder, in which a mass flow of reducing agents in the exhaust gas is increased in order to regenerate the NO_x storage catalytic converter, characterized in that, by means of a control unit, some of the cylinders are operated under lean conditions λ > 1 and some of the cylinders are operated under rich conditions λ < 1, but on average over all the cylinders λ is ≥ 1.
2. Method according to Claim 1, characterized in that some of the cylinders are operated at λ ≤ 0.95, in particular λ ≤ 0.85, during the regeneration.
3. Method according to Claim 1 or 2, characterized in that the cylinder-selective trimming takes place during a constant operating phase without any load changes.
4. Method according to one of the preceding claims, characterized in that half or nearly half of the cylinders are operated with a rich mix.
5. Method according to one of the preceding claims, characterized in that the control unit performs the cylinder-selective trimming in idling mode, in overrun mode and/or at an engine load of ≤ 25% of a maximum rated engine load.
6. Device for the NO_x and if appropriate SO_x regeneration of an NO_x storage catalytic converter which is arranged in an exhaust-gas purification system of an internal combustion engine with more than one cylinder and which, in order to be regenerated, is exposed to an increased mass flow of reducing agents in the exhaust gas, characterized in that the device comprises a control unit, by means of which, during the regeneration, some of the cylinders are operated under lean conditions λ > 1 and some of the cylinders are operated under rich conditions λ < 1, but on average over all the cylinders λ is ≥ 1.
7. Device according to Claim 6, characterized in that the internal combustion engine is a spark-ignition engine.
8. Device according to Claim 6, characterized in that the internal combustion engine is a direct-injection engine.

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