DE102014204439A1 - Exhaust gas aftertreatment with used up urea stock - Google Patents

Abstract

The invention introduces a novel method of operating an internal combustion engine (2) with an exhaust tract (3) comprising a nitrogen oxide storage catalyst (4) and an SCR catalyst (5). According to the invention, the method has at least the following steps: - operating the internal combustion engine (2) with a lean fuel mixture and adding aqueous urea solution with a first addition rate upstream of the SCR catalytic converter (4) from a reservoir (9); - Detecting a level of the reservoir (9); - comparing the detected level with a predetermined Schwellfüllstand; If the detected level is less than a predetermined threshold level or equal to the predetermined threshold level, operating the engine (2) with a rich fuel mixture and adding aqueous urea solution at a second rate of addition that is less than the first rate of addition. The invention also relates to an internal combustion engine (2), which operates according to the inventive method, and a motor vehicle (1) with such an internal combustion engine (2).

Description

The invention relates to a method for operating an internal combustion engine, an internal combustion engine, which operates according to the inventive method, and a motor vehicle with such an internal combustion engine.

Internal combustion engines such as internal combustion engines of motor vehicles generate harmful exhaust gases during operation, such as nitrogen oxides. In order to reduce the emission of such harmful exhaust gases, exhaust aftertreatment measures are taken. For this purpose, for example, it is known to provide a selective catalytic reduction (SCR) catalyst, through which the exhaust gas of the internal combustion engine is passed. Upstream of the SCR catalyst, a reducing agent is injected into the exhaust tract, which is transported by the hot exhaust gas stream and flows through the SCR catalyst together with the nitrogen oxides to be reduced. The SCR catalyst promotes a reaction between the reducing agent and the nitrogen oxides, which results in the harmless chemical products water and nitrogen. In this way, the nitrogen oxide emissions can be avoided or at least greatly reduced. The aqueous urea solution used for this purpose as reducing agent is carried in a reservoir and must be filled up regularly. During a very long journey, however, it may happen that the stock is used up or greatly reduced, so that a warning to the driver for refilling the urea reservoir should take place or should take place. Nevertheless, in order to comply with the emission regulations after falling below the minimum level, it is usual to use an operating mode called the "limp-home" mode in which the maximum speed and the torque of the internal combustion engine are limited until the stock is filled up. This affects the usability of the vehicle.

• – Betreiben der Brennkraftmaschine mit einem mageren Kraftstoffgemisch und Zugeben von wässriger Harnstofflösung mit einer ersten Zugaberate stromaufwärts des SCR-Katalysators aus einem Reservoir;
• – Erfassen eines Füllstandes des Reservoirs;
• – Vergleichen des erfassten Füllstandes mit einem vorbestimmten Schwellfüllstand;
• – wenn der erfasste Füllstand kleiner als ein vorbestimmter Schwellfüllstand oder gleich dem vorbestimmten Schwellfüllstand ist, Betreiben der Brennkraftmaschine mit einem fetten Kraftstoffgemisch und Zugeben von wässriger Harnstofflösung mit einer zweiten Zugaberate, die geringer als die erste Zugaberate ist.

The invention therefore introduces a novel method for operating an internal combustion engine. The internal combustion engine has an exhaust gas tract which has a nitrogen oxide storage catalytic converter (lean NOx trap) and an SCR catalytic converter arranged downstream of the nitrogen oxide storage catalytic converter. According to the invention, the method has at least the following steps:

• Operating the internal combustion engine with a lean fuel mixture and adding aqueous urea solution at a first rate of addition upstream of the SCR catalyst from a reservoir;
• - Detecting a level of the reservoir;
• - comparing the detected level with a predetermined Schwellfüllstand;
• If the detected level is less than a predetermined threshold level or equal to the predetermined threshold level, operating the engine with a rich fuel mixture and adding aqueous urea solution at a second rate of addition that is less than the first rate of addition.

The invention has the advantage of keeping the emission of nitrogen oxides within the legal framework. In the method according to the invention for operating an internal combustion engine, the internal combustion engine is operated during normal operation with a lean fuel mixture. With a lean fuel mixture, more oxygen is present than can be consumed by the existing fuel. The advantage of such an operation is the reduced fuel consumption of the internal combustion engine.

According to the invention, the consumption of aqueous urea solution is reduced by operating the internal combustion engine with a rich fuel mixture when the reservoir of aqueous urea solution is predominantly emptied or requires replenishment. With a rich fuel mixture, more fuel is present than can be burned by the oxygen of the combustion air present in the fuel mixture.

By simultaneously reducing the rate of addition of aqueous urea solution for a given engine power of the internal combustion engine at the same time as switching to the (at least temporary) operation with a rich fuel mixture, the remaining supply consumes correspondingly slower and there remains a length of time until the supply of aqueous urea solution must be replenished.

The nitrogen oxide storage catalytic converter of the exhaust tract functions under normal conditions as a passive nitrogen oxide adsorber (PNA, passive NOx adsorber), that is, the nitrogen oxides accumulate at low exhaust gas temperatures on the surface of the nitrogen oxide storage catalyst and dissolves therefrom at higher exhaust gas temperatures at which the SCR catalyst Reduction of nitrogen oxides can make. In operation with a rich fuel mixture, the nitrogen oxides stored in the nitrogen oxide storage catalyst are also directly reduced by a reaction with unburned hydrocarbons, ammonia can arise, which can participate in the SCR catalyst in the further reduction of nitrogen oxides as a reaction partner instead of the aqueous urea solution. As a result, according to the invention, the legal requirements for the emission of nitrogen oxides can be met and at the same time the consumption of aqueous urea solution can be reduced without having to limit the engine power of the internal combustion engine.

In particular, the predetermined Schwellfüllstand a level for a certain range z. B. 50 or 200 km correspond. In this case, predetermined or determined during operation of the internal combustion engine typical consumption data of aqueous urea solution for the estimation of the remaining range can be used.

Preferably, the engine is alternately operated with a rich and lean fuel mixture when the level is equal to or less than the predetermined threshold level. This helps to minimize the increase in fuel consumption. In the phases in which the internal combustion engine is operated with a lean fuel mixture, the nitrogen oxide storage catalyst can absorb and store the emitted nitrogen oxides. If the internal combustion engine is then operated with a rich fuel mixture, the stored nitrogen oxides can be broken down as described above.

In the method of the invention, moreover, a step of heating may be performed when the engine is operated on the rich fuel mixture. This can be used, for example, to specifically trigger the release of the nitrogen oxides stored in the nitrogen oxide storage catalyst or to increase the reactivity of the catalysts and the reagents.

In particular, fuel may be injected into the exhaust tract upstream of the nitrogen oxide storage catalyst. This can be done on the one hand to perform a heating, on the other hand, the injected here in the exhaust gas of the fuel fuel as described above participate in the reduction of nitrogen oxides by ammonia is formed in the nitrogen oxide storage catalyst, which uses in the SCR catalyst for the reduction of nitrogen oxides can be.

A second aspect of the invention introduces a novel internal combustion engine. The internal combustion engine includes an exhaust tract having a nitrogen oxide storage catalyst and an SCR catalyst disposed downstream of the nitrogen oxide storage catalyst, a reservoir for an aqueous urea solution, an injector for injecting the aqueous urea solution into the exhaust tract, and a control unit connected to the reservoir and the injector for controlling the internal combustion engine. According to the invention, the control unit is designed to carry out the method of the first aspect of the invention.

In this case, the exhaust gas tract can also have an injection device arranged upstream of the nitrogen oxide storage catalytic converter for injecting fuel into the exhaust gas tract. In this case, the control unit is designed to control a step of injecting fuel into the exhaust gas tract in the context of the method according to the invention.

The internal combustion engine may be equipped with a diesel particulate filter disposed downstream of the nitrogen oxide storage catalyst. A diesel particulate filter is used to filter soot particles and particulate matter out of the exhaust gas, thereby further improving the exhaust gas quality. The diesel particulate filter may be, for example, a Coated Diesel Particulate Filter (CDPF). A CDPF is a diesel particulate filter equipped with a coating that catalytically supports oxidation of the soot deposited in the diesel particulate filter.

The SCR catalyst may include an SCR-on-diesel particulate filter or be an SCR-on-diesel particulate filter. An SCR-on-Diesel Particulate Filter is a diesel particulate filter equipped with a catalytic coating that partially or completely takes over the function of the SCR catalyst.

The internal combustion engine of the invention is preferably a diesel internal combustion engine.

Another aspect of the invention relates to a motor vehicle with an internal combustion engine according to the invention.

The invention will be described in more detail below with reference to an illustration of an embodiment. The only 1 shows a detail of a motor vehicle 1 , An exhaust manifold (not shown) of an internal combustion engine 2 is with an exhaust tract 3 connected. The exhaust tract 3 has a nitrogen oxide storage catalyst 4 and an SCR catalyst 5 on. Between the nitrogen oxide storage catalyst 4 and the SCR catalyst 5 is an injector 6 for an aqueous urea solution arranged with a reservoir 9 for the aqueous urea solution is connected.

The exhaust gas of the internal combustion engine 2 happens first the nitrogen oxide storage catalyst 4 , which accumulates nitrogen oxides at its surface at low exhaust gas temperatures and releases at higher exhaust gas temperatures. The nitrogen oxide-containing exhaust gas is enriched by injection with aqueous urea solution with urea and thus passes, possibly after passing through an optional diesel particulate filter 8th , to the SCR catalyst. Is a diesel particulate filter 8th provided, it may also be arranged downstream of the SCR catalyst.

In some embodiments, the SCR catalyst may be implemented as SDPF, or both an SDPF and an SCR catalyst may be provided. The SCR catalyst reduces the nitrogen oxides contained in the exhaust gas by a reaction with the urea to nitrogen and water.

Optionally, an injection device 7 be provided for fuel. This is preferably upstream of the nitrogen oxide storage catalyst 4 arranged and can be provided to achieve by injection of the fuel, a heating of the exhaust tract. The fuel may also be injected to react by the reaction of the hydrocarbons of the fuel in the nitrogen oxide storage catalyst 4 Ammonia generated in the SCR catalyst 5 participates in the reduction of nitrogen oxides.

Further, a control unit 10 for controlling the internal combustion engine 2 provided except with the internal combustion engine 2 with the reservoir 9 for the aqueous urea solution and the injector 6 for the aqueous urea solution is connected. Is an injector 7 provided for fuel, the control unit 10 also be associated with this.

The control unit 10 is designed to carry out the method according to the invention. For this purpose, it records a level of the reservoir 9 and compares the detected level with a predetermined threshold level. If the level is too low, the control unit will stop 10 the internal combustion engine 2 from a lean fuel mixture operation to a rich fuel mixture operation. At the same time it reduces the consumption of aqueous urea solution. The operation with the rich fuel mixture lowers the emission of nitrogen oxides in the manner described above, so that the internal combustion engine 2 can be continued without limitation of their power or their maximum torque. The operation with a rich fuel mixture can only be made in phases; Between these phases, the internal combustion engine is then further operated with a lean fuel mixture, wherein the resulting nitrogen oxides are stored by the nitrogen oxide storage catalyst and released again in a subsequent operating phase with a rich fuel mixture and reduced in the SCR catalyst.

The invention has been explained in more detail with reference to an exemplary embodiment, but is not limited by the disclosed examples. Variations of the invention may be derived by those skilled in the art from the embodiment without departing from the scope of the invention as defined in the claims.

LIST OF REFERENCE NUMBERS

1

 motor vehicle

2

 Internal combustion engine

3

 exhaust tract

4

 Nitrogen oxide storage catalyst

5

 SCR catalyst

6

 Injection device for aqueous urea solution

7

 Injection device for fuel

8th

 diesel particulate Filter

9

 Reservoir for aqueous urea solution

10

 control unit

Claims (10)

A method for operating an internal combustion engine ( 2 ) with an exhaust tract ( 3 ) containing a nitrogen oxide storage catalyst ( 4 ) and a downstream of the nitrogen oxide storage catalyst ( 4 ) arranged SCR catalyst ( 5 ), the method having at least the following steps: - operating the internal combustion engine ( 2 ) with a lean fuel mixture and adding aqueous urea solution at a first rate of addition upstream of the SCR catalyst ( 5 ) from a reservoir ( 9 ); - Detecting a level of the reservoir ( 9 ); - comparing the detected level with a predetermined Schwellfüllstand; If the detected level is less than a predetermined threshold level or equal to the predetermined threshold level, operating the internal combustion engine ( 2 ) with a rich fuel mixture and adding aqueous urea solution at a second rate of addition that is less than the first rate of addition. The method of claim 1, wherein the predetermined threshold level is a level in the reservoir (10). 9 ) corresponds to a specified range. The method of one of the preceding claims, wherein the internal combustion engine ( 2 ) is operated alternately with a rich and a lean fuel mixture when the level is equal to the predetermined Schwellfüllstand or less than the predetermined Schwellfüllstand. The method of one of the preceding claims, further comprising the step of heating when the internal combustion engine ( 2 ) is operated with the rich fuel mixture. The method of any of the preceding claims, wherein fuel is injected into the exhaust stream upstream of the nitrogen oxide storage catalyst ( 4 ) is injected. An internal combustion engine ( 2 ) with an exhaust tract ( 3 ) containing a nitrogen oxide storage catalyst ( 4 ) and a downstream of the nitrogen oxide storage catalyst ( 4 ) arranged SCR catalyst ( 5 ), a reservoir ( 9 ) for an aqueous urea solution, an injection device ( 6 ) for injecting the aqueous urea solution into the exhaust gas tract (US Pat. 3 ) and one with the reservoir ( 9 ) and the injection device ( 6 ) connected control unit ( 10 ) for controlling the internal combustion engine ( 2 ), characterized in that the control unit ( 10 ) is adapted to carry out the method of one of the preceding claims. The internal combustion engine ( 2 ) of claim 6, wherein the exhaust tract ( 3 ) also upstream of the nitrogen oxide storage catalyst ( 4 ) arranged injection device ( 7 ) for injecting fuel into the exhaust tract ( 3 ) and the control unit ( 10 ) is configured to carry out the method of claim 5. The internal combustion engine ( 2 ) of one of claims 6 or 7, with a downstream of the nitrogen oxide storage catalyst ( 4 ) arranged diesel particulate filter ( 8th ). The internal combustion engine ( 2 ) of one of claims 6 to 8, wherein the SCR catalyst ( 5 ) contains an SCR-on-diesel particulate filter or is an SCR-on-diesel particulate filter. A motor vehicle ( 1 ) with an internal combustion engine ( 2 ) according to one of claims 6 to 9.

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