DE102006009028A1 - Assembly and method for nitrogen oxide reduction in the exhaust system of a motor vehicle - Google Patents

Abstract

A NOx reduction assembly in the exhaust system of a motor vehicle includes an SCR catalyst (12), a selective ammonia generation catalyst (16) disposed upstream of the SCR catalyst (12), adapted to be exhausted from the exhaust stream independently of the oxygen content in the exhaust Fuel generates hydrogen which reacts with the nitrogen oxides present in the exhaust gas stream to form ammonia, and a device (18) arranged upstream of the ammonia generation catalyst (16) for introducing fuel into the exhaust gas stream. A method according to the invention for nitrogen oxide reduction provides for determining the NO _x content in the exhaust gas, introducing fuel into the exhaust gas flow as a function of this, and generating ammonia independently of the oxygen content in the exhaust gas in the ammonia generation catalyst (16).

Description

The The invention relates to an assembly for nitrogen oxide reduction in the Exhaust system of a motor vehicle. Furthermore, the invention relates a method for nitrogen oxide reduction in a vehicle exhaust system.

To comply with environmental regulations, the exhaust gases of motor vehicles powered by internal combustion engines must be subjected to a cleaning. In particular, so-called SCR catalysts (also referred to as denitration catalysts), which selectively reduce nitrogen oxides (NO _x ) produced during combustion in the engine with the aid of ammonia (NH ₃ ) temporarily stored in the SCR catalyst, are increasingly used to reduce nitrogen oxides. The provision of the ammonia required for the selective catalytic reduction is accomplished by hydrolysis of urea which is usually added to the offgas in dissolved form. For this purpose, a storage tank arranged in the vehicle and a comparatively complicated metering system for the urea solution are required. Both causes increased costs and increased space requirements, which can be problematic especially in the case of passenger cars.

These mentioned disadvantages try to circumvent systems for in-vehicle ammonia production. At one of the EP 0 879 343 B1 The known system, rich operating phases of the engine or a periodic fuel injection, which serves the enrichment of the exhaust gas, used to generate ammonia in a three-way catalyst.

Even with one from the DE 103 00 298 A1 NH _{3 is produced} under rich operating conditions during the regeneration of a NO _x storage catalytic converter known arrangement.

In contrast creates the invention a system or a method in which an SCR catalyst If required, it can be supplied with ammonia at any time.

This is inventively achieved by an assembly for nitrogen oxide reduction in the exhaust system of a motor vehicle, with an SCR catalyst, a selective upstream of the SCR catalyst arranged ammonia generating catalyst which is designed so that it generates hydrogen regardless of the oxygen content in the exhaust gas from the exhaust gas supplied fuel which reacts with the nitrogen oxides present in the exhaust gas stream to ammonia, and a device arranged upstream of the ammonia generating catalyst for introducing fuel into the exhaust gas stream. The ammonia generation catalyst combines two functions. On the one hand, it converts fuel into hydrogen even in the presence of high oxygen concentrations, as a result of which the embodiment according to the invention differs from known systems. In these, namely, an oxidation of unburned fuel to CO ₂ and H ₂ O is usually carried out at an excess of oxygen. Hydrogen can only be formed in the rich mixture in known systems. On the other hand, in the embodiment according to the invention, the hydrogen formed in the ammonia generating catalyst is converted into ammonia with nitrogen oxides contained in the exhaust gas, with both reactions taking place in parallel. Since the ammonia production is thus based on a selective catalytic reaction, which is unaffected by the oxygen content in the exhaust gas, ammonia can be generated by the assembly according to the invention at any time, which is used in the downstream SCR catalyst for selective nitrogen oxide reduction. The system according to the invention is thus completely independent of the operating state of the engine. Furthermore, it is characterized by a lower fuel consumption than in the prior art, since the fuel supplied to the exhaust gas in the assembly according to the invention does not serve to degrease the exhaust gas (such as in a lean-burn engine), but merely to supply the hydrogen for ammonia production. In addition, the assembly according to the invention offers the advantage of ammonia production at significantly lower temperatures compared to systems with urea injection. The urea injection is usually carried out at temperatures of at least 200 ° C. In the system according to the invention, the fuel is supplied at a suitable SCR catalyst at much lower temperatures, for example from 150 ° C. Another advantage of the assembly according to the invention is that for the ammonia already a part (ideally about 50%) of the nitrogen oxides contained in the exhaust gas is needed. The remaining nitrogen oxides are reduced in the SCR catalyst, with the typical for the SCR reaction stoichiometry of NH ₃ to NO is one. Since the amount of nitrogen oxides which reaches the SCR catalyst is already reduced by the part required for the production of ammonia, the SCR catalyst can have a significantly smaller volume than the prior art because it has a significantly smaller amount of NO _x ( ideally only half).

According to one preferred embodiment a controller is provided which determines the amount of fuel introduced and controls the timing and duration of the contribution. In this way, the amount of ammonia generated can be the current one Needs adapted become.

At least upstream of the ammonia generating catalyst, a temperature sensor may be provided, in particular with the Steue connected.

Furthermore, at least one arranged upstream of the ammonia generating catalyst NO _x sensor should be provided which measures the instantaneous nitrogen oxide content in the exhaust gas. Depending on this nitrogen oxide content, the amount of fuel introduced into the exhaust gas line is then controlled or regulated. Alternatively, it is also possible to determine the instantaneous nitrogen oxide content by means of characteristic maps stored in the engine control unit as a function of the current engine operating point or to calculate it based on a model-based mathematical description of the emission behavior.

According to one another embodiment at least one oxygen sensor may be provided, which is also upstream of the Ammonia generating catalyst is arranged and in which it is for example, is a lambda probe.

Optionally, NO _x and / or NH ₃ sensors are arranged downstream of the ammonia generation catalyst and / or the SCR catalyst. These detect the ammonia or NO _x content in the exhaust gas flow and thus allow a particularly accurate control of the system.

Advantageous it is in the device for introducing fuel around an evaporation unit. This includes e.g. an arranged in a chamber Heating element, such as a glow plug, on. In the chamber becomes more fluid Fuel (e.g., from the fuel tank of the vehicle) evaporates and introduced into the exhaust stream in vapor form, whereby a particularly good distribution of the fuel is ensured in the exhaust gas. Such Evaporation unit is also much simpler than the known from the prior art metering devices for urea solution.

The ammonia generating catalyst preferably has a catalytically active coating comprising at least one noble metal, preferably from the group of Pt metals. This coating can be applied to a conventional substrate (eg cordierite) with an oxide washcoat (eg Al ₂ O ₃ , SiO ₂ , ZrO ₂ ,...).

Advantageously, the ammonia generation catalyst has a NO _x storage coating. Thus, NO _x can accumulate on the surface of the ammonia generation catalyst, which reacts with the introduced fuel to NH ₃ .

The NO _x storage coating preferably comprises at least one alkali metal and / or alkaline earth metal.

Upstream of the ammonia generating catalyst may be an oxidation catalyst for increasing the NO ₂ / NO ratio in the exhaust stream. Since only NO ₂ can be stored in the ammonia generation catalyst and this also reacts faster in the SCR catalyst than NO, the efficiency of the system can be increased in this way. It would also be conceivable to increase the NO ₂ content by a non-thermal plasma or to generate NO ₂ from NO directly in the ammonia generation catalyst.

According to a further embodiment of the invention, the ammonia-producing catalyst and the device for introducing fuel are arranged in a bypass line running parallel to a main exhaust gas line. In this embodiment, a smaller volume of the ammonia generating catalyst is sufficient. In addition, the NH ₃ / NO _x ratio can be regulated by variable distribution of the volume flows to the main exhaust gas line and the bypass line.

• – Bestimmen des NO_x-Gehalts im Abgas,
• – Einbringen von Kraftstoff in den Abgasstrom in Abhängigkeit vom bestimmten NO_x-Gehalt und
• – Erzeugen von Ammoniak im Ammoniakerzeugungskatalysator unab hängig vom Sauerstoffgehalt im Abgas.

According to another aspect of the invention, there is provided a method of nitrogen oxide reduction in an automotive exhaust system including an SCR catalyst, a selective ammonia generation catalyst disposed upstream of the SCR catalyst, and a fuel injection apparatus disposed upstream of the ammonia generation catalyst. The method according to the invention is characterized by the following steps:

• Determining the NO _x content in the exhaust gas,
• - introducing fuel into the exhaust stream depending on the determined NO _x content and
• - Generating ammonia in the ammonia generating catalyst inde pendent of the oxygen content in the exhaust gas.

As already mentioned, the determination of the NO _x content can take place by measurement or by calculation by means of data from the engine map. The method according to the invention enables a constant, demand-oriented generation of ammonia which (apart from a possible determination of the instantaneous NO _x content via the engine map) is independent of the operating state of the engine and of the oxygen content in the exhaust gas.

According to a first variant of the method, the fuel is introduced at irregular intervals, namely whenever the NO _x content in the exhaust gas requires the supply of ammonia. Here, in particular, the loading state of the SCR catalyst is taken into account.

According to one alternative method variant is the introduction of fuel continuously.

The amount of fuel introduced is advantageously varied as a function of at least the NO _x content in the exhaust gas flow, in order to adapt the amount of ammonia produced to the current demand, in particular in the case of a continuous introduction of fuel.

Preferably the fuel is introduced into the exhaust stream in vapor form, whereby a particularly good mixing with the exhaust more Features and advantages of the invention will become apparent from the following Description of a preferred embodiment with reference to the accompanying drawings. In these show:

1 schematically an assembly for nitrogen oxide reduction in the exhaust system of a motor vehicle according to a first embodiment of the invention, in which the inventive method is performed, and

2 schematically an assembly for nitrogen oxide reduction according to a second embodiment of the invention.

In 1 is an assembly 10 for nitrogen oxide reduction, which is used in the exhaust system of a motor vehicle with lean-burn engine (not shown). The assembly 10 includes an SCR catalyst 12 who is in an exhaust pipe 14 is arranged. The arrows indicate the flow direction of the exhaust gas. Upstream of the SCR catalyst 12 is an ammonia generation catalyst 16 provided that a device 18 upstream of the introduction of fuel in the form of an evaporation unit. The device 18 is connected to a fuel line, not shown, of the vehicle, which is preferably the fuel return line, in which the fuel is already preheated. In addition to the schematically indicated in the figure, the exhaust system, of course, other components, such as a manifold, an exhaust gas turbocharger, exhaust gas recirculation and / or other catalysts or particulate filter, whose representation has been omitted for clarity.

The ammonia generation catalyst 16 has a catalytically active coating which is applied to a conventional substrate (eg cordierite) with oxidic washcoat (eg Al ₂ O ₃ , SiO ₂ , ZrO ₂ ). This catalytically active coating comprises one or more noble metals, preferably from the group of Pt metals, and alkali or alkaline earth metals in a suitable composition. Characterized also receives the ammonia generating catalyst 16, a certain storage capacity for NO _x, which enables the deposition of nitrogen oxides on the catalyst surface and favors so the ammonia production, in which the formed from the introduced hydrogen fuel with nitrogen oxides (NO _x) to NH ₃ reacts.

Immediately upstream of the ammonia generation catalyst 16 are several sensors 20 arranged, which serve to detect the temperature, the oxygen content and the NO _x content in the exhaust stream and with a controller 22 keep in touch. Likewise, several of the functions mentioned can be integrated into a single sensor.

During operation of the vehicle is detected by the sensors 20 at least the nitrogen oxide content detected in the exhaust gas. Alternatively, as already mentioned, a determination of the NO _x content is possible via a characteristic field stored in the engine control unit. Depending on the measured or determined value is in the controller 22 determines a required amount of fuel through the device 18 in the exhaust pipe 14 upstream of the ammonia generation catalyst 16 is introduced. Out of this unburned fuel produces the ammonia generation catalyst 16 Hydrogen, which reacts with the aid of nitrogen oxides contained in the exhaust stream to ammonia. The overall reaction is selective, ie unaffected by the presence of oxygen in the exhaust stream, and can be summarized qualitatively in the following overall reaction equation: HC + NO _x → NH ₃ + CO ₂ + H ₂ O.

The produced ammonia leaves the ammonia generation catalyst 16 together with the exhaust gas and enters the SCR catalyst 12 where it reduces the nitrogen oxides remaining in the exhaust stream to nitrogen and water. In this case, the SCR catalyst 12 Temporarily store ammonia.

Optionally, downstream of the ammonia generation catalyst 16 and / or downstream of the SCR catalyst 12 NO _x and / or NH ₃ sensors 24 (shown in dashed lines), which detect the content of the respective substance in the exhaust stream and this data to the controller 22 hand off. This allows a more accurate control of fuel delivery.

In addition, to increase efficiency, upstream of the ammonia generating catalyst 16 an oxidation catalyst 26 be provided, which increases the NO ₂ / NO ratio in the exhaust gas. Also, this oxidation catalyst 26 is not mandatory and therefore in 1 shown in dashed lines.

It should be emphasized that the fuel injection into the exhaust gas flow is independent of the operating state of the engine or the oxygen content in the exhaust gas flow. It is not a fat exhaust gas required for ammonia production by the ammonia generation catalyst 16 to stimulate. The fuel delivery through the device 18 Consequently, the goal is not to degrease the exhaust gas (ie to provide for an oxygen deficiency), but only the goal for the formation of hydrogen in the ammonia generation catalyst 16 provide required fuel. In addition to the time of fuel injection is by the controller 22 also controlled the amount of introduced fuel and the duration of the introduction or regulated to produce exactly the amount of ammonia currently required. In this case, if necessary, by the sensors 20 detected temperature, the oxygen content in the exhaust gas and the current load state of the SCR catalyst 12 considered with ammonia.

The purely on-demand fuel injection has the consequence that the fuel at irregular intervals or is continuously introduced in varying amounts. This takes place the fuel injection only during lean operating conditions.

Particularly advantageous is the introduction of fuel vapor in the exhaust stream, which is distributed very evenly in the exhaust gas. Compared to urea hydrolysis, NH ₃ production is by the ammonia generation catalyst 16 with upstream device 18 Already at significantly lower temperatures possible, the only of the light-off temperature of the ammonia generating catalyst 16 depend. This is about 150 ° C, which means that with the assembly according to the invention a nitrogen oxide reduction is possible even at relatively low temperatures. In this way, the nitrogen oxide emissions during cold start can be significantly reduced.

2 shows an assembly 10 according to a second embodiment of the invention. The same or functionally identical components bear the same reference numerals, and in the following, only the differences from the first embodiment will be discussed.

At the assembly 10 according to 2 in the exhaust system of a lean burn engine 1 , in particular a diesel engine, is arranged a bypass line 28 provided upstream of the device 18 from the exhaust pipe serving as the main exhaust gas passage 14 branches. Here are the device 18 and the ammonia generation catalyst 16 in the bypass line 28 arranged parallel to the main exhaust line and immediately upstream of the SCR catalyst 12 again flows into this. About a in 2 not shown valve or the like, the volume flow ratio of the main exhaust gas line to bypass line can be 28 vary, whereby the NH ₃ / NO _x ratio can be regulated. Ideally, this is the proportion of the bypass line 28 directed exhaust gas 50%. In this case, a particularly small-sized ammonia generation catalyst 16 sufficient. It should be emphasized that in 2 just for the sake of clarity on the presentation of other components, such as the controller 22 or the sensors 20 . 24 , was waived.

Furthermore it is at the discretion of the skilled person, all the features described both individually and in combination, to the task of the invention to solve.

1

Lean burn engine

10

module

12

SCR catalyst

14

exhaust pipe

16

Ammonia generating catalyst

18

contraption for the introduction of fuel

20

sensors

22

control

24

NO _x / NH ₃ sensors

26

oxidation catalyst

28

bypass line

Claims (17)

Assembly for nitrogen oxide reduction in the exhaust system of a motor vehicle, with an SCR catalyst ( 12 ), one upstream of the SCR catalyst ( 12 ) arranged selective ammonia generation catalyst ( 16 ) which is adapted to generate, independently of the oxygen content in the exhaust gas from the exhaust gas flow, hydrogen which reacts with the nitrogen oxides present in the exhaust gas stream to form ammonia, and one upstream of the ammonia generating catalyst ( 16 ) arranged device ( 18 ) for introducing fuel into the exhaust stream. An assembly according to claim 1, characterized in that a controller ( 22 ), which controls the amount of fuel introduced and the timing and duration of the introduction. Assembly according to one of the preceding claims, characterized in that at least one upstream of the ammonia generating catalyst ( 16 ) arranged temperature sensor ( 20 ) is provided. Assembly according to one of the preceding claims, characterized in that at least one upstream of the Ammoniakzeugungskataly sators ( 16 ) arranged NO _x sensor ( 20 ) is provided. Assembly according to one of the preceding claims, characterized in that at least one upstream of the ammonia generating catalyst ( 16 ) arranged oxygen sensor ( 20 ) is provided. Assembly according to one of the preceding claims, characterized in that downstream of the ammonia generating catalyst ( 16 ) and / or the SCR catalyst ( 12 ) NO _x and / or NH ₃ sensors ( 24 ) are arranged. Assembly according to one of the preceding claims, characterized in that it is in the device ( 18 ) for introducing fuel around an evaporation unit. Assembly according to one of the preceding claims, characterized in that the ammonia generating catalyst ( 16 ) has a catalytically active coating comprising at least one noble metal, preferably from the group of Pt metals. Assembly according to one of the preceding claims, characterized in that the ammonia generating catalyst ( 16 ) has an NO _x storage coating. An assembly according to claim 9, characterized in that the NO _x storage coating comprises at least one alkali metal and / or alkaline earth metal. Assembly according to one of the preceding claims, characterized in that upstream of the ammonia generating catalyst ( 16 ) an oxidation catalyst ( 26 ) is arranged to increase the NO ₂ / NO ratio in the exhaust stream. Assembly according to one of the preceding claims, characterized in that the ammonia generating catalyst ( 16 ) and the device ( 18 ) for introducing fuel in a bypass line running parallel to a main exhaust gas line ( 28 ) are arranged. Nitrogen oxide reduction process in a vehicle exhaust system with an SCR catalyst ( 12 ), one upstream of the SCR catalyst ( 12 ) arranged selective ammonia generation catalyst ( 16 ) and one upstream of the ammonia generation catalyst ( 16 ) arranged device ( 18 ) for introducing fuel into the exhaust stream, characterized by the following steps: determining the NO _x content in the exhaust gas, introducing fuel into the exhaust gas flow as a function of the determined NO _x content and generating ammonia in the ammonia generation catalyst 16 ) regardless of the oxygen content in the exhaust gas. Method according to claim 13, characterized in that that this Fuel is introduced at irregular intervals. Method according to claim 13, characterized in that that this Introducing fuel continuously. Method according to one of claims 13 to 15, characterized in that the amount of introduced fuel is varied as a function of at least the NO _x content in the exhaust gas stream. Method according to one of claims 13 to 16, characterized that the Fuel is introduced in vapor form into the exhaust stream.