US20060101809A1

US20060101809A1 - Internal combustion engine comprising a reducing agent production unit and operating method therefor

Info

Publication number: US20060101809A1
Application number: US10/524,999
Authority: US
Inventors: Thilo-Ralf Bodo; Bernd Christner; Frank Duvinage; Christian Goerigk; Harald Menke; Sigfried Mueller-Lunz; Arno Nolte; Markus Paule
Original assignee: DaimlerChrysler AG
Current assignee: Daimler AG
Priority date: 2002-08-17
Filing date: 2003-07-30
Publication date: 2006-05-18
Also published as: DE10237777A1; WO2004022936A1; EP1529155A1

Abstract

An internal combustion engine with a reducing agent-generating unit for generation of an H₂-containing and/or NH₃-containing reducing gas, which can be added to an exhaust gas line upstream from an NO_xreducing catalytic converter, and a procedure for the operation of such an internal combustion engine are proposed. The reducing agent-generating unit has an NO_xgeneration step and an H₂generation step in serial arrangement. NH₃is formed at least temporarily by chemical reaction by the reducing agent-generating unit from NO_xproduced in the NO_xgeneration step. The invention is applicable in motor vehicles, especially in passenger vehicles with Diesel engines.

Description

BACKGROUND AND SUMMARY OF THE INVENTION

The invention concerns an internal combustion engine with a reducing agent-generating unit and an operating method for the engine.
Non-published German patent application 10128414.4 describes an internal combustion engine with reducing agent-generating unit. The reducing agent-generating unit serves for producing an H₂-containing, and NH₃-containing reducing gas, which can be added upstream of an NO_xcatalytic converter to the exhaust gas line of the internal combustion engine. An HC-containing (HC=hydrocarbon) fuel, as well as air and/or exhaust gas can be supplied to the reducing agent-generating unit. The generation of the H₂portion, and the NH₃portion in the reducing gas takes place in parallel controlled units, which makes the utilization of circuit components and the utilization of an intermediate storage unit necessary.
In contrast, an objective of this invention is to provide an internal combustion engine with a reduction-generating unit and an operating method for this, by means of which reducing agents for effective exhaust gas cleaning can be provided in a manner simple as to construction and process engineering.
This objective is accomplished in accordance with the invention.
The internal combustion engine of the invention includes a reducing agent-generating unit which has an NO_xgeneration step and an H₂generation step in serial arrangement. The serial arrangement allows for a constructionally simple coupling of the generation steps with a low number of control valves, and independent of each other to a high degree, and therefore an easy to control operation of the generation steps. The NO_xproduced by the NO_xgeneration step may be reduced as required by reducing the H₂from the H₂generation step to NH₃. Hereby, in connection with a suitable exhaust gas catalytic converter, an effective reducing agent for removing the nitrogen oxides present in the exhaust gas of the internal combustion engine is available. The H₂produced by the H₂generation step may also be utilized for the catalytic reduction of the nitrogen oxide contents in exhaust gas, especially at low temperatures.
The H₂generation step is preferably realized as a POX reactor (POX=partial oxidation). By the appropriate selection of the operating conditions of the POX reactor, the composition of the product gas may be purposefully set, so that for example a product gas rich in H₂, or a product gas rich in cracked short-chain hydrocarbons is obtained. Since short-chain hydrocarbons or hydrogen are or is more effective with respect to NO_xreducing than long-chain hydrocarbons, especially at low temperatures, a mineral oil used as fuel for the internal combustion engine may be turned into a more efficient reducing agent for nitrogen oxides by means of such a reactor. Furthermore, the various temperature ranges of the effectiveness of the reducing agents producible by the reducing agent-generating unit may also be utilized, and the composition of the reducing gas may be adapted to the temperature of the NO_xreducing catalytic converters. An NO_xreduction is thereby made possible in a wide temperature range.
The H₂yield of the H₂generation may be increased by including a water gas shift reaction (CO+H₂O ---->CO₂+H₂), or by a steam-reforming reaction (HC+H₂O ---->CO₂+H₂). These may also take place in the H₂generation step, or in a separate a reaction step preferably downstream from the generation step. The water necessary for the run of the water gas shift reaction, or the steam reforming reaction may be added to the relevant educt gas. If exhaust gas is added to the reducing agent-generating unit, the precondition for the run of the water gas shift reaction, or the steam reforming reaction, is already a given because of its water content.
Because of the easy oxidizability of the H₂produced by the reducing agent-generating unit, in addition a quick heating up of catalytic converters in the exhaust gas string can be achieved. By adding the produced H₂input of an exhaust gas catalyst, a rapid start-up of this catalytic converter may be achieved, which is of special importance in the reduction of pollutant emission at cold start. Similarly, catalytic converters may be effectively operated under thermally unfavorable conditions, such as in an underfloor position of a motor vehicle.
With the largely self-sufficiently operating reducing agent-generating unit, which is largely independent of the operation of the internal combustion engine, the reducing agent can thereby be made available on board of the associated motor vehicle and utilized for pollutant reduction. Since the reducing agent-generating unit can only be fed by the fuels, which are available on board of the motor vehicle anyhow, additional fuels and their storage become, or intermediate storage becomes, superfluous. Furthermore, the necessity is largely eliminated of converting the operation of the internal combustion engine for providing reducing agents for the NO_xreduction, e.g. to a rich combustion, which is associated with difficulties especially in Diesel engines. Overall, therefore, a reduction of the pollutant contents in exhaust gas is made possible in a constructionally simple manner, which is largely independent of the operation of the internal combustion engine.
In a refinement of the invention, the NO_xgeneration step is downstream from the H₂generation step. This arrangement may offer benefits in the operation of the reducing agent generation. For example, the reducing gas flowing out of the upstream H₂generation step in a hot state may cool down when passing through the NO_xgeneration step, so that the subsequent components are not being stressed too much thermally.
In a further refinement of the invention, the NO_xgeneration step is upstream of the H₂generation step. This arrangement may also offer benefits in the operation of the reducing agent generation. For example, the gas flowing out of the upstream NO_xgeneration step may be utilized for controlling the process taking place in the downstream H₂generation step.
In a further refinement of the invention an NH₃generation step is arranged downstream from the NO_xgeneration step. The NH₃generation step serves the on-board generation of NH₃, preferably adjusted to the requirements, so that this reducing agent does not have to be carried in a storage container for NO_xreduction.
In a further refinement of the invention, a fractionating unit is arranged to the reducing agent-generating unit in such a way that low-boiling components of a fuel used for operating the internal combustion engine are separable from the fractionating unit, which can be supplied to the H₂generation step. This embodiment has the advantage that a reducing agent-generating unit realized, for example, as POX reactor, is easier to operate. The low-boiling components separated from the fuel are better and more completely cracked by the POX reactor. Furthermore, the formation of soot and condensation problems in the POX reactor may largely be prevented and the operating temperatures can be reduced. The degree of efficiency and the H₂yield of the partial hydrocarbon oxidation can also be improved. Furthermore, by the fractioning largely sweet fuel components may be separated. The H₂generation step is therefore only supplied with low-boiling fuel components, which are free of sulphur, so that sulphur poisoning is minimized.
In a further refinement of the invention, the NO_xgeneration step is operable in two operating modes with the H₂generation step, such that in the first operating mode of the NO_xgeneration step, an NO_x-containing gas can be produced, and in the second operating mode an H₂-containing gas and NH₃-containing gas can be produced by the reducing agent-generating unit. This refinement allows an operation of the reducing agent-generating unit advantageously according to the need. In periods of time during which the component of the reduction gas is not needed, the operation of respective generation unit may be stopped.
In a further refinement of the invention an NO_xintermediate storage unit is arranged downstream from the NO_xgeneration step. In this embodiment, the NO_xgeneration step may also be operated with a low degree of efficiency. The NO_xavailable only in small concentrations in the product gas is accumulated in the NO_xintermediate storage unit, and after a certain time is available in large amounts for the reaction into NH₃.
In a further refinement of the invention, the NO_xintermediate storage unit is designed for the reaction of stored NO_xwith H₂into NH₃. With this double function of NO_xstorage and formation of NH₃, the reducing agent-generating unit can have an especially compact design. In particular a NO_xstorage catalytic converter may be used as an intermediate NO_xstorage unit. An NO_xstorage catalytic converter optimized, for example, by an increased rhodium content, with respect to the function of NO_xformation can be used within the framework of the invention.
In a further refinement of the invention, the H₂generation step is realized for the reaction of the NO_xsupplied into NH₃. If the H₂generation step is simultaneously supplied with NO_xfrom the upstream arranged NO_xgeneration step, and HC-containing fuel supplied thereto, in one single, preferably catalytic operation, H₂as well as NH₃can be produced. In the partial hydrocarbon oxidation taking place under reducing conditions, the reduction of NO_xinto NH₃is preferred due to thermodynamic reasons, because of which this reduction step can be put together in an advantageous fashion in one operation with the H₂generation. Thus, an NH₃and H₂-containing reducing gas is produced in one operation. Hereby, the generation of the NH₃and H₂-containing reducing gas is done preferably continuously.
In a further refinement of the invention, the NO_xgeneration step is designed for the generation of NO_xfrom air and/or oxygen-containing exhaust gas. Preferably, NO_xis produced in the NO_xgeneration step in a plasma process by an electric arc, or by a corona discharge. In conjunction with the downstream arranged reduction of NO_xthereby NH₃is exclusively produced from components of the air and the carried fuel, and therefore the storage of an NH₃releasing substance, such as urea, may be eliminated.
In a further refinement of the invention, the reducing NO_xcatalytic converter has a denox catalytic converter step for the reaction of NO_xwith H₂, and an SCR catalytic converter step for the reaction of NO_xwith NH₃. In both cases, the NO_xreduction may take place in lean exhaust gas conditions, which is the reason why the internal combustion engine is preferably operated continuously lean, and so that the full consumption benefit of the lean operation can be taken advantage of. In addition it is possible to benefit from the various temperature ranges of the efficiency of the two catalytic converter steps, which is why an efficient nitrogen oxide reduction can be achieved in a wide temperature range.
The procedure according to the invention is characterized by the following operations:
a) Generation of an NO_x-containing gas by one of the NO_xgeneration steps allocated to the reducing agent generating unit from the air and/or exhaust gas supplied to the generation step;
b) Intermediate storage of NO_xduring the passage of the NO_x-containing gas produced during operation a by an NO_xintermediate storage allocated downstream to the NO_xgeneration step, and arranged to the reducing agent-generating unit;
c) Generation of an H₂-containing gas by an H₂generation step allocated to the reducing agent-generating unit and arranged upstream from the NO_xintermediate storage unit from fuel, as well as air and/or exhaust gas supplied to the H₂generation step;
d) Reaction of NO_xstored in the NO_xintermediate storage unit with the gas produced in generation step cc into NH₃, so that an H₂-containing, and an NH₃-containing reducing gas is produced, whereby the operations a, and b are alternately performed with operations c and d.
With process control according the invention, the NO_xgeneration and NO_xintermediate storage unit takes place alternately with the generation of an H₂-containing, reducing gas, release of the intermediately stored NO_xand its reduction to NH₃. An H₂/NH₃-containing reduction gas is added intermittently to the reducing catalytic converter. Since, however, preferably an NH₃storing SCR catalytic converter is employed as NO_xreducing catalytic converter, this can nevertheless continuously reduce NO_xcontained in exhaust gas, since in the operating phases, in which no NH₃is added to the catalytic converter, NH₃added in the preceding operating phase and stored is used for NO_xreduction. By the NO_xintermediate storage unit in the operating phases of the NO_xgeneration, an enrichment of the produced NO_xtakes place, which when reducing gas with reducing composition is added, is released again in higher concentration from the NO_xintermediate storage unit, and turned into NH₃. Therefore, the reducing agent NH₃can be added to the NO_xreducing catalytic converter at a comparatively high concentration.
In refinement of the procedure the NO_xreaction into NH₃takes place in an NH₃generation step, which is arranged to the reducing agent-generating unit, and arranged downstream to the NO_xintermediate storage unit. Preferably the NH₃generation step contains an NH₃formation catalytic converter which catalyzes the reductive reaction of NO_xinto NH₃. A catalytic converter with a very high efficiency with respect to NH₃formation is for example described in the unpublished German patent application 10214686.1. If NO_xand H₂-containing reducing gas is added to such a catalytic converter, a reaction results from NO_xto NH₃from a high percentage.
In a further refinement of the procedure, the intermediate storage of NO_x, and the NO_xreaction to NH₃is performed by means of a catalytic NO_xintermediate storage. The catalytic NO_xintermediate storage preferably has an NH₃formation function in such a way that stored NO_xis reduced to NH₃at least partially under reducing, or stoichiometrical conditions. Such a behavior is shown for example by NO_xstorage catalytic converters, which are preferably employed here. By the functions of the NO_xintermediate storage, and the NH₃formation integrated into a catalytic component, a compact construction of the reducing agent-generating unit can be achieved.
A further procedure according to the invention is characterized by the following operations:
a) Generation of an NO_x-containing gas of an NO_xgeneration step allocated to the reducing agent-generating unit, from air and/or exhaust supplied to the NO_xgeneration step;
b) Generation of an H₂-containing and NH₃-containing reducing gas by an H₂generation step, allocated to the reducing agent-generating unit and arranged downstream from the NO_xgeneration step, from the NO_X-containing gas supplied to the H₂generation step, fuel supplied, as well as air supplied and/or exhaust gas supplied.
By means of this process control in accordance with the invention, a continuous generation of an H₂-containing and NH₃-containing reducing gas, and its supply to the NO_xreducing catalytic converter takes place. Preferably, the H₂generation step has a catalytic converter with NH₃formation function, and by the H₂generation step an H₂-containing and NH₃-containing reducing gas is produced. The H₂elimination reaction from the HC-containing fuel in the H₂generation step, and the reduction of the added NO_xto NH₃is performed in the same procedure. Thus an H₂and NH₃-containing reducing gas is generated in one operation. This advantageously simplifies the realization of the reducing agent-generating unit.
In refinement of the procedures according to the invention in one of the fractioning units arranged to the reducing agent generation unit, a fuel enriched in low-boiling components is produced, which is added to the reducing agent generation unit for the generation of reducing gas. The H₂generation step is thereby supplied with low-molecular hydrocarbons, whereby its educt gas stream is better homogenized, a change into coke is prevented, and the H₂yield is increased.
In refinement of the procedures according to the invention the NO_xreducing catalytic converter is divided into a denox catalytic converter step for the reaction of NO_xwith H₂, and into an SCR catalytic converter step for the reaction of NO_xwith NH₃, and depending upon its composition, the reducing gas is supplied to the exhaust gas on the input side to the SCR catalytic converter step (3 a), or on the input side to the denox catalytic converter step (3 b). Since the catalytic converter steps have different temperature ranges for their effectiveness, and the HC, H₂, NH₃generated by the reducing agent-generating unit have their optimal effectiveness at various temperatures, a high NO_xreduction in exhaust gas can thereby be achieved in a broad temperature range. The denox catalytic converter step may also be designed for the reduction of NO_xwith HC, which expands the application area of the reducing gas.
In further refinement of the procedure, the amount and/or the composition of the reducing gas generated by the reducing gas generation unit is set as a function of the operating state of the internal combustion engine. Preferably, the reducing agent generation unit provides more reducing gas at high-volume NO_xemissions of the internal combustion engine than at low. The procedure is preferably controlled in such a way that the effect of the relevant NO_xreducing catalytic converter brings the optimal benefit. At low charge of the internal combustion engine, or low exhaust gas temperature, preferably a reducing gas rich in H₂is produced. At a higher load of the internal combustion engine, or a higher exhaust gas temperature, the reducing agent-generating unit is preferably operated in such a way that the reducing gas contains more NH₃. Thereby in turn an SCR catalytic converter with higher NO_xconversion is operated in the exhaust gas train of the internal combustion engine.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained below in greater detail on the basis of drawings and related examples.
FIG. 1 is a schematic block diagram of an internal combustion engine with reducing agent-generating unit;
FIG. 2 is a schematic block diagram of a construction of a reducing agent-generating unit;
FIG. 3 is a schematic block diagram of the construction of a further embodiment of the reducing agent-generating unit;
FIG. 4 is a schematic block diagram of the construction of a further embodiment of the reducing agent-generating unit;
FIG. 5 is a schematic block diagram of the construction of a further embodiment of the reducing agent-generating unit; and
FIG. 6 is a schematic block diagram of the construction of an embodiment of the reducing agent-generating unit with an allocated fractioning unit.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1, an internal combustion engine 1 realized as a four-cylinder Diesel engine is shown here by way of example. In the following, simply the term motor will be used. The exhaust gas produced in the combustion process is conducted to the environment via the exhaust gas line 2. In exhaust gas line 2 here, by way of example the catalytic converter steps 3 a, and 3 b comprising the NO_xreducing catalytic converter 3 is arranged.
The catalytic converter step 3 a is realized as SCR catalytic converter, by means of which with NH₃as reducing agent an NO_xreduction takes place under lean exhaust gas conditions. An SCR catalytic converter on a V₂O₅/WO₃/TiO₂basis as full extrudate, or another catalytic converter suitable for the NO_xreduction with NH₃may be employed. The temperature range of the effectiveness of the catalytic converter step 3 a normally is in the range between 200° C. and 400° C.
The catalytic converter step 3 b is realized as denox catalytic converter, by means of which an NO_xreduction takes place under lean exhaust gas conditions with H₂and/or HC as a reducing agent. Preferably a precious metal-containing catalytic converter is employed, but a Cu substituted zeolite, or another catalytic converter suitable for the NO_xreduction with H₂or HC may also be employed. The temperature range of the effectiveness of the catalytic converter step 3 b, with H₂as reducing agent, normally is in the range between 80° C. and 200° C., with HC as reducing agent between 180° C. and 400° C.
The reducing agent-generating unit 20 serves for the generation of the reducing agent H₂and/or NH₃. For this purpose, it may be supplied with fuel, or air, or exhaust gas via the fuel supply line 9, or via the gas supply line 10 as needed. The reducing agent-generating unit 20 has a controlled heating system (not shown), which is mainly operated for start-up. The reducing gas produced by the reducing agent-generating unit 20, may be added to the exhaust gas on the input side of the catalytic converter steps 3 a, 3 b, via the addition line 8, and the addition locations 4, 5.
The operation of the motor, and the operation of the reducing agent-generating unit 20 is controlled by a motor control unit 6, which, for this purpose is connected with the motor 1, or with the reducing agent-generating unit 20 via control lines 7.
Of course, additional components may be arranged to the motor 1, or the exhaust gas system, which are not shown here for reasons of clear grouping. These may especially consist of further catalytic pollution abatement units, a particle filter, and sensors in the exhaust gas line 2, as well as the usual additional motor components such as injection system, exhaust gas turbocharger, components for the exhaust gas return, etc. Also not shown are switchable, or adjustable locking mechanisms in the addition line 8, and the addition lines 9, 10.
The reducing agent unit 20 is now operated in such a way that reducing gas is produced as a function of the NO_xemission of the motor 1, and is added to the exhaust gas on the input side of the catalytic converter steps/step 3 a and/or 3 b. For this, in the motor control unit 6, there exist, for example, performance characteristics, in which the NO_xemission is laid down as a function of the motor operating point. The procedure of the reducing gas generation is controlled and monitored by the motor control unit 6, whereby the motor control unit 6 has all the necessary information available regarding the reducing gas composition, and the operating status of the reducing agent-generating unit 20.
The motor control unit 6 controls the addition amounts of air, or exhaust gas, as well as fuel, and the operation of the reducing agent-generating unit 20 preferably in such a way, that with a low exhaust gas temperature, reducing gas is mainly supplied to the exhaust gas via the addition location 4 at the inlet of the denox catalytic converter 3 b. The reducing gas generation is hereby controlled in such a way, that the reducing gas predominantly contains H₂as reducing agent. Thereby, also at low exhaust gas temperatures, or at low temperatures of the catalytic converter step 3 b, an efficient NO_xreduction of the motor exhaust gas is achieved. The addition amount of the reducing gas thereby is set by the motor control unit 6 according to the NO_xcontents of the exhaust gas of the exhaust gas temperature, or the temperature of the catalytic converter step 3 b, as well as the H₂contents of the reducing gas. Preferably a molar ratio of about 3:1 of H₂:NO_xis set at the input side of the denox catalytic converter.
If, with increasing exhaust gas temperature, the catalytic converter step 3 b falls outside of the temperature range of its efficiency, the addition amount of predominantly H₂-containing reducing gas is reduced, or stopped at the addition location 4. At the same time, the operation at the reducing agent-generating unit is changed in such a way that this results in a higher NH₃portion, and the reducing gas is added to the exhaust gas at addition location 5 at the input of catalytic converter step 3 a. Since with increasing exhaust gas temperature the SCR catalytic converter of the catalytic converter step 3 a becomes gradually more efficient, now the NO_xreduction takes place predominantly at this catalytic converter.
At a further continuous increase of the exhaust gas temperature, also the SCR catalytic converter of the catalytic converter step 3 a may come outside of the temperature range of its effectiveness. In this case, the reducing gas generation is changed in such a way that mainly a cracked fuel with short-chain hydrocarbons is produced from the reducing agent-generating unit 20. This reducing gas may then be conducted to the further downstream located, and therefore less hot, denox catalytic converter of catalytic converter step 3 b. At this catalytic converter, at temperatures of about 300° C., an NO_xreduction with these hydrocarbons takes place.
Further catalytic converters, which are not shown here, may be arranged in the exhaust gas line 2, which can also be supplied with reducing gas, as needed, via an appropriate addition location. In particular, H₂-containing reducing gas may be apportioned to the exhaust gas at the inlet of a primary catalytic converter arranged near the motor for reducing emissions during a cold start of the motor 1. A rapid heating of the primary catalytic converter can be achieved in this way. As a consequence, pollutants can be removed from the exhaust gas already in an early phase of the motor warm-up period.
FIG. 2 schematically shows the construction of the reducing agent-generating unit 20 in a preferred embodiment. The reducing agent-generating unit 20 here comprises an H₂generation step 21, an NO_x generation step 22, an NO_x intermediate storage unit 23, and an NH₃generation step 24 in serial arrangement. The H₂generation step 21 can be supplied with fuel at measured volumes via the fuel supply line 9, and air, and/or exhaust gas via the gas supply line 10. The NO_xgeneration step 22 can also be supplied with air, and/or exhaust gas at measured volumes via the gas supply line 10. The means used for measuring the volume are not shown here.
The embodiment of the reducing agent-generating unit 20 represented is preferably operated alternately, so that alternately NH₃and/or H₂, or NO_xare being produced. For producing H₂, the H₂generation step 21 realized as a catalytic POX reactor, if needed, is first of all warmed-up to operating temperature by means of a not shown electric heater. Hereby, the operating temperature of the catalytic converter arranged in the POX reactor is about 600° C. to 1000° C. Afterwards, the POX reactor is supplied with fuel, and air, or exhaust gas in a weighted flow determined by the motor control unit. Thereby, a air/fuel ratio of preferably about λ =0.3 is set. At this lambda value the partial fuel oxidation in the POX reactor is practically free of soot. The reaction product consists of a reducing gas with a composition, which is highly dependent on the course of procedure, i.e. mainly on the temperature of the POX catalytic converters, and on the air/fuel ratio set. Typical content levels of the reducing agent H₂or CO are about 18%. The reducing gas may additionally contain a certain content of low-molecular hydrocarbons.
This reducing gas is now conducted through the NO_xgeneration step 22, which is not in operation during this phase of the reducing gas generation. After passing it through the NO_xgeneration step 22, the reducing gas flows through the NO_x intermediate storage unit 23, which contains an NO_xadsorber. The NO_xadsorber may for example be a ceramic honeycomb body, which is coated with a material, which in oxidizing conditions absorbs NO_xby adsorption, or absorption, and which releases NO_xagain in reducing conditions. For this, an NO_xadsorber material on a silver basis is suitable, for example. If the NO_xintermediate storage unit was loaded with NO_xbefore the reducing gas produced in the H₂generation step 21 flows through, it is consequently released. The reducing gas enriched with NO_xis further conducted to the NH₃generation step 24. This preferably contains a catalytic converter with a precious metal coating. The reduction of NO_xto NH₃is catalyzed by this catalytic converter, so that finally an NH₃and H₂-containing reducing gas leaves the reducing agent-generating unit 20 via the reducing gas line 8, and is added at one, or both of the addition locations 4, 5 to the exhaust gas of the motor 1 (see FIG. 1).
If the NH₃formation comes to a halt, for example by exhaustion of the amount of NO_xstored in the NO_xintermediate storage unit, if needed, the operation of the reducing agent-generating unit 20 for producing NO_xcan be switched. For this purpose, the supply of air, or exhaust gas, as well as the supply with fuel to the H₂generation step 21 is stopped. Subsequently, the NO_xgeneration step 22 is supplied with air and/or oxygen-containing exhaust gas. In the NO_x generation step 22 thereupon for example a plasma process is started, or an electric arc, or a corona discharge is ignited. By such a process NO_xis produced in the nitrogen and oxygen-containing atmosphere of the NO_xgeneration unit. Preferably an NTP procedure (NTP=non-thermal plasma) is started, and maintained for the desired time of the NO_xgeneration. The NO_xproduced by means of this process has a high NO₂portion of normally over 50%, which improves the subsequent storage after the supply of the generation gas to the NO_x intermediate storage unit 23. As described above, in this situation the supplied NO_xis to be absorbed by adsorption, or by absorption. The NO_xfree gas is further conducted through the NH₃generation step, from where it is added essentially unchanged to the exhaust gas of the motor 1 via the reducing gas line 8 at one of the addition locations 4, 5 (see FIG. 1). If the NO_x intermediate storage unit 23 is saturated with NO_x, or if the supply of reducing gas to a catalytic converter step is required for other reasons, the generation of NO_xis stopped, and the generation of reducing gas with a reducing effect as described above again takes place.
FIG. 3 schematically illustrates the construction of the reducing agent-generating unit 20 in a further preferred embodiment. The designation and the function of the of the components with the same effectiveness, corresponds to FIG. 2. As compared to the embodiment shown in FIG. 2, the reducing agent-generating unit 20 shown in FIG. 3 differs in the exchange of the H₂generation step 21, and the NO_xgeneration step 22. Also the embodiment of the reducing agent-generating unit shown in FIG. 3 is preferably operated alternately such, that alternately H₂and/or NH₃are produced, or NO_xis produced. In the operating phases with NO_xgeneration, however, in turn the H₂generation step 21 is not in operation, and the NO_x-containing product gas of the NO_xgeneration step 22 flows through the H₂generation step 21. Thereby, in the warmed-up H₂generation step 21 a regeneration of coking possibly caused by the cracking process may occur, which improves the operatability of the POX catalytic converter. Analogous to the function of the embodiment shown in FIG. 2, NO_xis withdrawn from the gas in the NO_x intermediate storage unit 23 by storage. After switching the operation of the reducing agent-generating unit 20 by terminating the NO_xgeneration, and starting operation of the H₂generation step 21, reducing H₂-containing reducing gas flows through the NO_x intermediate storage unit 23, whereby the release of the stored NO_xtakes place, as described above. In the downstream arranged NH₃generation step 24 thereupon a reaction of the stored NO_xinto NH₃takes place, and the reducing gas is enriched with NH₃. The produced reducing gas is, as described above, fed as needed at the inlet of a catalytic converter step into the exhaust gas line.
FIG. 4 schematically shows the construction of a reducing agent-generating unit 20 in a further preferred embodiment. Analogous to the embodiments in FIG. 2, and FIG. 3, this variation is also operated alternately. The differences in construction and in their function as compared to the embodiments in FIGS. 2 and 3 are described in the following. To the NO_x generation step 22, here an NO_xstorage catalytic converter step 26 is arranged downstream, which at the same time fulfills the function of the NO_xintermediate storage unit and the NH₃formation. In the supply of reducing reducing gas from the H₂generation step 21 to the NO_xstorage catalytic converter step 26, the NO_xwhich is stored there gets released and at the same time NH₃gets reduced. This effect, which occurs in the usual NO_xstorage catalytic converters, can be utilized advantageously. Preferably in this step 26 an NO_xstorage material is employed, which, with respect to the NH₃formation is for example optimized by an increased rhodium content. A further simplification is achieved in that the gas necessary for the generation of NO_xis supplied to the NO_xgeneration step 22 via the H₂generation step 21; therefore a branching of the gas supply line 10 can be eliminated. In comparison to the embodiments in FIG. 2 and FIG. 3 components can be eliminated hereby, whereby the reducing agent-generating unit 20 can be realized in a simpler way.
FIG. 5 schematically shows the construction of the reducing agent-generating unit 20 in a further preferred embodiment. As explained in the following, this embodiment can be operated on a continuous basis, i.e. the NO_xgeneration and NH₃generation can take place simultaneously. For this purpose a reactor step 25 is provided, in which simultaneously an H₂generation, preferably by a catalytic partial oxidation process, and a reduction of the NO_xsupplied to the NO_xgeneration step 22, can take place. The NO_xgeneration thereby is performed as described in the above embodiments. The necessary educt gas is supplied to the NO_xgeneration step 22 via the gas supply line 10 as needed. The gas supply line additionally may have a branch for reactor step 25. The reactor step 25 is supplied with hydrocarbon-containing fuel via the supply line 9. Preferably in the operation of reactor step 25 an air/fuel ratio of about λ=0.3 is set there. In this way, the thermodynamically preferred conditions are present, on the one hand for the H₂generation by partial oxidation, and on the other hand for the reduction of NO_xto NH₃, so that H₂as well as NH₃can be produced in one process. The volume ratio of these two components thereby can be set as needed by the appropriate setting of fuel, or NO_xsupplied to reactor step 25.
FIG. 6 schematically shows the construction of the reducing agent-generating unit 20 in a further preferred embodiment. The reducing agent-generating unit 20 contains, analogous to the embodiment shown in FIG. 4, an H₂generation step 21 arranged downstream to the NO_xgeneration step 22. The furthermore downstream arranged NO_xstorage catalytic converter step 26 serves for the NO_xintermediate storage and NH₃generation. The system is preferably operated in altering generation phases, and H₂, or NH₃generation phases. For the generation of NO_x, the NO_xgeneration 22 is supplied with air and/or exhaust gas via the gas supply line 10. Via a branch-off, the H₂generation step 21 can be supplied with air and/or exhaust gas as well. To the reducing agent-generating unit 20 furthermore a fractioning unit 27 is arranged to, in which from the fuel provided for the motor operation, low-boiling components are separated off in a fractioning process. For this purpose the fractioning unit 27 is supplied with fuel from the storage container of a motor not shown here, via the fuel supply line 9. Via the return line 9 b the fuel enriched in low-boiling components is returned again. The separated low-boiling fuel components are conducted to the H₂generation step 21 for H₂generation via the fuel supply line 9 a. By the fact, that the H₂generation step 21 is operated with low-boiling hydrocarbons, the H₂generation can take place with a higher yield, and the danger of coking of the POX catalytic converter is reduced. In addition, the possibility exists for preparing the low-boiling fuel components when needed only at low levels, and to add to the lean exhaust gas on the input side of a denox catalytic converter. Therefore, in the temperature range of about 200° C. to 350° C. an efficient NO_xreduction may be achieved. To increase the H₂yield in the generation of reducing gas, the reducing agent-generating unit 20 can generally be supplied with additional water. The water can thereby be taken from a storage container of the associated motor vehicle, or be obtained by condensation from the motor exhaust gas. The water may, for example be utilized for a steam reforming process in the H₂generation step 21. Apart from the increase of the H₂content in the produced reducing gas this brings the additional advantage, that this endothermal process improves the heat and energy balance of the H₂generation. Furthermore a larger range for temperature management in the H₂generation step 21, and the complete reducing agent-generating unit 20 is gained. The water may, however, also be supplied to a water gas shift step arranged downstream from the H₂generation step 21. The thereby with H₂enriched reducing gas has a higher selectivity in the NH₃generation, and in the NO_xreduction at a denox catalytic converter. At the same time the CO contents is reduced in the reducing gas, which prevents a poisoning of the denox catalytic converter by CO accumulation of active catalytic centers.
It is obvious that to the reducing agent-generating unit 20 heat exchangers may be arranged to, in order to improve the heat and energy balance of the total process, and the course of the procedure. Thus, for preheating of the supplied air, or the supplied exhaust gas for example, a heat exchanger may be provided in the gas supply line 10. Heat exchangers may, however, also be provided in the reducing agent-generating unit 20, for example in order to utilize the heat content of the hot product gas flow of the H₂generation step, or the NH₃generation step for preheating of the educt gas of a preliminary step.

Claims

1-18. (canceled)

19. An internal combustion engine comprising:

an exhaust gas line in which an NO_xreduction catalytic converter is arranged, and

a reducing agent-generating unit for generation of H₂-containing and NH₃-containing reducing gas which can be added upstream of the NO_xreduction catalytic converter in the exhaust gas line,

wherein the reducing agent-generating unit can be supplied with at least one of an HC-containing fuel, air, and exhaust gas, and

wherein the reducing agent-generating unit has an NO_xgeneration step and an H₂generation step in serial arrangement.

20. The internal combustion engine according to claim 19, wherein the NO_xgeneration step is arranged downstream from the H₂generation step.

21. The internal combustion engine according to claim 19, wherein the NO_xgeneration step is arranged upstream from the H₂generation step.

22. The internal combustion engine according to claim 19, further comprising an NH₃generation step arranged downstream from the NO_Xgeneration step.

23. The internal combustion engine according to claim 20, further comprising an NH₃generation step arranged downstream from the NO_Xgeneration step.

24. The internal combustion engine according to claim 21, further comprising an NH₃generation step arranged downstream from the NO_Xgeneration step.

25. The internal combustion engine according to claim 19, wherein the reducing agent-generating unit can be operated alternately in first and second operating modes in such a way that, during the first operating mode, an NO_X-containing gas can be produced and, during the second operating mode, an H₂-containing and NH₃-containing reducing gas can be produced.

26. The internal combustion engine according to claim 22, wherein a reducing agent-generating unit can be operated alternately in first and second operating modes in such a way that, in the first operating mode, an NO_X-containing gas can be produced and, in the second operating mode, an H₂-containing and NH₃-containing reducing gas can be produced.

27. The internal combustion engine according to claim 23, wherein a reducing agent-generating unit can be operated alternately in first and second operating modes in such a way that, in the first operating mode, an NO_X-containing gas can be produced and, in the second operating mode, an H₂-containing and NH₃-containing reducing gas can be produced.

28. The internal combustion engine according to claim 24, wherein a reducing agent-generating unit can be operated alternately in first and second operating modes in such a way that in the first operating mode of the NO_Xgeneration step, an NO_X-containing gas can be produced and, in the second operating mode, an H₂-containing and NH₃-containing reducing gas can be produced.

29. The internal combustion engine according to claim 25, further comprising an NO_Xintermediate storage unit arranged downstream from the NO_Xgeneration step.

30. The internal combustion engine according to claim 26, further comprising an NO_Xintermediate storage unit arranged downstream from the NO_Xgeneration step.

31. The internal combustion engine according to claim 27, further comprising an NO_Xintermediate storage unit arranged downstream from the NO_Xgeneration step.

32. The internal combustion engine according to claim 28, further comprising an NO_Xintermediate storage unit arranged downstream from the NO_Xgeneration step.

33. The internal combustion engine according to claim 29, wherein the NO_Xintermediate storage unit is designed for reaction of stored NO_Xwith H₂to NH₃.

34. The internal combustion engine according to claim 30, wherein the NO_Xintermediate storage unit is designed for reaction of stored NO_Xwith H₂to NH₃.

35. The internal combustion engine according to claim 31, wherein the NO_Xintermediate storage unit is designed for reaction of stored NO_Xwith H₂to NH₃.

36. The internal combustion engine according to claim 32, wherein the NO_Xintermediate storage unit is designed for reaction of stored NO_Xwith H₂to NH₃.

37. The internal combustion engine according to claim 21, wherein the H₂generation step is designed for reaction of supplied NO_Xinto NH₃.

38. The internal combustion engine according to claim 24, wherein the H₂generation step is designed for reaction of supplied NO_Xinto NH₃.

39. The internal combustion engine according to claim 28, wherein the H₂generation step is designed for reaction of supplied NO_Xinto NH₃.

40. The internal combustion engine according claim 32, wherein the H₂generation step is designed for reaction of supplied NO_Xinto NH₃.

41. The internal combustion engine according claim 36, wherein the H₂generation step is designed for reaction of supplied NO_Xinto NH₃.

42. The internal combustion engine according to claim 19, wherein the engine is a Diesel engine.

43. A process for operation of an internal combustion engine having a reducing agent-generating unit and an exhaust gas line in which an NO_Xreduction catalytic converter is arranged, whereby a reducing gas produced by the reducing agent-generating unit is added upstream of the NO_Xreducing catalytic converter to the exhaust gas, wherein generation of the reducing gas comprises:

generating an NO_X-containing gas from an NO_Xgeneration stage allocated to the reducing agent-generating unit from at least one of air and exhaust gas supplied to the NO_Xgeneration stage; and

intermediately storing NO_Xwhen conducting the NO_X-containing gas produced through an NO_Xintermediate storage unit which is arranged downstream from the NO_Xgeneration stage and allocated to the reducing agent-generating unit; or

generating an H₂-containing gas by an H₂generation stage allocated to the reducing agent-generating unit and arranged upstream from an NO_Xintermediate storage unit from fuel and air or exhaust gas supplied to the H₂generation stage; and

reacting NO_Xstored in the NO_Xintermediate storage unit with the gas produced into NH₃so that a reducing gas containing H₂and NH₃is produced.

44. The process according to claim 43, wherein reaction of NO_xinto NH₃takes place in the catalytic NH₃generation stage, which is allocated to the reducing agent generation unit and arranged downstream from the NO_Xintermediate storage unit.

45. The process according to claim 43, wherein intermediate storage of NO_Xand reaction of NO_Xinto NH₃is performed with a catalytic NO_Xintermediate storage unit.

46. The process according to claim 43, wherein the NO_Xreducing catalytic converter is divided into a denox catalytic converter stage for reaction of NO_Xwith H₂and an SCR catalytic converter stage for reaction of NO_Xwith NH₃, and wherein the reducing gas is supplied to the exhaust gas as a function of its composition at an input side to the SCR catalytic converter stage or on an input side to the denox catalytic converter stage.

47. The process according to claim 44, wherein the NO_Xreducing catalytic converter is divided into a denox catalytic converter stage for reaction of NO_Xwith H₂and an SCR catalytic converter stage for reaction of NO_Xwith NH₃, and wherein the reducing gas is supplied to the exhaust gas as a function of its composition at an input side to the SCR catalytic converter stage or on an input side to the denox catalytic converter stage.

48. The process according to claim 45, wherein the NO_Xreducing catalytic converter is divided into a denox catalytic converter stage for reaction of NO_Xwith H₂and an SCR catalytic converter stage for reaction of NO_Xwith NH₃, and wherein the reducing gas is supplied to the exhaust gas as a function of its composition at an input side to the SCR catalytic converter stage or on an input side to the denox catalytic converter stage.

49. A process for operation of an internal combustion engine having a reducing agent-generating unit and an exhaust gas line in which an NO_Xreduction catalytic converter is arranged, whereby a reducing gas produced by the reducing agent-generating unit is added upstream from the NO_Xreducing catalytic converter to the exhaust gas, wherein generation of the reducing gas comprises:

generating an H₂-containing gas and an NH₃-containing reducing gas from an H₂generation stage allocated to the reducing agent-generating unit and arranged downstream from the NO_Xgeneration stage based on fuel fed to the H₂generation stage, NO_X-containing gas produced, fuel supplied, and at least one of air and exhaust gas.

50. The process according to claim 49, wherein the NO_Xreducing catalytic converter is divided into a denox catalytic converter stage for reaction of NO_Xwith H₂and an SCR catalytic converter stage for reaction of NO_Xwith NH₃, and wherein the reducing gas is supplied to the exhaust gas as a function of its composition at an input side to the SCR catalytic converter stage or on an input side to the denox catalytic converter stage.