Patent Application
20060101809
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 NOx catalytic 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 H2 portion, and the NH3 portion 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 NOx generation step and an H2 generation 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 NOx produced by the NOx generation step may be reduced as required by reducing the H2 from the H2 generation 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 H2 produced by the H2 generation step may also be utilized for the catalytic reduction of the nitrogen oxide contents in exhaust gas, especially at low temperatures.
The H2 generation 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 NOx reducing 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 NOx reducing catalytic converters. An NOx reduction is thereby made possible in a wide temperature range.
The H2 yield of the H2 generation 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 H2 generation 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 H2 produced 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 H2 input 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 NOx reduction, 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 NOx generation step is downstream from the H2 generation step. This arrangement may offer benefits in the operation of the reducing agent generation. For example, the reducing gas flowing out of the upstream H2 generation step in a hot state may cool down when passing through the NOx generation step, so that the subsequent components are not being stressed too much thermally.
In a further refinement of the invention, the NOx generation step is upstream of the H2 generation step. This arrangement may also offer benefits in the operation of the reducing agent generation. For example, the gas flowing out of the upstream NOx generation step may be utilized for controlling the process taking place in the downstream H2 generation step.
In a further refinement of the invention an NH3 generation step is arranged downstream from the NOx generation step. The NH3 generation 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 NOx reduction.
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 H2 generation 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 H2 yield of the partial hydrocarbon oxidation can also be improved. Furthermore, by the fractioning largely sweet fuel components may be separated. The H2 generation 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 NOx generation step is operable in two operating modes with the H2 generation step, such that in the first operating mode of the NOx generation step, an NOx-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 NOx intermediate storage unit is arranged downstream from the NOx generation step. In this embodiment, the NOx generation step may also be operated with a low degree of efficiency. The NOx available only in small concentrations in the product gas is accumulated in the NOx intermediate 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 NOx intermediate storage unit is designed for the reaction of stored NOx with H2 into NH3. With this double function of NOx storage and formation of NH3, the reducing agent-generating unit can have an especially compact design. In particular a NOx storage catalytic converter may be used as an intermediate NOx storage unit. An NOx storage catalytic converter optimized, for example, by an increased rhodium content, with respect to the function of NOx formation can be used within the framework of the invention.
In a further refinement of the invention, the H2 generation step is realized for the reaction of the NOx supplied into NH3. If the H2 generation step is simultaneously supplied with NOx from the upstream arranged NOx generation step, and HC-containing fuel supplied thereto, in one single, preferably catalytic operation, H2 as well as NH3 can be produced. In the partial hydrocarbon oxidation taking place under reducing conditions, the reduction of NOx into NH3 is preferred due to thermodynamic reasons, because of which this reduction step can be put together in an advantageous fashion in one operation with the H2 generation. Thus, an NH3 and H2-containing reducing gas is produced in one operation. Hereby, the generation of the NH3 and H2-containing reducing gas is done preferably continuously.
In a further refinement of the invention, the NOx generation step is designed for the generation of NOx from air and/or oxygen-containing exhaust gas. Preferably, NOx is produced in the NOx generation step in a plasma process by an electric arc, or by a corona discharge. In conjunction with the downstream arranged reduction of NOx thereby NH3 is exclusively produced from components of the air and the carried fuel, and therefore the storage of an NH3 releasing substance, such as urea, may be eliminated.
In a further refinement of the invention, the reducing NOx catalytic converter has a denox catalytic converter step for the reaction of NOx with H2, and an SCR catalytic converter step for the reaction of NOx with NH3. In both cases, the NOx reduction 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 NOx-containing gas by one of the NOx generation steps allocated to the reducing agent generating unit from the air and/or exhaust gas supplied to the generation step;
b) Intermediate storage of NOx during the passage of the NOx-containing gas produced during operation a by an NOx intermediate storage allocated downstream to the NOx generation step, and arranged to the reducing agent-generating unit;
c) Generation of an H2-containing gas by an H2 generation step allocated to the reducing agent-generating unit and arranged upstream from the NOx intermediate storage unit from fuel, as well as air and/or exhaust gas supplied to the H2 generation step;
d) Reaction of NOx stored in the NOx intermediate 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 NOx generation and NOx intermediate storage unit takes place alternately with the generation of an H2-containing, reducing gas, release of the intermediately stored NOx and its reduction to NH3. An H2/NH3-containing reduction gas is added intermittently to the reducing catalytic converter. Since, however, preferably an NH3 storing SCR catalytic converter is employed as NOx reducing catalytic converter, this can nevertheless continuously reduce NOx contained in exhaust gas, since in the operating phases, in which no NH3 is added to the catalytic converter, NH3 added in the preceding operating phase and stored is used for NOx reduction. By the NOx intermediate storage unit in the operating phases of the NOx generation, an enrichment of the produced NOx takes place, which when reducing gas with reducing composition is added, is released again in higher concentration from the NOx intermediate storage unit, and turned into NH3. Therefore, the reducing agent NH3 can be added to the NOx reducing catalytic converter at a comparatively high concentration.
In refinement of the procedure the NOx reaction into NH3 takes place in an NH3 generation step, which is arranged to the reducing agent-generating unit, and arranged downstream to the NOx intermediate storage unit. Preferably the NH3 generation step contains an NH3 formation catalytic converter which catalyzes the reductive reaction of NOx into NH3. A catalytic converter with a very high efficiency with respect to NH3 formation is for example described in the unpublished German patent application 10214686.1. If NOx and H2-containing reducing gas is added to such a catalytic converter, a reaction results from NOx to NH3 from a high percentage.
In a further refinement of the procedure, the intermediate storage of NOx, and the NOx reaction to NH3 is performed by means of a catalytic NOx intermediate storage. The catalytic NOx intermediate storage preferably has an NH3 formation function in such a way that stored NOx is reduced to NH3 at least partially under reducing, or stoichiometrical conditions. Such a behavior is shown for example by NOx storage catalytic converters, which are preferably employed here. By the functions of the NOx intermediate storage, and the NH3 formation 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 NOx-containing gas of an NOx generation step allocated to the reducing agent-generating unit, from air and/or exhaust supplied to the NOx generation step;
b) Generation of an H2-containing and NH3-containing reducing gas by an H2 generation step, allocated to the reducing agent-generating unit and arranged downstream from the NOx generation step, from the NOX-containing gas supplied to the H2 generation 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 NOx reducing catalytic converter takes place. Preferably, the H2 generation step has a catalytic converter with NH3 formation function, and by the H2 generation step an H2-containing and NH3-containing reducing gas is produced. The H2 elimination reaction from the HC-containing fuel in the H2 generation step, and the reduction of the added NOx to NH3 is performed in the same procedure. Thus an H2 and 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 H2 generation step is thereby supplied with low-molecular hydrocarbons, whereby its educt gas stream is better homogenized, a change into coke is prevented, and the H2 yield is increased.
In refinement of the procedures according to the invention the NOx reducing catalytic converter is divided into a denox catalytic converter step for the reaction of NOx with H2, and into an SCR catalytic converter step for the reaction of NOx with 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 (3a), or on the input side to the denox catalytic converter step (3b). Since the catalytic converter steps have different temperature ranges for their effectiveness, and the HC, H2, NH3 generated by the reducing agent-generating unit have their optimal effectiveness at various temperatures, a high NOx reduction 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 NOx with 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 NOx emissions of the internal combustion engine than at low. The procedure is preferably controlled in such a way that the effect of the relevant NOx reducing 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 H2 is 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 NOx conversion is operated in the exhaust gas train of the internal combustion engine.
The invention is explained below in greater detail on the basis of drawings and related examples.
The catalytic converter step 3a is realized as SCR catalytic converter, by means of which with NH3 as reducing agent an NOx reduction takes place under lean exhaust gas conditions. An SCR catalytic converter on a V2O5/WO3/TiO2 basis as full extrudate, or another catalytic converter suitable for the NOx reduction with NH3 may be employed. The temperature range of the effectiveness of the catalytic converter step 3a normally is in the range between 200° C. and 400° C.
The catalytic converter step 3b is realized as denox catalytic converter, by means of which an NOx reduction takes place under lean exhaust gas conditions with H2 and/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 NOx reduction with H2 or HC may also be employed. The temperature range of the effectiveness of the catalytic converter step 3b, with H2 as 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 H2 and/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 3a, 3b, 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 NOx emission of the motor 1, and is added to the exhaust gas on the input side of the catalytic converter steps/step 3a and/or 3b. For this, in the motor control unit 6, there exist, for example, performance characteristics, in which the NOx emission 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 3b. The reducing gas generation is hereby controlled in such a way, that the reducing gas predominantly contains H2 as reducing agent. Thereby, also at low exhaust gas temperatures, or at low temperatures of the catalytic converter step 3b, an efficient NOx reduction 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 NOx contents of the exhaust gas of the exhaust gas temperature, or the temperature of the catalytic converter step 3b, as well as the H2 contents of the reducing gas. Preferably a molar ratio of about 3:1 of H2:NOx is set at the input side of the denox catalytic converter.
If, with increasing exhaust gas temperature, the catalytic converter step 3b 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 NH3 portion, and the reducing gas is added to the exhaust gas at addition location 5 at the input of catalytic converter step 3a. Since with increasing exhaust gas temperature the SCR catalytic converter of the catalytic converter step 3a becomes gradually more efficient, now the NOx reduction 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 3a 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 3b. At this catalytic converter, at temperatures of about 300° C., an NOx reduction 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.
The embodiment of the reducing agent-generating unit 20 represented is preferably operated alternately, so that alternately NH3 and/or H2, or NOx are being produced. For producing H2, the H2 generation 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 H2 or 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 NOx generation step 22, which is not in operation during this phase of the reducing gas generation. After passing it through the NOx generation step 22, the reducing gas flows through the NOx intermediate storage unit 23, which contains an NOx adsorber. The NOx adsorber may for example be a ceramic honeycomb body, which is coated with a material, which in oxidizing conditions absorbs NOx by adsorption, or absorption, and which releases NOx again in reducing conditions. For this, an NOx adsorber material on a silver basis is suitable, for example. If the NOx intermediate storage unit was loaded with NOx before the reducing gas produced in the H2 generation step 21 flows through, it is consequently released. The reducing gas enriched with NOx is further conducted to the NH3 generation step 24. This preferably contains a catalytic converter with a precious metal coating. The reduction of NOx to NH3 is catalyzed by this catalytic converter, so that finally an NH3 and 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
If the NH3 formation comes to a halt, for example by exhaustion of the amount of NOx stored in the NOx intermediate storage unit, if needed, the operation of the reducing agent-generating unit 20 for producing NOx can be switched. For this purpose, the supply of air, or exhaust gas, as well as the supply with fuel to the H2 generation step 21 is stopped. Subsequently, the NOx generation step 22 is supplied with air and/or oxygen-containing exhaust gas. In the NOx 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 NOx is produced in the nitrogen and oxygen-containing atmosphere of the NOx generation unit. Preferably an NTP procedure (NTP=non-thermal plasma) is started, and maintained for the desired time of the NOx generation. The NOx produced by means of this process has a high NO2 portion of normally over 50%, which improves the subsequent storage after the supply of the generation gas to the NOx intermediate storage unit 23. As described above, in this situation the supplied NOx is to be absorbed by adsorption, or by absorption. The NOx free gas is further conducted through the NH3 generation 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
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 H2 generation step, or the NH3 generation step for preheating of the educt gas of a preliminary step.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10237777.4 | Aug 2002 | DE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP03/08414 | 7/30/2003 | WO | 8/11/2005 |
