Patent Application
20150252706
The invention relates to an exhaust-gas aftertreatment device for an aftertreatment of an exhaust-gas of an internal combustion engine and a motor vehicle with such an exhaust-gas aftertreatment device, wherein the exhaust-gas aftertreatment device operates in accordance with the principle of the selective catalytic reduction.
Internal combustion engines that are constantly or occasionally operated with a lean air-fuel mixture produce nitrogen oxides NOx (mainly NO2 and NO), which necessitate NOx-reducing measures. A measure performed by the engine in order to reduce the NOx untreated emission in the exhaust-gas is the exhaust-gas recirculation, in which a part of the exhaust-gas of the internal combustion engine is recirculated into the combustion air, whereby the combustion temperatures are lowered and thus the NOx formation is reduced. However, the exhaust-gas recirculation is not always sufficient to comply with legal NOx limit values, which is why an active exhaust-gas aftertreatment is additionally required, which reduces the NOx final emission by catalytic reduction of NOx to nitrogen N2. A known NOx exhaust-gas aftertreatment provides for the use of NOx storage catalytic converters which store nitrogen oxides in the form of nitrates during lean operation (at λ>1) and, during short intervals with a rich exhaust-gas atmosphere (λ<1), desorb the stored nitrogen oxides and reduce them to nitrogen N2 in the presence of reducing agents, which are present in the rich exhaust-gas.
As a further approach for converting nitrogen oxides in exhaust-gases of combustion engines that are capable of a lean-burning operation, the use of catalytic converter systems is known which operate in accordance with the principle of the selective catalytic reduction (SCR selective catalytic reduction). These systems include at least an SCR catalytic converter which selectively converts, in the presence of a reducing agent, usually ammonia NH3, which is fed to the exhaust-gas, the nitrogen oxides of the exhaust-gas into nitrogen and water. In this case, the ammonia can be added in a metered manner from an aqueous ammonia solution to the exhaust-gas flow or can be obtained by way of thermolysis and hydrolysis from a precursor compound, for example urea in the form of an aqueous solution or solid pellets. A more recent approach to storing ammonia in the vehicle includes NH3 storage materials that reversibly bind ammonia as a function of the temperature. In particular metal amine storage systems are known in this context, for example, MgCl2, CaCl2 and SrCl2, which store ammonia in the form of a complex compound, in order to then be available for example as MgCl2(NH3)x, CaCl2(NH3)x or SrCl2(NH3)x. The ammonia can be released again from these compounds by the application of heat.
Also known are configurations in which an SCR catalytic converter is disposed downstream of a particulate filter, in many cases at an underfloor position of the vehicle. As is the case with all exhaust-gas catalytic converters, SCR catalytic converters too require a specific light-off temperature in order to provide a sufficient conversion performance. Depending on the coating of the SCR catalytic converter, exhaust-gas temperatures in the catalytic converter of at least 150° C. are typically necessary. Therefore underfloor SCR catalytic converters often require additional heating measures, which result in an undesirable increase in fuel consumption and, consequently, an increase in CO2 emissions.
In order to minimize the loss of temperature, a positioning of the SCR catalytic converter close to the engine is also known, in particular by integrating an SCR catalytic coating in the particulate filter. Such an SCR catalytically coated particulate filter (also called SCR particulate filter, SCR/PF, or SPF) thus combines the functions of trapping soot particles and catalytic reduction of nitrogen oxides under selective consumption of the reducing agent. Due to the positioning of the SCR catalytic converter or, respectively, the SCR/PF close to the engine, a rapid heating of this component to its operating temperature is achieved. This allows an early releasing of the reducing agent metering and thus an improved NOx conversion in the overall driving cycle. However, the temperature gradient across the particulate filter substrate compared to that of the flow-through substrate is greater because of the greater substrate length of the filter, which has an adverse effect on the NOx conversion. In individual cases, the substrate temperature in the rear area of the particulate filter can be so low that only low conversion rates are achieved in that area. Furthermore, the coating of the particulate filter substrate with the SCR catalytic material, in contrast to the coating of a flow-through substrate (honeycomb body), is limited to smaller amounts of coating, so that the exhaust-gas back pressure across the particulate filter is within acceptable ranges. Thus, the NOx efficiency of the SCR particulate filter is limited and a downstream SCR catalytic converter, in particular at an underfloor position of the vehicle, is still required. The downstream SCR catalytic converter also serves to prevent the emission of a reducing agent slip of the SCR unit which is close to the engine.
U.S. Patent Application Publication No. US 2008/0060348 A1 describes an exhaust-gas system having two series-connected SCR catalytic converters and having a particulate filter disposed between them. By an appropriate selection of the SCR catalytic coatings of the two SCR catalytic converters, the upstream SCR catalytic converter has a temperature window at lower temperatures than the downstream SCR catalytic converter. In accordance with an alternative embodiment, U.S. Patent Application Publication No. US 2008/0060348 A1 proposes to provide the particulate filter with an SCR catalytic coating and thereby eliminate the first upstream SCR catalytic converter.
From German Patent Application No. DE 10 2010 026 890 A1, which corresponds to U.S. Patent Application Publication No. US 2011/0011068 A1, an exhaust-gas system of a diesel engine is known, which has a first “SCR catalytic converter” (HC-SCR catalytic converter) which reduces nitrogen oxides in the presence of hydrocarbons, which are supplied to the exhaust-gas flow by a fuel metering. Downstream of the HC-SCR catalytic converter is an oxidation catalytic converter, a second SCR catalytic converter (NH3-SCR catalytic converter) and a diesel particulate filter downstream from that. The SCR catalytic coating of the NH3-SCR catalytic converter is provided on a wall-flow filter substrate.
German Patent Application No. DE 10 2010 039 972 A1, corresponding to U.S. Patent Application Publication No. US 2011/0064632 A1, describes a configuration which has a first oxidation catalytic converter, an SCR/DPF (selective catalytic reduction/diesel particulate filter) connected downstream thereof, and, downstream of that, an SCR catalytic converter and, optionally, a second oxidation catalytic converter.
All of the above-mentioned systems have in common that they include an SCR catalytic converter on a particulate filter substrate (SCR/PF) with a downstream-connected SCR catalytic converter on a flow-through substrate.
It is accordingly an object of the invention to provide an exhaust-gas aftertreatment device which allows a further reduction in NOx emissions without having to accept disadvantages in terms of fuel consumption or, respectively, CO2 emissions. It is a further object of the invention to provide a corresponding motor vehicle having such an exhaust-gas aftertreatment device.
With the foregoing and other objects in view there is provided, in accordance with the invention, an exhaust-gas aftertreatment device for an aftertreatment of an exhaust-gas of an internal combustion engine, including:
an SCR catalytic converter having an SCR catalytic coating for a selective reduction of nitrogen oxides in a presence of a reducing agent added to the exhaust-gas in a metered manner, the SCR catalytic converter having a flow-through substrate, the SCR catalytic coating of the SCR catalytic converter being disposed on the flow-through substrate; and
an SCR particulate filter downstream of the SCR catalytic converter, the SCR particulate filter having an SCR catalytic coating for a selective reduction of nitrogen oxides in a presence of the reducing agent added to the exhaust-gas in a metered manner, the SCR particulate filter having a particulate filter substrate, the SCR catalytic coating of the SCR particulate filter being disposed on the particulate filter substrate.
In other words, according to the invention, there is provided an exhaust-gas aftertreatment device for the aftertreatment of exhaust-gases of an internal combustion engine, wherein the exhaust-gas aftertreatment device includes:
an SCR catalytic converter having an SCR catalytic coating for the selective reduction of nitrogen oxides NOx in the presence of a reducing agent added to the exhaust-gas in a metered manner, wherein the SCR catalytic coating is disposed on a flow-through substrate, and
an SCR particulate filter (SCR-PF) having an SCR catalytic coating for the selective reduction of NOx in the presence of the reducing agent added to the exhaust-gas in a metered manner, wherein the SCR catalytic coating is disposed on a particulate filter substrate. The SCR particulate filter is in this case provided downstream of the SCR catalytic converter.
In contrast to known SCR exhaust-gas aftertreatment devices, the SCR catalytic converter in accordance with the invention is thus provided upstream of the SCR particulate filter, and is thus located at a position closer to the engine. By positioning the SCR catalytic converter close to the engine (close-coupled position) in front, i.e. upstream, of the SCR particulate filter, the NOx conversion performance of the SCR catalytic converter is improved due to the lower temperature gradient in the flow-through substrate of the SCR catalytic converter. This applies in particular in the case of low exhaust-gas temperatures, for example after a cold start of the internal combustion engine. The configuration according to the invention furthermore allows for the reduction or even the absence of heating measures of the SCR catalytic converter, as a result of which fuel advantages and thus lower CO2 emissions are achieved.
The upstream SCR catalytic converter also results in an improvement of the contact times between the NOx molecules of the exhaust-gas and the activity centers of the SCR catalytic coating, which also results in the improvement of the NOx conversion already at low temperatures. Since the flow-through substrate of the upstream SCR catalytic converter, when compared to the particulate filter substrate of the SCR particulate filter, permits a larger amount of SCR coating in relation to the substrate volume, the exhaust-gas back pressure of the entire exhaust-gas aftertreatment device, with the same total amount of the SCR catalytic coating, is reduced when compared to a single SCR particulate filter. At the same time lower NOx emissions are achieved with the same amount of coating when compared to a single SCR particulate filter without an upstream SCR catalytic converter. In addition, due to the high temperatures during the soot burn-off in the regeneration operation, the SCR coating of the SCR particulate filter is subject to high thermal aging processes which result in a slow deterioration of the NOx conversion. Through the use of the upstream SCR catalytic converter, the reduction of the NOx activity of the SCR particulate filter can be compensated.
In the context of the present invention, a flow-through substrate is understood to be a catalyst carrier which includes uninterrupted, continuous flow channels from an inflow face side to an outflow face side, wherein the flow channels are in particular disposed parallel to one another. This can be a ceramic monolith or a metallic catalyst carrier. In contrast, a particulate filter substrate is understood to be a carrier whose flow channels are closed. For example, the particulate filter substrate can be embodied in the form of a so-called wall-flow filter, whose parallel flow channels are closed alternately on the inlet side or on the outlet side. In this case, a flow channel closed on the outlet side is disposed adjacent to flow channels closed on the inlet side and vice versa. Exhaust-gas which flows into the flow channels closed on the outlet side, is thus forced to penetrate through the lateral channel walls, so as to enter the flow channels dosed on the inlet side and thus exit the filter. In this case, particulate constituents of the exhaust-gas, in particular soot particles, are retained on and in the porous channel walls. Particulate filter substrates are usually manufactured from a ceramic material.
In a preferred embodiment of the invention, at least the SCR catalytic converter is disposed at a position close to the engine. In this way, the light-off temperature of the SCR catalytic converter is quickly achieved after an engine cold start, and a cooling down of the catalytic converter during operation is avoided. This permits the elimination of additional heating measures for the targeted heat input into the catalytic converter. In the present case, what is meant by being positioned or disposed close to the engine, is a position within the exhaust-gas channel, wherein the position is located upstream of an underfloor position of a vehicle. In particular, the SCR catalytic converter dose to the engine is disposed so that a distance between a cylinder-side inlet opening of an exhaust-gas manifold of the exhaust-gas aftertreatment device and an inflow face side of the SCR catalytic converter is at most 100 cm, preferably at most 80 cm. In specific embodiments, this distance can even be reduced to values of at most 70 cm. The distance is in this case measured by the exhaust-gas travel length, i.e. the path length to be traveled by the exhaust-gas between the cylinder-side inlet opening of the exhaust-gas manifold and the inflow face side of the SCR catalytic converter.
In accordance with the invention there is thus provided, in combination with an internal combustion engine, an exhaust-gas aftertreatment device for an aftertreatment of an exhaust-gas of the internal combustion engine, wherein the exhaust-gas aftertreatment device includes:
an SCR catalytic converter having an SCR catalytic coating for a selective reduction of nitrogen oxides in a presence of a reducing agent added to the exhaust-gas in a metered manner, the SCR catalytic converter having a flow-through substrate, the SCR catalytic coating of the SCR catalytic converter being disposed on the flow-through substrate;
an SCR particulate filter downstream of the SCR catalytic converter, the SCR particulate filter having an SCR catalytic coating for a selective reduction of nitrogen oxides in a presence of the reducing agent added to the exhaust-gas in a metered manner, the SCR particulate filter having a particulate filter substrate, the SCR catalytic coating of the SCR particulate filter being disposed on the particulate filter substrate; and
at least the SCR catalytic converter being disposed in a position close to the internal combustion engine.
According to a feature of the invention, the internal combustion engine has an exhaust-gas manifold with a cylinder-side inlet opening, the SCR catalytic converter has an inflow face side, a distance between the cylinder-side inlet opening of the exhaust-gas manifold and the inflow face side of the SCR catalytic converter is at most 100 cm, preferably at most 80 cm.
In a further preferred embodiment of the invention, the downstream SCR particulate filter is also disposed at a position close to the engine, in which case the distance between the cylinder-side inlet opening of the exhaust-gas manifold and the inflow face side of the SCR particulate filter is at most 120 cm, preferably at most 100 cm.
According to an advantageous embodiment, the SCR catalytic converter has a smaller volume than the downstream SCR particulate filter. With this measure, a very quick light-off of the SCR catalytic converter is achieved after a cold start. In particular, the volume of the SCR catalytic converter is at most 75%, preferably at most 60% of the volume of the SCR particulate filter.
Thus, according to another feature of the invention, the SCR catalytic converter and the SCR particulate filter each have a respective volume, the volume of the SCR catalytic converter is smaller than the volume of the SCR particulate filter. In particular the volume of the SCR catalytic converter is at most 75% of the volume of the SCR particulate filter. Preferably, the volume of the SCR catalytic converter is at most 60% of the volume of the SCR particulate filter.
It is furthermore preferably provided that the SCR catalytic converter has at least the same or a larger amount of the SCR catalytic coating in relation to the substrate volume than the SCR particulate filter. This embodiment takes account of the fact that flow-through substrates can accommodate a larger amount of coating per substrate volume than filter substrates, without causing unacceptable exhaust-gas back pressures across the substrate. Through use of a largest possible amount of SCR catalytic coating of the SCR catalytic converter, it is possible to adhere to a particularly small catalytic converter volume. In particular, the SCR catalytic converter has a greater amount of SCR catalytic coating than the SCR particulate filter, the amount being greater by a factor of at least 1.2, preferably by a factor of at least 1.5.
Thus, according to a further feature of the invention, the flow-through substrate and the particulate filter substrate each have a respective substrate volume; and the SCR catalytic converter and the SCR particulate filter each have a respective amount of the SCR catalytic coating in relation to the respective substrate volume, the amount of the SCR catalytic coating in relation to the substrate volume of the flow-through substrate of the SCR catalytic converter is at least equal to or larger than the amount of the SCR catalytic coating in relation to the substrate volume of the particulate filter substrate of the SCR particulate filter. In particular, the amount of the SCR catalytic coating in relation to the substrate volume of the flow-through substrate of the SCR catalytic converter is larger by a factor of at least 1.2 than the amount of the SCR catalytic coating in relation to the substrate volume of the particulate filter substrate of the SCR particulate filter. Preferably, the amount of the SCR catalytic coating in relation to the substrate volume of the flow-through substrate of the SCR catalytic converter is larger by a factor of at least 1.5 than the amount of the SCR catalytic coating in relation to the substrate volume of the particulate filter substrate of the SCR particulate filter.
According to a further advantageous embodiment of the invention, the flow-through substrate of the SCR catalytic converter has a greater cell count (cell density) than the particulate filter substrate of the SCR particulate filter. Due to the greater cell count of the SCR catalytic converter, a large surface area of the cell walls of the flow channels is achieved, which facilitates the accommodation of a comparatively large amount of SCR coating. In particular, the flow-through substrate has a cell count which is greater by a factor of at least 1.1 and preferably by a factor of at least 1.2 than the cell count of the particulate filter substrate.
Thus, according to another feature of the invention, the flow-through substrate of the SCR catalytic converter and the particulate filter substrate of the SCR particulate filter each have a respective cell count, the cell count of the flow-through substrate is greater than the cell count of the particulate filter substrate. In particular, the cell count of the flow-through substrate is greater by a factor of at least 1.1 than the cell count of the particulate filter substrate. Preferably, the cell count of the flow-through substrate is greater by a factor of at least 1.2 than the cell count of the particulate filter substrate.
In particular, the cell count of the flow-through substrate of the SCR catalytic converter is at least 300 cpsi (cells per square inch), preferably at least 350 cpsi and particularly preferably at least 650 cpsi. In contrast, the particulate filter substrate of the SCR particulate filter has in particular a cell count of at least 250 cpsi, preferably at least 300 cpsi and particularly preferably at least 350 cpsi.
Further, the flow-through substrate of the SCR coating has a smaller wall thickness than the particulate filter substrate of the SCR particulate filter. The wall thickness of the flow-through substrate is preferably at most 6 mil (1 mil= 1/1000 inch=0.0254 mm), preferably at most 5.5 mil, more preferably at most 5 mil. In contrast, a preferred wall thickness of the particulate filter substrate of the SCR particulate filter is at most 30 mil, in particular at most 15 mil and more preferably at most 13 mil.
The porosity of the particulate filter substrate is in a preferred embodiment at most 65%, in particular at most 61%. The mean pore radius is preferably ≦25 μm, in particular ≦20 μm.
In accordance with an embodiment of the invention, the SCR catalytic converter and the SCR particulate filter are disposed in separate housings connected in series. According to a preferred embodiment, however, the SCR catalytic converter and the SCR particulate filter are disposed in a common housing, because this results in a further temperature advantage as well as a lower exhaust-gas backpressure.
Thus, according to a feature of the invention, the exhaust-gas aftertreatment device includes a common housing, wherein the SCR catalytic converter and the SCR particulate filter are disposed in the common housing.
In a preferred embodiment of the invention, the exhaust-gas aftertreatment device further includes a reducing agent metering device, which is configured to add the reducing agent, or a precursor compound thereof, in a metered manner to the exhaust-gas upstream of the SCR catalytic converter. In particular, it is a common metering device for both, the SCR catalytic converter and the downstream SCR particulate filter.
According to a further feature of the invention, the exhaust-gas aftertreatment device thus includes a reducing agent metering device, wherein the reducing agent metering device is configured to add the reducing agent or a precursor compound of the reducing agent in a metered manner to the exhaust-gas upstream of the SCR catalytic converter.
The reducing agent that is added in a metered manner is preferably ammonia NH3 or a precursor compound thereof, wherein in this case in particular urea is suitable. The urea may be used in the form of solid urea pellets, but preferably used in the form of an in particular an aqueous urea solution. The urea that is added in a metered manner reacts by way of thermolysis and hydrolysis while releasing NH3. It is within the scope of the invention that the reducing agent ammonia can in principle also be stored up through the use of NH3 storage materials that reversibly bind or, respectively, release ammonia as a function of the temperature. Corresponding metal amine storages have already been explained above.
According to another preferred embodiment of the invention, the exhaust-gas aftertreatment device further includes an oxidation catalytic converter. The oxidation catalytic converter is preferably disposed upstream of the SCR catalytic converter. In this manner it is achieved that the NO2/NO ratio of the exhaust-gas is increased, thus achieving an improved NOx conversion performance of the downstream SCR components. If furthermore the oxidation catalytic converter is provided downstream of the reducing agent metering, the oxidation catalytic converter additionally results in an improved homogenization of the supplied reducing agent in the exhaust-gas before it enters the SCR catalytic converter.
Thus, according to another feature of the invention, the exhaust-gas aftertreatment device includes an oxidation catalytic converter. The oxidation catalytic converter is preferably disposed upstream of the SCR catalytic converter.
The invention further relates to a motor vehicle with an intemal combustion engine for driving the vehicle and an exhaust-gas aftertreatment device according to the invention.
With the objects of the invention in view there is also provided, a motor vehicle including:
an internal combustion engine;
an exhaust-gas aftertreatment device for an aftertreatment of an exhaust-gas of the internal combustion engine, the exhaust-gas aftertreatment device including an SCR catalytic converter and an SCR particulate filter;
the SCR catalytic converter having an SCR catalytic coating for a selective reduction of nitrogen oxides in a presence of a reducing agent added to the exhaust-gas in a metered manner, the SCR catalytic converter having a flow-through substrate, the SCR catalytic coating of the SCR catalytic converter being disposed on the flow-through substrate; and
the SCR particulate filter being disposed downstream of the SCR catalytic converter, the SCR particulate filter having an SCR catalytic coating for a selective reduction of nitrogen oxides in a presence of the reducing agent added to the exhaust-gas in a metered manner, the SCR particulate filter having a particulate filter substrate, the SCR catalytic coating of the SCR particulate filter being disposed on the particulate filter substrate.
The internal combustion engine is an internal combustion engine that is operated permanently or at least temporarily in a lean-burn mode, in particular a diesel engine. The exhaust-gas aftertreatment device according to the invention can in principle also be advantageously used for Otto-cycle engines that are temporarily operated in a lean-burn mode, in particular Otto-cycle engines capable of stratified charge operation.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in an SCR exhaust-gas aftertreatment device and a motor vehicle with such an SCR exhaust-gas aftertreatment device, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.
Referring now to the figures of the drawings in detail and first, particularly, to
The motor vehicle 10 further has an exhaust-gas aftertreatment device according to the invention, which is overall designated by reference numeral 14, for the catalytic aftertreatment of an exhaust-gas of the internal combustion engine 12. The exhaust-gas aftertreatment device 14 includes an exhaust-gas manifold 16, which connects the individual cylinder outlets of the cylinders of the internal combustion engine 12 to an exhaust-gas channel 18. The exhaust-gas channel 18 has a section 20 close to the engine and has an underfloor section 22, which is shown here in a shortened form and which ends in an exhaust pipe that is not shown here.
Downstream of the exhaust-gas manifold 16, an oxidation catalytic converter 24 is disposed in the exhaust-gas channel 18. The oxidation catalytic converter 24 has a flow-through substrate which is coated with a catalytic coating which catalyzes the oxidation of exhaust-gas components. In particular, the catalytic coating is suited to convert unburned hydrocarbons HC and carbon monoxide CO into CO2 and H2O. In addition, the catalytic coating of the oxidation catalytic converter 24 is configured to oxidize NO and N2O to NO2 in order to increase the NO2/NO ratio. The catalytic coating of the oxidation catalytic converter 24 contains as a catalytic component in particular at least one element of the platinum group metals Pt, Pd, Rh, Ru, Os, or Ir or a combination of these, in particular Pt and/or Pd. The catalytic coating further contains a washcoat which includes a porous ceramic matrix having a large specific surface area, for example on the basis of zeolite that is doped with the catalytic component. The flow-through substrate of the oxidation catalytic converter 24 can be a metallic substrate or a ceramic monolith, in particular with a honeycomb-like structure having a plurality of continuous, parallel flow channels. Suitable ceramic materials include aluminum oxide, cordierite, mullite and silicon carbide. Suitable metal substrates are made for example from stainless steel or iron-chromium alloys.
Downstream of the oxidation catalytic converter 24, an SCR catalytic converter 26 is disposed in the section 20 of the exhaust-gas channel 18 that is close to the engine. The SCR catalytic converter 26 has, just like the oxidation catalytic converter 24, a flow-through substrate on metallic basis or ceramic basis, preferably on ceramic basis. Suitable ceramic or metallic materials correspond to those mentioned in connection with the oxidation catalytic converter. The flow-through substrate of the SCR catalytic converter 26 has a cell count of preferably ≧350 cpsi and a wall thickness of ≦5.5 mil. The walls of the parallel and continuous flow channels of the flow-through substrate of the SCR catalytic converter 26 are coated with an SCR catalytic coating. These in turn include a washcoat made of a porous ceramic matrix having a large specific surface area (for example a zeolite on the basis of aluminum silicate) and catalytic substances disposed thereon in a distributed manner. Suitable SCR catalytic substances include, in particular base metals such as Fe, Cu, Va, Cr, Mo, W, and combinations of these. These are deposited on the zeolite and/or the zeolite metals are partially replaced by ion exchange by the corresponding base metals.
Downstream of the SCR catalytic converter 26 is an SCR particulate filter 28 which is also disposed in the section 20 of the exhaust-gas channel 18 that is close to the engine. The SCR particulate filter has a particulate filter substrate, which is, for example, a wall-flow filter. The particulate filter substrate has parallel flow channels that are closed alternately on the inlet side and the outlet side. The particulate filter substrate is made of a porous ceramic material such as cordierite, α-aluminum oxide, silicon carbide, silicon nitride, zirconium oxide, mullite, spodumene, aluminum oxide-silicon oxide-magnesium oxide (alumina-silica-magnesia) or zirconium silicate. The cell count of the particulate filter substrate is preferably ≧300 cpsi, wherein the cell count is smaller by at least a factor of 1.1 than that of the flow-through substrate of the SCR catalytic converter 26. The wall thickness of the particulate filter substrate is preferably at most 15 mil and has a porosity of ≦61% with a mean pore radius of ≦20 μm. The flow channels of the particulate filter substrate are coated with an SCR catalytic coating, which in principle can have the same chemical composition as that of the SCR catalytic converter 26. However, the amount of the catalytic coating of the SCR particulate filter 28 in relation to the substrate volume is less than that of the SCR catalytic converter 26, in particular by a factor of at least 1.5. The particulate filter substrate of the SCR particulate filter 28 can be coated with the SCR catalytic coating over the whole area or only in sections, for example, only in an inlet-side section.
The SCR catalytic converter 26 and the SCR particulate filter 28 are disposed in a position close to the engine (close-coupled position). In particular, the distance D between a cylinder-side inlet opening of the exhaust-gas manifold 16 and an inflow face side of the SCR catalytic converter 26 is at most 80 cm. Crucial for the measurement of this distance D is the actual path length to be covered by the exhaust-gas (the distance D is in this case illustrated in a simplified manner).
In the embodiment shown in
The exhaust-gas aftertreatment device 14 further includes a reducing agent metering device 32, with which the reducing agent or a precursor compound thereof is added to the exhaust-gas in a metered manner. For example, the reducing agent is introduced into the exhaust-gas flow through the use of a nozzle upstream of the SCR catalytic converter 26. The reducing agent is typically ammonia NH3, which is added in a metered manner in the form of a precursor compound, in particular in the form of urea. Preferably, the urea, in the form of an aqueous solution, is fed from a reservoir, which is not shown, and is added in a metered manner. By way of thermolysis and hydrolysis, the urea is decomposed in the hot exhaust-gas to NH3 and CO2.
The metered adding of the reducing agent by the metering device 32 is usually carried out by a control unit which is not illustrated here and which controls the device 32 in dependence on an operating point of the engine 12, in particular in dependence on a current NOx concentration of the exhaust-gas. For the purpose of the control, the exhaust-gas aftertreatment device 14 can also have various exhaust-gas sensors and temperature sensors, such as NOx sensors upstream and/or downstream of the SCR components 26/28.
An exhaust-gas aftertreatment device 14 according to a second embodiment of the present invention is shown in
In contrast to the embodiment shown in
Further variants of embodiments of the exhaust-gas aftertreatment device 14, which are not shown here, envisage providing the oxidation catalytic converter 24 downstream of the reducing agent metering device 32. In addition, further exhaust-gas aftertreatment components can be present, for example, at the underfloor position 22 of the exhaust-gas channel 18.
Up to a point in time t1, the curves of the NOx final emissions (3 and 4) correspond to that of the NOx untreated emission 1, 2. Up to this point in time, the SCR catalytic coatings have in each case not yet reached their light-off temperature, so that no significant NOx conversion takes place. From the point in time t1 on, the light-off temperature of the SCR components is reached, so that the NOx final emissions 3 and 4 are considerably lower than the NOx untreated emissions 1, 2. In the further course of the curves, also the curves of the final emissions 3 and 4 separate from one another, wherein the NOx final emissions of exhaust-gas aftertreatment according to the invention are significantly below the comparison system. From this it is evident, that despite an identical amount of catalytic material, the exhaust-gas aftertreatment device according to the invention has an improved NOx conversion.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2012 023 049.4 | Nov 2012 | DE | national |
This application is a continuation, under 35 U.S.C. §120, of copending International Application No. PCT/EP2013/072611, filed Oct. 29, 2013, which designated the United States; this application also claims the priority, under 35 U.S.C. §119, of German Patent Application No. DE 10 2012 023 049.4, filed Nov. 26, 2012; the prior applications are herewith incorporated by reference in their entirety.
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/EP2013/072611 | Oct 2013 | US |
| Child | 14714190 | US |
