US20060251548A1

US20060251548A1 - Exhaust aftertreatment device

Info

Publication number: US20060251548A1
Application number: US11/123,903
Authority: US
Inventors: Ray Willey; Bret Zimmerman; David Kubinski; Diana Brehob
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2005-05-06
Filing date: 2005-05-06
Publication date: 2006-11-09
Also published as: US7833495B2; US20070128088A1; GB2425740A; GB0608498D0; US8133842B2; US20110035941A1

Abstract

An exhaust aftertreatment device for an internal combustion engine is disclosed which contains multiple, parallel channels of a porous material, such as cordierite or silicon carbide, in which about some of the channels are plugged at an upstream end and other channels remain unplugged. In one embodiment, the substrate has an SCR coating and the engine has a urea supply system. Other embodiments include using TWC and LNT formulations. Several washcoat configurations and other specific geometries and dimensions to encourage crossflow, diffusion, and adsorption are disclosed herein.

Description

FIELD OF THE INVENTION

The present invention relates to a device for treating exhaust gases from an internal combustion engine. In particular, the exhaust aftertreatment device contains parallel channels of a porous material with about half of the channels being plugged on the upstream end. The device is coated with a SCR (selective catalytic reduction) washcoat.

BACKGROUND OF THE INVENTION

Achieving low NOx (NO, nitric oxide, plus NO2, nitrogen dioxide) emissions from lean-burning engines, such as diesels is a challenge. To treat NOx emitted from diesel engines, SCRs and LNTs (lean NOx traps) have been developed. LNTs operate in a lean/rich cycle in which NOx is purged during lean operation and NOx is released and reacted in a shorter period of rich operation. A disadvantage of LNTs is that they significantly degrade diesel fuel economy. Although SCRs do not consume a large amount of extra fuel to react NOx, urea is supplied to the SCR to cause the NOx reaction. For vehicular use, an onboard urea tank and delivery system is used.
It has been found that SCRs are very effective at converting NOx to N2 and O2 under steady state conditions. However, during transient conditions, such as tip-ins (driver demand for a rapid increase in torque), a high concentration of NOx passes through the SCR unreacted. SCRs have not achieved the extremely low NOx emission levels of gasoline engines with three-way catalysts, largely due to the large NOx breakthrough during transients.
A need exists for a catalyst system which provides very low NOx emissions without incurring a large fuel consumption penalty.

SUMMARY OF THE INVENTION

Low NOx conversion efficiency of prior art SCRs during transient engine operation is overcome by a substrate comprised of multiple, porous, parallel channels in which about half of the channels are plugged preferably on the upstream end. The substrate is made of cordierite or silicon carbide with a porosity greater than 10%, but preferably 35 to even 65%. The substrate is placed in an engine exhaust with the plugged end preferably closer to the engine. The substrate is wash coated with copper zeolite or other SCR type coatings which provide molecular storage of NH3 for the reduction of NOx. The substrate contains 100 to 600 cells per square inch, with greater than 250 cells per square inch being preferred. In another embodiment, the number of cells in the substrate nears 1000 cells per square inch.
In one embodiment, flow restrictors are placed in the open channels at the downstream end, i.e., the opposite end in which the plugs are placed. Preferably, the flow restrictors are placed in channels without plugs and not in channels with plugs, but flow restrictions could be placed in both the open or primary flow channels as well as the plugged or secondary flow channels to facilitate fabrication and avoid high cost.
A method of manufacturing an exhaust aftertreatment device in disclosed in which a porous substrate of multiple, parallel channels is formed. Some, but not all of the channels are plugged near one end of the channels. A washcoat is applied to the substrate after the plugging of some of the channels. The substrate is formed of cordierite. Alternatively, silicon carbide is used. The channels are square in cross-section and alternative channels are plugged on the upstream end. The plug material is the same as the substrate material.
A SCR according to the present invention provides superior control of NOx during transient operating conditions compared to prior art SCRs when equipped on a diesel vehicle (FIGS. 7 and 8). The NOx breakthrough during transients is reduced one-third to over three-fourths depending on the nature of the tip-in. By controlling transient emissions, overall diesel cycle NOx emissions are halved with the SCR of the present invention compared to the prior art.
Additionally, this concept improves steady state NOx efficiency by providing improved diffusion and subsequent kinetics by the effective storage mechanisms having a deeper penetration into and through the washcoat material applied to either or both sides of the porous substrate material. Washcoats and their placement in the channels can be varied to facilitate cross flow from the primary to the secondary channels. Traditional channel flow diffusion and kinetics with a shared packed bed or wall flow diffusion and kinetics are coupled in the present invention allowing more efficient systems at converting NOx.

BRIEF DESCRIPTION OF THE INVENTION

The advantages described herein will be more fully understood by reading an example of an embodiment in which the invention is used to advantage, referred to herein as the Detailed Description, with reference to the drawings wherein:
FIG. 1 is a schematic of an engine equipped with an exhaust aftertreatment device according to an aspect of the present invention;
FIG. 2 is a schematic of a diesel particulate filter of the prior art;
FIG. 3 is a schematic of a SCR catalyst according to an aspect of the present invention;
FIG. 4 shows an upstream end and a downstream end of a SCR catalyst according to an aspect of the present invention;
FIG. 5 is a schematic showing a series of aftertreatment devices according to an aspect of the present invention;
FIG. 6 shows upstream ends of two serial SCR catalysts according to an aspect of the present invention;
FIG. 7 is a graph of tailpipe NOx concentration from a diesel vehicle during a drive cycle, the aftertreatment system of the diesel engine comprising a SCR catalyst according to the prior art; and
FIG. 8 is a graph of tailpipe NOx concentration from a diesel vehicle during a drive cycle, the aftertreatment system of the diesel engine comprising a SCR catalyst according to the present invention.

DETAILED DESCRIPTION

A 4-cylinder internal combustion engine 10 is shown, by way of example, in FIG. 1. Engine 10 is supplied air through intake manifold 12 and discharges spent gases through exhaust manifold 14. An intake duct upstream of the intake manifold 12 contains a throttle valve 32 which, when actuated, controls the amount of airflow to engine 10. Sensors 34 and 36 installed in intake manifold 12 measure air temperature and mass air flow (MAF), respectively. Sensor 31, located in intake manifold 14 downstream of throttle valve 32, is a manifold absolute pressure (MAP) sensor. A partially closed throttle valve 32 causes a pressure depression in intake manifold 12. When a pressure depression exists in intake manifold 12, exhaust gases are caused to flow through exhaust gas recirculation (EGR) duct 19, which connects exhaust manifold 14 to intake manifold 12. Within EGR duct 19 is EGR valve 18, which is actuated to control EGR flow and optional intercooler 17. Fuel is supplied to engine 10 by fuel injectors 26, in a port fuel injected alternative. In a second alternative, fuel is supplied through fuel injectors 30, a configuration commonly called direct injection. Although it is typical for fuel to be supplied by one or the other of port injectors 26 or direct fuel injectors 30, other alternatives include: carburetion (not shown because such carburetor would be located upstream of throttle valve 32) and any combination of carburetion, port injection, and direct injection. Each cylinder 16 of engine 10 contains a spark plug 26 in embodiments using spark ignition. In another alternative, engine 10 is a diesel or compression ignition engine in which ignition spontaneously occurs upon compression. The crankshaft (not shown) of engine 10 is coupled to a toothed wheel 20. Sensor 22, placed proximately to toothed wheel 20, detects engine 10 rotation.
Continuing to refer to FIG. 1, electronic control unit (ECU) 40 is provided to control engine 10. ECU 40 has a microprocessor 46, called a central processing unit (CPU), in communication with memory management unit (MMU) 48. MMU 48 controls the movement of data among the various computer readable storage media and communicates data to and from CPU 46. The computer readable storage media preferably include volatile and nonvolatile storage in read-only memory (ROM) 50, random-access memory (RAM) 54, and keep-alive memory (KAM) 52, for example. KAM 52 may be used to store various operating variables while CPU 46 is powered down. The computer-readable storage media may be implemented using any of a number of known memory devices such as PROMs (programmable read-only memory), EPROMs (electrically PROM), EEPROMs (electrically erasable PROM), flash memory, or any other electric, magnetic, optical, or combination memory devices capable of storing data, some of which represent executable instructions, used by CPU 46 in controlling the engine or vehicle into which the engine is mounted. The computer-readable storage media may also include floppy disks, CD-ROMs, hard disks, and the like. CPU 46 communicates with various sensors and actuators via an input/output (I/O) interface 44. Examples of items that are actuated under control by CPU 46, through I/O interface 44, are fuel injection timing, fuel injection rate, fuel injection duration, throttle valve 32 position, spark plug 26 timing (in an alternate embodiment related to a spark ignition engine), EGR valve 18 position, and urea injector 64 opening. Various other sensors 42 and specific sensors (engine speed sensor 22, in-line torque sensor, cylinder pressure transducer sensor, engine coolant sensor 38, manifold absolute pressure sensor 31, exhaust gas component sensors 24 and 25, air temperature sensor 34, and mass airflow sensor 36) communicate input through I/O interface 44 and provide signals from which engine rotational speed, vehicle speed, coolant temperature, manifold pressure, pedal position, cylinder pressure, throttle valve position, air temperature, exhaust temperature, exhaust stoichiometry, exhaust component concentration, and air flow can be computed. Some ECU 40 architectures do not contain MMU 48. If no MMU 48 is employed, CPU 46 manages data and connects directly to ROM 50, RAM 54, and KAM 52. The present invention could utilize more than one CPU 46 to provide engine control and ECU 60 may contain multiple ROM 50, RAM 54, and KAM 52 coupled to MMU 48 or CPU 46 depending upon the particular application.
Continuing with FIG. 1, exhaust gases from engine 10 pass through exhaust aftertreatment device 38. Sensor 24, in exhaust manifold 14 located upstream of exhaust aftertreatment device 38, is an exhaust gas component sensor. In one embodiment, exhaust aftertreatment device 38, is a SCR catalyst. Urea tank 60 contains urea 62 which is supplied to the engine exhaust through injector 64. Urea injector 64 is controlled by ECU 40 (electrical connection between the two not shown). Downstream of SCR 38 is exhaust gas component sensor 25, which senses ammonia. Urea 62 is supplied to SCR 38 to cause NOx to react to N2 and O2. It is desirable to provide no more urea 62 to SCR 38 than is reacted within SCR 38. Unreacted urea 62 breaking through SCR 38 is detected by sensor 25. Urea sensor 64 can be feedback controlled to avoid urea 62 breakthrough.
Exhaust aftertreatment device 38, shown in FIG. 1, is alternatively a LNT. In this embodiment, sensors 24 and 25 are NOx sensors. Alternatively, sensor 24 is a wide-range exhaust gas oxygen sensor or a combination NOx and wide-range exhaust gas oxygen sensor.
As shown in FIG. 1, engine 10 is naturally aspirated. However, the present invention is not limited to naturally-aspirated engines. The present invention is compatible with pressure charging of the intake gases as accomplished by a turbocharger, supercharger, or any combination of these devices and other know pressure charging device.
In FIG. 2, a schematic of a portion of a diesel particulate filter (DPF) substrate 70 is shown. DPFs are known to contain multiple, parallel channels 72. Half of the channels contain plugs 76 at a first end. Channels 72 not plugged at the first end are plugged at the opposite end. Walls 74 of channels 72 are porous to allow gases to pass through, but particulate matter (soot) is trapped on walls 74. As can be seen in FIG. 2, the exhaust gas must pass through a wall 74 to exit substrate 70. Thereby, all exhaust gases are filtered. DPFs are known to be constructed of cordierite and of silicon carbide.
In FIG. 3 is shown an exhaust aftertreatment device according to the present method. Substrate 80, made of cordierite, contains multiple, parallel channels 82 with porous walls 84. About half of channels 82 contain plugs 86 at one end. Plugs 86 allow almost no soot to pass through, but are porous enough to allow some smaller molecules, such as ammonia, NH3, to pass. In one embodiment, the plugs are made of the substrate material. But, because they are thicker than the walls, they provide a greater barrier to larger particles such as soot to pass. In contrast to a DPF, in substrate 80, about half of channels 82 do not have plugs. Substrate 80 contains a washcoat 90. Some of the inlet gases traverse through substrate 80 without passing through walls 84. Some gases do pass through walls 84 prior to exiting substrate 80. To encourage more flow through walls 84, one embodiment includes flow restrictors 88 providing more pressure drop in unplugged channels. In the present invention, restrictors 88 do not occlude the cross-section of the channel. In contrast, plugs 86 do extend across the cross-section of the channel.
In one embodiment, the primary channel walls, i.e., without plugs, have washcoat 90. Secondary channels, i.e, those containing plugs are impregnated with the washcoat materials. In this way, three diffusion and kinetic mechanisms favoring NOx conversion are encouraged: primary channel flow, secondary channel flow, and very slow packed bed flow through the wall. To this end, it is desirable to have small channels: preferably above 250 cells per square inch. Furthermore, to facilitate flow through walls 84, porous walls (>50% porosity) are desired. It is desirable to have the ratio of primary (through nonplugged channels) flow to secondary (plugged channels) flow to be around the ratio of 2:1. One alternative is to select the porosity to provide such flow ratio.
To further encourage flow into and through wall 84, the length of substrate 80 is extended as much as possible within manufacturing feasibility, i.e., to avoid substrate cracking. A long substrate increases back pressure and encourages cross-flow diffusion and kinetics. Preferably, the substrate length is more than 1.5 times the diameter of the substrate. This provides an alternative or complementary embodiment to the use of restrictors 82.
In yet another embodiment, primary channels (no plugs) utilize coarser grain washcoat and thicker walls, whereas secondary channels (with plugs) utilize finer grain washcoat and thinner walls. Furthermore, secondary channels are less than fully coated to encourage cross-flow.
In a preferred embodiment, substrate 80 is coated with an acidic material, such acidic material selected to render substrate 80 an SCR. There are many different and evolving SCR washcoats which are suitable depending on activation temperature, maximum temperature, porosity requirements, contamination, etc. The application of these washcoats to maximize wall-flow diffusion by regions, thicknesses, and zone coating are a part of this invention.
An upstream cross-section 92 of substrate 80 is shown in FIG. 4, in which the cross-section of the channels are square and every other channel is plugged, as in a checkerboard fashion. Also in FIG. 4 is the downstream cross-section of substrate 80, in which no channels are plugged. The illustration in FIG. 4 is for example only and not intended to be limiting. Alternatively, the channels are hexagonal or other tessellating shapes.
In the example shown in FIG. 4, half of the channels are plugged. However, alternatively less than half of the channels are plugged. In yet another alternative, more than half of the channels are plugged, which increases diffusion by increasing pressure drop.
Multiple aftertreatment units 38, 39, and 41 are shown in FIG. 5. In one embodiment, units 38, 39, and 41 contain substrates 80 according to the present invention. Alternatively, aftertreatment unit 39 is a conventional SCR and aftertreatment unit 41 is a conventional TWC (three-way catalyst). The inventors of the present invention contemplate many alternatives in which devices 39 and 41 are any of a LNT, SCR, DPF, TWC, and diesel oxidation catalyst to achieve an overall highly efficiency catalyst system.
In FIG. 6, two SCRs of the present invention are shown in which the upstream face 96 of one of the units has all the interior channels plugged and all of the exterior channels unplugged. Directly behind an SCR of this configuration is a second SCR in which the upstream face 98 has all interior channels unplugged ad the exterior channels plugged. This configuration causes flow to traverse from the exterior channels toward the interior channels as it moves from the first SCR to the second SCR.
In an alternative embodiment, substrate 80 has alternately tapered channels such that crossover flow is encouraged. In particular, the nonplugged channels are wider on an upstream end and reduce in diameter along the length of the substrate. The plugged channels increase in diameter from the plugged end to a nonplugged end (downstream end).
In an alternate embodiment, substrate 80 is made of silicon carbide. Silicon carbide is known to be less brittle, thus more durable, than cordierite, with a penalty of higher cost and weight. Silicon carbide, being more durable, is more able to be extruded to a longer length. A longer length allows more opportunity for diffusion. A length of at least 1.5 times the diameter is preferred.
It is desirable to decrease the size of substrate 80 to facilitate packaging. The cross-sectional size of substrate 80 is designed such that a tolerable pressure drop across substrate 80 is experienced at the highest flow conditions. In one embodiment, fewer than half of the channels are plugged at one end. It is desirable that each open channel have contact with a plugged channel, encouraging flow through the walls. By plugging fewer than half of the channels, flow through substrate 80 is restricted less than if half are plugged. The advantage is that the cross-sectional area of substrate 80 can be reduced. Alternatively, by plugging more than half of the channels, the pressure drop is increased, thereby promoting better diffusion.
In FIG. 3, flow is shown entering at the end containing plugs 86. This is desirable for an application in which there might be particulate matter in the exhaust gases, such as diesel engines. If plugs 86 were installed in the downstream end of the substrate 80, particulate matter would collect in substrate 80. It is intended to filter particulate matter in a DPF in which collected particles are regularly incinerated to regenerate the DPF. If there is no intention of regenerating substrate 80, it is desired to avoid collection of particles by adopting the orientation shown in FIG. 3. In another embodiment in which there are few particles, such as a gasoline engine, it may be found to be advantageous to place substrate 80 in the flow in the opposite direction to that shown in FIG. 3.
In a preferred embodiment, substrate 80 contains a washcoat 90 which causes it to be a SCR. An injector 64 coupled to a urea tank 62 (injector and tank shown in FIG. 1) sprays an initial aqueous solution containing dissolved NH3. Under exhaust heat and reaction with water in the exhaust deposit NH3 typically to the acidic sites of the SCR wash coat. The urea solution coats and saturates substrate 80 and upon adsorption leaves the solid acidic matter on the surface of substrate 80 as well as inside the porous walls and eventually on secondary walls also. It is the nature of NH3 to be both physio-adsorbed and chemi-adsorbed. In use, the acidic material attracts the reductant, which is basic. In the case of an SCR, urea converted to NH3 is typically the reductant material. The reductant is adsorbed on the solid acidic material of the washcoat available to react with NOx on a continuous basis.
Referring to FIGS. 7 and 8, the NOx concentration at the tailpipe of a diesel equipped vehicle is shown for a prior art SCR (FIG. 5) and a SCR of the present invention (FIG. 7). The spikes in the both figures correspond to tip-ins, driver demand for additional torque for acceleration, hill climb, highway passing, or other purposes. The magnitude of the spikes in FIG. 6 is greatly diminished over those shown in FIG. 5. The resulting NOx emission over the drive cycle for a SCR of the present invention is a marked improvement over the prior art.
Alternately, substrate 80 has a LNT washcoat 90. Such washcoat contains three components: a first component that facilitates oxidation of NO to NO2, a second component that traps NO2 on the surface (NO2 forming a nitrate on the surface), and a third component of precious metals, often rhodium, which reduces released NOx (under rich operating conditions) to N2.
In another alternative, substrate 80 is used in place of traditional gasoline TWC substrates. This provides a precious metal cost save due to the enhanced diffusion and kinetics over traditional channel flow designs of TWCs. This embodiment provides diminished tip-in emission spikes.
This invention is also useful in non-automotive applications where diffusion, slow kinetics, and transients are problematic for catalysis, synthesis, and other chemical processes.
Although the invention has been described in connection with preferred embodiments thereof, it will be appreciated by those skilled in the art that modifications, substitutions, and additions and deletions may be made, without departing from the spirit or scope of the invention as defined in the appended claims.

Claims

1. An exhaust aftertreatment device, comprising:

a substrate comprised of multiple channels of a porous material, said channels being substantially parallel, said substrate being disposed in an exhaust stream; and

plugs placed in a portion of said channels near one end of said substrate wherein another portion of said channels is unplugged along the entire channel length.

2. The device of claim 1 wherein said substrate further comprises a washcoat of an acidic material which is capable of adsorbing a reductant.

3. The device of claim 2 wherein said washcoat renders said substrate to be a SCR catalyst and said reductant is ammonia.

4. The device of claim 1 wherein said plugs are placed in about half of said channels.

5. The device of claim 1 wherein said plugs are placed in said channels evenly spaced on said end of said substrate.

6. The device of claim 1 wherein the device is placed in an internal combustion engine exhaust with said plugs receiving said engine exhaust.

7. The device of claim 1 wherein said substrate material has a porosity greater than 10% prior to applying a washcoat material.

8. The device of claim 1 wherein said substrate is comprised of cordierite.

9. The device of claim 1 wherein said substrate is comprised of silicon carbide.

10. The device of claim 1, further comprising: flow restrictors on said unplugged channels, said flow restrictors being located at an opposite end of said substrate from said plugs.

11. A method of manufacturing an exhaust aftertreatment device, comprising:

forming a porous substrate of multiple, parallel channels;

plugging some, but not all, of said channels near a first end of said channels; and

washcoating said substrate after said plugging wherein a second end of said channels are unplugged wherein said channels without plugs are unplugged along the entire channel length.

12. The method of claim 11, further comprising: forming flow restrictors in channels without plugs, said flow restrictors being placed in said second end of said channels.

13. The method of claim 11 wherein about half of said channels are plugged.

14. The method of claim 11 wherein said substrate has a porosity greater than 20% prior to said washcoating.

15. The method of claim 11 wherein said substrate is comprised of cordierite.

16. The method of claim 11 wherein said substrate has 250 to 600 cells per square inch.

17. The method of claim 11 wherein said substrate has a porosity greater than 50% prior to said washcoating.

18. An exhaust aftertreatment device, comprising: a substrate having multiple channels of a porous material, a centerline of said channels being substantially parallel, said channels further comprising a first group of unplugged channels and a second group of channels in which plugs are placed in said channels near one end of said substrate, said substrate being situated in an exhaust stream of an internal combustion engine wherein said first group of unplugged channels are unplugged along the entire length of said substrate.

19. The device of claim 18, further comprising: a urea injector coupled to an exhaust of said engine, said injector located upstream of said substrate.

20. The device of claim 19, further comprising:

an ammonia sensor coupled to said exhaust of said engine downstream of said substrate; and

an electronic control unit coupled to said engine, said urea injector, and said ammonia sensor, said electronic control unit commanding said urea injector based on an output of said ammonia sensor.

21. The device of claim 1 wherein a solid acidic material is deposited on a surface of said substrate thereby providing sites for urea reductant to be adsorbed.

22. The device of claim 1 wherein said substrate is made of cordierite having 80 to 600 cells per square inch.

23. The device of claim 1 wherein said first group and said second group are substantially similar in number.

24. The device of claim 1 wherein said first group is substantially greater in number than said second group.

25. The device of claim 1 wherein said substrate is made of cordierite having at least 10% porosity prior to washcoating.

26. The device of claim 1 wherein said substrate has a porosity greater than 50%.

27. The device of claim 1 wherein said substrate comprises tapered channels wherein said first group decreases in diameter from an upstream end to a downstream end and said second group increases in diameter from said upstream end to said downstream end.

28. The device of claim 1 wherein a length of said substrate is greater than 1.5 times a diameter of said substrate.

29. The device of claim 1 wherein said first group of channels has a washcoat and said second group of channels is impregnated with washcoat materials.

30. The device of claim 1 wherein said substrate is washcoated so as to render said substrate a LNT.

31. The device of claim 1 wherein said substrate is washcoated so as to render said substrate a TWC.

32. The device of claim 1 wherein said substrate is washcoated so as to render said substrate an oxidation catalyst.

33. The device of claim 1 wherein said channels are square in cross section and each of said first group of channels is adjacent to members of said second group of channels.

34. The device of claim 1 wherein said first group of channels comprises an outer ring of said substrate and said second group of channels comprises and inner portion of said substrate.