US 20060251548 A1
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.
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.
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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.
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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.
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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.
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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.
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.
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 (
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.
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:
A 4-cylinder internal combustion engine 10 is shown, by way of example, in
Continuing to refer to
Exhaust aftertreatment device 38, shown in
As shown in
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
In the example shown in
Multiple aftertreatment units 38, 39, and 41 are shown in
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 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
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.