US20090120081A1

US20090120081A1 - Exhaust gas treatment system

Info

Publication number: US20090120081A1
Application number: US12/188,869
Authority: US
Inventors: Shiguang Zhou; Joel John Beltramo; Charles Ernest Gray; Eric Hermann
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2007-11-08
Filing date: 2008-08-08
Publication date: 2009-05-14

Abstract

An exhaust gas aftertreatment system having a housing and a plurality of separate diesel particulate filter bricks disposed in the housing. A first one of such bricks is disposed upstream of a second one of the bricks. The second one of the bricks has channels with closed downstream ends and open upstream ends and channels with closed upstream ends and open downstream ends and the upstream brick has channels with closed downstream ends and open upstream ends and bypass channels with open upstream ends and open downstream ends.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional application No. 60/986,351 filed Nov. 8, 2007, the entire subject matter thereof being incorporated herein by reference.

TECHNICAL FIELD

This invention relates generally to exhaust gas treatment systems and more particularly to particulate filters used in such systems.

BACKGROUND AND SUMMARY

As is known in the art, most current diesel exhaust gas treatment systems today include a DOC (diesel Oxidation catalyst) followed by a DPF (Diesel particular Filter). The DPF includes a substrate (sometimes referred to as a substrate brick or brick) with the outlet end closed on the inlet channel and the inlet end closed on the outlet channel as shown in FIG. 1. Exhaust gas flows through the inlet channel, crosses the wall of cells, and then exits through the outlet channel. The particles are filtrated in the inlet channel.
As is also known in the art, particulate filters are used in the exhaust systems of internal combustion engines, especially diesel engines, (diesel particulate filters or DPF) to trap and remove particulate matter which is primarily formed of carbon based material. As the engine exhaust passes through the DPF, the particulates are trapped in the filter and accumulate over time. This leads to an increase in the resistance of the exhaust gas flow through the DPF, and therefore, to an increase in the backpressure on the engine. This increase in backpressure has an adverse effect on engine operation, and especially on fuel consumption. In order to reduce backpressure to acceptable levels, the DPF is periodically regenerated by burning off the accumulated particulates, most of which are combustible.
As is also known in the art, the Ring off crack (ROC) of the DPF substrate may result in reduced emission effectiveness. The inventors have discovered that ROC is caused by high temperature and uneven temperature distribution resulting in high temperature gradients across the brick; high aspect ratios (length/diameter) of the brick resulting in lower brick strength; improper regeneration temperature control and soot loading detection errors resulting in soot over-loading in the brick.
The inventors have recognized that most packaging conditions do not allow the optimum brick aspect ratio (<1.2) with the current design concept; some current control strategies are incapable of detecting soot over-loading and controlling the regeneration temperature within the allowable limits, further, reducing soot-loading target will cause high regeneration frequency.
In accordance with the invention, an exhaust gas aftertreatment system is provided having a filter with a plurality of bricks with channels having common, porous walls, a first one of the bricks being disposed upstream of a second one of the bricks, the upstream brick having closed outlet ends and open inlet ends and bypass channels with open inlet end and open outlet ends.
In one embodiment, each one of the bricks has channels with common, porous walls, the common walls being porous to allow untrapped portions of particles in the exhaust gas to pass through such walls.
In one embodiment, the second one of the bricks has channels with closed downstream ends and open upstream ends and channels with closed upstream ends and open downstream ends and the upstream brick has channels with closed downstream ends and open upstream ends and channels with open upstream ends and open downstream ends.
In accordance with another embodiment, an exhaust gas aftertreatment system is provided having: a housing; and a plurality of separate diesel particulate filter bricks disposed in the housing, a first one of such bricks being disposed upstream of a second one of the bricks, the second one of the bricks having channels with closed downstream ends and open upstream ends and channels with closed upstream ends and open downstream ends and the upstream brick having channels with closed downstream ends and open upstream ends and bypass channels with open upstream ends and open downstream ends.
In accordance with another embodiment, an exhaust gas aftertreatment system is provided having: a first particulate filter stage brick having a plurality of channel therein, such channels having common walls, upstream ends of the channels facing an inlet flow of the exhaust gas, a first portion of the channels being bypass channels having open upstream ends and open downstream ends and a second portion of the channels being particulate trapping channels having open upstream ends and closed downstream ends to trap portions of particulates in the exhaust gas, the common walls being porous to allow untrapped portions of particles in the exhaust gas to pass between the channels; a second particulate filter stage brick disposed having a plurality of channel therein, such channels having common walls, upstream ends of the channels facing an inlet flow of the exhaust gas processed by the first particulate filter stage, a first portion of the channels being exhaust channels having open downstream ends and closed upstream ends and a second portion of the channels being particulate trapping channels having open upstream ends and closed downstream ends to trap portions of particulates in the exhaust gas, the common walls being porous to allow untrapped portions of particles in the exhaust gas to pass between the channels.
In one embodiment, the bypass channels of the first particulate filter stage brick face the particulate trapping channels of the second particulate filter stage brick stage and wherein the particulate trapping channels of the first particulate filter stage brick face the exhaust channels of the second particulate filter stage brick stage.
In accordance with another embodiment, a method is provided for treating exhaust gas, comprising: passing the exhaust gas through a filter having a plurality of bricks with channels having common, porous walls, a first one of the bricks being disposed upstream of a second one of the bricks, the upstream brick having channels with closed outlet ends and open inlet ends and bypass channels with open inlet end and open outlet ends; and increasing temperature of the upstream brick generating an exothermal energy release of energy from the upstream brick to the downstream brick concurrently as a portion of the exhaust gas flows through the bypass channels in the upstream brick to the downstream brick resulting in regeneration in the downstream brick.
The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims.

DESCRIPTION OF DRAWINGS

FIG. 1 is a sketch of a DPF according to the PRIOR ART;

FIGS. 2A and 2B are sketches of a DPF according to the invention; and

FIG. 3 is a flowchart of a regeneration process occurring in the DPF of FIG. 2.

Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

Referring now to FIGS. 2A and 2B, an exhaust gas aftertreatment system, here a diesel particulate filter (DPF) 10 is shown. The DPF 10 includes a housing 12 and a plurality of, here for example two, longitudinally separated diesel particulate filter bricks 14, 16 disposed in the housing 12. A first one of such bricks 14 is disposed upstream of a second one of the bricks 16. More particularly, the second one of the bricks 16 has outlet ends closed on inlet channels 18 a and inlet ends closed on outlet channels 18 b. The upstream brick 14 has channels 20 a with closed outlet ends and channels 20 b (herein sometimes referred to as bypass channels) with open inlet ends and open outlet ends. It is noted that there is a gap between the two bricks 14, 16.
More particularly, the first one of such bricks 14 serves as a first particulate filter stage brick and the second one of such bricks 16 serves as a second particulate filter stage brick. With both bricks 14, 16 the channels 18 a, 18 b, 20 a, 20 b have porous common walls 24.
Considering in more detail the first particulate filter stage brick 14, the upstream ends of the channels face an inlet flow of the exhaust gas. A first portion of the channels 20 b have open upstream ends and open downstream ends and a second portion of the channels 20 a have open upstream ends and closed downstream ends to trap portions of particulates in the exhaust gas. The common walls 24 are, as noted above, porous to allow untrapped portions of particles in the exhaust gas to pass into the first portion of the channels 20 a through walls 24 and exit at channels 20 b. It is noted that the first portion of the channels 20 b being open at both the upstream end and the downstream end allow a portion of the exhaust gas to pass directly through such first portion of the channels 20 b and hence are referred to as bypass channels.
The second particulate filter stage 16 has upstream ends of a first portion of the channels 18 a of the second particulate filter stage brick 16 facing an inlet flow of the exhaust gas processed by the first particulate filter stage brick 14 have open upstream ends and closed downstream ends to trap portions of particulates in the exhaust gas and a second portion 18 b of the second particulate filter stage brick 16 have closed upstream ends and open downstream ends to exhaust the filtered gas. More particularly, upstream open ends of the first portion of the channels 18 a of the second particulate filter stage brick 16 face the bypass channels 20 b of the exhaust gas processed by the first particulate filter stage brick 14. The common walls 24 of the second particulate filter stage brick 16 are porous to allow untrapped portions of particles in the exhaust gas processed by the first particulate filter stage brick 14 and passed to the first portion of the channels 18 a of the second particulate filter stage brick 16 to pass into the second portion of the channels 18 b of the second particulate filter stage brick 16.
Thus, with such an arrangement, there is more than one substrate or brick in the DPF can or housing. The rear or downstream brick is similar to current DPF brick with the outlet end closed on the inlet channel (sometimes referred to as cells) and the inlet end closed on the outlet channel. The front or upstream brick or bricks have some channels (sometimes referred to as cells) closed at outlet ends and the rest of the cells open at both ends as flow bypasses.
With such an arrangement, by having separate brick, and hence shorter in length bricks, the aspect ratio of each brick can be below 1.2. The aspect ratio of each substrate or brick can be determined with the number of substrates, diameter and required DPF volume. For example, a 12″×8″ substrate has a 1.5 aspect ratio (12/8=1.5) in one current design and it can be replaced with two 6″×8″ substrates with a 0.75 aspect ratio (6/8=0.75) By shortening the substrate, the length over which thermal stresses can build is dramatically reduced.
With such an arrangement, optimal distribution of soot (i.e., particulate) loading may be provided by proper design and hence proper distribution among the bricks to thereby reduce the temperature during regeneration in any one brick. Adjusting the ratio of closed cells to bypass cells (i.e., channels open at both ends) in the upstream bricks provides optimal soot distribution between bricks. For a 10 liter DPF system in FIG. 1, with 100 g soot loading target, all the soot is contained in one substrate. With the DPF 10 in FIG. 2, this 100 g soot can be allocated into two substrates (5 liter each) with 50 g of soot in each using a two stage DPF system in accordance (FIG. 2B) with the invention that has about 50% of closed cells and 50% bypass cells in the front or upstream brick. The brick inside temperature can be greatly reduced in a 5 liter substrate with 50 g soot comparing with a 10 liter substrate with 100 g soot.
With such an arrangement, multi stage regenerations are provided and reduce the risk of uncontrolled regeneration. The first stage regeneration is 100% active regeneration at the front or upstream brick, and the rest of the stage regenerations will occur at the following bricks (it being understood that the system may have more than one brick 14 spaced one behind the other) using the exothermal and hydrocarbon (HC) slip from the upstream brick. The regenerations start in series between bricks and occur in different bricks that will greatly reduce the temperature. The temperature can increase to above 1200 C at a “drop to idle” regeneration condition with 100 g soot loaded in the DPF system of FIG. 1, but will maintain around 900 C with the DPF 10 (FIG. 2) according to the invention.
With such an arrangement, active regeneration occurs in the front or upstream brick and passive regeneration occurs in the rear or downstream brick using exothermal of front brick. Thus, regeneration occurs in series between bricks.
More particularly the regeneration process is described in the flowchart of FIG. 3. Thus, first there is a soot-loading estimate by the system using any conventional method. Next, regeneration is triggered using any conventional method. Next, as a result of the triggering of the regeneration process, there is an increase in the inlet temperature of the upstream brick 14 to the target temperature. As a result of the increased temperature in the upstream brick, there is an exothermal energy release from the upstream brick to the downstream brick concurrently as a portion of the exhaust gas flow through the bypass channels in the upstream brick to the downstream brick and as a result, regeneration starts at the downstream brick. It is noted that during the regeneration in both the upstream and downstream bricks, there is a reduction in backpressure. The process continues until regeneration is completed.
A number of embodiments of the invention have been described. Nevertheless, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, other embodiments are within the scope of the following claims.

Claims

1. An exhaust gas aftertreatment system, comprising a filter having a plurality of bricks with channels having common, porous walls, a first one of the bricks being disposed upstream of a second one of the bricks, the upstream brick having channels with closed outlet ends and open inlet ends and bypass channels with open inlet end and open outlet ends.

2. The system recited in claim 1 wherein each one of the bricks has channels with common, porous walls, the common walls being porous to allow untrapped portions of particles in the exhaust gas to pass through such walls.

3. The system recited in claim 2 wherein the second one of the bricks has channels with closed downstream ends and open upstream ends and channels with closed upstream ends and open downstream ends and the upstream brick has channels with closed downstream ends and open upstream ends and channels with open upstream ends and open downstream ends.

4. An exhaust gas aftertreatment system, comprising:

a housing;

a plurality of separate diesel particulate filter bricks disposed in the housing, a first one of such bricks being disposed upstream of a second one of the bricks, the second one of the bricks having channels with closed downstream ends and open upstream ends and channels with closed upstream ends and open downstream ends and the upstream brick having channels with closed downstream ends and open upstream ends and bypass channels with open upstream ends and open downstream ends.

5. An exhaust gas aftertreatment system, comprising:

a first particulate filter stage brick having a plurality of channel therein, such channels having common walls, upstream ends of the channels facing an inlet flow of the exhaust gas, a first portion of the channels being bypass channels having open upstream ends and open downstream ends and a second portion of the channels being particulate trapping channels having open upstream ends and closed downstream ends to trap portions of particulates in the exhaust gas, the common walls being porous to allow untrapped portions of particles in the exhaust gas to pass between the channels;

a second particulate filter stage brick disposed having a plurality of channel therein, such channels having common walls, upstream ends of the channels facing an inlet flow of the exhaust gas processed by the first particulate filter stage, a first portion of the channels being exhaust channels having closed upstream ends and open downstream ends and a second portion of the channels being particulate trapping channels having open upstream ends and closed downstream ends to trap portions of particulates in the exhaust gas, the common walls being porous to allow untrapped portions of particles in the exhaust gas to pass between the channels.

6. The exhaust gas aftertreatment system recited in claim 5 wherein the bypass channels of the first particulate filter stage brick face the particulate trapping channels of the second particulate filter stage brick stage and wherein the particulate trapping channels of the first particulate filter stage brick face the exhaust channels of the second particulate filter stage brick stage.

7. A method for treating exhaust gas, comprising:

passing the exhaust gas through a filter having a plurality of bricks with channels having common, porous walls, a first one of the bricks being disposed downstream of a second one of the bricks, the upstream brick having closed outlet ends and open inlet ends and bypass channels with open inlet end and open outlet ends;

increasing temperature of the upstream brick generating an exothermal energy release of energy from the upstream brick to the downstream brick concurrently as a portion of the exhaust gas flows through the bypass channels in the upstream brick to the downstream brick resulting in regeneration in the downstream brick.