Exhaust aftertreatment system having a diesel particulate filter manufactured for reducing thermal gradients

Abstract

A diesel particulate filter includes a honeycomb structure having a plurality of elongated cell walls defining a plurality of passages and a skin layer surrounding the honeycomb structure. The filter is regenerated by oxidizing particulate matter trapped within the honeycomb structure by heating at least a portion of the honeycomb structure. A thermal gradient between the honeycomb structure and the skin layer is limited to less than a crack causing thermal gradient by utilizing at least one of maintaining a ratio of a heat capacity of the skin layer to a heat capacity of the cell walls at less than about 5, blocking at least those passages comprising a perimeter of the honeycomb structure, and heating the skin layer from an external side.

Description

TECHNICAL FIELD

The present disclosure relates generally to reducing thermal gradients in an exhaust aftertreatment system, and more particularly to reducing thermal gradients between a honeycomb structure and a skin layer of a diesel particulate filter during a regeneration process.

BACKGROUND

Recent governmental regulations have prompted development and application of exhaust aftertreatment systems to reduce particulate matter emissions from both on-highway and off-highway vehicles. Exhaust aftertreatment systems for diesel engines typically include a diesel particulate filter (DPF). A DPF generally consists of a ceramic honeycomb structure that is surrounded by a non-permeable skin layer and includes numerous channels that are blocked at alternate ends. This structure forces exhaust gas to flow through the porous walls between the channels, leaving particulate matter deposited on the walls. Periodically, or once a substantial amount of particulate matter is collected within the DPF, it must be cleaned out to prevent blockage. The process of removing the accumulated particulate matter from the DPF is referred to generally as regeneration.

While a variety of strategies of both active and passive regeneration are known, a common method includes quickly heating the particulate matter to a temperature at which it combusts. This involves heating the exhaust gas, and as a result, the DPF, to very high temperatures. Since the skin layer of the DPF is more dense than the porous walls of the honeycomb structure, it has a much higher heat capacity. During the regeneration process the porous walls heat up much more rapidly than the skin layer and create large thermal gradients. The stress caused by these large thermal gradients may result in the formation of cracks in the DPF. Ultimately, these cracks may lead to failure of the DPF.

U.S. Pat. No. 7,073,327 teaches a diesel particulate filter having a reduced temperature gradient during a regeneration process. Specifically, partition walls defining cells of the filter have an increased wall thickness, and a cell density of the filter is increased. Setting the thickness and density to appropriate values provides a reduced temperature gradient in the filter. This reference does not, however, contemplate a temperature gradient between a honeycomb structure of the filter and a skin layer surrounding the filter.

The present disclosure is directed to one or more of the problems set forth above.

SUMMARY OF THE INVENTION

In one aspect, a method of regenerating a diesel particulate filter includes a step of oxidizing particulate matter trapped within the honeycomb structure by heating at least a portion of the honeycomb structure. A thermal gradient between the honeycomb structure and the skin layer is limited to less than a crack causing thermal gradient. The limiting step includes at least one of maintaining a ratio of a heat capacity of the skin layer to a heat capacity of the cell walls at less than about 5, blocking at least those passages comprising a perimeter of the honeycomb structure, and heating the skin layer from an external side.

In another aspect, an exhaust aftertreatment system includes a can having a gas inlet and a gas outlet. The exhaust aftertreatment system also includes a substrate having a honeycomb structure and a skin layer surrounding the honeycomb structure disposed within the can. The honeycomb structure comprises a plurality of elongated cell walls extending from the gas inlet to the gas outlet and defining a plurality of passages, wherein the cell walls are permeable relative to the skin layer. The exhaust aftertreatment system further includes means for limiting a thermal gradient between the honeycomb structure and the skin layer to less than a crack causing thermal gradient during a regeneration process. The limiting means include at least one of means for maintaining a ratio of a heat capacity of the skin layer to a heat capacity of the cell walls at less than about 5, means for blocking at least those passages comprising a perimeter of the honeycomb structure, and means for heating the skin layer from an external side.

In still another aspect, a diesel particulate filter includes a honeycomb structure having a plurality of elongated cell walls defining a plurality of passages and a skin layer surrounding the honeycomb structure. The diesel particulate filter also includes at least one crack avoidance feature of: the skin layer having a heat capacity less than about five times the heat capacity of the cell walls and at least those passages comprising a perimeter of the honeycomb structure being blocked. The crack avoidance feature limits a thermal gradient between the honeycomb structure and the skin layer to less than a crack causing thermal gradient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective diagrammatic view of a diesel particulate filter according to the present disclosure;

FIG. 2 is a top diagrammatic view of one embodiment of the diesel particulate filter of FIG. 1 according to the present disclosure;

FIG. 3 is a top diagrammatic view of another embodiment of the diesel particulate filter of FIG. 1 according to the present disclosure;

FIG. 4 is a top diagrammatic view of yet another embodiment of the diesel particulate filter of FIG. 1 according to the present disclosure;

FIG. 5 is a graph of thermal gradient versus skin thickness according to the embodiment of FIG. 2; and

FIG. 6 is a graph of thermal gradient versus number of cells blocked according to the embodiment of FIG. 4.

DETAILED DESCRIPTION

An exemplary embodiment of a diesel particulate filter (DPF) 10 is shown generally in FIG. 1. DPF 10 includes a can 12 having a gas inlet 14 and a gas outlet 16. A honeycomb structure 18 is disposed within the can 12 and includes a plurality of elongated, permeable cell walls 20 extending from the gas inlet 14 to the gas outlet 16. The honeycomb structure 18 may be composed of a ceramic material, such as, for example, cordierite or porcelain. The cell walls of the honeycomb structure 18 are uniformly thin and define a plurality of passages that are blocked at alternate ends, in a checkerboard fashion. For example, a passage 22 is blocked at the gas inlet 14. Alternatively, a passage 24 is not blocked at the gas inlet 14 and is, therefore, blocked at the gas outlet 16. The passages may comprise a square shape, as shown, or may comprise any other geometric shape.

Turning to FIG. 2, a top view of one embodiment of DPF 10 is shown. The view of FIG. 2 may be of either the gas inlet 14 or the gas outlet 16 (both of FIG. 1) of the DPF 10. The DPF 10 is provided with a skin layer 30 interconnected with and extending continuously around the honeycomb structure 18. The skin layer 30 provides structural stability to the honeycomb structure and may also comprise a ceramic material, or any other suitable material, and may be non-permeable relative to the cell walls. Additionally, a mat layer 32 may be provided between the can 12 and the skin layer 30 to cushion the honeycomb structure 18 against shock and vibration. The mat layer 32 occupies space between the honeycomb structure 18 and the surrounding can 12 and may also serve to insulate against heat loss from the honeycomb structure.

A thickness of the skin layer 30 is selected to maintain a ratio of a heat capacity of the skin layer to a heat capacity of the cell walls 20 at less than about 5. Preferably, that ratio is maintained at less than about 2.5. In these and other examples, “about” indicates rounding to one significant digit. For example, 10.4 is about 10, 2.54 is about 2.5, etc.

FIG. 3 shows a top view of a second embodiment of DPF 10. As seen in the view of FIG. 3, at least those passages comprising a perimeter of the honeycomb structure 18, such as, for example, passages 40, 42, and 44, are blocked. Blocking may occur at the gas inlet 14 (FIG. 1), gas outlet 16 (FIG. 1), or both and may be achieved by masking or filling the passages. A single row of passages extending around the perimeter of the honeycomb structure 18 may be blocked. Alternatively, two, three, or more rows around the perimeter of the honeycomb structure 18 may be blocked.

Turning now to FIG. 4, a top view of a third embodiment of DPF 10 is shown. A heating layer 50 may be provided around the skin layer 30. The heating layer 50 may be formed of any substance useful for generating or conducting heat. For instance, heater 50 may be a thin film electric resistance heater of a type known in the art. Alternatively, a heater (not shown) may be provided external to the mat 32 and or can 12.

INDUSTRIAL APPLICABILITY

A DPF 10 generally consists of a ceramic honeycomb structure 18 that is surrounded by a non-permeable skin layer 30 and includes numerous channels, such as, for example, channels 22 and 24, that are blocked at alternate ends. This structure forces exhaust gas to flow through the porous walls between the channels, leaving particulate matter deposited on cell walls 20. Once a large amount of particulate matter is collected within the DPF 10, it must be cleaned out to prevent blockage. The process of removing the accumulated soot or particulate matter from the DPF 10 is referred to generally as regeneration.

A common method of regeneration includes quickly heating the particulate matter to a temperature at which it combusts. This involves heating the exhaust gas, and as a result, the DPF 10, to very high temperatures. Since the skin layer 30 of the DPF 10 is more dense than the porous cell walls 20 of the honeycomb structure, it has a much higher heat capacity. During the regeneration process the porous cell walls 20 heat up much more rapidly than the skin layer 30 and create large thermal gradients. The stress caused by these large thermal gradients may result in the formation of cracks in the DPF 10. These thermal gradients may also be referred to as “crack causing thermal gradients.” Ultimately, these cracks may lead to failure of the DPF 10.

Utilizing one or more of the diesel particulate filter embodiments of the present disclosure maintains a thermal gradient between the cell walls and the skin layer below the crack causing thermal gradient. For example, the DPF of FIG. 2 provides a skin layer thickness that maintains the heat capacity ratio of the skin layer 30 to the cell walls 20 at less than about 2.5. If the cell walls 20 are about 0.3 mm thick and about 50% porous, providing a skin layer thickness of about 0.5 maintains the heat capacity ratio at less than about 2.5.

FIG. 5 is a graph 60 of thermal gradient 62, shown on the vertical axis, versus skin thickness 64, shown on the horizontal axis, according to the embodiment of FIG. 2. Depicted on the graph 60 is a sample gradient 66 showing thermal gradients between the honeycomb structure 18 and skin layer 30 for various thickness values of the skin layer. Wherein a thermal gradient between about 500° C./cm and 600° C./cm may be a crack causing thermal gradient, it can be seen that maintaining a skin layer thickness below about 1.5 mm may be desired. It may be further desirable to maintain a thickness of the skin layer 30 at about 0.5 mm.

The DPF 10 of FIG. 3 provides for blocking at least those cell passages comprising a perimeter of the honeycomb structure 18. Namely, one or more rows of cell passages along the exterior of the honeycomb structure 18 may be plugged. Blocking these cell passages reduces the heat transfer that is passed to these peripheral cells, and, therefore, the crack causing thermal gradients between the cell walls 20 and the skin layer 30.

FIG. 6 is a graph 70 of thermal gradient 72, shown on the vertical axis, versus number of cells blocked 74, shown on the horizontal axis, according to the embodiment of FIG. 3. Depicted on the graph 70 is a sample gradient 76 showing thermal gradients between the honeycomb structure 18 and skin layer 30 for various numbers of cell rows blocked. Since it may be desirable to maintain the thermal gradient during regeneration to below between about 500° C./cm and 600° C./cm, it may be desirable to block at least one to two rows of cells along the periphery of the honeycomb structure.

The DPF of FIG. 4 provides a heating layer 50 external to the skin layer 18. Heating the skin layer 30 externally helps compensate for the large difference in heat capacity between the skin layer 30 and the honeycomb structure 18. Reducing the heat gradient between the two layers during the regeneration process helps prevent occurrence of a crack causing thermal gradient.

While each embodiment alone may prevent crack causing thermal gradients, it may be desirable to use one of the embodiments in conjunction with one of the other embodiments. For example, a skin layer 30 of a desired thickness may be provided on a honeycomb structure 18 having at least those cell passages comprising a perimeter of the honeycomb being blocked. Alternatively, a skin layer 30 may be set to a desired thickness of a DPF 10 that includes a heater provided external the skin layer.

It should be understood that the above description is intended for illustrative purposes only, and is not intended to limit the scope of the present invention in any way. Thus, those skilled in the art will appreciate that other aspects of the invention can be obtained from a study of the drawings, the disclosure and the appended claims.

Claims

1. A method of regenerating a diesel particulate filter, wherein the diesel particulate filter includes a honeycomb structure having a plurality of elongated cell walls defining a plurality of passages and a skin layer surrounding the honeycomb structure, comprising:

oxidizing particulate matter trapped within the honeycomb structure by heating at least a portion of the honeycomb structure; and

limiting a thermal gradient between the honeycomb structure and the skin layer to less than a crack causing thermal gradient, wherein the limiting step includes at least one of maintaining a ratio of a heat capacity of the skin layer to a heat capacity of the cell walls at less than about 5, blocking at least those passages comprising a perimeter of the honeycomb structure, and heating the skin layer from an external side.

2. The method of claim 1, wherein the limiting step includes maintaining a ratio of the heat capacity of the skin layer to the heat capacity of the cell walls at less than about 2.5.

3. The method of claim 2, wherein the limiting step further includes limiting a thickness of the skin layer to less than about 0.5 mm.

4. The method of claim 1, wherein the limiting step includes at least two of maintaining a ratio of a heat capacity of the skin layer to a heat capacity of the cell walls at less than about 5, blocking at least those passages comprising a perimeter of the honeycomb structure, and heating the skin layer from an external side.

5. The method of claim 1, wherein the limiting step includes maintaining a ratio of a heat capacity of the skin layer to a heat capacity of the cell walls at less than about 5 and blocking at least those passages comprising a perimeter of the honeycomb structure.

6. The method of claim 5, wherein the limiting step further includes maintaining a ratio of a heat capacity of the skin layer to a heat capacity of the cell walls at less than about 2.5.

7. The method of claim 6, wherein the limiting step further includes limiting a thickness of the skin layer to less than about 0.5 mm.

8. An exhaust aftertreatment system, comprising:

a can having a gas inlet and a gas outlet;

a substrate having a honeycomb structure and a skin layer surrounding the honeycomb structure disposed within the can, the honeycomb structure comprising a plurality of elongated cell walls extending from the gas inlet to the gas outlet and defining a plurality of passages, wherein the cell walls are permeable relative to the skin layer; and

means for limiting a thermal gradient between the honeycomb structure and the skin layer to less than a crack causing thermal gradient during a regeneration process, wherein the limiting means include at least one of means for maintaining a ratio of a heat capacity of the skin layer to a heat capacity of the cell walls at less than about 5, means for blocking at least those passages comprising a perimeter of the honeycomb structure, and means for heating the skin layer from an external side.

9. The exhaust aftertreatment system of claim 8, wherein the limiting means include means for maintaining a ratio of the heat capacity of the skin layer to the heat capacity of the cell walls at less than about 2.5.

10. The exhaust aftertreatment system of claim 9, wherein the cell walls and the skin layer comprise a ceramic material.

11. The exhaust aftertreatment system of claim 10, wherein the maintaining means include limiting a thickness of the skin layer to less than about 0.5 mm.

12. The exhaust aftertreatment system of claim 8, wherein the limiting means include at least two of means for maintaining a ratio of a heat capacity of the skin layer to a heat capacity of the cell walls at less than about 5, means for blocking at least those passages comprising a perimeter of the honeycomb structure, and means for heating the skin layer from an external side.

13. The exhaust aftertreatment system of claim 8, wherein the limiting means include maintaining a ratio of a heat capacity of the skin layer to a heat capacity of the cell walls at less than about 5 and blocking at least those passages comprising a perimeter of the honeycomb structure.

14. The exhaust aftertreatment system of claim 13, wherein the limiting means further include maintaining a ratio of a heat capacity of the skin layer to a heat capacity of the cell walls at less than about 2.5.

15. The exhaust aftertreatment system of claim 14, wherein the limiting means further include limiting a thickness of the skin layer to less than about 0.5 mm.

16. A diesel particulate filter, comprising:

a honeycomb structure having a plurality of elongated cell walls defining a plurality of passages;

a skin layer surrounding the honeycomb structure;

at least one crack avoidance feature of: the skin layer having a heat capacity less than about five times the heat capacity of the cell walls and at least those passages comprising a perimeter of the honeycomb structure being blocked; and

wherein the crack avoidance feature limits a thermal gradient between the honeycomb structure and the skin layer to less than a crack causing thermal gradient.

17. The diesel particulate filter of claim 16, wherein the cell walls and the skin layer comprise a ceramic material.

18. The diesel particulate filter of claim 17, wherein the crack avoidance feature includes the skin layer having a thickness of less than about 0.5 mm.