Electrical Heating Unit for Exhaust Gas System and Method for its Manufacture

Abstract

A method for the manufacture of an electrical heating unit for use in an exhaust gas system and the use of an electrical heating unit obtainable by this method are described. The method comprises forming the electrical heating unit as a single piece by additive layer manufacturing. An exhaust gas system comprising the electrical heating unit and a downstream catalyst article is also described.

Claims

1. A method for the manufacture of an electrical heating unit for use in an exhaust gas system, the method comprising forming the electrical heating unit as a single piece by additive layer manufacturing, wherein the electrical heating unit comprises: a tubular body comprising a wall for, in use, conducting a flow of an exhaust gas to be heated; a first electrode extending through a hole in the wall of the tubular body without contacting the wall of the tubular body; a second electrode in contact with the wall of the tubular body; and a resistive heating element arranged within the tubular body extending from the first electrode to the wall of the tubular body to, in use, heat the flow of an exhaust gas, wherein the electrical heating unit is formed from a metal having a melting point of at least 1000° C.

2. The method according to claim 1, wherein the metal is selected from the group consisting of an FeCr alloy, an NiCr alloy, stainless steel and Inconel.

3. The method according to claim 1, wherein the second electrode is arranged coaxially around the first electrode without contacting the first electrode, preferably wherein the method further comprises providing an insulating sleeve between the first electrode and the second electrode.

4. The method according to claim 1, wherein the resistive heating element is in the form of a substantially planar continuous strip having a length greater than a diameter of the tubular body.

5. The method according to claim 4, wherein the resistive heating element is in the form of a pair of spirals that meet near a centre of the tubular body, a first spiral extending from the first electrode and a second spiral extending from the wall of the tubular body.

6. The method according to claim 1, wherein the resistive heating element contacts at least 5% of a circumference of the wall of the tubular body.

7. The method according to claim 1, wherein the resistive heating element has a thickness in an axial direction of the tubular body of at least 5mm.

8. The method according to claim 1, wherein the resistive heating element has a width in a plane perpendicular to an axial direction of the tubular body of at least 5mm.

9. The method according to claim 1, wherein the resistive heating element occupies at least 25% of a cross-sectional area of the tube in a plane perpendicular to an axial direction of the tubular body.

10. The method according to claim 1, wherein the resistive heating element has a repeated lattice structure providing a plurality of through-channels through the resistive heating element, preferably wherein the repeated lattice structure further provides a plurality of through-channels orthogonal to an axial direction of the tubular body.

11. The method according to claim 1, wherein the step of forming the electrical heating unit as a single piece by additive layer manufacturing includes the removal of bridging material between the first and second electrodes as formed.

12. The method according to claim 1, wherein the method further comprises coating the resistive heating element with a catalyst composition, preferably a catalyst composition comprising one or more platinum group metals.

13. An electrical heating unit obtainable by the method of claim 1.

14. An electrical heating unit for use in an exhaust gas system, the electrical heating unit comprising: a tubular body comprising a wall for, in use, conducting a flow of an exhaust gas to be heated; a first electrode extending through a hole in the wall of the tubular body without contacting the wall of the tubular body; a second electrode in contact with the wall of the tubular body; and a resistive heating element arranged within the tubular body extending from the first electrode to the wall of the tubular body to, in use, heat the flow of an exhaust gas, and wherein the resistive heating element has a repeated lattice structure providing a plurality of through-channels through the resistive heating element, wherein the electrical heating unit is formed from a metal having a melting point of at least 1000° C.

15. An exhaust gas system comprising the electrical heating unit according to claim 14 and a downstream catalyst article.

Description

[0061] The invention will now be described further with reference to the following figures in which:

[0062] FIG. 1 shows a cross-section of an exhaust gas treatment system comprising the electrical heating unit of the invention.

[0063] FIG. 2 shows an end-section through an electrical heating unit of the invention.

[0064] FIGS. 3A-3D show different configurations of the electrical heating unit of the invention.

[0065] FIG. 1 shows an exhaust gas treatment system 1. The exhaust gas treatment system 1 comprises a duct 5 through which, in use, an exhaust gas 10 to be treated is flowed. The duct 5 includes a catalyst article 15 for treating the exhaust gas 10.

[0066] The catalyst article 15 is typically a flow-through monolith comprising a catalyst washcoat provided on a porous honeycomb body. A catalyst article 15 typically has an optimum operating temperature and, in the context of exhaust gases from a gasoline or diesel engine, this will generally be at temperatures in excess of 200° C.

[0067] Upstream of the catalyst article 15 there is provided a heating component 20. This is provided as a section 25 of the duct 5, typically welded to an inlet duct portion 30 and a catalyst-containing duct portion 35.

[0068] The section 25 comprises a tubular body 40 directly connected to a return electrode 45. The tubular body 40 is also connected to a supply electrode 50 via a resistive heating element 55. The return electrode 45 has a tubular form and is provided coaxially around the supply electrode 50. The return electrode 45 is spaced from the supply electrode 50 by an insulating ring 60 to prevent short circuits. The return electrode 45 and the supply electrode 50 are connected to an DC power source 65.

[0069] The resistive heating element 55 is provided across the lumen 70 of the duct 5 to heat the exhaust gas 10. Advantageously, therefore, it takes a long path crossing between the supply electrode 50 and the tubular body 40. This extends the surface area of the resistive heating element 55, increasing the amount of heating that can be provided to the gas. A preferred 2D design is a pair of spirals meeting in the middle of the lumen 70. More complex 3D designs, such as stacked spirals can also be envisioned to extend the length of the resistive heating element 55. It is important, nonetheless that the rigidity of the resistive heating element 55 is maintained.

[0070] In use, the DC power source 65 provides a voltage across the supply electrode 50 and the return electrode 45, causing the resistive heating element 55 to heat up. The heating effect heats the exhaust gas 10 to an operating temperature of the catalyst article 15, such as in excess of 200° C. This causes the temperature of the catalyst article 15 to quickly reach a desired operating temperature and for the catalyst washcoat on the catalyst article 15 to catalyst the decomposition of species in the exhaust gas 15.

[0071] The section 25 is formed by additive layer manufacture (ALM). This means that the entirety of the tubular body 40, the supply and return electrodes 45, 50 and the resistive heating element 55 are formed of a single material, such as FeCr-alloy and in a single step. It may be necessary to construct bridging support components, such as between the supply and return electrodes 45, 50, but these are removed before use.

[0072] Advantageously, the resistive heating element 55 can itself have a porous structure, preferably a regular repeating matrix. Such a matrix permits the exhaust gas 10 to flow through the resistive heating element 55 as well as within the lumen 70 which is not occupied by the resistive element 55. This increases the efficiency and rate of heat transfer that can be achieved. Such an expanded lattice or matrix can be readily achieved by ALM without compromising on the structural integrity of the resistive heating element 55.

[0073] FIGS. 3A-3D show different configurations of the resistive heating element 55. In particular:

[0074] FIG. 3A shows a resistive heating element 55 formed by a pair of spirals that meet near a centre of the lumen 70 of the tubular body 40, a first spiral extending from the supply electrode 50 and a second spiral extending from the wall of the tubular body 40. The resistive heating element 55 has a very fine expanded structure providing a plurality of regular through-channels parallel to and orthogonal to the axis of the tubular body 40. In order to increase the rigidity of the resistive heating element 55 it is connected to around 40% of the tubular wall 40.

[0075] FIG. 3B shows a resistive heating element 55 formed by a pair of spirals that meet near a centre of the lumen 70 of the tubular body 40, a first spiral extending from the supply electrode 50 and a second spiral extending from the wall of the tubular body 40. Compared to FIG. 3A, the resistive heating element 55 occludes a greater percentage of the lumen of the tubular body 40. The resistive heating element 55 has a very fine expanded structure providing a plurality of regular through-channels parallel to and orthogonal to the axis of the tubular body 40 (i.e. into and in the plane of the figure). In order to increase the rigidity of the resistive heating element 55 it is connected to around 40% of the tubular wall 40.

[0076] FIG. 3C shows a resistive heating element 55 formed by a pair of spirals that meet near a centre of the lumen 70 of the tubular body 40, a first spiral extending from the supply electrode 50 and a second spiral extending from the wall of the tubular body 40. Compared to FIGS. 3A and 3B, the resistive heating element 55 has a coarser expanded structure providing a plurality of regular through-channels only parallel to the axis of the tubular body 40. In order to increase the rigidity of the resistive heating element 55 it is connected to around 40% of the tubular wall 40.

[0077] FIG. 3D shows a resistive heating element 55 formed by a pair of spirals that meet near a centre of the lumen 70 of the tubular body 40, a first spiral extending from the supply electrode 50 and a second spiral extending from the wall of the tubular body 40. Compared to FIGS. 3A and 3B, the resistive heating element 55 has a coarser expanded structure providing a plurality of regular through-channels parallel to and orthogonal to the axis of the tubular body 40. In order to increase the rigidity of the resistive heating element 55 it is connected to around 40% of the tubular wall 40.

[0078] The invention will now be described further in relation to the following non-limiting examples:

[0079] Heaters were printed using a laser sintering additive process having an FeCrAlloy 1.4767 powder feed. The electrode patterns are shown in FIGS. 3A-3D. Some of the Heaters were designed with structure that could not be made with wire fed or foil processes. Heaters have been tested in air flow at low and high power. High power tests have had hydrocarbons burnt upstream of the heater to generate temperatures akin to those experienced in real exhaust systems. Heaters have shown a constant electrical resistance across a range of powers tested. Heaters have shown a proportional relationship between power in and temperature out, so a heat transfer coefficient may be calculated.

[0080] A high power supply was purchased for Engine Testing work and fitted with safety devices for shutdown when test cell/DPG rig is open. A steady state test has been developed and was used. There are limitations to the steady state test; however, the test represents a heated gas feed which has a composition and transient elements of automotive exhaust. The DPG test has a high airflow compared to the size of the part, and has a fast heat ramp from ambient to 250° C. Electrical power can be activated in a consistent way test-to-test. High Power testing has been completed on the reference EHC from Vitesco-Emitec and on all of the designs of prototype heaters made using ALM by Sandvik.

[0081] Resistive heating of a gas flow for an exhaust application shows that heating depends on the power applied. Thus, a heat transfer coefficient can be applied to each heater design. For this application a low heat transfer coefficient is good. Three of the ALM parts tested showed a lower heat transfer coefficient than the reference resistive heater.

[0082] The ALM heaters had lower resistance than the reference part. Low resistance heaters have a robustness challenge to cope with high electrical current. High current is required to deliver the power required for the desired temperature for heat up function. High current caused some parts to fail; whether through a melting event, or alternatively through a fracture event. Therefore, it is required that the designs have a suitable combination of heat transfer coefficient and resistance.

TABLE-US-00001 Heat transfer Design coefficient Comment Reference   1100 Wm.sup.−2K.sup.−1 Emitec wrapped foils 1*   610 Wm.sup.−2K.sup.−1 Wide spirals, honeycomb through-channels 2** 12,240 Wm.sup.−2K.sup.−1 Thin multiple spirals-mechanical failure during test 3*   790 Wm.sup.−2K.sup.−1 Medium spiral, fine lattice (very fine expanded structure) 4*   760 Wm.sup.−2K.sup.−1 Medium spiral, large lattice 5  8,180 Wm.sup.−2K.sup.−1 Large lattice mesh (not a single discharge route) 6  7,480 Wm.sup.−2K.sup.−1 Fine lattice mesh (not a single discharge route)

[0083] Examples 1, 3 and 4 demonstrated desirable properties for the application. Example 2 suffered a failure suggesting that the specific configuration lacked sufficient rigidity.

[0084] Although preferred embodiments of the invention have been described herein in detail, it will be understood by those skilled in the art that variations may be made thereto without departing from the scope of the invention or of the appended claims.