GB1560311A

GB1560311A - Combustion chamber components

Info

Publication number: GB1560311A
Application number: GB823477A
Authority: GB
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1978-02-13
Filing date: 1978-02-13
Publication date: 1980-02-06

Description

(54) IMPROVEMENTS IN OR RELATZG TO COMBUSTION CHAMBER COMPONENTS (71) I, THE SECRETARY OF STATE FOR DEFENCE. Whitehall, London, SW1A 2HB, a British Corporation Sole, do hereby declare the invention, for which I pray that a patent may be granted to me, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to combustion chamber components for internal combustion engines. In particular, but not exclusively, the invention relates to thermally insulated piston crowns.

The advantages to be gained from the application of thermal insulation to the bounding surfaces of a combustion chamber are well known and include such features as cooler operating condtions for all the associate,d components, higher permissible temperatures within the chamber, faster warm-up time, and consequent reduction of hydrocarbon and carbon monoxide levels in the engine exhaust.

Various forms of thermal insulation for regions bounding the combustion chamber have been proposed, but attachment of thermal insulation to a piston crown in particular, has proved difficult because the inertia forces generated when the piston is in reciprocating motion act to separate the insulating material from the piston. Layers of heat-resistant materials such as silicon nitride based ceramics have been used to provide insulation at the piston crown but, the thermal conductivity of these materials is such that fairly thick layers must be used to achieve effective insulation, with the result that the high inertia forces generated, by these consequently heavy layers tends to increase the problems of attachment.Furthermore, the thermal expansion rate of a ceramic material differs considerably from that of the piston metal to which it must be attached and consequently the layers are prone to cracking and eventual disintegra tion in operation. Some light-weight piston insulating devices have been used, such as an air-spaced flame plate fitted to the piston crown, but these are difficult to attach with robustness.

It is an object of the present invention to provide combustion chamber components having a thin, light-weight, robust and securely attached layer of thermal insulation material which is compatible with the component material.

According to the present invention, a combustion chamber component having a body, and a combustion face exposed in operation to combustion processes, includes a metallic retiform layer interjacent the body and the combustion face.

The retiform layer is preferably formed from a parent metal or metal alloy having a melting point sufficient to prevent softening at the operating temperatures encountered in the combustion chamber of an internal combustion engine, which may be either petrol or diesel driven. For example, an 80/20 nickel-chrome alloy which has a melting point of about 1400 C, would be suitable for all normal engines.

The retiform layer provides thermal insulation between the combustion face and the body, the degree of which is dependent upon the void content of the layer, which void content is preferably as high as possible consistent with rigidity of the layer. Various structural forms of suitably high void/metal ratio are possible. For example, the retiform layer may be of honeycomb-like construction, or may comprise a random network of thin metallic tubes such as that produced by electro-deposition of metal upon a decomposable, reticulated foam (as described in British Patent Specification No. 1,1 19A04), or alternatively may comprise a network of predominantly parallel tubes or struts arranged in a direction normal to the surface of the layer, such as may be manufactured by electro-deposition of metal upon a multiperforated, decomposable plate.

The high void content of such structural forms further endows the retiform layer with extreme lightness, a quality which is of particular advantage when the combustion chamber component is subject to high accelerations, as in the case of a piston.

The retiform layer may be attached to the body of the combusion chamber component by means of a high temperature adhesive, or by brazing, or by friction welding, and the outer surface of the retiform layer rendered impermeable to ingress of combustion gases, to provide the aforesaid combustion face, by the provision of a thin skin of the parent metal or metal alloy. The skin may be integrally fabricated with the retiform layer or attached by means of brazing, friction welding or, in the case of tubular network structures, may be formed by spraying with liquid metal or by smearing the surface of the layer so as to close the interstices.

The retiform layer may also be provided with a skin at its interface with the body in order to increase the area available for adhesive attachment.

The thickness selected for the retiform layer is of course dependent upon the temperature requirements of the particular engine to which the component is to be applied.

The retiform layer may be of uniform thickness and of any shape required for a specific combustion chamber component.

Non-planar shapes may be pressed from a planar sheet of suitably reticulated metal or metal alloy, or alternatively may be fabricated by electro-deposition upon an appropriately shaped substrate.

An embodiment of the invention will now be described by way of example with reference to the drawings accompanying the provisional specification, of which: Figure 1 is a sectional diagram of a piston and Figure 2 is a part-section of a former for a typical retiform layer.

Figure 1 illustrates an engine piston 1 located within a cylinder bore 2 and having a skirt 3 and a crown 4. A metallic retiform layer 5 having a combustion face 7 is attached to the upper surface of the crown 4 by means of a high temperature adhesive cement 6 such as Fortafix (Registered Trade Mark) type RM/8/3MED, Zircon grade.

The retiform layer 5 is of nickel-chrome alloy and typically has a void content which is about 85% of its total volume and a thermal conductivity of about 3 % to 5% of the parent alloy.

Preferably such layer is constructed integrally with the combustion face 7 by electrodeposition upon a purpose-made former as illustrated in Figure 2, which comprises a polyurethane disc 8 axially pierced with a regularly spaced arrangement of perforations 9. The disc 8 is destroyed by heating, after deposition has taken place.

Alternatively a suitable layer can be fabricated from a sheet of Retimet (Registered Trade Mark), a reticulated nickelchrome alloy manufactured as described in British Patent Specification No. 1,289,690, which material has a 95 % void content and a thermal conductivity which is only about 1% of the parent alloy. The fabrication is performed by first compressing a sheet of Retimet of, for example, 0.5 inch thickness at a pressure of 3000 lblsq. in.

to about one third of its original thickness, to provide a material having the required characteristics and capable of remaining rigid at the pressures likely to be encountered during operation of the piston. An appropriately sized disc is then cut and a thin skin of the parent alloy attached to one face so as to close the interstices of the structure either by friction welding or by brazing thereby providing the combustion face 7.

The inertia forces exerted by the retiform layer 5 and the combustion face 7 when the piston 1 is in reciprocal motion are extremely small and well below the breaking strain of conventional adhesives; the weight of such layer, but a 5 in diameter piston for example, being less than 3 ozs.

The face temperatures of piston crowns in highly rated turbocharged diesel engines can be in excess of the melting point of aluminium thus necessitating, in the absence of thermal insulation, the use of steel pistons or two-part pistons having nimonic crowns. Application of the present invention in such circumstances permits the continued use of aluminium pistons with their attendant advantages of lower weight and cost. Furthermore, the presence of the insulating layer will allow the face temperature to rise more rapidly from a cold start, thus leading to faster reduction of smoke emission levels.

The invention has been particularly described above in respect of the piston of an internal combustion engine, as the light weight of the metallic retiform layer makes it especially suitable for thermally insulating such a reciprocating component. However, it will be apparent that a similar retiform layer may be applied to other components adjacent the combusion chamber ie cylinder head, valves, exhaust port etc. in order to insulate these components, thereby reducing their operating temperatures and also reducing the heat repected to the engine coolant.

WHAT I CLAIM IS:- 1. A combustion chamber component

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. by electro-deposition of metal upon a multiperforated, decomposable plate. The high void content of such structural forms further endows the retiform layer with extreme lightness, a quality which is of particular advantage when the combustion chamber component is subject to high accelerations, as in the case of a piston. The retiform layer may be attached to the body of the combusion chamber component by means of a high temperature adhesive, or by brazing, or by friction welding, and the outer surface of the retiform layer rendered impermeable to ingress of combustion gases, to provide the aforesaid combustion face, by the provision of a thin skin of the parent metal or metal alloy. The skin may be integrally fabricated with the retiform layer or attached by means of brazing, friction welding or, in the case of tubular network structures, may be formed by spraying with liquid metal or by smearing the surface of the layer so as to close the interstices. The retiform layer may also be provided with a skin at its interface with the body in order to increase the area available for adhesive attachment. The thickness selected for the retiform layer is of course dependent upon the temperature requirements of the particular engine to which the component is to be applied. The retiform layer may be of uniform thickness and of any shape required for a specific combustion chamber component. Non-planar shapes may be pressed from a planar sheet of suitably reticulated metal or metal alloy, or alternatively may be fabricated by electro-deposition upon an appropriately shaped substrate. An embodiment of the invention will now be described by way of example with reference to the drawings accompanying the provisional specification, of which: Figure 1 is a sectional diagram of a piston and Figure 2 is a part-section of a former for a typical retiform layer. Figure 1 illustrates an engine piston 1 located within a cylinder bore 2 and having a skirt 3 and a crown 4. A metallic retiform layer 5 having a combustion face 7 is attached to the upper surface of the crown 4 by means of a high temperature adhesive cement 6 such as Fortafix (Registered Trade Mark) type RM/8/3MED, Zircon grade. The retiform layer 5 is of nickel-chrome alloy and typically has a void content which is about 85% of its total volume and a thermal conductivity of about 3 % to 5% of the parent alloy. Preferably such layer is constructed integrally with the combustion face 7 by electrodeposition upon a purpose-made former as illustrated in Figure 2, which comprises a polyurethane disc 8 axially pierced with a regularly spaced arrangement of perforations 9. The disc 8 is destroyed by heating, after deposition has taken place. Alternatively a suitable layer can be fabricated from a sheet of Retimet (Registered Trade Mark), a reticulated nickelchrome alloy manufactured as described in British Patent Specification No. 1,289,690, which material has a 95 % void content and a thermal conductivity which is only about 1% of the parent alloy. The fabrication is performed by first compressing a sheet of Retimet of, for example, 0.5 inch thickness at a pressure of 3000 lblsq. in. to about one third of its original thickness, to provide a material having the required characteristics and capable of remaining rigid at the pressures likely to be encountered during operation of the piston. An appropriately sized disc is then cut and a thin skin of the parent alloy attached to one face so as to close the interstices of the structure either by friction welding or by brazing thereby providing the combustion face 7. The inertia forces exerted by the retiform layer 5 and the combustion face 7 when the piston 1 is in reciprocal motion are extremely small and well below the breaking strain of conventional adhesives; the weight of such layer, but a 5 in diameter piston for example, being less than 3 ozs. The face temperatures of piston crowns in highly rated turbocharged diesel engines can be in excess of the melting point of aluminium thus necessitating, in the absence of thermal insulation, the use of steel pistons or two-part pistons having nimonic crowns. Application of the present invention in such circumstances permits the continued use of aluminium pistons with their attendant advantages of lower weight and cost. Furthermore, the presence of the insulating layer will allow the face temperature to rise more rapidly from a cold start, thus leading to faster reduction of smoke emission levels. The invention has been particularly described above in respect of the piston of an internal combustion engine, as the light weight of the metallic retiform layer makes it especially suitable for thermally insulating such a reciprocating component. However, it will be apparent that a similar retiform layer may be applied to other components adjacent the combusion chamber ie cylinder head, valves, exhaust port etc. in order to insulate these components, thereby reducing their operating temperatures and also reducing the heat repected to the engine coolant. WHAT I CLAIM IS:-

1. A combustion chamber component

having a body, and a combustion face exposed in operation to combustion processes, and including a metallic retiform layer inter- jacent the body and the combustion face.

2. A combustion chamber component as claimed in Claim 1 wherein the retiform layer comprises a random network of metal or metal alloy tubes or rods.

3. A combustion chamber component as claimed in Claim 1 wherein the retiform layer comprises a regular array of metal or metal alloy tubes or rods disposed perpendicularly to the combustion face.

4. A combustion chamber component as claimed in Claim 1 wherein the retiform layer comprises metal or metal alloy membranes disposed in a honeycomb arrangement.

5. A combustion chamber component as claimed in any one of the preceding claims wherein the retiform layer has a void content of at least 75% of its total volume.

6. A combustion chamber component as claimed in any one of Claims 2 to 5 wherein the retiform layer has a thermal conductivity no greater than 10% of that of the parent metal or metal alloy.

7. A combustion chamber component as claimed in any one of Claims 2 to 6 wherein the metal alloy is a nickelebrome alloy.

8. A combustion chamber component as claimed in any one of Claims 1 to 3. and 5 to 7 wherein the retiform layer is manufactured by a process of electro-deposition upon a thermally decomposable substrate.

9. A combustion chamber component as claimed in Claim 8 as dependent upon Claim 2 wherein the substrate comprises an open-cell plastics material foam.

10. A combustion chamber component as claimed in Claim 8 or dependent upon Claim 3 wherein the substrate comprises a plastics material former having multiple perforations in a direction normal to the resulting retiform layer.

11. A combustion chamber component as claimed in Claims 8 or 10 wherein the combustion face is manufactured integrally with the retiform layer by electro-deposition.

12. A combustion chamber component as claimed in Claims 4 and 8 to 10 wherein the combustion face is attached to the retiform layer by brazing or welding.

13. A combustion chamber component as claimbed in any one of the preceding claims wherein the retiform layer is adhesively attached to the body.

14. A combustion chamber component as claimed in any one of Claims 1 to 13 wherein the retiform layer is brazed or welded to the body.

15. A combustion chamber component comprising a piston substantially as hereinbefore described with reference to the Figures 1 and 2 accompanying the provisional specification.