GB2172084A

GB2172084A - Pistons

Info

Publication number: GB2172084A
Application number: GB08505573A
Authority: GB
Inventors: William David Alexander
Original assignee: SOUTH WESTERN IND RES
Current assignee: SOUTH WESTERN IND RES
Priority date: 1985-03-05
Filing date: 1985-03-05
Publication date: 1986-09-10
Also published as: GB2172084B; GB8505573D0

Abstract

This invention relates to pistons and in particular, but not exclusively, to pistons for diesel engines. A piston (10) comprises a main body (11) and a ceramic crown (12). The body is formed with a circular, central recess (13) in its upper portion (14) which recess receives an extension (15) of the crown (12). The extension is held therein by a generally cylindrical insert (16) which comprises a corrugated stainless steel cylinder which is dimensioned so that the extension (15) is a press fit within the recess (13). <IMAGE>

Description

SPECIFICATION Pistons This invention relates to pistons and in particular, but not exclusively, to pistons for diesel engines.

Considerable research has been carried out to find ways of insulating the combustion chamber of internal combustion engines, because it is expected to increase efficiency and, perhaps more importantly, to enable engines to burn a wider range of fuels including, for example, synthetic fuels. However, there are considerable problems in providing a separate piston crown of, say ceramic material, which can be secured to the main piston body in a manner which allows for the different coefficients of thermal expansion of the materials, but does not require a large number of elements or assembly space.

From one aspect the invention consists in a piston including a heat insulating crown having an extension, a main body having a recess for receiving the extension within the recess, whilst allowing for differential expansion of the crown and the main body.

Preferably the interconnecting means spaces the extension from the main body to provide a low effective mechanical modulus between the extension and the main body and/or to define a partial thermal barrier between the extension and the body. The interconnecting means may be resilient in at least one direction and they may provide a seal between the body and the crown to prevent leakage of gas past the piston.

In an alternative construction the interconnecting means are constituted by an insert disposable between the body and the extension to hold the extension within the recess.

The insert may be generally cylindrical and may be corrugated or similarly profiled. The extension may also be profiled to cooperate with the insert and it may be provided with flanges to locate in cooperating recesses or formations on the body to restrain longitudinal movement of the insert and hence of the crown.

From another aspect the invention consists in a piston including a crown portion having an extension, a main body having a recess for receiving the extension and interconnecting means disposed within the recess to provide a press fit between the extension and the body.

Although the invention has been defined above it is to be understood that it includes any inventive combination of the features set out above or in the following description.

The invention may be performed in various ways and a specific embodiment will now be described, by way of example, with reference to the accompanying drawings, in which: Figure la is a longitudinal sectional view through piston; Figure 1b is a longitudinal section through Fig. 1a on the line l-l.

Figure 2a is is an enlarged scrap view of A on Fig. la.

Figure 2b is a corresponding view to Fig. 3 of an alternative embodiment.

In the Figs. a piston 10 comprises a main metallic body 11 and a ceramic crown 12.

The body 11 and the crown 12 combine to form a piston of generally conventional shape and each may be adapted to include any desired design features.

The body is formed with a circular central recess 13 in its upper portion 14. The recess receives an extension 15 of the crown 12 and the extension is held therein by a generally cylindrical insert 16.

The insert comprises a corrugated stainless steelcylinder which is dimensioned so that the extension 15 is a press fit within the recess 13.

As has been mentioned above the coefficients of thermal expansion of ceramics and the metal used to make the main body are quite different although the ceramic coefficient is always lower. Thus when the piston heats up in use the insert must be able to take up any differences in relative size between the extension 15 and the recess 13. The insert partly achieves this because it provides a region of low effective mechanical modulus between the two elements of the piston and partly because it provides a partial thermal break between the two elements reducing the temperature to which the main body rises and hence reducing the difference in actual dimensional changes of the main body with respect to the ceramic insert. The corrugated nature of the cylinder also allows it to flex radially in a resilient manner so that it can contract and expand its radial thickness.It is thought that it is important to provide some direct heat flow paths between the extension 15 and the main body 11 so that the insert 16 does not become overheated and suffer plastic deformation.

Another feature of the insert 16 is that it provides a very lengthy seal between the crown 12 and main body 11 and prevents leakage of gas between them as could occur when the recess 13 breaks through the upper portion 14 of the main body 11. In larger pistons the recess may not need to break through in this way and alternative methods of interconnecting the extension 15 and the main body 11 may be applicable. In addition other forms of insert could be applicable, for example a resilient mass of low thermal coefficient of expansion or a number of resilient rings.

It will be appreciated that in each of these designs the number of elements which make up the piston are very few indeed and further they are extremely simply interconnected. The design therefore significantly reduces both manufacturing and assembly costs as against many earlier proposals.

Fig. 2b shows an alternative arrangement in which the crown extension 15 is corrugated to receive the projections 17 of the insert 16, which is itself clipped on the piston body at 11 at 18, 19, by means of radial projections 20, 21 and cooperating recesses 22, 23. This construction allows for a more positive, location of the ceramic crown 12.

Claims

1. A piston having a heat insulating crown having an extension, a main body having a recess for receiving the extension and interconnecting means for holding the extension within the recess, whilst allowing for differential expansion of the crown and the main body.

2. A piston as claimed in claim 1, wherein the interconnecting means spaces the extension from the main body to provide a low coefficient of thermal expansion between the extension and the main body.

3. A piston as claimed in claim 1 or claim 2 wherein the interconnecting means define a partial thermal barrier between the extension and the body.

4. A piston as claimed in any one of claims 1 to 3, wherein the interconnecting means are resilient in at least one direction.

5. A piston as claimed in any one of the preceding claims, wherein the interconnecting means provide a seal between the body and the crown to prevent leakage of gas passed the piston.

6. A piston as claimed in any one of claims 1 to claim 3, wherein the interconnecting means is constituted by loosely cooperating threads on the body and extension.

7. A piston as claimed in any one of claims 1 to 5, in which the interconnecting means is constituted by an insert disposable between the body and the extension to hold the extension within the recess.

8. A piston as claimed in claim 7 in which the insert is generally cylindrical.

9. A piston as claimed in claim 7 or claim 8 wherein the insert is corrugated or similarly profiled.

10. A piston as claimed in any one of claims 7 to 9, wherein the insert is made of stainless steel.

11. A piston as claimed in any one of the preceding claims in which the extension is profiled to cooperate with the insert.

12. A piston as claimed in claim 11, where the extension is provided with flanges to locate in cooperating recesses or formations on the body to restrain longitudinal movement of the insert and hence of the crown.

13. A piston including a crown portion having an extension, a main body having a recess for receiving the extension and interconnecting means disposed within the recess to provide a press fit between the extension and the body.

14. A piston substantially as hereinbefore described, with reference to the accompanying drawings.

15. A diesel engine incorporating a piston as claimed in any one of the preceding claims.