GB2092709A

GB2092709A - Securing piston crown

Info

Publication number: GB2092709A
Application number: GB8203391A
Authority: GB
Original assignee: AE PLC
Current assignee: AE PLC
Priority date: 1981-02-07
Filing date: 1982-02-05
Publication date: 1982-08-18
Also published as: GB2092709B

Abstract

A piston for an internal combustion engine has a body part 11, formed separately from an insert 17 defining the crown of the piston. The insert is made of a material which is more resistant to the effect of temperature than the body part. A retaining member 19 is provided for holding the insert in position on the body part and is diffusion bonded to the body part as to compress the insert between the retaining member and the body part. <IMAGE>

Description

SPECIFICATION Improvements in or relating to pistons This invention relates to pistons such as are used in combustion engines and more particularly, but not exclusively, in diesel engines.

A number of proposals have been made for reinforcing the crown of such pistons when made of aluminium alloy because, as the output of such a diesel engine is increased for a given size, higher temperatures occur in the combustion space, and these higher temperatures, especially when a piston has a re-entrant combustion bowl in the crown, tend to cause cracking of an aluminium alloy piston crown.

In order to overcome this difficulty, it has been proposed to provide a reinforced piston crown in which a reinforcement part is held by the body of the piston, the reinforcement part being of a material which is more resistant to the effects of temperature than the aluminium alloy of the body part. One difficulty which arises is caused by the need to secure the reinforcement part to the body of the piston in such a way that no relative movement can take place between them. This difficulty is aggravated by the wide temperature range over which the piston operates, from ambient temperature to its operating temperature, and the different coefficients of thermal expansion of the reinforcement part and the material of the body of the piston.

For example, the reinforcement part may be secured to the body by means of a screw-threaded steel ring or by means of a retaining steel ring welded to a part which is integral with the piston body. Both methods of securing the reinforcement present difficulties; the screw thread engagement of a steel ring with an aluminium body presents difficulties owing to the aluminium having a higher thermal expansion coefficient than the steel ring, and due to the relative weakness of a screw thread in an aluminium component. Where the reinforcement part is retained by a ring welded in position, it is necessary to cast a ferrous insert into the aluminium body to which the ring can be welded; besides the problems inherent in casting in the ferrous member, there is the problem of placing the reinforcement part in compression to the desired degree before welding the ring in place.

According to the present invention, there is provided a piston comprising a body part of aluminium alloy and a crown reinforcement part which is carried on the body part to form a crown therewith and which is of a material more resistant to the effects of temperature than the aluminium alloy of the body part, and a retaining member which holds the crown reinforcement member in position on the body part and which is so diffusion bonded to the body part that the crown reinforcement part is compressed between the retaining member and the body part.

It is found that in diffusion bonding the retaining ring of the body part of the piston, a small amount of these two members is extruded from the joint line so that the insert is nipped between the retaining ring and the body part to a controlled extent.

Three embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1, which shows to the left of the centre line an elevation of a first embodiment of a piston, and to the right of the centre line a cross-section at right angles to the axis of a gudgeon pin of the piston, and including the longitudinal axis of the piston, and Figs. 2 and 3, which show in cross-section part of second and third embodiments of a piston.

Referring to Figure 1, the first embodiment of a piston comprises a body part 10 which includes a piston skirt 1 gudgeon pin bosses 12 which may be of any known or convenient kind and which provide a gudgeon pin bore 13, a ring band 14 connected to the upper end of the skirt along its periphery and formed with piston ring grooves 15, and a transverse portion 1 6. The ring band 14 and transverse portion 1 6 together define a substantially cylindrical well which receives a crown reinforcement part in the form of an insert 17, which defines a combustion bowl 18, and which is formed of a material which is resistant to the high temperatures experienced by the piston crown and to the effect of fuel injection.The insert is aiso preferably, but not essentially, of a material having a lower thermal conductivity than the material of the piston body, so that heat is conducted less rapidly from the combustion chamber to the piston body than would otherwise be the case.

The material of the insert 1 7 may be a ceramic, for example silicon nitride, or it may be an austenitic ferrous or nickel based alloy, or a maraging ferrous alloy; certain copper alloys, e.g.

beryllium copper, may also be usefully employed for the insert, in spite of their high conductivity.

The insert 1 7 is retained in position by means of a retaining ring 19, which is secured to the ring carrier portion 1 4 of the piston body around the well by diffusion bonding on the joint line shown at 20. The retaining ring 17 includes an annular rabbet which overlies and bears upon a corresponding rabbet extending around the insert 17.

The piston body may be made of high silicon aluminium alloy, e.g. Lem 13 alloy, and the retaining ring 19 may be made of HE30 aluminium alloy.

The diffusion bonding process may be carried out as described in British Patent No. 1,532,628. It has been found that in carrying out this process a small amount of metal is extruded on the bond line during the process, so that the axial dimension of the piston body part 10 and the retaining ring 19, after bonding, is approximately 0.25 mm less than before bonding. This places the insert 17 in compression to an extent which can be accurately controlled. The compressional force acts in a direction parallel to the axis of the piston. Thus, when the piston is heated to operating temperature, and the aluminium alloy expands to a greater extent than the material of the insert, the latter will remain in compression, and relative movement of the parts will be avoided.

If desired, a layer of densely interangled sintered stainless steel wire 21 such as that marketed under the Registered Trade Mark "Feltmetal" FM 134, which is formed of stainless steel wire specification Al SI 347, may be inserted between the base of the insert 1 7 and the transverse wali 1 6 to the piston, and/or between the transverse surface of the retaining ring 1 9 and the insert. Furthermore, an annular layer of the same stainless steel wire material may be inserted between the circumference of the insert 1 7 and the ring-carrying portion 14 of the piston body.

This stainless steel wire material acts as a thermal insulator and reduces the transmission of heat from the insert to the aluminium alloy parts.

Referring next to Figure 2, the second embodiment of piston has an austenitic stainless steel insert 1 7A and the line of the diffusion bond 20A is further from the crown of the piston than in the piston of Figure 1. One or more of the ring grooves 15 may be machined in the retaining ring 19.

Referring next to Figure 3, the third embodiment of piston has an austenitic stainless steel insert 1 7B with stainless steel wire insulating layers 21A, 21 B, 21 C between the insert 1 7B and the wall 16, around the circumference of the insert, and between the insert and the retaining member 19, respectively. There may be several superimposed layers of stainless steel wire material, as shown at 21 C in Figs. 2 and 3.

The pistons described above with reference to the drawings have the advantage that it provides an insert over at least part of the piston crown, which is of a material resistant to the effects of high temperature, and which is secured in position in compression to a controlled extent, during the process of diffusion bonding, thus the likelihood of stress relaxation occurring on change of temperature, allowing the insert to move relative to the body part of the piston, is overcome.

Moreover, the area over which the retaining ring is bonded to the body part can be chosen so that the stresses across the bond line are acceptable.

Other variations within the scope of the invention will readily be apparent to those skilled in the art.

Claims

1. A piston comprising a body part of aluminium alloy and a crown reinforcement part which is carried on the body part to form a crown therewith and which is of a material more resistant to the effects of temperature than the aluminium alloy of the body part, and a retaining member which holds the crown reinforcement member in position on the body part and which is so diffusion bonded to the body part that the crown reinforcement part is compressed between the retaining member and the body part.

2. A piston according to claim 1 , wherein the compressional force acting on the crown reinforcement part is in a direction parallel to the axis of the piston.

3. A piston according to claim 1 or claim 2, wherein the crown reinforcement part is of generally circular cross-section and includes an annular rabbet which engages a corresponding annular rabbet on the retaining member.

4. A piston according to any one of claims 1 to 3, wherein the crown reinforcement part has a generally cylindrical outer surface which is received in a corresponding well provided in the body part and wherein the retaining member is a ring diffusion bonded to the body part around the well and having a portion overlying and bearing upon the crown reinforcement part.

5. A piston according to any one of claims 1 to 5, wherein the crown reinforcement member defines a combustion bowl.

6. A piston according to any one of claims 1 to 5, wherein there is provided a layer of thermally insulating material between the crown reinforcement insert and the body part and/or the retaining member.

7. A piston according to claim 6, wherein the thermally insulating material is an interangled stainless steel wire.

8. A piston according to any one of claims 1 to 6, wherein the crown reinforcement part is of a ceramic material or an austenitic ferrous alloy or an austenitic nickel alloy or a maraging ferrous alloy or a copper alloy.

9. A piston according to claim 8, wherein the crown reinforcement part is of silicon nitride or beryllium copper.

10. A piston substantially as hereinbefore described with reference to Figure 1 or Figure 1 as modified by Figure 2 or Figure 3 of the accompanying drawings.