A conventional piston for an internal combustion engine comprises a crown which, in some cases, defines a combustion recess, a guidance skirt, and bosses to receive a gudgeon pin. The combustion recess cooperates with the space above the crown of the piston in the cylinder in which it reciprocates to form a combustion chamber in which the fuel charge is burnt. In some cases, it is necessary to provide pistons of this type with chambers. The term "chamber"is used herein to denote a substantially enclosed space having relatively small openings whereas the term "recess"is used to denote a space having a relatively large opening. For example, pistons have been proposed which have chambers in the crown thereof, the chambers having openings which communicate with a combustion recess in the crown. These chambers (see US Patent No. 4,898,135, where the chambers have restricted openings and also much larger"slot openings", and US Patent No. 5,322,042 where the chambers only have restricted openings) are intended to provide reaction chambers to shelter radical species of the fuel from being exhausted from the cylinder so that they can return to the cylinder to seed subsequent fuel charges.

Such seeding improves the completeness of the combustion of the fuel.

A known method of providing a piston with chambers involves manufacturing the piston in two portions which are subsequently joined together. These portions are a piston body portion, defining most of the piston including a lower portion of the combustion recess, and an upper generally annular portion, defining a side surface of the combustion recess, a top surface of the piston, and a portion of the outer side surface of the piston. The body portion has a generally annular upper surface on to which a matching lower surface of the upper portion is fitted and the two portions are secured together. References herein to surfaces as being"generally annular"are not intended to exclude surfaces which have elliptical outer edges (as most pistons are slightly elliptical in transverse cross- section) and also are not intended to exclude surfaces which have their inner edges non-circular and/or non- central.

In the known method described above, the chambers are formed by providing recesses in the lower surface of the upper portion, or in the upper surface of the lower portion, or in both surfaces. Thus, in the assembled piston, the body portion defines a bottom surface of the combustion recess and also lower surfaces of the chambers, and the upper portion defines a side surface of the combustion recess and upper surfaces of the chambers. The restricted openings to the chambers may be formed by boring or drilling through the material of the piston after the two portions thereof have been secured together. This is a difficult process which must begin in the combustion recess. Also the openings are fine (having a diameter between 1 mm and 0.2 mm, typically 0.4 mm) and are relatively long, eg 9 mm. Boring or drilling such openings is difficult, especially where the piston material is reinforced with hard particles or fibres and a typical piston may require as many as 3 openings per chamber with there being up to 6 chambers. Thus, in this method, the manufacture of such pistons is complex.

WO 94/20241 discloses a method of forming chambers in a piston in which each chamber is formed by a box. The box is mounted in a die cavity and the piston is cast around the box. The box is provided with projections which enter recesses in the wall of the die cavity and support the box.

After removing the piston from the die cavity, the projections are machined off. The openings to the chamber can be provided by tubes projecting from the box.

This method requires the manufacture of boxes and complicates the casting process.

It is an object of the present invention to provide a method of manufacturing a piston having a chamber therein which is less complex and produces a more reliable piston.

The invention provides a piston having a crown which defines a combustion recess which has an opening in an end surface of the piston, the piston also defining at least one chamber having at least one opening into the combustion recess, wherein the piston comprises a body portion which defines a socket which has an opening in said end surface of the piston, the body portion having a central surface within said socket, the central surface defining a portion of said combustion recess, the body portion also having a chamber-defining surface which extends around said central surface thereof within said socket, the piston also comprising a generally annular portion which is received in said socket and is secured to the body portion of the piston, the generally annular portion having a surface which cooperates with said central surface in defining the combustion recess, the generally annular portion also having a chamber-defining surface which overlies the chamber-defining surface of the body portion, characterised in that said chamber-defining surfaces cooperate in defining said chamber, and said opening to the chamber is defined by a groove in one of said chamber-defining surfaces.

In a piston according to the invention, the restricted opening to the chamber can be formed without boring or drilling, as the groove can be formed by machining, eg surface broaching, into a surface which can be arranged to be flat and radial relative to a longitudinal axis of the piston. Alternatively, the groove may be formed by other methods, such as stamping.

In a piston according to the invention, said chamber may be defined by a recess in said chamber-defining surface of the body portion and by a portion of said chamber- defining surface of the generally annular portion which covers the recess. Alternatively, said chamber may be defined by a recess in said chamber-defining surface of the generally annular portion of the piston, or by cooperating recesses in both chamber-defining surfaces.

Said groove may extend generally radially of the piston or may extend in a direction which is parallel to the radial direction.

The chamber may have further openings into the combustion recess provided by further grooves in one of the chamber-defining surface. All the grooves providing openings to the chamber may extend parallel to one another, thereby making them easier to form by machining.

Said chamber-defining surfaces may cooperate in defining further chambers distributed around the combustion chamber, said further chambers also having openings into the combustion recess provided by grooves in one of said chamber-defining surfaces.

The body and generally annular portions may be secured together by a threaded connection, by brazing or by any other suitable method.

Said body and generally annular portions of the piston may be formed from different materials, eg reinforced material may be utilised for the generally annular portion to strengthen the rim of the combustion recess.

In order to ensure good gas flow through the opening defined by the groove, it is preferred that the groove has a transverse cross-section with an aspect ratio of approximately unity, ie the width of the groove approximately equals its depth.

There now follows a detailed description, to be read with reference to the accompanying drawing, of a piston which is illustrative of the invention.

The drawing is a diagrammatic view, with parts broken away to show the construction, of portions of the illustrative piston, indicating how the portions are assembled.

The illustrative piston 10 is assembled from a body portion 12, and a generally annular portion 14.

The body portion 12 defines most of the piston.

Specifically, the body portion 12 defines a guiding skirt (not shown) of the piston 10, two bosses (not shown) having bores therein to receive a gudgeon pin by which the piston 10 is connected to a connecting rod, a side surface 12a of a crown of the piston 10, which defines piston ring grooves 16 in which piston rings 18 are received, and a generally annular end surface 12b which forms an outer part of an end surface of the piston 10. The surface 12a is slightly elliptical, in transverse cross-section, as is conventional.

The body portion 12 also defines a socket 20 which has an opening in said end surface of the piston 10. The socket 20 has a circular opening 20a which is surrounded by the surface 12b. The socket 20 has a side wall comprising a smooth cylindrical upper wall 20b, a screw-threaded lower wall 20c of smaller diameter than the wall 20b, and a radial step 20d joining the surfaces 20b and 20c. The bottom of the socket 20 is formed by surfaces of the body portion 12 within the socket 20. These surfaces are a central surface 12c and a chamber-defining surface 12d.

The central surface 12c is disposed centrally of the socket 20 and is profiled to define a lower portion of a combustion recess of the piston 10. Specifically, the central surface 12c forms a generally-conical peak surrounded by an annular trough. The chamber-defining surface 12d extends around the central surface 12c within the socket 20. The surface 12d is annular and extends radially of the socket 20. The chamber-defining surface 12d also has four recesses 22 therein (only two of which are visible in the drawing). The recesses 22 are equally distributed circumferentially about the central surface 12c. Each recess 22 extends circumferentially along an arc so that the recess maintains a constant distance from the centre of the socket 20. Each recess 22 is generally rectangular in transverse cross-section.

The chamber-defining surface 12d also has grooves 24 formed therein. Each groove 24 extends radially of the socket 20 from one of the recesses 22 to the central surface 12c. There are three grooves 24 per recess 22, one adjacent each end of the recess 22, and one disposed between the ends but nearer to one end than the other.

These grooves 24 are generally U-shaped in transverse cross-section, having a depth of 0.38 mm and a width at the surface 12d also of 0.38 mm, and are formed by machining into the surface 12d. In variations of the illustrative piston 10, the grooves 24 may have other transverse cross- sections, eg square, rectangular, V-shaped, etc. However, re-entrant cross-sections are normally to be avoided because of the difficulty in machining such a shape.

The generally annular portion 14 of the illustrative piston 10 is received in the socket 20. The portion 14 has a side surface which comprises a lower screw-threaded outer surface 14a, an upper smooth cylindrical surface 14b of greater diameter than the surface 14a, and a radial step 14c joining the surfaces 14a and 14b. The surface 14a is threadedly-received in the surface 20c of the socket 20 to secure the portion 14 to the body portion 12 by a threaded connection. A pin 26 is driven through a cylindrical hole 28 through the portion 14 and into a cylindrical recess 30 in the portion 12 to prevent the portion 14 from turning in the socket 20. The surface 14b is a close fit within the surface 20b, and the step 14c matches and fits over the surface 20d.

The portion 14 also has an annular upper surface 14d which cooperates with the surface 12b in defining the end surface of the piston 10. This surface 14d also defines the opening of the combustion recess of the piston 10. The portion 14 also has an interior surface 14e which cooperates with said central surface 12c of the portion 12 in defining the combustion recess. The portion 14 also has a lower annular chamber-defining surface 14f which overlies the chamber-defining surface 12d of the body portion 12.

The assembled piston 10, thus, has a crown which defines a combustion recess which has an opening in an end surface of the piston. In the assembled piston 10, said chamber-defining surfaces 12d and 14f, of the portions 12 and 14, cooperate in defining four chambers of the piston.

Each chamber is defined by one of the recesses 22 and by a portion of the surface 14f which covers the recess 22.

Each chamber has three restricted openings into the combustion recess, each opening being defined by one of the grooves 24 and being covered by a portion of the surface 14f which overlies the groove 24.