GB2158185A - Reinforced light metal pistons - Google Patents

Abstract

In a reinforced piston for an internal combustion engine, the uppermost piston ring groove (14) is as close as possible to the crown (11) of the piston. At least the lower surface (17) of the groove (14) is defined by an encast insert (20) and overlying this is a portion (18) of the crown reinforced by fibres, thus allowing this portion to be thin and the spacing between the ring groove (14) and the upper surface (11) of the crown to be minimised. The reinforced crown portion also holds the insert against axial separating movement. The crown can be provided with a combustion bowl (12) whose entrance (28) is reinforced by the fibres. In an alternative embodiment the reinforced part of the piston crown may be formed as a precast insert. <IMAGE>

Description

SPECIFICATION Reinforced pistons The invention relates to reinforced pistons for internal combustion engines.

In general, pistons for internal combustion engines are provided with a ring band which extends around a crown of the piston and in which are formed two or more piston ring grooves. A piston ring is provided in each groove and engages an associated cylinder or liner during operation of the engine. Since the piston rings are commonly made of a ferrous material and the piston is commonly made of a light metal or metal alloy such as aluminium or an aluminium alloy, it is customary to form at least one of the piston ring grooves (usually the uppermost) in a ferrous insert which is cast into, and surrounded by, the light alloy, in order to reduce piston ring groove wear.

There is, extending from above the uppermost piston ring to the level of the crown, a dead volume which is in communication with the combustion chamber formed above the piston. It has been found that, when this dead volume is reduced, there is a corresponding reduction in the exhaust emission and an improvement in the fuel consumption of the associated engine. One previous proposal for reducing this volume has been by a reduction in the clearance between the piston and the associated cylinder or liner. However, in highly rated engines, this can lead to a buildup of carbon in this volume, which causes bore polishing and can lead to high oil consumption.

In view of this, it has recently been proposed to reduce the dead volume by locating the uppermost piston ring as close as possible to the level of the crown. This involves moving the associated piston ring groove insert as high as possible up the piston, even to the extent of having the top of the piston ring groove insert flush with the top of the piston crown.

Although such a design reduces the dead volume to a minimum, it has a potentially very serious failure condition if the insert separates from the remainder of the piston.

For this reason, such pistons require very careful handling and inspection to ensure that the connection between the insert and the remainder of the piston is maintained fully effective until the engine is operated. Although this reduces the possibility to a minimum, the potential dangers of separation of the piston and the insert are so great as to prevent widespread acceptance of this proposal, despite the attractions of a reduced dead volume. Even when the insert is overlaid by a thin layer of the light alloy, this potential failure condition still exists, because the strength of the light alloy may not be sufficient to maintain the insert in position.

According to the invention, there is provided a piston for an internal combustion engine formed of a light metal or metal alloy and comprising a crown, a ring band extending around the crown and including two or more piston ring grooves, each piston ring groove having upper and lower generally radially extending surfaces, the piston ring groove closest to the crown having at least the lower radially extending surface thereof formed by a wearresistant insert which is cast into the piston, at least a portion of the crown including a fibre reinforcement which contacts the insert and which is between the insert and an upper surface of the crown, to allow the upper piston ring groove to be close to the upper surface of the crown.

The following is a more detailed description of four embodiments of the invention, by way of example, reference being made to the accompanying drawings in which: Figure 1 is a side elevation of a piston for an internal combustion engine, the upper part of the piston being shown in section and including an insert and a reinforcement forming between them a piston ring groove, Figure 2 is a view similar to Figure 1 but in which the reinforcement extends across the whole of the crown of the piston to reinforce a combustion bowl of the piston, Figure 3 is a cross-section of a part of a first variation of the piston of Figure 1 in which the piston ring groove is formed wholly within the insert, with the reinforcement holding the insert in position, and Figure 4 is a cross-section of a part of a second variation of the piston of Figure 1 in which the reinforcement has a generally flat surface which defines a lower surface of the piston ring groove.

Referring first to Figure 1, the piston has a crown 10 formed with an upper crown surface 11 and a combustion bowl 1 2. A ring band 1 3 extends around the crown and is formed with three piston ring grooves 14.

Although three grooves 14 are shown, there may be two or four such grooves. A skirt 1 5 depends from the ring band.

The piston is cast from a light metal, such as aluminium or an aluminium alloy, in a manner to be described in more detail below.

The uppermost piston ring groove 14 has upper and lower generally radially extending surfaces 16, 1 7 respectively. The upper radially extending surface 1 6 is formed by a portion 1 8 of the crown which is reinforced with fibres. This fibre reinforced portion 1 8 is generally annular and extends around the periphery of the crown to form an outer part of the upper surface 11 of the crown and the commencement of the ring band 1 3. The fibres act to strengthen these parts and render them more resistant to the high temperatures encountered by the crown in operation.

The lower radially extending surface 1 7 of the upper piston ring groove 14, together with a base 1 9 extending between the upper and lower radially extending surfaces 16, 1 7 is defined by an insert 20 of a ferrous material, such as cast iron or steel or ni-resist.

This insert 20 is in the form of a generally flat ring with an upstanding inner edge. The upper surface of the ring forms the lower radially extending surface 1 7 of the groove 14 while the outer surface of the edge forms the base 19 of the groove 14.

The lower surface of the ring and the inner surface of the edge are formed with shallow recesses 21 which interlock with corresponding projections 22 formed by the light metal of the piston. In the embodiment shown in Figure 1, the recesses 21 extend circumferentially around these surfaces with the recess 21 on the edge tending to resist movement of the insert 20 in a direction parallel to the axis of the piston and the recesses 21 on the lower surface of the ring tending to prevent radial movement of the insert 20.

The free end of the edge of the insert 20 contacts the reinforced portion 1 8 so that the reinforced portion 1 8 further prevents the insert separating from the remainder of the piston in an axial direction. Because this part of the piston is reinforced, and so strengthened, the axial thickness can be kept to a minimum, so allowing the upper piston ring groove 14 to be as close as possible to the crown surface 11, so reducing to a minimum the dead volume above a piston ring in the uppermost groove 14, i.e. the annular volume around the piston between the upper surface of such a piston ring and the level of the piston crown.

The piston of Figure 1 can be manufactured in any convenient way. For example, a suitably shaped wad or mat of fibres, such as alumina fibres, may be placed with an annular ferrous insert 20 of generally rectangular cross-section at the base of a mould. Molten aluminium or aluminium alloy is then gravity fed into the mould and a suitably shaped upper mould part brought into the lower mould part to close the die and to apply a force of many tons to the solidifying metal.

This forces the molten aluminium or aluminium alloy into the wad or mat of fibres and also into the recesses in the insert 20, so ensuring that the fibres and the insert 20 are fully incorporated into the casting so produced.

The insert 20 may be provided with a bond metal coating to improve bonding between the insert 20 and the aluminium or aluminium alloy. In this case, the wad or mat of fibres may be pre-heated to keep the bond metal on the insert 20 molten, facilitating good bond pick-up during the casting of the aluminium or aluminium alloy.

The piston ring grooves 14 are then machined in the casting by any suitable machining technique. One beneficial effect of this, in relation to the uppermost groove 14, arises as follows. When a ferrous insert 20, particularly one of cast iron is encast, it is desirable that all the molten metal contacting surfaces are adequately washed by the molten metal during casting to remove impurities. When a piston is cast 'crown-down', as described above, it is difficult to ensure adequate washing of the upper radially extending surface of the insert (which, in crown-down casting, is the lower of the two radially extending surfaces). The method described above, overcomes this problem because the machining step cuts away both the upper radially extending surface and the adjacent fibres or whiskers, so removing any imperfections in this region.

Alternatively, the reinforced crown portion of the piston may be formed separately from the remainder of the piston by a squeeze casting process, such as that described above and then the reinforced portion incorporated into the piston with the insert.

Referring next to Figure 2, parts common to Figures 1 and 2 are given the same reference numerals and are not described in detail. In the embodiment of Figure 2, the reinforced crown portion 18 extends over the whole of the crown of the piston 10 and, at the centre of the piston, is of such a thickness that the combustion bowl 12 is formed wholly within the reinforced crown portion 18. Thus, the combustion bowl entrance 28, which, being thin is particularly susceptible to damage at elevated operating temperatures, is reinforced and is better able to withstand such elevated temperatures.

Alternatively, as shown in broken line at 32 in Figure 1, the reinforced crown portion 1 8 may extend to the combustion bowl entrance 28 but not reinforce the base of the combustion bowl. In this way, the combustion bowl entrance 28 which is the part of the combustion bowl most susceptible to heat damage, is reinforced.

Referring next to Figures 3 and 4, these Figures show alternative ways of forming the upper piston ring groove 14, parts common to Figure 1 and to Figures 2 and 3 being given the same reference numerals and not being described in detail. In addition, since the pistons of Figures 2 and 3 can be manufactured in any of the ways described above with reference to Figure 1, the complete manufacture of the pistons of Figures 2 and 3 will not be described in detail.

In Figure 3, the insert 20 is generally annular with upper and lower limbs 23, 24 which define the upper and lower radially extending surfaces 16 17. These limbs 23, 24 are interconnected at their radially inner ends to form the base 1 9. The outer surface of the insert 20 is provided with recesses 21 which interlock with projections formed by the remainder of the piston, in the manner described above with reference to Figure 1.

The fibre reinforced portion 1 8 of the piston is shaped so that it overlies an upper surface 25 of the upper limb 23 of the insert, so holding the insert 20 on the piston and also protecting it from the elevated temperatures encountered in operation.

Although a single insert 20 is shown in Figure 3, it will be appreciated that this insert may be formed in two parts, separated at the line 30 in Figure 3.

Referring next to Figure 4, in this embodiment the insert 20 is in the shape of a flat ring. An upper surface of the ring forms the lower radially extending surface 1 7 of the piston ring groove 14. A lower surface 26 of the insert 20 carries an annular connecting member 27 which includes a plurality of apertures therethrough. The material of the remainder of the piston extends through these apertures to lock the insert 20 to the piston body.

The reinforcement 1 8 is shaped to define the upper radially extending surface of the groove 14 and also the base 1 9 of the groove 14. The inner end of the ring-like insert 20 extends into the remainder of the piston and its upper surface is contacted by the reinforcement 18 so that the reinforcement holds the insert against axial movement.

As has been described above, the reinforced crown portion 1 8 of the piston can be manufactured separately from the remainder of the piston and then incorporated into the piston.

One example of such manufacture will now be described with reference to Figure 4.

The reinforced crown portion 1 8 is cast, for example squeeze cast, separately from reinforcing fibres, such as alumina fibres sold under the trade mark SAFFIL, and a hypereutectic aluminium-silicon alloy, with the optional addition of graphite. After casting the reinforced crown portion 1 8 is pre-machined to a generally rectangular cross-section, as shown in chain-dotted line in Figure 4, i.e.

oversize for its required final shape.

The pre-machined reinforced crown portion 18 is then encast into the piston, for example by a squeeze casting process. To ensure attachment of the portion to the cast piston material, the portion 1 8 may be provided with passages 31 extending therethrough into which the piston material passes to form an interlock. Additionally or alternatively, the reinforced crown portion 1 8 may be plated with copper to facilitate the jnterconnection on casting. Additionally or alternatively, a labyrinth can be incorporated at the edges of the crown portion 1 8 so that this can readily fuse in the molten casting metal, thus giving a good bond.

As shown in Figure 4 in broken line, the insert 20 is also manufactured over-size and is incorporated into the piston in this condition.

The insert 20 may be provided with apertures 29 into which the piston material penetrates on casting to improve the interlock between the insert 20 and the piston body.

After casting, the complete piston is finishmachined to the shape shown in full line in Figure 4. Thus the depth and diameter of the reinforced crown portion 18 are reduced and the piston ring groove 14 is formed in this portion 1 8. At the same time, the outer edge of the insert is machined flush with the piston surface and the upper surface 1 7 of the insert 20 is also machined away to provide a flat groove surface.

In all the embodiments described above with reference to the drawings, the reinforced portion of the piston is generally annular in shape. It will be appreciated, however, that this need not necessarily be the case. The reinforcement may have any convenient shape and may contact a greater proportion of the surface of the insert, than the reinforcement of Figures 1 to 3.

Although the inserts 20 described above with reference to Figures 1 and 2 are provided with circumferentially extending recesses, it will be appreciated that these recesses may be arranged in any convenient manner. For example, they might be arranged in the form of an intersecting grid or network of recesses with one set of recesses extending circumferentially and another set of recesses extending radially and axially. For example, the recesses and projections may be arranged in any of the ways described in British Patent Specification No.2,104,622.

Although recesses and connecting members have been described above for connecting the inserts to the remainder of the piston, it will be appreciated that this connection may be formed in any suitable way either in addition to or as an alternative to the recesses and connecting members. For example, the socalled 'Al-Fin' bond may be used. Alternatively, there may be provided welds or other forms of metallurgical connection between the insert and the remainder of the piston. In the embodiments of Figures 1, 2 and 3, the connection may be omitted.

Claims (14)

1. A piston for an internal combustion engine formed of a light metal or metal alloy and comprising a crown, a ring band extending around the crown and including two or more piston ring grooves, each piston ring groove having upper and lower generally radially extending surfaces, the piston ring groove closest to the crown having at least the lower radially extending surface thereof formed by a wear-resistant insert which is cast into the piston, at least a portion of the crown including a fibre reinforcement which contacts the insert and which is between the insert and an upper surface of the crown, to allow the upper piston ring groove to be close to the upper surface of the crown.

2. A piston according to claim 1, wherein the upper radially extending surface of the upper piston ring groove is formed by the fibre reinforced portion of the crown.

3. A piston according to claim 2, wherein a base connects the radially inner ends of the upper and lower radially extending surfaces, and wherein the base is formed either by the insert or by the reinforced portion.

4. A piston according to claim 2 or claim 3, wherein the insert has a radially inner end located within the remainder of the piston, the fibre reinforced portion contacting the inner end.

5. A piston according to claim 1, wherein the upper radially extending surface of the upper piston ring groove is formed by the insert.

6. A piston according to claim 5, wherein the insert is formed in two parts, one part forming the lower radially extending surface and the other part forming the upper radially extending surface.

7. A piston according to claim 5 or claim 6, wherein the insert is cast from a ferrous material and has upper and lower limbs which define respectively the upper and lower radially extending surfaces, there being a base connecting said limbs.

8. A piston according to claim 7, wherein the reinforced portion of the piston contacts the outer surface of the upper limb.

9. A piston according to any one of claims 1 to 8, wherein the insert and the surrounding light metal or metal alloy form an interlocking projection and recess to prevent separation of the insert in a direction normal to the piston axis.

10. A piston according to claim 9, wherein the insert and the light metal or metal alloy form at least one further interlocking projection and recess to prevent separation of the insert in a radial direction.

11. A piston according to claim 10, wherein the insert is provided with a mesh or grid of intersecting recesses which engage with corresponding projections formed by the light metal or metal alloy.

1 2. A piston according to claim 9 or claim 10, wherein the insert has a plurality of apertures therethrough, the material of the remainder of the piston extending into said apertures to lock the insert to the piston body.

1 3. A piston according to any one of claims 1 to 12, wherein the fibre reinforced portion of the crown is an annular portion extending around the periphery of the crown.

14. A piston according to any one of claims 1 to 12, wherein the fibre reinforced portion of the crown extends across the whole of the crown.

1 5. A piston according to claim 1 3 or claim 14, wherein the piston includes a combustion bowl having an entrance extending therearound and wherein the fibre reinforced portion forms at least said combustion bowl entrance.

1 6. A piston according to any one of claims 1 to 15, wherein the fibre reinforced portion of the piston is formed separately from the remainder of the piston, wherein the piston is formed by casting, and wherein the fibre reinforced portion is incorporated during said casting of the piston.

1 7. A piston according to any one of claims 1 to 15, wherein the piston is formed by placing a wad or mat of reinforcing fibres in a casting mould, together with the insert, and then casting the piston around the insert and into the fibre reinforcement by a squeeze casting process.

1 8. A piston for an internal combustion engine substantially as hereinbefore described with reference to Figure 1 or Figure 2 or Figure 3 or Figure 4 of the accompanying drawings