GB2252980A

GB2252980A - Pistons

Info

Publication number: GB2252980A
Application number: GB9202470A
Authority: GB
Inventors: Michael Ledsham Price Rhodes; Andrew Timothy Cole; Geoffrey Ernest Josey
Original assignee: T&N Technology Ltd
Current assignee: Federal Mogul Technology Ltd
Priority date: 1991-02-20
Filing date: 1992-02-05
Publication date: 1992-08-26
Anticipated expiration: 2012-02-05
Also published as: GB9202470D0; GB2252980B; GB9103482D0

Abstract

Pistons and a method for the manufacture thereof are described. The pistons have a ceramic insulating member within a porous sintered ferrous material body which is incorporated into a light alloy piston casting by a pressure casting technique.

Description

PISTONS The present invention relates to pistons for internal combustion engines and to a method for the manufacture thereof particularly, though not exclusively, for compression ignition engines.

Pistons having insulation in the crown region are well known. Insulation is used to achieve an increase in the combustion chamber operating temperature in order to secure reductions in undesirable emissions and parasitic power losses due, for example, to cooling air fans and coolant pumps.

The form of insulation used varies widely; GB 2,205,923 describes a piston having a ferrous or nickel alloy crown cap with an encapsulated ceramic insulating member, the cap being produced by the technique of investment casting. The cap is attached to the piston body by pressure casting of the body material around projections on the underside of the cap. This method of producing the insulated crown cap requires considerable investment in equipment for its manufacture.

GB 2,220,004 describes a method of making a piston having insulation in the form of cavities or air-gaps in the crown region. The cavities are produced by pressing and sintering a crown piece insert from a ferrous powder, the crown piece having encapsulated therein a cavity-shaped metallic member which melts and infiltrates the porous material adjacent the member position to form a sealed cavity. The sintered crown-piece is then incorporated into a piston by pressure casting the piston body material around it such that the residual porosity is filled by the molten body material. It has been found that the relationship between the cavity volume and crown-piece porosity volume and size is critical to forming a sealed cavity against ingress of the pressure cast piston body material, usually an aluminium alloy.In some piston crown geometries, such a relationship is difficult or impossible to achieve.

We have now found that a ceramic insulating member may be encapsulated within a pressed and sintered crown piece within acceptable limits of displacement and breakage. We have found a method of manufacture for crown pieces which overcomes the economic disadvantages and manufacturing control difficulties of prior art pistons and methods.

According to a first aspect of the present invention there is provided a method for the manufacture of a thermally insulated piston for an internal combustion engine, the method comprising the steps of encapsulating a thermal insulation member of a desired shape and size of a ceramic material within a mass of metal powder, compacting the metal powder around the insulating member to produce a crown-piece compact, sintering the crown-piece compact and incorporating the sintered crown piece into a piston by a pressure casting technique.

The insulation member may be made from any ceramic material having the desired thermal properties. Such ceramics may include, for example, zirconia, alumina and silicon nitride.

The insulation member may itself be formed from ceramic powder. It may, for example, be mixed with a binder and formed into the desired shape using any of the known techniques such as die pressing, isostatic pressing and injection moulding. The insulation member may be sintered prior to incorporation into the crown piece or may be sintered in situ in the crown piece during sintering of the latter. Sintering simultaneously and in situ in the crown piece has the advantage that the pre-formed insulation member may retain some plasticity and be less prone to rupture during compaction of the encapsulating powder mass prior to sintering.

Preferably, the encapsulating metal powder may be a ferrous powder, for example, stainless steel and may be compacted either by die pressing or by isostatic pressing.

The crown-piece may be machined prior to incorporation into the piston casting. Machining may be carried out in either the green state or after sintering.

In one embodiment of the method the crown piece may be incorporated into the piston by the technique of squeezecasting.

According to a second aspect of the present invention, there is provided a light-alloy piston for an internal combustion engine, the piston having a crown region which includes a porous, ferrous alloy crown piece which has encapsulated therein a ceramic material insulating member and the porosity of said crown piece is infiltrated with the light alloy in which the crown piece is encast and which forms the body of the piston.

The light alloy may be an aluminium alloy or a magnesium alloy.

The piston may be a single piece piston having an integral skirt component or may be of the articulated type having a separate skirt member.

In order that the present invention may be more fully understood, examples will now be described by way of illustration only with reference to the accompanying drawings, of which: Figures l(a) to l(e) show schematically the steps in the production of a piston crown insert having a ceramic insulating member therein; Figures 2(a) to 2(c) show schematically the incorporation of the crown insert of Figure l(e) into a piston; and Figures 3, 4 and 5 which show cross-sections through examples of alternative embodiments of pistons according to the present invention.

Referring now to Figure l(a) to l(e) and where the same features have common reference numerals. A female powder pressing die portion is indicated at 10 together with a quantity of metal powder 12, for example 304L stainless steel, in the bottom (Figure l(a)). An annular ceramic ring member 14 is placed on the top of the powder 12 and a further quantity of metal powder 16 is placed on top of the ring 14 and first powder 12 (Figures l(b) and (c)).The powder is compacted by a male die punch 18 to an extent sufficient to leave interconnected porosity in the compacted powder matrix 20 (Figure l(d)). The compacted insert 22 is extracted from the die 10 and sintered to increase the strength, still leaving interconnected porosity to be infiltrated in subsequent processing steps (Figure l(e)).

At this stage the insert 22 may be machined to produce any desired features such as, for example, undercrown curvature or a combustion bowl. Alternatively, the insert may be produced by isostatic pressing within a shaped flexible mould to produce the desired form.

The sintered insert 22, is now placed in the female portion 30 of a squeeze casting die into which is introduced a metered quantity of molten light alloy 32. A male die punch 34 applied sufficient pressure to the molten metal to cause complete infiltration of the interconnected porosity within the insert and to prevent any porosity from developing during solidification of the alloy. The solidified piston 36 is finally extracted from the die 30, the piston having a strengthened crown region 38 by virtue of the sintered stainless steel contained therein and also having additional insulation by virtue of the ceramic ring member 14.

Limited cracking of the ceramic member during compaction of the surrounding powder is not a problem since the effective insulating ability of the material is largely unaffected. Because such cracks are filled with the light alloy which infiltrates the porous steel crown-piece during squeeze casting, any subsequent stress raising or crack formation and propagation potential would be suppressed by the strength of the surrounding, infiltrated stainless steel matrix 20.

Figures 3, 5 and 6 show ceramic insulating members having different geometries.

Figure 3 shows a member 40 having a flat ring form similar to that described above with reference to Figures 1 and 2. The crown insert has a combustion bowl 42 formed therein together with the top piston ring groove 44 in the outer peripheral edge.

Figure 4 shows an insulating member 50 in the form of a short cylinder.

Figure 5 shows an insulating member 60 in the form of a flat disc below the combustion bowl 42.

It will be appreciated by those skilled in the art that more than one insulating member may be incorporated into the crown insert. For example, it may be desirable to include both the forms of insulating members 50 and 60 together in one insert.

Claims

1 A method for the manufacture of a thermally insulated piston for an internal combustion engine, the method comprising the steps of encapsulating a thermal insulation member of a desired shape and size of a ceramic material within a mass of metal powder, compacting the metal powder around the insulating member to produce a crown-piece compact, sintering the crown-piece compact and incorporating the sintered crown piece into a piston by a pressure casting technique.

2 A method according to Claim 1, wherein the insulation member is a ceramic selected from the group comprising zirconia, alumina and silicon nitride.

3 A method according to either Claim 1 or Claim 2, wherein the insulation member is itself formed via a powder route.

4 A method according to Claim 3 wherein the insulating member is sintered in situ during sintering of the crown-piece compact.

5 A method according to any one preceding Claim, wherein the mass of metal powder is compacted by die pressing or by isostatic pressing.

6 A method according to any one preceding Claim, wherein the pressure casting technique is squeeze casting.

7 A light-alloy piston for an internal combustion engine, the piston having a crown region which includes a porous, ferrous alloy crown piece which has encapsulated therein a ceramic material insulating member and the porosity of said crown piece is infiltrated with the light alloy in which the crown piece is encast and which forms the body of the piston.

8 A piston according to Claim 7, wherein the ceramic is selected from the group comprising zirconia, alumina and silicon nitride.

9 A piston according to either Claim 7 or Claim 8, wherein the encapsulating metal powder is a ferrous powder.

10 A piston according to Claim 9, wherein the ferrous powder is a stainless steel.

11 A piston according to any one of preceding Claims 7 to 10, wherein the light alloy is either an aluminium alloy or a magnesium alloy.

12 A method substantially as herein before described with reference to the accompanying specification and drawings.

13 A light alloy piston substantially as herein before described with reference to the accompanying specification and drawings.