GB2188122A

GB2188122A - Pistons

Info

Publication number: GB2188122A
Application number: GB08706238A
Authority: GB
Inventors: Micheal Ledsham Price Rhodes
Original assignee: AE PLC
Current assignee: AE PLC
Priority date: 1986-03-20
Filing date: 1987-03-17
Publication date: 1987-09-23
Also published as: GB8606998D0; GB2188122B; US4876947A; EP0238146A2; EP0238146B1; GB8706238D0; DE3773019D1; EP0238146A3

Description

GB 2 188 122 A SPECIFICATION comprises a crown portion which also includes

a piston ring belt and gudgeon-pin bosses and which Pistons crown portion comprises a ferrous alloy and a separate articulated skirt portion which also includes The present invention relatesto pistons and 70 gudgeon-pin bores the crown portion and the skirt particularlyto pistons of the articulated type. portion having a common gudgeon-pin wherein at Pistons generally comprisethree distinct portions; least part of the skirt portion surfaces which the crown portion which includesthe piston ring co-operate with an associated cylinder orcylinder groove belt, the gudgeon-pin boss portion andthe linercomprise a material selected from the group skirt portion. Pistons, heretofore, have mainly been 75 comprising aluminium alloys, plastics material and produced from aluminium alloys. With ever coated steel or cast iron.

increasing demandsfor improved power output,fuel In preferred embodiments of the present invention economy and emission reductions aluminium alloys the rubbing surface area of the skirt portion with the are showing strength limitations in the crown and associated cylinder or cylinder liner may be provided pin boss regions especially in diesel engines. Much 80 in the form of axially spaced-apart circumferentially attention is now being paid to pistons in steel and extending bearing lands.

cast-iron. Such lands in cast monolithic skirt constructions Steel orcast-iron is betterableto withstandthe may be provided in theform of separate distinct thermal and mechanical loads imposed on the crown lands supported by struts depending from the in the highest rated engines. The piston ring grooves 85 associated gudgeon-pin boss and the lands not are more wear-resistant and may be positioned being connected to each other in any way otherthan nearerto the crown top surface because of lower via the struts and gudgeon-pin bosses. It has been thermal conductivity. Gudgeon-pin bosses in ferrous found that such a construction permits a closerfit material are also better able to withstand the firing between the skirt and associated cylinder and thus loads at higher temperatures without cracking than 90 assists in lowering the noise generated by the piston.

are aluminium alloys. The closerfit is permitted duetothe enhanced One disadvantage with a single pieceferrous alloy conformability of the skirt lands with the cylinder piston may be in increased weight although with surfacewhich tends to distort dueto local modern designs and production techniques forthin temperature variations. Thistype of skirt wall casting the weight may be no more than a 95 construction may be applicableto cast aluminium comparable aluminium alloy piston. The use of alloys and to ferrous castings having surface non-ferrous or plastics materials in the skirt-portion, treatments or coatings on the bearing lands.

however, has certain intrinsic advantages. These Separated bearing lands may alternatively be advantages generally relate to the bearing properties provided by removal of intervening skirt portions.

of such materials which are generally superiorto 100 Spaced-apart bearing lands may also be provided ferrous alloys, especially in lubricated contactwith a by relieving of the intervening material between the cast-iron cylinder bore. This does not, however, lands withouttotal removal thereof. This may be precludethe use of ferrous alloyskirt portionswhich appropriateto fabricated skirt constructions which may be employed in some engine applications or will be described in greater detail hereinafter.

may have surface treatments orcoatingsto improve 105 Spaced-apart bearing lands provide lowerfriction the compatibility of the piston skirtwith the cylinder dueto lower rubbing areasthan conventional skirts bore. where high-area oil films increasethe viscous drag Articulated pistons have several advantages. between piston and cylinder. Spaced-apart lands Crown and hence piston ring stability is greatly also tend to be inherently better lubricated dueto enhanced since side loads imposed on the crown 110 unimpeded access of oil to the contact surfaces.

portion are small and attributable mainlyto gudgeon Fabrication of the skirt portion from sheet orstrip pin friction. Contact between crown and cylinder may afford a greaterchoice of materials. Many bore is usually also greatly reduced duetothe plastics materialsJor example, are availablewhich removal of significant sidethrusts and hence are amenableto being bonded to steel, aluminium or sideways motion and piston lands aretherefore able 115 copper alloy strip and sheet. Indeed manysuch to run with smaller clearances. This latterfeature is materials are available and which are produced as beneficial with respeetto improved fuel efficiency journal bearing materials. Examples of such plastics and lower emissions. Because the skirt portion of the material bonded to steel via a porous bronze sintered piston is effectively isolated from the crown portion interlayer include various exceptfor conduction of heatvia the gudgeon pin the 120 polytetrafluoroethylene-based plastics, alloys of skirt portion of an articulated piston may run polyphenylenesul phone and polyetherketones, significantly coolerthan in a one-piece piston. polyetheretherketones and amide-imide resins Because of the lower running temperature of the having fillers of other dry-bearing or self-lubricating skirt portion there is considerable freedom of choice materials.

of materials. Thus the materials of the crown portion 125 The choice of materials forthe fabrication of skirt and the skirt portion may be more effectively portions from sheet or strip is not only limited to optimised in respect of the duties required of each. plastics-lined materials as described above. Many Skirt design freedom may also be enhanced by metallic bearing materials which are either not relative independence from the crown portion. amenable to casting or do not have the requisite According to the present invention a piston 130 mechanical properties in the castform are available 2 GB 2 188 122 A 2 in bimetal form bonded to a steel backing. Such been cold pressure welded by roll-bonding an bearing materials include, for example, alloys based aluminium-silicon 11 %-copper 1 % alloy 45. The alloy on aluminium-tin-copper, has been machined away in regions 46,47 and 48to aluminium-silicon-copper, leavetwo lands 49 of the alloy 45 extending part-way aluminium-lead-tin-copper, 70 around the finally wrapped member43.

aluminium-silicon-tin-copper, copper-lead-tin, etc. Figure4 shows a piston comprising a stainless These alloys have superiorwear and steel crown portion 11 and a steel skirt portion 12.

seizure-resistance. The crown portion is essentially as described with In orderthatthe present invention may be more reference to the preceding three figures. The ski rt fully understood examples will now be described by 75 portion 12 comprises an investment casting having way of illustration only with reference to the pin boss pieces 50, thin wall, arcuate load bearing accompanying drawings of which: skirt lands 51 and 52 supported on the boss pieces 50 Figure 1 shows a view of a piston according to the by struts 53 and 54 depending therefrom. The boss present invention sectioned in two mutually pieces have bores 55 of substantially the same bore perpendicular planes, one plane being thatwhich 80 19 as those of the crown portion bosses 18. The boss includes both the gudgeon pin axis and the piston pieces 50 further include circlip grooves 56 for axial axis; the half-section on the left being viewed in the location of the gudgeon-pin 30 (not shown).

direction of the gudgeon-pin axis and the In further alternative embodiments of the half-section on the right being viewed normal to the invention the bearing lands 24 and 25 of Figure 1 or gudgeon-pin in the direction of arrows'A'. 85 lands 32 and 33 of Figure 2 may be profiled so asto Figure2 shows a first alternative piston sectioned generate oil films for hydrodynamic lubrication of as in Figure 1; the piston in operation in accordance with known Figure3shows a second alternative piston practice. Alternatively the portions 22 may not be sectioned as in Figure 2. The right-hand half-section machined away butthe periphery of the skirt casting is viewed in direction 'B'with referenceto the left 90 21 may be suitably profiled as disclosed in US section; and 4,535,682 or PCT/G1385/0055. Anyotherknown Figure4which shows a sectionedviewof a piston profiles maybe employed to generate favourable in two mutually perpendicular planes, the section on distribution of the lubricant.

the left being through a plane including both the The skirt portion 12 of Figure 1 may be gudgeon-pin and piston axes and that on the right is 95 manufactured from aluminium alloytubethe bores viewed in the direction of arrow'C'parallel to the 28 being formedjor example, in upset portions of gudgeon-pin axis. thetube.

Referring nowto Figure 1 and where a piston is The wrapped member43 of Figure 3 may not denoted generally at 10. The piston comprisestwo necessarily beformed from a bimetal material but separate components; a crown portion 11 and a skirt 100 may beformed from a metal-backed material having portion 12. The crown portion 11 consists of a thereon a lining comprising a plastics material. One stainless steel casting having piston ring grooves 13, such plastics material suitablefor such applications 14 and 15, a combustion bowl 16, pillars 17 is based on polyetherketone (PEK) or depending from the bowl 16, the pillars 17further polyetheretherketone (PEEK). PEEKwith additions of including gudgeon pin bosses 18 having bores 19. 105 polytetrafluoroethylene (PTFE), bronze and graphite The skirt portion 12 consists of an aluminium alloy is particularly suitable. Furthermorethe member43 casting 21 of basically cylindrical form of which may not have the lining material 45 be it metallic or portions 22 have been machined away to leave plastics based completely removed to expose the bearing lands 24 and 25 to support the thrust side backing metal but may be profiled away only loads. The skirt portion 12 further includes 110 sufficientlyto produce bearing lands, pads orother gudgeon-pin bores 28 having circlip grooves 29for profilesto generate hydrodynamic oil lubrication location of a common gudgeon pin 30 (notshown). films in accordance with known practice as disclosed The bores 19 and 28 are both of substantially equal in US patent 4,535, 682 of common ownership diameter. herewith.

The piston of Figure 2 again shows a steel crown 115 Fabricated crown portion structures may also be portion casting 11 similarto that of Figure 1. The skirt employed in place of casting. ltwill be evidenttothe portion 12 however comprises a complex aluminium person skilled in the art that where features such as, alloy die-casting which comprises pin-boss pieces 30 for example, circlips have been described forthe having bores 31, lands 32 and 33which are axial location of gudgeon-pins that any other known supported by struts 34 and 35 depending from the 120 method or device such as buttons in the pin ends boss pieces 30,the bores 19 and 31 again being of may be used.

substantially equal diameterfor support by a Although the invention has been described with common gudgeon-pin 30 (not shown). reference to pistons having steel or cast-iron crown Figure 3 shows a piston having a steel crown portions the scope of the invention is intended to portion 11 and a fabricated skirt portion 12. The skirt 125 encompass articulated pistons having aluminium portion comprisestwo boss pieces 40 having bores alloycrown portions. Crown portions having air 41 towhich has been brazed orwelded atjoints42a gaps incorporated into the combustion bowls are wrapped, generailycylindrical member43. The also envisaged. Pistons having crowns and member43 has been wrapped from a flat bimetal combustion bowls of the type envisaged are sheet material having a steel backing 44to which has 130disclosed in GB 2, 125,517 of common ownership 3 GB 2 188 122 A 3 herewith.

Claims

1. A piston, the piston comprising a crown portion which also includes a piston ring belt and gudgeon-pin bosses and which crown portion comprises a ferrous alloy and a separate articulated skirt portion which also includes gudgeon-pin bores the crown portion and the skirt portion having a common gudgeon-pin wherein at least part of the skirt portion surfaces which co-operate with an associated cylinder or cylinder linercomprise a material selected from the group comprising aluminium alloys, plastics material and coated steel or cast iron.

2. A piston according to Claim 1 wherein the portions of the skirt which co-operate with the associated cylinder or cylinder liner comprise circumferentially extending bearing lands.

3. A piston according to either Claim 1 or Claim 2 wherein the circumferentially extending bearing lands are arcuate, load-bearing skirt lands supported by struts depending from boss pieces.

4. A piston according to Claim 3 wherein the skirt portion is an aluminium alloy die casting.

5. A piston according to Claim 3 wherein the skirt portion is a ferrous alloy casting.

6. A piston according to either Claim 1 or Claim 2 wherein the skirt portion is fabricated from sheet or strip material which comprises at least two layers.

7. A piston according to Claim 6 wherein the sheet or strip material comprises a metallic-backed plastics material.

8. A piston according to Claim 7 wherein the plastics material is selected from the group comprising polytetrafluoroethylene-based material, alloys of polyphenylenesu 1 phone and polyetherketones, polyetheretherketones and amide-imide resins.

9. A piston according to Claim 6 wherein the material comprises a steelbacked aluminium alloy.

10. A piston according to Claim 9 wherein the aluminium alloy is selected from the group comprising aluminium-tin-copper, aluminium-silicon-copper, aluminium-lead- tin-copper and alu mini u m-silicon-tin-copper.

11. A piston substantially as hereinbefore described with reference to the specification and Figure 1 of the drawings.

12. A piston substantially as hereinbefore described with reference to the specification and Figure 2 of the drawings.

13. A piston substantially as hereinbefore described with reference to the specification and Figure 3 of the drawings.

14. A piston substantially as herein before described with reference to the specification and Figure 4 of the drawings.

Printed for Her Majesty's Stationery Office by Croydon Printing Company (U K) Ltd,8187, D8991685. Published by The Patent Office, 25 Southampton Buildings, London, WC2A l^from which copies maybe obtained.