Improvements in or relating to pistons for internal combustion engines

Abstract: A piston (10) carries on its skirt surface (13) a number of separate strips (15) of a plastics material, such as a polyether­ etherketone alloy, so that the strips are hydrodynamically lubri­ cated and transmit lateral thrust from the piston to an associ­ ated cylinder or liner (37). The strips (15) are just proud of the skirt surface and may be contained in recesses (16) formed in the skirt surface.

Description: The invention relates to a piston for an internal combustion engine of the kind comprising a crown, a gudgeon pin bore and a skirt. Conventional pistons for internal combustion engines are provided with generally cylindrical skirts which transmit the lateral loads on the piston to an associated cylinder or liner. A lubricant film is provided between the piston skirt and the associated cylinder or liner in order to prevent metal-to-metal contact between these parts (which would lead to seizure of the engine). However, the presence of this lubricant film between these parts produces a frictional force which increases the engine losses. In cases where maximum engine power output is of considerable importance, such as in racing cars, it is known to mount on the skirt surface, studs or buttons of a plastics material which engage directly with the associated cylinder or liner; there is no oil film between the parts. Since the coefficient of friction of the plastics material on the material of the cylinder or liner is low and the area of contact is small, there is a corresponding reduction in the frictional losses generated, and thus an increase in the power output of the engine. These studs or buttons have the disadvantage that, since their contact with the associated cylinder or liner is direct and unlubricated, they are subject to wear. This is not a problem in engines for racing cars, where the life of the engine, or the time for which the engine is run, is of limited and short duration. However, such a limited life is not acceptable in engines for private and commercial vehicles, where a substantial piston life is essential. FR-A-1384266 discloses a piston having two rings of blocks extending around the piston and held in respective grooves extending around the piston, which has no piston skirt. GB-A-2050563 discloses a piston for an internal combustion engine comprising a crown, a gudgeon pin bore and a skirt. The skirt carries two inserts of ferritic steel, austenitic stainless steel, brass or bronze. The inserts are encast into the piston and the skirt, adjacent the upper and lower edges of the inserts, is cut away so that the inserts are spaced outwardly of that part of the skirt, although the remainder of the skirt is of the same diameter as the inserts. At least some of the lateral thrust forces on the piston during reciprocation in an associated cylinder or liner are transmitted through the inserts, over which a lubricating oil film passes. US-A-4535682 discloses a piston for an internal combustion engine of the kind comprising a crown, a gudgeon pin bore, a skirt having portions lying on opposite sides of a plane including the piston axis and the gudgeon pin bore axis, at least three bearing surfaces being on each skirt portion and including portions for forming a lubricating film on said bearing surfaces, two of said at least three bearing surfaces being towards the piston crown and being spaced, while at least one remaining bearing surface is towards the lower edge of the skirt opposite the space between the crown end surfaces, said at least three bearing surfaces being spaced radially outwardly of, and surrounded by, the associated skirt portion, so that sliding contact between the skirt and the associated cylinder is confined to said surfaces, there being provided hydrodynamic ramps extending between the skirt and each said bearing surface to maintain a pressurized oil film between said at least three bearing surfaces and the associated cylinder, so that sliding contact between the skirt and the associated cylinder is confined to said at least three bearing surfaces. According to the invention, there is provided a piston for an internal combustion engine of the kind comprising a crown, a gudgeon pin bore, a skirt having portions lying on opposite sides of a plane including the piston axis and the gudgeon pin bore axis, at least three bearing surfaces being on each skirt portion and including portions for forming a lubricating film on said bearing surfaces, two of said at least three bearing surfaces being circumferentially aligned, being towards the piston crown and being spaced, while at least one remaining bearing surface is towards the lower edge of the skirt opposite the space between the crown end surfaces, said at least three bearing surfaces being spaced radially outwardly of, and surrounded by, the associated skirt portion, so that sliding contact between the skirt and the associated cylinder is confined to said surfaces, there being provided hydrodynamic ramps arranged between the skirt and each said bearing surface to maintain a pressurized oil film between said at least three bearing surfaces and the associated cylinder , so that sliding contact between the skirt and the associated cylinder is confined to said at least three bearing surfaces and lateral thrust forces on the piston during reciprocation in an associated cylinder are transmitted through said bearing surfaces, characterized in that each bearing surface and the associated ramps are formed by a strip of a plastics material, the skirt being formed with a plurality of recesses, each recess being surrounded by the skirt surface and each recess carrying an associated strip of plastics material. The following is a more detailed description of some 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 first form of piston carrying three strips of plastics material on each side of the skirt thereof, Figure 2 is a section of a part of the skirt of the piston of Figure 1, showing one of the three strips carried in a recess in the skirt, Figures 3A and 3B are alternative sections on the line III-III of Figure 1 showing respective forms of slot provided in the piston skirt for receiving the ends of the strips, Figure 4 is a similar view to Figure 2 but showing the strip connected to the skirt by a snap-fit coupling, and Figure 5 is a section through a strip of the kind shown in Figures 1 to 3 showing bevelled edges to the strip. Referring first to Figure 1, the first form of piston for an internal combustion engine comprises a crown 10 surrounded by a ring band 11 provided with three piston ring grooves 12. The skirt 13 depends from the ring band and a gudgeon pin bore 14 extends through the piston. The piston may be forged or cast, particularly squeeze cast, from aluminium or an aluminium alloy such as that known as LO-EX. On each side of a plane including the piston axis 34 and normal to the gudgeon pin bore axis 35, the skirt 13 carries a plurality of strips 15 of a plastics material. In the embodiment of Figure 1, three such strips 15 are shown although it will be appreciated that any other suitable arrangement of strips 15 may be used. One suitable material for the strips is a polyetheretherketone alloy of the kind described in British Patent Application No. 84.06547 (Publication No. 2136439). Such an alloy is a mixture of polyetherketone, together with polytetrafluoroethylene (PTFE), graphite and bronze. The polyetherketone may be polyetheretherketone and the bronze may be a tin bronze, for example an 11% tin bronze. Each strip 15 is generally rectangular in shape with longer sides 15a interconnected by shorter sides 15b. The strips 15 have a width and a length which are substantially greater than their thickness so that they are flexible about axes across their width. As seen in Figure 2, the strips are inserted in corresponding recesses 16, each recess 16 being shaped to receive the associated strip 15 and having a radial depth which is such that at least a part of the associated strip 15 is radially outwards of the skirt 13 surrounding the recess 16. This outward extent or "height" may be, for example, less than 125 microns and may, for example, be at least 25 microns. The strips 15 are arranged with their longer sides 15a extending circumferentially and with the interconnecting shorter sides extending parallel to the piston axis. As seen in Figure 1, the strips 15 are held in position by inserting the ends of each strip into slots 17 formed in the piston skirt 13. These slots 17 may be formed entirely within the thickness of the skirt 13 (see Figure 3A) or may pass right through the skirt 13 so that the associated strip extends into the interior of the piston (see Figure 3B). In an alternative construction shown in Figure 4, each strip 15 and the skirt 13 may be provided with inter-engaging parts 18, 19 which are a snap-fit together to hold the strip on to the skirt. As shown, the strip 15 is provided with a projection 18 having a head which can be snapped into an aperture 19 provided at the back of the associated recess 16 so that the head engages in the aperture. The three strips 15 are arranged as follows (see Figure 1). Two of the strips 15 are located towards the ring band 11 and are equidistantly spaced on either side of a plane including the piston axis 34 and normal to the gudgeon pin bore axis 35. The remaining strip 15 is spaced axially downwardly from these two strips and is in register with the gap between the two upper strips 15 so that it is disposed symmetrically about the plane. The surface of each strip 15 may be such that the surface intersects planes including the piston axis 34 along straight lines, as shown in Figures 2 and 4. Alternatively, as shown in Figure 5, the surface of each strip 15 may be formed with circumferentially extending bevelled portions 20 which extend between a central area 21 of each strip 15 and the longer sides 15a respectively. The angle of these two bevels 20 may be no more than 2 degrees. Alternatively, the surfaces of the strip 15 may be of curved cross-section in planes including the piston axis; with the curvature being such that the maximum tangential angle is no more than 2 degrees. In use, the piston of Figure 1 is attached to a connecting rod and reciprocates in an associated cylinder or liner with a film of oil being provided on the surface of the cylinder or liner by an oil supply and an oil control ring carried in the lowermost piston and ring groove 12. The outward radial spacing of the strips 15 from the surrounding skirt 13 is greater than the thickness of this oil film and so the lateral thrust forces on the piston, caused by the angle of the connecting rod relative to the piston, are transmitted to the associated cylinder or liner through the three strips 15 alone, and not through the remainder of the skirt 13. Since the frictional forces generated between the piston and the cylinder are proportional to the area of the oil film, this reduction in the area of the oil film, as compared to a conventional piston where the oil film extends over the whole skirt surface, reduces the frictional forces between the piston and the associated cylinder and so increases the performance of the associated engine. Although the reduction in area is substantial, hydrodynamic lubrication is nevertheless maintained and mixed or boundary lubrication avoided. Since, in general, the piston will tilt relative to the associated cylinder or liner, during reciprocation, the surfaces of the strips 15 will not be parallel to the surface of the associated cylinder or liner during such reciprocation, but will be slightly angled. This will cause the cylinder or liner and the surfaces 15 to form a hydrodynamic wedge between them which will assist in forcing lubricant over the strip surface to maintain efficient hydrodynamic lubrication between these parts. Where the strips 15 are bevelled or curved (see Figure 5), this assists in the formation of hydrodynamic wedges 36 of lubricant between the strips 15 and the associated cylinder or liner 37 which provide a pressurized film of oil over the surfaces of the strips 15. The fact that the strips 15 have a greater circumferential dimension than axial dimension ensures that this film is maintained over the whole axial length of the strips 15 because the oil cannot readily escape in a circumferential direction. In addition, the location of the strips 15 on the surface, with the lowermost strip in register with the gap between the two upper strips, ensure that each strip 15 has an uninterrupted flow of oil over its surface. In general, a reduction in the area of the oil film between the piston and the associated cylinder or liner tends to increase the noise generated by the piston during reciprocation, because the greater the volume of oil between the piston and the associated cylinder or liner, the greater the damping action of this oil film. However, the use of these plastics strips 15 provides a noise-dampening effect which mitigates this disadvantage or may even reduce the amount of noise produced by the piston of Figure 1 in comparison with a conventional piston with full skirt. This is assisted by the fact that oil will seep into the recesses 16 behind the strips 15 and form an oil cushion between the back surfaces of the strips and the adjacent skirt surface allowing limited and noise-dampening radial movement of the strips 15. A further problem in pistons is that of direct contact between the piston and the associated cylinder or liner. This can occur on starting, before the oil film 36, Figure 5 has had a chance to become established, or at top and bottom dead centre where the piston is momentarily stationary. In a conventional piston, these direct contacts can lead to marking of the piston surface and increased frictional forces as a result of the direct metal-to-metal contact. In the piston described above with reference to Figures 1 to 5, however, any contact between the piston and the associated cylinder or liner is contact between the cylinder or liner and the plastics strips 15. Due to their low inherent coefficient of friction, this produces little or no increase in the frictional forces and allows the piston to run smoothly over the surface of the cylinder without scoring or wear until the lubricant film is established or until the piston has passed top or bottom dead centre. Because the strips 15 are held only loosely in their associated recesses, it is possible for them to tilt slightly about an axis parallel with the gudgeon pin bore axis. This "tilting pad" effect can further assist in ensuring hydrodynamic lubrication over the surfaces of these strips 15, even where the bevels 20 are not provided. It will be appreciated that the strips may be formed on the piston other than as described above For example, the piston skirt may be provided with recessed portions in which the strips are formed by direct injection moulding. The recesses may be provided with an aperture in which is formed part of the strips which lock the strips to the associated recesses. Where the strips are to be profiled, this is achieved by suitable shaping of the injection moulding device so that the profile is formed with the strips.

Claims: A piston for an internal combustion engine of the kind comprising a crown (10), a gudgeon pin bore (14), a skirt (13) having portions lying on opposite sides of a plane including the piston axis and the gudgeon pin bore axis, at least three bearing surfaces (15) being on each skirt portion and including portions for forming a lubricating film on said bearing surfaces, two of said at least three bearing surfaces being circumferentially aligned, being towards the piston crown (10) and being spaced, while at least one remaining bearing surface (15) is towards the lower edge of the skirt (13) opposite the space between the crown (10) end surfaces (15), said at least three bearing surfaces (15) being spaced radially outwardly of, and surrounded by, the associated skirt portion, so that sliding contact between the skirt (13) and the associated cylinder is confined to said surfaces, there being provided hydrodynamic ramps (20) arranged between the skirt (13) and each said bearing surface (15) to maintain a pressurized oil film between said at least three bearing surfaces (15) and the associated cylinder (37), so that sliding contact between the skirt (13) and the associated cylinder (37) is confined to said at least three bearing surfaces (15) and lateral thrust forces on the piston during reciprocation in an associated cylinder (37) are transmitted through said bearing surfaces (15), characterized in that each bearing surface and the associated ramps are formed by a strip (15) of a plastics material, the skirt (13) being formed with a plurality of recesses (16), each recess (16) being surrounded by the skirt surface and each recess (16) carrying an associated strip (15) of plastics material. A piston according to claim 2, characterised in that each strip (15) is rectangular in shape having longer circumferentially extending edges (15a) interconnected by shorter axially extending edges. A piston according to claim 1 or claim 2, characterised in that the or each strip (15) has ends which are inserted in respective slots (17) provided in the piston skirt (13) to connect the strip to the skirt. A piston according to claim 1 or claim 2, characterised in that the or each strip (15) and the skirt (13) are provided with inter-engaging parts (18, 19) which are a snap-fit together to hold the or each strip (15) on the skirt. A piston according to any one of claims 1 to 4, characterised in that the or each strip (15) has upper and lower circumferentially extending bevels (20) leading from a central portion (21) of the associated strip to respective longer sides of the associated strip (15), the angle of each bevel being no more than 2 degrees so that the bevels (20) form, with the associated cylinder or liner (37), a converging hydrodynamic wedge which forces oil over the surface of the associated strip. A piston according to any one of claims 1 to 5, characterised in that the or each strip (15) has a rear surface which is spaced from an adjacent part of the skirt for the receipt of lubricant therebetween to damp lateral movements of the piston.

Inventors: PHILBY JONATHAN DAVID, PHILBY, JONATHAN DAVID
Applicants: AE PLC
Application year: 1985
Main classification: F16J1
IPC codes: F16J1, F02F3, F02F7, F02F5