IMPROVEMENTS TO PISTON & CONNECTING ROD ASSEMBLIES Connecting rods of internal combustion engines are usually linked to their associated pistons by means of a gudgeon pin carried by the piston via pin bosses. In this arrangement, all loads to the crankshaft of the engine from the piston are transmitted through the gudgeon pin and the pin bosses of the piston. The gudgeon pin and the pin bosses, therefore, have to take the inertia forces of the piston when it moves upwardly during the last half of the exhaust stroke and downwardly during the first half of the induction stroke (of a 4-stroke cycle), and the much larger loads from the piston when it moves downwardly during the combustion/expansion stroke and upwardly during the compression stroke, which can pose considerable problems for engines with high cylinder pressures. Moreover, the load transmission paths from the piston to the connecting rod are not direct and during the expansion stroke this leads to considerable bending loads on the piston crown, the pin bosses, the gudgeon pin and the small end of the connecting rod. These bending loads, particularly under the increasing trend of higher gas pressures, can present limitations, both structural and hydrodynamic, in the pin bosses and connecting rod small end. These limitations lead in turn to limitations in cylinder pressure and reduced engine performance in terms of emissions, fuel consumption and torque/power.

Also, some lightweight materials, that offer major weight savings for the piston and connecting rod and hence reduce dynamic loading on the engine structure, have bending load limits that may preclude their use at high gas pressure loading on the piston crown.

Various embodiments of a piston and connecting rod assembly according to the invention are proposed that allow the relatively large downwardly directed piston loads during the combustion/expansion and compression stroke to be transmitted directly through the centreline of the piston and connecting rod axes, thereby reducing bending loads and providing more favourable hydrodynamic load carrying areas in the load transmission path between piston and connecting rod.

In the broadest aspect as set out in Claim 1, the invention is a piston and connecting rod assembly, comprising a piston, the lower surface of the crown of which is provided with a concave load bearing area, a connecting rod, the upper surface of the small-end of which is provided with an opposing convex load bearing area, and a gudgeon pin carried by the piston and received in the bore of the small-end, the arrangement being such that when the pin is centrally located in the small-end bore, there is a constant radial all round clearance between the pin and the bore, and a clearance is defined between the load bearing areas, the all round clearance between the pin and the bore being greater than the clearance between the load bearing areas so that when the load bearing areas contact, there remains a clearance between the gudgeon pin and the bore of the small-end, and when the gudgeon pin contacts the bore of the small-end, there remains a clearance between the load bearing areas.

Other aspects of the invention are outlined in the further subordinate claims.

Background explanations and specific embodiments of the invention are now described by way of example with reference to the accompanying drawings in which: Figure 1 shows a conventional monolithic piston, gudgeon pin and connecting rod assembly.

Figure 2a shows the forces on the gudgeon pin of Figure 1 at top dead centre of the exhaust stroke.

Figure 2b shows forces on the gudgeon pin of Figure 1 during the combustion/expansion stroke.

Figure 3a shows the bending and resisting load paths for the assembly of Figure 2a at top dead centre of the exhaust stroke. It can be seen that the forces are transmitted from the piston periphery, via the gudgeon pin, to the connecting rod small end and shank. The offset of the applied load from the piston and the resisting load in the connecting rod gives rise to bending forces on the gudgeon pin and only loading is exerted on the pin bosses and the small end of the connecting rod.

Figure 3b shows the bending and resisting load paths for the assembly of Figure 2b during the compression and combustion/expansion strokes. It can be seen that the forces are transmitted from the piston periphery, via the gudgeon pin, to the connecting rod small end and shank. The offset of the applied load on the piston crown bosses and the resisting load in the connecting rod gives rise to bending forces on the gudgeon pin. This exerts loading on the piston pin bosses and on the small end of the connecting rod.

Figure 4a shows a section through an articulated piston and connecting rod assembly according to the invention; in this figure, the piston is moving downwardly during the induction stroke.

Figure 4b shows the assembly of Figure 4a when the piston is moving downwardly during the combustion/expansion stroke or upwardly during the compression stroke or the first half of the exhaust stroke.

Figure 5a shows a section at right angles to that shown in Fig. 4a of an assembly similar to that of Figure 4a except that the gudgeon pin bosses of the piston crown are split.

Figure 5b shows a view of the underside of the crown of the assembly of Figure 5a, without the piston skirt.

Figure 5c shows a view of the underside of the crown of the assembly of Figure 5a, with the skirt in position.

Figure 6a shows a section through the small-end of a connecting rod forming part of an assembly according to the invention showing oil paths (indicated with arrows).

Figure 6b shows a side view of the small end of Figure 6a, the load bearing area of which is provided with oil spreader grooves.

Figure 6c shows a view on the top of the small-end of Figure 6a showing an arrangement of oil spreader grooves.

Figure 7a shows a monolithic piston and connecting rod assembly according to the invention.

Figure 7b shows a view on the underside of the piston shown in Figure 7a Figure 8a is a perspective view of the cradle of the crown part of the assembly in Fig. 5a, 5b & 5c.

Figure 8b shows a view at right angles to that shown in Figure 8a.

Figure 8c shows a perspective view of the skirt of the assembly of Figures 5a, 5b & 5c.

With reference to Figure 4a, an articulated piston comprises a crown 1 and a separate skirt 2. The crown is provided with bosses 3 which engage a gudgeon pin 4. The skirt is provided with bosses 2a which also engage the gudgeon pin. In this way the crown and skirt are connected together. The gudgeon pin is located with all-round clearance 5 within the bore 5a of the small end 6 of a connecting rod 7. The crown is provided on its undersurface with a concave arcuate load bearing area 8 and the small end is provided with a complementary convex arcuate load bearing area 9. The small end is thus held loosely within the piston with the two load bearing areas in opposition with a clearance 10 between them.

The all round clearance between the gudgeon pin and the bore of the small end, is larger than the clearance between the load bearing areas of the piston and small-end. Such an arrangement operates in the following manner.

At the beginning of the induction stroke, the piston inertia resists the downwardly motion of the connecting rod. As shown in Figure 4a, this causes the upper side of the gudgeon pin to engage the bore of the small end and for an enlarged clearance 5b to be formed at the underside of the gudgeon pin and for an enlarged clearance 10 to be formed between the load bearing areas 8 and 9. Thus, during the induction stroke, the relatively small load A (compared to gas forces during the combustion/expansion strokes) is transmitted from the piston to the gudgeon pin and thus to the connecting rod in essentially conventional fashion involving bending forces.

At the beginning of the combustion/expansion stroke, the piston starts to move downwardly relative to the connecting rod. As shown in Figure 4b, this causes the load bearing areas 8 and 9 to move into contact and for a clearance 5 to be re-established around the gudgeon pin. Thus during the combustion/expansion stroke, the relatively high gas load C is transmitted directly from the piston to the connecting rod and the bending forces on the piston crown are significantly reduced, and there are no bending forces on the gudgeon pin and small end. The actual load in the connecting rod is the algebraic sum of the downward forces C (from the gas pressures acting on the piston crown) and the inertia forces A + B due to the total piston masses..

Again, during the first half of the exhaust and the compression stroke, the piston is pushed by the connecting rod and the load bearing areas move into contact to transmit the gas loads.

The small end is provided with an oil feed passage 11 extending from the bushes 12 in the bore of the small end, to the load bearing area 9 of the small end. The crown has radial apertures 13 in the pin bosses 3, allowing the oil from the small end of the connecting rod to pass unimpeded into a peripheral oil receiving cavity 14 between the pin bosses and the inner radius of the ring carrier 15. The skirt 2 has a closure plate 16 having a central aperture which accommodates the connecting rod small end with some clearance to define a drain path 16a for the oil from the cavity 14. The clearance also allows for connecting rod angularity during piston motion. Alternatively, or in addition, the closure plate may be provided with apertures to form oil drain paths from the cavity 14.

Preferably, the centres of curvature of the arcuate load bearing areas lie on the central axis of the connecting rod small end, so that the connecting rod can assume its natural motion without imposing any unnatural tipping motion to the piston crown, and the radius 12 (see Figure 5a) of the load bearing area 8 of the crown is marginally larger than that of the load bearing area 9 of the small end.

With reference to Figure 5a, each boss 3 of the crown 1 may be split along a major diameter of the bore 5a which receives the gudgeon pin, thereby forming a removable cap 18 which can be retained to the upper boss half 19 by a set bolt 20. The closure plate 16 of the skirt is also visible in this section.

With reference to Figure 5b, the load bearing area 8 in the under surface of the crown is visible, as are the set bolts 20.

With reference to Figure 5c, this shows the closure plate 16 of the skirt having an aperture for receiving the connecting rod shank. A clearance 21 is formed between the plate and the rod. The pin bosses 22 of the skirt can be seen adjacent to the bosses 3 of the crown 1.

With reference to Figure 6a, this shows an embodiment of a connecting rod small end which may be used in the invention. The small end has a pair of spaced bushes 23,24 which receive the gudgeon pin. Oil is fed to the space 25 between the bushes by a feeder passage in the shank of the connecting rod and then passes via an oil feed passage 26 in the small end to the load bearing area 9 of the small end and via laterally extending oil passageways 27 to the sides of the small end.

With reference to Figure 6b, the feeder passage 28 in the connecting rod shank is visible. The passage intersects the bore of the small end tangentially at the space 25 between the bushes and is stopped at its upper end by a ball. The load bearing area 9 of the small end may be provided with oil distribution grooves 30, shown in Figures 6b and 6c. Oil distribution grooves may be provided on the load bearing area of the crown.

With reference to Figures 7a and 7b, these show a monolithic piston, i. e. a piston in which the crown 1 and the skirt 2 are integral. The piston body is provided with bosses 3 which receive the ends of a gudgeon pin 53. The gudgeon pin is received with all round clearance within the bore of the small end of a connecting rod 56. A smaller clearance is defined between the concave arcuate load bearing area 51 of the crown and the convex arcuate load bearing area 52 of the small end. In similar fashion to the assembly of Figures 4a and 4b, the load during the compression stroke, the exhaust stroke and the combustion/expansion strokes is transmitted by contact between the load bearing area of the crown and the load bearing area of the small end of the connecting rod, and the load during the induction stroke is transmitted via the gudgeon pin 53.

A separate closure plate 54, retained within a groove in the inner surface of the skirt by a circlip 55, is used to form an oil retention gallery, the closure plate being a close fit around the connecting rod. Oil is supplied along the connecting rod shank to the small-end arcuate load bearing areas via a passage 57 and to the cavity 58 behind the ring grooves via lateral passages 59 and 60. The closure plate has a window 61 which allows assembly of the connecting rod small end into the piston body. The closure plate window has the necessary clearance with the connecting rod to allow for the angular swing of the connecting rod.

Alternatively, the closure plate may be made of a flexible material and the inner surface of the skirt is provided with a groove which receives the edge of the plate, the plate being held in position by flexure of the plate. As a further alternative, the inner surface of the skirt is provided with a screw threaded groove and the edge of the plate is screw threaded for reception by the groove.

With reference to Figures 8a-8c, these show perspective views of the crown and skirt parts of the assembly shown in Fig5a, 5b & 5c, in particular showing the retaining piston boss cap 18 and upper half 19 and the retaining set screws 20. Fig 8c shows the skirt aperture 195 (for the conrod) in the closure plate 16.

It is possible for the piston and connecting rod assemblies or parts of the assemblies, previously described, to be made in fibrous materials such as carbon-carbon. Other possible materials for the piston and or connecting rod are described in the following sections.

Piston and connecting rod assemblies or parts of the assemblies, as previously described, can be made in fibrous reinforced materials such as carbon fibre reinforced plastics.

Piston and connecting rod assemblies or parts of the assemblies, as previously described, can be made in metallic materials and alloys such as steel, cast-iron or aluminium alloys.

The piston crown portion of piston and connecting rod assemblies, as previously described, can be made in a particular metallic material or metallic alloys such as steel, cast-iron or aluminium alloys, whilst the piston skirt is made in a different metal or metallic alloy material.

The piston crown portion of piston and connecting rod assemblies, as previously described, can be made in a fibrous material such as carbon- carbon, whilst the piston skirt is made in a metal or metallic alloy material such as steel, aluminium alloy or cast-iron.

The piston crown portion of piston and connecting rod assemblies, as previously described, can be made in a fibrous material such as carbon- carbon, whilst the piston skirt is made in a fibre reinforced material such as carbon fibre reinforced plastics.

The connecting rod of connecting rod/piston assemblies, as previously described, can be made from metallic alloys, carbon-carbon or fibre reinforced plastic composite materials.