The present invention relates to piston and piston sealing ring assemblies.

In a conventional piston assembly used in, for example, an internal combustion engine, a piston is slidable within a cylinder and connected to a crank shaft by a connecting rod. One end of the connecting rod is secured to a pivot pin known as a gudgeon fixed into the piston and the other end of the connecting rod is secured to a crank on the crank shaft. Reciprocation of the piston within the cylinder causes rotation cf the crank shaft and vice versa.

A conventional piston head is provided with circumferential grooves in which a series of separate sealing rings are received. The ends of the rings do not overlap but are positioned close to each other, clearance being provided between the two ends to enable thermal expansion and contraction to be accommodated. The springs are formed so that in their relaxed state they define a portion of a circle of greater diameter than the cylinder in which the piston is movable. Thus the rings spring outwards into contact with the .cylinder wall. The grooves in which the rings are received are located close to the top or crown of the piston. The piston also includes a skirt which depends from the crown of the piston below the piston ring grooves and the gudgeon pin extends through this skirt. The arrangement is assembled by pushing a gudgeon pin through an aperture in the skirt into engagement with a bearing in the connecting rod.

As the gudgeon pin penetrates the skirt, it is necessary in a conventional piston assembly for the axis of the gudgeon pin to be displaced from the crown of the piston by a distance greater than the distance occupied by the sealing grooves. It is known that the pivot axis between the connecting rod and the piston head should be as high as possible relative to the piston crown so as to minimise the angle of side swing applied to the connecting rod as the crank shaft rotates. Thus the provision of a gudgeon pin which extends through the piston skirts is a significant restraint on the piston assembly design.

Various proposals have been made to enable the pivot axis between the connecting rod and the piston head to be above the lowermost piston ring. For example, German Patent Specification No. DE-A-3514928

describes an arrangement in which the connecting rod is provided with a cross-member opposite ends of which are received in respective recesses defined in the piston head, those recesses not penetrating the piston skirt. The recesses are defined by a sub-assembly that is secured by bolts to the crown of the piston. The cross-member typically comprises an arcuate member of constant radial thickness which is received between two arcuate surfaces defined by the recesses. Thus the pivot axis may be located at a relatively high point either within the piston head or above the crown of the piston head. This arrangement does enable the position of the pivot axis to be optimised but it is nevertheless necessary to secure the recess-defining sub-assembly by bolts in the restricted area defined within the piston skirt.

German Patent Specification No. DE-A-2903325 discloses another arrangement in which a connecting rod is provided with a cross-member that serves the same purpose as the gudgeon pin of a conventional piston assembly. The cross-member is received in a pair of recesses which are permanently secured beneath the main skirt of the piston, the cross-member being inserted by pushing one end of the cross-member into one of the recesses with the axes of the connecting rod and piston head mutually inclined and subsequently manoeuvring the connecting rod and piston head until the axes are parallel, whereafter the head can be pushed sideways on the cross-member to fully interengage the two components. This approach avoids the need for securing a sub-assembly within the piston head but the space required to enable the cross- member to be manoeuvred into place results in it being necessary for at least one of the recesses to penetrate the skirt of the piston. Thus the cross-member must be located on the side of the piston ring grooves remote from the crown of the piston.

Japanese Patent Specification No. JP-A-51021565 describes a technique for producing high quality pistons by forging half -pistons and subsequently welding the half-pistons together. This technique was proposed however for use in the manufacture of conventional piston heads having skirts which are penetrated by apertures intended to receive a gudgeon pin. The objective was to minimise the wastage of material in the press formation of a piston.

Further problems arise with conventional piston assemblies as a result of the form of the piston rings. In particular, substantial

frictional losses result from friction between the rings and the cylinder walls. The rings are spring-biased outwards against the cylinder wall and the spring-biased force must be sufficient to provide acceptable sealing for all positions of the piston relative to the cylinder. After the piston has moved to top dead center and a charge of fuel has been ignited however high pressure gases enter the grooves in which the piston rings are received and substantially increase the radial force applied by the rings to the cylinder wails. This substantially increases frictional losses and also wear. As the control of emissions from internal combustion engines becomes increasingly significant it will be necessary to reduce those emissions and this will increase the significance of leakage past the rings of a conventional piston assembly. Solutions to this problem have been suggested in a number of documents, for example GB-A-2153964, GB-A-2144518, and GB-A-2117868. Essentially these solutions rely upon the provision of auxiliary piston rings and complex piston assemblies with precision machine parts, but all based on the conventional piston ring of the type described above.

Various proposals were made in the past to provide fundamentally different piston ring structures. For example, British Patent

Specification GB-A-934302 describes the use of helical sealing rings, but these rings had an active seal located at both ends of the ring and throughout several convolutions. This required precision machined helical grooves and end slots cut into the piston wall. The suggested arrangement does not however deal with the problem of gas leakage resulting in very high contact pressure and friction between the rings and the cylinder wall. British Patent Specification GB-A-901990 describes another arrangement in which a helical bearer ring is provided between helical sealing rings. The bearer ring was not provided to support and locate the sealing rings however but rather to carry side thrust from the piston to the cylinder bore. The arrangement thus provided a wider bearing surface between the piston assembly and the cylinder although the area of contact between the bearer ring and the cylinder wall would not be well lubricated. Again the problem of high contact pressure and friction was not addressed.

It is an object of the present invention to obviate or mitigate one or more of the problems outlined above.

According to the present invention, there is provided a piston

assembly comprising a piston head, and a connecting rod one end of which is secured to the piston head so as to be pivotal relative to the piston head about a predetermined axis, the said one end of the connecting rod supporting a cross-member opposite ends of which are received in respective recesses defined in the piston head, and the recesses defining arcuate bearing surfaces which are contacted by corresponding arcuate bearing surfaces defined by the cross-member, wherein the piston head is formed from at least two components and each recess is defined by a respective one of the components, the components being permanently secured together after insertion of the cross-member into the recesses such that subsequent separation of the piston head and connecting rod is prevented.

Preferably the predetermined axis is located near or even above the crown of the piston. The predetermined axis may be inclined to a line perpendicular to the direction of reciprocation of the piston within the cylinder. The inclination of the axis in this manner causes a small angular oscillating rotation of the piston as it reciprocates in the cylinder.

The connecting rod cross-member may bear against a bearing pad located on the underside of the piston crown. The bearing pad may be supported on a wedge arranged to adjust the disposition of the bearing pad relative to the piston head. A single screw-adjustable wedge or one or more spring-loaded automatically adjustable wedges may be provided.

The piston head may be in for example, three parts, comprising two half -piston shells and a piston top which is secured on the shells. Such an arrangement enables for example different materials to be used for the half-shells and the piston top. The piston top may be secured to the half-shells by, for example, friction welding.

The piston head may have bearing pads supported in a depending skirt, the bearing pads bearing against the cylinder wall to reduce wear.

The piston assembly may incorporate a piston sealing ring comprising at least one helical ring seal, the ends of which overlap and at least one helical ring seat, the ends of which overlap, the ring seal and ring seat being received in a common slot, the ring seat being biased to abut against the base of the slot, and the ring seal being biased to project from the slot into contact with the cylinder wall, the ring seat and ring seal bearing against each other such that the ring seat

obstructs the flow of gas to an annular cavity defined between the ring seal and the base of the slot.

Thus, excessive pressure exerted on the cylinder wall by the necessary sealing ring is reduced by restricting high pressure gas penetrating behind the ring seal.

Preferably either the ring seal or the ring seal and ring seat are locked at one end against rotation relative to the piston assembly. With such an arrangement rotation of the piston relative to the cylinder causes the sealing ring assembly to tighten and slacken depending on the direction of rotation, thereby enabling the seal pressure to be increased in the region of top dead center and reduced in the region of bottom dead center. This again reduces frictional losses. Furthermore, rotational movement of the sealing rings with the piston causes the cylinder wall to be wiped clear with a greater efficiency than in conventional assemblies, thereby reducing the risk of wear particles seriously damaging the cylinder wall surface.

The sealing ring assembly may be locked against rotation by, for example, radial slots engaged by projections formed on the ends of the rings or by axial slots engaged by formations on the rings.

The rings may extend so that their ends just overlap, for example extending approximately 1.1 times around the piston. Rings may be smooth, one effectively forming a continuation convolution of the other such that the opposite ends of the seal rings contact each other and opposite ends of the seat ring contact each other. Alternatively, each ring may comprise a plurality of convolutions, the convolutions of the seal ring being interleaved with convolutions of the seat ring. The rings may be corrugated along their lengths. Generally, a single pair of rings, that is one seat ring and one seal ring, will be received in a single groove defined in the piston head.

The present invention also provides a sealing ring assembly for incorporation in a conventional piston head, the sealing ring assembly comprising at least one helical ring seal, the ends of which overlap, and at least one helical ring seat, the ends of which overlap, wherein the ring seal and ring seat are intended to be received in a common slot opening into the annular gap defined between the piston and a cylinder within which the piston is movable, the ring seat being biased to abut against a base of the slot and the ring seal being biased to project

radially from the slot across the gap, wherein the ring seat and ring seal bear against each other such that the ring seat obstructs the flow of gas to an annular cavity defined between the ring seal and the base of the slot.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Fig. 1 is a schematic illustration of a first embodiment of the present invention- Fig. 2 is an exploded view of a piston head of a second embodiment of the present invention;

Fig. 3 is a partial section on the line III-III of Fig. 2;

Fig. 4 is a perspective view of the small end of a connecting rod intended to be inserted in the piston head of Fig. 2;

Fig. 5 illustrates a low friction side thrust bearing pad to be inserted in the piston head illustrated in Fig. 2;

Fig. 6 is a perspective view of component parts of a piston head of a third embodiment of the present invention;

Fig. 7 is a bottom view of the third embodiment of the present invention shown without fitting the piston head or the connecting rod;

Fig. 8 is a sectional view through a partially exploded fourth embodiment of the present invention;

Fig. 9 is a side view of portions of the fourth embodiment of the present invention;

Fig. 10 is an exploded view of the piston head of the fourth embodiment of the present invention;

Fig. 11 is an exploded view of the small end of a connecting rod intended for insertion into the piston head of Fig. 10;

Fig. 12 is a side view of a piston top suitable for incorporation in an embodiment of the present invention and incorporating slots for receiving helical sealing rings;

Fig. 13 is a view of the sealing rings intended for insertion into the piston top of Fig. 12;

Fig. 14 illustrates adjacent ends of rings incorporated in the piston top of Fig. 12;

Fig. 15 is ^' a partial section through one of the ring assemblies of Fig. 13, on the line 15-15;

Fig. 16 is a perspective view of adjacent ends of rings shown in

Fig. 14;

Fig. 17 illustrates characteristics of a conventional ring assembly;

Fig. 18 illustrates characteristics of a ring assembly of the type illustrated in Figs. 12, 13 or 14;

Fig. 19 is an exploded view of a conventional piston head adapted for use with spiral piston rings;

Fig. 20 is a perspective view of a seal ring for mounting on the piston head of Fig. 19;

Fig. 21 is a perspective view of a seat ring for use with the seal ring of Fig. 20;

Fig. 22 is a side view partially in section of the piston head of Fig. 19 supporting the ring structures of Figs. 20 and 21;

Fig. 23 shows an alternative ring arrangement;

Fig. 24 is an exploded view of components shown in Fig. 23; and

Fig. 25 illustrates a further embodiment of the invention.

Referring to Fig. 1, the illustrated structure comprises a connecting rod 1 having a cross member 2 formed on its small end. The cross member is received in a piston head defined by a first half-shell 3 and a second half-shell 4. The half-shell 3 defines a first recess 5 and a second recess which is a duplicate of recess 5 is defined in the half -shell 4. A bearing pad 6 is arranged to be supported by each of the half-shells 3 and 4 beneath a piston top 7.

The illustrated components are assembled by bringing the half- shells 3 and 4 together and fastening/bonding/welding them together so that the cross-member 2 is retained within the recesses defined by the half-shell. The piston top is then welded to the assembled half-shells. The top end of the connecting rod bears against an arcuate surface defined by the bearing pad 6 and the undersides of the cross member 2 bear against arcuate surfaces defined by the insides of the recesses remote from the piston top 7. Thus the connecting rod is permanently secured in the piston head and can rotate about an axis defined by the line about which the lower surface of the bearing pad 6 is a surface of revolution.

Having described in general terms an embodiment of the present invention with reference to the schematic illustration of Fig. 1, a second embodiment of the invention will now be described with reference to Figs. 2 to 5.

Referring to Fig. 2, this shows examples all of the components except for the side thrust bearings and the connecting rod making up the piston head of the second embodiment. The basic components comprise a first half-shell 8, a second half-shell 9, a piston top 10, a bearing pad 11 and a wedge 12. The recess defined by the half shell 8 is limited on its lower side by a bearing surface 13 and there is an equivalent bearing surface which is not visible defined within the recesses formed within half -shell 9.

The piston top 10 is formed with recesses 14 in its upper surface to provide valve clearance and a suitably shaped central upper surface 15. A projection 16 is defined on the underside of the piston top and intended for reception in a groove 17 provided in the top of each of the half -shells 8 and 9. The underside of the piston top is provided with corrugations 18 which serve to assist the flow of cooling gas through the piston head assembly via holes 19 defined by the half shells 8 and 9.

Each of the half -shells define a groove 20 intended to receive a conventional oil return ring (not shown). Beneath the groove 20 a series of further grooves are formed in a downwardly depending skirt portion of the two half-shells 8, 9, the grooves being formed in the left-hand portion of the half-shell 8 as shown in Fig. 2 and in the right-hand portion of half-shell 9 as shown in Fig. 2. The left-hand portion of the half-shell 8 in Fig. 2 supports a pair of flanges 21 and 22, the flange 21 being intended to engage in a recess 23 in the half shell 9 and the flange 22 being intended to engage in a slot 24 in the half-shell 9. Corresponding formations are identified by the same reference numerals on the half -shell 8 and it will be appreciated that these formations are engaged by equivalent formations to the flanges 21 and 22 provided on the half-shell 9. Guide pins 25 engageable in apertures 26 ensure the accurate alignment of the two half-shells. Once aligned and fitted together the flanges 21 and 22 can be welded e.g. laser welded, to secure the two half -shells together.

Before assembly, the small end of the connecting rod shown in Fig. 4 is inserted between the bearing pad 11 and the bearing surfaces 13. An upper surface 27 of a cross-member 28 formed on the small end of the connecting rod 29 rests against the bearing pad 11. Bearing surfaces (not visible in Fig. 4) on the undersides of the cross member, that is the

surface diametrically opposite the surface 27 on each of the two ends of the cross member, contact the bearing surfaces 13. The bearing pad 11 is initially supported on small lips 30 and between flanges 31. This arrangement allows the bearing pad 11 to slide with the small end of the connecting rod into the recesses defined by the two half-shells. After assembly, the position of the wedge 12 is adjusted using two set screws only one of which is visible in the drawing and is labelled 32. One set screw is provided at each end of the wedge so that it may be positioned appropriately to maintain the spacing between the bearing plate 11 and the bearing surfaces 13 so as to provide a good sliding fit with the cross member of the connecting rod. This arrangement provides free side-play for the connecting rod while maintaining close control of the connecting rod.

The connecting rod 29 is thus rotatable about an axis relative to which the surface 27 and the surfaces 13 are surfaces of revolution. That axis is indicated in Fig. 4 by broken line 33. The axis 33 is inclined at an angle of, for example 10° to a plane which is perpendicular to the intended axis along which the piston is reciprocated. As a result, as the connecting rod swings about the axis 33 the surface 27 effectively moves across a frusto-conical surface and therefore the piston is caused to rotate to a small extent relative to the cylinder.

Referring now to Fig. 5, this shows a "low friction" pad 36 made from a cylindrical rod formed to present a curved face which is a close match to the shape of the adjacent cylinder walls. A low friction pad is inserted in a pair of slots 35 machined in the grooves beneath the slot 20 which receives a sealing ring. The final shape of the pads 36 is such that the center of gravity of each pad lies on the piston side of the contact surface on the cylinder. Thus when the pads 36 is not carrying substantial side load thrust from the piston (which would trap it against the slot edges shown) then the pad is free to partially rotate about its major axis until this rotation is resisted by contact with the cylinder wall. This action allows the pads to trap an oil wedge for each stroke.

After assembly of the two half -shells, the outer diameter of the piston skirt in which the slot 20 is formed is machined down to the desired diameter. Although after final assembly no machining

modifications can be made inside the slots 35, the pads 34 can be easily fitted or removed when the piston is not within a cylinder simply by rotating the pads 36 through approximately 180° so that the ends come free. The effective thickness of the pads 36 (cylinder surface to rod center line mid-way along its length) may be produced and arranged to allow selective assembly allowing each piston unit to be matched to a respective cylinder. This allows for close fitting to cylinders to enable quiet running to be achieved and for the correction of wear.

The surfaces 13 which bear against the underside of the cross- member on the connecting rod must be accurately machined and cutting tools may be guided by the bearing surface of the bearing pad 11. Machining may take place with the wedge 12 in an intermediate adjustment position. The surfaces 13 will approximate to part of a cylinder but will vary from this proportionally towards the end to allow for arc movement of the lower bearing face of the connecting rod cross- member.

Mass reducing holes 36 may be distributed about the piston head. The provision of such holes permits crank case gas flow initiated by angled guide fins 37 combined with the sweeping movements of the connecting rod cross-member 28 to cool the piston head. This is particularly valuable if component parts of the piston head are fabricated from materials having low thermal conductivity, for example low mass engineering plastics. It will of course be appreciated that the multi-component structure of the piston head does enable the selection of different materials for the different components to provide desirable combinations of characteristics.

It will be seen that the faces of the two half-shells which come together in final assembly are stepped. The half -shells are intended to be cast using precision techniques so that small asperities contact at the joint. Liquid gasket bonding filler may be used to seal the joint. Final machining of the radially outer surface of the piston head ensures appropriate dimensions.

Referring now to Figs. 6 and 7, this embodiment of the invention is similar to that of Figs. 2 to 5 but differs in that the joint face between the two half-shells lies on a flat surface which intersects the vertical center line of the piston but is angled approximately across the corners of the connecting rod clearance space as seen from below. This

still allows low friction pads, if required, to be fitted in a joint free socket. The two half-shells may be held together for example by a section 38 of extrusion engageable in a circumferentially extending slot 39, the extrusion defining end flanges which engage behind appropriate mating surfaces on the two half-shells. In Fig 7, the joint line between the two half-shells is indicated by numeral 40. The piston top is not shown in Figs. 6 and 7 but it will be appreciated that the top incorporates at least one groove for receiving a sealing ring. Other features of the embodiment of Figs. 6 and 7 are identified by the same reference numerals as used in the case of Figs. 2 to 5. The outer periphery of the widest part of the piston head is indicated by numeral 41. Small protrusions 42 at the joint surface edge assist welding and bonding in the appropriate position. Again after final assembly with the cross-member of the connecting rod inserted in the two recesses defined by the half-shells the two half -shells are welded or otherwise secured together and then turned down to the final desired outside diameter. The 'outline' arrows in Fig. 7 indicate the direction of removal from dies or mould. The double black arrows indicate direction of assembly onto pad 11.

Referring now to Figs. 8 to 11, this illustrates a still further embodiment of the invention. Again the same reference numerals are used as in Figs. 2 to 5 for equivalent components. In this case the single piece wedge 12 of Fig. 2 is replaced by a double section wedge having identical wedge portions 43 biased apart by a spring 44. The spring 44 ensures that the wedges 43 press the bearing pad 11 downwards onto the top of the connecting rod so as to maintain the appropriate clearance between the connecting rod, the pad 11 and the bearing surfaces 13. In the arrangement of Figs. 8 to 11, the two half-shells have a flat joint face, a friction welded top 10 and thrust fins 45.

Annular ridges 46 are provided on the underside of the piston top 10 to permit cooling but additionally to form point contact welds with protrusions 47 on the top of the two half-shells. This is achieved simultaneously with the friction welding of the conical surface 48 on the piston top to the conical surface 49 defined by the two half -shells. A thin guide fin 50 is provided on each side of the final assembly. The guide fin which is normally provided with a small working clearance gap to the cylinder wall provides stiffness and additional tensile strength

to the final assembly, promotes cooling ventilation of the underside of the piston crown and the body of the piston head, protects the sealing rings from excessive side movement, and on contact with the cylinder wall provides a short slide path to the oil film to reduce dry friction contact.

The thrust fins 45 are perforated with holes and are stiff, angled and slightly curved. The connecting rod is shown in Fig. 11 but is not shown in Fig. 8 to avoid over complication of the drawings. An oil spray path is provided through the connecting rod however to provide additional cooling to the underside 46 of the piston top. As shown in Fig. 11, bearing pads 51 may be positioned in recesses defined on the underside of the cross member of the connecting rod. Such bearing pads may be inserted into a mould or die in which the connecting rod 29 is formed.

Referring again to Fig. 10, bushes 52 are used to accurately locate the two half-shells during assembly, the two half-shells being secured together by tubular rivets 53 extending through the bushes.

The spring 44 moves the two wedges 43 apart only when they are not under load, for example as a result of wear and/or hot "creep" of the case with high loading. A cutting edge 54 is defined by the bearing pad 11, the cutting edge being effective only on initial assembly after which it is blunted before final assembly. The cutting edge self cuts the underside of the piston half-shell with which it comes into contact as the two half shells are pushed together and thus serves to locate the bearing pad 11 relative to the half-shells, giving freedom to slide but without side play.

Referring now to Figs. 12 to 16, a sealing ring assembly is illustrated which may be used in a piston structure of the type described above or in a more conventional piston structure. As shown in the drawings, a piston top 55 has a pair of grooves 56, 57 formed therein, each groove communicating with a respective aperture 58, 59 extending radially inwards but not communicating to the inside of the piston 55. Each of the grooves 56, 57 receives a pair of rings, the disposition of the rings being as shown in Fig. 13. It will be seen that the topmost groove 56 receives a seal ring 60 and a seat ring 61 which is disposed above the seal ring 60. In contrast the groove 57 receives a seal ring 62 beneath which is located a seat ring 63. In the illustrated

case the piston top 55 and the associated sealing ring assemblies are mounted on a piston of the type indicated in Figs. 2 to 11 and accordingly an axis 33 is shown in Fig. 12 which corresponds to the pivot axis of a connecting rod located beneath the piston top 55. Thus the piston top 55 oscillates through a small angle of rotation as the piston reciprocates in a cylinder. However the rings may be used less beneficially in a piston not having the angular oscillation.

Fig. 14 illustrates the disposition of the rings in the groove 56 in the region of overlap of the ends of those rings. It will be seen that the ends of the seal ring 60 overlap to a small extent. It will also be seen that the ends of the seat ring 61 overlap to a small extent. The groove 56 is formed with abutment surfaces 64 which are spaced from the free ends of the two rings to allow for thermal expansion and contraction as indicated by arrows 65. The other ends of the rings are secured in position by shoes 66 as shown in Fig. 16 which engage in the aperture 58 (see Fig. 12). Fig. 15 shows the disposition of the two rings. It will be seen that the seal ring 60 is biased to project out of the groove 56 whereas the seat ring 61 is biased into the groove 56. The seat ring 61 thus prevents high pressure gas from above the piston from entering the open space defined in the groove 56 on the radially inner side of the seal ring 60. Indentations 67 formed on the upper side of the seat ring 61 ensure that the seat ring is urged into a position in which it prevents the high pressure gas above the piston from pushing the seal ring 60 outwards against the facing cylinder wall with excessive force.

It will be appreciated that the inclination of the axis 33 causes the piston top 55 to partially rotate about the cylinder axis as it reciprocates. As the piston moves from top dead center towards bottom dead center the piston top can be caused to rotate in an anti-clockwise direction when looking downwards on the top of the piston. This "unwinds" the seal ring 60, increasing the force with which the seal ring is pressed against the cylinder wall. Simultaneously, the seal ring 62 is pushed during the same motion, thereby increasing the force with which it bears against the cylinder wall. Part way down the cylinder from the point of maximum side swing of the connecting rod, rotation of the piston head is in the opposite direction, thereby causing the seal rings 60 and 62 to tighten, reducing the pressure that it exerts on the cylinder

wall. Thus the seal effectiveness is increased at the top 'half of the stroke where maximum gas pressure is applied to the top of the piston. This pressure is however never excessive as high pressure gas above the piston is not allowed to force the seal ring against the cylinder wall and furthermore the higher pressure between the seal ring and the cylinder is only maintained during approach and leaving T.D.C. stroke of the piston, being reduced on the approach and leaving B.D.C. when leakage past the seal ring is not a major problem due to the relatively low gas pressures to which the piston is exposed. Leakage past the top seal 60 raises the pressure between the two seals 60 and 62 and since from this side of the seals there is access to 'inside' the seals this increase in mean pressures also increases the contact force in the seals until an equilibrium pressure is reached suited to the mean effective pressure alone the piston. Ring friction can be considerably reduced during 'light running'. Thus, it is possible to optimise, or at least radically improve the sealing ring performance.

Fig. 17 illustrates the sealing ring contact force between a conventional sealing ring and a cylinder wall, the solid line indicating peak torque conditions, the broken line part load conditions, and the chain line light or no load conditions. Ring friction is closely related to sealing ring contact force and thus it may be seen that a fairly high minimum friction is indicated for all speeds and loads.

Fig. 18 illustrates the performance which may be achievable given a sealing ring structure such as that shown in Figs. 12 to 16 operating in a piston assembly of the type illustrated in Figs. 2 to 11, that is one in which the piston partially rotates as it reciprocates. The line represented by a series of small zeros corresponds to the full line in Fig. 17. The other three lines correspond to high, part and light load conditions as in the case of the corresponding curves of Fig. 17. It will be seen that all three curves fall fairly dramatically halfway through the working stroke due to the change in direction of the piston rotation as the connecting rod passes its maximum side swing position, the magnitude of the change in the ring friction with the change in direction of rotation of the piston being greater for greater pressure loadings. Thus, it can be seen that a very useful reduction in friction is available under peak torque conditions, and substantial reductions may be achieved in part and light load conditions.

Referring now to Figs. 19 to 22, an alternative helical sealing ring arrangement is illustrated. In contrast to the arrangement of Figs. 12 to 16, where the two ends of the seat ring contact each other and the two ends of the seal ring contact each other so that the seal ring is positioned in the same position as would be a further convolution of the seat ring, in the arrangement of Figs. 19 to 22 the seal ring 68 comprises three convolutions and the seat ring 69 comprises five convolutions. The two rings are interleaved as shown in Fig. 22 and placed in a wide slot 70 defined in a conventional piston head supported on a gudgeon pin 71 held in position by circlips 72. Ends 73 of the two rings are nested together in engagement with a recess 74 defined in one side of the slot 70. Corrugations 74 are formed along the length of the seat ring 69 and corrugations 75 are formed along the length of the seal ring 68. The corrugations towards the bottom of the seat ring 69 extend parallel to the axis of motion of the piston whereas the corrugations in the upper portion of the seat ring 69 extend radially as shown in Fig. 22. As in the case of the embodiment of Figs. 12 to 16, there is shown at the top right hand corner of Fig. 22 the seal ring 68 is biased radially outwards whereas the seat ring 69 is biased radially inwards. Once again therefore high pressure gas cannot penetrate readily behind the radially inner edges of the seal ring 68. Furthermore, assuming that the piston head is provided with an inclined rotation axis as in the case of the embodiment of Figs. 2 to 5, the beneficial coiling and uncoiling of the sealing ring assembly enables the performance of the type illustrated in Fig. 18 to be achieved.

It may be that beneficial effects can be achieved using a conventional form of sealing ring in a configuration such as that illustrated in Figs. 23 and 24. As shown in Fig. 23, a sealing ring 76 is received in a groove 77 of uniform width and depth. In a conventional manner a small but stepped gap 78 is provided between the abutting ends of the ring 76. One end of the ring 76 is pinned using a hardened steel pin 79 which is pressed into the piston body and engages in a slot 80 in the piston ring. Such an arrangement does not provide the benefit of the helical seal ring in combination with the helical seat ring but does provide the benefit obtainable from rotation of the piston as it reciprocates.

During manufacture of 'mated' seal and seats as described normal

assembly would include a 'lapping ' stage made to simulate operating conditions so that a gas tight combination of pairs of seat and seal is obtained.

Referring to Fig. 25, this illustrates a simplified embodiment of the invention using only the benefits of a larger than conventional bearing surface and a pivot axis placed high within the piston but within the portion of the piston defining seal ring lands 81. A unitary connecting rod 29 is formed with a generally cylindrical cross-member 82. Two piston halves 82 and 84 which are substantially identical are formed in aluminium alloy with ceramic bushes 85 inserted one in each side into blind holes 86. When assembled, a gap is left between the two bushes 85 to allow cooling oil from the top of the rod 82 to be sprayed into cavities 87. Guide members in the form of fins 88 have radially outer edges which define bearing surfaces that contact the wall of a cylinder in which the piston is finally inserted. The member 88 carry only small side loads. More substantial members 89 also define bearing surfaces extending to enlarged ends 90 which carry the greater proportion of the side thrust on the piston. The surfaces 90 are given a low friction coating as is the part of the ring land below slot 20.

On final assembly, the joint faces of piston halves 83 and 84 are turned down to the desired diameter and the ring slots machined prior to applying a low friction coating.

It will be appreciated that a more conventional piston skirt than that illustrated in Fig. 25 could be provided to support side thrust on the pistons.