A piston and an associated manufacturing method

Abstract

A conrod (10 Fig. 1a), a gudgeon / wrist pin 30 and piston 40 together form a reciprocating assembly group of an internal combustion engine, and in use, oil under pressure is delivered from a crankpin v

Description

The invention relates to a piston for use in an internal combustion engine; and a method of manufacturing a piston. As is well known, internal combustion engines have undergone continuous development since their invention in the late 18th Century and their industrial application during the 19th - 21st Centuries. Internal combustion engines have been hugely successful products, and their use has become extremely widespread. In the UK alone there are estimated to be approximately 33.5 million existing passenger cars the vast majority of which are powered by internal combustion engines of either the spark ignition or compression ignition type, these terms being familiar to the person of skill in the art. Within the spark ignition and compression ignition types of internal combustion engine both 2-stroke and 4-stroke modes of operation are well known. Furthermore there exist in industrialised economies millions of light goods vehicles (LGVs), heavy goods vehicles (HGVs), motorcycles, scooters, ambulances, buses / coaches, other land vehicles (including trains, snowmobiles, ski-doos, go-karts, quad bikes and specialist vehicles such as agricultural, refuse collection, fire control, construction, mining and military vehicles) and water craft such as ships, boats, hovercraft and jet skis the majority of which are powered by internal combustion engines. Internal combustion engines also have been used in many aircraft, especially light aircraft manufactured up to approximately the early 1990s; and some helicopters, with opposed and radial cylinder designs being known in such craft. Although their use in vehicles represents the most prevalent application of internal combustion engine technology, these engine types also are used in numerous machines that do not have a primary transportation function and instead may be stationary or have only a limited range of travel. Internal combustion engines hence are widely used in pumps, auxiliary power units and electricity generators, as well as in domestic or estate machines such as lawn mowers, garden / estate tractors, leaf blowers, brush cutters, pressure washers, log splitters and chainsaws. Further exemplary applications of internal combustion engines are in fixed or largely immobile machines such as dredging platforms, power shovels and draglines. Internal combustion engines have been the subject of extensive academic and practical research and development over approximately two centuries. Despite this research effort and the widespread engines have remained unsolved. In some cases these drawbacks have become more acute as the power output and efficiency demands imposed on engines have increased over time. One such drawback relates to oscillation, overheating and wear of the skirts of the pistons that are essential components of internal combustion engines. An internal combustion engine piston is a cylindrical metal component that in use reciprocates longitudinally inside a cylinder bore formed in an engine block or functionally similar part of an engine. The piston and cylinder bore are carefully engineered so the piston is a sliding fit within the cylinder bore while providing an essentially sealed space, often referred to as a "combustion chamber", at one essentially closed end of the cylinder bore. Sealing of the combustion chamber is effected by typically two or more compression rings, of larger diameter than the piston itself, set into grooves in the outer surface of the piston in a manner that is well known to those of skill in the art. An oil control ring, the nature of which also is well-known, is set into a further groove in the piston outer surface for the purpose of scraping away a relatively small amount of oil that accumulates on the part of the cylinder adjacent the section of the piston, and called a "skirt", that lies on the opposite side thereof to the combustion chamber. The reciprocating movement of the piston is caused by cyclical detonation of charges of a volatile fuel and air mixture in the combustion chamber adjacent one end of the piston. The force of such detonation, and in particular the resulting rapid expansion of combustion gases, acts primarily on a part of the piston called the crown to drive the piston along the cylinder bore away from the point of detonation. The diameter of the piston in the vicinity of the crown typically is slightly less than that of the skirt. As mentioned the compression rings create the necessary seal that is required to ensure the energy of detonation of the fuel / air mixture acts on the piston. The skirt, being of slightly larger diameter than the crown, is the part of the piston intended to be a sliding fit in the cylinder as the reciprocal piston movement occurs. As explained below direct metal-to-metal contact of the skirt on the cylinder wall, as arises in prior art engines, is problematic in various ways. Following the generation of impulse energy from the detonation at least one (and in most engines, more than one) valve opens in the cylinder bore to allow the scavenging of waste gases from the cylinder and re-charging of the cylinder with more of the air / fuel mixture of a desired composition. The movement of the piston is converted into rotary motion through the combined operation of a wrist pin (sometimes called a gudgeon pin in the UK) retained in usually a pair of laterally spaced wrist pin bosses defining laterally spaced journals adjacent the crown of the piston; and an elongate, rigid connecting element referred to as a conrod that is pivotably connected at one end to the wrist pin. The conrod is pivotably connected at its opposite end, often called the crank pin end, to a crankshaft. As a result of the pivoting connections and the rigidity of the conrod the longitudinal reciprocal motion of the piston becomes typically an irregular locus in the conrod, and rotating motion in the crankshaft (which is supported at least at its ends in rotary bearings). The shape of the crankshaft on rotation of it causes the piston following the detonation to reciprocate, under a return force transmitted by the conrod, back towards the detonation point in the cylinder. This effect in many engines is enhanced by the action of a flywheel that is connected to the crankshaft and increases its rotational inertia. When the piston reaches the detonation point again a further detonation of a fresh charge of fuel / air mixture occurs and the piston is driven back along the cylinder. As is well known, in a 2-stroke engine the detonation occurs each time the piston returns to the closed end of the combustion chamber. In a 4-stroke engine the detonation occurs every alternate time the piston reaches the detonation position. The detonation may, depending on the engine type, result from the timed generation of a spark (spark-ignition engine) or from heating of air (caused by compression in turn deriving from the piston movement) that is timed to coincide with the injection of charges of fuel into the combustion chamber (compression-ignition engine). The piston, conrod and crankshaft movement cycle repeats, in most examples several times per second, for as long as desired, unless the supply of combustible fuel to the cylinder becomes consumed or steps are taken to switch off the engine. Internal combustion engines may include a single piston, wrist pin and conrod assembly; or multiple such sets of parts that are spaced apart from one another inside the engine. Adaptations to accommodate multiple pistons are well known. In multi-cylinder engines the design of the crankshaft causes the motions of plural pistons in respective cylinders to be timed. Control apparatus forming part of the engine ensures the detonations adjacent the pistons are also appropriately timed. The overall effect is of smooth sequencing of the detonations and piston movements, so that the power output of the engine can be optimised; and harmonic oscillations, noise and other forms of energy wastage and component wear are minimised. The crankshaft usually, in so-called "in-line" and V-formation designs, extends essentially horizontally, typically in or near a lowermost part of the hollow interior of the engine. In e.g. opposed cylinder engines (also called "flat" or "boxer" engines) the crankshaft is approximately central in the engine. Regardless of its location the crankshaft or a component connected to it protrudes from a crankcase that houses the crankshaft. As a result of such protrusion the rotary motion of the crankshaft is available to be applied as a rotary output of the engine and (in many instances) to be fed via one or more clutches and gear sets to power e.g. rotatable ground-engaging wheels or caterpillar tracks of a vehicle, or other output elements in other machine types, in a controlled manner. The smallest internal combustion engines such as those used in some types of garden machinery may generate power outputs of less than 1 kW, whereas the most powerful (as used in various forms of racing cars, powerboats, heavy goods vehicles and construction, agricultural and mining machines) might generate outputs of several hundred kW or, in certain instances, more than 1 MW. Some ships moreover are powered by very large internal combustion engines that generate outputs of several tens of MW. All internal combustion engines however relative to their sizes generate large amounts of heat, as a result of detonation and combustion of the fuel; because of friction of their various moving parts; and, in the case of a compression ignition engine, intentionally through rapid compression of air in order to cause the desired fuel detonations. The heat generated by internal combustion engines, aside from that caused intentionally in compression ignition engines, for the most part is undesirable. A relatively small amount of the generated heat may be recycled in a vehicle for comfort heating and windshield mist clearing purposes. The remainder is potentially deleterious. This is partly because the efficiency of an engine may reduce if it is not operated at an optimal temperature related to the materials from which the engine is made and the composition of the fuel / air mixture. Moreover if overheated the metal parts from which an engine is made can distort or undergo unwanted thermal treatments. In severe cases these effects can reduce efficiency or cause permanent damage to the engine. Such damage can manifest itself as catastrophic engine failure. This is strongly undesirable for various cost / economics and inconvenience reasons; and in some situations is highly hazardous to humans. In view of these factors it normally is desirable to cool an internal combustion engine. This is partly achieved in many designs by causing a cooling medium such as water or air to pass over and through various parts of the engine and conduct heat away. Lubricants that are present inside the engine in order to assure smooth, efficient sliding of the various moveable parts that contact each other in the engine also are formulated to have chosen thermal characteristics. As a result an engine lubricant has an important additional function in conducting unwanted heat away from various components. In many engines such a lubricant in liquid form is pumped around a flow circuit by a rotary pump. The pump is powered by drawing rotary energy directly or indirectly from e.g. a camshaft that operates the valves mentioned above. The pump includes a pickup tube that is open at one end and submerged in a sump of oil (or another lubricant) that in many engines lies beneath the crankshaft. The location of the sump and the structure of the crankshaft mean it is relatively straightforward to design the flow circuit so that the bearings of the conrod crank pin end are lubricated by pumped oil, causing the crank pins forming part of the crankshaft to be efficiently lubricated, cooled and supported in a manner referred to as "floating". This term implies that the crank pins are in use supported on and surrounded by pressurised lubricant that is interposed as a result of the lubricant pumping between the crank pins and bearing inserts that form part of the pivoting connection of the conrod crank pin end to the crankshaft. The effect of this is to support the crank pins evenly around at least parts of their circumferences notwithstanding the variety of forces they experience during the engine operation described above. The ability of the lubricant to support the crank pins in turn results from the essentially inelastic nature of compressed liquids such as lubricants. On the other hand the location and design of the piston and attached components mean it is in practice impossible in existing engine designs to pump liquid lubricant to the skirt of the piston. Instead a limited amount of lubricant is washed onto the skirt using a fling or squirting technique and (in small amounts) from the oil control ring, which includes recesses that distribute a thin layer of oil scraped off the cylinder interior as the piston reciprocates. This limited lubrication of the piston skirt is a disadvantage considering the requirement as explained for the piston to slide repeatedly back and forth along the cylinder bore with the skirt intended to be in contact with or very close to the cylinder wall. In particular the oil fling / squirting technique does not permit floating lubrication of a piston skirt because it does not give rise to a pressurised supply to the skirt surface. The lubricant distributed by the oil control ring is very small in volume and also unpressurised. Its effect on the piston skirt therefore is minimal. The compromised nature of piston skirt lubrication in existing engine designs means that the piston, especially if it is worn such that the skirt is not a snug sliding fit in the cylinder, may oscillate while reciprocating e.g. from side to side or according to a complex harmonic pattern. This causes unwanted contact with the cylinder wall and accelerates wear of the skirt. Wear of the skirt in turn worsens the oscillation problem. Moreover the ability of flung or squirted lubricant to convey heat away from the skirt of a piston is limited. Manufacturers of pistons sometimes coat the surfaces of the piston skirts with thermally or otherwise bonded layers intended to counteract wear of the skirts that arises from oscillations of the pistons in the cylinder bores. Investigations by the Applicant however have revealed that such surface coatings often last only a very short time in practice. This is a consequence of the sliding contact between the piston skirts and the cylinder walls that arises in prior art engines. As a result of this the coatings can become worn away after only a few minutes of use in for example a high-performance (e.g. race car) or high-torque (e.g. construction or mining machine) engine. When this happens the coatings break up with the result that the skirt diameter reduces and e.g. side-to-side oscillation of the pistons becomes more likely. This in turn means that increased contact of the metal of the skirt with the cylinder wall occurs, and this can be highly deleterious. Moreover particles of the worn-away coatings become contaminants in the lubricant. The coatings may be expensive to apply and therefore are rarely used in low-cost engines. Furthermore the use of the coatings does nothing to address the need to convey heat away from the piston skirts. One area in which the inadequate lubrication of piston skirts is economically highly damaging is in the mining industry. Frequently in this industry very large machines such as draglines, power shovels, rotary shovels, bucket-wheel excavators and haul trucks can include internal combustion engines having large-diameter cylinders and pistons. These are highly prone to unwanted oscillations which cause premature wear of the piston skirts. Such wear means that engine rebuilds often are required several times a year in such machines. Aside from the cost this incurs in terms of lost working time, typically the pistons and compression rings are discarded as part of a rebuild. Consequently the engine rebuilds are somewhat wasteful despite the fact that large and / or high-performance pistons are expensive, may be made from highly alloyed materials and often are in short supply. A further engine example in which piston wear causes a need for frequent engine rebuilds is a race engine as may be used in a race car, racing motorcycle or powerboat. In these engines very high speeds of reciprocation cause relatively rapid wear of the pistons. Even internal combustion engines which reciprocate relatively slowly, as used in some ships, require major overhauls for example every two or three years (with expected wear of pistons and associated rings being the primary determinant of overhaul schedules). Taking a large ship out of service for the purpose of rebuilding its engine may be extremely expensive. It is an aim of the invention to address or at least ameliorate one or more drawbacks of the prior art. Disclosed but not specifically claimed herein is a wrist pin, for an internal combustion engine, that is a rigid, elongate cylinder having formed therein at least one first wrist pin oil feed passage extending from a first wrist pin opening, on the outer surface of the cylinder preferably but not necessarily approximately half-way along its length, to a terminus adjacent the diametral centre of the wrist pin; and at least one second wrist pin oil feed passage extending from the terminus of the first wrist pin oil feed passage towards a further wrist pin opening on the outer surface of the cylinder that is spaced along the length of the cylinder from the first wrist pin opening, the first and second wrist pin oil feed passages being fluidically connected one to the other at or near the terminus in order to permit flow of fluid from the first wrist pin opening along the first wrist pin oil feed passage to the terminus and thence along the second wrist pin oil feed passage to the further opening. Such a wrist pin advantageously allows a pressurised feed of a liquid lubricant to flow from e.g. a longitudinally central region of the wrist pin towards an end thereof. This in turn allows the pressurised feeding of lubricant from the longitudinal mid-point of a wrist pin to the vicinity of a wrist pin boss of an attached piston. Consequently a wrist pin according to this aspect of the disclosure when supplied with pressurised lubricant via a connected conrod makes possible the floating support of the wrist pin relative to an attached piston. This significantly improves the efficiency, cooling and service life of a wrist pin compared with one that is lubricated by prior art techniques. According to the invention there is provided a piston, for use in a cylinder of an internal combustion engine, comprising a cylindrical body defining a piston crown and at least a first skirt part that in use depends from the piston crown, the skirt part(s) defining a hollow space and supporting in the space at least one wrist pin journal for receiving therein a wrist pin, at least a first said skirt part including formed therein a piston oil feed passage for feeding pressurised oil from the hollow space to the first skirt part, the hollow space including an inner opening of the piston oil feed passage and the piston oil feed passage terminating in an outer opening at or near the outer surface of the first skirt part. In many practical embodiments preferably a mutually spaced pair of wrist pin journals would be present, but embodiments are also possible in which for example a single wrist pin journal is provided. Advantageously a piston according to the invention is capable of supplying a pressurised feed of a lubricant such as an oil from the interior of the piston to the outer surface of the skirt. This in turn for the first time permits floating support of the skirt, and hence of the piston as a whole, in a manner that (a) damps unwanted oscillations of the piston; (b) lubricates the sliding movement of the piston; and (c) conducts heat away from the piston as the reciprocal movement of the piston wipes lubricant off the skirt. The floating supporting of the skirt in eliminating oscillations of the piston renders more efficient the piston movement and also significantly reduces wear with the result that the vibration-free life of the piston is prolonged. Further it no longer is necessary to apply coatings to the piston skirt, thereby reducing costs and avoiding the problems caused by wearing off of the coatings. A piston as defined herein is of particular benefit in designs of engine in which hitherto the piston wear caused by contact with the cylinder wall has created a need for regular engine rebuilds, as explained herein. As mentioned it often is the norm when carrying out a rebuild of such an engine to discard the worn pistons. This is a significant consumption of engineering components that can be dramatically reduced, or even eliminated altogether, through use of the piston as defined herein. Disclosed herein is a conrod, for an internal combustion engine, comprising a crank pin end, a small end and a longitudinally extending rod rigidly interconnecting the crank pin end and the small end, the rod being perforated internally along its length by a conrod oil feed passage that includes crank pin end and small end openings respectively at the crank pin end and the small end, whereby oil fed under pressure into the conrod oil feed passage at the crank pin end conveys along the conrod oil feed passage to emerge therefrom under pressure at the small end. An advantage of such a conrod is that when it is used in conjunction with a pressurised feed of a lubricant to a crank pin bearing assembly it allows the supply of a pressurised feed of lubricant along the length of the conrod to the gallery at the small end. The disclosed wrist pin and the piston according to the invention may be assembled in combination with one another and with a conrod as described herein in order to define a conrod, wrist pin and piston assembly. In particular in such an assembly the various openings forming part of the lubricant feed passages may be arranged to be in register with one another or with one or more reservoirs of pressurised lubricant at least at times during reciprocation of the various parts. This means a repeatedly replenished feed, or more preferably a continuous feed, of pressurised lubricant may be supplied to the outer surface of the skirt, in turn ensuring that floating lubrication, and / or support and / or vibration damping and / or cooling effects arise in an essentially constant manner. Clearly in view of this the wrist pin and the piston defined herein may be regarded as sub-components of the assembly comprising the conrod, wrist pin and piston as explained; or the piston may be considered as individually inventive since it offers advantages even when not forming part of an assembly as aforesaid. In an assembly as described herein the wrist pin may be fixed relative to the conrod and pivotable only relative to the piston; or pivotable relative to both the conrod and the piston. Also disclosed but not specifically claimed here is an operational method. Advantages of such a method are as specified above in respect of the conrod, wrist pin, piston and assembly. In particular the method disclosed herein is widely applicable in a variety of internal combustion engine designs, and solves the long-standing problems of piston skirt lubrication, cooling and stabilisation / vibration damping. The method of the invention preferably is put into effect using apparatus as defined herein. According to a further aspect of the invention there is provided a manufacturing method as defined in Claim 6 hereof. Such a method advantageously may be practised in respect of a newly manufactured piston or as a modification of an existing piston, thereby allowing the invention to be applied as a retro-fit option in an existing engine. Optional features of the various aspects of the invention are defined in the dependent claims hereof. One optional feature of particular importance is a recessed oil distribution gallery formed in the outer surface of the skirt and extending about a part of the periphery of the skirt, wherein the outer opening opens into the recessed gallery. These features ensure that an adequate supply of lubricant is constantly provided to the piston skirt. For the avoidance of doubt the terms "oil" and "lubricant" are used essentially synonymously herein to refer to a flowable, preferably but not necessarily liquid, medium that is capable of performing The various aspects of the invention, and embodiments as described herein, may be useable in virtually all types of internal combustion engine, including but not limited to those specifically mentioned herein. There now follows a description of preferred embodiments of the invention, by way of non-limiting example, with reference being made to the accompanying drawings in which: Figure la is a schematic, isometric view of a conrod according to the disclosure hereof; Figure lb is an enlarged, isometric view of the region A of Figure la; Figures 2a and 2b are isometric views similar to Figures la and lb, showing various normally hidden features in dotted lines and with Figure 2b showing in enlargement the region B of Figure 2a; Figure 3a and 3b respectively are an isometric external plan view and a side elevational view, showing normally hidden features in dotted lines, of an embodiment of a wrist pin according to the disclosure hereof; Figures 4a, 4b and 4c are respectively a solid isometric view from underneath, a similar isometric view showing normally hidden features in dotted lines and a side elevational view of an embodiment of a piston according to the disclosure hereof; Figure 5 is an isometric view of a perforated bearing insert that may be used in conjunction with the other components described herein; and Figure 6 is an isometric view of a conrod, wrist pin and piston assembly in accordance with the disclosure hereof. Referring initially to Figures la, lb, 2a and 2b there is shown a conrod 10 for an internal combustion engine. The engine in which such a conrod 10 may be used is not particularly limited. The engine type and intended use may be any of those mentioned herein, with the invention being especially suitable for (among others) high-output engines such as those used in performance and racing cars and motorcycles; and large engines such as those used in mining machinery and ships. In such engines a pressurised lubricant feed as results from the invention is strongly beneficial. Conrod 10 is shown extending vertically upwardly, which (subject to orientation changes caused by its motion in use) is the orientation it generally adopts when it is used in an in-line engine. As is familiar to the person of skill in the art and as implied above however different orientations are possible. Hence when included in for example an opposed cylinder engine the conrod extends generally horizontally; when included in a V-configuration engine it may extend at an angle of approximately 30° - 45° (or another angle, depending on the engine design) to the vertical; and when included in a radial engine it may adopt any of a number of orientations again as determined by the engine design. In the remainder of this description references to directions and dimensions being "vertical" are to the longitudinal orientation visible in Figures 1 and 2, with derivative terms (including prepositions such as "above" and "below") being construed accordingly. As explained however the illustrated orientation of the illustrated components is not limiting of the invention. Furthermore unless the context requires otherwise references herein to singular components are to be taken as potentially embracing plural examples as would be envisaged by the person of skill in the art. Hence for example the illustration of a single conrod, wrist pin and piston does not preclude application of the invention as described herein in an engine having more than one example of such components. As is conventional conrod 10 includes a crank pin end 11, a small end 12 and a rigid, longitudinally extending rod 13 interconnecting the crank pin end 11 and the small end 12. The conrod 10 is made from a metal alloy and in many, but not all, cases would be formed through a combination of forging and machining, again as is conventional. Other ways of making the conrod 10 are however known and are within the scope of this disclosure. In common with most conrod designs, the rod 13 tapers in the frontally lateral (left-to-right or right-to-left) sense visible in Figures 1 and 2 from a relatively great width adjacent the crank pin end 11 to a relatively narrow width adjacent the small end 12. The rod 13 also is of reduced dimensions in the central part of a thickness direction orthogonal to the direction of the taper, as illustrated, in order to minimise the amount of metal required in its manufacture. The region of reduced thickness is signified in Figure la by numeral 16, and causes the conrod 10 over a major part of its length to be of I-beam cross-section. H-beam, and other, conrod crosssections also are familiar to the person of skill in the art, as are conrods having other degrees of taper or no taper at all. Yoke-ended conrods also are known. The illustrated conrod shape is not limiting of the disclosure hereof, which extends to all variants as mentioned and / or as occur to the person of skill in the art. As is best shown in Figures 2a and 2b, the rod 13 is in embodiments internally perforated along its length by a conrod oil feed passage 14. Conrod oil feed passage 14 includes at its lowermost end a crank pin end opening 17. This opens into the annulus 18 that in a conventional conrod forms part of the crank pin end and rotatably surrounds a crank pin of a crankshaft. In a typical vehicle engine conrod the crank pin end is formed as two parts being a forged big end 19, that is integral with the rod 13, and a rod cap 21, the big end 19 and rod cap 21 each defining a semicircular part of the annulus 18. The rod cap is in the illustrated embodiment secured to the big end using bolts that are omitted from the figures for clarity and which are familiar to the person of skill in the art. The bolts tightly secure the rod cap 21 onto the big end 19 such that the mating line between them is rendered invisible in the views making up Figures la and 2a. In some other engine types, to which the disclosure pertains, the whole conrod (including the crank pin end) is formed as a single piece. In such engines the crankshaft instead is made up from multiple subcomponents that are secured together e.g. using bolts once the crank pins have been inserted into the eye defined at the conrod crank pin end. This method of constructing the crankshaft and conrod connection is commonplace in motorcycle engines, for example. Regardless of its exact design the aim of the construction is to allow the crank pin end 11 to be assembled in a manner presenting contiguous arcuate surfaces that define a circular annulus 18 that encircles the crank pin. Semi-circular bearing inserts 22, 23 (which in Figures 1 and 2 are not visible as separate components, but in practice are two half-shells) also typically are included encircled by the annulus 18 resulting from the shapes of the inner surfaces of the big end 19 and rod cap 21. A non-limiting example of the design of bearing insert 22 is described below with reference to Figure 5. At its upper end the conrod oil feed passage 14 opens as a small end opening 24 in the inner surface of an annular eye 26 defined by the small end 12. It will be apparent from the foregoing that pressurised lubricant such as oil in the vicinity of the crank pin end opening 17 may be caused to feed up the oil feed passage 14 to emerge via small end opening 24 as a pressurised flow of lubricant. As explained, in many internal combustion engines a source of pressurised lubricant exists in the vicinity of the crank pin in order to provide floating supporting of the crank pin relative to the crank pin end of the conrod. Hence as a result of the invention it is possible to supply pressurised lubricant to the top of the conrod, and in particular to moving parts inside the depending part of an attached piston adjacent the skirt(s). The conrod oil feed passage 14 in the illustrated embodiment is a straight, circular cross-section bore. Such a passage may readily be formed by machining, especially drilling and (as required) reaming the passage 14 when the big end 21 is not attached to the crank pin end 11. Consequently it is possible to form the conrod oil feed passage 14 as a modification to an existing conrod, and it is not essential to form the conrod oil feed passage 14 at the time of manufacturing the conrod. It need not be the case that the conrod oil feed passage 14 is uniform as described. On the contrary desired fluid flow effects may be achieved e.g. by varying the diameter and / or the cross-sectional shape of the conrod oil feed passage 14 along its length and / or by including a non-straight variant of the passage 14. The person of skill in the art will be versed in techniques for forming non-uniform variants of the conrod oil feed passage 14. Furthermore it is not essential that the conrod oil feed passage 14 internally perforates the conrod 10. In alternative embodiments it could be located e.g. on the surface of the conrod and extend along it in the manner of a pipe. Forming the conrod oil feed passage 14 internally within the rod 13 however may be preferable from the standpoint of ease of forming the passage, resistance of forces and maintenance of symmetry of the conrod 10. The eye 26 is intended to encircle a wrist pin. The small end opening 24 in embodiments opens in the eye at a location corresponding to the longitudinal mid-point of such a wrist pin when it is assembled together with the conrod 10. However in some other embodiments the wrist pin may not be positioned longitudinally symmetrically relative to the eye 26. Hence it is not essential in all embodiments that the small end opening lies centrally along an associated wrist pin. The inner surface 27 of the eye 26 includes formed therein as an arcuate recess a lubricant gallery 28. Small end opening 24 opens into the gallery 28. Gallery 28 may be formed by machining the metal of surface 27, or by a variety of other methods as will be known to the person of skill in the art. The gallery 28 may act as a reservoir of lubricant that in effect is pumped to it along the conrod oil feed passage 14 and out of the small end opening 24 as a result of the supply of pressurised lubricant in the vicinity of the crank pin end 11. In the illustrated embodiment the gallery 28 extends as a circle around the entire inner surface of the eye 26. However this need not be the case, and galleries that extend only part-way around the inner surface are possible. Moreover the gallery 28 does not need to adopt the constant cross-section shown, and it does not have to present a rectilinear cross-section as exemplified in Figures 2a and 2b. Hence the gallery 28 may be of numerous dimensions and shapes, and its dimensions and shape(s) may alter at various locations along its length. Furthermore the gallery need not be continuous and instead may be interrupted by e.g. lands that divide it into a series of sub-galleries for example. Referring now to Figures 3a and 3b there is shown an embodiment of a wrist pin 30 that may be used in combination with the conrod 10 of Figures la, lb, 2a and 2b. Figure 3a is an external, isometric view of the wrist pin 30 and Figure 3b is a side elevational view showing various internal features (that normally are obscured) by way of dotted lines. Wrist pin 30 as illustrated is a rigid cylindrical rod of (in virtually all embodiments) a metal or metal alloy. The wrist pin 30 is elongate in the left-right direction visible in Figures 3a and 3b. It may be formed for example by machining a metal bar. Numerous other ways of forming the wrist pin 30 are possible and will occur to the person of skill in the art. A constant circular cross-section bore 31 is formed centrally in the wrist pin 30, extending along its longitudinal axis from one end of the wrist pin 31 to the other. Bore 31 may be formed by drilling and, as desired, reaming the material of the wrist pin 30. This forming method however is not limiting, and a variety of other ways of creating the bore 31 are known and will occur to the person of skill in the art. The generally cylindrical shape of the wrist pin 30 and the central bore 31 are conventional in wrist pin designs. At its longitudinal mid-point the outer surface of the wrist pin 30 is formed with a first opening 32 that defines an open end of a first wrist pin oil feed passage 33. Over most of its length first wrist pin oil feed passage 33 is in the illustrated embodiment a straight, circular, radially extending, constant cross-section bore that may be formed in the material of the wrist pin 30 e.g. by drilling or by another viable technique as will occur to the person of skill in the art. First wrist pin oil feed passage 33 extends towards the transverse centre of the wrist pin 30 and terminates a short distance radially outwardly from the central bore 31. As a result first oil feed passage 33 does not communicate with central bore 31. The cross-section, dimensions and uniformity of first wrist pin oil feed passage 33 may vary in ways that will occur to the person of skill in the art. At its end nearest to the transverse centre of the wrist pin 30 the first wrist pin oil feed passage terminates in a connection passage 34. Connection passage 34 as illustrated in Figure 3b optionally may be of a different cross-section and / or dimensions than the major part of first wrist pin oil feed passage 33. Connection passage 34 acts as a terminus to the first wrist pin oil feed passage 33 and fluidically connects it to a second wrist pin oil feed passage 36. Second wrist pin oil feed passage 36 in the illustrated embodiment is a straight, circular, constant cross-section bore extending from connection passage (terminus) 34 to a further wrist pin opening 37 formed in the outer surface of the wrist pin 30 at a location spaced along the wrist pin from first wrist pin opening 32. The cross-section, dimensions and uniformity of second wrist pin oil feed passage 36 may vary in ways that will occur to the person of skill in the art. Second wrist pin oil feed passage 36 is inclined relative to first wrist pin oil feed passage 33 as indicated in order to extend as described. As a result of intersection of the second wrist pin oil feed passage 36 with the surface of the wrist pin 30 further wrist pin opening 37 is formed as an oval in the surface of the wrist pin 30. In the illustrated embodiment the second wrist pin oil passage is inclined at an angle of approximately 30° to the horizontal as represented in Figure 3b, but in other embodiments different angles of inclination are possible. Wrist pin 30 includes formed extending about its external periphery and to either side of its longitudinal mid-point an annular recess defining a first wrist pin lubricant gallery 38. The purpose of this is described below. It will be apparent that when filled with oil or another liquid lubricant the first wrist pin lubricant gallery 38 acts as a buffer store of oil and thereby a continuous, pressurised supply of oil during operation of an engine including the wrist pin 30. A further annular recess is defined extending about the external periphery of wrist pin 30 and acts in use as a further wrist pin lubricant gallery 39. This is similar to first wrist pin lubricant gallery 38, except that it is wider in the length direction of the wrist pin 30 in order to accommodate the greater length of the further wrist pin opening 37 in the lengthwise direction along the wrist pin 30. Further wrist pin lubricant gallery 39 also acts as a reservoir of liquid lubricant in use of the wrist pin. This effect is explained further below. The wrist pin galleries 38 and 39 may be formed by machining (e.g. lathe turning) the wrist pin outer surface. However other methods of forming these features also are possible and will occur to the person of skill in the art. In the illustrated embodiment the first and further wrist pin oil galleries 38 and 39 are formed to the same depth in the outer surface of the wrist pin 30, but this need not necessarily be the case. Moreover the galleries 38, 39 need not be of the rectilinear cross-sectional shapes shown; nor need they be uniform along their lengths either in terms of dimensions or cross-sections. The wrist pin galleries 38, 39 are illustrated as extending about the entire periphery of the wrist pin, and this form lends itself well to creation by machining techniques as indicated. However in embodiments one or both the wrist pin galleries 38, 39 may extend for only part of the peripheral distance, and / or they may include lands or other features that render them discontinuous. Additionally in some embodiments it may not be necessary to provide both the galleries 38 and 39. In such embodiments a single gallery 38 or 39 may be provided; and in other embodiments the galleries 38, 39 may be dispensed with entirely. Furthermore the wrist pin galleries 38, 39 need not be the only features formed in the surface of the wrist pin 30. Referring to Figures 4a, 4b and 4c there is shown a piston 40 in accordance with an embodiment hereof. Piston 40 is intended to be used in combination with the conrod 10 and wrist pin 30 described above, although it provides unexpected advantages even when considered independently of such components. Piston 40 is intended for use in a cylinder of an internal combustion engine. To the extent that features of the piston 40 needed to achieve this are not specifically described herein they will be apparent to the person of skill in the art. Piston 40 is formed from a heat-resistant metal alloy, and as is conventional may be manufactured through a billet cutting and machining process or through a combination of casting and machining or forging and machining operations. Piston 40 comprises an essentially cylindrical body 41 defining a piston crown 42 of essentially conventional design. In view of its conventional nature crown 42 is not described in detail herein. Crown 42 defines a cylindrical outer surface. In the illustrated embodiment this includes formed therein first and second (upper and lower in the illustrated orientation) mutually adjacent compression ring grooves 43, 44 for receiving first and second compression rings in a conventional manner. More or fewer compression ring grooves may be provided, depending on the design of the engine in which the piston 40 is installed. Adjacent the second compression ring groove 44 in a downward direction when the piston 40 is oriented as illustrated is an oil control ring groove 46. Oil control ring groove 46 also is formed in the outer surface of the piston crown 42 and is intended to receive an oil control ring in a conventional manner. Oil control ring groove 46 may in some embodiments be augmented by further such grooves, or may be absent from certain other embodiments. Piston 40 includes depending from the crown 42 respective first and second skirt parts 47, 48. In the illustrated embodiment each skirt part 47, 48 is a part-circular wall that depends downwardly from the crown 42. The first and second skirt parts 47, 48 are disposed on diametrically opposite sides of the crown 42, and depend downwardly from its outer edge. Other shapes and positions of skirt parts are possible within the scope of the disclosure. A single continuous skirt is not precluded, although it nowadays is relatively commonplace for pistons to have diametrically opposed partial skirts as is the preferred embodiment and as is illustrated in the figures. A hollow space 49 exists between the first and second skirt parts 47, 48. In the illustrated embodiment by reason of the shapes and positions of the skirt parts 47, 48 the space 49 is part-cylindrical as illustrated. The first and second skirt parts 47, 48 support within the space 49 a mutually spaced pair of wrist pin journals or bosses 51, 52. Each wrist pin journal 51, 52 includes a downwardly depending journal eye 51a, 52a (Figure 4a) in which are receivable the respective ends of a wrist pin such as wrist pin 30 described above. The wrist pin journals 51, 52 are in the illustrated embodiment cast integrally with the remainder of the piston 40 and are spaced apart in a direction that is orthogonal to the axis of spacing of the skirt parts 47, 48 from one another. On their radially inner edges the journals 51, 52 merge into respective first and second support webs 53, 54. These are spaced apart in the hollow space in the same direction as the mutual spacing of the journals 51, 52. The webs 53, 54 in the illustrated embodiment also are cast integrally with the remainder of the piston 40. In other embodiments however this need not be the case, and the piston may be assembled from discrete parts that can be secured one to another e.g. by welding. Piston 40 includes first and second, straight, constant circular cross-section piston oil feed passages 56, 57. These extend in opposite directions from inner openings 61, 62 (Figure 4b) on diametrically opposite sides of the second wrist pin journal 52 outwardly within second support web 54. The first and second piston oil feed passages 56, 57 each terminate in an outer opening 58, 59 at the outer surface of a respective said first or second skirt part 47, 48. Since as shown in Figures 4a, 4b and 4c the support web 54 extends in a plane that is offset from the lateral mid-point of each skirt part 47, 48, the outer openings 58, 59 are similarly laterally offset. The arrangement of the piston oil feed passages however may differ from that shown. More or fewer than the two passages 56, 57 may be included. They may pass through parts of the piston 40 other than the second support web 54, they may be non-straight and they may have non-constant and / or non-circular cross-sections in order to achieve desired flow and / or cooling effects. One particular option within the scope of this disclosure is an arrangement of the passages that provides a pressurised lubricant feed to only one side of a piston. In this regard in many engine designs one side of the piston may experience the major part of the thrust generated by the action of the crankshaft and the force of fuel / air detonation. In particular when the piston includes distinct skirt parts on opposite sides of the piston as illustrated it may be desired to provide a pressurised lubricant feed, using the principles as disclosed herein, to only one of the skirt parts. Such arrangements could dispense with the passages and galleries feeding pressurised lubricant to the other skirt part. It would readily occur to the person of skill in the art how to achieve this benefit. Other forms of asymmetry may be included in the piston design, and such options are within the scope of this disclosure. Notwithstanding the possibility of creating these variants, the illustrated passages 56, 57 are desirable since they readily may be formed by drilling a piston in a direction along the interior of a support web such as second support web 54. Consequently they readily may be provided as modifications of existing pistons which therefore may be upgraded in accordance with the disclosure hereof. Each of the outer openings 58, 59 in the illustrated embodiment opens into a respective recessed lubricant distribution gallery 63, 64 that extends along a length of the surface of a respective skirt part 47, 48. Each lubricant distribution gallery 63, 64 acts in use of the piston as described below as a reservoir of pressurised lubricant. The lubricant distribution galleries 63, 64 readily may be formed e.g. by machining (e.g. milling) the surface of the respective associated skirt part 47, 48, or may be formed in other ways such as casting. Machining of the lubricant distribution galleries has the advantage of permitting them to be formed in existing pistons as upgrades in accordance with the disclosure hereof. Figure 5 is an isometric view of one form of bearing insert such as insert 22 mentioned above. In common with known conrod bearing inserts the inserts 22, 23 indicated in Figure 2a are arcuate, metal semi-circles that are fitted in pairs inside the circular interior of the crank pin end 11 of the conrod 10. The inserts 22, 23 serve as replaceable wear surfaces on the inside of the annulus 18. This means that the big end 19 and rod cap 21 of the conrod 10 are protected from being worn away by contact with the crank pin. The bearing inserts 22, 23 are made from a material (typically a metal alloy) which is chosen to have particular wear characteristics. Bearing insert 22 differs from a plain bearing insert, and hence from insert 23 (which may be of a conventional design). In this regard insert 22 has a central, through-going aperture 66 formed at the mid-point of the arcuate length of the insert 22; and includes a longitudinally extending groove 67 formed as a recess in the in-use inner surface of the insert 22. Both the aperture 66 and the groove 67 are laterally central in the illustrated insert 22. This need not necessarily be the case, but in the majority of designs of the insert 22 it is expected that the aperture 66 and groove 67 are centralised in the side-to-side sense. As is visible in Figure 5 the positioning of the aperture 66 and groove 67 means that the aperture 66 perforates the longitudinal mid-point of the groove 67. When the inserts 22, 23 are installed in the crank pin end 11 of the conrod 10 the aperture 66 is aligned with the opening 17 of conrod oil feed passage 14. Retaining features may as desired be included as parts of the bearing insert 22 and / or the bearing insert 23 and / or the big end 19 and / or the rod cap 21 in order to ensure and maintain correct positioning of the aperture 66 as described. As mentioned the components thus far described may be provided individually and bring about technical advantages when so considered. However their utility is greatly increased when they are assembled together as a conrod, wrist pin and piston assembly in which the dimensions and locations of various features are chosen so as to create a fluid flow path for pressurised lubricant extending from the big end of the conrod to the lubricant distribution galleries 63, 64 of the piston. Such a fluid flow path permits the supplying of lubricant under pressure to the skirt parts 47, 48 of the piston 40 in a manner providing for floating support of the skirt parts and hence of the piston 40 as a whole. This improves vibration attenuation, and / or lubrication and / or cooling effects significantly compared to prior art conrod, wrist pin and piston assemblies. An assembly of the conrod 10, wrist pin 30 and piston 40 as described herein is shown in Figure 6. Figure 6 shows firstly a conrod 10 such as that described herein having its crank pin end 11 as constituted by big end 19 and rod cap 21 fitted together encircling and being rotatably captive relative to the crank pin of a crankshaft. The crank pin is for clarity omitted from the figures but may readily be envisaged. Bearing inserts 22, 23 are interposed between the parts of the conrod 10 and the crank pin, in an essentially conventional manner. The aperture 66 formed in bearing insert 22 is visible in Figure 6. This aperture as explained communicates via opening 17 with conrod oil feed passage 14. Groove 67 of bearing insert 22 is not visible in Figure 6 and as noted is an optional feature. It readily may be envisaged based on Figure 5. As previously explained, it is readily possible to include a feed of pressurised lubricant in a crankshaft and this exits a passage formed inside the crankshaft via a crank pin opening formed in the surface of the crank pin. This gives rise to floating supporting of the crank pin relative to the conrod 10 during reciprocating movement of the crankshaft as is conventional during operation of an engine. The arrangement of the aperture 66, groove 67 (when present), aperture 17 and conrod oil feed passage 14 at the same time gives rise to a feed of pressurised lubricant up the interior of the conrod. This pressurised lubricant emerges from the conrod at the small end opening, whence it passes into the gallery 28 located in the eye 26 defining the small end 12 of the conrod 10. As implied the pumping of lubricant preferably is continuous, and the small end gallery 28 helps to achieve this such that an essentially continuous, pressurised feed of lubricant emerges at the small end 12 of the conrod 10 even if the feed of lubricant along the passage 17 is for some reason not uniform. The eye 26 encircles and is rotatably captive relative to a wrist pin 30. Wrist pin 30 preferably is as described and illustrated herein but if it is not nonetheless a benefit of the design of the conrod arises. This is that even in the absence of the various wrist pin features the conrod is able to provide a pressurised lubricant feed to the wrist pin. This permits efficient floating support of the wrist pin in a manner ensuring good lubrication and cooling. The eye 26, as is expected during operation of the engine in which the conrod, wrist pin and piston assembly is installed, rotatingly reciprocates relative to the wrist pin, which may be of the fixed (relative to the conrod 10) or fully pivotable wrist pin types. During such relative movement of the eye 26 and wrist pin 30 the first wrist pin opening 32 travels back and forth along the gallery 28 and remains open to the gallery 28. Consequently liquid lubricant under pressure becomes continuously pumped into the first wrist fin lubricant (oil) feed passage 33 and, via the connection passage 34, from there into the second wrist pin lubricant (oil) feed passage 36. Thereafter the pressurised lubricant emerges from the wrist pin via second wrist pin opening 37 and fills further wrist pin lubricant gallery 39 under pressure. The inner openings 61, 62 of the piston oil feed passages 56, 57 are in fluid communication with the further wrist pin lubricant gallery 39. As a result pressurised lubricant flows along the piston oil feed passages 56, 57 to emerge via the outer openings 58, 59. This flow of lubricant fills the lubricant distribution galleries 63, 64 (of which one, 63, is visible in Figure 6) and in turn provides for floating support of the skirt parts 47, 48 of the piston 40 relative to the cylinder in which it reciprocates. The provision of floating support in this manner reduces vibrations of the piston in the cylinder, thereby improving engine efficiency and prolonging piston skirt life. Furthermore the longitudinal reciprocal motion of the piston 10 repeatedly wipes lubricant from the lubricant distribution galleries 63, 64 onto the outer surfaces of the skirt parts 47, 48. This lubricates the skirt parts 47, 48 more effectively than the prior art oil fling technique, and promotes effective cooling of the skirt parts since volumes of lubricant repeatedly are wiped off the bottoms of the skirt parts 47, 48 as the piston 10 moves. The oil that is wiped off the skirt parts 47, 48 in this way is continuously replenished by the flow of lubricant via the piston oil feed passages 56, 57 into the lubricant distribution galleries 63, 64. The wiped off lubricant may trickle under gravity into the oil sump forming part of the engine and thereafter may be recirculated by the oil pickup and pump that also form part of the engine. As noted although a single conrod, wrist pin and piston assembly is illustrated, the disclosure hereof extends to a wide variety of multi-cylinder versions. It will be apparent to the person of skill in the art how to modify the components shown in the drawings so that they can form part of a multicylinder installation. Furthermore the illustrated embodiments include a relatively simple kinematic chain comprising as main components the crankshaft, conrod, wrist pin and piston. However in some engine designs, especially those in which the motion of the crankshaft must be transmitted over a considerable length (as may be a requirement in some stationary engines and in some ships) the kinematic chain may include additional parts such as a crosshead pin and crosshead bearing that are constrained to move longitudinally in a crosshead guide arrangement. The principles of the disclosure hereof readily may be applied to such an arrangement, such that pressurised lubricant is fed from a crankpin feed via a passage formed in an intermediate rod so as to emerge under pressure via one or openings and, as desired, one or more galleries so as to provide floating support of the crosshead pin in the crosshead bearing. From the crosshead bearing the pressurised lubricant may be conveyed via a passage formed in a piston rod to the interior of a piston. The thus-fed lubricant may flow under pressure from the interior of the piston to its skirt(s) via passages and optional galleries that are similar to features 56, 58. 57 and 59 visible in Figure 4b hereof in order to provide a pressurised lubricant feed to the exterior of the piston in its bore. Consequently the principles of the invention may readily be extended to multi-component connections between the crankshaft and the piston while continuing to provide the benefits of floating support of the pistons and, as desired, the crosshead pin. The method of operation of the assembly of Figure 6 as described above as well as the components forming part of the assembly and the assembly itself are associated with significant advantages, as explained. Moreover each of the components of the assembly may be manufactured either as new parts or by modifying existing engine parts. The disclosure hereof extends to methods of manufacturing the described components. In the preferred but non-limiting embodiments described the manufacturing steps may conveniently be straightforward ones such as drilling, milling and lathe turning of metal parts as will be apparent to the person of skill in the art. However as also explained a wide variety of variations on both the designs of the parts and the methods by which they are made is possible within the scope of the disclosure. Although the various galleries 28, 38, 39, 63 and 64 are illustrated as being circular arcs that extend circumferentially, this need not be the case. Different paths of the galleries across the surfaces in which they are formed may be adopted, for example in order to achieve particular flow effects in the lubricant. As a non-limiting example in this regard one or more of the galleries may be formed to resemble a sinusoid. Moreover it is not essential that the galleries are uniform in cross-section, or that their cross-sections adopt regular polygonal shapes as illustrated. Overall the galleries may adopt a wide variety of forms as will occur to the person of skill in the art; and all such variants are included within the disclosure hereof. Preferences and options for a given aspect, feature or parameter of the invention should, unless the context indicates otherwise, be regarded as having been disclosed in combination with any and all preferences and options for all other aspects, features and parameters of the invention. The listing or discussion of an apparently prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge.

Claims

1. A piston, for use in a cylinder of an internal combustion engine, comprising a cylindrical body defining a piston crown and at least a first skirt part that in use depends from the piston crown, the skirt part(s) defining a hollow space and supporting in the space at least one wrist pin journal for receiving therein a wrist pin, the piston including formed therein at least a first piston oil feed passage for feeding pressurised oil from the hollow space to the first skirt part, the hollow space including an inner opening of the piston oil feed passage and the piston oil feed passage terminating in an outer opening at or near the outer surface of the first skirt part.

2. A piston according to Claim 1 including supported in the space a mutually spaced pair of wrist pin journals.

3. A piston according to Claim 1 or Claim 2 including a recessed lubricant distribution gallery formed in the outer surface of the first skirt part and extending about a part of the periphery of the first skirt part, wherein the outer opening opens into the recessed gallery.

4. A piston according to any of Claims 1 to 3 additionally including a second skirt part depending from the piston crown on an opposite side thereof to the first skirt part, the piston including first and second piston oil feed passages respectively extending on opposite sides of a transverse centre line of the piston, the piston further including a corresponding pair of inner openings in the hollow space in communication with a said piston oil feed passage; the first piston oil feed passage terminating in a first outer opening at or near the outer surface of the first skirt part; and the second piston oil feed passage terminating in a second outer opening at or near the outer surface of the second skirt part.

5. A piston according to Claim 4 including a pair of recessed oil distribution galleries formed in the outer surface of respectively the first and second skirt parts and extending about respective parts of the of the peripheries of the skirt parts, wherein a respective outer opening of a said piston oil feed passage opens into one of the pair of recessed oil distribution galleries.

6. A method of manufacturing a piston according to Claim 1 or Claim 2 comprising the steps of forming at least a first piston oil feed passage in a piston so as to interconnect the hollow space and the first skirt part, such that the hollow space includes an inner opening of the piston oil feed passage and the piston oil feed passage terminates in an outer opening at or near the outer surface of the first skirt part.

7. A method of manufacturing a piston according to Claim 3, the method comprising the steps of Claim 6 and the additional step of forming a recessed lubricant distribution gallery in the outer surface of the first skirt part and extending about at least part of the periphery of the first skirt part, such that the outer opening opens into the recessed gallery.

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