Background An internal combustion engine typically has one or more reciprocating pistons, which are lubricated to reduce the friction as the piston slides within a cylinder bore. Lubricated sliding contacts, such as between piston rings of the piston and an inner surface of the cylinder bore, have frictional losses due to the shear forces generated in the lubricant, contact between surface asperities, and boundary contacts caused by additives in the lubricant.

It is desirable to reduce the friction between the piston rings and the inner surface of the cylinder in order to increase the efficiency of the engine and reduce wear between engine components. The friction between the components may be determined by a number of factors, which include the operational parameters of the engine and the configuration of each of the sliding surfaces. For example, the frictional coefficient between sliding components may be determined using the Stribeck curve, which is used to categorise the frictional properties between two surfaces as a function of the viscosity of the lubricant and the relative speed between the components per unit load. As such, friction may be minimised by operating at the minimum point on the Stribeck curve, which defines the transition between hydrodynamic lubrication and mixed lubrication. However, it is difficult to maintain operation at the minimum point on the Stribeck curve across the full piston stroke as a result of the low relative speed between the piston and the cylinder at the extremes of the range of movement of the piston.

Statements of Invention

According to an aspect of the present disclosure, there is provided a cylinder for receiving a reciprocating piston, the cylinder defining a bore portion having first and second ends between which the piston travels in an axial direction,

wherein the bore portion comprises a plurality of axially spaced apart recesses formed in a piston facing surface of the bore portion at a plurality of axial positions, with at least one recess being provided at each axial position; wherein the at least one recess at each axial position defines a total recess volume at each axial position,

wherein a recess volume-to-spacing ratio is defined by a ratio of the total recess volume at a particular axial position to an axial spacing between the at least one recess at the particular axial position and a neighbouring at least one recess, and

wherein the recess volume-to-spacing ratio varies progressively along the axial length of the bore portion.

The cylinder may be provided in a reciprocating machine. For example, the cylinder may be provided in a reciprocating engine (such as internal combustion engine), a reciprocating pump or any other machine with a piston that slidably reciprocates in a cylinder of the machine.

The recesses may be distributed in the axial direction. The recesses may also be distributed in a circumferential direction of the bore portion if there is more than one recess at each axial position.

The recess volume-to-spacing ratio may increase towards the first and second ends of the bore portion. The recess volume-to-spacing ratio may increase away from a point of the bore portion substantially mid- way between the first and second ends of the bore portion, e.g. the point at which velocity of piston is maximum. The point at which the maximum piston velocity occurs may be close to the midpoint between the first and second ends.

The progressive variation of the recess volume-to-spacing ratio may comprise a plurality of changes to the recess volume-to-spacing ratio, e.g. with a plurality of changes in each direction away from the point substantially mid-way between the first and second ends of the bore portion.

The axial spacing for a particular axial position may be measured consistently in the same direction, e.g. in a first axial direction or a direction away from the point substantially mid- way between the first and second ends of the bore portion.

The progressive variation of the recess volume-to-spacing ratio may be centred on the travel of a piston ring of the piston. For example, the point of the bore portion substantially mid-way between the first and second ends of the bore portion may be the point at which the piston ring axial velocity is highest. The first and second ends of the bore portion may be defined by the extent of travel of the piston ring. Recesses may be absent at the first and second ends of the bore portion where the piston changes direction. For example, no recesses may be provided at the extreme axial locations at which the piston ring of the piston changes direction.

The recess volume-to-spacing ratio at the particular axial position may be dependent on (e.g. a function of) a piston-velocity ratio at the particular axial position. The piston velocity ratio may be a ratio of the piston velocity at the particular axial position to the maximum velocity of the piston during that particular piston stroke in a steady state (e.g. with a constant crankshaft velocity). The maximum velocity of the piston may occur at a point of the bore portion between the first and second ends of the bore portion. By way of example, the recess volume-to-spacing ratio at the particular axial position may be a function of the reciprocal of (e.g. inversely proportional to) the piston velocity ratio at the particular axial position. Each recess may have a width in the axial direction of the cylinder. The widths of the recesses may vary in the axial direction to at least partially contribute to the progressive variation of the recess volume-to-spacing ratio.

Each recess may have a depth perpendicular to the axial direction of the cylinder. The depth may also be perpendicular to the circumferential direction of the cylinder. The depths of the recesses may vary in the axial direction to at least partially contribute to the progressive variation of the recess volume-to-spacing ratio.

Each recess may have a length in a direction with a circumferential component. For example, the lengths of the recesses may be orientated in a direction perpendicular to the axial direction of the cylinder (e.g. in the circumferential direction) or inclined relative to the axial and circumferential directions. The lengths of the recesses may vary in the axial direction to at least partially contribute to the progressive variation of the recess volume-to-spacing ratio. The axial spacing between neighbouring recesses may vary in the axial direction to at least partially contribute to the progressive variation of the recess volume-to-spacing ratio.

A plurality of circumferentially spaced apart recesses may be provided at each of the axial positions. A circumferential spacing between neighbouring recesses may vary in the axial direction to at least partially contribute to the progressive variation of the recess volume-to- spacing ratio. The progressive variation of the recess volume-to-spacing ratio may occur between successive neighbouring axial positions. Additionally or alternatively, the progressive variation of the recess volume-to-spacing ratio may occur between a plurality of axial regions along the length of the bore portion, each region comprising a plurality of axially spaced apart recesses. The recess volume-to-spacing ratio may be substantially constant within a particular axial region. Three or more such axial regions may be provided.

An internal combustion engine, reciprocating machine or cylinder liner may comprise the above-mentioned cylinder.

According to another aspect of the present disclosure, there is provided a method of manufacturing a cylinder for receiving a reciprocating piston, the cylinder defining a bore portion having first and second ends between which the piston travels in an axial direction, wherein the method comprises:

forming a plurality of axially spaced apart recesses in a piston facing surface of the bore portion at a plurality of axial positions, with at least one recess being provided at each axial position;

wherein the at least one recess at each axial position defines a total recess volume at each axial position,

wherein a recess volume-to-spacing ratio is defined by a ratio of the total recess volume at a particular axial position to an axial spacing between the at least one recess at the particular axial position and a neighbouring at least one recess, and

wherein the recess volume-to-spacing ratio varies progressively along the axial length of the bore portion.

To avoid unnecessary duplication of effort and repetition of text in the specification, certain features are described in relation to only one or several aspects or embodiments of the invention. However, it is to be understood that, where it is technically possible, features described in relation to any aspect or embodiment of the invention may also be used with any other aspect or embodiment of the invention.

Brief Description of the Drawings For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which: Figure 1 is a schematic cross-sectional view through an engine according to arrangements of the present disclosure;

Figure 2 is a schematic cross-sectional view through a cylinder according to arrangements of the present disclosure;

Figure 3(a) is a schematic cross-sectional view of a cylinder according to a first arrangement of the present disclosure and Figure 3(b) is a corresponding graph showing the variation of the recess volume-to-spacing ratio along the axial length of the bore portion; Figure 4(a) is a schematic cross-sectional view of a cylinder according to a second arrangement of the present disclosure and Figure 4(b) is a corresponding graph showing the variation of the recess volume-to-spacing ratio along the axial length of the bore portion; and

Figure 5 is a graph showing the coefficient of friction with a progressively textured surface according to an arrangement of the present disclosure compared to cylinders without a progressively textured surface.

Detailed Description Figure 1 shows a simplified cross-section of an engine 101. The engine comprises pistons 109, which reciprocate in cylinders 103 and drive rotation of a crankshaft 110 via piston rods 112. The depicted engine 101 is a four-cylinder engine having an overhead camshaft. However, the engine 101 may be any type of engine, for example a single overhead camshaft (SOHC) engine, a double overhead camshaft (DOHC) engine, an overhead valve (OHV) engine, or other appropriate type of engine. Furthermore, whilst the engine 101 shown in Figure 1 is a four- cylinder engine, the engine 101 may comprise any appropriate number of cylinders 103, for example the engine 101 may be a three-cylinder engine, a six-cylinder engine or an eight- cylinder engine. The cylinders 103 may be arranged in any appropriate configuration, such as in-line, horizontally opposed or V-form. Each of the cylinders 103 defines a bore portion 103' having first and second ends 103a, 103b between which the piston 109 travels in an axial direction. The piston 109 may change direction at the first and second ends 103a, 103b. In particular, each of the cylinders 103 comprises an inner bore surface 105 configured to engage piston rings 107 of the engine piston 109. The inner bore surface 105 may be an inner surface of a cylinder bore formed directly into a cylinder block of the engine 101 , as shown in Figure 1. Alternatively, the inner bore surface 105 may be an inner surface of a cylinder liner that is assembled into the cylinder block.

During operation of the engine 101, each of the pistons 109 reciprocates within the cylinder 103 between a top dead centre position and a bottom dead centre position. In the context of the present disclosure, the term "top dead centre" refers to the furthest point of a piston's travel, at which it changes from an upward stroke, i.e. away from the crankshaft 110 of the engine 101, to a downward stroke, i.e. towards the crankshaft 110 of the engine 101. The term "bottom dead centre" refers to the furthest point of a piston's travel, at which it changes from a downward to an upward stroke. In a similar manner, the term "top" end of the cylinder 103 refers to the second end 103b of the cylinder 103 at which the piston 109 reaches top dead centre, and the term "bottom" end of the cylinder 103 refers to the first end 103a of the cylinder 103 at which the piston 109 reaches bottom dead centre. In the arrangement shown in Figure 2, the piston 109 has a top piston ring 107A and a bottom piston ring 107B. However, the piston 109 may have any appropriate number of piston rings 107, for example the piston 109 depicted in Figure 1 has a middle piston ring. Each of the piston rings 107 may be configured to perform a different function, for example top piston ring 107A may be a compression ring configured to provide a seal between the top and bottom of the cylinder 103 on either side of the piston 109, and the bottom piston ring 107B may be an oil scraper ring configured to remove oil from the inner surface 105 of the cylinder 103.

In the arrangement shown in Figure 2, the top and bottom piston rings 107A, 107B each comprise a circumferential surface configured to engage the inner surface 105 of the cylinder 103. A contact zone between any one of the piston rings 107 may be defined by the portion of the circumferential surface of the piston ring 107 that engages the inner surface 105 of the cylinder 103.

During the operation of the engine 101, the linear speed of the piston 109 varies between a minimum speed, for example a zero speed when the piston is stationary relative to cylinder 103 at top dead centre or bottom dead centre, and a maximum speed as the piston 109 moves between top centre and bottom dead centre. (The maximum piston speed occurs at a point 103c between the first and second ends 103a, 103b of the bore portion.) As a result of the change in speed of the piston 109, the coefficient of friction between the piston rings 107 and the inner bore surface 105 of the cylinder varies as the piston 109 travels within the cylinder bore.

In order to reduce the friction between the sliding components of the engine 101, such as the piston rings 107 and the inner bore surface 105 of the cylinder, a lubricant may be used.

Accordingly, during operation of the engine, a lubricant film may be formed between the circumferential surface of the piston ring 107 and the inner surface 105 of the cylinder 103, for example as a result of the motion between the respective surfaces. The lubricant film may be used to separate the inner surface 105 and the circumferential surface of the piston ring 107 so that there is no physical contact between the two surfaces.

The frictional coefficient between sliding components may be determined using the Stribeck curve, which is used to categorise the frictional properties between two surfaces as a function of the viscosity of the lubricant and the relative speed between the components per unit load. Friction may be minimised by operating at the minimum point on the Stribeck curve, which defines the tribological transition between hydrodynamic lubrication and mixed lubrication. However, it is difficult to maintain operation at the minimum point on the Stribeck curve across the full piston stroke as a result of the cyclical acceleration and deceleration of the piston 109. For example, it is difficult to maintain hydrodynamic lubrication towards the top and bottom ends of the piston stroke owing to the low relative speeds between the piston 109 and the cylinder 103. In particular, at the ends of the travel of the piston 109, where the piston speed drops to zero, a lubricant film between the piston rings 107 and the inner bore surface 105 of the cylinder 103 can collapse as there is no motion to form a hydrodynamic lubricant film. The collapse of the film is dependent on how fast the lubricant can drain away from a contact zone between the piston rings 107 and the inner bore surface 105 of the cylinder 103.

As shown in Figure 2, the bore portion of the cylinder 103 comprises a plurality of discrete recesses 129 indented into the inner surface 105. The recesses 129 may comprise any type of opening or depression in the inner surface 105 that enables a fluid, such as a lubricant, to be held within the opening as the piston ring 107 moves over the opening. For example, the recesses 129 may comprise a plurality of pockets shaped to retain lubricant, and/or decrease the rate at which lubricant drains away from the contact zones. The pockets may be of any shape, for example the pockets may be square, rectangular, circular, rounded oblongs or any other shape. In one arrangement, the pockets may be of a similar shape to each other. In another arrangement, the plurality of pockets may comprise a number of differently formed/shaped pockets, for example the plurality of pockets may comprise a number of round-bottomed pockets and a number of square-bottomed pockets that are configured to trap lubricant. As depicted, the recesses 129 are formed at a plurality of axial positions along the length of the bore portion, e.g. forming a plurality of axially separated rows of recesses 129. At least one recess 129 may be provided at each axial position and in the particular arrangement shown, there is a plurality of recesses 129 circumferentially distributed at each axial position. Each recess 129 has a width W in the axial direction of the cylinder, a depth D perpendicular to the surface 105 and a length L in a direction with a circumferential component. As shown, the lengths L of the recesses may be orientated in a direction perpendicular to the axial direction of the cylinder (i.e. in the circumferential direction), however it is also envisaged that the lengths may be inclined relative to the axial and circumferential directions. Regardless of how the recesses 129 are orientated, the recesses 129 may be elongate with the lengths L being greater than the widths W.

To maximise effectiveness, lubricant may be restricted from "leaking" out of the pocket as the piston ring 107 travels over it. This may be achieved by having a contact zone that is larger than the width W of the recess 129. In other words, the width W of a recess 129 may have a maximum value that is less than the corresponding size of the contact zone.

As mentioned above, the recesses 129 are axially distributed along the length of the bore portion. The recesses 129 may thus be spaced apart in an axial direction with an axial spacing Sx. The recesses 129 may also be circumferentially distributed about the circumference of the bore portion with a circumferential spacing S _c . (The recess dimensions and spacings depicted in the Figures are schematic and may be much smaller than those depicted.)

Each recess 129 has a volume V associated with it, e.g. the volume defined by the recess below the bore surface 105. Furthermore, the recesses 129 at each axial position define a total recess volume VT for each axial position. A recess volume-to-spacing ratio R for recesses at a particular axial position may be defined as a ratio of the total recess volume VT at that particular axial position to the axial spacing S _x between the particular axial position and a neighbouring axial position with one or more recesses. The recess volume-to-spacing ratio R is thus a function of the axial position, x, as given below: ff (x) = V _T (*)/¾(*)·

When determining the recess volume-to-spacing ratio R, the axial spacing S _x for a particular axial position may be measured using the neighbouring axial position consistently in the same direction, e.g. in a first axial direction or a direction away from the point 103c substantially midway between the first and second ends 103a, 103b of the bore portion.

The recess volume-to-spacing ratio R may vary across the length of the bore portion, in particular the recess volume-to-spacing ratio R may vary progressively along the axial length of the bore portion. This variation may be achieved by changing one or more of the widths W, depths D, lengths L, axial spacing S _x and circumferential spacing S _c of the recesses 129 in the axial direction. The progressive variation of the recess volume-to-spacing ratio R may comprise a plurality of changes to the recess volume-to-spacing ratio R moving along the length of the bore portion. For example, there may be a plurality of changes to the ratio R in each direction away from the maximum piston velocity point 103c.

In the particular example shown, the widths W, depths D, lengths L, axial spacing S _x and circumferential spacing S _c of the recesses 129 all vary. However, it is equally envisaged that only a subset of these parameters may vary with another subset remaining substantially constant along at least a portion of the bore portion. It is also envisaged that a first subset of the parameters may vary in the axial direction so as to increase the recess volume-to-spacing ratio R and a second subset of the parameters may vary in the same axial direction so as to reduce the recess volume-to-spacing ratio R. In this case, one of the subsets may dominate the other to provide the overall desired variation in the recess volume-to-spacing ratio. For example, the lengths L of the recesses 129 may increase in a particular axial direction, the widths W and depths D may reduce in the same axial direction, and the variation of the widths and depths may dominate so that the recess volumes V and recess volume-to-spacing ratio R reduce in the particular axial direction. With reference to Figures 3 and 4, the recess volume-to-spacing ratio R may increase towards the ends 103a, 103b of the bore portion. In particular, the recess volume-to-spacing ratio R may increase away from the axial location 103c of the bore portion 103' substantially mid-way between the first and second ends 103a, 103b of the bore portion. As mentioned above, the axial location 103c of the bore portion may be the axial point at which the velocity of the piston 109 is maximum. The point at which the maximum piston velocity occurs may be close to, although not exactly at, the midpoint between the first and second ends 103a, 103b. The maximum velocity of the piston may occur at a point that is closer to the second end 103b of the bore portion (that is furthest from the crankshaft 110) than the first end 103a (that is closest to the crankshaft 110). The minimum value of the recess volume-to-spacing ratio R may occur at the axial location 103c. The recess volume-to-spacing ratio R may thus be centred on the axial location 103c. However, as the axial location 103c may not be exactly central with respect to the first and second ends 103a, 103b, the recess volume-to-spacing ratio R may vary as a function of the axial position that is not symmetrical about the axial location 103c.

The variation of the recess volume-to-spacing ratio R may be based on the travel of the piston ring 107 of the piston or, where there is more than one piston ring, a particular one of the piston rings, e.g. piston ring 107A. For example, the point 103c substantially mid-way between the first and second ends 103a, 103b of the bore portion may be the point at which the axial velocity of the piston ring (or particular piston ring) is highest. Likewise, the location of the first and second ends 103a, 103b of the bore portion may be set by the extent of travel of the piston ring or particular piston ring.

As mentioned above, the recesses 129 are axially distributed across the length of the bore portion between the first and second ends 103a, 103b. However, there may be no recesses at the first and second ends 103a, 103b of the bore portion where the piston (e.g. piston ring) changes direction. There may also be no or few recesses at or in the region of the axial location 103c. This may be due to the changing axial spacing S _x , which may result in the provision of recesses 129 in a central region being sparse.

The recess volume-to-spacing ratio R may be a function of a piston-velocity ratio P at the same axial position. The piston velocity ratio P at a particular axial location may be a ratio of the piston velocity magnitude Vp at the particular axial position to the maximum velocity magnitude of the piston Vp,max during that particular piston stroke in a steady state in which the crankshaft rotational speed Ω is constant. (The piston velocity ratio P thus varies with axial position x and is independent of the crankshaft speed Ω.) Accordingly, (x) = ν _Ρ (χ, Ω)/ν _{Ρι) αΑ} .(Ω); and It will be appreciated that the piston velocity ratio P will vary between 0 and 1 with the ratio P being zero at the first and second ends 103a, 103b of the bore portion and 1 at the maximum velocity point 103c. By way of example, the recess volume-to-spacing ratio R may be a function of the reciprocal of the piston velocity ratio P, e.g. R may be inversely proportional to P. However, the recess volume-to-spacing ratio R may vary with any function in which R reduces as the piston moves away from the first and second ends 103a, 103b of the piston's travel.

It will be appreciated that the recess parameters (e.g. dimensions and spacings) may vary in a stepwise fashion since they are determined at discrete axial locations where the recesses 129 are provided. Nevertheless, as shown in Figures 3 and 4, the overarching recess volume-to-spacing ratio R may vary with a continuous function and the values of R for the discrete locations, at which the recesses 129 are provided, may be obtained with reference to the function R. As depicted in Figure 3, the progressive variation of the recess volume-to-spacing ratio R may occur between successive axially neighbouring recesses 129. Alternatively, as depicted in Figure 4, the progressive variation of the recess volume-to-spacing ratio R may occur between a plurality of axial regions 130a, 130b, 130c, 130d along the length of the bore portion. Each region 130a-d may comprise a plurality of axially spaced apart recesses 129. The recess volume-to-spacing ratio R may be substantially constant within a particular axial region.

Although four regions are shown in Figure 4, three or more such axial regions 130a-d may be provided along the length of the bore portion.

Figure 5 is a Stribeck graph showing how a coefficient of friction μ between a piston and cylinder varies depending on a lubrication parameter (in this case the product of piston speed U and lubricant viscosity η) for various cylinder arrangements. The first cylinder arrangement (i) comprises a non-textured and smooth cylinder bore. The second, third and fourth cylinder arrangements (ii)-(iv) comprise a textured cylinder bore with recesses evenly spaced apart along the length of the cylinder. In the second, third and fourth arrangements (ii)-(iv), the axial spacing between recesses is 500um, lOOOum and 3000um respectively. The fifth cylinder arrangement (v) has a textured cylinder bore with recesses that have a progressively varying volume-to-spacing ratio as described above. As is apparent from Figure 5, the fifth cylinder arrangement (v) on average exhibits a lower coefficient of friction μ than the other

arrangements. The present invention advantageously provides appropriately sized and spaced recesses to promote a hydrodynamic lubrication regime over a greater range of piston velocities. As a result, friction between the piston and cylinder is reduced. Although the cylinder 103 has been described above with reference to an internal combustion engine, the cylinder 103 may also be provided in a reciprocating machine. For example, the cylinder may be provided in a reciprocating engine, a reciprocating pump or any other machine with a piston that slidably reciprocates in a cylinder of the machine.

It will be appreciated by those skilled in the art that although the invention has been described by way of example, with reference to one or more examples, it is not limited to the disclosed examples and alternative examples may be constructed without departing from the scope of the invention as defined by the appended claims.