Reciprocating arrangement with anti-leak formations

Abstract

A reciprocating arrangement for use in a fuel injection system comprises a reciprocating member 20 inside, and sharing a common longitudinal axis L with, a longitudinal bore (22, figure 2a). The outer surface of the reciprocating member is separated from an inner surface of the bore by a clearance which defines a leak path for fluid in a leak direction substantially parallel to the common longitudinal axis. The outer surface of the reciprocating member comprises an anti-leak formation comprising at least one groove 30 having an inlet portion 32 and a redirection portion 34 contiguous with one another. The inlet portion is defined by the outer surface of the reciprocating member sloping inwardly relative to the longitudinal axis to cause a local reduction in diameter. The redirection portion defines a blind end of the groove and comprises a redirection surface 36 shaped to redirect a flow of fluid entering the groove via the inlet portion such that a velocity flow vector of the flow of fluid includes a component in a direction substantially opposite to the leak direction when exiting the groove.

Description

TECHNICAL FIELD The invention relates to a reciprocating arrangement comprising a reciprocating member with an anti-leak formation. BACKGROUND Internal combustion engines comprise a number of reciprocating arrangements of a reciprocating member located coaxially within a longitudinal bore. These reciprocating arrangements include pistons, pumps and plungers. While it is common for the reciprocating members of these arrangements to have a tight clearance with the bore in which they are located, a flow path, or flow channel, still exists around the outside of the reciprocating member. It is therefore possible for fluid to leak past the reciprocating member along this fluid path. Reducing the clearance further makes the performance of the reciprocating member more sensitive to geometric variations, thereby creating a need for tighter manufacturing tolerances. This increases the manufacturing costs associated with the production of the reciprocating member. There is therefore a limit as to how tight the clearance between the reciprocating member and the bore can be. Where there is a relatively low pressure differential across the reciprocating member, one current solution is to manage leakage using a physical seal between the reciprocating member and the bore. However, this has significant repercussions for the ability of the reciprocating member to accelerate quickly and so is not a practical solution for devices that require quick response times, such as pressure regulator valves. It is an objective of the invention to address some of the issues identified with the prior art. SUMMARY OF THE INVENTION In a first aspect, the invention provides a reciprocating arrangement for use in a fuel injection system. The reciprocating arrangement comprises: a reciprocating member located inside a longitudinal bore, the reciprocating member and the longitudinal bore sharing a common longitudinal axis. The outer surface of the reciprocating member is separated from an inner surface of the bore by a clearance, which defines a leak path for fluid, wherein a leak flow of fluid travelling along the leak path does so in a leak direction substantially parallel to the common longitudinal axis. The outer surface of the reciprocating member comprises an anti-leak formation comprising at least one groove, wherein each groove comprises an inlet portion and a redirection portion, which are contiguous with one another. The inlet portion is defined by the outer surface of the reciprocating member sloping inwardly relative to the longitudinal axis to cause a local reduction in the diameter of the reciprocating member. The redirection portion defines a blind end of the groove and comprises a redirection surface that is shaped to redirect a flow of fluid entering the groove via the inlet portion such that a velocity flow vector of the flow of fluid includes a component in a direction substantially opposite to the leak direction when exiting the groove. The redirection surface of each groove may comprise a hemispherical or otherwise part-spherical wall. Alternatively, the redirection surface of the groove may comprise a planar wall, wherein the planar wall is arranged at an angle of between 0 and 20 degrees to a direction perpendicular to the common longitudinal axis. The slope of the outer wall of the reciprocating member at the inlet portion of the groove may define an angle of between 10 and 30 degrees to the common longitudinal axis. This corresponds to the length of the groove being between 1.7 and 5.7 times the width of the groove. Preferably, the length of the groove may be between 2 and 4 times the width of the groove. This corresponds to the slope of the outer wall of the reciprocating member at the inlet portion of the groove defining an angle of between 14.0 and 26.6 degrees to the common longitudinal axis. The inlet portion of the groove may have a length of between 0.5 mm and 2 mm in the direction of the common longitudinal axis. The redirection portion of the groove may be arranged adjacent to the clearance. Each groove may extend circumferentially around the reciprocating member. The anti-leak formation may comprise a plurality of grooves, wherein the grooves are separated from each other along the common longitudinal axis by a separation length. The separation length between neighbouring grooves represents a region where the reciprocating member has its natural diameter, unaffected by the grooves of the anti-leak formation. Preferably, the reciprocating member comprises at least three grooves. Each groove may have a fluid resistance between 0.24 MPa / (m3s~1) and 2 MPa / (m3s’1). The anti-leak formation as a whole may have a fluid resistance of at least 1.2 MPa / (m3s-1) . BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be described with reference to the following drawings, in which: Figure 1 shows a cross-sectional view of a prior art regulator piston; Figure 2a shows a cross-sectional view of a regulator piston comprising an anti-leak formation in situ in a bore; Figure 2b shows a cross-sectional view of an upper section of the regulator piston of Figure 2a; Figure 3 shows a cross-sectional close-up view of a groove of an anti-leak formation of the regulator piston of Figure 2a; and Figure 4 shows a cross-sectional close-up view of an alternative groove of the anti-leak formation of the regulator piston of Figure 2a. DETAILED DESCRIPTION Figure 1 shows an example reciprocating arrangement 10 as currently represented by the state of the art. The reciprocating arrangement 10 comprises a reciprocating member 20 in the form of a regulator piston. The regulator piston 20 is located substantially coaxially within a cylindrical bore 22 and is able to travel along the common longitudinal axis L defined by the bore 22 and the piston 20. As discussed above, there is an annular clearance 24 between the piston 20 and the bore 22. This clearance 24 may typically be around 40 pm in width. This allows the piston 20 to move freely, but leaves a leak path for fluid in the bore 22 to leak past the piston 20. The outside surface of the piston 20 is marked by circumferential grooves 26. The grooves 26 are used for debris relief and are characterised by a sudden change in the diameter of the piston 20. Turning now to Figures 2a and 2b, which illustrate a reciprocating member 20 in the form of a regulator piston in situ in a longitudinal bore 22 (Figure 2a) and an upper portion of the reciprocating member 20 in isolation (Figure 2b). The piston 20 comprises an anti-leak formation 28, which aims to prevent leakage of fluid past the piston 20 and through a leak path defined by a clearance 24 between the piston 20 and the bore 22. The anti-leak formation 28 comprises at least one asymmetric groove 30 in the outer surface of the piston 20. The embodiment shown in Figures 2a and 2b comprises five grooves 30, but, depending on the shape and configuration of the reciprocating arrangement 10, one groove 30 may be sufficient to achieve the desired effect. However, it is preferred that the anti-leak formation 28 comprises at least three grooves 30, and preferably comprises at least five grooves 30 for optimal function. The anti-leak formation 28 works by redirecting a portion of leaking fluid flow such that it is substantially opposed to the leaking flow. This reduces the leak flow beyond the antileak formation 28. The asymmetry of the groove 30 means that fluid travelling in the opposite direction to the leak flow is not resisted in the same way. In this way, the general working of the anti-leak formation 28 employs a similar philosophy to a Tesla valve, which acts as a fluidic diode. Fluid diodicity describes the tendency of a flow path to allow fluid flow along one direction, but to oppose fluid flow in the opposite direction. Fluid diodicity, Di, can be thought of as a ratio of the resistance to fluid flow along a flow path in a reverse direction, Rr, to the resistance to fluid flow along the flow path in a forward direction, Rf. In other words: The fluid resistances can be thought of as analogues of electrical resistance, which is defined by Ohm’s law as the drop in voltage, or electromotive force, per unit of current through an electrical component. The fluid resistance can therefore be defined as the ratio of the drop in the fluid pressure, ^p, to the flow rate, Q: For a constant flow rate, the fluid diodicity therefore simplifies to a ratio of the drops in fluid pressure in the reverse direction to the forward direction: In this description, the direction of leak flow is taken to be the forward direction, such that fluid diodicity values greater than one indicate that leak flow is being opposed. In some cases, the fluid diodicity of the anti-leak formation 28 may be the characteristic of greatest importance. For example, if a flow of lubricant originated one side of the piston 20, it may be desirable for flow of the lubricant to only travel one way, and for flow counter to this direction to be resisted. In other cases, the anti-leak formation 28 may be intended solely to resist a leak flow through the clearance 24 past the piston 20. In this case, fluid resistance is the defining characteristic of the system. This can either be thought of in terms of the fluid resistance of the anti-leak formation 28 as a whole, or in terms of the individual fluid resistances of the constituent grooves 30 of the anti-leak formation 28. Naturally, if the grooves 30 have a higher fluid resistance, the anti-leak formation 28 can comprise fewer grooves 30 for the same resistive effect, but different configurations of different reciprocating arrangements 10 may have different requirements and so in some embodiments, it may be preferable for the anti-leak formation 28 to comprise more grooves 30 having a lower fluid resistance, or vice versa. In preferred embodiments, the fluid resistance of each groove 30 may be at least 0.24 MPa / (m3s'1), and the fluid resistance of the anti-leak formation 28 may be at least 1.2 MPa / m3S’1). As discussed above, the grooves 30 work by redirecting a portion of the leak flow such that it opposes the leak flow, thereby resisting the passage of the leak flow beyond the groove. The leak flow therefore defines a leak direction. As the leak flow will be along the clearance 24, the leak direction will be substantially parallel to the common longitudinal axis L of the piston 20 and the bore 22. In the example shown in Figures 2a and 2b, the leak flow travels downwardly along the clearance 24 and so has a substantially downward velocity flow vector: the leak direction is therefore downward in the example of Figures 2a and 2b. Through the interaction with the grooves 30, a portion of the leak flow is redirected such that it leaves the groove 30 with a velocity flow vector including a component in a direction substantially opposite to the leak direction. In other words, the redirected portion of the leak flow has a velocity flow vector that is angled at least slightly upwardly in the example of Figures 2a and 2b. It will be apparent to the skilled person that the shape of each groove 30 is critical to its ability to redirect the leak flow. Figure 3 shows a close-up view of an embodiment of a groove 30, which better shows its shape. In general, the groove 30 comprises two main regions, an inlet portion 32 and a redirection portion 34. The inlet portion 32 collects and diverts a flow of fluid from the leak flow in the clearance 24, while the redirection portion 34 is responsible for redirecting the diverted flow of fluid such that it opposes the leak flow. As can be seen in Figure 3, the inlet portion 32 is defined by the outer surface of the piston 20 sloping inwardly relative to the longitudinal axis L so that the diameter of the piston 20 undergoes a local reduction in the region of the inlet portion 32. In determining the angle of slope Q of the inlet portion 32 relative to the longitudinal axis L, a balance must be preserved between avoiding too great a pressure drop in the flow of fluid as it enters the inlet portion 32, as this will slow the flow of fluid down and reduce its resistive effect when redirected against the leak flow, and ensuring that a sufficient flow of fluid is diverted into the groove 30 for the resistive effect of the redirected flow of fluid to be felt. Increasing the angle of slope Q of the inlet portion 32 causes the width of the leak path to increase more quickly, increasing the pressure drop when the diverted flow of fluid enters the inlet portion 32. However, decreasing the angle of slope Q of the inlet portion 32 reduces the total volume defined by the groove 30, reducing its capacity for diverted fluid. Preferably therefore, the angle of slope Q of the inlet portion 32 is between 10° and 30°. The angle of slope Q may also be thought of in terms of a preferred aspect ratio between the length of the inlet portion 32 in direction parallel to the longitudinal axis L and the width of the inlet portion 32, defined by the reduction in the radius of the piston 20 at the downstream end of the inlet portion 32. For upper and lower bounds of 30° and 10° for the angle of the slope Q, this corresponds to a range of 1.7 to 5.7 for the aspect ratio of the length of the inlet portion 32 to the width of the inlet portion 32. Preferably, the aspect ratio may be between 2 and 4 (i.e. the inlet portion may be between two times and four times longer than it is wide). This corresponds to a range of between 14.0° and 26.6° for the angle of the slope 0. As well as the angle of slope 0 being important, the length of the inlet portion 32, defined with respect to the direction of the longitudinal axis L, also has an impact on the ability of the groove 30 to effectively redirect the leak flow. Again, there is a balance to be struck between enabling a sufficient portion of the leak flow to be diverted into the groove 30 for the desired resistive effect on the leak flow, and avoiding the inlet portion 32 being too long such that the fluid pressure of the diverted flow of fluid entrained therein reduces to such a degree that the redirected flow of fluid has insufficient speed to resist the leak flow. The preferred length of the groove may change depending on the characteristics of the leak flow (its flow rate, the width of the clearance 24, the viscosity of the leaking fluid, etc). In some embodiments, the length of the inlet portion is preferably between 0.5 mm and 2 mm. The redirection portion 34 is contiguous with the inlet portion 32 and defines a blind end that terminates the groove 30. For the ease of description, the two portions of the groove 30 may be considered to be separated by an imaginary horizontal plane P which intersects the reciprocating member 20 at the point at which the diameter of the reciprocating member 20 resumes its natural diameter (i.e., the diameter of the parts of the reciprocating member 20 that are not reduced by the presence of the grooves 30), although the skilled person will appreciate that this is a somewhat arbitrary construction due to the continuous nature of the space inside the groove 30. The diverted flow of fluid that has entered the inlet portion 32 continues flowing into the groove 30 until it enters the redirection portion 34. The redirection portion 34 comprises a redirection surface 36 at the blind end of the groove 30. Interaction of the diverted flow of fluid with the redirection surface 36 redirects the flow of fluid to oppose the leak direction. As explained above, this means that a velocity flow vector of the redirected flow of fluid incudes a component in a direction substantially opposite to the leak direction. Naturally, for the resistive effect of the redirected flow of fluid to be as strong as possible, it is preferable for the velocity flow vector of the redirected flow of fluid to be in a direction that is as close to opposite the leak direction as possible, while still being able to exit the groove 30. However, it will be appreciated by the skilled person that some level of resistive effect will be provided if the velocity flow vector includes any component in a direction substantially opposite the leak direction. The redirection portion 34 is preferably located adjacent to the clearance 24. To define this concept more clearly, it may be considered that the reciprocating member 20 has two diameters at the point at which the plane P is defined: a first, outer diameter, which corresponds to the natural diameter of the reciprocating member 20 and a second, inner diameter defined by the radially innermost extent of the inlet portion 32. The redirection portion 34 occupies substantially all of the space in between these two diameters so that there is no significant barrier to flow of fluid out of the redirection portion 34 caused by the body of the reciprocating member 20. This aids with the resistive effect provided by the groove 30. Figure 3 shows an embodiment in which the redirection surface 36 takes the form of a hemispherical surface. Strict adherence to an exactly hemispherical redirection surface 36 may not be necessary, however, and it may be the case in some embodiments that much the same effect can be achieved by a part-spherical redirection surface 36. A hemispherical redirection surface 36 is particularly effective at redirecting the flow of fluid to oppose the leak flow and additionally can induce turbulence into the redirected flow to further impede the leak flow. In other embodiments, as shown in Figure 4, the redirection surface 36 may simply take the form of a planar wall. In this case, the planar redirection surface 36 may be angled at an angle a to the radial direction of the reciprocating member 20, or equivalently, the plane P (as shown in dashed lines in Figure 4). The value of the angle a may lie between 0° (i.e. a planar surface that is parallel to the radial direction of the piston 20, in which case the redirection portion 34 is comprised entirely of the redirection surface 36) and 20°. The skilled person will be aware that other configurations of the redirection surface 36, or redirection portion 34 more generally, may also provide a similar redirective effect on the diverted flow of fluid. It is preferred that each groove 30 extends circumferentially around the entirety of the piston 20, so that no leak paths exist that bypass the groove 30. A circumferential groove 30 may also be more straightforward to machine during manufacturing of the piston 20. However, it may also be the case that the anti-leak formation 28 includes a number of part-circumferential grooves 30, axially spaced from each other in the direction of the longitudinal axis L and also angularly offset from each other around the circumference of the piston 20 so the anti-leak formation 28 as a whole provides total circumferential coverage around the piston 20. In addition to the lengths of the grooves 30 themselves, an additional consideration in embodiments where the anti-leak formation 28 comprises multiple grooves may be the separation length 38 between adjacent grooves 30. The separation length 38 refers to the length between adjacent grooves 30 where the diameter of the reciprocating member 30 reverts to its natural diameter, unaffected by the grooves 30 of the anti-leak formation 28. The separation length 38 may influence the resistive effect of the anti-leak formation 28 by influencing the extent to which a redirected flow from a groove 30 will interact with the zone of mixing between the redirected flow and the leak flow from an adjacent groove 30. In the drawings, the embodiments shown have all comprised anti-leak formations 28 with grooves 30 that are all oriented in the same way. In other words, all the grooves 30 of the anti-leak formations 28 are oriented to resist a leak flow past the reciprocating member in the same direction. However, depending on the configuration of the reciprocating assembly 10, the anti-leak formation 28 may also comprise grooves 30 that are oriented in the opposite sense, so that the anti-leak formation 28 resists leak flows past the reciprocating member in the opposite direction. The skilled person will appreciate that, while this description has made specific reference to the reciprocating assembly 10 and the reciprocating member 20 being a regulator valve and a regulator piston, respectively, that the principles of the anti-leak formation are applicable to any reciprocating assembly 10 where there is a desire to prevent a leak flow of fluid through a clearance 24 past a reciprocating member 20. While some directional terms, such as ‘down’ and ‘up’, have been used in the description, the skilled person will also understand that these have been in relation to the orientation of the components as shown in the Figures, and are not necessarily limiting, as the components may exist in other orientations when in use.

Claims

1. A reciprocating arrangement (10) for use in a fuel injection system comprising:a reciprocating member (20) located inside a longitudinal bore (22), the reciprocating member (20) and the longitudinal bore (22) sharing a common longitudinal axis (L),wherein the outer surface of the reciprocating member (20) is separated from an inner surface of the bore (22) by a clearance (24), which defines a leak path for fluid, wherein a leak flow of fluid travelling along the leak path does so in a leak direction substantially parallel to the common longitudinal axis (L),wherein the outer surface of the reciprocating member (20) comprises an anti-leak formation (28) comprising at least one groove (30),wherein each groove (30) comprises an inlet portion (32) and a redirection portion (34), wherein the inlet portion (32) and redirection portion (34) are contiguous with one another, andwherein the inlet portion (32) is defined by the outer surface of the reciprocating member sloping inwardly relative to the longitudinal axis (L) to cause a local reduction in the diameter of the reciprocating member (20), andwherein the redirection portion (34) defines a blind end of the groove and comprises a redirection surface (36) that is shaped to redirect a flow of fluid entering the groove (30) via the inlet portion (32) such that a velocity flow vector of the flow of fluid includes a component in a direction substantially opposite to the leak direction when exiting the groove (30).

2. The reciprocating arrangement (10) of Claim 1, wherein the redirection surface (36) of each groove (30) comprises a hemispherical or otherwise part-spherical wall.

3. The reciprocating arrangement of Claim 1, wherein the redirection surface (36) of the groove (30) comprises a planar wall, wherein the planar wall is arranged at an angle of between 0 and 20 degrees to a direction perpendicular to the common longitudinal axis (L).

4. The reciprocating arrangement (10) of any preceding claim, wherein the slope of the outer wall of the reciprocating member (20) at the inlet portion (32) of the groove (30) defines an angle of between 10 and 30 degrees to the common longitudinal axis (L).

5. The reciprocating arrangement (10) of Claim 4, wherein the slope of the outer wall of the reciprocating member (20) at the inlet portion (32) of the groove (30) defines an angle of between 14.0 and 26.6 degrees to the common longitudinal axis (L).

6. The reciprocating arrangement of any preceding claim, wherein the inlet portion (32) of the groove (30) has a length of between 0.5 mm and 2 mm in the direction of the common longitudinal axis (L).

7. The reciprocating arrangement (10) of any preceding claim, wherein the redirection portion (34) of the groove (30) is arranged adjacent to the clearance (24).

8. The reciprocating arrangement (10) of any preceding claim, wherein each groove (30) extends circumferentially around the reciprocating member (30).

9. The reciprocating arrangement (10) of any preceding claim, wherein the anti-leak formation (28) comprises a plurality of grooves (30), wherein the grooves (30) are separated from each other along the common longitudinal axis (L) by a separation length (38).

10. The reciprocating arrangement (10) of Claim 9, wherein the reciprocating member (20) comprises at least three grooves (30).

11. The reciprocating arrangement (10) of any preceding claim, wherein each groove (30) has a fluid resistance of between 0.24 MPa / (m3s-1) and 2 MPa / (m3s-1).

12. The reciprocating arrangement (10) of any preceding claim, wherein the anti-leak formation (28) has a fluid resistance of at least 1.2 MPa / (m3S’1).12

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