Control valve assembly and fuel injector incorporating a control valve assembly

A control valve assembly for controlling fuel pressure in a control chamber of a fuel injector. The control valve assembly comprising a valve member arranged in a bore provided in a valve housing, at least one of the valve member and the valve housing being moveable with respect to the other, wherein the valve member comprises a fuel-receiving cavity arranged to receive fuel that distorts at least a portion of the valve member so as to increase an external dimension thereof.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a national stage application under 35 U.S.C. 371 of PCT Application No. PCT/EP2014/053670 having an international filing date of 26 Feb. 2014, which designated the United States, which PCT application claimed the benefit of European Patent Application number 13158969.9 filed on 13 Mar. 2013, the entire disclosure of each of which are hereby incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to a control valve assembly for controlling fuel pressure in a control chamber. In particular, the invention relates to a control valve assembly for controlling fuel pressure in the control chamber of a fuel injector for use in a fuel injection system of an internal combustion engine. The invention also relates to a fuel injector incorporating a control valve assembly.

BACKGROUND TO THE INVENTION

In common rail fuel systems for diesel engines, fuel is delivered from an accumulator to each cylinder of an engine by a dedicated fuel injector. Each fuel injector comprises a valve needle that is moveable relative to a seating to permit or arrest the delivery of fuel. Movement of the valve needle towards and away from the seating is controlled by balancing opposed closing and opening forces. Fuel in a delivery chamber exerts an opening force on an end of the valve needle nearest the seating, which acts to lift the needle away from the seating. Fuel in a control chamber exerts a closing force on an end of the valve needle remote from the seating, which acts to force the needle against the seating.

The balance of the opening and closing forces is controlled by controlling fuel pressure in the control chamber. When the pressure in the control chamber is high, the closing force on the needle is sufficient to overcome the opening force exerted by the fuel in the delivery chamber, and the valve needle is retained in a closed position against the valve seating. When the fuel pressure in the control chamber is relatively low, the closing force is lower than the opening force, and the valve needle is lifted from the seating into an open position, such that fuel is injected into the engine cylinder.

FIG. 1illustrates in cross-section a known control valve assembly10for use in controlling the fuel pressure in such a control chamber12. A valve member14is moveable with respect to a housing16between a closed position and an open position by means of an actuator18, such as a solenoid. In the closed position, the control chamber12is in communication with a high-pressure fuel supply20. Fuel pressure in the control chamber12is therefore high. In the open position of the control valve10, the control chamber12and the high-pressure fuel supply20are in communication with a fuel passage22that leads to a low-pressure drain (not shown). Pressure in flow passages24in the control valve and in the control chamber12is therefore relatively low. In this way, the control valve assembly10controls the pressure in the control chamber12, which in turn controls the fuel injection.

In such control valves, the fuel in the flow passages24is prone to leakage, which leads to fuel loss, and hence leads to energy loss. This energy loss is undesirable because it decreases efficiency of the engine and results in increased CO2emissions.

Leakage can occur during opening and/or closing of the valve10, known as dynamic leak, and/or when the control valve10is in the closed position, known as static leak. Static leak is particularly significant as the control valve10is in the closed position for the majority of its lifetime. A major source of static leak is a clearance26between the valve member14and the surrounding housing16. The clearance26must exist to allow for sliding movement of the valve member14with respect to the housing16; however, the clearance26provides an undesirable pathway through which high-pressure fuel may leak out of the flow paths22of the control valve10.

It will be appreciated that when the control valve10is in the closed position, the flow paths22within the control valve contain high-pressure fuel. The high operating pressures of today's fuel injectors mean that fuel in the flow paths22is typically at pressures of at least 2000 bar; high enough that fuel within the flow paths22exerts a pressure on the surrounding components that is sufficient to cause significant distortion.

Specifically, the high-pressure fuel in the flow paths22exerts an outward radial force on the housing16, and an inward radial force on the valve member14. The distortion forces the two components apart in the region of the flow paths22. Away from the flow paths22, the strain relaxes. However, the relaxation is gradual, and thus the housing16and the valve member14are still subject to distortion in the region of the clearance26between the valve member14and the housing16. This distortion increases the size of the clearance26between the valve member14and the housing16, and hence increases the tendency for leakage.

It is with a view to addressing the aforementioned disadvantage that the present invention provides an improved control valve assembly for a fuel injection system for an internal combustion engine.

SUMMARY OF THE INVENTION

Against this background, a first aspect of the invention resides in a control valve assembly for controlling fuel pressure in a control chamber of a fuel injector, the control valve assembly comprising a valve member arranged in a bore provided in a valve housing, at least one of the valve member and the valve housing being moveable with respect to the other, wherein the valve member comprises a fuel-receiving cavity arranged to receive fuel that distorts at least a portion of the valve member so as to increase an external dimension thereof.

The invention provides a control valve assembly in which undesirable distortion effects caused by fuel in the bore of the control valve can be counteracted. The undesirable distortion effects tend to increase a diameter of the bore, causing leakage between the valve member and the valve housing. In the control valve of the invention, these effects are counteracted by means of the fuel-receiving cavity provided in the valve member. Fuel received in the fuel-receiving cavity causes an intentional distortion of the valve member, which increases an external diameter of the valve member, compensating for the increase in the diameter of the bore. By counteracting the distortion effects, leakage of fuel from the control valve between the valve member and the valve housing is reduced. This decreases the energy consumption and hence increases the efficiency of an engine provided with the control valve assembly.

Preferably, at least one of the valve member and the valve housing is engageable with a valve seating to control fuel pressure within the control chamber, and fuel pressure within the fuel-receiving cavity is variable depending on whether the valve member is engaged with the valve seating. In this way, the pressure in the fuel-receiving cavity can be varied to match the pressure in the control valve, such that the intentional distortion of the valve member occurs only to the extent that is required.

In preferred embodiments, the valve member comprises an annular wall surrounding the fuel-receiving cavity, so that fuel received in the fuel-receiving cavity exerts an outward radial force on at least a portion of the annular wall, so as to increase its external diameter. The annular wall is easily and uniformly distorted by the fuel in the fuel-receiving cavity, which reduces stresses exerted on the valve member and the valve housing as a result of the intentional distortion.

The valve member may comprise a valve body arranged in a close sliding fit in the bore, and at least a portion of the fuel-receiving cavity may extend into the valve body. Leakage between the valve member and the valve housing is particularly significant at a region of close-sliding fit, so extending at least a portion of the fuel-receiving cavity into the valve body advantageously helps to ensure that distortion in this region is counteracted, thereby reducing leakage still further.

Preferably, the bore is arranged to receive fuel from a high-pressure fuel supply, In this case fuel received in the bore may act to distort the valve housing and/or the valve member so as to increase a radial clearance defined between the valve body and a wall of the bore. In this embodiment, the bore may define a fuel gallery for receiving fuel from the high-pressure fuel supply, and fuel received in the fuel gallery may act to distort the valve housing and/or the valve member so as to increase the radial clearance.

In this case, fuel received in the fuel-receiving cavity preferably acts to distort the valve body so as at least partially to counteract the increase in the radial clearance caused by the fuel received in the bore.

In preferred embodiments, the fuel-receiving cavity is in fluid communication with the fuel gallery, such that fuel pressure in the fuel-receiving cavity is substantially the same as fuel pressure in the fuel gallery. Ensuring the same fuel pressure in the fuel gallery and the fuel-receiving cavity guards against a situation where, for example, the pressure in the fuel-receiving cavity is too low, so that the external diameter of the valve member is not sufficiently increased, and the clearance remains large enough to cause significant leakage, and also guards against a situation where the pressure in the fuel-receiving cavity is too high, such that the external diameter of the valve member increases too much, introducing an unacceptable degree of friction between the valve member and the valve housing.

Preferably, the fuel-receiving cavity is arranged in fluid communication with the fuel gallery by means of an inlet, such as a cross-drilling, that is in fluid communication with the fuel-receiving cavity and the fuel gallery.

In preferred embodiments, when fuel is absent from the bore and the fuel-receiving cavity, the valve body is of substantially constant external diameter.

For ease of manufacture, the fuel-receiving cavity is preferably a drilled cavity. To prevent fuel escaping from the fuel-receiving cavity, the drilled cavity may be plugged by an insert.

To control pressure in the control chamber of the injection valve, at least one of the valve member and the bore is preferably moveable with respect to the other between a first position in which the control chamber, the fuel-receiving cavity and a high-pressure fuel supply are arranged in mutual fluid communication, and a second position in which the control chamber is in fluid communication with a low-pressure fuel drain. For example, the valve member may be moveable with respect to the valve housing. Alternatively, the valve housing may be moveable with respect to the valve member.

The invention also extends, in a second aspect, to a fuel injector comprising a control valve assembly as described above, a control chamber, and an injection nozzle, the control chamber being arranged to control movement of a valve needle to control the injection of fuel from the injection nozzle.

It will be appreciated that preferred and/or optional features of the first aspect of the invention may be incorporated alone or in appropriate combination in the second aspect of the invention also.

Throughout the remainder of this document, terms such as ‘above’, ‘below’, ‘upwardly’, ‘downwardly’ and so on are used with reference to the orientation of the control valve in the accompanying drawings. However, it will be appreciated that a control valve according to the present invention could be used in any orientation. Terms such as ‘upstream’ and ‘downstream’ are used with reference to the direction of fuel flow in use of the control valve, during opening or closing of the control valve, or otherwise as the context demands.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Referring toFIG. 2, a fuel injector30for use in delivering fuel to an engine cylinder of an internal combustion engine (not shown) comprises an injection nozzle32, a control chamber34and a control valve36according to the invention. The injection nozzle32comprises a valve needle38that is moveable with respect to an injection valve seating (not shown) between an open position, in which delivery of fuel into the engine cylinder is permitted, and a closed position, in which delivery of fuel is arrested.

The control chamber34contains fuel that exerts a closing force on an end40of the valve pin38thereby acting to force the valve pin38against the injection valve seating. The control valve36controls the pressure of the fuel in the control chamber34, so as to control the closing force and to move the valve pin38between the open position and the closed position.

FIG. 3illustrates a control valve36according to the invention in isolation from the rest of the fuel injector30. The control valve comprises a bore42defined in a valve housing44, exemplified here as a control valve block, and a valve member46, exemplified here as a valve stem, that is slidably received in the bore42.

Drillings48in the valve housing define a high-pressure fuel supply that supplies high-pressure fuel to the bore42of the control valve36, and to the control chamber34. A primary drilling50delivers high-pressure fuel to the bore42of the control valve36, and a secondary drilling52, branching off from the primary drilling downstream of the control valve36, delivers fuel to the control chamber34. The bore42is arranged to receive high-pressure fuel from the high-pressure fuel supply48via an annular fuel gallery54, which is defined by a spacing provided between the valve stem46and a wall56of the bore42.

Above the fuel gallery54, the wall56of the bore42comprises a valve seating, generally indicated at58, in the form of a frustoconical surface60. The valve stem46comprises a corresponding frustoconical surface62, of complementary shape and dimensions, such that the valve stem46is engageable with the valve seating58.

Moving upwardly, above the valve seating58, the bore42is in communication with a low-pressure fuel drain (not shown) via outlet openings64in the wall of the bore42. The low-pressure fuel drain communicates with further drillings66in the valve housing44that drain fuel away from the bore42, so as to increase the volume of the fuel and hence to decrease the pressure of the fuel in the control valve36.

Continuing upwardly, the uppermost portion68of the valve stem46is arranged in contact with an actuator70such as a solenoid or a piezoelectric actuator. The actuator70acts on the uppermost portion68of the valve stem46, so as to move the valve stem46either downwardly and into engagement with the valve seating58, or upwardly and out of engagement with the valve seating58.

In this way, and as will later be described in detail, the valve stem46is moveable within the bore42between a first or closed position, shown inFIG. 3, in which the valve stem46is in abutment with the valve seating58, and a second, or open position, shown inFIG. 4, in which the valve stem46is spaced apart from the valve seating58.

Considering now the valve stem46in further detail, a lower portion of the valve stem46, located below the fuel gallery54, defines a valve body72of the valve stem46. The valve body72is substantially cylindrical in shape, and is of substantially constant radius. The valve body72is arranged in a close sliding fit within the bore42. Specifically, the portion of the bore42that surrounds the valve body72is of substantially constant radius, that radius being slightly greater than the radius of the valve body72. In this way, a small clearance74is provided between the valve body72of the valve stem46and the wall56of the bore42, so as to allow for relative sliding movement between the valve stem46and the valve housing72. Typically, the clearance74is of the order of a few microns.

The valve body72of the valve stem46is substantially hollow. Specifically, the valve body72of the valve stem46comprises an annular wall76that surrounds a fuel-receiving cavity78. The internal cavity has a diameter that is between approximately 30% and approximately 60% of the diameter of the valve stem.

The fuel-receiving cavity78is formed, for example, by drilling into a lowermost end80of the valve body72, and plugging the lowermost end with an insert82. The insert82is sealed to an inner surface84of the annular wall76in a leak-tight fashion, for example by welding.

Moving from the lowermost end80of the valve body72upwards, the fuel-receiving cavity78extends longitudinally through the length of the valve body72. The fuel-receiving cavity78continues beyond the valve body72of the valve stem46, such that it extends into a region of the valve stem46of reduced radius that is surrounded by the fuel gallery54of the bore42. The fuel-receiving cavity78terminates in this region of reduced radius, such that the fuel-receiving cavity78does not extend beyond the fuel gallery56.

An uppermost end86of the fuel-receiving cavity78is provided with one or more inlets88in the form of cross-drillings. The inlets88are arranged to extend between the fuel-receiving cavity78and the fuel gallery54, such that fuel can flow from the fuel gallery54into the fuel-receiving cavity78. It will be appreciated that fuel is prevented from flowing out of the fuel-receiving cavity78by the insert82provided in the lowermost end80of the valve stem46.

Continuing upwardly from the cross-drillings88, the central portion of the valve stem46leads into the frustoconical surface62of the valve stem46, in which region the radius of the valve stem46gradually increases until the radius is substantially the same as the radius of the valve body72. Above the frustoconical surface62, in the region of the low-pressure passages66and the uppermost region of the valve stem46, the valve stem46is provided with a blind drilling90. This blind drilling90restricts flow to the low-pressure drain, as described in the Applicant's published patent application WO 2004/005702.

The flow of fuel to, from and within the control valve36will now be described, with particular reference toFIGS. 3 and 4, which show the control valve36in the closed and open positions respectively.

Referring firstly toFIG. 3, when the control valve36is in the closed position, the frustoconical surface62of the valve stem46abuts the frustoconical surface60of the valve seating58. In this way, the fuel path between the fuel gallery54and the low-pressure passage66is blocked. Therefore, while the high-pressure fuel supply48, the control chamber34, the fuel gallery54, and the fuel-receiving cavity78are in mutual fluid communication, and high-pressure fuel can flow between them, fluid communication with the low-pressure passage66to the drain is broken. Thus, the fuel in the control valve36cannot drain to low pressure. Fuel in the control valve36, and hence in the control chamber34, therefore remains at a relatively high pressure.

It will be appreciated that when the control valve36is in the closed position, the fuel gallery54and fuel-receiving cavity78contain high-pressure fuel. The high-pressure fuel in the fuel gallery54exerts an inward radial force on the valve stem46and an outward radial force on the wall56of the bore42in the region of the fuel gallery54. This causes elastic distortion of the valve stem46and the wall56of the bore42, which tends to decrease an external diameter of the valve stem46and increase an internal diameter of the bore42.

As has been described, although this elastic distortion relaxes away from the fuel gallery54, it does so gradually, and residual strains exists in the valve stem46and the wall56of the bore42. Thus, at the valve body72of the valve stem46, elastic strain tends to decrease the external diameter of the valve stem46and increase the internal diameter of the bore42, thereby increasing the clearance74between the valve stem46and the bore42in the region of the valve body72. This strain is largest close to the fuel gallery54, and gradually decreases towards the lowermost end80of the valve stem46.

However, when the control valve36is in the closed position, the fuel-receiving cavity78in the valve stem46also receives high-pressure fuel from the fuel gallery54, via the inlets88. This high-pressure fuel in the fuel-receiving cavity78exerts an outward radial force on the annular wall76of the valve body72of the valve stem46.

The outward radial force exerted by the fuel in the fuel-receiving cavity78deforms the annular wall76of the valve stem46outwardly. Specifically, the outward radial force elastically deforms the annular wall76so as to increase its external dimensions appreciably. Since the annular wall76is of uniform thickness, and the pressure provided by the fuel is uniform, the effect of the distortion is to appreciably increase an external diameter of the valve body72of the valve stem46.

The tendency of high-pressure fuel in the fuel gallery54to decrease the external diameter of the valve stem46and increase the diameter of the bore42is therefore counteracted by the tendency of the fuel in the fuel-receiving cavity78to increase the external diameter of the valve stem46. The net result is that, in a control valve36according to the invention, the clearance74between the valve stem46and the bore42of the control valve36is not appreciably increased by the presence of the high-pressure fuel. The risk of leakage between the valve stem46and the bore42in a control valve36according to the invention is therefore significantly lower than in known control valves.

As previously mentioned, the control valve36spends the majority of its life in the closed position. Thus, reducing the tendency for leakage between the valve stem46and the wall56of the bore42when the control valve36is in the closed position has a significant impact on the total operational leakage of the control valve36.

Referring now toFIG. 4, when the control valve36is in the open position, the valve stem46is spaced apart from the valve seating58, such that flow of fuel between the fuel gallery54and the low-pressure drain66is permitted. Thus, when the control valve36is in the open position, the high-pressure fuel supply48, the control chamber34, the fuel gallery42, the fuel-receiving cavity78and the low-pressure fuel drain66are in mutual fluid communication. Fuel can therefore flow to the low-pressure fuel drain66, and the fuel in the control valve36and in the control chamber34is at a relatively low pressure.

Thus, when the control valve36is in the open position, fuel in the fuel gallery42and the fuel-receiving cavity78is at relatively low pressure. The fuel in the fuel gallery42exerts a relatively low outward radial force on the wall56of the bore42and a relatively low inward force on the valve stem46, causing a relatively low level of distortion. Similarly, the fuel in the fuel-receiving cavity78exerts a relatively low outward force on the annular wall76of the valve stem46(in comparison with when the control valve36is closed), causing a correspondingly low distortion that counteracts the distortion provided by the fuel in the fuel gallery42.

It will be appreciated that the provision of the cross-drillings88between the fuel gallery54and the fuel-receiving cavity78means that the fuel pressure in the fuel gallery54is always at equilibrium with the fuel pressure in the fuel-receiving cavity78. In this way, the fuel pressure in the fuel-receiving cavity78varies with the pressure in the fuel gallery54, and hence varies depending on whether the control valve36is open or closed. The fuel in the fuel-receiving cavity78will therefore only distort the annular wall76of the valve body to the extent that is necessary to maintain a substantially constant clearance74between the valve body72of the valve stem46and the wall56of the bore42.

This equilibrium between the fuel gallery54and the fuel-receiving cavity78guards against a situation where, for example, the pressure in the fuel-receiving cavity78is too low, so that the external diameter of the valve stem46is not sufficiently increased, and the clearance74between the valve stem46and the bore42remains large enough to cause significant leakage. The equilibrium also guards against a situation where the pressure in the fuel-receiving cavity78is too high, such that the external diameter of the valve stem46increases too much, introducing an unacceptable degree of friction between the valve stem46and the wall56of the bore42, or even causing damage to the valve stem46and/or the housing44.

In use of the injection valve30, the control valve36is initially biased in the closed position by a spring (not shown), which mechanically biases the valve stem46against the valve seating58. In this closed position, fluid communication between the control chamber34and the low-pressure drain66is broken. The high-pressure fuel supply48, the control chamber34, the fuel gallery54and the fuel-receiving cavity78are in mutual fluid communication, such that they all contain high-pressure fuel.

High-pressure fuel in the fuel gallery54exerts relatively high radial forces on the wall56of the bore42and the valve stem46that tends to increase the clearance74between the wall56of the bore42and the valve body72of the valve stem46; however, this is compensated for by high-pressure fuel in the fuel-receiving cavity78that exerts a relatively high outward radial force on the annular wall76of the valve body72of the valve stem46, thereby increasing its external diameter by a relatively large amount. In this way, the provision of the fuel-receiving cavity78reduces the clearance74and guards against leakage between the wall56of the bore42and the valve stem46when the control valve36is in the closed position.

With the control valve36in the closed position, high-pressure fuel in the control chamber34exerts a relatively high closing force on the valve pin38of the injection nozzle32, thereby retaining the valve pin38against the injection valve seating. The injection nozzle32is therefore retained in a closed position, such that fuel is prevented from entering the engine cylinder.

To trigger an injection event, the actuator70is actuated by application of an electric current or voltage. The actuator70acts against the bias of the spring to lift the valve stem46of the control valve36upwardly, into the open position, such that the valve stem46is lifted out of engagement with the valve seating58. In the open position, the high-pressure fuel supply, the control chamber34, the fuel gallery54, the fuel-receiving cavity78and the low-pressure drain are arranged in mutual fluid communication, such that they all contain fuel at relatively low pressure.

In the open position, the relatively low-pressure fuel in the fuel gallery54exerts relatively low radial forces on the wall56of the bore42and the valve stem46that tend to increase the clearance74between the wall56of the bore42and the valve body72of the valve stem46by only a relatively small amount. This is compensated for by relatively low-pressure fuel in the fuel-receiving cavity78that exerts a relatively low outward radial force on the annular wall76of the valve body72of the valve stem46, thereby increasing its external diameter by a correspondingly small amount. In this way, the provision of the fuel-receiving cavity78guards against leakage between the wall56of the bore42and the valve stem46when the control valve36is in the open position.

With the control valve36in the open position, relatively low-pressure fuel in the control chamber34exerts a relatively low closing force on the valve pin38of the injection nozzle32. This low closing force is insufficient to retain the valve pin38against the injection valve seating. The injection nozzle32is therefore moved to an open position, such that fuel is injected into the engine cylinder.

When the injection event is complete, the electric current or voltage is removed from the actuator70of the control valve36. The spring biases the control valve36into the closed position once more, and the control valve36remains in the closed position until the next injection event.

FIGS. 5 and 6illustrate a second embodiment of a control valve36according to the invention, in which like numbers correspond to like parts. In this alternative embodiment, the control valve36comprises a valve member46in the form of a valve pin, which is received in a bore provided in a housing44, the housing being in the form of a valve stem44. The valve stem44is located within a further bore92that is provided in a valve block94. The valve stem44is slidable within the further bore92, such that the valve stem44is moveable with respect to the valve pin46and with respect to the valve block94.

In contrast to the first embodiment, in which the valve member46(in the form of a valve stem) is moveable while the valve housing44(in the form of a control valve block) remains stationary, in the second embodiment, the valve member46(in the form of the valve pin) remains stationary, while the valve housing44(in the form of the valve stem) is moveable. It will be appreciated that, in both embodiments, relative sliding movement takes place between the valve member46and the housing44. Said another way, in both embodiments, at least one of the valve member46and the housing44is moveable with respect to the other.

Referring still toFIGS. 5 and 6, movement of the valve stem44is actuated by means of an actuator70that is arranged at an uppermost end of the valve stem44. The valve stem44is biased downwardly into engagement with the valve seating58in a first or closed position by the spring96, illustrated inFIG. 5, and the actuator70acts against the spring96to move the valve stem44upwardly into an open position, illustrated inFIG. 6.

At the lowermost end of the control valve36, remote from the actuator70, the further bore92comprises a valve seating58in the form of a frustoconical surface60. In the region of the valve seating58, the valve stem44comprises a corresponding frustoconical surface62, of complementary shape and dimensions, such that the valve stem44is engageable with the valve seating58.

Below the valve seating58, a high-pressure fuel supply48opens into the bore42in the valve stem44. The high-pressure fuel enters the valve stem44at its lowermost end via an inlet97. Above the valve seating58, a low-pressure fuel passage66allows fuel from the bore42to flow to a low-pressure drain (not shown). As illustrated inFIG. 7, a low-pressure path99additionally exists between the valve stem44and the valve block94. To provide this low-pressure fuel path99, the valve stem44may be of tri-lobe configuration, as illustrated inFIG. 7, or an outer surface of the valve stem44may be provided by linear or planetary grooves100, as illustrated inFIGS. 8aand8b.

In this way, the valve seating58is intermediate the high-pressure fuel supply inlet96and the low-pressure fuel passage66. Thus, when the valve stem44is arranged in the closed position, shown inFIG. 5, fluid communication between the high-pressure fuel supply48and the low-pressure drain is broken. When the valve stem44is arranged in the open position, shown inFIG. 6, fluid communication between the high-pressure fuel supply48and the low-pressure drain is open.

Considering now the valve pin46in further detail, a lower portion of the valve pin46defines a valve body72of the valve pin46, which is substantially cylindrical in shape, and of substantially constant radius. The valve body72is arranged in a close sliding fit within the bore92of the valve stem44, and a small clearance74is provided between the valve body72of the valve pin46and the wall56of the surrounding bore42, so as to allow for sliding movement between the valve pin46and the surrounding bore92. Typically, the clearance74is of the order of a few microns.

The valve pin46is provided with a fuel-receiving cavity78that extends through the length of the valve body72. Specifically, the valve body72of the valve pin46comprises an annular wall76surrounding the fuel-receiving cavity78. In this embodiment, the fuel-receiving cavity78opens onto a lowermost end of the valve pin46, such that the end of the valve pin46defines a fuel gallery in the form of an inlet98through which fuel can flow into the fuel-receiving cavity78. In this way, the fuel-receiving cavity78is in fluid communication with the bore42of the valve stem44.

When the valve stem44is arranged in a closed position, shown inFIG. 5, the bore42of the valve stem44and the fuel-receiving cavity78both contain high-pressure fuel. The high-pressure fuel in the bore42exerts an outward radial force on the wall56of the bore42, and an inward radial force on the valve pin46, tending to increase the clearance74between them in a manner that has already been described with reference to the first embodiment. However, the high-pressure fuel in the fuel-receiving cavity78exerts an outward radial force on the annular wall76of the valve pin46, increasing its external diameter and thereby counteracting the tendency for the clearance74to increase.

When the valve stem44is arranged in an open position, shown inFIG. 6, the bore42of the valve stem44and the fuel-receiving cavity78both contain low-pressure fuel. The low-pressure fuel in the bore42exerts a relatively small outward radial force on the wall56of the bore42, and a relatively small inward radial force on the valve pin46, tending to increase the clearance74between them to a lesser degree. The low-pressure fuel in the fuel-receiving cavity78exerts a relatively low outward radial force on the annular wall76of the valve pin46, increasing its external diameter to a correspondingly lesser extent, and thereby counteracting the tendency for the clearance74to increase.

It will be appreciated that, in a manner similar to that described with regard to the first embodiment, the provision of the inlet98between the bore42and the fuel-receiving cavity78means that the fuel pressure in the bore42is always at equilibrium with the fuel pressure in the fuel-receiving cavity78. In this way, the fuel pressure in the fuel-receiving cavity78varies with the pressure in the bore42, and hence varies throughout the cycle of opening and closing of the control valve36. The fuel in the fuel-receiving cavity78will only distort the annular wall76of the valve body72to the extent that is necessary to maintain a constant clearance74between the valve body72of the valve stem44and the wall56of the bore42.

Thus, in both embodiments, the invention provides effective means for reducing the static leak between the valve member46and the valve housing44, thereby reducing fuel and energy loss, and increasing the efficiency of the engine.

Although in the first embodiment described above the valve body72of the valve stem44is integral with a remaining portion of the valve stem44, and the cavity78is a drilled cavity plugged by an insert82, it will be appreciated that the cavity may be introduced into the valve stem by any suitable means. For example, the valve body may be formed separately from the remainder of the valve stem in the form of a cylinder that is closed at one end. The valve body may then be attached to the remainder of the valve stem, for example by welding, so as to provide an internal cavity in the valve stem.

Fuel need not necessarily be received into the bore via a fuel gallery, but may be received into the bore by any suitable arrangement of flow passages within the control valve. The fuel in any of the flow passages in the control valve may act to the increase the radial clearance between the valve body and the wall of the bore. If the fuel gallery is present, it may be of any suitable shape, and arranged at any suitable location.

Although in the embodiments described the fuel supply is a high-pressure fuel supply, it will be appreciated that distortion effects will be also present at lower fuel pressures, and thus the fuel may be supplied at any pressure above ambient pressure.

It should be appreciated that various other modifications and improvements can be made without departing from the scope of the invention as defined in the claims.