Patent Description:
In known fuel pump assemblies, a return spring may be used to maintain contact between a plunger and a drive arrangement that drives the pumping action of a pump plunger through a pumping cycle. During the pumping cycle fuel is drawn into a pump chamber at low pressure and is delivered, once pressurised, to the downstream parts of a fuel injection system (e.g. a common rail). The drive arrangement is driven by means of an engine-driven drive shaft which typically carries a cam. Contact between the plunger and the drive arrangement may entail preserving engagement between a cam follower and the cam, for example. In pump variants employing a slipper-tappet mechanism as the drive arrangement, the return spring must maintain contact between a tappet and a rider. Contact must be maintained through the full pumping cycle, on both a pumping stroke of the plunger between bottom-dead-centre (BDC) and top-dead-centre (TDC) and on a return stroke of the plunger between TDC and BDC. The return spring must be capable of providing a return force that is sufficient to maintain contact between the plunger and its drive arrangement for all operating conditions of the pump assembly.

One problem which exists in pump assemblies of this type is that the return spring is susceptible to dynamic effects and, as the camshaft speed is increased, the force that is required to maintain contact between the plunger and the drive arrangement is increased dramatically compared to lower camshaft speeds. This requires the spring to be designed for the highest speeds that the pump could ever be driven at, as determined by the manufacturer's specification. In practice, these "overspeeds" are rarely, and sometimes never, encountered in use. In this respect, in a context of increasing demands on fuel pump designs in terms of higher pump speeds and stroke volumes, it is becoming increasingly difficult to meet the varied design constraints of providing the required dynamic force at TDC, whilst maintaining fatigue resistance over a large number of cycles. Document <CIT> discloses a high-pressure fuel pump comprising a spring system used to maintain a contact force between a cam follower and a plunger.

According to an aspect of the invention, there is provided a fuel pump assembly for an internal combustion engine, the fuel pump assembly comprising a plunger arranged to reciprocate within a plunger bore under the influence of a drive arrangement driven by means of a drive shaft, to perform a pumping cycle comprising a pumping stroke and a return stroke, the pumping stroke comprising movement of the plunger from a bottom dead centre (BDC) position to a top dead centre (TDC) position to pressurise fuel within a pump chamber, and the return stroke comprising movement of the plunger from the TDC position to the BDC position. The fuel assembly includes a spring assembly including a return spring configured to apply a return force to the plunger to effect the return stroke, wherein the return spring is cooperable, at a first end, with a first spring member coupled to the plunger and movable at a first speed dependent on the speed of rotation of the drive shaft and, at a second end, with a second spring member which is movable at a damped speed relative to the first speed so that the return spring has a variable stroke length depending on the speed of rotation of the drive shaft.

The present invention provides an advantage over known pump assemblies where the return spring has to be selected to ensure that, even for the highest and uncommon speeds of rotation of the engine, a sufficient return force is applied to ensure the plunger and the various components of the drivetrain are retained in contact with one another. As the speed of rotation of the engine increases, the force required to maintain contact between components of the drivetrain, through which drive is imparted to the plunger on rotation of the shaft, also increases and so, even though the highest of speeds are only achieved rarely, the spring must be capable of providing a high return force even when engine speeds are lower. The effect of this is that return springs are 'overdesigned' and encounter an unnecessarily high stress range for many circumstances. The present invention avoids this problem by providing a spring assembly which has a variable stroke length (i.e. provides a variable return force), depending on engine speed, so that only at the highest engine speeds is the spring at maximum compression. In this way spring life is improved considerably. The variable stroke length is achieved by damping movement of one end of the return spring, relative to the other end, by means of a damper arrangement.

The spring assembly thus typically includes a damper arrangement which acts on the second spring member to determine the damped speed of movement, with the extent of damping depending on the speed of rotation of the drive shaft.

By way of example, the damper arrangement includes a damper chamber for receiving a fluid which applies a damping force to the return spring to limit the stroke length of the return spring depending on the speed of rotation of the drive shaft.

In one embodiment, the second spring member is a spring retainer member which receives the second end of the return spring.

The spring retainer member may take the form of a shroud for receiving the second end of the return spring.

The spring retainer member may be at least partially received within the damper chamber. Typically, for example, one or more dead coils of the return spring may be received in the spring retainer member in a press fit or interference fit, or by securing the or each dead coil by means of a fastener. For example, a surface of the spring retainer member may be exposed to the contents of the damper chamber (e.g. fluid or gas).

In some embodiments, the damper arrangement may include at least one inlet and at least one outlet for allowing fluid to flow into and out of the damper chamber, respectively.

A clearance is defined between the movable member and a wall of the damper chamber to allow fluid to flow out of the damper chamber. This may be provided in addition to the aforementioned outlet.

In other embodiments, the damper chamber may be a sealed chamber filled with fluid or gas.

The fluid within the damper chamber may conveniently be lubricating oil, such as that used to lubricate other parts of the drivetrain for the pump assembly/engine.

Alternatively, the damper chamber may be filled with gas.

In other embodiments the spring assembly may include an additional return spring which has a fixed stroke length which does not vary depending on the speed of rotation of the drive shaft.

Also, the fuel pump assembly may comprise a tappet assembly which acts as the drive assembly.

The above and other aspects of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:.

It should be understood that throughout this description, references to upper and lower ends of components, and other such directional or relative references are made in relation to the orientations of the components shown in the Figures but are not intended to be limiting.

<FIG> and <FIG> show a known common rail fuel pump assembly <NUM> ("the pump" hereinafter) for use in a compression-ignition internal combustion engine. The pump <NUM> is an in-line pump arrangement comprising a main pump housing <NUM> and first and second pump elements, <NUM>, <NUM> respectively, which are driven by means of a common, engine-driven drive shaft <NUM> which extends through the main pump housing <NUM> and rotates at a speed proportionate to the speed of the engine. A low pressure suction pump <NUM> is mounted to the side of the main pump housing <NUM> to deliver relatively low pressure fuel to the pump <NUM>. The drive shaft <NUM> carries first and second cam drive arrangements, <NUM>, <NUM> respectively, which are either mounted on, or form an integral part of, the drive shaft <NUM>. The drive shaft <NUM> is reciprocally connected to each of the pump elements <NUM>, <NUM> via a respective intermediate drive assembly in the form of a first or second tappet assembly, referred to generally as <NUM>, <NUM>. Each tappet assembly <NUM>, <NUM> includes a respective tappet, <NUM>, <NUM>. As can be seen most clearly in <FIG>, and describing only the first tappet assembly <NUM>, the tappet <NUM> is coupled to a roller assembly consisting of a pair of hollow rollers comprising an outer roller <NUM> and an inner roller <NUM>. The outer roller <NUM> rolls over the surface of the associated cam arrangement <NUM>. A pin <NUM> secures the tappet <NUM> to the associated roller assembly <NUM>, <NUM> and the inner roller <NUM> rolls on the pin <NUM> within the outer roller <NUM>.

It will be appreciated that this arrangement of the tappet assembly <NUM>, <NUM> and the roller assembly <NUM>, <NUM> is just one example of how the drive assembly for a plunger is driven through rotation of the drive shaft <NUM>.

It is helpful to consider the operation of the pump assembly in <FIG> and <FIG> to understand the technical problem which the invention sets out to address. Two separate pump elements in the form of the first and second pumping plungers, <NUM> and <NUM>, are shown in <FIG> and <FIG>, but for the purpose of the following description only one of the plungers <NUM> will be described in detail.

The first pumping plunger <NUM> extends through a substantially tubular turret <NUM> which forms a part of a pump head housing <NUM> mounted to the main pump housing <NUM>. The turret <NUM> downwardly extends from the pump head housing <NUM> and defines a substantially cylindrical plunger bore <NUM>, the turret <NUM> projecting into the body of the main pump housing <NUM> and terminating in a lower turret surface <NUM>. The plunger bore <NUM> is configured to receive the plunger <NUM>, the lower end of which extends from the turret <NUM>.

At the uppermost end of the plunger <NUM> (in the illustration shown), the plunger <NUM> defines, together with the bore <NUM> in the pump head <NUM>, a pump chamber <NUM> (as shown in <FIG>) for receiving fuel to be pressurised by the plunger <NUM> when the pump assembly is in use. Likewise, the second plunger <NUM> has an associated pump chamber <NUM>.

The pump chamber <NUM> is fitted with an inlet valve <NUM> and an outlet valve (not shown) to control, respectively, fuel flow into and out of the pump chamber <NUM> through the pump cycle. The configurations of such valve assemblies are well known in the art and, given that they are not central to the invention, will not be described in detail here, save that they are used to control flow of the fuel from a pump inlet <NUM> through to the pump chamber <NUM> and from the pump chamber <NUM> through to a pump outlet <NUM> to the common rail (not shown). Each valve includes a spring (not identified), which acts to close the valve to prevent the passage of fuel therethrough.

The plunger <NUM> is moveable between a bottom-dead-centre positon (hereinafter, "BDC position") and a top-dead-centre position (hereinafter, "TDC position"), defining a pumping stroke, and between the TDC position and the BDC positon, defining a return stroke. A pumping stroke followed by a return stroke defines a pumping cycle for the plunger <NUM> and pump assembly <NUM>. <FIG> shows the plunger <NUM> on the right hand side of the assembly with the plunger at the BDC position, while the plunger <NUM> on the left hand side of the assembly is moving towards TDC position.

A spring abutment member in the form of an annular spring plate <NUM> forms a collar around the plunger <NUM> in a lower region of the plunger and is attached thereto such that their respective motions are coupled together. The spring plate <NUM> defines an abutment surface <NUM> for one end of a plunger return spring ("return spring" hereinafter) <NUM> in the form of a helical coil spring. Accordingly, the spring plate <NUM> acts as a seat member for the return spring <NUM>. The other end of the return spring <NUM> engages a fixed abutment surface defined by the underside of the pump head housing <NUM>. The return spring <NUM> is thus permanently engaged with both the spring plate <NUM> and the pump head housing <NUM>.

When the plunger is in the TDC position (as for the left hand plunger <NUM> in <FIG>), both the inlet and outlet valves, <NUM>, <NUM> to the respective pump chamber <NUM> are closed, thereby preventing fuel from flowing into or out of the pump chamber <NUM>. As the drive shaft <NUM> rotates and the tappet assembly <NUM> rides over the cam <NUM>, the return spring <NUM> acts on the plunger <NUM> to urge the plunger <NUM> away from the TDC position, through the return stroke to the BDC position. This causes an increase in the volume of the pump chamber <NUM>, decreasing the pressure within it and establishing a pressure drop across the inlet valve <NUM>. This pressure drop allows the inlet valve <NUM> to open against the force of the inlet valve spring and fuel enters the pump chamber <NUM> until the pressure across the inlet valve <NUM> equalises, causing it to close. This typically occurs just after the plunger <NUM> reaches the BDC position. During the return stroke the fuel is supplied to the pump chamber <NUM> at a pressure of around <NUM> bar (<NUM> kPa). Throughout the return stroke the return spring <NUM> serves to ensure that contact is maintained between the various drivetrain components, including maintaining contact between the plunger <NUM> and the tappet assembly <NUM> and between the tappet assembly <NUM> and the cam <NUM>.

Once the plunger <NUM> reaches the BDC position, it begins the pumping stroke as the drive shaft <NUM> continues to rotate. During the pumping stroke fuel in the pump chamber <NUM> is pressurised as the volume of the pump chamber <NUM> is reduced with the advancing plunger <NUM>. During this phase of operation the inlet valve <NUM> of the pump chamber <NUM> is caused to close due to the pressure drop across it and the pressure in the pump chamber <NUM> is increased, typically to at least <NUM> bar (<NUM> MPa) and sometimes as high as <NUM> bar (<NUM> MPa). A pressure drop is created across the outlet valve (not shown), allowing it to open against the force of the outlet valve spring and fuel exits the pump chamber <NUM> and flows into the common rail fuel volume. As the plunger <NUM> reaches the TDC position, the pressure across the outlet valve (not shown) equalises, causing it to close.

Throughout the pumping stroke the force from the return spring <NUM> continues to act through the drivetrain components to ensure contact is maintained between the tappet <NUM>, the shoe <NUM> and the cam <NUM>, whilst importantly minimising slippage between the shoe <NUM> and the cam <NUM>. Similarly, in pumps incorporating a slipper-tappet mechanism as part of the drive arrangement, the return spring <NUM> must maintain sufficient force between the tappet <NUM> and a cam rider, or 'slipper', to avoid rotation of the rider relative to the housing.

One problem which occurs in the aforementioned pump assembly is that the helical compression springs which are used for the return spring <NUM> are highly susceptible to dynamic effects. As the speed of rotation of the drive shaft <NUM> increases, the force which is required to maintain contact between the drivetrain components increases drastically. This means that the return spring <NUM> must be designed for the highest speed of operation that the pump could ever be subjected to, as determined by the engine manufacturer's specification. In practice, these "overspeeds" are rarely, and sometimes never, encountered in use. As the spring force is proportional to the stress in the return spring <NUM>, the stress range for the return spring <NUM> increases with the stroke of the spring: the "stroke length" of the return spring is defined, for any given stroke of the plunger, as the extension of the spring from its minimum length of extension (when fully compressed at TDC) to its maximum length of extension (when fully expanded at BDC). In other words, when the stroke of the return spring <NUM> is greater, the stresses in the return spring <NUM> are higher.

In the existing pump shown in <FIG> and <FIG>, the return spring <NUM> must travel through its full stroke for all speeds of the drive shaft <NUM>. These strokes of the return spring <NUM> contribute to a reduction in the fatigue life of the return spring <NUM>: often the return spring <NUM> dictates the overall lifetime and reliability of the whole pump assembly. However, at lower speeds the requirement for the force from the return spring <NUM> is lower, so it would be possible for the spring to have a reduced stroke in the lower-speed range, which would reduce spring stresses for at least some circumstances of pump operation.

The present invention solves this problem through the pump assembly shown in and described with reference to <FIG>.

Referring to <NUM>, an embodiment of the pump assembly of the invention includes similar parts to those described previously, with reference to <FIG> and <FIG>, with the exception of the arrangement of the return spring and how this functions. Similar parts to those described previously will be referred to with like reference numerals, increased by <NUM>. As described previously for <FIG> and <FIG>, the plunger <NUM> carries a first spring member in the form of a spring abutment plate <NUM> at its lower end with which one end <NUM> of the return spring <NUM> is engaged. At the other end <NUM> of the return spring <NUM>, remote from the spring plate <NUM>, the spring <NUM> is received within a second spring member in the form of a spring retainer member <NUM>. The spring retainer member takes the form of an annular shroud <NUM>. The shroud <NUM> has an open end which opens downwardly in the illustration shown, towards the spring abutment plate <NUM>, and a closed end which defines an internal abutment surface <NUM>. The shroud <NUM> defines an internal receiving volume <NUM>, with the end <NUM> of the spring <NUM> being received within the receiving volume <NUM> and being in abutment with the internal abutment surface <NUM>. The return spring <NUM> is therefore compressed between the abutment surface <NUM> of the spring plate <NUM> and the internal abutment surface <NUM> of the shroud <NUM>.

The shroud <NUM> defines a movable abutment member for the return spring <NUM> and forms a part of a damping arrangement, referred to generally as <NUM>, further including a hollow annular member <NUM>. The annular member <NUM> is carried by the turret <NUM> on the pump head and is open at one end and closed at the other, with the open end facing the spring <NUM>. A damper chamber <NUM> is defined within the annular member <NUM> and is defined by a cylindrical wall of the annular member <NUM>. The shroud <NUM> is at least partially received within the annular member <NUM> in a slidable manner: the extent to which the shroud <NUM> is received in the damper chamber <NUM> depends on engine speed as described further below. The shroud <NUM> therefore forms a 'plug' at the open end of the annular member <NUM>, with the position of the shroud <NUM> within the annular member <NUM> being variable. The end <NUM> of the return spring <NUM> is securely coupled to the shroud <NUM> so that neither one can move relative to the other. For example, the end <NUM> of the spring <NUM> may be received within the shroud <NUM> in an interference fit or the dead coils (i.e. the coils which are not active) at the end <NUM> of the spring may be fastened inside the shroud <NUM> using fasteners to attach the shroud <NUM>.

A clearance gap <NUM> exists between the inner surface of the wall of the damper chamber <NUM> and the outer surface of the shroud <NUM> to allow minimal leakage of fluid from the damper chamber <NUM>, as described further below. Different positions for the shroud <NUM> within the damper chamber <NUM> can also be seen by comparing <FIG> and <FIG>, discussed below.

Referring to <FIG> (in which the plunger and the attached components are hidden), the damper chamber <NUM> is filled with lubricating fluid in the form of engine oil which is delivered to the chamber <NUM> via inlet or feed ports <NUM> provided in the underside of the pump head housing <NUM> (the inlet ports are not visible in <FIG>) when the volume of the pump chamber <NUM> is expanding through the return stroke. Two inlet ports <NUM> are provided in the underside of the pump head housing <NUM>, at diametrically opposed positions around the circumference of the turret <NUM>. A plurality of outlet ports <NUM> are provided in the wall of the damper chamber <NUM> to allow lubricating fluid within the damper chamber <NUM> to be ejected from the damper chamber <NUM> when the chamber volume is compressed during the pumping stroke.

In <FIG>, only two of the outlet ports <NUM> are visible in the cross section, whereas in <FIG> all four of the outlet ports <NUM> are visible. In practice the number of inlet and outlet ports <NUM>, <NUM> may vary, depending on the particular configuration of the pump assembly and the material properties of the fluid within the damper chamber <NUM>. In one embodiment, the lubricating fluid takes the form of engine oil which is used to lubricate other parts of the drivetrain for the pump assembly. This provides a particularly convenient solution for routing the lubricating fluid into and out of the damper chamber <NUM>, relative to other parts of the pump assembly <NUM> which require lubrication. In other embodiments, other lubricating fluids may be used, or even gas, as described further below.

As described above, the plunger <NUM> undergoes pumping cycles in use, each cycle comprising a pumping stroke and a return stroke. <FIG> and <FIG> shows the pump assembly <NUM> with the plunger at the TDC position at the end of the pumping stroke. The force applied to the spring plate <NUM> by the return spring <NUM> varies approximately sinusoidally with rotation of the cam <NUM>, with extremes of the force being applied at the TDC and BDC positions and with maximum force being provided at the TDC position when the return spring <NUM> is maximally compressed (as shown in <FIG> and <FIG>).

Referring to <FIG>, the drive shaft <NUM> is rotating at a relatively low speed and, as the plunger <NUM> moves up the plunger bore <NUM> during the pumping stroke towards the TDC position, the spring plate <NUM>, being affixed to the plunger <NUM>, moves towards the lower surface <NUM> of the turret <NUM>. The spring plate <NUM> moves at a first speed dependent on the speed of rotation of the drive shaft <NUM>. As the spring plate <NUM> moves towards the lower surface <NUM> of the turret <NUM>, the return spring <NUM> is progressively compressed. The shroud <NUM>, containing the upper end <NUM> of the return spring <NUM>, is displaced upwardly into the damper chamber <NUM>, causing fluid to be 'squeezed out' or displaced through the outlet ports <NUM> and through the clearance <NUM> defined between the shroud <NUM> and the chamber wall.

Because the plunger <NUM> is only moved upwards relatively slowly (with the drive shaft rotating at a relatively low speed), the volume of fluid displaced from the damper chamber <NUM> through the outlet ports <NUM> is relatively high, with a relatively long time being available for fluid to be displaced during the pumping stroke (at lower speeds). The force due to remaining fluid within the damper chamber <NUM>, which acts against the moving shroud <NUM>, and hence the return spring <NUM>, is therefore relatively low throughout the pumping stroke so that the return spring <NUM> compresses relatively little. The speed of movement of the shroud <NUM> in this phase is damped, relative to the speed of movement of the lower end <NUM> of the spring <NUM> at the spring plate <NUM>, but with only relatively little damping. As a result of this limited compression of the spring <NUM>, the force due to the return spring <NUM> which acts through the spring abutment plate <NUM> and the drivetrain components (the tappet assembly <NUM>, the roller assembly <NUM>, <NUM> and the pin <NUM>), and onto the cam <NUM>, is relatively low. Nevertheless, as the speed of rotation of the drive shaft <NUM> is relatively low, the force is still sufficient to retain the components of the drive train in contact with one another. In other words, the return force applied by the return spring <NUM>, which acts through the spring plate <NUM>, to the tappet assembly <NUM> and through the roller assembly (not identified in <FIG>) to the cam, is sufficient to ensure that all of these parts remain in contact with neighbouring parts as the plunger retracts through the return stroke, and so there is no impact damage caused due to parts "lifting off", or separating and subsequently reconnecting.

Referring to <FIG>, at higher speeds of rotation of the drive shaft <NUM>, the plunger is driven upwardly at a higher speed so that the volume of fluid within the damper chamber <NUM> which, during the return stroke, is able to exit the outlet ports <NUM> and through the clearance between the shroud <NUM> and the chamber wall, is reduced compared to lower speeds as there is a relatively short time for fluid to be displaced from the damper chamber <NUM>. As a result, the force of the remaining fluid within the damper chamber <NUM>, which opposes the moving shroud <NUM>, is higher so that the upward displacement of the shroud <NUM> is less than for lower speeds of drive shaft rotation, with the shroud moving at a more heavily damped speed compared to the lower speed scenario of <FIG>. The return spring <NUM> is therefore compressed by a relatively large amount for higher speeds of drive shaft rotation, compared to the extent of compression of the return spring <NUM> for lower speeds. As a result, the force of the return spring <NUM> which acts through the spring abutment plate <NUM> and the drivetrain components (the tappet assembly <NUM>, the roller assembly <NUM>, <NUM> and the pin <NUM>), and onto the cam, is relatively high for higher engine speeds. Thus, as for the lower speed scenario, any impact damage of parts of the drive train is avoided.

The extent to which the return spring <NUM> is compressed is often referred to as the "stroke" of the spring, being a measure of the difference between the length of the spring at the BDC position (when fully expanded) compared to its length at the TDC position (when fully compressed for that stroke). It will be appreciated that the speed of movement of the shroud <NUM>, which moves at a damped speed relative to movement of the lower end <NUM> of the spring <NUM>, is dependent or set by the extent of the fluid that is displaced from the damper chamber <NUM>. In practice this damping effect, or the damping force applied to the shroud <NUM> due to the fluid in the chamber <NUM>, is dependent on the square of the velocity (V) of the moving drive assembly <NUM> (the well known "drag equation") so that there is an increasing damping effect on the shroud <NUM> as the speed of rotation increases, thereby causing the return spring <NUM> to be compressed by a greater amount for higher speeds (and thus providing a higher return force).

Although at lower speeds the return spring <NUM> is compressed less at the TDC position, and the loading of the abutment plate <NUM> onto the tappet <NUM> and other components of the drive train is reduced through the return stroke, because the speed of rotation of the shaft is lower the reduced force imparted by the return spring <NUM> is still sufficient to ensure contact is maintained between the drive train parts. However, a benefit is obtained because the return spring <NUM> is compressed to a lesser amount at the TDC position, dependent on speed of cam rotation, compared to the situation where the maximum compression is achieved for every stroke (regardless of the speed of cam rotation). The reduction in the stroke of the return spring <NUM> for lower speeds of rotation of the drive shaft means there is a lower alternating stress within the return spring <NUM> depending on engine speed, yielding a higher fatigue life for the spring.

At the TDC position, the volume of the pump chamber <NUM> is at its minimum volume and fuel pressure within the pump chamber <NUM> is pressurised to a sufficiently high level to cause the pump outlet valve to open, delivering pressurised fuel to the downstream parts of the fuel injection system. Through the subsequent return stroke, the return spring <NUM> applies a return spring force to the plunger <NUM>, via the abutment plate <NUM>, which serves to drive the plunger <NUM> towards the BDC position, being a reduced force when the speed of rotation of the drive shaft is lower. Through the return stroke, the volume of the pump chamber <NUM> is expanded so that fuel at relatively low pressure is drawn into the pump chamber <NUM> through the inlet valve (<NUM> in <FIG>), ready for pressurisation in the subsequent pumping stroke.

As the plunger <NUM> is withdrawn from the plunger bore <NUM> during the return stroke there is a continual supply of lubricating fluid into the damper chamber <NUM> through the inlet ports <NUM>, and the ejection of fluid through the outlet ports <NUM>, and through the clearance between the shroud <NUM> and the chamber wall, eases as the shroud <NUM> is drawn downwards to increase the volume of the damper chamber <NUM>.

It will be appreciated that in order to ensure there is a sliding fit between the shroud <NUM> and the internal wall of the damper chamber <NUM>, a small amount of leakage fluid from the damper chamber <NUM> will occur through the pump cycle through the clearance gap <NUM> between the outer surface of the shroud <NUM> and the inner surface of the annular chamber <NUM>. In another embodiment (not shown), if the clearance gap <NUM> between the damper chamber <NUM> and the outer surface of the shroud <NUM> is sized correctly, it is possible to avoid providing the outlet ports <NUM> in the wall of the damper chamber <NUM> altogether and for the outflow of fluid from the damper chamber <NUM>, during the pumping stroke, to be governed only by the rate of flow of fluid through the clearance gap <NUM>. In any case, it is important that the quantity of damper fluid within the damper chamber <NUM> is maintained through the inflow of fluid through the inlet ports <NUM>, so the clearance gap <NUM> cannot be too large.

In another embodiment of the invention, the damper chamber <NUM> need not be formed within a separate component (annular member <NUM>) consisting of the walled chamber shown in <FIG>, <FIG> and <FIG> and, instead, the damper chamber <NUM> may be formed directly within the pump head housing <NUM> by removing an annular region of the turret <NUM>. However, this may be less desirable as it requires the expensive, hard material of the turret <NUM> to be discarded after is has been formed which may not be cost effective.

In another embodiment of the invention, it is possible to remove the inlet and outlet ports <NUM>, <NUM> to the damper chamber <NUM> altogether so that the damper chamber is sealed. However, this solution would require the use of a low profile seal for the chamber <NUM> which may not be desirable.

Other embodiments of the invention may fill the damper chamber <NUM> with a gas, rather than the lubricating fluid such as the fluid which serves to lubricate other components of the drive train.

Although in the embodiments described above a spring plate <NUM> is provided as a separate component that is attached to the plunger <NUM>, it would be possible to form the spring plate <NUM> integrally with the plunger <NUM>.

In still further embodiments it is possible to provide an additional spring (not shown) to the return spring of previous embodiments, but one which has a fixed stroke length regardless of the speed of rotation of the drive shaft. In this case the additional return spring may be arranged around the turret <NUM> to engage with the underside of the pump head (i.e. a fixed abutment surface for the return spring) and a surface of the spring abutment plate <NUM>. However, the use of an additional spring adds cost to the assembly which may be undesirable.

Claim 1:
A fuel pump assembly (<NUM>) for an internal combustion engine, the fuel pump assembly (<NUM>) comprising:
a plunger (<NUM>) arranged to reciprocate under the influence of a drive assembly (<NUM>) driven by means of a drive shaft (<NUM>), to perform a pumping cycle comprising a pumping stroke and a return stroke, the pumping stroke comprising movement of the plunger (<NUM>) from a bottom dead centre (BDC) position to a top dead centre (TDC) position to pressurise fuel within a pump chamber (<NUM>), and the return stroke comprising movement of the plunger (<NUM>) from the TDC position to the BDC position;
a spring assembly including a return spring (<NUM>) configured to apply a return force to the plunger (<NUM>) to effect the return stroke, wherein the return spring (<NUM>) is cooperable, at a first end (<NUM>), with a first member (<NUM>) coupled to the plunger (<NUM>) and movable at a first speed dependent on the speed of rotation of the drive shaft (<NUM>) and, at a second end (<NUM>), with a second member (<NUM>) which is movable at a damped speed relative to the first speed so that the return spring has a variable stroke length depending on the speed of rotation of the drive shaft (<NUM>); and
a damper arrangement (<NUM>, <NUM>) which acts on the second member (<NUM>) to set the damped speed, wherein the damper arrangement includes a damper chamber (<NUM>) for receiving a fluid which applies a damping force to the return spring (<NUM>), characterized in that
a clearance gap is defined between the second member and a wall of the damper chamber to allow the fluid to flow out of the damper chamber (<NUM>).