Patent Application
20230323845
This invention relates to a fuel pump assembly for use in an internal combustion engine. In particular, the invention relates to a fuel pump assembly for use in a compression ignition (diesel) internal combustion engine.
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.
It is against this background that the invention has been devised.
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 may be 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, or as an alternative 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.
It will be appreciated that this arrangement of the tappet assembly 19, 21 and the roller assembly 27, 29 is just one example of how the drive assembly for a plunger is driven through rotation of the drive shaft 18.
It is helpful to consider the operation of the pump assembly in
The first pumping plunger 14 extends through a substantially tubular turret 28 which forms a part of a pump head housing 30 mounted to the main pump housing 12. The turret 28 downwardly extends from the pump head housing 30 and defines a substantially cylindrical plunger bore 32, the turret 28 projecting into the body of the main pump housing 12 and terminating in a lower turret surface 34. The plunger bore 32 is configured to receive the plunger 14, the lower end of which extends from the turret 28.
At the uppermost end of the plunger 14 (in the illustration shown), the plunger 14 defines, together with the bore 32 in the pump head 30, a pump chamber 36 (as shown in
The pump chamber 36 is fitted with an inlet valve 40 and an outlet valve (not shown) to control, respectively, fuel flow into and out of the pump chamber 36 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 42 through to the pump chamber 36 and from the pump chamber 36 through to a pump outlet 44 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 14 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 14 and pump assembly 10.
A spring abutment member in the form of an annular spring plate 50 forms a collar around the plunger 14 in a lower region of the plunger and is attached thereto such that their respective motions are coupled together. The spring plate 50 defines an abutment surface 52 for one end of a plunger return spring (“return spring” hereinafter) 54 in the form of a helical coil spring. Accordingly, the spring plate 50 acts as a seat member for the return spring 54. The other end of the return spring 54 engages a fixed abutment surface defined by the underside of the pump head housing 30. The return spring 54 is thus permanently engaged with both the spring plate 50 and the pump head housing 30.
When the plunger is in the TDC position (as for the left hand plunger 16 in
Once the plunger 14 reaches the BDC position, it begins the pumping stroke as the drive shaft 18 continues to rotate. During the pumping stroke fuel in the pump chamber 36 is pressurised as the volume of the pump chamber 36 is reduced with the advancing plunger 14. During this phase of operation the inlet valve 40 of the pump chamber 36 is caused to close due to the pressure drop across it and the pressure in the pump chamber 36 is increased, typically to at least 200 bar (20 MPa) and sometimes as high as 2500 bar (250 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 36 and flows into the common rail fuel volume. As the plunger 14 reaches the TDC position, the pressure across the outlet valve (not shown) equalizes, causing it to close.
Throughout the pumping stroke the force from the return spring 54 continues to act through the drivetrain components to ensure contact is maintained between the tappet 23, the shoe 24 and the cam 20, whilst importantly minimising slippage between the shoe 24 and the cam 20. Similarly, in pumps incorporating a slipper-tappet mechanism as part of the drive arrangement, the return spring 54 must maintain sufficient force between the tappet 23 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 54 are highly susceptible to dynamic effects. As the speed of rotation of the drive shaft 18 increases, the force which is required to maintain contact between the drivetrain components increases drastically. This means that the return spring 54 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 54, the stress range for the return spring 54 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 54 is greater, the stresses in the return spring 54 are higher.
In the existing pump shown in
The present invention solves this problem through the pump assembly shown in and described with reference to
Referring to 3, an embodiment of the pump assembly of the invention includes similar parts to those described previously, with reference to
The shroud 70 has an open end which opens downwardly in the illustration shown, towards the spring abutment plate 150, and a closed end which defines an internal abutment surface 76. The shroud 70 defines an internal receiving volume 72, with the end 64 of the spring 154 being received within the receiving volume 72 and being in abutment with the internal abutment surface 76. The return spring 154 is therefore compressed between the abutment surface 152 of the spring plate 150 and the internal abutment surface 76 of the shroud 70.
The shroud 70 defines a movable abutment member for the return spring 154 and forms a part of a damping arrangement, referred to generally as 80, further including a hollow annular member 82. The annular member 82 is carried by the turret 128 on the pump head and is open at one end and closed at the other, with the open end facing the spring 154. A damper chamber 84 is defined within the annular member 82 and is defined by a cylindrical wall of the annular member 82. The shroud 70 is at least partially received within the annular member 82 in a slidable manner: the extent to which the shroud 70 is received in the damper chamber 84 depends on engine speed as described further below. The shroud 70 therefore forms a ‘plug’ at the open end of the annular member 82, with the position of the shroud 70 within the annular member 82 being variable. The end 64 of the return spring 54 is securely coupled to the shroud 70 so that neither one can move relative to the other. For example, the end 64 of the spring 154 may be received within the shroud 70 in an interference fit or the dead coils (i.e. the coils which are not active) at the end 64 of the spring may be fastened inside the shroud 70 using fasteners to attach the shroud 70.
A clearance gap 88 exists between the inner surface of the wall of the damper chamber 82 and the outer surface of the shroud 70 to allow minimal leakage of fluid from the damper chamber 84, as described further below. Different positions for the shroud 70 within the damper chamber 84 can also be seen by comparing
Referring to
In
As described above, the plunger 114 undergoes pumping cycles in use, each cycle comprising a pumping stroke and a return stroke.
Referring to
Because the plunger 114 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 84 through the outlet ports 92 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 84, which acts against the moving shroud 70, and hence the return spring 154, is therefore relatively low throughout the pumping stroke so that the return spring 154 compresses relatively little. The speed of movement of the shroud 70 in this phase is damped, relative to the speed of movement of the lower end 64 of the spring 154 at the spring plate 150, but with only relatively little damping. As a result of this limited compression of the spring 154, the force due to the return spring 154 which acts through the spring abutment plate 150 and the drivetrain components (the tappet assembly 119, the roller assembly 27, 29 and the pin 24), and onto the cam 20, is relatively low. Nevertheless, as the speed of rotation of the drive shaft 18 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 154, which acts through the spring plate 150, to the tappet assembly 119 and through the roller assembly (not identified in
Referring to
The extent to which the return spring 154 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 70, which moves at a damped speed relative to movement of the lower end 64 of the spring 154, is dependent or set by the extent of the fluid that is displaced from the damper chamber 84. In practice this damping effect, or the damping force applied to the shroud 70 due to the fluid in the chamber 84, is dependent on the square of the velocity (V) of the moving drive assembly 119 (the well known “drag equation”) so that there is an increasing damping effect on the shroud 70 as the speed of rotation increases, thereby causing the return spring 154 to be compressed by a greater amount for higher speeds (and thus providing a higher return force).
Although at lower speeds the return spring 154 is compressed less at the TDC position, and the loading of the abutment plate 150 onto the tappet 123 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 154 is still sufficient to ensure contact is maintained between the drive train parts. However, a benefit is obtained because the return spring 154 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 154 for lower speeds of rotation of the drive shaft means there is a lower alternating stress within the return spring 154 depending on engine speed, yielding a higher fatigue life for the spring.
At the TDC position, the volume of the pump chamber 36 is at its minimum volume and fuel pressure within the pump chamber 36 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 154 applies a return spring force to the plunger 114, via the abutment plate 150, which serves to drive the plunger 114 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 36 is expanded so that fuel at relatively low pressure is drawn into the pump chamber 36 through the inlet valve (40 in
As the plunger 114 is withdrawn from the plunger bore 132 during the return stroke there is a continual supply of lubricating fluid into the damper chamber 84 through the inlet ports 90, and the ejection of fluid through the outlet ports 92, and through the clearance between the shroud 70 and the chamber wall, eases as the shroud 70 is drawn downwards to increase the volume of the damper chamber 84.
It will be appreciated that in order to ensure there is a sliding fit between the shroud 70 and the internal wall of the damper chamber 84, a small amount of leakage fluid from the damper chamber 84 will occur through the pump cycle through the clearance gap 88 between the outer surface of the shroud 70 and the inner surface of the annular chamber 82. In another embodiment (not shown), if the clearance gap 88 between the damper chamber 84 and the outer surface of the shroud 70 is sized correctly, it is possible to avoid providing the outlet ports 92 in the wall of the damper chamber 84 altogether and for the outflow of fluid from the damper chamber 84, during the pumping stroke, to be governed only by the rate of flow of fluid through the clearance gap 88. In any case, it is important that the quantity of damper fluid within the damper chamber 84 is maintained through the inflow of fluid through the inlet ports 90, so the clearance gap 88 cannot be too large.
In another embodiment of the invention, the damper chamber 84 need not be formed within a separate component (annular member 82) consisting of the walled chamber shown in
In another embodiment of the invention, it is possible to remove the inlet and outlet ports 90, 92 to the damper chamber 84 altogether so that the damper chamber is sealed. However, this solution would require the use of a low profile seal for the chamber 84 which may not be desirable.
Other embodiments of the invention may fill the damper chamber 84 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 150 is provided as a separate component that is attached to the plunger 114, it would be possible to form the spring plate 150 integrally with the plunger 114.
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 128 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 150. However, the use of an additional spring adds cost to the assembly which may be undesirable.
It will be appreciated by a person skilled in the art that the invention could be modified to take many alternative forms to that described herein, without departing from the scope of the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2017720.0 | Nov 2020 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/081302 | 11/10/2021 | WO |
