GB2342701A - Control of a variable displacement axial piston pump - Google Patents

Abstract

The displacement of a high pressure pump, in which plungers 22 are reciprocated in a rotatable housing 18 by a swash plate 28 which is pivotable about an axis extending in a direction tangential to the path of movement of the plungers 22, is controlled by a balancing piston 68 which opposes the torque applied to the swash plate 28 by the plungers 22, a control piston 72 movable under the action of fluid within a control chamber 70 to control the pivot angle of the swash plate 28, and electromagnetically controlled inlet and outlet valves 74,76 controlling communication between the control chamber 70 and a high pressure outlet 56 of the pump and a low pressure drain respectively. Also, axial drillings 114 through the plungers 22 allow dilation of the plungers 22 in use, reducing the clearance and therefore fluid leakage between each plunger 22 and the housing 18.

Description

HIGH PRESSURE PUMP This invention relates to a pump for use in pressurizing fluid to a high pressure. In particular the invention relates to a high pressure pump of the swash plate type in which the rate at which fluid is delivered by the pump can be controlled. The invention is particularly suitable for use in supplying fuel to a common rail of the fuel system of a compression ignition interna combustion engine, but it will be understood that the pump is suitable for use in other applications where fluid must be pressurised to a high level.

Common rail fuel systems require the common rail to be charged to a very high pressure, typically 2000 bar or higher. The fuel pumps used to achieve these pressures conveniently have low levels of cyclical variation in their output so that the rail pressure can be held at a relatively constant level, and conveniently have a low drive torque. Further, the pumps are conveniently of relatively small dimensions.

In a swash plate pump, it is known to vary the pumping rate by varying the angle of the swash plate. Such an arrangement is advantageous in that the cam rate varies approximately linearly with the pumping stroke. In order to balance the tipping moments applied to the swash plate by the pumping plungers, the swash plate is typically pivotable about its diameter. However, such an arrangement has the disadvantage that the dead volume of the pump is high, particularly at low delivery rates.

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According to the present invention there is provided a high pressure pump comprising a swash plate, a rotatable plunger housing provided with a plurality of bores within which pumping plungers are slidable, the pumping plungers cooperating with the swash plate so as to reciprocate within the bores upon rotation of the plunger housing, the-swash plate being pivotable to control the pumping stroke of the pumping plungers, and a control arrangement controlling the position of the swash plate, wherein the control arrangement comprises a balancing piston arranged to apply a torque to the swash plate countering the torque applied to the swash plate by the pumping plungers, a control piston moveable under the action of fluid within a control chamber to permit movement of the swash plate to a desired position, and a valve arrangement controlling the flow of fluid to or from the control chamber.

As a balancing piston is used to overcome the torque applied to the swash plate by the pumping plungers, the swash plate need not be pivotable about its diameter.

The valve arrangement conveniently comprises an inlet valve controlling communication between the control chamber and a high pressure outlet of the pump, and an outlet valve controlling communication between the control chamber and a low pressure drain. The inlet and outlet valves are conveniently electromagnetically controlled.

Conveniently, the swash plate is pivotable about an axis extending in a direction tangential to the path of movement of the pumping plungers.

Such an arrangement is advantageous in that the dead volume of the pump at low pumping rates is reduced.

Such a pump is particularly suitable for use in charging a common rail of a common rail type fuel system to a high pressure, for example 2000 bar.

The invention will further be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a sectional view illustrating a pump in accordance with an embodiment of the invention, with the pump operating at a high pumping rate ; Figure 2 is a diagrammatic view illustrating operation of the Figure 1 embodiment; Figure 3 is a view similar to Figure 1 illustrating some modifications, and showing the pump in a very low or zero pumping rate condition; Figure 4 is an enlargement of part of Figure 2; Figure 5 is a sectional view, to an enlarged scale, illustrating one of the pumping plungers of the arrangement of Figure 3 and its associated pad; Figures 6a and 6b illustrate the valve member of one of the control valves of the arrangement of Figure 1; and Figure 6c is a view similar to Figure 6b illustrating a modification.

The high pressure fuel pump illustrated in Figures 1 and 2 comprises a housing 10 within which a drive shaft 12 is rotatable, the drive shaft 12 being supported for rotation within the housing 10 by means of bearings 14 which conveniently include seal arrangements in order to restrict leakage of fuel from within the housing 10. The drive shaft 12 includes splines 16 arranged to cooperate with corresponding formations formed upon a plunger housing 18 so as to drive the plunger housing 18 for rotation within the housing 10. As illustrated, the housing 10 is conveniently of two-part form, the parts be secured to one another by any appropriate technique, for example using screw-threaded bolts, a seal member being trapped between the parts of the housing 10 in order to minimize leakage of fuel from the housing 10.

The plunger housing 18 is provided with five through bores 20 within which pumping plungers 22 are reciprocable. The pumping plungers 22 include, at their ends which project from the bores 20, part-spherical regions 22a which engage within corresponding part-spherical recesses provided in respective load transmitting pads 24, spring clips 26 being secured to the pads 24, the spring clips 26 engaging the part-spherical regions 22a of the plungers 22 to secure the pads 24 to the pumping plungers 22. It will be appreciated that the pads 24 are free to articulate about the part spherical regions 22a of the plungers 22. The pads 24 are engageable with and slidable upon a surface of a swash plate 28 which is located within the housing 10, the swash plate 28 being pivotable relative to the housing 10 about a pivot axis defined by a pivot pin 30 which extends through openings formed in the swash plate 28 and into corresponding openings formed in the housing 10, screw-threaded nuts 32 being used to secure the pivot pin 30 in position and to form a fluid tight seal with the housing 10 in order to avoid leakage of fuel from the housing 10, in use.

The swash plate 28 is mounted such that the axis of pivotal movement extends approximately tangentially to the circular path through which the plungers move upon rotation of the drive shaft 12. Such an arrangement is advantageous in that the dead volume of the pump is reduced and is substantially independent of the angle of the swash plate 28.

The pads 24 are each engageable with an annular member 34, the parts of the pads 24 with which the end regions 22a of the pumping plungers 22 engage extending through openings formed in the annular member 34.

The annular member 34 is arranged to engage a load transmitting member 36 which encircles the drive shaft 12, a helical compression spring 38 being engaged between the load transmitting member 36 and the plunger housing 18, the spring 38 acting to bias the pads 24 into engagement with the swash plate 28. The parts of the annular member 34 and load transmitting member 36 which are arranged to engage one another are shaped to be of frusto-conical or part spherical form. In use, upon rotation of the plunger housing 18, it will be appreciated that the attitude of the annular member 34 changes, with the result that different parts of the annular member 34 engage the load transmitting member 36.

The end of the plunger housing 18 remote from the swash plate 28 engages a closure plate 40 which is of outer diameter substantially equal to that of the plunger housing 18, the closure plate 40 substantially closing each of the bores 20. As illustrated, the closure plate 40 is provided with a series of drillings, each drilling being located to communicate with a respective one of the plunger bores 20 with the result that each plunger bore 20 includes a relatively small diameter opening 42. The closure plate 40 is secured to the plunger housing 18 by means of a suitable annular member 44.

The closure plate 40 abuts a port plate 46 which, as shown in Figure 2, is provided with a pair of arcuate grooves 48,50 shaped to align with the openings 42 provided in the closure plate 40. The groove 48 is positioned to communicate only with the plunger bores 20 associated with pumping plungers 22 which, at any particular time, are being withdrawn from the respective bores 20, the groove 50 being positioned to communicate only with the plunger bores 20 associated with plungers 22 which, at that instant, are moving inwardly under the action of the swash plate 22. The groove 48 is of relatively large width, the groove 50 being of smaller width, the width of the groove 50 being chosen such that the effective cross-sectional area of the groove 50 is less than the effective area of the bores 20 containing fuel at high pressure at any particular instant to avoid lifting of the plunger housing 18 from the port plate 46.

The port plate 46 is secured to the housing by an appropriate means, dowels being used to hold the port plate 46 in the desired position during assembly. The spring 38 applies a force to the plunger housing 18 urging the plunger housing 18 and the closure plate 40 towards the port plate 46, the spring 38 supplementing the action of the fuel under pressure within the bores 20, in use, in applying a sufficiently high force to the plunger housing 18 and closure plate 40 to form a substantially fluid tight seal between the port plate 46, the closure plate 40 and the plunger housing 18. The engagement between the splines 16 and the plunger housing 18 includes sufficient slack to allow the plunger housing 18 to adopt a position in which it can compensate for slight manufacturing inaccuracies.

The port plate 46 is conveniently constructed from a material which is hard wearing under low lubrication conditions. In an alternative arrangement, the port plate may be omitted, and the grooves formed directly in the housing 10.

If desired, one or more grooves connected to a low pressure drain may be provided alongside the groove 50 to limit the spread of high pressure gradients.

The groove 48 in the port plate 46 communicates through a supply passage 52 provided in the housing 10 with an inlet port 54 whereby fuel is supplie to the high pressure fuel pump. The groove 50 communicates through a passage 56 provided in the housing 10 with an outlet port 58 whereby fuel under high pressure is supplie to a common rail, in use.

Fuel from the passage 56 is further supplie through passages 60,62 formed in the two parts of the housing 10, and a radially expanding seal tube 64 located at the connection between the two parts of the housing 10, to a bore provided in the housing 10 within which a balancing piston 68 is slidable, the bore and piston 68 together defining a chamber 66.

The balancing piston 68 is located to be engageable with a cranked region of the swash plate 28. It will be appreciated that, in use, if the pressure at the outlet of the high pressure pump varies, then the fuel pressure within the chamber 66 applied to the inner end of the balancing piston 68 will also vary, and as a result, the force applied by the balancing piston 68 to the swash plate 28 varies. Similarly, if the outlet pressure of the pump varies, then the fuel pressure within the plunger bores 20 which, at that instant, are in communication with the outlet of the pump will also vary, thus the torque applied to the swash plate 28 by the pumping plungers 22 varies. The position of the balancing piston 68 and the dimensions of the balancing piston 68 are chosen to ensure that, at all times, the torque applied to the swash plate 28 by the balancing piston 68 acts against and is greater than or equal to the torque applied to the swash plate 28 by the pumping plungers 22.

The housing 10 further includes a bore 70 within which a control piston 72 is slidable. The control piston 72 is engageable with the cranked part of the swash plate 28, the balancing piston 72 being located and of dimensions such that the force applied to the swash plate 28 by the control piston 72 acts against and is able to overcome that applied by the balancing piston 68. The control piston 72 and bore 70 together define a control chamber, to or from which fuel is able to flow under the control of a valve arrangement including an inlet electromagnetically controlled valve 74 and an outlet electromagnetically controlled valve 76. The control piston 72 is conveniently offset from the axis of the balancing piston 68, the pistons conveniently overlapping each other thereby allowing the pump to be of reduced dimensions.

The inlet electromagnetically controlled valve 74 is arranged to control communication between the passage 56 and the control chamber, such communication permitting fuel to flow to the control chamber, the valve 76 controlling communication between the control chamber and a low pressure drain to permit fuel to escape from the control chamber. As illustrated, an edge filter arrangement 78 is located within an upper part of the passage 56 of relatively large diameter to permit filtering of the fuel before the fuel is supplied to the valves 74,76 to reduce the risk of damage thereto.

As illustrated most clearly in Figure 4, the inlet valve 74 comprises a valve housing 80 which defines an inlet chamber 82 which communicates with the passage 56. A valve member 84 is located within the chamber 82, the valve member 84 being biased by means of a spring 86 and by the action of the fuel pressure within the chamber 82 towards a position in which a seating region 88 thereof engages a region of the housing 10 surrounding a passage 90 which communicates with the control chamber. The valve member 84 is constructed of an appropriate magnetic material such that in addition to acting as the valve member, it also forms the armature of an electromagnetic actuator including a stator component 92 and a winding 94. In use, energization of the actuator results in the valve member 84 being attracted towards the stator 92 against the action of the spring 86 to lift the region 88 of the valve member 84 away from the end of the passage 90, thus permitting fuel to flow from the passage 56 through the chamber 82 to the passage 90 and control chamber. Such an application of fuel increases the fuel pressure within the control chamber, urging the control piston 72 towards the right in the orientation illustrated in Figure 1, thus urging the swash plate 28 towards a position in which the pumping stroke of each plunger 22 is increased. The delivery rate of the pump is thus increased. Once it is determined that the swash plate 28 has reached the desired position, de-energization of the actuator of the inlet control valve 74 results in the valve member 84 returning to its original position under the action of the spring 86 thus terminating the supply of fuel to the control chamber.

As illustrated, the valve housing 80 is secured against the upper surface of the housing 10 in such a manner as to restrict or prevent leakage of fuel from the chamber 82, the valve housing 80 being secured in position by means of a control valve housing 96 which is secured to the housing 10.

A domed disc 98 is arranged to transmit the clamping force through the stator 92 and a camping member 100 to the valve housing 80.

The outlet electromagnetically controlled valve 76 communicates through a passage 102 with the control chamber. The valve 76 may be of any appropriate form, but in the arrangement illustrated takes the form of a valve member 104 slidable within a bore 106, the valve member 104 including a region of enlarged diameter engageable with a seating defined around an end part of the bore 106. The valve member and bore together define a chamber which communicates through a passage 108 with the passage 102. An armature 110 is carried by the valve member 104 and moveable under the influence of the magnetic field generated by an electromagnetic actuator, the valve member 104 being biased towards the position shown in which the enlarged region thereof engages its seating by means of an appropriate helical compression spring.

In use, the drive shaft 12 is arranged to rotate, the drive shaft 12 being driven, for example using an appropriate drive belt, from the cam shaft of an engine. Alternatively, the pump may be aligned with the cam shaft and rotated directly by it, for example through an Oldham coupling. Rotation of the drive shaft 12 causes rotation of the plunger housing 18, such rotation causing reciprocation of the plungers 22 within the bores 20 due to the cooperation between the pads 24 and the swash plate 28. Those plungers 22 which are moving in an outward direction draw fuel from the inlet port 52 into the bores 20 thus charging the bores 20 to a relatively low pressure. As each plunger 22 reaches its outmost position, the bore thereof moves out of communication with the groove 48. Subsequently, each plunger 22 commences inward movement thus commencing pressurization of the fuel within the respective bore 20. Shortly after such pressurization has commenced, the bore 20 moves to a position in which it communicates with the groove 50, thus further inward movement of the plunger 22 delivers the fuel under high pressure to the outlet passage 56.

Delivery continues until the plunger 22 reaches its innermost position which, in the orientation illustrated in Figure 1, is when the plunger 22 reaches its lowermost position. Once the plunger 22 has reached its innermost position, the bore 20 moves out of communication with the groove 50 and the plunger 22 then commences outward movement thus reducing the pressure of any remaining fuel within the bore 20. Shortly after commencing outward movement, the bore 20 returns into communication with the groove 48 thus charging of the bore 20 with fuel under relatively low pressure takes place.

It will be appreciated that the pumping events of adjacent plungers overlap one another. Any reverse torque tendency due to the unloading of high pressure at the end of the pumping stroke of each plunger is masked by the pumping events of the other plungers. As a result, the pump drive torque is approximately constant. Further, by allowing the pressure in each bore to fall prior to connection to the groove 48, and by allowing the pressure to rise prior to connection to the groove 50, the generation of hydraulic shocks can be reduced.

As described hereinbefore, the torque applied to the swash plate 28 by the pumping plungers 22 is countered by the action of the balancing piston 68. If it is determined that the pump is delivering fuel at an undesirable rate, then in order to adjust the delivery rate of the pump, the angular position of the swash plate 28 is adjusted. This is achieved by varying the volume of the control chamber. For example, if it is determined that the fuel delivery rate of the pump is too high, then in order to reduce the pumping rate, the outlet electromagnetically controlled valve 76 is energized to permit fuel from the control chamber to escape past the seating of the outlet electromagnetically controlled valve and through a passage 112 provided in the housing 10 to a low pressure drain. As a result, the fuel pressure within the control chamber falls and the force applied to the swash plate 28 by the control piston 72 is reduced.

Consequently, the swash plate 28 moves in an anti-clockwise direction about the pivot pin 30 to reduce the pumping stroke of each plunger 22.

As a result of the reduction in the pumping stroke of each plunger 22, less fuel is delivered during each rotation of the drive shaft 12, thus for a given speed of rotation of the drive shaft 12, fuel is delivered by the pump at a reduced rate. When the desired pump delivery rate has been achieved, the outlet electromagnetically controlled valve 76 is closed.

If it is determined that the delivery rate of the pump is too low, then in order to increase the pumping rate, the inlet electromagnetically controlled valve 74 is actuated to lift the valve member 84 away from the end of the passage 90, thus permitting fuel at high pressure from the passage 56 to flow to the control chamber. As a result, the fuel pressure within the control chamber 70 rises and the force applied to the swash plate 28 by the control piston 72 increases. The increased force applied by the control piston 72 urges the swash plate 28 towards the position shown in Figure 1 in which the pumping stroke of each pumping plunger 22 is maximised, increasing the volume of the control chamber. Again, the valve is closed once the desired swash plate position, and hence pump delivery rate, has been achieved.

As the control and balance pistons have substantially the same stroke, but the balancing piston is of diameter greater than that of the control piston, adjustment of the delivery rate of the pump to increase the delivery rate results in more fuel being expelled from the chamber 66 than is supplie to the control chamber of the control piston. As a result, additional fuel is transferred to the pump outlet, thus improving the speed of response of the pump to changes in desired delivery rate. Similarly, when the delivery rate is to be reduced, fuel flows to the chamber 66, reducing the fuel pressure at the pump outlet.

In the arrangement illustrated in Figure 1, each pumping plunger 22 is provided with a relatively small diameter axially extending drilling 114 which communicates with a conventional load off-setting chamber defined between the pumping plunger 22 and pad 24. The pad 24 is provided with a drilling permitting fuel to flow to a further load off-setting chamber defined between the pad 24 and swash plate 28. The provision of the drilling 114 in each plunger 22 serves to cause dilation of the pumping plungers 22, in use, during inward movement of each plunger 22, such dilation reducing the clearance between the pumping plunger 22 and plunger housing 18, thus reducing the rate of fuel leakage. The reduction in fuel leakage can be improved by increasing the diameter of the drilling 114 as illustrated in the modification shown in Figure 3 and also as shown in Figure 5. Increasing the diameter of the drilling 114 further has the advantage that manufacture of the pumping plunger 22 is simplified. In order to reduce the dead volume produced by the use of such a large diameter drilling 114, a multi-turn"Spirol"spiral spring pin 116 is conveniently located within each drilling 114, the resilience of the spring pin 116 retaining the spring pin 116 in position despite the cyclical expansion and contraction of the drilling 114, in use. It will be appreciated, however, that the spring pin 116 may be replace by, for example, a loose rod or a rod retained within the plunger 22 by local knurling.

In order to reduce leakage from the control and balancing pistons, blind drillings and spring pins may be provided in these components.

The arrangement of Figure 3 differs from that of Figures 1 and 2 in several other respects. Firstly, rather than providing the parts of the housing 10 with grooves arranged to receive a seal member, the housing parts are arranged to trap therebetween a gasket member 10a for example in the form of a thin metal sheet, the gasket member carrying a printed rubberlike sealing material, in appropriate locations, to form a substantially fluid tight seal between the parts of the housing 10.

A further distinction is that the cover plate 40 is omitted, and instead, the bores 20 within which the pumping plungers 22 are reciprocable are of blind form rather than being through bores as in the arrangement of Figures 1 and 2, the blind bores of the Figure 3 arrangement communicating, at their blind ends, through drillings provided in the plunger housing 18 with the grooves 48,50 provided in the port plate 46.

Although there are structural differences between the arrangement of Figure 3 and that of Figures 1 and 2, it will be appreciated that in use, the two arrangements operate in the same manner.

Figures 6a and 6b are views from beneath and a side sectional view, respectively, of the valve member 84 of the inlet electromagnetically controlled valve 74 of the arrangement of Figures 1 and 2. As described hereinbefore, in this arrangement, the valve member 84 includes a region 88 arranged to form a seal with the housing 10 around an end of the passage 90. The valve member 84 further includes a series of integral legs 84a which are provided to assist in ensuring that, when the valve member 84 engages the housing 10, the valve member 84 is relatively stable and extends parallel to the adjacent surface of the housing 10.

The arrangement illustrated in Figure 6c differs from that of Figures 6a and 6b in that the region 88 takes the form of a cylindrical projection rather than being of frusto-conical shape as in the arrangement of Figure 6b, the frusto-conical arrangement including an axially extending blind drilling such that the region 88 defines an annular structure.

In the arrangement described hereinbefore, the spring 38 serves to retract the plungers from the bores 20, in use, and as a result, the pump can draw fuel into the bores, thus the provision of a lift or transfer pump can be avoided. If a transfer pump is desired, then this may be driven from the non-driven end of the drive shaft 12.

It will be appreciated that a number of modifications to the pump are possible, and fall within the scope of the present application. For example, the pump may have fewer or a greater number of pumping plungers. The use of a relatively large number of plungers permits the output of the pump to be smooth, and also permits the pump to be of relatively short axial length.

If desired, the balancing piston may be provided in a separate housing part for manufacturing ease.

A position sensor may be provided in order to permit the position of the swash plate, and hence the delivery rate of the pump to be monitored.

In order to avoid rapid, violent changes in the pump delivery rate, appropriate restrictors may be provided to limit the rate at which fuel flows to or from the control chamber and the chamber 66. Alternatively, the valves may be controlled in such a manner as to gradually change the fuel pressure within the control chamber. Further, if it is desired to reduce the common rail pressure, both valves may be opened to allow fuel to escape to the low pressure reservoir.

In the event of electronic failure, gradual leakage of fuel from the control chamber would return the pump to its zero output level, thus the pump is fait safe.

Although in the description hereinbefore, the pump is intended for use in supplying fuel to a common rail of an engine fuel system, it will be appreciated that the invention is also applicable to other applications.

Claims (5)

1. CLAIMS 1. A high pressure pump comprising a swash plate, a rotatable plunger housing provided with a plurality of bores within which pumping plungers are slidable, the pumping plungers cooperating with the swash plate so as to reciprocate within the bores upon rotation of the plunger housing, the swash plate being pivotable to control the pumping stroke of the pumping plungers, and a control arrangement controlling the position of the swash plate, wherein the control arrangement comprises a balancing piston arranged to apply a torque to the swash plate countering the torque applied to the swash plate by the pumping plungers, a control piston moveable under the action of fluid within a control chamber to permit movement of the swash plate to a desired position, and a valve arrangement controlling the flow of fluid to or from the control chamber.
2. 2. The pump as claimed in Claim 1, wherein the valve arrangement comprises an inlet valve controlling communication between the control chamber and a high pressure outlet of the pump, and an outlet valve controlling communication between the control chamber and a low pressure drain.
3. 3. The pump as claimed in Claim 2, wherein the inlet and outlet valves are controlled electromagnetically.
4. 4. The pump as claimed in any of Claims 1 to 3, wherein the swash plate is pivotable about an axis extending in a direction tangential to the path of movement of the pumping plungers.
5. 5. A high pressure pump substantially as hereinbefore described with reference to the accompanying drawings.