EP3105451A1

EP3105451A1 - Fuel pump

Info

Publication number: EP3105451A1
Application number: EP14812519.8A
Authority: EP
Inventors: Paul Garland; Toby Pedley
Original assignee: Delphi International Operations Luxembourg SARL
Current assignee: Delphi International Operations Luxembourg SARL
Priority date: 2014-02-13
Filing date: 2014-12-16
Publication date: 2016-12-21
Also published as: WO2015120930A1; KR20160120300A; CN106232980A; GB201402535D0

Abstract

According to an aspect of the present invention there is provided a pump for supplying high- pressure fuel to a common rail fuel injection system. The pump has a head housing and a pumping chamber is defined within the head housing. A bore for receiving a pumping element is arranged for reciprocal movement within the bore. The pump comprises means for compensating for dilation of the pumping element and/or the bore under operational loads.

Description

FUEL PUMP

TECHNICAL FIELD

The present invention relates to a pump for supplying high-pressure fuel; to a pump head for a high-pressure fuel pump; and to a pumping element for a high-pressure fuel pump.

BACKGROUND

In common-rail fuel injection systems, and particularly diesel fuel injection systems, it is desirable to supply fuel to the fuel injectors at very high pressures. One known type of fuel pump assembly suitable for the supply of fuel to a common rail at high pressure includes at least one pumping element such as a plunger, which is driven within a bore by means of a cam drive arrangement so as to pressurise fuel within a pumping chamber for delivery to the common rail. A radial clearance typically exists between the bore and the plunger. The size of the radial clearance is usually a compromise between the requirements of pumping efficiency and anti-seizure of the parts of the pump head. If the clearance is too large, dynamic leakage past the plunger can reduce pumping efficiency. If, however, the clearance is too small, rubbing between the parts can occur, causing overheating which may lead to seizure or fusing of the bore wall and the plunger. In addition the clearance must accommodate geometric variations in the dimensions of the bore and plunger resulting from the manufacturing process. These problems are exacerbated when the pump operates at high pressures, for example in excess of 2,000 bar or 2,500 bar, to improve combustion of the fuel.

At least in certain embodiments, the present invention sets out to overcome or ameliorate at least some of the problems associated with known pump heads. In particular, at least in certain embodiments, the present invention sets out to provide an effective pump head in which both leakage of fuel and seizure of the parts of the pump head are avoided.

SUMMARY OF THE INVENTION

Aspects of the present invention relate to a pump for supplying high-pressure fuel; to a pump head for a high-pressure fuel pump; and to a pumping element for a high-pressure fuel pump.

According to a further aspect of the present invention there is provided a pump for supplying high-pressure fuel to a common rail fuel injection system, the pump comprising:

a head housing;

a pumping chamber defined within the head housing; a bore extending from the pumping chamber; and

a pumping element arranged for reciprocal movement within the bore;

wherein the pump comprises means for compensating for dilation of the pumping element and/or the bore under operational loads. By providing dilation compensating means, the deformation of the pumping element and/or the bore under operational loads can be accommodated. At least in certain embodiments, dynamic leakage past the pumping element can be reduced whilst reducing the risk of the pumping element seizing.

The pumping element reciprocates along a longitudinal axis arranged coincident with a central axis of the pumping element and the bore. The bore can have a bore cross section, and the pumping element can have a pumping element cross section. As described herein, the cross section of the pumping element and/or the cross section of the bore can change under operational loads. When the bore is unloaded, (i.e. when the bore is not exposed to operational loads), the cross section of the bore can vary along said longitudinal axis. Alternatively, or in addition, when the pumping element is unloaded (i.e. when the pumping element is not exposed to operational loads) the cross section of the pumping element can vary along said longitudinal axis. The dilation compensating means can be in the form of a bore having a non-uniform cross section along said longitudinal axis when the bore is unloaded; and/or in the form of a pumping element having a non-uniform cross section along said longitudinal axis when the pumping element is unloaded.

A clearance can be defined between the bore and the pumping element. When the pump is unloaded, (i.e. when the pump is not exposed to operational loads), the clearance can vary along the longitudinal axis. The dilation compensating means can be in the form of a non- uniform clearance along the longitudinal axis when the pump is unloaded. The clearance between the bore and the pumping element can increase along said longitudinal axis in a direction extending away from the pumping chamber. The clearance can be smallest adjacent to or proximal to the pumping chamber. This arrangement can help to reduce dynamic leakage between the pumping element and the bore. The clearance can extend in a radial direction and be formed around the circumference of the pumping element.

The dilation compensating means can extend axially along only a portion of the pumping element and/or the bore. The dilation compensating means can be localised in an axial direction. Thus, the dilation compensating means does not have to extend along the length of the pumping element and/or the bore. The operational loads result from hydraulic forces occurring during a pressurization stroke of the pumping element within the bore. As the pumping element advances towards a top dead centre position, the fuel in the pumping chamber is pressurised which results in the application of hydraulic forces to the pumping element and the bore. At least in certain embodiments, the dilation compensating means can compensate for dilation of the pumping element and/or the bore resulting from the peak (i.e. maximum) hydraulic forces occurring during a pressurization stroke of the pumping element.

When the pumping element and the bore are unloaded (i.e. not exposed to operational loads), the dilation compensating means can be in the form of a first radial clearance defined between the bore and the pumping element at a first axial location and a second radial clearance defined between the bore and the pumping element at a second axial location. The second radial clearance can be larger than the first radial clearance, the first axial location being positioned closer to the pumping chamber than the second axial location. By providing a second radial clearance which is larger than the first radial clearance, the deformation of the pumping element and/or the bore under operational loads can be accommodated. Increasing the clearance along the length of the pumping element would result in increased leakage and lower pumping efficiency; however, the clearance in the bore proximal to the pumping chamber can be reduced compared with prior art pumps since bore dilation at high pressure can reduce or eliminate the risk of seizure in this region. The result is a bore with smaller than normal clearance at the high pressure end to control leakage, but larger than normal clearance at the low pressure end where it is needed to avoid seizure. The volumetric efficiency of the pump can thereby be improved. Under operational loads, the first radial clearance can increase and the second radial clearance can decrease in comparison to the unloaded state.

The bore can comprise a first portion and a second portion. The first portion can be disposed proximal to the pumping chamber at a high-pressure end of the bore. The second portion can be spaced apart from the pumping chamber. The second portion can be disposed at a low-pressure end of the bore (i.e. at the opposite end of the bore from the first portion). The second portion of the bore can be flared outwardly in a direction extending away from said pumping chamber. The term 'flare angle' is used herein to define the opening angle of a sidewall of the bore with reference to a longitudinal axis of the bore. The second portion of the bore can have a flare angle greater than 0°, for example in the range 0.001 ° to 0.020°. The second portion of the bore can comprise a straight taper. The second portion of the bore can be frustoconical. In a variant, the second portion of the bore can be curved outwardly. For example the second portion of the bore can be bell-shaped or campanulate.

The first portion and the second portion of the bore can both be flared outwardly. The first and second portions can have different flare angles. For example, the flare angle of the first portion can be less than the flare angle of the second portion. Alternatively, the first portion can be a right cylinder and the second portion can be flared outwardly.

In an alternate arrangement, the first portion can comprise a first right cylindrical portion having a first diameter; and the second portion can comprise a second right cylindrical portion having a second diameter. The first diameter can be less than the second diameter. An intermediate taper or a step can be defined between said first and second right cylindrical portions. The bore can be tapered outwardly along its entire length, having a larger diameter at the high pressure end than at the low pressure end.

The bore can have an opening disposed opposite to the pumping chamber. The first axial location can be disposed at or proximal to said opening. Thus, the pumping chamber can be disposed at a first end of the bore and the opening can be disposed at a second end of the bore. The opening can have an enlarged diameter to increase the radial clearance between the bore and the pumping element.

The pumping element can comprise a tapered section which is tapered in a direction extending away from the pumping chamber. The pumping element can comprise a first end which partially defines the pumping chamber and a second end opposite to the first end. The pumping element can comprise a tapered section which is tapered towards the second end. The pumping element can be tapered along only a portion of its length. Alternatively, the tapered section can extend from the first end to the second end. The pumping element can comprise a first right cylindrical region having a first diameter and a second right cylindrical region having a second diameter, the first right cylindrical region can be positioned closer to the pumping chamber than the second right cylindrical region, and the first diameter can be larger than the second diameter. According to a further aspect of the present invention there is provided a pump head for a high-pressure fuel pump, the pump head comprising:

a head housing; a pumping chamber defined within the head housing; and

a bore extending from the pumping chamber for receiving a pumping element;

the bore being configured to compensate for dilation of the pumping element under operational loads.

The bore can comprise a first portion and a second portion, the first portion being disposed proximal to the pumping chamber. The second portion of the bore can be flared outwardly. Alternatively, the second portion can have a larger diameter than the first portion. The second portion can be spaced apart from the pumping chamber, for example at the opposite end of the bore from said first portion.

According to a still further aspect of the present invention there is provided a high-pressure fuel pump for use in a common rail fuel injection system, the fuel pump comprising a pump head as described herein. According to a yet still further aspect of the present invention there is provided a pumping element for a high-pressure fuel pump, wherein the pumping element comprises:

a first end configured to define a part of a pumping chamber; and

a second end disposed opposite to the first end;

wherein the pumping element is configured to compensate for dilation of the bore under operational loads. The pumping element can comprise a tapered section which is tapered towards the second end.

It will be appreciated that the pump head and/or the pumping element can undergo deformation due to the operational loads. The relative sizes of the first and second radial clearances are defined in an unloaded state. The profile of the bore and/or the pumping element is/are defined in an unloaded state.

Throughout this description, terms such as 'upper', 'lower' and so on relate to the orientation of the components as shown in the accompanying drawings and are used for ease or reference only. It will be appreciated that the invention could be used in any suitable orientation.

BRIEF DESCRIPTION OF THE DRAWINGS

One or more embodiments of the present invention will now be described, by way of example only, with reference to the accompanying figures, in which: Figure 1 is a schematic cross-sectional view of a pump head of a known high- pressure fuel pump for use in a fuel injection system, the plunger being in a top dead centre position;

Figure 2 is a schematic cross-sectional view of the pump head of Figure 1 , the plunger being in a bottom dead centre position;

Figure 3 is an enlarged cross-sectional view showing a part of the pump head of Figure 2 in greater detail, the pump head being in an unloaded state;

Figure 4 is an enlarged cross-sectional view showing a part of the pump head of Figure 1 in greater detail, the pump head being under operational loads;

Figure 5 is a cross-sectional view showing a part of the pump head according to a first embodiment of the present invention, the pump head being in an unloaded state;

Figure 6 is a cross-sectional view showing a part of the pump head according to the first embodiment of the present invention, the pump head being under operational loads;

Figure 7 is a cross-sectional view showing a part of the pump head according to a first variant of the first embodiment of the present invention;

Figure 8 is a cross-sectional view showing a part of the pump head according to a second variant of the first embodiment of the present invention;

Figure 9 is a cross-sectional view showing a part of the pump head according to a second embodiment of the present invention; and

Figure 10 is a cross-sectional view showing a part of the pump head according to a variant of the second embodiment of the present invention.

DETAILED DESCRIPTION

Figures 1 to 4 are schematic cross-sectional views of a known high-pressure fuel pump for use in a fuel injection system.

Referring to Figures 1 and 2, a pump 1 is provided which comprises a pump head housing 3. The pump head housing 3 comprises a housing body portion 5 and a cylindrical housing projection, also known as a turret portion 7. The turret portion 7 projects from the housing body portion 5. The pump head housing 3 comprises a bore 9 extending into the housing body portion 5 and through the turret portion 7. The bore 9 is defined by a bore wall 1 1. A pumping element in the form of a pumping plunger 13 is slidably received within the bore 9. The plunger 13 comprises a high pressure end 15 (the upper end of the plunger 13 in the orientation shown in Figures 1 and 2) and a low pressure end 17 (the lower end of the plunger 13 in the orientation shown in Figures 1 and 2). The low pressure end 17 of the pumping plunger 13 cooperates with a follower (not shown) which is driven by a cam mounted on a drive shaft (not shown). As the drive shaft rotates, the cam imparts an axial force on the follower, causing the plunger 13 to reciprocate within the bore 9, between a top dead centre position (i.e. the uppermost position of the plunger within the bore), as represented in Figure 1 , and a bottom dead centre position (i.e. the lowermost position of the plunger within the bore), as represented in Figure 2.

The pump head housing 3 defines a pumping chamber 19 at an upper end 21 of the bore 9. The pumping chamber 19 is defined by the pump head housing 3, an inlet valve 22, an outlet valve 25 and the high-pressure end 15 of the plunger 13. Low-pressure fuel is fed to the pumping chamber 19 by a low-pressure lift pump in a fuel tank or alternatively by a transfer pump built into the high-pressure fuel pump (not shown). The pump head housing 3 includes an exit drilling 23 in fluid communication with the pumping chamber 19. In use, fuel is pressurised within the pumping chamber 19 by the reciprocal motion of the pumping plunger 13 within the bore 9 along a longitudinal axis X. The pressurised fuel is fed from the pumping chamber 19, along the exit drilling 23, and through the outlet valve 25 to downstream components of a fuel injection system, such as a common rail.

Figures 3 and 4 show in greater detail the bore 9 and the plunger 13 of the known pump 1 . As shown in Figure 3, when the pump 1 is not operating (i.e. the components are in an unloaded state), the bore wall 1 1 of the pump 1 and the plunger 13 are substantially cylindrical. The plunger 13 is disposed in the bore 9 with a close clearance. The clearance between the bore wall 1 1 and the plunger 13 in the conventional pump 1 is typically comprised between 5 and 6 microns. The clearance is enlarged in the Figures to improve clarity. When the pump 1 is operating, the pump head housing 3 and the plunger 13 are exposed to hydraulic forces during the pressurization stroke, which can result in an elastic deformation of the pump head housing 3 and/or the plunger 13. These forces are represented by the arrows A in Figure 4 (the length of each arrow A representing the magnitude of the force). During the pressurization stroke, the plunger 13 is compressed axially by the cam force which acts to compress fuel in the pumping chamber 19. According to Poisson's ratio, this causes the plunger 13 to be compressed in an axial direction (i.e. to shorten) and undergo elastic expansion in a radial direction, thereby reducing the clearance between the bore wall 1 1 and the plunger 13. Simultaneously, a pressure gradient is established in the clearance between the bore wall 1 1 and the plunger 13. The resulting radial force is greatest at the top end of the bore 9 (i.e. proximal to the pumping chamber 19) and decreases towards the bottom end of the bore 9 (as illustrated by the arrows A in Figure 4). This causes the upper end 21 of the bore 9 to dilate, thereby increasing the effective clearance between the bore wall 1 1 and the plunger 13. However, since the pressure varies along the length of the bore 9, the dilation of the bore 9 varies along its length. In particular, the dilation is greatest at the top of the bore 9 and smallest (or not present) at the lower end of the bore 9. As shown in Figure 4, the radial pressure also acts on the plunger 13 and can suppress or even overcome the radial expansion of the plunger 13 caused by the axial forces. The radial forces may even be sufficient to reduce the diameter of the plunger 13 proximal to the pumping chamber 19. The result of these forces is to reduce the clearance at the lower end of the bore during the pressurization stroke, as shown in Figure 4. A pump 101 in accordance with a first embodiment of the present invention will now be described with reference to Figures 5 and 6. The pump 101 has a pump head housing 103 which comprises a bore 109 defined by a bore wall 1 1 1 . A pumping element in the form of a pumping plunger 1 13 is slidably received within the bore 109. The plunger 1 13 comprises a high pressure end 1 15 and a low pressure end 1 17 opposite to the high pressure end 1 15. A pumping chamber 1 19 is defined by the pump head housing 103, an inlet valve 122, an outlet valve 125 and the high-pressure end 1 15 of the plunger 1 13. The bore 109 and the plunger 1 13 shown in Figures 5 and 6 in accordance with the first embodiment of the present invention are adapted to be incorporated in a conventional pump 1 as described above. The clearance between the components in the pump 101 is enlarged in the Figures to improve clarity.

Figure 5 shows a part of the pump head housing 103 comprising the bore 109 and the plunger 1 13 when the pump 101 is not operating, i.e. in an unloaded state in which no strains are applied to the bore wall 1 1 1 or to the plunger 1 13. In this unloaded state, the plunger 1 13 is substantially cylindrical. The cylindrical plunger 1 13 has a diameter comprised between 5 and 9 mm, and in particular equal to 6.5 mm. The plunger 1 13 has a longitudinal length comprised between 25 and 60 mm, and in particular equal to 35 mm. The plunger 1 13 is manufactured by centre-less grinding in the present embodiment. The bore 109 comprises an opening 123 opposite to the pumping chamber 1 19. The bore 109 comprises an upper portion 125 which includes the pumping chamber 1 19, and a lower portion 127, which includes the opening 123. As shown in Figure 5, in the unloaded state as defined above, the upper portion 125 is substantially cylindrical, and the lower portion 127 has a flared shape. The lower portion 127 has a diameter which increases linearly towards the opening 123. In other words, the lower portion 127 is frustoconical. The difference between the diameter of the opening 123 and the diameter of the upper portion 125 of the bore 109 in the unloaded state is between 1 and 2 microns. The flaring of the lower portion 127 can be manufactured by honing or grinding the bore 109 until the desired clearance between the bore 109 and the plunger 1 13 is reached. To avoid edge loading in the event of a side-load at the low pressure end 1 17 of the plunger 1 13, the upper and lower portions 125, 127 of the bore 109 are blended together to form a smooth interface.

Figure 6 shows a part of the pump head housing 103 comprising the bore 109 and the plunger 1 13 during a pressurization stroke, i.e. when the pumping plunger 1 13 is driven from the bottom dead centre position to the top dead centre position, thereby pressurizing the fuel contained in the pumping chamber 1 19. During the pressurization stroke, the plunger 1 13 is exposed to hydraulic forces, as detailed above for the prior art arrangement. These forces are illustrated by the arrows A in Figure 6. The result of these forces is to reduce the clearance at the opening 123 of the bore 109. The flared shape of lower portion 127 of the bore 109 means that the clearance can be increased in the localised region where it is expected to reduce whilst the pumping chamber 1 19 is at high pressure. This configuration can help to avoid seizure of the plunger 1 13 and/or the bore wall 1 1 1 which may otherwise lead to fusing between the plunger 1 13 and the bore wall 1 1 1.

The operation of the pump 101 in accordance with the first embodiment of the present invention will now be described. In use, low-pressure fuel is fed to the pumping chamber 1 19 by a low-pressure lift pump in a fuel tank or alternatively by a transfer pump built into the high-pressure fuel pump (not shown). As the drive shaft rotates, the cam imparts an axial force on the follower, causing the pumping plunger 1 13 to reciprocate within the bore 109, between the bottom dead centre position and the top dead centre position. Fuel is therefore pressurised within the pumping chamber 1 19 by the reciprocal motion of the pumping plunger 1 13 within the bore 109. Then, pressurised fuel is fed from the pumping chamber 1 19, along the exit drilling, and through the outlet valve to the common rail of the fuel injection system.

During the pressurization stroke, the low pressure end 1 15 of the plunger 1 13 dilates due to the axial forces. The clearance between the low pressure end 1 15 of the plunger 1 13 and the bore wall 1 1 1 is thereby reduced. The flared shape of the lower portion 127 of the bore 109 accommodates this dilation. The high pressure end 1 17 of the plunger 1 13 is constricted by the radial forces, thereby increasing the clearance between the plunger 1 13 and the bore wall 1 1 1 . By modifying the profile of the lower end of the bore 109, the risk of seizure of the pump 101 can be reduced. Moreover, smaller clearances can be implemented between the plunger 1 13 and the bore wall 1 1 1 , particularly at the high pressure end 1 17. Dynamic leakage can be reduced and, at least in certain embodiments, volumetric efficiency of the pump 101 can be improved. A further advantage of this arrangement is that a relatively short flared bore 109 can be made to perform with equal or better efficiency than longer bores with plain cylindrical bores. This helps to reduce the overall height of the pump head housing 103 which is an advantage in packaging on the engine.

In a first variant, represented in Figure 7, the lower portion 127 is bell-shaped, or campanulate. In other words, the bore wall 1 1 1 is curved outwards. The lower portion 127 can, for example, be profiled by controlling honing of the bore wall 1 1 1. The operation of the first variant is unchanged from that of the pump 101 in accordance with the first embodiment of the present invention.

In a second variant, represented in Figure 8, the upper portion 125 is right cylindrical and has a first constant diameter D1 , and the lower portion 127 is right cylindrical and has a second constant diameter D2. The upper portion 125 and the lower portion 127 are concentric. The diameter D2 of the lower portion 127 is larger than the diameter D1 of the upper portion 125. The upper portion 125 and the lower portion 127 are connected via a connecting portion 129. The connecting portion 129 has a diameter which gradually increases from the upper portion 125 to the lower portion 127. The operation of the second variant is unchanged from that of the pump 101 in accordance with the first embodiment of the present invention.

A second embodiment 201 of the pump according to the present invention is shown in Figure 9 and 10. Only the differences in relation to the first embodiment 101 are described below. As shown in Figure 9 and 10, the bore 209 is substantially cylindrical. The pumping plunger 213 has a tapered section which is tapered towards the second end 217.

In the embodiment represented in Figure 9, the pumping plunger 213 has an upper region 231 and a lower region 233. The upper region 231 is substantially right cylindrical. The lower region 233 is tapered, having a larger diameter proximal to the upper region 213 than at the low pressure end 217. The difference between the diameter in the upper region 213 and the diameter at the low pressure end 217 comprises between 1 and 2 microns. The plunger is tapered by grinding in the present embodiment.

It will be appreciated that operation of the pump 201 according to the second embodiment is unchanged from that of the first embodiment. However, in the second embodiment, the pumping plunger 213 is profiled to increase the clearance between the low-pressure end 217 of the pumping plunger 213 and the bore wall 21 1 . The plunger 213 according to the second embodiment can be manufactured more readily than the tapered bore 109 of the first embodiment. The bore 209 of the pump head housing 203 can be made cylindrical throughout. The effect is virtually unchanged, but in this case some allowance should be made for the motion of the plunger 213 which affects clearance dynamically through its stroke.

In a further variant, as represented in Figure 10, the plunger 213 is tapered along its entire length, having a larger diameter at the high pressure end 215 than at the low pressure end 217.

It will be appreciated that features of the pumps 101 , 201 according to the first and second embodiments described herein could be combined. For example, a modified pump could comprise a tapered bore and a tapered pumping element of the type described herein.

Claims

CLAIMS:

1 . A pump for supplying high-pressure fuel to a common rail fuel injection system, the pump comprising:

a head housing (103, 203);

a pumping chamber (1 19, 219) defined within the head housing (103, 203);

a bore (109, 209) extending from the pumping chamber (1 19, 219); and

a pumping element (1 13, 213) being arranged for reciprocal movement within the bore (109, 209);

wherein the pump comprises means for compensating for dilation of the pumping element (1 13, 213) and/or the bore (109, 209) under operational loads and wherein, when the pumping element and the bore are not under operational loads, said dilation compensating means is in the form of a first radial clearance defined between the bore (109, 209) and the pumping element (1 13, 213) at a first axial location and a second radial clearance defined between the bore (109, 209) and the pumping element (1 13, 213) at a second axial location, and wherein the second radial clearance is larger than the first radial clearance, the first axial location being positioned closer to the pumping chamber ( 1 19, 219) than the second axial location.

2. A pump as claimed in claim 1 , wherein the first axial location is disposed in a first portion (125) of the bore (109, 209) and the second axial location is disposed in a second portion (127) of the bore (109, 209), the first portion (125) being disposed proximal to the pumping chamber (1 19, 219) and the second portion (127) being spaced apart from said pumping chamber (1 19, 219).

3. A pump as claimed in claim 2, wherein said second portion (127) of the bore (109) is flared outwardly.

4. A pump as claimed in claim 3, wherein said second portion (127) of the bore (109) is frustoconical.

5. A pump as claimed in claim 3, wherein said second portion (127) of the bore (109) is curved outwardly.

6. A pump as claimed in any one of claim 2 to 5, wherein said second portion (127) of the bore (109) defines an opening (123) disposed opposite to the pumping chamber (1 19), the second axial location being disposed at or proximal to said opening (123).

7. A pump as claimed in claim 2, wherein the first portion (125) of the bore (109) comprises a first right cylindrical portion having a first diameter (D1 ) and the second portion (127) of the bore (109) comprises a second right cylindrical portion having a second diameter (D2); wherein the first diameter (D1 ) is less than the second diameter (D2).

8. A pump as claimed in any one of the preceding claims, wherein the pumping element (213) comprises a first end (215) which partially defines the pumping chamber (219) and a second end (217) opposite to the first end (215), wherein the pumping element (213) comprises a tapered section (233) which is tapered towards the second end (217).

9. A pump as claimed in claim 8, wherein the tapered section (233) extends from the first end (215) to the second end (217).

10. A pump as claimed in any one of claims 1 to 7, wherein the pumping element (213) comprises a first right cylindrical region having a first diameter and a second right cylindrical region having a second diameter, the first right cylindrical region being positioned closer to the pumping chamber (219) than the second right cylindrical region, and wherein the first diameter is larger than the second diameter.