WO2016055250A1

WO2016055250A1 - Pump for supplying fuel at high pressure to an internal combustion engine

Info

Publication number: WO2016055250A1
Application number: PCT/EP2015/071329
Authority: WO
Inventors: Angelo Forte; Sunilkumar Vemula
Original assignee: Robert Bosch Gmbh
Priority date: 2014-10-09
Filing date: 2015-09-17
Publication date: 2016-04-14

Abstract

Pump (1) for supplying fuel at high pressure to an internal combustion engine, of the type comprising: a head (4); an elongate piston (7) which engages, in an axially slidable manner, a first cylindrical cavity (8) created in the head (4), forming, inside the same head (4), a variable-volume chamber (9) intended to receive the fuel to be pumped to the internal combustion engine; and a ring seal (18) which is positioned inside the first cylindrical cavity (8) so as to be engaged in a passing manner by the piston (7), the pump also comprising a sleeve (19) having a rectilinear through hole (20) which can be engaged in a passing and axially slidable manner by a portion of the piston (7).

Description

Description Title

PUMP FOR SUPPLYING FU EL AT HIGH PRESSU RE TO AN INTERNAL COMBUSTION ENGINE

The present invention relates to a pump for supplying fuel at high pressure to an internal combustion engine.

In greater detail, the present invention relates to a high-pressure pump for common rail systems for supplying fuel to an internal combustion engine, to which the following description will explicitly refer without thereby losing any of its generality.

As is known, high-pressure pumps of the most widely used common rail systems at the present time comprise a pump body, structured to be fixed to the cylinder block of the internal combustion engine; a driving shaft fitted in an axially rotatable manner inside the pump body and capable of being connected to the driving shaft of the internal combustion engine so as to be made to rotate by the latter; and a head positioned so as to close the upper end of a large cylindrical cavity, which extends in the pump body coaxially with a reference axis which is locally perpendicular to the axis of rotation of the driving shaft, and has its lower end facing the periphery of a lobed cam fixed to the driving shaft.

Additionally, the high-pressure pumps of common rail systems comprise an elongate pump piston which is fitted in an axially slidable manner in a cylindrical cavity extending inside the head coaxially to the cylindrical cavity of the pump body, so as to form a variable-volume chamber inside the head, and which also projects from the lower end of the head until it reaches and bears on the peripheral surface of the lobed cam; a coil spring which is positioned inside the cylindrical cavity of the pump body, with a first end bearing on the head and a second end bearing on the lower end of the pump piston, and is structured so as to oppose the rising of the piston inside the cylindrical cavity of the head with consequent reduction of the volume of the variable-volume chamber, while simultaneously keeping the lower end of the piston constantly bearing on the peripheral surface of the lobed cam; and a large ring seal which is positioned inside the cylindrical cavity of the head, near the lower end of the head, so as to be engaged in passing by the piston, and which is structured so as to surround the piston in order to prevent the seepage of the fuel toward the cylindrical cavity of the pump body during the descent and rise of the piston.

Finally, high-pressure pumps of common rail systems are provided with an intake valve and a delivery valve which are fixed to the upper part of the head in the variable-volume chamber. The intake valve controls the ingress of the fuel from the low-pressure pump into the variable-volume chamber of the head, while the delivery valve controls the outflow of the fuel at high pressure from the variable- volume chamber of the head toward the internal combustion engine.

Regrettably, none of the annular sealing gaskets currently in use is capable of completely eliminating losses of fuel from the head, owing to the very high pressures reached by the fuel during the rise of the piston inside the head.

The object of the present invention is to provide a ring seal which is more effective in preventing the seepage of fuel from the head toward the cylindrical cavity pump body.

In line with this objective, according to the present invention, a pump for supplying fuel at high pressure to an internal combustion engine, as defined in claim 1 and preferably, but not necessarily, in any of the claims dependent thereupon, is provided.

The present invention will now be described with reference to the attached drawings, which show a non-limiting exemplary embodiment of the invention, in which: - Figure 1 is a perspective view, with parts in cross section and parts removed for clarity, of a pump for supplying fuel at high pressure to an internal combustion engine, made according to the teachings of the present invention;

- Figure 2 is a front view of a detail of the pump shown in Figure 1;

- Figure 3 is a partially sectional perspective view of the ring seal shown in Figure 2; while

- Figure 4 is a partially sectional perspective view of a different embodiment of the ring seal shown in Figure 3.

With reference to Figure 1, the number 1 indicates the whole of a pump for supplying fuel at high pressure to an internal combustion engine (not shown), which can advantageously be used in systems for supplying fuel to an internal combustion engine.

In greater detail, the pump 1 can advantageously be used in common rail fuel supply systems, which are used for supplying diesel fuel at high pressure to a diesel engine.

The pump 1 comprises an outer pump body 2 which is preferably structured so as to be fixed to the body of the internal combustion engine (not shown); a driving shaft 3 which is fitted in an axially rotatable manner inside the pump body 2, preferably with the interposition of suitable bearings and/or bronze bushes, so that it can rotate freely about its longitudinal axis A; and a head 4 which is positioned so as to close the upper end/inlet of a large cylindrical cavity 5 which extends in the pump body 2 transversely to the axis of rotation A of the driving shaft 3, and has its lower end/inlet directly facing the periphery of a lobed cam 6 provided on the driving shaft 3.

In greater detail, in the illustrated example the pump body 2 is preferably structured so as to be fixed rigidly to the cylinder block of a generic internal combustion engine (not shown). On the other hand, the cylindrical cavity 5 extends in the pump body 2 coaxially with a reference axis B which is preferably locally perpendicular to the axis of rotation A of the driving shaft 3.

Additionally, the driving shaft 3 is preferably structured so as to project at one end from the pump body 2, at which end it can be mechanically connected to the drive shaft of the internal combustion engine (not shown) so that the driving shaft 3 can be made to rotate by the internal combustion engine. Preferably, the lobed cam 6 is also made in one piece with the driving shaft 3.

With reference to Figure 1, the pump 1 also comprises a pump piston 7 of elongate shape, which engages in an axially slidable manner with a

corresponding cylindrical cavity 8 extending inside the head 4 coaxially with a reference axis C which is preferably locally perpendicular to the axis of rotation A of the driving shaft 3, and which preferably, but not necessarily, also coincides with the longitudinal axis B of the cylindrical cavity 5, so as to form, inside the head 4, a small closed variable-volume chamber 9 for accommodating the fuel to be pumped to the internal combustion engine (not shown).

The pump 1 is also provided with an intake valve 10 and a delivery valve 11, which are located on the upper part of the head 4, near the variable-volume chamber 9, and are structured so as to control the inflow and outflow of the fuel to and from the variable-volume chamber 9 resulting from the variations of the internal volume of the variable-volume chamber 9.

With reference to Figures 1 and 2, the pump piston 7 also projects from the lower end of the head 4 and extends into the cylindrical cavity 5 coaxial with the axis C, until it reaches the cam 6, in such a way that the driving shaft 3, rotating about its longitudinal axis A, can impart to the piston 7 a reciprocating rectilinear motion inside the cylindrical cavity 8, resulting in a cyclic variation of the volume of the variable-volume chamber 9 provided in the head 4. In greater detail, in the illustrated example the lower end of the piston 7 preferably bears on the peripheral surface of the cam 6, preferably, but not necessarily, with the interposition of a cam follower 12.

Preferably, the pump 1 also comprises a coil spring 13, which is positioned inside the cylindrical cavity 5 with a first end bearing on the head 4 and a second end bearing on the lower end of the pump piston 7 and/or on the cam follower 12, and is structured so as to oppose the rising of the piston 7 inside the cylindrical cavity 8 and the consequent reduction of the volume of the variable-volume chamber 9, while simultaneously keeping the lower end of the piston 7, or the cam follower 12 if present, constantly bearing on the peripheral surface of the lobed cam 6.

With reference to Figure 1, in the illustrated example, in particular, the intake valve 10 is structured so as to allow the selective inflow of the fuel at low pressure into the variable-volume chamber 9 when the piston 7 moves toward the driving shaft 3, causing an increase in the volume of the variable-volume chamber 9, and is interposed between the variable-volume chamber 9 and a fuel supply line (not shown) which is structured so as to carry the fuel at low pressure entering the variable-volume chamber 9.

In greater detail, in the illustrated example the intake valve 10 is preferably interposed between the variable-volume chamber 9 and the delivery end of a gear pump of a known type, which can draw the fuel from a generic fuel tank (not shown) and then send it into the pump 1 at a pressure of several bar.

On the other hand, the delivery valve 11 is structured so as to allow the selective outflow of the fuel at high pressure from the variable-volume chamber 9 when the piston 7 moves away from the driving shaft 3, causing a reduction in the volume of the variable-volume chamber 9, and is interposed between the variable-volume chamber 9 and a fuel discharge line 14 which is structured so as to carry the fuel at high pressure leaving the variable-volume chamber 9 toward the internal combustion engine (not shown). The intake valve 10 and the delivery valve 11 are two components which are already well known in the field of high-pressure fuel pumps for common rail systems, and therefore will not be described further except to specify that they are preferably housed inside the head 4.

On the other hand, the cam follower 12 is preferably structured so as to engage the lower end of the cylindrical cavity 5 in an axially slidable manner, and preferably comprises a cup-shaped body 15 which has a shape substantially complementary to that of the cylindrical cavity 5, and is inserted in an axially slidable manner into the lower end of the cylindrical cavity 5, so as to bear stably on the lower end of the piston 7 while being able to move in a reciprocating rectilinear manner inside the pump body 2; and a cylindrical body 16, which is positioned to bear rotatably on the peripheral surface of the cam 6, with the longitudinal axis locally parallel to the axis of rotation A of the driving shaft 3, and is housed in a freely axially rotatable manner inside a seat suitably formed on the end wall of the cup-shaped body 15.

The rotating cylindrical body 16 is therefore interposed between the cup-shaped body 15 and the peripheral surface of the cam 6, while the lower end of the piston 7 preferably bears directly on the end wall of the cup-shaped body 15 on the opposite side from the rotating cylindrical body 16.

With reference to Figures 1, 2 and 3, the pump 1 is also provided with a large ring seal 18, which is positioned inside the cylindrical cavity 8, preferably near the lower end of the head 4, so as to be engaged in a passing manner by the piston 7, and is structured so as to surround the piston 7 to prevent the fuel from seeping from the variable-volume chamber 9 and reaching the cylindrical cavity 5 of the pump body 2 during the descent and rise of the piston 7 along the cylindrical cavity 8.

In greater detail, the ring seal 18 comprises a sleeve 19 of substantially cylindrical shape, which is located inside the cylindrical cavity 8 in such a way that its longitudinal axis D is locally coincident with the longitudinal axis C of the piston 7, and is provided centrally with a rectilinear through hole 20 which extends coaxially with the longitudinal axis D and is designed so as to be engaged in a passing and axially slidable manner by a portion of the piston 7.

The sleeve 19 therefore has a first axial end 19a facing the variable-volume chamber 9, and a second axial end 19b facing the cylindrical cavity 5 and the driving shaft 3.

In the illustrated example, in particular, the sleeve 19 is preferably made of plastic material, and is preferably located inside an annular seat 21 suitably formed along the cylindrical cavity 8, being set back from the lower end of the head 4.

In greater detail, in the illustrated example the sleeve 19 is preferably made of polytetrafluoroethylene.

With reference to Figures 2 and 3, the sleeve 19 is provided internally with a plurality of projecting annular lips, which are spaced apart inside the central through hole 20, and are structured so as to remain stably bearing on the lateral surface of the piston 7 without a break in continuity over the whole perimeter of the piston 7, so as to prevent the seepage of fuel via the through hole 20 toward the cylindrical cavity 5.

In greater detail, a first group of annular sealing lips 22 is positioned inside the central hole 20, being set back from the mouth 20a of the central hole 20 located in the center of the axial end 19a of the sleeve; while a second group of annular sealing lips 23 is positioned inside the central hole 20, being set back from the mouth 20b of the central through hole 20 located in the center of the axial end 19b of the sleeve 19.

In other words, the mouth 20a of the central hole 20 faces the variable-volume chamber 9, while the mouth 20b of the central hole 20 faces the cylindrical cavity 5 and the driving shaft 3.

With reference to Figures 2 and 3, the sleeve 19 is also provided with a large outer annular projection 24, which is positioned on the outer lateral surface 19c of the sleeve 19, being set back from the axial end 19a of the sleeve 19 facing the variable-volume chamber 9, so as to be substantially coplanar with the annular lips 22 provided inside the central hole 20, and is structured so as to remain stably bearing on the surface of the head 4.

The outer annular projection 24 is also provided with one or more transverse through slits 25, each of which is designed so as to allow the fuel which seeps from the variable-volume chamber 9 and then descends along the lateral surface of the piston 7 to the sleeve 19 to pass beyond or penetrate the annular projection 24 and to seep additionally along the outer lateral surface 19c of the sleeve 19, inside the seat 21.

In greater detail, in the illustrated example the annular projection 24 is positioned coaxially with the longitudinal axis D of the sleeve 19, and therefore with the longitudinal axis C del piston 7, and is preferably provided with a plurality of transverse through slits 25 (four through slits 25 in the illustrated example), which are preferably spaced at equal angular intervals around the longitudinal axis D of the sleeve 19.

Still with reference to Figures 2 and 3, the sleeve 19 is also provided externally with at least two projecting annular sealing ridges 26 which are positioned on the outer lateral surface 19c of the sleeve 19, with one adjacent to the other, being set back from the axial end 19b of the sleeve facing the cylindrical cavity 5 and the driving shaft 3, so as to be locally substantially coplanar with the annular sealing lips 23, and are both structured so as to remain stably bearing on the surface of the head 4 without any break in continuity along the whole perimeter of the sleeve 19, so as to prevent any fuel seepage toward the cylindrical cavity 5.

Like the outer annular projection 24, in the illustrated example the two outer annular ridges 26 are also preferably coaxial with the longitudinal axis D of the sleeve 19, and therefore with the longitudinal axis C of the piston 7.

With reference to Figures 2 and 3, the ring seal 18 preferably also comprises a first expansion spring 27 of annular shape, which is fitted into the body of the sleeve 19 so as to be substantially coaxial with the axis D, and locally

substantially coplanar with the inner annular lips 22 and with the outer annular projection 24, and is structured so as to urge the annular lips 22 in a resilient manner against the surface of the piston 7, and to urge the annular projection 24 against the surface of the head 4.

In the illustrated example, in particular, the expansion spring 27 is positioned inside an annular channel 28, which is formed on the axial end 19a of the sleeve 19 facing the variable-volume chamber 9, so as to surround the mouth 20a of the central through hole 20 without a break in continuity.

With reference to Figures 2 and 3, the ring seal 18 preferably also comprises a second expansion spring 29 of annular shape, which is fitted into the body of the sleeve 19 so as to be substantially coaxial with the axis D, and locally

substantially coplanar with the inner annular lips 23 and with the two outer annular ridges 26, and is structured so as to urge the annular lips 23 in a resilient manner against the surface of the piston 7, and to urge the annular ridges 26 against the surface of the head 4.

In the illustrated example, in particular, the expansion spring 29 is positioned inside an annular channel 30, which is formed on the axial end 19b of the sleeve 19 facing the cylindrical cavity 4 and the driving shaft 3, so as to surround the mouth 20b of the central through hole 20 without a break in continuity.

With reference to Figures 2 and 3, the sleeve 19 is also provided, in its interior, with a small annular pressure chamber, which is interposed between the inner annular lips 23 and the two outer annular ridges 26, and communicates with the outer lateral surface 19c of the sleeve 19 upstream of the two outer annular ridges 26, or rather between the outer annular projection 24 and the two outer annular ridges 26, so as to receive the fuel which penetrates the outer annular projection 24 and seeps along the outer lateral surface 19c of the sleeve.

In other words, the annular pressure chamber is formed inside the body of the sleeve 19, so as to surround the through hole 20 and the inner annular lips 23. Preferably, the annular pressure chamber is also positioned between the annular lips 23 and the expansion spring 29.

In greater detail, with reference to Figures 2 and 3, in the illustrated example the sleeve 19 has, along the portion of its outer lateral surface 19c delimited by the outer annular projection 24 by the two outer annular ridges 26, a series of oblique blind holes 31 which are angularly spaced around the longitudinal axis D of the sleeve 19, and extend obliquely inside the body of the sleeve 19, toward the axial end 19b of the sleeve, so as to reach the plane P in which the inner annular lips 23 and the outer annular ridges 26 lie.

Preferably, the oblique blind holes 31 are also spaced at equal angular intervals around the longitudinal axis D of the sleeve 19. In the illustrated example, in particular, the oblique blind holes 31 are preferably shaped so as to reach the plane P in which the inner annular lips 23 and the outer annular ridges 26 lie, between the inner annular lips 23 and the annular channel 30 which accommodates the expansion spring 29. In greater detail, in the illustrated example, the oblique blind holes 31 are preferably rectilinear holes, and extend inside the body of the sleeve 19 parallel to the generatrix of the same cone coaxial with the longitudinal axis D of the sleeve 19. With reference to Figures 2 and 3, finally, the sleeve 19 is also preferably provided with a further outer annular projection 32, which is positioned on the outer lateral surface 19c of the sleeve 19, between the mouths of the oblique blind holes 31 and the two outer annular ridges 26, or immediately upstream of the outer annular ridges 26, and is structured so as to project radially toward the head 4 without coming into contact with the surface of the head 4, so that it can form, on the outer lateral surface 19c of the sleeve, an annular constriction s for creating a laminar fuel flow. In other words, the annular constriction s is designed so as to create a sudden pressure drop in the fuel which penetrates the annular projection 32 in the direction of the two outer annular ridges 26. The operation of the pump 1 can easily be deduced from the above description, and requires no further explanation.

However, as regards the ring seal 18, the fuel which seeps from the variable- volume chamber 9 descends along the lateral surface of the piston 7 until it reaches the upper axial end 19a of the sleeve 19. When the sleeve 19 is reached, the fuel from the variable-volume chamber 9 seeps into the seat 21 until it reaches the outer annular projection 24. Passing the through slit or slits 25, the fuel from the variable-volume chamber 9 then penetrates the outer annular projection 24, and seeps along the outer lateral surface 19c of the sleeve 19, until it reaches the oblique blind holes 31, in which it accumulates.

Since the fuel seeping from the variable-volume chamber 9 usually has a pressure of more than about ten bar, the fuel present in the oblique blind holes 31 tends to expand the sleeve 19 locally, thus significantly increasing the radial force with which the inner annular lips 23 press against the surface of the piston 7, and the outer annular ridges 26 press against the surface of the head 4.

In turn, the larger radial force exerted by the inner annular lips 23 on the outer surface of the piston 7 increases the capacity of the annular sealing lips 23 to remove or detach, during the rise of the piston 7 inside the head 4, the film of lubricating oil adhering to the outer surface of the piston 7.

The constriction s formed by the outer annular projection 32 upstream of the two outer annular ridges 26 causes a reduction in the pressure of the fuel reaching the outer annular ridges 26, so as to increase the pressure difference between the fuel flowing over the outer annular ridges 26 and the fuel located inside the oblique blind holes 31. This pressure difference causes a further increase in the radial force with which the two outer annular ridges 26 press on the surface of the head 4.

The particular structure of the ring seal 18 offers numerous benefits.

The ring seal 18 can make use of the pressure of the fuel which seeps from the variable-volume chamber 9 to increase the grip of the sleeve 19 on the surface of the piston 7.

Finally, the pump 1 can evidently be modified and varied without departure from the scope of the present invention.

For example, with reference to Figure 4, in a different embodiment, instead of being formed by the oblique blind holes 31, the annular pressure chamber of the sleeve 19 can consist of a large annular channel 34 of flared shape, which, starting from the outer lateral surface 19c of the sleeve, extends obliquely inside the body of the sleeve 19, toward the axial end 19b of the sleeve, so as to reach the plane P in which the inner annular lips 23 and the outer annular ridges 26 lie, and so as to surround the inner annular lips 23 without any break in continuity.

In the illustrated example, in particular, the annular channel 34 extends inside the body of the sleeve 19, following a substantially frustoconical profile coaxial with the axis D and converging toward the mouth 20b of the central through hole 20, until it reaches the plane P in which the inner annular lips 23 lie, between the inner annular lips 23 and the annular channel 30 which accommodates the expansion spring 29.

Still with reference to Figure 4, in a less complex embodiment which is not shown, the sleeve 19 may also have, in place of the two outer annular sealing ridges 26, a large annular sealing projection 35 which is structured so as to remain stably bearing on the surface of the head 4, without a break in continuity along the whole perimeter of the sleeve 19.

Claims

1. A pump (1) for supplying fuel at high pressure to an internal combustion engine, of the type comprising: a head (4); an elongate piston (7) which engages, in an axially slidable manner, a first cylindrical cavity (8) created in the head (4), forming, inside the same head (4), a variable-volume chamber (9) intended to receive the fuel to be pumped to the internal combustion engine; and a ring seal (18) which is positioned inside the first cylindrical cavity (8) so as to be engaged in a passing manner by the piston (7);

the ring seal (18) comprising a sleeve (19) which has a rectilinear through hole (20) that can be engaged in a passing and axially slidable manner by a portion of the piston (7); said sleeve (19) being provided, inside the through hole (20), with at least a first annular sealing lip (22) set back from a first mouth (20a) of the through hole (20) facing said variable-volume chamber (9), and with at least a second annular sealing lip (23) set back from a second mouth (20b) of the through hole (20), opposite said first mouth (20a);

the pump (1) being characterized in that said sleeve (19) is provided with annular sealing means (26, 35) which are positioned on the outer lateral surface of the sleeve (19c), being set back from the axial end (19b) of the sleeve facing in the opposite direction from said variable-volume chamber (9), so as to be locally substantially coplanar with said at least one second annular sealing lip (23); and with an annular pressure chamber (31, 34) which is formed inside the body of the sleeve (19), between said annular sealing means (26, 35) and said at least one second annular sealing lip (23), and communicates with the outer lateral surface of the sleeve (19c) upstream of said annular sealing means (26, 35) so as to receive the fuel which seeps along the outer lateral surface of the sleeve (19c).

2. The pump as claimed in claim 1, wherein the sleeve (19) also comprises an outer annular projection (24) which is positioned on the outer lateral surface of the sleeve (19c), being set back from the axial end (19a) of the sleeve facing toward the variable-volume chamber (9), and can bear on the surface of the head (4); said outer annular projection (24) being provided with one or more slits (25) capable of allowing the fuel to pass beyond the aforesaid outer annular projection (24) and seep over the outer lateral surface (19c) of the sleeve (19); the pressure chamber (31, 34) of the sleeve (19) being in communication with the outer lateral surface of the sleeve (19c) between said annular sealing means (26, 35) and said outer annular projection (24).

3. The pump as claimed in claim 2, wherein said outer annular projection (24) is positioned so as to be locally substantially coplanar with said at least one first annular sealing lip (22).

4. The pump as claimed in any of the preceding claims, wherein said annular sealing means (26, 35) comprise at least a projecting annular ridge (26) or an annular projection (35), which is structured so as to remain stably bearing on the surface of the head (4), without a break in continuity along the whole perimeter of the sleeve (19).

5. The pump as claimed in any of the preceding claims, wherein the ring seal (18) also comprises a first expansion spring (29) of annular shape, which is fitted into the body of the sleeve (19) so as to be locally substantially coplanar with said annular sealing means (26, 35) and with said at least one second annular sealing lip (23), and is structured so as to urge said annular sealing means (26, 35) in a resilient manner against the surface of the head (4), and to urge said at least one second annular sealing lip (23) against the surface of the piston (7).

6. The pump as claimed in claim 5, wherein the annular pressure chamber (31, 34) of the sleeve (19) is positioned between said first expansion spring (29) and said at least one second annular sealing lip (23).

7. The pump as claimed in any of claims 2 to 6, wherein the ring seal (18) also comprises a second expansion spring (27) of annular shape, which is fitted into the body of the sleeve (19) so as to be locally substantially coplanar with said outer annular projection (24) and with said at least one first annular sealing lip (22), and is structured so as to urge said outer annular projection (24) in a resilient manner against the surface of the head (4), and to urge said at least one first annular sealing lip (22) against the surface of the piston (7).

8. The pump as claimed in any of the preceding claims, wherein the annular pressure chamber (31, 34) of the sleeve (19) comprises a series of oblique blind holes (31) which are formed on the outer lateral surface of the sleeve (19c) upstream of said annular sealing means (26, 35), being angularly spaced around the longitudinal axis (D) of the sleeve (19), and extend obliquely inside the body of the sleeve (19) toward the axial end (19b) of the sleeve facing in the opposite direction from said variable-volume chamber (9), so as to reach the plane (P) on which said annular sealing means (26, 35) and said at least one second annular sealing lip (23) lie.

9. The pump as claimed in claim 8, wherein said oblique blind holes (31) are spaced at equal angular intervals around the longitudinal axis (D) of the sleeve (19).

10. The pump as claimed in claim 8 or 9, wherein said oblique blind holes

(31) are rectilinear and extend inside the body of the sleeve (19) parallel to the generatrix of the same cone coaxial with the longitudinal axis (D) of the sleeve (19).

11. The pump as claimed in any of claims 1 to 7, wherein the annular pressure chamber (31, 34) of the sleeve (19) comprises a large annular channel of flared shape (34) which, starting from the outer lateral surface of the sleeve (19c), extends obliquely inside the body of the sleeve (19) toward the axial end (19b) of the sleeve facing in the opposite direction from said variable-volume chamber (9), so as to reach the plane (P) on which said annular sealing means

(26, 35) and said at least one second annular sealing lip (23) lie.

12. The pump as claimed in any of the preceding claims, wherein the sleeve (19) is also provided with a further outer annular projection (32), which is positioned on the outer lateral surface of the sleeve (19c), upstream of said annular sealing means (26, 35), and projects radially toward the head (4) so as to form, on the outer lateral surface of the sleeve (19c), an annular constriction for creating a laminar fuel flow (s).

13. The pump as claimed in any of the preceding claims, wherein the sleeve (19) is made of plastic material, preferably polytetrafluoroethylene.

14. The pump as claimed in any of the preceding claims, wherein said pump (1) also comprises a pump body (2) and a driving shaft (3) fitted in an axially rotatable manner inside the pump body (2); the head (4) being at least partially housed inside a second cylindrical cavity (5) which extends in the pump body (2) transversely to the driving shaft (3), and has one end facing a cam (6) provided on said driving shaft (3); said piston (7) being structured so as to project from the head (4) and extend inside the second cylindrical cavity (5) until it reaches said cam (6), so that the driving shaft (3), rotating around its longitudinal axis (A) can impart a reciprocating rectilinear motion to the piston (7) inside the first cylindrical cavity (8).