The present invention relates to a cam actuated high pressure pump and more particularly to a plunger arrangement limiting fuel leaks.

BACKGROUND OF THE FNVENTION

A diesel internal combustion engine (ICE) comprises a fuel injection equipment wherein fuel sucked in a low pressure tank is pressurized at 2000 bars or higher in a cam actuated high pressure pump, prior to be delivered to a high pressure reservoir, or common rail, to which are connected a plurality of fuel injectors. A command unit controls the equipment as a function of the demand of fuel from the ICE.

In the high pressure pump, fuel is pressurized in a compression chamber defined between the head end of a plunger and the blind end of a bore. The plunger reciprocates in the bore therein performing a pumping cycle during which the inner volume of the compression chamber is varied. As said volume reduces, part of the fuel, instead of being pressurized, leaks between the plunger and the bore through a clearance that enlarges as the pressure in the chamber rises.

To limit or reduce said leaks to the minimum acceptable still enabling plunger to bore lubrication several solutions have been tested such as plungers provided with a recess opening on their head end. In operation the recess is filled with fuel and, when the pressure rises the peripheral wall of the recess slightly expands and closes, at least partially, the clearance.

Unfortunately, the recess augments the quantity of pressurized fuel in said volume then, the pressure of said volume alternatively rises and drops, this representing important volumetric loss.

SUMMARY OF THE FNVENTION

Accordingly, it is an object of the present invention to resolve the above mentioned problems in providing a high pressure fuel pump having a pumping head wherein a compression chamber is defined between the blind end of a bore and the head end of a plunger slidably adjusted in said bore and adapted to perform therein a pumping cycle varying the volume of said compression chamber.

The pump is further provided with inlet and outlet valve assemblies for controlling fuel flowing in and out of said compression chamber.

Advantageously, the plunger is provided with a blind bore defining a chamber open in the head end of the plunger and being in fluid communication with the compression chamber. The end face of the plunger is limited to a peripheral annular face surrounding said opening. Said bore further defines a peripheral cylindrical wall and a bottom face and wherein, a filler member is arranged inside said chamber.

The filler member has a slightly narrower section than the inner diameter of the bore so that, an annular gap is defined between the filler member and the inner face of the cylindrical wall of the blind bore.

Said gap is smaller than 20μιη, preferably around ΙΟμιη to the radius. Also, the filler member is made of low density material.

In particular, the filler member is made of aluminum or of an inert polymer or of ceramic material.

More precisely, the filler member is unfixed to the plunger, the filler member being free to move inside the chamber.

Also, the space between the blind bore and the filler member is filled with fuel.

The filler member has a height slightly smaller than the depth of the blind bore so that, when the filler member is inside the chamber. The upper face of the filler member is in flush surface continuity with the top peripheral annular face of the plunger defining a bottom clearance between the under face of the filler member and the bottom face of the bore.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now described by way of example with reference to the accompanying drawings in which:

Figure 1 is a general section of a high pressure pump as per the invention.

Figure 2 is magnified portion of the compression chamber of the pump of figure 1. Figure 3 is a further magnified view of the plunger of the pump of figure

1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In reference to figure 1 is described a high pressure fuel pump 10 adapted to be arranged in a diesel fuel injection equipment not represented. The pump 10 comprises a body 12 wherein a camshaft 14 arranged between two bearings is adapted to rotate about a cam axis Y. On the body 12 is fixedly arranged a pumping head 16 wherein fuel entering via an inlet 18 in a compression chamber 20 exits after being pressurized via an outlet 22.

In use, fuel compression occurs during a pumping cycle wherein a plunger 24 reciprocally slides in a bore 26 of the pumping head 16 between a bottom dead centre (BDC) position and a top dead centre (TDC) position. A functional annular clearance C 1 is kept between the plunger 24 and the bore 26 to ensure sliding and lubrication of the plunger. The plunger 24 extends along a pumping axis X, perpendicular to the cam axis Y, from a low pressure end 28 protruding outward the pumping head 16 and cooperating via a cam follower 30 with a cam 32 of the camshaft 14, to a high pressure end 34 that is inside the bore and which extreme transverse face 36, or top face 36 of the plunger, partly defines the compression chamber 20.

The inlet 18 in controlled by an inlet valve member 38 arranged at the end of the bore 26 and, the outlet 22 radially arranged relative to the compression chamber 20 in controlled by an outlet valve member 40. In the representation of figure 1 the outlet valve member 40 is a check valve with a ball biased by a coil spring in a closed position against a conical seating face. The pump represented on figure 1 is chosen as a non-limiting example and, in alternative embodiments the inlet and outlet channels are arranged differently for instance both being parallel, or slightly angled, to the plunger axis.

The compression chamber 20 is the space fully defined between said extreme transverse face 36 of the plunger, the cylindrical lateral face of the bore and the under face of the inlet valve member 38 or, in alternative embodiments the transverse end face of the bore. As visible on figure 2 the short drilling between the pumping chamber and the outlet ball valve seat is also part of the pressurized volume.

Finally a pump spring 42 compressed between the pumping head 16 and the a spring seat fitted with the plunger, the plunger 24 pushing the cam follower 30 toward BDC against the cam 32.

In use, when the camshaft 14 rotates, the cam 32 imparts to the plunger 24 said reciprocating movement performing said pumping cycle during which the inner volume of the compression chamber 20 is varied.

The region of the compression chamber 20 now detailed in reference to figures 2 and 3 shows that the outlet 22 radially opens in the final portion 44 of the bore and that the high pressure end 34 of the plunger, when in TDC, extends in said enlarged final portion 44. In order to avoid closing the outlet 22 with the plunger 24, said final portion 44 of the bore has a slightly enlarged section.

Also, the plunger 24 is provided on its high pressure end 34 with a deep recess forming a cylindrical blind bore 46 axially X extending inside the plunger from the top face 36 toward a bottom face 48. The blind bore 46 has a diameter D46 and a length L46, or depth L46, said bore 46 defining an open chamber 50 limited by a peripheral cylindrical wall 52 and the bottom face 48. The chamber 50 is open at the very top of the plunger and therefore the top face 36 of the plunger is limited to the peripheral annular face surrounding the opening of the inner bore 46 in the compression chamber 20.

Inside the inner bore 46 is arranged a filler member 54 having a cylindrical outer face 56 of diameter D54, slightly smaller than the diameter D46 of the bore and, a height H54 slightly smaller than the depth L46 of the bore, the filler member 54 extending from an under face 58 to a upper face 60. In place in the chamber 50, said dimensions D54, H54, being slightly smaller than the corresponding dimensions of the chamber, define an annular gap G between the cylindrical outer face 56 of the filler member and the cylindrical peripheral wall 52 of the bore and, a bottom clearance C2 between the under face 58 of the filler member and the bottom face 48 of the bore, the upper face 60 of the filler member being substantially in flush surface continuity with the top annular face 36 of the plunger. There may be a small, fluid- filled clearance C2, but it is not essential to the design. Ideally, height H54 is equal to or slightly smaller than depth L46 of the bore but, provided the filler member 54 does not interfere with the inlet valve at TDC, then it would be feasible to make the height H54 greater than the depth L46.

In use, said gap G and said bottom clearance C2 are filled with fuel and, to ensure this filling, the chamber 50 is first filled with fuel then, the filler member is inserted in the bore thus evacuating the fuel via the gap G and ensuring that no air is retained under the filler member and that, when fully inserted, the bottom clearance C2 is filled with fuel and no air remains captured therein.

It is to be noted that the filler member 54 is not fixed or attached by any means to the plunger. The filler member 54 is therein free to move.

In use, when the plunger 24 reciprocates, the fuel captured in the gap G and in the bottom clearance C2 keep the filler member 54 inside the chamber. Said fuel acts as a suction pad retaining the filler member 54 in place.

Furthermore, the high pressure generated in the compression chamber 20 applies on the upper face 60 of the filler member a force that contributes to its positioning inside the chamber 50. To further ease said retention into the chamber 50, the filler member 54 is made of a light material such as aluminum, or ceramic, or even an inert polymer such as PEEK, the weight of the filler member being minimized it reduces the inertia of the filler member and, when the plunger reverses its axial movement, around TDC, the suction of the fuel in the bottom clearance C2 and the pressure force on the upper face 60 overcome the inertia force of the filler member that tends to extract the filler member from the inner bore 46 and, consequently, the filler member remains in the chamber 50.

Also, the pressure in the compression chamber 20 is transmitted to the fuel in the gap G and consequently, the peripheral wall 52 of the bore expands when the plunger approaches TDC, said outwardly radial expansion of the wall 52 partially closing the functional sliding clearance CI around the plunger and prevents increase of fuel leakage between the plunger 24 and the bore 26.

Tests have been successfully performed with filler members 54 made out of aluminum. The bottom clearance C2 was smaller than 20μιη and, the gap G was less than 20μιη to the diameter, or ΙΟμιη to the radius. Plunger diameter was 6.5mm and, a range of bore sizes from 3.5 to 5.1mm, giving a minimum wall thickness of 0.7mm. The depth L46 was approximately 8 mm Also, on the figures the filler member 54 has rounded edges alternative geometries such as chamfers or sharp edges are possible.

LIST OF REFERENCES

X pumping axis

Y cam axis

BDC bottom dead centre position

TDC top dead centre position

D46 diameter of the inner bore

L46 length/depth of the bore

D54 diameter of the filler member

H54 length/height of the filler member

CI functional clearance

G gap

C2 bottom clearance

10 pump

12 pump body

14 camshaft

16 pumping head

18 inlet

20 compression chamber

22 outlet

24 plunger

26 bore

28 low pressure end of the plunger

30 cam follower

32 cam

34 high pressure end of the plunger

36 transverse face - top face

38 inlet valve member

40 outlet valve member

42 pump spring final enlarged portion of the bore inner blind bore - deep recess bottom face

chamber

peripheral wall

filler member

outer face

under face of the filler member upper face of the filler member