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Fluid pump

Fluid pump description/claims

The present invention relates to a fluid pump. More specifically the invention relates to a ‘unit pump’ type fuel injection pump for use in a compression ignition internal combustion engine in which a pumping plunger reciprocates within a plunger bore in order to pressurize fuel within a pumping chamber. In addition, the invention relates to a pumping plunger for use within such a pump.

FIG. 1 shows a sectional view of a known fuel injection pump, indicated generally as 2, which is suitable for use as a means of supplying pressurized fuel to a fuel injector of an internal combustion engine. Such a pump is generally known in the art as a mechanical ‘unit pump’. The fuel pump 2 includes a generally tubular pump housing 4 having an axially disposed bore 6 within which a pumping plunger 8 is slidable. The plunger 8 has a lower end 10 (in the orientation shown in FIG. 1) that is coupled to a lower spring plate 12. Although not shown in FIG. 1, in practice the lower spring plate 12 may couple to a tappet member against which a driving means (also not shown) acts, a cam for example, to transmit reciprocating motion to the plunger 8.

A biasing means in the form of a helical spring 17 is received over the plunger 8 such that the spring 17 is disposed between the pump housing 4 and the lower spring plate 12. An upper end 18 of the spring 17 abuts a spring plate 20 that is attached to a lower end of the pump housing 4, and a lower end 22 of the spring 17 abuts the lower spring plate 12, the spring 17 thus serving to bias the plunger 8 downwards in the orientation shown.

An upper end of the pump housing 4 defines a cup-shaped recess 26 into which a lower end of an outlet valve arrangement 28 is received. The lower end of the outlet valve arrangement 28 closes off the plunger bore 6 and defines a pumping chamber 30 between it and the upper end of the plunger 8. The other end of the outlet valve arrangement 28 remote from the recess 26 defines an outlet connection 31.

The inner surface of the bore 6 in the region of the pumping chamber 30 defines two opposed ports: a fill port 32 and a spill port 33. The fill port 32 is shown to the left in FIG. 1 and is connectable to a fuel supply circuit such that fuel is permitted to fill the pumping chamber 30 through the fill port 32. The spill port 33 is shown on the right in FIG. 1 and is connectable to a drain connection such that high pressure fuel from the pumping chamber 30 can escape to low pressure through the spill port 33.

In use, the plunger 8 is driven on a pumping stroke during which fuel within the chamber 30 is pressurized. When the pressure of fuel within the chamber 30 reaches a predetermined pressure, the outlet valve arrangement 28 opens to permit pressurized fuel to flow through the outlet connection 31. Although not shown in FIG. 1, a fuel conduit may be attached to the outlet connection 31 to convey fuel to a fuel injector, for example.

In order to terminate fuel delivery, the plunger 8 is provided with a spill groove 41. As the plunger 8 is driven further on its pumping stroke, a point will be reached at which the spill groove 41 overlaps the spill port 33 so that pressurized fuel can escape from the pumping chamber 30 along the spill groove 41 to the spill port 33 which terminates pressurization and, thus, fuel delivery from the pump 2.

Following termination of pumping, the plunger 8 is driven further such that it passes a top dead centre position and thus commences a return stroke under the force of the spring 17. During the return stroke, fuel is permitted to fill the chamber 30 through the fill port 32 which is connected to a source of fuel at a relatively low pressure.

In order to vary the delivery volume of the pump 2, the plunger 8 is provided with a control arm 40 which extends radially away from the approximate mid point of the plunger 8. Angular movement of the control arm 40 varies the angular position of the plunger 8.

In use, the control arm 40 engages a fuel delivery rack (not shown) via a control pin 42 that depends downwardly from a radially outer end of the control arm 40. The position of the fuel delivery rack is determined by the engine governor and the rack, in turn, acts on the control arm 40 to cause radial movement of the plunger 8 about its longitudinal axis. The radial position of the plunger 8 determines the point of the pumping stoke that the spill groove 41 registers with the low pressure spill port 33, thus terminating fuel pressurization earlier, or later, in the pumping stroke depending on the direction and extent of angular movement of the plunger 8. The variation of the effective stroke between the upper surface of the plunger 8 and the spill groove 41 varies the fuel delivery to the associated engine.

In circumstances where mechanical unit pumps are used in low emission engines, for example in relatively small cubic-capacity road vehicles, some unit pumps are fitted with constant-pressure unloading delivery valves.

The general configuration and function of such delivery valves is known in the art and an example of which is shown in FIG. 1 as the outlet valve arrangement 28. Their purpose is to trap fuel in a fuel passageway downstream of the outlet connection 31 at a predetermined pressure at the end of the pumping stroke. By way of explanation, when pressure in the pumping chamber 30 collapses at the end of a pumping stroke, fuel pressure in the fuel passageway is at an injectable pressure level. It would be undesirable for the pressure of fuel to remain at this level since there would be a risk of secondary injection events. Conversely, it would be undesirable to permit pressure uncontrolled collapse in the fuel passageway. Therefore, the constant pressure unloading valve 28 allows a sufficient amount of fuel to pass back from the high pressure fuel passageway through to the pumping chamber 30, and thus to the spill port 33, so as to reduce the pressure in the fuel passageway by a predetermined amount. The controlled pressure reduction suppresses pressure waves emanating from the pump in the direction of the associated injector so as to avoid undesirable secondary injection events. In addition, cavitation is guarded against since fuel pressure downstream of the outlet connection is not permitted to collapse completely and in an uncontrolled manner.

Although such valves have associated benefits, the characteristic fuel delivery curve shape produced by mechanical unit pumps incorporating the aforementioned outlet valve arrangements usually exhibits considerably higher fuel delivery at rated load when the pumps are running at low speed compared to when the pumps are running at high speed. This characteristic tends to cause excessive smoking from the associated engine which can limit the use of such pumps for low emission engines despite the advantage they provide in eliminating the risk of secondary injections.

It is against this background that the invention provides, in a first aspect, a pump for fluid, comprising a pump body defining a bore, and a pumping plunger having an end region which is slidably received within the bore, a spill port defined by the pump body in a wall of the bore which port is coverable by the end region of the pumping plunger as the plunger is driven on a pumping stroke, in use, so as to initiate fluid pressurization within a pumping chamber, and a spill groove defined by the pumping plunger and being arranged to uncover the spill port during a pumping stroke so as to cause depressurization of the pumping chamber. The pumping plunger further defines a speed sensitive feature configured to permit a controlled leak of fluid from the pumping chamber to the spill port towards the end of a pumping stoke, the leak rate being dependent on the speed of operation of the pump.

The speed sensitive feature is adapted preferably to be effective during a rated load operating condition of the pump and has the effect of reducing the delivery of the pump at lower pump speeds compared to a pump having plungers without the speed sensitive feature. However, at higher pump speeds delivery remains substantially unaffected. The invention has particular utility in internal combustion engines in which it is important to minimize exhaust emissions, for example small capacity road vehicles. Moreover, the speed sensitive feature is configured to be effective towards the end of a pumping stroke which enables the relationship between fuel delivery and injection timing for high speed operation to be preserved. Although the speed sensitive feature may be effective over a broad range of angular movement of the plunger, it is preferred that it is effective during approximately the last 20 percent of the range of angular movement of the plunger which corresponds approximately to the rated load operating condition of the pump.

Preferably the spill feature may be a groove extending around a portion of the outer surface of the pumping plunger at an angle to the longitudinal axis of the pumping plunger. Similarly, the speed sensitive feature may also be a groove disposed in the surface of the plunger, and preferably may be in advance of the spill groove.

Furthermore, the speed sensitive groove may communicate with the spill groove and, preferably, is configured to extend away from the spill groove in a direction substantially perpendicular to the longitudinal axis of the plunger.

Although the pumping plunger may be manufactured so to have various cross sections, for example oval, elliptical, or square, for convenience it is preferred that the pumping plunger is substantially circular in cross section. As a result of this, the spill groove is substantially helical in shape since it is machined in the surface of the plunger at an angle to its longitudinal axis.

The precise dimensions of the speed sensitive groove are to a large extent dependent on the size of the pump, the associated plunger and the desired effect on the pump delivery. However, for a plunger that is typically approximately 7 to 10 millimetres in diameter it is preferred that the depth of the speed sensitive groove is approximately 30 to 60 micrometres and, more preferably between 40 and 50 micrometres. It should be appreciated, however, that some pumping plungers may have greater diameters than this in certain applications, large capacity marine engines for example.

In a second aspect the invention provides a pumping plunger comprising an elongate rod defining an end face and a side surface, and including a spill groove provided in the side surface of the plunger defining an angle with a longitudinal axis of the plunger, and a speed sensitive feature in communication with the spill groove and configured to permit a controlled flow of fluid therethrough, in use, at an end of a pumping stroke of the plunger.

• Provisional Patent
• Utility Patent

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