A unit fuel injector comprises the combination of a fuel pump and a fuel injector. The fuel pump has a pump housing which is formed with a longitudinal bore, in which is mounted an axially slidable plunger, and a cylindrical pump chamber into which one end of the plunger projects.

For ease of construction, it has been customary to manufacture the pump housing in two parts. The two parts of the pump housing have mutually abutting end surfaces defining a sealing face, and the pump chamber is defined by a cylindrical recess which is formed in the end surface of one of the parts and is closed off by the end surface of the other part.

This form of construction has given rise to a problem caused by the very high pressures - typically in excess of 1500 bar - to which the walls of the pump chamber may be subjected. The changes of pressure occurring in the pump chamber during successive operating cycles of the fuel pump cause the walls of the chamber to expand and contract

radially, and this gives rise to relative displacement of the two parts from which the pump housing is constructed causing their mutually abutting end surfaces to wear.

A similar problem can arise in other forms of high pressure assembly having a sealing face at which wear can occur.

It is one object of the invention to provide a high pressure assembly which at least alleviates this problem.

SUMMARY OF THE INVENTION According to a first aspect of the invention there is provided a high pressure assembly for fluid, having a chamber which is formed in a housing comprised of at least two parts, wherein respective parts of the housing have abutting surfaces defining a sealing face between the parts, and the chamber is formed in both said parts and extends across the sealing face.

In a preferred embodiment of the invention the high pressure assembly is the fuel pump of a unit fuel injector.

According to a further aspect of the invention there is provided a unit fuel injector comprising the combination of a fuel pump and a fuel injector, wherein the fuel pump

has a pump housing comprised of at least two parts, respective parts of the pump housing having mutually abutting surfaces defining a sealing face, and the fuel pump has a pump chamber which is formed in both said parts of the pump housing and extends across the sealing face.

With this construction, the pressure of fuel inside the pump chamber is distributed over respective inside wall surfaces of both parts of the pump housing and this has the effect of reducing relative displacement of the two parts and so reduces any consequential wear of their mutually abutting end surfaces.

The walls of the pump housing may have a substantially uniform thickness in a region to either side of the mutually abutting end surfaces.

In a preferred embodiment, the pump chamber comprises a first cylindrical recess formed in the end surface of one of said parts of the pump housing and a second cylindrical recess formed in the end surface of the other said part of the pump housing wherein a plunger for the fuel pump is mounted in said one part and projects into said first cylindrical recess, and the second cylindrical recess has a depth bearing a predetermined relationship to the diameter of the recess.

The unit fuel injector may include means to control a flow of fuel between the fuel pump and the fuel injector, and the control means preferably includes an electro- magnetically-actuated valve coupled to the pump chamber via a control duct formed in one or both said parts of the pump housing.

BRIEF DESCRIPTION OF THE DRAWINGS A unit fuel injector in accordance with the invention will now be described, by way of example only, with reference to the accompanying drawing which shows a longitudinal sectional view through the unit fuel injector.

BEST MODE OF CARRYING OUT THE INVENTION Referring to the drawing, the uni fuel injector comprises a fuel pump, referenced generally at 10, in combination with a fuel injector, referenced generally at 20.

The fuel pump 10 comprises a pump housing 11 formed with a longitudinal bore 12, in which is mounted an axially slidable plunger 13, and a cylindrical pump chamber 14 into which one end of the plunger 33 projects.

As will be described in greater detail hereinafter, the pump housing 11 is of two-part construction, being comprised of a first part 11', in which the plunger is mounted, and a second part 11". The two parts 11',11" of

the pump housing have mutually abutting end faces E, and E_ and the pump chamber 14 is formed in both parts of the housing, extending to either side of the abutting end faces, as shown in the drawing.

The plunger 13 is connected to a respective rocker inside the engine cylinder head by means of a shoe 15, and a compression spring 16 resiliently biasses the plunger towards its outermost axial position, shown in the drawing.

In this position, a cross-bore 17 in the plunger is aligned with a radially extending duct 18 formed in the pump housing enabling fuel to be supplied to the pump chamber 14 from a constant-pressure fuel supply inlet port I via respective annular and longitudinal ducts 18',18" in the pump housing, and a longitudinal duct 17 ^" in the plunger.

During the compression phase, the rocker will depress the plunger 13 in opposition to the action of coil spring 16, forcing the end of the plunger further into the pump chamber 14 and pressurising the fuel which has been supplied thereto.

As will now be explained, the pressurised fuel in the pump chamber is delivered to the fuel injector 20, in a controlled manner, for injection into the combustion

chamber of the engine.

Referring again to the drawing, the fuel injector 20 comprises an outer casing 21 housing the serial arrangement of a cylindrical body 22, an intermediate disc member 23 and an injector body 24. The casing 21 is screw-fitted to the first part 11' of the pump housing and the casing 21 and the injector body 24 have complimentary tapered surfaces which cause the two parts 11',11" of the pump housing and the afore-mentioned components 22, 23 and 24 of the fuel injector to be clamped rigidly together when the casing 21 is fitted to the pump housing.

The assembly is clamped into the cylinder head so that the outlet end of the fuel injector 20 is located inside the combustion chamber.

The fuel injector 20 has a needle valve comprising a valve member 25 which is axially-slidable in a guide bore 26 formed in the injector body 24. The valve member 25 is resiliently biassed towards the normally-closed position of the valve (downwardly in the drawing) by a compression spring 27 housed within an axial bore in the cylindrical body 22. The compression spring 27 acts on an end plate 28 fitted to one end of the valve member which projects through a central hole in disc member 23.

An extension 26' of the guide bore 26 is connected to the pump chamber 14 of the fuel pump by a duct 29. When the pressure of fuel in the pump chamber 14 rises, caused by a downwards displacement of plunger 13, the pressure of fuel in bore 26' may also rise. This increased pressure in bore 26' displaces the valve member 25 in opposition to the action of the compression spring 27 (upwardly in the drawing), opening the needle valve and causing fuel to be discharged through the outlet of the fuel injector 20 (not shown) .

In order to exercise control over the flow of fuel from the fuel injector 20, the pressure of fuel in duct 29 is regulated, in known manner, using an electromagnetically- actuated spill valve (not shown in the drawing) which may be mounted on an extension of the pump housing (shown, in part, in the drawing) or alternatively mounted in a separate assembly which is fixed to the pump housing. The electromagnetically-actuated spill valve is coupled to duct 29 by a further duct 30 formed in the pump housing 11. When, in operation, the electromagnetically-actuated valve is opened the pressure of fuel in ducts 29 and 30 falls, causing a fall of pressure in bore 26', and the valve member 25 is returned to the normally-closed position under the action of compression spring 27. In this mode of operation, fuel from the pump chamber is

routed along a further duct (not shown) in the pump housing and is returned via an outlet port 0 to the fuel supply system for the unit fuel injector, and fuel which has leaked into the bore 31 in body 22 can also reach the outlet port O via a V-groove (referenced V in the drawing) provided in the high pressure sealing face F.

When the electromagnetically-actuated spill valve is closed, the pressure of fuel in ducts 29 and 30, and in bore 26, will rise causing the valve member to be displaced to the open position allowing fuel to be discharged from the outlet of the fuel injector 20.

Accordingly, by supplying a suitable electrical control signal to the electromagnetically-actuated spill valve, the needle valve of the fuel injector can be opened and closed in accordance with a desired operation (such as a pulsed operation) so as to regulate the flow of fuel from the fuel injector 20 during the compression stage in each operating cycle of the fuel pump 10.

During each compression stage of the pump, the walls of pump chamber 14 will be subjected to very high pressures - typically in excess of 1500 bar - and this causes the walls of the chamber to expand and contract radially during successive operating cycles of the pump.

In accordance with the invention, the pump chamber is formed in both constituent parts 11 ",11" of the pump housing and extends across the abutting end faces E.,E„ of those parts. Thus, a cylindrical recess 14' is formed in the respective end face E, of part 11', and a cylindrical recess 14" is formed in the respective end face E„ of the other part 11". With this construction, the pressure of fuel in the pump chamber 14 is distributed over the inside wall surfaces of both parts 11 ',11" of the pump housing and this has the beneficial effect of reducing their relative displacement when the walls of the housing expand and contract, with a consequent reduction of wear on their abutting end surfaces.

In order to attain the optimum improvement, the relationship between the depth d of the recess 14", formed in part 11", and the diameter D of the recess can be determined by numerical analysis, such as finite element analysis. Furthermore, although the two recesses 1 ',14" may have the same diameter, this need not necessarily be the case; the optimum diameter of recess 14" will depend on such factors as the outside diameter of part 11" and the depth d of the recess 14".

In order to further reduce wear, the walls of the pump housing have a substantially uniform thickness over a

defined region to either side of the abutting end surfaces, and to accomplish this a section of the external screw thread provided on part 11' is removed, as shown in broken outline in the drawing.

INDUSTRIAL APPLICABILITY It will be understood that although the invention has been described with reference to a unit fuel injector for a diesel engine, the injector is suitable for use with fuels other than diesel, e.g. petrol or methanol.

Furthermore, although the invention has been described with reference to a unit fuel injector, the invention is applicable to other kinds of high pressure assembly having a sealing face at which wear can occur.