The invention relates to valve arrangements, particularly, though not exclusively, a valve arrangement forming the fuel injector of a combustion engine. The invention has particular applicability to fuel injectors for gaseous fuels.

There is an increasing need for a fuel injector capable of accurately regulating a supply of gaseous fuel. Electronically-controlled fuel injectors for use with liquid, hydrocarbon fuels are, of course, known. However, such known fuel injectors do not have the capability to accommodate the relatively high flow rates needed for gaseous fuels.

DISCLOSURE OF THE INVENTION According to the invention there is provided a valve arrangement characterised by a first valve for controlling a flow of fluid from the inlet to the outlet of a main conduit, a subsidiary conduit connecting the main conduit to a further outlet and an electromagnetically-controlled valve operable to control a discharge of the fluid from the further outlet so as to control the differential pressure of fluid across the first valve thereby enabling

the first valve to control said flow of fluid.

It will be understood that the term fluid is intended herein to embrace both liquids and gases.

In a preferred embodiment, the valve arrangement forms a fuel injector for ^' injecting gaseous fuel into the combustion chamber of an engine. The electromagnetically- controlled valve can only accommodate a relatively low fuel flow rate. However, this valve does exercise control over the operation of the main valve which can accommodate the higher flow rates needed for gaseous fuels, these being typically two to four times higher than the flow rates needed for liquid fuels.

The electromagnetically-controlled valve is controlled by a suitable electrical control signal which may be a pulsed control signal; for example, a variable width, fixed frequency signal, a variable frequency, fixed width signal or a combination of these.

In a preferred embodiment the electromagnetically controlled valve is an electromagnetically-controlled plate valve.

The first valve may comprise a valve body formed with said main conduit and a valve member resiliently biassed towards a valve seat provided around the outlet and in a particular embodiment the valve member comprises a hollow piston and the subsidiary conduit is coupled to the interior of the hollow piston and communicates with the main conduit through an opening in a wall of the hollow piston.

BRIEF DESCRIPTION OF THE DRAWING A valve arrangement in accordance with the invention is now described, by way of example only, with reference to the accompanying drawing which shows a part-schematic, cross-sectional view of the valve arrangement.

BEST MODE OF CARRYING OUT THE INVENTION The valve arrangement shown in the drawing is in the form of a fuel injector for injecting gaseous fuel, such as methane or hydrogen, into the inlet port of the engine manifold or directly into the engine cylinder early in the cycle before compression begins. The valve arrangement is well suited to this application since it has the capability to accommodate the relatively high fuel flow rates needed for engines fuelled by gas. However, it will be understood that the illustrated valve arrangement could alternatively be used as a fuel injector for liquid, hydrocarbon fuels.

Referring to the drawing, the valve arrangement comprises a servo valve, shown generally at 10, and an electro¬ magnetically-controlled pilot valve, shown generally at 20.

The servo valve 10 comprises a valve body 11 having a main conduit 12 that interconnects a fuel inlet 13, by which fuel is admitted to the conduit under pressure from the fuel supply rail (not shown), and a fuel outlet 14, by which fuel exits the conduit for delivery to the combustion chamber of an engine.

The servo valve has a valve member in the form of a hollow piston 15 which, in the illustrated, normally-closed position bears against a valve seat 16, provided in the valve body around the outlet 14, blocking a flow of fuel along the main conduit. To that end, the valve member is resiliently biased against the valve seat by a compression spring 17 acting between respective abutment surfaces of the valve body and the piston.

The pilot valve 20 is located in a subsidiary conduit, referenced S in the drawing, that communicates with the main conduit 12 via the interior of piston 15 and an orifice 19 in the piston wall or, alternatively via a further conduit 19" in the valve body 11.

As will be explained in greater detail hereinafter, the

electromagnetically-controlled pilot valve is operable to accurately control the differential pressure of fuel across the servo valve causing the latter to regulate the flow of fuel from the inlet 13 to the outlet 14.

In this embodiment, the pilot valve 20 is an electro¬ magnetically-controlled plate valve. This comprises a housing 21 having an inlet 22 and an outlet 23, the inlet and the outlet being interconnected by a conduit 24 in the housing forming part of the subsidiary conduit S.

A flow of fuel along conduit 24 is regulated by an electromagnetically-controlled plate valve comprising an electromagnet including a solenoid coil 25 supported on a coil former 26. The coil former is located inside an annular cavity 27 defined by concentric parts 28 and 29 of housing 21. Both parts 28 and 29 are made of a magnetisable material, thus providing a magnetic circuit round the solenoid coil. The electromagnetically- controlled valve further includes a valve plate 30, also made of a magnetisable material, which is urged by a coil spring 31 into contact with a valve seat in the form of disc 32 in which is formed an aperture 33. The valve plate 30, when in the position shown in the drawing, blocks the aperture 33 in disc 32 thereby preventing a flow of fuel along conduit 24 to the fuel outlet 23.

Upon energisation of solenoid coil 25, the valve plate 30 is magnetically attracted towards the adjacent end face of part 28 thus unblocking the aperture 33 in the disc 32.

Upon de-energisation of the solenoid coil 25, the valve plate is returned by the action of the coil spring 31 into contact with the disc 32, thereby blocking the flow of fuel through the aperture. The flow of fuel through . aperture 33 can be accurately regulated by suitably energising the electromagnetically-controlled plate valve. To that end, the solenoid coil 25 may be supplied with a pulsed control signal which regulates the time intervals during which the solenoid coil is energised (and fuel is able to flow through aperture 33) and the time intervals during which the solenoid coil is de-energised (and fuel is prevented from flowing through aperture 33) .

The pulsed control signal may be a variable width, fixed frequency signal or a variable frequency, fixed width signal, or a combination of these.

In operation, the spring 17 of servo valve 10 and the pressure of fuel in the subsidiary conduit S, which acts on internal surfaces of the piston 15, exerts a closing force on the piston, directed downwardly in the drawing, whereas the pressure of fuel in the main conduit 12, which

acts on external surfaces of the piston, exerts an upwardly directed opening force.

When the pilot valve 20 is closed, the downwardly directed closing force exceeds the upwardly directed opening force and the piston 15 assumes the illustrated closed condition, preventing a flow of fuel along the main conduit. However, when pilot valve 20 opens, fuel is discharged from the subsidiary conduit S via the outlet 23, and so the pressure in the subsidiary conduit falls. The relative areas of the respective internal and external surfaces of the piston 15 are such that the resulting change of differential pressure across the servo valve causes the opening force to exceed the closing force and so the piston moves away from the valve seat 16. This causes the servo valve to assume the open condition, allowing fuel to flow along the main conduit.

When the pilot valve is closed, the pressure of fuel in the subsidiary conduit rises, reversing the change of differential pressure across the servo valve, causing the latter to resume the closed condition and block the flow of fuel along the main conduit.

It will be apparent that the pilot valve, which can only accommodate a relatively low fuel flow rate nevertheless

exercises accurate control over the servo valve which regulates the main fuel flow.

The described embodiment has a pilot valve in the form of an electromagnetically-controlled plate valve. It will be appreciated, however, that other forms of electromagnetically-controlled valve could alternatively be used.

INDUSTRIAL APPLICABILITY It will be understood that although the valve arrangement of this invention has been described in the context of a fuel injector, preferably (though not exclusively) an injector for gaseous fuel, the valve arrangement finds application in many other industrial fields where there is a need to control a flow of fluid, both liquid and gas.