TECHNICAL FIELD The present invention relates to a system for regulating the capacity of an injection engine high- pressure fuel pump.

BACKGROUND ART As is known, fuel injection internal combustion engines comprise, for each cylinder, at least one injector supplied with high-pressure fuel. In modern injection engines, the high-pressure fuel is fed to a vessel or distributor known as a"common rail", which is connected to the various injectors and supplied by a high-pressure, normally piston, pump.

The high-pressure pump, in turn, is supplied from the fuel tank by means of a low-pressure pump. In modern vehicle engines, fuel supply pressure ranges between 1,300 and 1,600 bars, and the low-pressure pump supplies fuel at about 5-bar pressure.

In known supply devices, the fuel pressure in the distributor is maintained constant by a regulating system comprising a drain valve located downstream from the

high-pressure pump and for feeding any surplus pumped fuel back into the tank. Known regulating systems of this sort have various drawbacks.

In particular, since the surplus fuel not required by the engine is simply recirculated, the high-pressure pump must pump the maximum amount of fuel at all times.

On the one hand, this obviously results in energy being wasted to operate the pump to no purpose, and, on the other, compression-heated fuel is recirculated continually into the tank, thus increasing the temperature of the fuel in the tank.

Increasing the temperature of the fuel to be pumped in turn increases fuel leakage of the pump pistons, thus impairing the efficiency of the pump. To reduce the efficiency loss of the pump, it has been proposed to reduce the temperature of the recirculated fuel by means of a cooler. Being fairly expensive and bulky, however, such coolers are unsuitable for low-powered engines or engines housed in confined spaces. Which is one of the reasons injection feed is not normally adopted on low- powered engines, and in particular is one of the reasons diesel engines are unsuitable for low-power applications.

DISCLOSURE OF INVENTION It is an object of the present invention to provide a system for regulating the capacity of a high-pressure pump, and which provides for maximum efficiency, is low- cost, and eliminates the aforementioned drawbacks typically associated with known regulating systems. It is

a further object of the invention to provide a regulating system which is compact and suitable for low-powered injection engines.

According to the present invention, there is provided a system for regulating the capacity of a high- pressure pump for supplying fuel to an injection engine, characterized in that the capacity of said high-pressure pump is regulated by choking means for choking the intake of said high-pressure pump.

In particular, the choking means comprise a variable-capacity electromagnetic valve communicating with an intake conduit of the high-pressure pump.

Capacity is proportional to the displacement of an armature of a control solenoid controlled by an electronic control unit as a function of parameters of the instantaneous power required or the engine.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred, non-limiting embodiment of the invention will be described by way of example with reference to the accompanying drawings, in which: Figure 1 shows a diagram of an injection engine fuel supply device comprising the regulating system according to the invention; Figure 2 shows a graph of the variable-capacity characteristic of the regulating system.

BEST MODE FOR CARRYING OUT THE INVENTION Number 5 in Figure 1 indicates as a whole a device for supplying fuel to an injection engine, e. g. a vehicle

multicylinder diesel engine. Device 5 comprises a high- pressure pump 6, e. g. of known type having three radial cylinders 7 in which operate three corresponding pistons 8 activated by a common cam 9 by means of a faced ring 11.

Each cylinder 7 has an intake valve 12 communicating with a low-pressure intake conduit 13; and a delivery valve 14 communicating with a high-pressure delivery conduit 16. Intake conduit 13 receives fuel from a normal fuel tank 17 via a filter 18 and along an input conduit 19 of a low-pressure pump 20.

Pump 20 may be electric, activated by a respective electric motor, or mechanical, e. g. a gear pump activated by the shaft of the injection engine itself. Cam 9 and ring 11 of high-pressure pump 6 are lubricated by part of the incoming fuel from intake conduit 13, which is fed back into the tank along a recirculating conduit 15.

Delivery conduit 16 of pump 6 communicates with a vessel or distributor 21 known as a"common rail", which communicates with a series of electromagnetic injectors 22, each of which is controlled to inject, at each cycle and into a corresponding cylinder of the injection engine, a quantity of fuel metered according to the instantaneous power requested of the engine.

According to the invention, supply device 5 comprises a system for regulating the capacity of high- pressure pump 6 and comprising choking means for choking the intake of pump 6. The choking means substantially

comprise a variable-capacity electromagnetic valve 23 communicating with intake conduit 13 of pump 6 and providing for laminar flow of the fuel supplied to the pump.

More specifically, electromagnetic valve 23 is controlled by an armature 24 of a solenoid 26, and is of the type in which capacity is proportional to the excitation current of solenoid 26, and therefore to the displacement of armature 24. Figure 2 shows a graph 25 of the capacity Q characteristic of valve 23 as a function of the excitation current I of control solenoid 26 of valve 23. Graph 25 shows a straight portion P extending between a point M, corresponding to the maximum capacity of valve 23 for full-load operation of the engine, and a point m corresponding to the minimum capacity of valve 23 for minimum-load operation of the engine. Another portion R of graph 25 corresponds to displacement of armature 24 to shut down valve 23 completely.

Solenoid 26 is controlled by an electronic control unit 27 as a function of signals indicating various parameters of the instantaneous power requested of the injection engine, and which may comprise an engine speed signal, an accelerator pedal position signal. etc. Valve 23 therefore enables high-pressure pump 6 to operate as though it were a variable-displacement type.

Electromagnetic valve 23 is associated with an overpressure, i. e. pressure control, valve 31 along an input conduit 68 supplying fuel to electromagnetic valve

23. Overpressure valve 31 communicates via calibrated conduits 62 and 64 with a conduit 28 for internally lubricating pump 6, and is connected to an input conduit 30 communicating with the delivery side of low-pressure pump 20; and electromagnetic valve 23 is connected to intake conduit 13 of pump 6 by an output conduit 29.

Overpressure valve 31 is calibrated to ensure constant-pressure, preferably 5-bar, fuel supply to electromagnetic valve 23. Another output of valve 31 is connected to a recirculating conduit 32 parallel to low- pressure pump 20 and communicating with input conduit 19 of pump 20. Any surplus fuel supplied to pump 20 is drained or recirculated by valve 31 along recirculating conduit 32 and back into conduit 19.

The system for regulating high-pressure pump 6 operates as fcllows.

When the injection engine is running, low-pressure pump 20 draws fuel from tank 17 through filter 18 and along input conduit 19, and feeds the fuel to input conduit 30 of overpressure valve 31, which provides for maintaining a constant 5-bar fuel pressure in input conduit 68 of electromagnetic valve 23, and for recirculating any surplus fuel back into conduit 19 along recirculating conduit 32. The energy expended to bring the surplus fuel to 5-bar pressure is therefore very small and does not produce a noticeable increase in the temperature of the recirculated fuel.

Controlled by electronic unit 27, solenoid 26 opens

electromagnetic valve 23 to supply intake conduit 13 of high-pressure pump 6 with the amount of fuel corresponding to the instantaneous power required of the injection engine according to the incoming engine speed and accelerator pedal position signals, etc. High- pressure pump 6 therefore operates at variable capacity, only brings to 1,600-bar pressure the amount of fuel required instantaneously by injectors 22, and does not recirculate high-pressure surplus fuel.

The advantages, as compared with known systems, of the regulating system according tc the invention will be clear from the foregoing description. In particular, electromagnetic valve 23 provides for reducing the energy expended to compress the surplus fuel to high pressure; eliminates the increase in temperature of the fuel in tank 17 and the efficiency loss of pump 6 caused by the resulting increase in leakage; and eliminates the need for a fuel cooler and associated control members, thus enabling supply device 5 to be also employed on low- powered engines or engines housed in confined spaces.

Clearly, changes may be made to the regulating system as described herein without, however departing from the scope of the accompanying Claims. For example, conduit 19 may be provided with a water separator; and common rail 21 may be provided with a pressure regulating or safety valve.