This invention relates to a fuel pump for supplying fuel under pressure for delivery to a combustion space of a compression ignition internal combustion engine.
It is known to provide each injector of a fuel system with a corresponding fuel pump arranged to supply fuel under pressure only to that fuel injector. The pump and associated fuel injector may be mounted upon one another or spaced apart from one another, a fuel pipe being used to connect to pump outlet to the injector. The pumps used in such applications typically comprise a pumping plunger reciprocable within a bore under the action of a cam and tappet arrangement. The pump and associated components are designed such that, at any instant, the speed of movement of the plunger is governed by the speed of rotation of the cam and by the shape of the cam profile.
In order to reduce the levels of noise and particulate emissions produced by an engine, it is desirable, under some conditions, to arrange for each injection of fuel to the engine to include a period during which fuel is injected at a reduced rate. For example, each injection may include an initial part during which fuel is delivered at a relatively low rate followed by a period during which fuel injection occurs at a higher rate. In known arrangements, this has been achieved by appropriate shaping of the cam profile.
According to the present invention, there is provided a fuel pump comprising a pumping plunger reciprocable within a bore, and a drive arrangement for driving the pumping plunger, the drive arrangement including a compressible load transmission arrangement for transmitting movement of the drive arrangement to the pumping plunger, the load transmission arrangement comprising first and second surfaces which define, therebetween, a chamber for fluid, and supply means for permitting the chamber to be supplied with fluid, in use, and control means for controlling the flow of fluid from the chamber.
The provision of the load transmission arrangement results in some of the movement of the drive arrangement being absorbed, by compression of the fluid and by fluid escaping from the chamber, and hence in the plunger moving at a reduced rate than would otherwise be the case for a period. As a result, an associated injector can be operated in such a manner as to include, in its injection cycle, a period during which fuel is injected at a reduced rate.
The stiffness of the load transmission arrangement may vary with, for example, the applied load, and a plunger movement may be achieved which is not possible simply by modifying the profile of the cam.
The first and second surfaces are conveniently resiliently biased apart, for example by one or more disc springs. The supply means conveniently includes a non-return valve whereby fluid can be supplied to the chamber upon separation of the surfaces under the action of the resilient biasing. The non-return valve may comprise a perforated diaphragm. The disc springs are conveniently arranged such that, in use, when the load applied to the device exceeds a predetermined level the disc springs deflect to define a fluid flow path whereby fluid can escape from the chamber in a controlled manner.
Preferably, the fluid supplied to the engine is engine lubricating oil.