Fuel-injection pump with variable cylinder capacity for diesel engine injection systems
The present invention relates generally to fuel-injection pumps for diesel engine injection systems. 
     More particularly the invention concerns an injection pump of the type comprising at least one cylinder in which a pumping piston driven by an eccentric is slidable between a bottom dead center position and a top dead center position and defines a pumping chamber in the cylinder which is connected to a fuel intake duct and to a fuel-delivery duct through an intake valve and a delivery valve respectively, and in which the electric pump is associated with the intake duct for supplying fuel from a tank to the injection pump. 
     The object of the present invention is to provide an injection pump of the type described above which is arranged, by simple and practical means, to supply fuel at a variable flow rate by means of the variation of its cylinder capacity. 
     In order to achieve this object, the subject of the invention is a fuel-injection pump of the type defined at the beginning, characterised in that equalising means are interposed between the piston and the eccentric to enable the position of the bottom dead center of the piston to be varied, and in that electronic modulation means are operatively associated with the electrical pump for varying the pressure of the fuel taken in by the pumping chamber of the injection pump and consequently the bottom dead center position of the piston and the length of its pumping stroke. 
     By virtue of this concept, the cylinder capacity is varied so as to vary the flow rate by means of the modification of the initial position of the piston, so varying its useful stroke, by means of the modulation of the pressure of the electrical supply pump. 
     This modulation is conveniently controlled by means of a microprocessor connected to a pressure sensor in the delivery duct of the pump. 
     The invention is applicable both to radial injection pumps and to in-line injection pumps. 
    
    
     The invention will now be described in detail with reference to the appended drawing, provided purely by way of non-limiting example, in which a diagram of an injection pump according to the invention with variable cylinder capacity is shown schematically. 
     With reference to the drawing, part of a fuel-injection pump for a diesel-engine injection system is generally indicated 1. In the embodiment illustrated, the pump 1 is of radial type and, in known manner, includes a plurality (normally three) of pumping units arranged radially around a rotor with an eccentric 2. Each of the units, only one of which is shown at 3 in the drawing, comprises a cylinder 4 in which a piston 5 driven so as to reciprocate in a straight-line by means of the eccentric 2 is slidable. 
     The interior of the cylinder 4 is divided by the piston 5 into an upper pumping chamber 6 and a lower chamber 7 whose base is defined by a cup-shape tappet member 8 urged against the surface of the camshaft 2 by a thrust spring 9. An equalising spring 10 is interposed between the base of the cup-shaped member 8 and the floating piston 5 and tends to keep the piston 5 in equilibrium with the pressure of the fuel supplied to the pumping chamber 6, when it is at its bottom dead center position. As will be seen below, this pressure is variable so as to allow the initial position of the piston 5 and hence its useful pumping stroke to vary. 
     The pumping chamber 6 is connected on the one hand to a fuel-intake duct 11 and on the other to a fuel-delivery duct 12 through an intake valve 13 and a delivery valve 14 respectively. The fuel is delivered to the intake duct 11 by means of a supply pump 15 connected to a fuel tank 16 and driven by means of an electric motor 17. This electric motor 17 is energised through a PWM modulator circuit 18 in turn controlled by means of a microprocessor 19 in dependence on the fuel pressure in the delivery duct 12. For this purpose, the microprocessor 19 is supplied by means of a pressure transducer 20 inserted in the delivery duct 12. 
     A pressure-control device 21, constituted by a pressure regulator, or by a calibrated maximum-pressure valve is also associated with the delivery duct 12 in conventional manner. 
     In operation, it is necessary for the pressure in the pumping chamber 6 always to be higher than that in the lower chamber 7. For this purpose, it suffices to keep the latter at amospheric pressure. 
     The microprocessor 19 regulates the operation of the electric supply pump 15-17 in dependence on the fuel-delivery conditions through pressure signals from the transducer 20 and thus modulates the pressure of the fuel taken in by the pumper chamber 6. This enables the bottom dead center position of the piston 5, that is its useful stroke, to be modified, thus varying the cylinder capacity of the pump 1 so as to achieve a variable fuel flow rate. 
     Naturally, the principle of the invention remaining the same, the forms of embodiment and details of construction may be varied widely with respect to those described and illustrated without thereby departing from the scope of the present invention. 
     Thus, for example, although the embodiment illustrated relates to a radial pump, the invention is also applicable to in-line pumps.