This invention relates to a fuel injector for supplying liquid fuel to an air inlet duct of a spark ignition engine and of the kind comprising a valve seat member in which is formed a discharge orifice and about which on one side of the seat member is formed an annular seating element, a circular plate-like valve member which is urged into engagement with said seating element by means of a spring to prevent flow of fuel through said discharge orifice, an electromagnet which when energized attracts the valve member away from the seating element to allow fuel flow through the orifice and an annular spacer member which surrounds the valve member.
An example of such a fuel injector is seen in European Patent Application Publication No. EP-A-0328277 wherein spring is of the coiled compression type and is located within a bore formed in a central core member of the electromagnet. The bore also serves to convey fuel from an inlet of the injector to adjacent the valve member so that when the latter is lifted from the seating element, fuel can flow through the outlet orifice. The end of the spring adjacent the valve member is located within a recess formed in the adjacent face of the valve member. The base wall of the recess in a practical example of the injector is made parallel to the surface of the valve member which engages the seating element and the rim of the valve member is rounded. A small clearance exists between the rim and the internal surface of the spacer member.
In the use of the injectors it is found that the performance of some injectors is not consistent with the flow of fuel through the injector for a given fuel pressure at the inlet and for a given length of energizing pulse, varying throughout the life of the injector. The variation of fuel delivery is undesirable.