An electromagnetic fuel injector comprises a cylindrical tubular body displaying a central feeding channel, which functions as a fuel conduit and ends with an injection nozzle regulated by an injection valve controlled by an electromagnetic actuator. The injection valve is provided with a needle, which is rigidly connected to a mobile keeper of the electromagnetic actuator in order to be displaced by the action of the electromagnetic actuator between a closed position and an open position of the injection nozzle against the bias of a spring which tends to hold the needle in the closed position. The valve seat is defined in a sealing element, which is shaped as a disc, lowerly and fluid-tightly closes the central channel of the support body and is crossed by the injection nozzle.
Patent application EP1635055A1 describes an electromagnetic fuel injector in which a guiding element rises from the sealing element, such guiding element having a tubular shape, accommodating the needle therein in order to define a lower guide of the needle itself and displaying a smaller external diameter with respect to the internal diameter of the feeding channel of the supporting body so as to define an external annular channel through which pressurised fuel flows. Four through feeding holes, which lead towards the valve seat to allow the flow of pressurised fuel towards the valve seat itself, are obtained in the lower part of the guiding element. The needle ends with an essentially spherical shutter head, which is adapted to fluid-tightly rest against the valve seat and slidingly rests on an internal cylindrical surface of the guiding element so as to be guided in its movement. The injection nozzle is of the “multi-hole” type, i.e. it is defined by a plurality of through injection holes, which are obtained from a chamber formed downstream of the valve seat; in this way, the optimal geometries of the injection nozzle may be obtained for the various applications by appropriately orienting the single injection holes.
Experimental tests have shown that the drive time-injected fuel quantity curve (i.e. the law linking the drive time to the quantity of injected fuel) of the electromagnetic injector described above is on the whole rather linear, but displays an initial step (i.e. displays a step increase for short drive times and therefore for small quantities of injected fuel); furthermore, the extent of such initial step is higher proportionally to the fuel feeding pressure.
Consequently, the electromechanical injector described above may be used in a direct injection internal combustion Otto cycle engine (i.e. fed with petrol, LPG, methane or the like), in which the fuel feeding pressure is limited (lower than 200-250 bars) and the injector is not normally driven to inject small amounts of fuel). However, the electromagnetic injector described above cannot be used in a small direct injection internal combustion Diesel cycle engine (i.e. fed with Diesel fuel or the like), in which the feeding pressure of the fuel is rather high (up to 800-900 bars) and the injector is constantly driven so as to perform a series of pilot injectors before a main injection.