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A fuel injection system for an internal combustion engine is provided for providing an injection event including a first stage and a second stage via a single nozzle. The nozzle is connected by its inlet port to a source of variable fuel pressure and it includes a needle valve for performing the fir

A fuel injection system for an internal combustion engine is provided for providing an injection event including a first stage and a second stage via a single nozzle. The nozzle is connected by its inlet port to a source of variable fuel pressure and it includes a needle valve for performing the first stage of injection, and a poppet valve for performing the second stage of injection. The first and second stages of injection are selectable by controlling the fuel pressure in the inlet port which is common for both the needle valve and the poppet valve.

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The invention claimed is: 1. A fuel injection system for an internal combustion engine, for providing an injection event comprising a first stage and a second stage via a single nozzle which is connected by its inlet port to a source of variable fuel pressure, the nozzle including a needle valve fo

The invention claimed is: 1. A fuel injection system for an internal combustion engine, for providing an injection event comprising a first stage and a second stage via a single nozzle which is connected by its inlet port to a source of variable fuel pressure, the nozzle including a needle valve for performing the first stage of injection, and a poppet valve for performing the second stage of injection, wherein the first stage of injection and the second stage of injection are selectable by controlling the fuel pressure in the inlet port which is common for both the needle valve and the poppet valve. 2. A fuel injection system according to claim 1, wherein the poppet valve has a poppet and a poppet return resilient means which biases the poppet valve to close the nozzle by abutting the poppet against a poppet seat formed on the nozzle, wherein the area of the poppet valve enclosed within the diameter of the poppet seat is exposed to pressure in the inlet port such that a higher pressure in the inlet port can overcome the force of the poppet return resilient means and of the pressure outside the nozzle and open the poppet valve for injection of fuel into the engine. 3. A fuel injection system according to claim 2, wherein the poppet valve has a bore connected to the inlet port and terminated at the end proximate to the poppet by at least one injection orifice which connects the bore to the engine combustion chamber, and that the needle valve is installed in the bore and is slidably engaged with the bore by a precision-matched needle guide, the needle valve forming a seat, of a diameter smaller than the diameter of the guide, which can engage with the bore to close the fluid communication between the bore and the injection orifice, the needle guide forming a needle spring chamber in which a needle return resilient means is installed to act against the poppet valve and the needle valve to bias the needle valve towards closing the fluid communication between the bore and the injection orifice, wherein the force of the needle return resilient means and the areas enclosed by the diameter of the needle guide and by the diameter of the needle seat are chosen such that a higher pressure in the inlet port can overcome the force of the needle return resilient means and open the needle valve for injection of fuel from the bore through the injection orifice into the engine. 4. A fuel injection system according to claim 3, wherein the forces of the poppet return resilient means and the needle return resilient means and the diameters of the needle guide, needle valve seat and the poppet valve seat are chosen such that, at a given pressure of the medium outside the nozzle, the pressure of the fuel in the inlet port which is necessary to open the poppet valve, is higher than the pressure in the inlet port which is necessary to open the needle valve. 5. A fuel injection system according to claim 3, wherein the nozzle comprises a return channel, wherein the needle spring chamber is connected to the return channel through an outlet control orifice. 6. A fuel injection system according to claim 5, wherein the nozzle and the poppet valve are designed to slidably engage through a precision-matched poppet guide and that the return channel is arranged to open onto the poppet guide, wherein the poppet valve can close the fluid communication between the outlet control orifice and the return channel, depending on the axial position of the poppet valve. 7. A fuel injection system according to claim 5, wherein an edge of the needle valve can close the fluid communication between the needle spring chamber and the outlet control orifice, depending on the axial position of the needle valve. 8. A fuel injection system according to claim 3, wherein a communication between the needle spring chamber and the inlet port, is open or closed depending on the axial position of the poppet valve. 9. A fuel injection system according to claim 8, wherein during the closed position of the poppet valve when its poppet is abutted against the poppet seat, the needle spring chamber is connected to the return channel via the outlet control orifice and is disconnected from the inlet port. 10. A fuel injection system according to claim 9, wherein the opening travel of the poppet valve that is necessary to hydraulically connect the needle spring chamber to the inlet port, is at least as long as the opening travel of the poppet valve necessary to hydraulically disconnect the outlet control orifice from the return channel. 11. A fuel injection system according to claim 3, wherein there is a bottom guide section with a closely matched clearance between the poppet valve and the nozzle body along the entire periphery of the guide section, wherein the fuel to be injected through the area between the poppet and the poppet seat is delivered from the bore through at least one spray orifice. 12. A fuel injection system according to claim 11, wherein there is a clearance portion which is adjacent to the poppet and has a bigger clearance to the nozzle body than the clearance in the bottom guide section, wherein the fuel to be injected through the area between the poppet and the poppet seat is delivered from the bore through at least one spray orifice that opens out onto the clearance portion. 13. A fuel injection system according to claim 5, wherein the return channel is connected to a transfer volume. 14. A fuel injection system according to claim 5, wherein there is a return conduit and that the return channel is connected to the return conduit. 15. A fuel injection system according to claim 14, wherein a hydraulic differential valve is installed between the return channel and the return conduit, the valve being designed such that it is closed when the pressure in the valve is below a feed pressure which is characteristic to a running engine and that it is open when the pressure in the valve is at or above a feed pressure which is characteristic to a running engine. 16. A fuel injection system according to claim 15, wherein a plurality of injectors are connected by their return channels to a single hydraulic differential valve.