A FUEL INJECTOR FOR AN ENGINE

Abstract: A fuel injector comprising a piston (8) actuated by a piezoelectric element (9). A pressure control chamber (15) is formed between the piston (8) and the top face of the needle (3) and connected to a high pressure fuel source via a fuel passage (18) having a restricted flow area. The pressure control chamber (15) is filled with fuel under pressure. The rear face of the piston (8), which is positioned opposite to the pressure control chamber (15), is exposed to a high pressure fuel chamber (27) filled with fuel under pressure. The driving force acting on the piston due to the pressure of fuel in the pressure control chamber (15) is cancelled by the driving force acting on the piston (8) due to the pressure of fuel in the high pressure fuel chamber (27).

Description: The present invention relates to a fuel injector for an engine, according to the preamble of claim 1. In document US-A-4 579 283 there has been disclosed a generie fuel injector as for an engine. According to this fuel injector a high pressure fuel chamber is provided on the rear face of the piston and opposite to the pressure control chamber for balancing the pressure acting on the piston. The pressure control chamber and the high pressure fuel passage communicating with the high pressure fuel source. This fuel injector design, even though it is very functional, requires a plurality of constructional members and measures and is thus very difficult to realize technologically. It is the object of the invention to produce a fuel injector of this kind with a simplified design, while its functional advantages are maintained. This object is achieved by the features in the characterizing part of claim 1. According to these features the high pressure fuel chamber is not fed directly via the high pressure fuel passage communicating with the high pressure fuel source, but via a communication with the pressure control chamber, which is formed as a clearance between the piston and the cylinder formed around it. This clearance is sufficient for balancing the pressure acting on both ends of the piston to a large extent and thus makes it possible to carry out a precise control of the fuel injection; it is at the same time much easier when manufacturing the fuel injector according to the invention to change only the design of the piston or of the cylinder, instead of achieving a branching off of the high pressure fuel passage by using additional constructional components or by working extensively on the fuel injectior body. The present invention may be more fully understood from the description of preferred embodiment of the invention set forth below, together with the accompanying drawing. The figure illustrates a embodiment of a fuel injector. Referring to this Figure, reference numeral 1 designates a housing of the fuel injector, 2 a needle bore, 3 a needle inserted into the needle bore 2, 4 nozzle holes, 5 a pressure receiving face formed on the needle 3, 6 a needle pressure chamber formed around the pressure receiving face 5, 7 a cylinder, 8 a piston slidably inserted in the cylinder 7, and 9 a piezoelectric element for activating the piston 8. A rod 30 having a diameter which is smaller than the diameter of the piezoelectric element 9 is fixed to the piston 8, and the piston 8 is connected to the piezoelectric element 9 via the rod 30. The seal ring 26 is inserted between the rod 30 and a rod bore 31, and the disc-shaped spring 12 is inserted between the rod 30 and the housing 1. A pressure control chamber 15 defined by the piston 8 is formed in the cylinder 7. This pressure control chamber 15 is connected to a pressure control chamber 16 defined by the top face of the needle 3 within the needle bore 2, and consequently, the pressure control chambers 15, 16 are formed between the piston 8 and the top face of the needle 3. A comparison spring 17 is arranged in the pressure control chamber 16 to continuously urge the needle 3 toward the nozzle holes 4, and the pressure control chamber 16 is connected to the needle pressure chamber 6 via an annular fuel passage 18 having a restricted flow area and formed between the needle 3 and the needle bore 2. The need pressure chamber 6 is connected on one hand to the nozzle holes 4 via an annular fuel passage 19 formed around the needle 3, and on the other hand, to a fuel inlet 21 via a fuel passage 20. The fuel inlet 21 is connected to a reservoir tank 22 storing fuel under a high pressure therein, and fuel under a high pressure discharged from a fuel pump 23 is fed into the reservoir tank 22 via a flow control valve 24. An annular high pressure fuel chamber 27 is formed around the rod bore 31, and the rear face 28 of the piston 8 is exposed to the high pressure fuel chamber 27. The cylinder 7 has a cylindrical shape having a uniform cross-section over the entire length thereof, and the piston 8 has a cylindrical shape having a uniform cross-section over the entire length thereof. An annular fuel passage 32 having a restricted flow area is formed between the cylinder 7 and the piston 8, and the high pressure fuel chamber 27 is connected to the pressure control chamber 15 via the fuel passage 32 having a restricted flow area. Fuel under a high pressure fed into the fuel inlet 21 from the reservoir tank 22 is fed into the needle pressure chamber 6 via the fuel passage 20. The fuel under a high pressure in the needle pressure chamber 6 is fed into the pressure control chambers 15, 16 via the fuel passage 18 having a restricted flow area, and the fuel under a high pressure in the pressure control chamber 15 is fed into the high pressure fuel chamber 27 via the fuel passage 32 having a restricted flow area. Therefore, the pressure of the fuel in the high pressure fuel chamber 27 becomes equal to that in the pressure control chambers 15, 16. The pressure of the fuel in the high pressure fuel chamber 27 acts on the rear face 28 of the piston 8. The surface area of the rear face 28 of the piston 8 is slightly smaller than the cross-sectional area of the piston 8, the upward driving force acts on the piston 8 due to the pressure of fuel fed from the fuel pump 23, but this driving force is weak, and the load acting to contract the piezoelectric element 9 is low. Since the surface area of the rear face 28 of the piston 8 can be formed to be very close to the cross-section area of the piston 8, by reducing the diameter of the rod 30, it is possible to considerably decrease the load acting on the piezoelectric element 9. When electric charges in the piezoelectric element 9 are discharged, the piezoelectric element 9 contracts, and at this time, the piston 8 is moved upward due to the spring force of the disc-shaped spring 12. As a result, since the volume of the pressure control chambers 15, 16 is increased, the pressure of the fuel in the pressure control chambers 15, 16 becomes low, and when the pressure of the fuel in the pressure control chambers 15, 16 becomes low, the needle 3 is moved upward due to the pressure of fuel in the pressure receiving face 5 of the needle 3, and thus the fuel injection from the nozzle holes 4 is started. When the pressure of the fuel in the pressure control chambers 15, 16 becomes low, and the needle 3 is moved upward, the volume of the pressure control chambers 15, 16 is decreased, and further, the fuel under high pressure in the needle pressure chamber 6 is gradually fed into the pressure control chambers 15, 16 via the fuel passage 18 having a restricted flow area. As a result, although the pressure of the fuel in the pressure control chambers 15, 16 is increased, the spring force of the compression spring 17 and the flow area of the fuel passage 18 are determined such that the needle 3 remains open during the fuel injection time, and thus the fuel injection continues to be carried out. When electric power is charged to the piezoelectric element 9, since the piezoelectric element 9 expands, the piston 8 is moved downward, and as a result, since the volume of the pressure control chambers 15, 16 is decreased, the pressure of the fuel in the pressure control chambers 15, 16 becomes high. When the pressure of the fuel in the pressure control chambers 15, 16 becomes high, the needle 3 is moved downward and closes the nozzle holes 4, and thus the fuel injection is stopped. Also, when the needle 3 is moved downward, the volume of the pressure control chambers 15, 16 is increased, and further, the fuel in the pressure control chambers 15, 16 is returned to the needle pressure chamber 6 via the fuel passage 18 having a restricted flow area. As a result, the pressure of the fuel in the pressure control chambers 15, 16 approaches the pressure of the fuel in the needle pressure chamber 6. During the above-mentioned operation of the fuel injector, the driving force acting on the piston 8 from the pressure control chamber 15 side due to the pressure of the fuel fed from the fuel pump 23 is substantially cancelled by the driving force acting on the piston 8 from the high pressure fuel chamber 27 side due to the pressure of the fuel fed from the fuel pump 23. Consequently, even if the pressure of the fuel fed from the fuel pump 23 is changed, this change does not have a substantial influence on the piezoelectric element 9, and therefore, since this change does not cause a change in the amount of the expansion of the piezoelectric element 9, a precise control of the fuel injection can be obtained. In addition, the driving force acting on the piezoelectric element 9 due to the pressure of the fuel fed from the fuel pump 23, is extremely weak. Consequently, an energy needed to expand the piezoelectric element 9 is reduced, and thus it is possible to minimize the size of the piezoelectric element 9 and reduce the consumption of electric power. When the piezoelectric element 9 contracts, the piston 8 is moved upward due to the spring force of the disc-shaped spring 12, and therefore, the high pressure fuel chamber 27 must have a relatively large volume, or the fuel passage 32 must have a relatively large cross-sectional area so that, when the piston 8 is moved upward, the pressure of the fuel in the high pressure fuel chamber 27 is not increased to an extent such that the upward movement of the piston 8 is prevented.

Claims: A fuel injector, comprising a needle (3) having a pressure receiving face (5) formed thereon, said needle (3) being slidably inserted in a needle bore (2) which has a needle pressure chamber (6) formed around said pressure receiving face (5) and connected to a high pressure fuel source (22, 23, 24), and said needle (3) having one end for controlling an opening operation of a nozzle hole (4) and having another end opposite to said one end, a piston (8) slidably inserted in a cylinder (7), the piston (8) having one end and a rear face (28) opposite to said one end of said piston (8), the other end of said needle (3) and the one end of said piston (8) defining a pressure control chamber (15, 16) therebetween, a fuel passage (18) having a restricted flow area and connecting said pressure control chamber (15, 16) to said needle pressure chamber (6), a high pressure fuel chamber (27) to which the rear face (28) of said piston (8) is exposed, said high pressure fuel chamber (27) being filled with fuel under pressure and having a pressure which is substantially equal to that of the fuel under pressure in said pressure control chamber (15, 16), and actuating means (9) for actuating said piston (8) to increase a volume of said pressure control chamber (15, 16), to thereby cause said nozzle hole (4) to be opened by said needle (3) and to decrease the volume of said pressure control chamber (15, 16), to thereby cause said nozzle hole (4) to be closed by said needle (3), characterized in that, a clearance between said piston (8) and said cylinder (7) forms another fuel passage (32) having a restricted flow area and extending between said high pressure fuel chamber (27) and said pressure control chamber (15, 16) to connect said high pressure fuel chamber (27) to said pressure control chamber (15, 16). A fuel injector according to claim 1, characterized in that said piston (8) has a cylindrical shape having a uniform cross-section over the entire length thereof, and said cylinder (7) has a cylindrical shape having a uniform cross-section over the entire length thereof. A fuel injector according to claim 1 or 2, characterized in that said needle (3) is slidably inserted in a needle bore (2), and a compression spring (17) is arranged in said needle bore (2) to urge said needle (3) toward said nozzle hole (4). A fuel injector according to any one of claims 1 to 3, characterized in that said actuating means (9) is connected to said piston (8) via a rod (30), with a seal ring (26) being inserted between said rod (30) and a rod bore (31), and a disc shaped spring (12) being inserted between said rod (30) and a housing (1).

Inventors: HASHIMOTO, EIJI, MITSUYASU, MASAKI
Applicants: TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1988
Main classification: F02M47
IPC codes: F02M47, F02M51, F02M61, F02M63