Accumulator-type fuel injection system

Abstract: A fuel injection system includes a fuel injector (6) having a body, a needle valve (5) movably provided in the body and a fuel accumulator having two chambers (3, 4) for storing a fuel at high and low pressures. An injection chamber (19) is formed in the injector lower body and a control chamber (22) is formed in the injector upper body. An injection passage (17) connects the high pressure chamber (3) with the injection chamber (18) and a control passage (21) connects the low pressure chamber (4) with the control chamber (22). The needle valve (5) is lifted upon leaking of the fuel to the accumulator from the control chamber (22) and the fuel is injected from the injection chamber (19) upon lifting of the needle valve (5). The fuel is recycled to the fuel tank from the control chamber (22).

Description: BACKGROUND OF THE INVENTION This invention relates to an accumulator-type fuel injection system according to the first part of claim 1. Accumulator-type fuel injection systems are known in the art. One typical example of such systems is schematically illustrated in Figure 7 of the accompanying drawings. Referring to Figure 7, a fuel in a fuel tank "a" is pumped by a high pressure pump "b" and the pressurized fuel is transferred from the pump "b" to an accumulator chamber "c", which is generally referred to as a common rail, and stored therein under a pressurized condition. The pressure in the accumulator chamber "c" is detected by a pressure sensor "d" and fed back to an ECU (Engine Control Unit) "j" such that an amount of fuel supplied to the accumulator chamber "c" by the pump "b" is controlled to an appropriate value. The high pressure fuel in the accumulator chamber "c" is introduced to an injector "g" through a feed line "f". Upon opening and closing of an electromagnetic valve "q" (i.e., three-way valve, which will be described later: Figure 8) located in the injector "g", the pressurized fuel is injected into a combustion chamber of an engine (not shown) such as a diesel engine from an injection opening or injection hole "h" at an end of the injector. A recycle line "i" extends to the fuel tank "a" from the injector "g" to return an excessive fuel to the fuel tank "a" from the injector "g". The high pressure pump "b", accumulator chamber "c" and injector "g" are controlled by the control unit "j" respectively. Referring to Figure 8 of the accompanying drawings, illustrated is a schema of the injector "g". As depicted, the pressurized fuel stored in the accumulator chamber "c" in the fuel feed line "f" is partly fed to an injection chamber "m" through an injection line "k" branched from the fuel feed line "f" and the remainder is introduced to a control chamber "o" through a branch line "n". A front or lower end portion of a needle valve "1" is located in the injection chamber "m" and a rear or upper end portion of the needle valve "l" is located in the control chamber "o". The fuel in the control chamber "o" is directed to the recycle line "i" from a leak line "p". At connection of the leak line "p" and control line "n", provided is the three-way valve "q" (electromagnetic valve). The three-way valve "q" connects the control line "n" with the control chamber "o" in its feed state and connects the control chamber "o" with the leak line "p" in its leak state. The fuel pressure in the injection chamber "m" acts on a cone portion "r" of the needle valve "l" to lift the needle valve (or open the needle valve) whereas the fuel pressure in the control chamber "o" acts on the tail of the needle valve to lower it (or close it). If the fuel pressure in the chamber "m" is balanced with that in the chamber "o" and the three-way valve "q" is switched to the leak state, the pressure balance is lost so that the needle valve "l" moves upward and the fuel in the injection chamber "m" is injected from the injection opening "h" formed at the nozzle tip. After that, by switching the three-way valve "q" to the feed state, the upper and lower pressures are balanced again so that the needle valve "l" moves downward and closes the opening "h" to complete the fuel injection. The injection chamber "m" for holding the fuel to be injected and the control chamber "o" for lifting/lowering the needle valve are communicated with the accumulator chamber "c" by the fuel feed line "f" and its branches "k" and "n". Therefore, if the pressure in the accumulator chamber "c" is raised to obtain a higher fuel injection pressure (i.e., a higher pressure in the injection chamber "m"), it also results in a higher pressure in the control chamber "o". As a result, an amount of the fuel leaking to the leak line "p" from the control chamber "o" through the three-way valve "q" increases. This raises an wasted work of the pump "b" and deteriorates a fuel consumption rate of a vehicle. In addition, if a high pressure is applied to the three-way valve "q", a lateral pressure is inherently generated because of the structure of the three-way valve. If the lateral pressure is generated, a larger drive power is required to maintain a desired operation. As a result, an electric power or current consumed by the three-way valve "q" increases. Furthermore, if the pressure in the control chamber "o" is raised, scratching and/or deformations occur in the three-way valve "q". This shortens and deteriorates durability and liability of the three-way valve. Also, this hinders smooth movement of the three-way valve so that response characteristics of the valve are degraded. The pressure in the injection room "m" varies with closing/opening of the needle valve "l". This pressure change is transferred to the three-way valve "q" via the passages "k" and "n" and might also damage the three-way valve "q". The disadvantages of the above-mentioned injection system are partly overcome by a system disclosed in US-A-4 603 671 which shows the features mentioned in the first part of claim 1. In this injection system fuel feeding to the two accumulators is completely independent and separate. Therefore, a plurality of pumps are required. Also pressure control for the two accumulators is completely independent. Thus, the fuel pressure is difficult to control. DE-A-3 618 447 discloses a fuel injection system having two accumulators and a single pump shared by the two accumulators. However, the pump directly supplies the fuel to the low pressure accumulator. Pressure control is carried out by a relief valve. This relief valve closes at a predetermined pressure. Therefore, it is not possible to create high and low pressure situations until a predetermined pressure is reached. Further, control is impossible when the predetermined pressure is exceeded. Another known arrangement is disclosed in Japanese Patent Application, Publication No. 6-108948. An object of this invention is to provide a fuel injection system for an engine not having drawbacks of a prior fuel injection system. Another object of this invention is to provide a fuel injection system for an engine having an accumulator injection pump which can inject a high pressure fuel with less fuel leakage. Still another object of the present invention is to provide a fuel injection system having two accumulators which will not be damaged even if operated under a super-high or ultra-high pressure. These objects are achieved by a fuel injection system according to claim 1. The needle valve is lifted upon leaking of the fuel to the accumulator from the control chamber and the fuel is injected from the injection chamber upon lifting of the needle valve. The high pressure fuel is only fed to the injection chamber via the injection passage whereas the low pressure fuel is only fed to the control chamber via the control passage. Therefore, the high pressure fuel is sprayed from the injector upon lifting of the needle valve. The low pressure fuel is used to activate the needle valve and then allowed to leak or escape to a recovery line which extends to the fuel accumulator under low pressure. Since the leak fuel is a low pressure fuel and the injection fuel is a high pressure fuel, the object of the invention is achieved. A bypass passage may be provided for connecting the injection passage with the control passage, a first open/close valve may be provided in the injection passage upstream of a bypass passage-injection passage connection and a second open/close valve may be provided in the leak passage. First, the first and second open/close valves are both closed. Then, the second valve is only opened to allow the fuel to leak from the control chamber to the leak passage. This causes the needle valve to be lifted. At this point, the low pressure fuel is only fed to the injection chamber through the bypass line. Therefore, the low pressure fuel is sprayed from the injector. After a while, the first open/close valve is also opened to introduce a high pressure fuel into the injection chamber. Then, this high pressure fuel is sprayed from the injector. The low pressure fuel injection is performed during an initial combustion period and the high pressure fuel injection is performed thereafter. The low pressure fuel injection results in a relatively slow or moderate combustion so that quick combustion temperature rise is prevented and in turn generation of NOx is reduced. A throat portion may be formed in the injection passage between the first open/close valve and the bypass passage-injection passage connection. The high pressure fuel unavoidably advances through the throat portion so that an only limited amount of high pressure fuel is fed to the injection chamber. Therefore, the fuel pressure injected from the injector gently rises even after feeding of the high pressure fuel. This further contributes to reduction of NOx. These and other objects and advantages of the fuel injection system of the present invention will become more apparent as the following detailed description and the appended claims are read and understood with the accompanying drawings. Figure 1 illustrates a schematic view of an accumulator-type fuel injection system according to the present invention; Figure 2 is a flowchart of operations of an open/close valve provided between a high pressure chamber and a low pressure chamber shown in Figure 1; Figure 3 schematically illustrates another fuel injection system (second embodiment) according to the present invention; Figure 4 illustrates injection characteristics of the fuel injection system shown in Figure 3; Figure 5 depicts a schematic construction of a third embodiment system according to the present invention; Figure 6 illustrates injection characteristics of the fuel injection system shown in Figure 5; Figure 7 shows a conventional fuel injection system; and Figure 8 shows another conventional fuel injection system. Preferred embodiments of the present invention will be described in reference to the accompanying drawings. Referring to Figure 1, an accumulator-type fuel injection system includes a high pressure pump 2 for pressurizing a fuel in a fuel tank 1 and delivering it to a predetermined destination, a high pressure chamber 3 connected with the high pressure pump 2 for storing the fuel under high pressure (e.g., about 1,200 bar), a low pressure chamber 4 connected with the high pressure chamber 3 for storing the fuel under low pressure (e.g., about 600 bar) and an injector 6 for utilizing the fuel from the high pressure chamber 3 as an injection fuel and the fuel from the low pressure chamber 4 as a working fluid for a needle valve 5. In the high pressure pump 2, a plunger 8 moves reciprocally against a spring 9 upon rotation of a camshaft 7 connected with a crankshaft or a similar member of an engine so that the fuel pumped from the fuel tank 1 by a pump 10 is pressurized in a cylinder 11 and transferred it to the high pressure chamber 3. An entrance passage 12 is bypassed from a discharge passage 13 of the high pressure pump 2 by a return passage 14. An open/close valve 15 is provided in the return passage 14. A check valve 16 is provided in the discharge passage 13. The high pressure chamber 3 holds the high pressure fuel transmitted from the high pressure pump via the discharge passage 13 at the pressure of approximately 1,200 bar. An injection passage 17 extends to the injector 6 from the high pressure chamber 3 and a communication passage 18 also extends to the low pressure chamber 4 from the high pressure chamber 3. Therefore, part of the fuel in the high pressure chamber 3 is introduced to the low pressure chamber 4 through the communication passage 18 and the remaining fuel is introduced through the injection passage 17 to an injection chamber 19 formed in the lower end portion of the injector 6 near the tip of the needle valve 5. The fuel in the injection chamber 19 is used as a fuel to be injected: it is injected from an injection opening 20 upon lifting of the needle valve 5. A pressure sensor 21a is provided in the high pressure chamber 3. The low pressure chamber 4 stores the fuel transmitted from the high pressure chamber 3 via the communication passage 18 at a low pressure (approximately 600 bar). A control passage 21 extends to the injector 6 from the low pressure chamber 4. The fuel flows from the low pressure chamber 4 through the control passage 21 and enters a control chamber 22 formed in the injector 6 near the tail of the needle valve 5. This fuel is used to activate the needle valve (referred to as "working fluid") and ultimately discharged to a recovery unit 24 via a leak passage 23. A pressure sensor 25 is provided in the low pressure chamber 4 and an open/close valve 26 (e.g., electromagnetic valve) is provided in the communication passage 18. The injector 6 uses the high pressure fuel from the high pressure chamber 3 as the injection fuel and uses the low pressure fuel from the low pressure chamber 4 as the working fluid. The needle valve 5 is housed in the injector 6 such that it can move reciprocally in the axial direction of the needle valve or the longitudinal direction of the injector 6. The injection chamber 19 is formed on the lower or front end side of the needle valve 5 and the control chamber 22 is formed on the upper or rear end side. Specifically, the injection chamber 19 surrounds a cone portion 27 of the needle valve 5 and the control chamber 22 is defined by a cylinder 29 formed to guide a piston portion 28 of the needle valve 5. The needle valve 5 is normally biased downward by a coil spring 30 to close the injection opening 20. A fork way 31a/31b extends from the control chamber 22. An orifice 32 is formed in one route 31a and a check valve 33 is provided in the other route 31b. These two routes 31a and 31b meet to form a single route 34. This route 34 is selectively connected with the leak passage 23 or the control passage 21 by a three-way valve 35 (electromagnetic valve). The three-way valve 35 can take a feed condition to communicate the control passage 21 with the passage 34 (and in turn the control chamber 22) and a leak condition to communicate the leak passage 23 with the passage 34 (and in turn the control chamber 22). The leak passage 23 allows the fuel in the control chamber 22 to escape to the recovery unit 24 from the control chamber 22. The needle valve 5 of the injector 6 is forced upwardly by a pressure of the fuel in the injection chamber 19 and at the same time forced downwardly by a fuel pressure in the control chamber 22 and a coil spring 30. The needle valve 5 is maintained at a closed position upon balancing of the two opposite force. If the three-way valve 35 is switched to the leak condition, then the balanced situation is lost and the needle valve 5 moves upward so that the fuel in the injection chamber 19 is sprayed from the injection opening 20 at the tip of the nozzle. After that, if the three-way valve 35 is switched to the feed condition, then the needle valve 5 is moved downward to close the injection opening 20 upon balancing of the upward and downward forces. The pressure sensor 21a of the high pressure chamber 3, the pressure sensor 25 of the low pressure chamber 25, a sensor 36 for an engine speed and a cam angle, a sensor 37 for determination of cylinder into which the fuel be injected, a sensor 38 for an engine load and other sensors for various engine and driving information such as amount of intake air are all coupled to a control unit 39 (ECU). The control unit 39 controls the three-way valve 35, the valve 26 of the passage 18 and the valve 15 of the high pressure pump 2 based on information fed from these sensors. Specifically, the control unit 39 controls the valve 15 of the high pressure pump 2 in accordance with data detected by the pressure sensor 21a of the high pressure chamber 3 such that part of the fuel pressurized by the plunger 8 is caused to return to the inlet passage 12 through the return passage 14 so as to maintain the fuel pressure in the high pressure chamber 3 at a substantially constant value (about 1,200 bar). The control unit 39 also keeps the fuel pressure in the low pressure chamber 4 at a substantially constant value (about 600 bar) in the following manner. Referring to Figure 2, first, the pressure P1 of the high pressure chamber 3 and the pressure P2 of the low pressure chamber 4 are detected by the associated pressure sensors 21a and 25 respectively to determine whether P1/n is greater than P2. Here, n is a ratio of the pressure in the high pressure chamber 3 to that in the low pressure chamber 4 and it is generally set to 2 to 4. If P1/n is greater than P2, the valve 26 on the communication line 18 is opened whereas if it is not, the valve 26 is closed. Such control can maintain the pressure ratio "n" to a certain value. As understood from the flowchart shown in Figure 2, keeping the fuel pressure in the high pressure chamber 3 to a particular value (e.g., about 1,200 bar) by opening and closing of the valve 15 of the high pressure pump 2 results in keeping the fuel pressure in the low pressure chamber 4 to one-"n"th the fuel pressure of the high pressure chamber 3. In this particular embodiment, "n" = 2. Accordingly, the pressure in the low pressure chamber 4 is maintained to about 600 bar. The control unit 39 determines the condition of the three-way valve 35 of the injector 6, the feed condition or the leak condition, according to data acquired from the engine speed and cam angle sensor 36 and other sensors. Upon switching to the leak condition, the needle valve 5 is lifted and the fuel in the fuel chamber 19 is sprayed from the injection opening 20 formed at the tip of the nozzle, whereas upon switching to the feed condition, the needle valve 5 is lowered to close the injection hole 20, as mentioned earlier. Now, operations of the system will be described. The fuel in the high pressure chamber 3 of about 1,200 bar is introduced to the injection chamber 19 through the injection passage 17 and injected upon lifting of the needle valve 5. On the other hand, the fuel in the low pressure chamber 4 of about 600 bar is supplied to the control chamber 22 through the control passage 21 as the three-way valve 35 is switched to the feed condition. Then, the low pressure fuel is used to activate the needle valve 5 and allowed to leak into the leak passage 23 under low pressure as the three-way valve 35 is switched to the leak condition. Since the high pressure fuel of the high pressure chamber 3 is utilized as the injection fuel and the low pressure fuel of the low pressure chamber 4 is utilized as the working fluid for the needle valve 5, the fuel injection pressure is raised and the leakage fuel pressure is decreased. In other words, high pressure injection is achieved while achieving leakage fuel reduction. In this manner, the amount of leaking fuel is suppressed even if the injection pressure is raised. This reduces dead work of the high pressure pump 2 so that the fuel consumption rate is improved. The consumption current of the three-way valve 35 (electromagnetic valve) also drops. Since the low pressure fuel is fed into the three-way valve 35 as the working fluid, scratching in the three-way valve 35 and/or deformations do not occur. This extends the durability of the three-way valve 35 and improves the reliability. Also, this ensures smooth movement/operation of the three-way valve 35 so that the response characteristic of the three-way valve 35 is maintained even if the injection pressure is raised. The pressure in the injection chamber 19 varies with opening/closing of the needle valve 5, but this pressure change is moderated by the injection passage 17, high pressure chamber 3 and low pressure chamber 4 before it is transmitted to the three-way valve 35 via the control passage 21. Therefore, the three-way valve 35 is not damaged by the pressure fluctuation. In this manner, the pressure in the control passage 21 is maintained to a constant value regardless the opening/closing of the needle valve 5. Accordingly, precise control of the three-way valve 35 is possible in both feed and leak conditions. Another embodiment according to the present invention will be described with reference to Figure 3. The fundamental construction of the system of this embodiment is the same as that shown in Figure 1 so that the same reference numerals are used and detailed description of these elements is omitted. Hereunder, only differences as compared with the previous embodiment will be described. Referring to Figure 3, the injection passage 17 of the injector 6 is communicated with the control passage 21 by a bypass passage 41 having a one-way valve 40. A first open/close valve 43 is provided in the injection passage 17a upstream of a bypass passage-injection passage connection 42. The three-way valve 35 serves as a second open/close valve since it allows the fuel in the control chamber 22 to escape to the recovery unit 24 through the leak passage 23 when it is in the leak condition. Operations of this embodiment will now be described. In Figure 3, first, the first open/close valve 43 in the injection passage 17a is closed and the three-way valve 35 is set to the feed condition. Then, the three-way valve 35 is changed to the leak condition. This causes the fuel in the control chamber 22 to proceed to the recovery unit 24 through the leak passage 23 and in turn causes the needle valve 5 to lift thereby injecting the fuel from the injection chamber 19. At this point, the low pressure fuel of the low pressure chamber 4 has been supplied to the injection chamber 19 via the control passage 21, the bypass passage 41 and the injection passage 17. Therefore, the fuel injection nozzle performs low pressure injection. After that, as the first open/close valve 43 is opened, the high pressure fuel of the high pressure chamber 3 is supplied to the injection chamber 19 through the injection passages 17a and 17. Thus, the fuel injection nozzle now performs high pressure injection. Such changes in the injection pressure are illustrated in Figure 4. As seen in this illustration, the three-way valve 35 is set to the leak state at the time T1 to conduct the low pressure injection (P L ), the first open/close valve 43 is opened at the time of T2 to conduct the high pressure injection (P H ), and the first open/close valve 43 is closed and the three-way valve 35 is switched to the feed state at the time of T3 to complete the fuel injection. This operation provides a two-stage fuel injection. This stepwise fuel injection suppresses preliminary combustion or initial-stage combustion since less fuel is fed during the low pressure injection. Accordingly, quick rise of combustion chamber temperature is prevented and generation of NOx is reduced particularly during the initial stage of combustion. Still another embodiment of the present invention will be described with reference to Figure 5. The basic structure of this system is also the same as the preceding embodiment so that the same reference numerals are assigned and detailed description of these parts will be omitted. Only difference lies in that provision of a throttling portion 44 in the injection passage 17b between the first open/ close valve 43 and the injection passage-bypass passage connection 42. When the first open/close valve 43 is opened for high pressure fuel injection, the fuel in the injection passage 17b is throttled by the throat portion 44 so that increase in the fuel pressure becomes gentle. This is illustrated in Figure 6. As understood from this diagram, the three-way valve 35 is set to the leak state at the time of t1 for the low pressure injection (p l ), the first open/close valve 43 is opened at the time of t2 for gentle transition to the high pressure injection (p h ), and the first open/close valve 43 is closed and the three-way valve 35 is changed to the feed state at the time of t3 for completion of the fuel injection. The high pressure injection is performed in a non-sharp manner as indicated by the relatively gentle slope between t2 and t3 in Figure 6, so that the combustion temperature does not rise suddenly during the initial combustion period and generation of NOx is further suppressed.

Claims: A fuel injection system comprising: a fuel injector (6) having a body, a needle valve (5) movably provided in the body, an injection chamber (19) formed in the body in its lower end portion and a control chamber (22) formed in the body in its upper end portion; fuel accumulator means for storing a fuel, the fuel accumulator means having a high pressure chamber (3) and a low pressure chamber (4); an injection passage (17) for connecting the high pressure chamber (3) with the injection chamber (19); a control passage (21) for connecting the low pressure chamber (4) with the control chamber (22); means for selectively feeding the fuel to the control chamber (22) and the injection chamber (19) from the fuel accumulator means; and a leak passage (23) for recycling the fuel to the fuel accumulator means from the control chamber (22), the needle valve (5) being lifted upon leaking of the fuel to the accumulator means from the control chamber (22), the fuel being injected from the injection chamber (19) upon lifting of the needle valve (5), characterised in that the high pressure chamber (3) is connected to a pump (2) so that the pump (2) feeds the fuel to the high pressure chamber (3) only, and that a controllable electromagnetic valve (26) is provided in a communication passage (18) connecting the high and low pressure chambers (3, 4) whereby the fuel supplied to the low pressure chamber (4) is a fuel accumulated in the high pressure chamber (3). The fuel injection system of claim 1, further including: a bypass passage (41) for connecting the injection passage (17) with the control passage (21); a first open/close valve (43) provided in the injection passage (17) upstream of a bypass passage-injection passage connection (42); and a second open/close valve (35) provided in the leak passage (23). The fuel injection system of claim 2, further including a throat portion (44) formed in the injection passage (17) between the first open/close valve (43) and the bypass passage-injection passage connection (42). The fuel injection system according to any one of claims 1 to 3, wherein the high pressure chamber (3) maintains the fuel at about 1,200 bar and the low pressure chamber (4) maintains the fuel at about 600 bar. The fuel injection system according to any one of claims 1 to 4, wherein the selectively feeding means includes a three-way valve (35) provided in the control passage (21), and the control passage (21) is communicated with the control chamber (22) upon switching the three-way valve (35) to a first condition whereas it is disconnected from the control chamber (22) upon switching the three-way valve (35) to a second condition.

Inventors: NISHIMURA TERUKAZU, TAKAHASI SUSUMU, NISHIMURA, TERUKAZU, TAKAHASI, SUSUMU, Nishimura, Terukazu, c/o Isuzu Motors Ltd., Takahasi, Susumu, c/o Isuzu Motors Ltd.
Applicants: ISUZU MOTORS LTD, ISUZU MOTORS LIMITED
Application year: 1996
Main classification: F02M47
IPC codes: F02D41, F02M59, F02M63, F02M45, F02M47