Fuel injection system for engine

Abstract: A fuel injection system for an engine controls a behavior of a nozzle needle for controlling a fuel injection timing and a fuel injection amount. The system controls a hydraulic pressure applied to the nozzle needle for displacing the nozzle needle between its fully opened position and its fully closed position. The system so controls the hydraulic pressure applied to the nozzle needle as to quickly displace the nozzle needle from the fully opened position to a given position which is located between the fully opened and closed positions, on the other hand, slowly displace the nozzle needle from the given position to the fully closed position so as to decrease an impact load applied to a valve seat for the nozzle needle when the nozzle needle is seated onto the valve seat.

Description: The present invention relates generally to a fuel injection system for an engine according to the preamble of claim 1, and more specifically, to a common-rail fuel injection system for a diesel engine. A common-rail fuel injection system according to the preamble of claim 1 is disclosed in US-A-4,545,352. In this known common-rail fuel injection system, high pressure fuel is accumulated in a so-called common rail working as a surge tank to be injected into engine cylinders via opening and closing operations of respective fuel injectors. As shown in Fig. 1, a common-rail fuel injection device 100 of this type includes an injection nozzle 101 through which the high pressure fuel from the common rail is injected into the corresponding engine cylinder, and a three-way solenoid valve 102 which controls a fuel injection timing and a fuel injection amount. The injection nozzle 101 includes a nozzle needle 103 operative to open and close injection holes, a hydraulic piston 104 operative to drive the nozzle needle 103, and a control chamber 105 operative to control a hydraulic pressure to be applied to the hydraulic piston 104. As shown in Fig. 2, a pressure control valve 107 is provided in the control chamber 105. The pressure control valve 107 comprises an orifice 109 extending through the pressure control valve 107 at its center. A reference numeral 108 denotes a portion of the three-way solenoid valve 102, defining a communication passage 106 and working as a valve seat for the pressure control valve 107. Practically, the orifice 109 only works to control the flow of the hydraulic pressure from the control chamber 105 into the communication passage 106 of the three-way solenoid valve 102 as will be clear from the following explanation with reference to Fig. 3. Fig. 3 is a timechart showing a relationship between a hydraulic pressure in the control chamber 105, a lift position of the nozzle needle 103 and a load applied to a valve seat for the nozzle needle 103. At the start of the fuel injection, which corresponds to Fig. 2, the three-way solenoid valve 102 allows the communication passage 106 to communicate with a low pressure side. Accordingly, the pressure control valve 107 is seated on the valve seat 108 to allow the high pressure fuel within the control chamber 105 to slowly flow out via the orifice 109 in a controlled fashion, as shown in Fig. 3(A). When the hydraulic pressure in the control chamber 105 drops to a valve opening pressure for the nozzle needle 103, the hydraulic piston 104 starts to slowly move up, resulting in lifting up the nozzle needle 103 as shown in Fig. 3(B). This means that the nozzle needle 103 starts to separate from its valve seat in a nozzle body 110 to allow the start of the fuel injection via the injection holes into the corresponding engine cylinder. On the other hand, at the end of the fuel injection, the three-way solenoid valve 102 allows the communication passage 106 to communicate with a high pressure side, i.e. the common rail. Accordingly, the high pressure fuel is applied to the pressure control valve 107 to urge the same toward the hydraulic piston 104. Thus, the pressure control valve 107 is separated from the valve seat 108 to allow immediate introduction of the high pressure fuel into the control chamber 105 via an annular gap formed between the outer periphery of the pressure control valve 107 and the peripheral wall of the control chamber 105. Accordingly, in this case, the orifice 109 does not function to control the flow of the high pressure fuel from the communication passage 106 into the control chamber 105. As a result, as shown in Fig. 3(A), the pressure in the control chamber 105 immediately increases to a valve closing pressure for the nozzle needle 103. This leads to a quick overall downward movement of the hydraulic piston 104 to force the nozzle needle 103 onto the valve seat in the nozzle body 110. With the foregoing structure, this prior art common-rail fuel injection system is capable of providing the desired so-called delta type fuel injection characteristics, that is, the fuel injection rate is small at the start of the injection and gradually gets larger, while the sharp cut-off of the fuel injection is attained at the end of the injection. This prior art common-rail injection system, however, has the following disadvantages. As described above, the high pressure fuel is immediately introduced into the control chamber 105 at the end of the fuel injection. Accordingly, as shown in Fig. 3(A), the hydraulic pressure in the control chamber 105 inevitably becomes overshot so that the nozzle needle 103 is forced down to a level exceeding a position of the nozzle needle 103 at the start of the fuel injection, as shown in Fig. 3(B). This causes the disadvantage that an excessive impact load P = {(upper peak value) - (lower peak value)} is applied to the valve seat for the nozzle needle 103, as shown in Fig. 3(C). This necessitates associated portions around the valve seat in the nozzle body 110 to be made thicker so as to provide a strength which is large enough to withstand the applied impact load P. Mere provision of the larger thickness around the valve seat, however, inevitably increases a length of each injection hole so that an increased resistance against the flow of the injected fuel is obtained. On the other hand, in order to avoid such an increased resistance with the increased thickness, a volume of a sack chamber 111 should be enlarged. This, however, causes the following problems. The sack chamber 111 is located downstream of the valve seat for the nozzle needle 103 and is formed with the injection holes at its downstream end portions. Accordingly, the fuel in the sack chamber 111 is likely to flow out into the corresponding engine cylinder via the injection holes even after the completion of the fuel injection, i.e. even after the nozzle needle 103 is seated on the valve seat. This means that the enlarged volume of the sack chamber 111 may lead to serious disadvantages such as increases of fuel consumption rate, exhaust gas temperature and hydrocarbon. Under these circumstances, enlarging the thickness around the valve seat can not be taken as measures for solving the problem of the excessive impact load P in view of the other serious problems caused thereby. Therefore, it is the object of the present invention to provide an improved fuel injection system for an engine that can eliminate the above-noted disadvantages of the prior art system: This object is solved by the advantageous features indicated in the characterizing part of claim 1. The present invention will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention In the drawings: Fig. 1 is a sectional view showing a conventional fuel injection device to be used in a common-rail fuel injection system for a diesel engine; Fig. 2 is a sectional view showing a portion of the fuel injection device in Fig. 1, wherein an arrangement of associated members for controlling a hydraulic pressure applied to a hydraulic piston is shown; Fig. 3 is a timechart showing a relationship of variations among a hydraulic pressure in a pressure control chamber, a lift position of a nozzle needle and a load applied to a valve seat for the nozzle needle, which is derived by the prior art of Figs. 1 and 2; Fig. 4 is a sectional view showing a common-rail fuel injection system for a diesel engine according to a first preferred embodiment of the present invention; Fig. 5 is a sectional view showing a portion of the fuel injection system in Fig. 4, wherein an arrangement of associated members for controlling a hydraulic pressure applied to a hydraulic piston is shown; Fig. 6 is a sectional view showing portions of a nozzle body and a nozzle needle incorporated in the fuel injection system in Fig. 4; Fig. 7 is a sectional view showing the arrangement in Fig. 5, wherein one operating state of the associated members for controlling the hydraulic pressure applied to the hydraulic piston is shown; Fig. 8 is a sectional view showing another operating state of the associated members in Fig. 7; Fig. 9 is a sectional view showing still another operating state of the associated members in Fig. 7; Fig. 10 is a sectional view showing a further operating state of the associated members in Fig. 7; Fig. 11 is a sectional view showing a still further operating state of the associated members in Fig. 7; Fig. 12 is a timechart showing a relationship of variations among a hydraulic pressure in a pressure control chamber, a lift position of the nozzle needle and a load applied to a valve seat for the nozzle needle, according to the first preferred embodiment of the present invention; Fig. 13 is a sectional view showing a modification of the arrangement in Fig. 7; Fig. 14 is a sectional view showing another modification of the arrangement in Fig. 7; Fig. 15 is a sectional view showing one operating state of an arrangement of associated members for controlling a hydraulic pressure applied to a hydraulic piston according to a second preferred embodiment of the present invention; Fig. 16 is a sectional view showing another operating state of the associated members in Fig. 15; Fig. 17 is a sectional view showing still another operating state of the associated members in Fig. 15; Fig. 18 is a sectional view showing a further operating state of the associated members in Fig. 15; and Fig. 19 is a timechart showing variations in a lift position of a nozzle needle, according to the second preferred embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, a first preferred embodiment of a fuel injection system for an engine according to the present invention will be described with reference to Figs. 4 to 12. Fig. 4 shows a common-rail fuel injection system for a diesel engine according to the first preferred embodiment. A fuel injection device 1 is provided for each engine cylinder (not shown) and constantly fed with the high pressure fuel at an inlet port 58 from a common rail 11. The common rail 11 works as a pressure accumulator for storing the high pressure fuel supplied from a high pressure fuel supply pump (not shown) and feeds the high pressure fuel to each of the fuel injection devices 1. The fuel injection device 1 includes a nozzle needle 2, a nozzle body 3, a hydraulic piston 4 and a nozzle holder 5, which cooperatively constitute an injection nozzle. The fuel injection device 1 further includes a three-way solenoid valve 6. The nozzle needle 2 is slidably received in the nozzle body 3 and, as shown in Fig. 6, formed at its longitudinal end with a stepped contact portion 21 which is selectively seated on and separated from a valve seat 33 of the nozzle body 3 by means of the operations of the hydraulic piston 4. Specifically, the nozzle needle 2 is mechanically connected at its another longitudinal end to the hydraulic piston 4. When the hydraulic piston 4 is forced toward the three-way solenoid valve 6, the contact portion 21 is separated from the valve seat 33, on the other hand, when the hydraulic piston 4 is forced toward the nozzle needle 2, the contact portion 21 is seated onto the valve seat 33. As shown in Fig. 12(B), the nozzle needle 2 is lifted up and down between levels A and E during the fuel injection, i.e. between the beginning and end of the fuel injection, which will be described later in detail. The nozzle body 3 slidably supports the nozzle needle 2 therewithin and includes a pressure chamber 31, injection holes 32, the valve seat 33 and a sack chamber 34. The pressure chamber 31 is defined between the inner peripheral wall of the nozzle body 3 and the outer periphery of the nozzle needle 2 and is constantly fed with the high pressure fuel from the common rail 11 via the inlet port 58 and a fuel feed passage 51 which connects the inlet port 58 to the pressure chamber 31. The valve seat 33 is provided upstream of the injection holes 32 with respect to the flow direction of the high pressure fuel. Accordingly, when the contact portion 21 of the nozzle needle 2 is seated on the valve seat 33 to block a communication between the pressure chamber 31 and the sack chamber 34, no fuel is injected into the engine cylinder via the injection holes 32. On the other hand, when the contact portion 21 of the nozzle needle 2 is separated from the valve seat 33 to establish the communication between the pressure chamber 31 and the sack chamber 34, the high pressure fuel is injected into the engine cylinder via the injection holes 32. As shown in Fig. 4, the hydraulic piston 4 is drivingly connected to the nozzle needle 2 via a push rod 41 constantly urged toward the valve seat 33 by the force of a coil spring 42. The operations of the hydraulic piston 4 will be described later in detail. The nozzle holder 5 is formed therein with the inlet port 58, the fuel feed passage 51 and a cylindrical stepped bore 59. The stepped bore 59 includes first and second chambers 52 and 53. The first chamber 52 is arranged at one end of the nozzle holder 5 remote from the valve seat 33 and opens toward the three-way solenoid valve 6. The second chamber 53 is of a smaller diameter than that of the first chamber 52 and extends toward the valve seat 33 to slidably receive therein the cylindrical hydraulic piston 4. As clearly shown in Fig. 5, the first chamber 52 is opened at an end surface 54 of the nozzle holder 5 and defined between an annular step 55 of the stepped bore 59 and an end surface 60 of the three-way solenoid valve 6. The annular step 55 and the end surface 60 respectively serve as valve seats for a pressure control valve member 7. The pressure control valve member 7 is slidably received in the first chamber 52 and is formed with an orifice 73 at its center. The orifice 73 extends through the pressure control valve member 7 in the longitudinal direction of the nozzle needle 2 or the hydraulic piston 4, that is, from a side of an end surface 72 facing the three-way solenoid valve 6 into a cylindrical central recess 75 formed at a side of an end surface 71 facing the hydraulic piston 4. The outer periphery 74 of the pressure control valve 7 and the peripheral wall of the first chamber 52 cooperatively provide a fluid-tight sealing effect therebetween. A coil spring 8 is received in the recess 75 of the pressure control valve member 7 at its one end and in a cylindrical central recess 41 of the hydraulic piston 4 at its other end so as to urge both members 7 and 4 in axially opposite directions, that is urging the pressure control valve member 7 toward the valve seat 60 of the three-way solenoid valve 6 and urging the hydraulic piston 4 toward the valve seat 33. The pressure control valve member 7 and the hydraulic piston 4 cooperatively define therebetween a pressure control chamber 76 for controlling a hydraulic pressure to be applied to the hydraulic piston 4. As will be described later in detail, the orifice 73 works to control the hydraulic pressure within the pressure control chamber 76 both at the start of the fuel injection and at the termination thereof. As shown in Fig. 4, the three-way solenoid valve 6 includes a coil 61, an inner valve member 62, an outer valve member 63 and a valve body 64. The inner valve member 62 is slidably received in the outer valve member 63. The outer valve member 63 is slidably received in the valve body 64 and formed therein with a hydraulic passage 65. The valve body 64 is formed therein with a communication passage 66, a high pressure passage 67, a low pressure or drain passage 68 and a valve chamber 69 which slidably receives the outer valve member 63. The communication passage 66 communicates with the first chamber 52 at its one end and with the valve chamber 69 at its other end. The high pressure passage 67 communicates with the fuel feed passage 51 at its one end and with the valve chamber 69 at its other end. Accordingly, the high pressure fuel is constantly fed into the high pressure passage 67 via the fuel feed passage 51. The drain passage 68 communicates with the valve chamber 69 at its one end and with a low pressure side 12 at its other end. When the coil 61 is energized, the cooperation of the inner and outer valve members 62 and 63 blocks the communication between the high pressure passage 67 and the communication passage 66, while, establishes the communication between the communication passage 66 and the drain passage 68 via the valve chamber 69 in a known manner. Accordingly, the high pressure fuel in the pressure control chamber 76 is discharged into the low pressure side 12 via the orifice 73. On the other hand, when the coil 61 is de-energized as shown in Fig. 4, the cooperation of the inner and outer valve members 62 and 63 blocks the communication between the communication passage 66 and the drain passage 68, while, establishes the communication between the high pressure passage 67 and the communication passage 66 via the hydraulic passage 65 in a known manner. Accordingly, the high pressure is applied to the pressure control valve member 7 from the side of the communication passage 66. Now, the operation of the first preferred embodiment will be described with reference to Figs. 4 to 12. Fig. 7 shows the state where the coil 61 of the three-way solenoid valve 6 is de-energized so that the high pressure is applied to the pressure control valve member 7 from the communication passage 66 and further the hydraulic pressure across the pressure control valve member 7 is balanced, that is, the hydraulic pressure within the pressure control chamber 76 is maximum. In this condition, the hydraulic piston 4 is forced to a position where the nozzle needle 2 is seated on the valve seat 33, which corresponds to a lift position A in Fig. 12(B). This lift position A is a fully closed valve position which is attained when the hydraulic piston 4 moves to the position at a predetermined distance Dp from the annular step 55. Since the nozzle needle 2 is seated on the valve seat 33, the communication between the pressure chamber 31 and the sack chamber 34 is blocked so that no fuel is injected from the injection holes 32. Further, since the hydraulic pressure across the pressure control valve member 7 is balanced, the pressure control valve member 7 is forced by the force of the spring 8 to rest on the valve seat 60 of the three-way solenoid valve 6. When the coil 61 of the three-way solenoid valve 6 is energized in the state of Fig. 7, the communication passage 66 is communicated with the low pressure side 12 so that the high pressure fuel in the pressure control chamber 76 is gradually discharged via the orifice 73 to gradually decrease the hydraulic pressure in the pressure control chamber 76 as shown in Fig. 12(A). When the hydraulic pressure in the pressure control chamber 76 is reduced to a predetermined valve opening pressure, i.e. the hydraulic pressure in the nozzle body 3 applied to the nozzle needle 2 at a side axially opposite to the pressure control chamber 76 is balanced with the sum of the forces of the coil springs 8 and 42 and the hydraulic pressure in the pressure control chamber 76 applied to the hydraulic piston 4, the hydraulic piston 4 starts to gradually displace upward or toward the pressure control valve member 7 as shown in Fig. 8. Simultaneously, the contact portion 21 of the nozzle needle 2 starts to gradually separate from the valve seat 33 as shown in Fig. 12(B) so that the pressure chamber 31 is communicated with the sack chamber 34 to start the fuel injection via the injection holes 32. Subsequently, as the hydraulic pressure in the pressure control chamber 76 gets smaller, the hydraulic piston 4 is further forced toward the pressure control valve member 7 to allow the nozzle needle 2 to gradually reach a lift position B in Fig. 12(B). This lift position B is a fully opened valve position which is attained when the hydraulic piston 4 is displaced extremely toward the pressure control valve member 7, i.e. the hydraulic pressure in the pressure control chamber 76 is minimum. As shown in Fig. 12(A) and (B), until the hydraulic pressure in the pressure control chamber 76 reaches a predetermined valve closing pressure, the nozzle needle 2 remains at a lift position C which is equal in level to the lift position B. As shown in Fig. 9, when the coil 61 of the three-way solenoid valve 6 is de-energized, the high pressure fuel is introduced into the communication passage 66 to urge the pressure control valve member 7 toward the hydraulic piston 4. Since the force of the coil spring 8 is set very small, the pressure control valve member 7 is immediately displaced from the valve seat 60 to be seated onto the annular step 55 as shown in Fig. 10. This displacement of the pressure control valve member 7 causes an immediate pressure increase in the pressure control chamber 76 to the valve closing pressure as shown in Fig. 12(A). Accordingly, the hydraulic piston 4 is quickly forced toward the valve seat 33 to displace the nozzle needle 2 to a lift position D which is located immediately before the valve seat 33 or immediately adjacent to the valve seat 33. After the pressure control valve member 7 is seated on the annular step 55, the high pressure fuel is introduced into the pressure control chamber 76 via the orifice 73. Since the orifice 73 throttles the flow of the high pressure fuel introduced into the pressure control chamber 76, the hydraulic pressure in the pressure control chamber 76 is gradually increased to slowly displace the nozzle needle 2 further toward the valve seat 33 via the hydraulic piston 4. As appreciated, the introduction speed of the high pressure fuel into the pressure control chamber 76 is adjusted by changing a diameter of the orifice 73. When the hydraulic piston 4 reaches the position at the distance of Dp from the annular step 55 as shown in Fig. 11, the nozzle needle 2 returns to a lift position E which is equal in level to the lift position A as shown in Fig. 12(B) so that the contact portion 21 of the nozzle needle 2 is seated on the valve seat 33 to cut-off the fuel injection via the injection holes 32. Since the hydraulic pressure in the pressure control chamber 76 is gradually increased by means of the orifice 73 to slowly displace the nozzle needle 2 toward the valve seat 33 after the nozzle needle 2 reaches the lift position D, no overshooting of the hydraulic pressure is generated in the pressure control chamber 76 as shown in Fig. 12(A), as opposed to the prior art of Fig. 3(A). As a result, an impact load P = {(upper peak value) - (lower peak value)} is significantly lowered as shown in Fig. 12(C) in comparison with the impact load P in Fig. 3(C). As shown in Fig. 12(C), the load applied to the valve seat 33 is lowered during the fuel injection since the contact portion 21 of the nozzle needle 2 is separated therefrom, which, however, can not be reduced to zero due to the high pressure fuel from the common rail 11 being applied thereto during the fuel injection. As appreciated from the foregoing description of the first preferred embodiment, the hydraulic pressure applied to the hydraulic piston 4 is so controlled as to reduce the speed of the movement of the nozzle needle 2 toward the valve seat 33 after the nozzle needle 2 reaches immediately before the valve seat 33. Accordingly, the impact load P applied to the valve seat 33, which otherwise becomes excessively high, is significantly reduced. Further, since the speed of the nozzle needle 2 is lowered only after the nozzle needle 2 reaches immediately before the valve seat 33, the sharp cut-off of the fuel injection is effectively ensured satisfying the required fuel injection characteristics. Fig. 13 shows a modification of the first preferred embodiment. In Fig. 13, the same or like members or components are designated by the same reference numerals as in the first preferred embodiment. In this modification, an annular gap of a predetermined width is provided between the outer periphery 74 of the pressure control valve member 7 and the peripheral wall of the first chamber 52. Accordingly, in this modification, it is so designed that the fluid-tight sealing is securely provided between the end surface 71 of the pressure control valve member 7 and the annular valve seat 55 and between the end surface 72 of the pressure control valve member 7 and the valve seat 60 when the pressure control valve member 7 is selectively seated on the respective valve seats. The width of the annular gap should be set small enough to ensure substantially the same operation of the pressure control valve member 7 as in the first preferred embodiment. Fig. 14 shows another modification of the first preferred embodiment, wherein the same or like members or components are designated by the same reference numerals as in the first preferred embodiment. In this modification, the annular step 55 is formed tapering toward the second chamber 53 and a corresponding tapering surface 77 is formed on the pressure control valve member 7. In this modification, the fluid-tight sealing may be provided between the outer periphery 74 of the pressure control valve member 7 and the peripheral wall of the first chamber 52 as in the first preferred embodiment, or, instead of this, the fluid-tight sealing may be provided between the end surface 72 of the pressure control valve member 7 and the valve seat 60 and between the tapering annular surface 77 of the pressure control valve member 7 and the tapering annular step 55. Now, a second preferred embodiment of the fuel injection system according to the present invention will be described with reference to Figs. 15 to 19. In these figures, the same or like members or components are designated by the same reference numerals as in the first preferred embodiment. Further, the other structures not shown in these figures are the same as in the first preferred embodiment. In the second preferred embodiment, as shown in Fig. 15, the first chamber 52 includes first and second pressure control valve members 7a and 7b instead of the pressure control valve member 7 in the first preferred embodiment, and accordingly may have a longer axial length than that in the first preferred embodiment. The first pressure control valve member 7a is disposed between the hydraulic piston 4 and the second pressure control valve member 7b so as to form a first pressure control chamber 76a between the first valve member 7a and the hydraulic piston 4 and a second pressure control chamber 76b between the first and second valve members 7a and 7b. The first and second valve members 7a and 7b have the same diameter which is smaller than that of the first chamber 52 to provide annular gaps of a predetermined width between the peripheral wall of the first chamber 52 and the outer periphery of each of the first and second valve members 7a and 7b. The first valve member 7a has a recessed portion 78a at a side facing the second valve member 7b which has a corresponding projected portion 78b received in the recessed portion 78a. The coil spring 8 is disposed between the first and second valve members 7a and 7b for urging them in opposite directions, i.e. urging the first valve member 7a toward the hydraulic piston 4 and urging the second valve member 7b toward the communication passage 66. The first valve member 7a has an orifice 73a axially extending through the center of the first valve member 7a from a side of an end surface 72a or the second pressure control chamber 76b to a side of an end surface 71a or the first pressure control chamber 76a. Similarly, the second valve member 7b has an orifice 73b axially extending through the center of the second valve member 7b from a side of an surface 72b or the communication passage 66 to a side of an end surface 71b or the second pressure control chamber 76b. The orifices 73a and 73b are arranged in alignment with each other. Now, operations of the second preferred embodiment will be described with reference to Figs. 15 to 19. Fig. 15 shows the state where the coil 61 of the three-way solenoid valve 6 is de-energized so that the high pressure is applied to the first chamber 52 from the communication passage 66 and further the hydraulic pressures in the first and second pressure control chambers 76a and 76b are maximum. In this condition, the hydraulic piston 4 is forced to a position where the nozzle needle 2 is seated on the valve seat 33, which corresponds to a lift position A in Fig. 19. This lift position A is a fully closed valve position which is attained when the hydraulic piston 4 moves a predetermined distance Dp from the annular step 55 or from the end surface 71a of the first valve member 7a. Since the nozzle needle 2 is seated on the valve seat 33, the communication between the pressure chamber 31 and the sack chamber 34 is blocked so that no fuel is injected from the injection holes 32. Further, since the hydraulic pressure across the second valve member 7b is balanced, the second valve member 7b is forced by the force of the spring 8 to rest on the valve seat 60 of the three-way solenoid valve 6. When the coil 61 of the three-way solenoid valve 6 is energized in the state of Fig. 15, the communication passage 66 is communicated with the low pressure side 12 so that the high pressure in the first pressure control chamber 76a is gradually discharged via the orifices 73a and 73b and the high pressure in the second pressure control chamber 76b is gradually discharged via the orifice 73b. Accordingly, the hydraulic pressures in the first and second pressure control chambers 76a and 76b are gradually decreased. When the hydraulic pressure in the first pressure control chamber 76a is reduced to a predetermined valve opening pressure, the hydraulic piston 4 starts to gradually displace upward or toward the first valve member 7a. Simultaneously, the contact portion 21 of the nozzle needle 2 starts to gradually separate from the valve seat 33 or gradually displace from the lift position A as shown in Fig. 19 so that the pressure chamber 31 is communicated with the sack chamber 34 to start the fuel injection via the injection holes 32. After moving the predetermined distance Dp, the hydraulic piston 4 contacts the end surface 71a of the first valve member 7a to urge the latter toward the second valve member 7b. Simultaneously, the decreasing hydraulic pressure in the second pressure control chamber 76b allows the hydraulic piston 4 to slowly displace the first valve member 7a from the annular step 55 to reach the state as shown in Fig. 16. In Fig. 16, the hydraulic piston 4 and the first valve member 7a are displaced extremely toward the second valve member 7b to force the nozzle needle 2 to a lift position B in Fig. 19. This lift position B is a fully opened valve position which is attained when the hydraulic pressure in the second pressure control chamber 76b is minimum. Until the hydraulic pressure in the second pressure control chamber 76b reaches a predetermined valve closing pressure, the nozzle needle 2 remains at a lift position C which is equal in level to the lift position B. When the coil 61 of the three-way solenoid valve 6 is de-energized in the state of Fig. 16, the high pressure fuel is introduced into the communication passage 66 to urge the second valve member 7b toward the first valve member 7a. Since the force of the coil spring 8 is set very small, the second valve member 7b is immediately separated from the valve seat 60 as shown in Fig. 17. This displacement of the second valve member 7b allows the high pressure fuel in the communication passage 66 to be immediately introduced into the second pressure control chamber 76b via the annular gap provided between the outer periphery of the second valve member 7b and the peripheral wall of the first chamber 52. Accordingly, an immediate pressure increase over the valve closing pressure is caused in the second pressure control chamber 76b to quickly displace the first valve member 7a to be seated onto the annular step 55, which is also shown in Fig. 17. This displacement of the first valve member 7a forces the hydraulic piston 4 toward the valve seat 33 so that the hydraulic piston 4 reaches a position on a level with the annular step 55 as seen in Fig. 17. Simultaneously, the nozzle needle 2 is quickly displaced to a lift position D in Fig. 19 which is located immediately before the valve seat 33 or immediately adjacent to the valve seat 33. After the first valve member 7a is seated on the annular step 55, the high pressure fuel is introduced into the first pressure control chamber 76a via the first orifice 73a. Since the first orifice 73a throttles the flow of the high pressure fuel introduced into the first pressure control chamber 76a, the hydraulic pressure in the first pressure control chamber 76a is gradually increased to slowly displace the nozzle needle 2 further toward the valve seat 33 via the hydraulic piston 4. When the hydraulic piston 4 moves the predetermined distance Dp from the annular step 55 as shown in Fig. 18, the nozzle needle 2 returns to a lift position E which is equal in level to the lift position A as shown in Fig. 19 so that the contact portion 21 of the nozzle needle 2 is seated on the valve seat 33 to cut-off the fuel injection via the injection holes 32. Since the hydraulic pressure in the first pressure control chamber 76a is gradually increased by means of the orifice 73a to slowly displace the nozzle needle 2 toward the valve seat 33 after the nozzle needle 2 reaches the lift position D, an impact load applied to the valve seat 33, which is excessively high in the prior art of Fig. 3(C), is significantly lowered similar to the impact load P in the first preferred embodiment of Fig. 12(C). After the hydraulic pressure across the second valve member 7b is balanced, the second valve member 7b is seated on the valve seat 60 as shown in Fig. 15. As appreciated from the foregoing description of the second preferred embodiment, the similar effects as in the first preferred embodiment are attained for controlling the hydraulic pressure applied to the hydraulic piston 4 to finally control the behavior of the nozzle needle 2. In the second preferred embodiment, the annular step 55 and the valve seat 60 may respectively form inclined surfaces or curved surfaces for abutment with the corresponding surfaces of the first and second valve members 7a and 7b. On the other hand, the first and second valve members 7a and 7b may respectively form inclined surfaces or curved surfaces for abutment with the corresponding surfaces of the annular step 55 and the valve seat 60. Various changes and modifications may be made. For example, the three-way solenoid valve 6 may be replaced by a plurality of solenoid valves of another type. The nozzle needle body 3, the nozzle holder 5 and the valve body 64 may be formed integral, or may be formed by two members or by more than four members. The push rod 41 may be omitted so that the hydraulic piston 4 directly drives the nozzle needle 2. Further, the coil spring 8 may be omitted. This means that, without the coil spring 8, the similar effects can be attained in view of controlling the hydraulic pressure applied to the hydraulic piston 4.

Claims: Fuel injection system for an engine, comprising: [a] a fuel injection means (1) including a valve member (2) and a valve seat (33), said valve member (2) being movable between a first position where said valve member (2) is separated from said valve seat (33) to allow a fuel injection via an injection opening (32) into said engine, and a second position where said valve member (2) is seated on said valve seat (33) to inhibit the fuel injection via said injection opening (32); [b] a pressure control chamber (76) which provides a fluid pressure for controlling the movement of said valve member (2) between said first and said second position; and [c1] a control means for gradually decreasing the fluid pressure within said control chamber (76) from a high to a low pressure at the beginning of the fuel injection and for sharply increasing the fluid pressure within said control chamber (76) from a low to a high pressure at the end of the fuel injection; characterized in that [c2] said control means comprises means (7,73) for stopping the sharp increase of the fluid pressure within said control chamber (76) from the low to the high pressure when said valve member (2) is located at a third position between said first and said second position, and for gradually increasing the fluid pressure within said control chamber (76) in order to move said valve member (2) from said third to said second position. System according to claim 1, characterized in that said third position is located immediately adjacent to said second position. System according to claim 1, characterized in that said third position is located immediately before said second position when said valve member (2) is moved toward said second position. System according to one of claims 1 through 3, characterized in that said control means includes pressure control valve means (7) having a pressure throttle means, and a pressure switching means (61) for selectively applying a high hydraulic pressure to said pressure control valve means (7), said pressure control valve means (7) applying the high hydraulic pressure to said valve member (2) via said pressure throttle means for gradually increasing the hydraulic pressure applied to said valve member (2) in order to displace said valve member (2) from said third position to said second position. System according to one of claims 1 through 4, characterized in that said valve member (2) is a nozzle needle (2) and the controlled hydraulic pressure is applied to said nozzle needle (2) via driving means mechanically connected to said nozzle needle (2) at a side opposite to said valve seat (33). System according to claim 5, characterized in that said driving means includes a cylindrical piston (4) and said control means includes a cylindrical stepped bore (59) having therein an annular step (55) which defines a first section and a second section having a smaller diameter than that of said first section, said second section being located closer to said valve seat (33) than said first section and slidably receiving therein said cylindrical piston (4), said control means further including a pressure control valve means (7) movably disposed in said first section so as to define a pressure control space (76) between said cylindrical piston (4) and said pressure control valve means (7) for controlling the hydraulic pressure applied to said cylindrical piston (4), said pressure control valve means (7) including pressure throttle means therein, and wherein said control means further includes a pressure switching means for selectively applying a high hydraulic pressure to said first section in order to quickly displace said pressure control valve means (7) toward said cylindrical piston in order to contact with said annular step so as to allow the high hydraulic pressure to be introduced into said pressure control space (76) only through said pressure throttle means. System according to claim 6, characterized in that the displacement of said pressure control valve means (7) toward said cylindrical piston (4) quickly displaces said cylindrical piston (4) so that said nozzle needle (2) is quickly displaced from said first position to said third position, and wherein the introduction of the high hydraulic pressure into said pressure control space (76) through said pressure throttle means gradually increases the hydraulic pressure in said pressure control space (76) in order to slowly displace said cylindrical piston (4) so that said nozzle needle (2) is displaced from said third position to said second position. System according to claim 7, characterized in that said pressure switching means alternatively establishes a communication between said first section and a high pressure side in order to apply the high hydraulic pressure to said first section and a communication between said first section and a low pressure side in order to discharge the high hydraulic pressure from said first section into said low pressure side. System according to claim 8, characterized in that said pressure control valve means (7) includes a cylindrical valve member movably disposed in said first section to define said pressure control space between said cylindrical valve member and said cylindrical piston (4), and said pressure throttle means includes an orifice (73) extending through said cylindrical valve member (4) into said pressure control space (76) from a side opposite to said pressure control space (76), and wherein said cylindrical valve member is quickly displaced toward said cylindrical piston (4) to contact with said annular step (55) so as to allow the high hydraulic pressure to be introduced into said pressure control space (76) only through said orifice (73) when said pressure switching means establishes the communication between said first section and said high pressure side, the quick displacement of said valve member sharply increasing the hydraulic pressure in said pressure control space (76) to quickly displace said cylindrical piston (4) such that said nozzle needle (2) is quickly displaced from said first position to said third position, and wherein the introduction of the high hydraulic pressure into said pressure control space (76) through said orifice (73) gradually increases the hydraulic pressure in said pressure control space (76) to slowly displace said cylindrical piston (4) such that said nozzle needle (2) is slowly displaced from said third position to said second position. System according to claim 9, characterized in that an outer periphery of said cylindrical valve member and a peripheral wall of saaid first section cooperatively provide a fluid-tight sealing therebetween. System according to claim 10, characterized in that said cylindrical valve member has a diameter smaller than that of said first chamber to provide an annular gap of a predetermined width therebetween. System according to claim 11, characterized in that said pressure control valve means includes first and second cylindrical valve members movably disposed in said first section in alignment with said cylindrical piston (4), said first valve member being disposed between said cylindrical piston (4) and said second valve member in order to define said pressure control space (76) between said first valve member and said cylindrical piston (4) and a further pressure control space between said first and said second valve member, and said pressure throttle means includes first and second orifices, said first orifice extending through said first valve member into said pressure control space (76) from said further pressure control space and said second orifice extending through said second valve member into said further pressure control space from a side opposite to said further pressure control space, and wherein said second cylindrical valve member is quickly displaced towards said first valve member to immediately introduce the high hydraulic pressure into said further pressure control space through an annular gap formed between an outer periphery of said second valve member and a peripheral wall of said first section when said pressure switching means establishes the communication between said first section and said high pressure side, the immediate introduction of the high hydraulic pressure into said further pressure control space sharply increasing the hydraulic pressure therein to quickly displace said first valve member to contact with said annular step (55) so as to allow the high hydraulic pressure to be introduced into said pressure control space (76) only through said first orifice, the quick displacement of said first valve member directly pushing said cylindrical piston (4) such that said nozzle needle (2) is quickly displaced from said first position to said third position, and wherein the introduction of the high hydraulic pressure into said pressure control space (76) through said first orifice gradually increases the hydraulic pressure in said pressure control space (76) to slowly displace said cylindrical piston (4) such that said nozzle needle (2) is slowly displaced from said third position to said second position. System according to one of claims 6 through 12, characterized in that a coil spring (8) is disposed between said cylindrical valve member and said cylindrical piston (4) to urge said cylindrical piston (4) toward said valve seat and said cylindrical valve member in a direction opposite to said valve seat. System according to claim 12, characterized in that a coil spring (8) is disposed between said first and said second cylindrical valve member to urge said first cylindrical valve member towards said cylindrical piston (4) and said second cylindrical valve member in a direction opposite to said cylindrical piston. System according to one of claims 8 through 14, characterized in that said pressure control valve means blocks the communication between said first section and said low pressure side when said pressure switching means establishes the communication between said first section and said low pressure side such that said first section is communicated with said low pressure side only through said pressure throttle means to gradually decrease the hydraulic pressure in said pressure control space (76).

Inventors: IWANAGA TAKASHI, IWANAGA, TAKASHI, IWANAGA, TAKASHI, RAIONZU MANSHION
Applicants: NIPPON DENSO CO, NIPPONDENSO CO., LTD.
Application year: 1992
Main classification: F02M47
IPC codes: F02M47, F02M45