Abstract:
Fuel injectors are disclosed for use in internal combustion engines including a housing which includes a working chamber, a piston, including a plunger, for reciprocal movement within the working chamber, a hydraulic valve having one side facing the control chamber and the other side facing a poppet chamber, and including a poppet defining the poppet chamber in communication with an inlet port and the working chamber, the poppet providing a throttling slot between the working chamber and the poppet chamber, a spring urging the hydraulic valve towards the closed position, a control valve between the control chamber and the spill port, and a bypass channel between the poppet chamber and the control chamber, whereby during initial movement of the hydraulic valve from its closed position to its open position the primary bypass channel is closed and when the hydraulic valve reaches an intermediate position the primary bypass channel is open.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a system for injecting fuel into compression ignition internal combustion engines. 
     BACKGROUND OF THE INVENTION 
     Certain fuel injection systems for engines have been designed as unit injectors which incorporate hydraulically driven pressure intensifiers with a stepped plunger for injecting fuel into the engine&#39;s cylinder, wherein the fuel delivery and timing are controlled by an electronically controlled valve. In addition, the spray pattern is controlled by means of modulating the base oil pressure supplied to the unit injector and/or by means of modulating the nozzle opening pressure. 
     The present invention concerns hydraulically actuated electronically controlled unit injection (HEUI) systems which are well known in the art. The most relevant art includes U.S. Pat. No. 5,785,021, the contents of which are incorporated herein by reference thereto. 
     U.S. Pat. No. 5,785,021 discloses a fuel injection system which comprises a pressure intensifier which is associated with hydraulically controlled differential valves. These valves comprise a poppet valve opening into a working chamber of the pressure intensifier. A throttling slot is provided between the poppet valve chamber and the working chamber, with either at least a bypass channel between the poppet valve chamber and a control chamber of the valve, or a bore connecting the working chamber to the control chamber of the valve. 
     Furthermore, International Application No. PCT/AU98/00073 discloses a fuel injection system in which a pressure intensifier is associated with a hydraulically controlled differential valve, which in turn defines a poppet opening into a working chamber of the pressure intensifier. The pressure intensifier comprises a plunger with an external groove for connection of a locking chamber of a nozzle with a compression chamber of the plunger during an injection cut-off position of the plunger, and for connection of the locking chamber to a control channel during other positions of the plunger. The pressure in the control channel is controlled by a hydraulic control system, which, in a preferred embodiment, is common for a set of injectors of the engine. In this manner, the injection system can be used for varying the shape of an injection curve and for providing a varying fuel injection pressure. 
     A primary object of the present invention is to provide an improved fuel injection system. In particular, it is an object of the present invention to provide improvements which increase the range of electronic control of an injection curve shape of the unit injector, improve the stability of fuel delivery in consecutive cycles of injections and between the unit injectors of a multi-cylinder engine, simplify the unit injector&#39;s design, and improve the injection end quality. 
     SUMMARY OF THE INVENTION 
     These and other objects have now been realized by the invention of a fuel injector for use in an internal combustion engine comprising a fuel injector housing including an inlet port for the fuel, a spill port for release of the fuel, a working chamber, a piston disposed for reciprocal movement within the working chamber, a plunger attached to the piston, a nozzle for injecting the fuel into the internal combustion engine in response to the action of the plunger, a hydraulic valve movable between a closed position, an open position, and at least one intermediate position therebetween, the hydraulic valve including a first side and a second side, the first side of the hydraulic valve facing a control chamber and the second side of the hydraulic valve facing a poppet chamber, whereby the hydraulic valve moves between the control chamber and the poppet chamber, the hydraulic valve including a poppet defining the poppet chamber, the poppet including a first side and a second side, the first side of the poppet defining the poppet chamber and being in communication with the inlet port and the second side of the poppet being in communication with the working chamber, the poppet disposed with respect to the working chamber so as to provide a throttling slot between the working chamber and the poppet chamber, biasing means for urging the hydraulic valve towards the closed position in which the poppet chamber is not in communication with the inlet port, a control valve disposed between the control chamber and the spill port, and a primary bypass channel for connecting the poppet chamber to the control chamber, whereby during at least a portion of the initial movement of the hydraulic valve from the closed position to the open position the primary bypass channel is closed and when the hydraulic valve reaches the at least one intermediate position the primary bypass channel is open. In accordance with a preferred embodiment, the hydraulic valve includes a groove proximate to the first side of the hydraulic valve, whereby when the hydraulic valve is closed the groove opens the primary bypass channel, when the hydraulic valve is in the at least one intermediate position the first side of the hydraulic valve opens the primary bypass channel, and when the hydraulic valve is in a second intermediate position between the closed position and the at least one intermediate position the primary bypass channel is closed. 
     In accordance with one embodiment of the fuel injector of the present invention, the fuel injector includes a secondary bypass channel connecting the poppet chamber to the control chamber. In a preferred embodiment the fuel injector includes a secondary valve disposed is the secondary bypass channel for altering the flow area of the secondary bypass channel. Preferably, the fuel injector includes an engine management system for controlling the secondary valve. 
     In accordance with another embodiment of the fuel injector of the present invention, the fuel injector includes a tertiary bypass channel connecting the poppet chamber to the control chamber. 
     In accordance with another embodiment of the fuel injector of the present invention, the secondary valve cannot completely close the secondary bypass channel. 
     In accordance with another embodiment of the fuel injector of the present invention, the fuel injector includes a hydraulic control system for controlling the secondary valve. Preferably, the fuel injector includes an engine management system for controlling the hydraulic control system. 
     In accordance with another embodiment of the fuel injector of the present invention, the fuel injector includes a solenoid for actuating the secondary valve. 
     In accordance with the present invention, these objects have also been realized by the invention of a fuel injector for use in an internal combustion engine comprising a fuel injector housing including an inlet port for the fuel, a spill port for release of the fuel, a working chamber, a piston disposed for reciprocal movement within the working chamber, a compression chamber, a plunger attached to the piston and defining at least a portion of the compression chamber, a nozzle for injecting the fuel into the internal combustion engine in response to actuation by the plunger, a needle movable between a first position closing the nozzle and a second position opening the nozzle, a locking chamber, first biasing means disposed within the locking chamber for urging the needle into the first position, an outlet chamber connecting the nozzle to the compression chamber, a non-return valve for permitting the fuel to enter the compression chamber from the inlet port, a cut-off channel connecting the locking chamber to the compression chamber, a control channel for connecting the cut-off channel to the spill port, a secondary control valve for controlling the flow from the control channel to the spill port, a link channel connecting the control channel to either the inlet port or to a hydraulic control system of the internal combustion engine, the link channel and the secondary control valve being disposed such that when the secondary control valve is open, the pressure in the control channel is less than the pressure between the link channel and either the inlet port or the hydraulic control system of the internal combustion engine, the plunger being movable between a first position and at least one second position, whereby when the plunger is in the first position the cut-off channel is connected to the compression chamber and when the plunger is in the at least one second position the cut-off channel is connected to the control channel. 
     In accordance with a preferred embodiment of the fuel injector of the present invention, the fuel injector includes a hydraulic valve movable between a closed position, an open position, and at least one intermediate position therebetween, the hydraulic valve including a first side and a second side, the first side of the hydraulic valve facing a control chamber and the second side of the hydraulic valve facing a poppet chamber, whereby the hydraulic valve moves between the control chamber and the poppet chamber, the hydraulic valve including a poppet defining the poppet chamber, the poppet including a first side and a second side, the first side of the poppet defining the poppet chamber and being in communication with the inlet port and the second side of the poppet being in communication with the working chamber, the poppet disposed with respect to the working chamber so as to provide a throttling slit between the working chamber and the poppet chamber, second biasing means for urging the hydraulic valve towards the closed position in which the poppet chamber is not in communication with the inlet port, a control valve disposed between the control chamber and the spill port, and a primary bypass channel for connecting the poppet chamber to the control chamber, whereby during at least a portion of the initial movement of the hydraulic valve from the closed position to the open position the primary bypass channel is closed and when the hydraulic valve reaches the at least one intermediate position the primary bypass channel is open. In a preferred embodiment, the hydraulic valve includes a groove proximate to the first side of the hydraulic valve, whereby when the hydraulic valve is closed the groove opens the primary bypass channel, when the hydraulic valve is in the at least one intermediate position the first side of the hydraulic valve opens the primary bypass channel, and when the hydraulic valve is in a second intermediate position between the closed position and the at least one intermediate position the primary bypass channel is closed. Preferably, the fuel injector includes a secondary bypass channel connecting the poppet chamber to the control chamber. Most preferably, the fuel injector includes a secondary valve disposed is the secondary bypass channel for altering the flow area of the secondary bypass channel. In a preferred embodiment, the fuel injector includes an engine management system for controlling the secondary valve. 
     In accordance with one embodiment of the fuel injector of the present invention the fuel injector includes a tertiary bypass channel connecting the poppet chamber to the control chamber. 
     In accordance with another embodiment of the fuel injector of the present invention, the secondary valve cannot completely close the secondary bypass channel. 
     In accordance with another embodiment of the fuel injector of the present invention, the fuel injector includes a hydraulic control system for controlling the secondary valve. Preferably, the fuel injector includes an engine management system for controlling the hydraulic control system. 
     In accordance with another embodiment of the fuel injector of the present invention, the fuel injector includes a solenoid for actuating the secondary valve. 
     In accordance with another embodiment of the fuel injector of the present invention, the secondary valve includes a control chamber connected to the control channel, and the fuel injector includes third biasing means urging the secondary valve to close the secondary bypass channel, whereby increasing the pressure in the control chamber overcomes the third biasing means to open the additional bypass channel, and lowering the pressure in the control chamber permits the secondary valve to reduce the flow in the secondary bypass channel. Preferably, the control valve and the secondary control valve comprise solenoid valves. 
     In accordance with one embodiment of the fuel injector of the present invention, the first biasing means has a variable stiffness. 
     In accordance with one embodiment of the present invention there is provided a fuel injection system for an internal combustion engine with a fuel injector, the injector comprising an inlet port; a spill port; a pressure intensifier comprising a piston forming a working chamber and a plunger adapted for injecting fuel through a nozzle; an hydraulic valve comprising a control chamber and a poppet chamber and having a poppet located between the inlet port and the working chamber and opening into the working chamber, wherein the poppet provides a throttling slot; means for biasing the hydraulic valve towards its closed position; a control valve installed between the control chamber and the spill port; a bypass channel for connection of the poppet chamber to the control chamber; and wherein the hydraulic valve is adapted to control the flow area of the bypass channel such that the bypass channel is open when the hydraulic valve is in its closed and open positions, or near these positions, and is closed during the other positions of the hydraulic valve. 
     In a preferred embodiment of the present invention, there is also a third bypass channel connecting the poppet and control chambers, such that when the additional bypass channel is closed by the secondary control valve, the third bypass channel defines the opening rate of the hydraulic valve during the positions of the hydraulic valve when it keeps the bypass channel closed. 
     In accordance with another embodiment of the present invention there is provided a fuel injection system for an internal combustion engine with a fuel injector, the injector comprising an inlet port; a spill port; a pressure intensifier comprising a piston forming a working chamber and a spill chamber and a plunger forming a compression chamber, wherein the working chamber is adapted to be connected either to the inlet port or to the spill port according to the commands from an engine management system in order to enable the pressure intensifier to perform injections; a nozzle with a needle, a locking chamber, means biasing the needle to close the nozzle and an outlet chamber connected to the compression chamber; a non-return valve, the inlet of the non-return valve being connected to the inlet port and the outlet of the non-return valve being connected to the compression chamber; a cut-off channel connected to the nozzle locking chamber; a control channel; an additional control valve installed between the control channel and the spill port; a link channel connecting the control channel to the inlet port; and in which the flow areas of the link channel and the additional control valve are such that when the additional control valve is open the pressure in the control channel becomes less than the pressure upstream of the link channel; the plunger being adapted to connect the cut-off channel to the compression chamber at an injection cut-off position of the plunger, and adapted to connect the cut-off channel to the control channel at other positions of the plunger. 
     The differences between the injector and injection system of an aspect of the present invention and that of the specifications mentioned above reside firstly in the inclusion of an additional bypass channel, connecting the poppet chamber to a control chamber, and a secondary valve which is adapted to control the flow area of the additional bypass channel in accordance with a pressure level in an hydraulic control system or in a control channel, where an hydraulic valve is adapted to control the flow area of a bypass channel for connection of the poppet chamber to the control chamber; secondly, the hydraulic valve controls the flow area of the bypass channel such that the bypass channel is open when the hydraulic valve is in its closed and open positions, or near these positions, and closed during the other positions of the hydraulic valve (HDV). By controlling the pressure in the hydraulic control system or in the control channel, the secondary valve can be controlled to open or close the additional bypass channel. When such pressure is increased, the secondary valve opens the additional bypass channel, and vice versa. As the opening speed of the hydraulic valve is dependent upon the flow area of the bypass channels, it is possible to control the opening speed of the hydraulic valve during its initial opening by controlling the position of the secondary valve and the flow area of the bypass channel. Slower hydraulic valve opening delays the injection pressure build-up. During the final part of the opening of the hydraulic valve the bypass channel is open and therefore the pressure in the control chamber is increased, which helps to fully open the hydraulic valve and reduce its hydraulic restriction. 
     It is preferable to use an hydraulic control system that is common for a set of injectors on an engine to control the positions of the secondary valves. The pressure in this common hydraulic control system is controlled by an engine management system. This helps to ensure uniform injection patterns throughout the engine cylinders, to simplify the injection system design and help keep the cost down as in this case only one pressure regulator is required and it can be mounted anywhere on an engine. Alternatively, the hydraulic control system can be replaced by a direct solenoid control of the secondary valves, which can be executed by a single solenoid and a mechanical arrangement transmitting the solenoid action to all of the injectors of an engine. 
     Another embodiment of the present invention resides in the provision of a link channel between a control channel and an inlet port, or between the control channel and an hydraulic control system, and in the provision of an additional control valve between the control channel and a spill port, in which a plunger is adapted to disconnect the control channel from a cut-off channel during a cut-off position of the plunger, and further where the flow areas of the additional control valve and the link channel are such that when the additional control valve is open the pressure in the control channel becomes less than the pressure in the hydraulic control system or in the inlet port. The pressure in the hydraulic control system is typically controlled by an engine management system. Thus, during a position of the plunger other than the cut-off position, a pilot or a boot injection is possible by means of opening the additional control valve. During the cut-off position of the plunger the control channel is disconnected from the cut-off channel, and the additional control valve is thus not subjected to a high pressure, and the volume of the cut-off channel is kept to a minimum. 
     Different embodiments of the present invention enable a wider range of control of the injection curve shape independently of the common rail (actuating) pressure, simplification of the unit injector design, and an improvement in the injection end quality and injector reliability. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will now be described in more detail in the following detailed description, which, in turn, refers to the accompanying drawings, in which various embodiments of the unit injection system in accordance with the present invention are shown in different stages of operation, as follows: 
     FIG. 1 is a side, elevational cross-sectional view of a first embodiment of the present invention; 
     FIG. 2 is a side, elevational, cross-sectional view of a second embodiment of the present invention; 
     FIG. 3 is a side, elevational, partial, enlarged, cross-sectional view of the hydraulic differential valve shown in FIG. 1; 
     FIG. 4 is a side, elevational, cross-sectional view of a third embodiment of the present invention; and 
     FIG. 5 is a side, elevational, cross-sectional view of a fourth embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     The embodiment of the present invention shown in FIG. 1 shows a source of fuel pressure  1 , inlet port  2 , spill port  3 , an hydraulic valve  4 , preferably in the form of an hydraulically controlled differential valve (HDV), a control chamber  5 , a pressure intensifier which comprises a piston  6  and plunger  7 , with an external groove  8  and an edge  9 , working chamber  10 , spill chamber  11  and compression chamber  12 , spill channel  13 , nozzle  14 , needle  15 , spring  16 , locking chamber  17  and outlet chamber  18 , non-return valve  19 , the inlet of which is connected to the inlet port  2 , and the outlet of which is connected to the compression chamber  12 , cut-off channel  20 , control valve  21  installed between the control chamber  5  and the spill port  3 , control channel  22 , an additional control valve  23  installed between the control channel  22  and the spill port  3  and a link channel  24  connecting the control channel  22  to the inlet port  2 . 
     The hydraulic valve  4  controls the flow area from the inlet port  2  to the working chamber  10  and opens towards the working chamber. The hydraulic valve  4  has a poppet  25  with a seating face  26  and forms a poppet chamber  27  and a throttling slot  28 . There is a bypass channel  29  and an additional bypass channel  30  for connection of the poppet chamber  27  to the control chamber  5 . The hydraulic valve  4  is biased towards its closed position by a spring  31 . The compression chamber  12  is connected with the outlet chamber  18 . The compression chamber  12  may also be connected with the cut-off channel  20  through the external groove  8  of the plunger  7 , depending on the plunger&#39;s position. The cut-off channel  20  may be connected to the control channel  22  through the groove  8  of the plunger  7  depending on the plunger&#39;s position. The spill channel  13  may be connected to the spill chamber  11 , depending on the plunger&#39;s position. 
     There is also a secondary valve  32  installed in the additional bypass channel  30  and biased by a spring  33  to close the additional bypass channel. The secondary valve has a control chamber  34  connected to an hydraulic control system  35 . 
     The hydraulic valve  4  is designed such that its upper edge  36  (See FIG. 3) can open or close the bypass channel  29 , depending on the position of the hydraulic valve. With the hydraulic valve closed the upper edge  36  closes the bypass channel  29  as shown in FIG.  2 . In a certain position of the hydraulic valve during its opening stroke the edge  36  (FIG. 3) opens the bypass channel and keeps it open as the hydraulic valve opens further. 
     The hydraulic valve also has a groove  37  with an edge  38  which can control the flow area of the bypass channel  29  such that when the hydraulic valve is closed, the edge  38  opens the bypass channel, and at a certain point of the opening stroke of the hydraulic valve the edge  38  closes the bypass channel. In the preferred embodiment, during the opening stroke of the hydraulic valve the edge  38  closes the bypass channel before the upper edge  36  opens the bypass channel again, so that it remains closed on a part of the opening stroke of the hydraulic valve. 
     A second embodiment of the present invention is shown in FIG.  2  and is identical to that shown in FIG. 1 except that there is a third bypass channel  39  connecting the poppet chamber  27  to the control chamber  5 . 
     An alternate form of the present invention is shown in FIG. 4 which is identical to that shown in FIG. 1 except that there is no groove on the hydraulic valve  4  and no third bypass channel, and the secondary valve  32  is designed such that it cannot completely close the additional bypass channel  30 . In addition, the link channel  24  connects the control channel  22  to the hydraulic control system  35  instead of connecting channel  22  to the inlet port. 
     Another alternate form of the present invention is shown in FIG. 5 which is identical to that shown in FIG. 1 except that the control chamber  34  of the secondary valve  32  is connected to the control channel  22  instead of being connected to the hydraulic control system. 
     The fuel injection system of the depicted embodiments works as follows. 
     Referring to FIG. 1, in the initial position the control valve  21  is inert and closes off the connection between the control chamber  5  and spill port  3 . In the case where the pressure in the hydraulic control system  35  is set to a low level by an engine management system (not shown), the spring  33  overcomes the force exerted by the pressure in the control chamber  34  on the secondary valve  32  and keeps the additional bypass channel  30  closed as shown. The hydraulic valve  4  is pushed by the spring  31  in the direction of closing the hydraulic valve until it reaches a first intermediate position where the upper edge  36  of the hydraulic valve (Ref. FIG. 3) closes the bypass channel  29 . Then the hydraulic valve stays in the first intermediate position as the fuel cannot escape from the control chamber  5  with the control valve  21  and the bypass channels  29  and  30  are closed. Referring to FIG. 1, the piston  6  and plunger  7  are kept in the bottom position by the fuel pressure in the working chamber  10 , the locking chamber  17  is connected by means of the cut-off channel  20  and the plunger&#39;s external groove  8  with compression chamber  12 , and the nozzle  14  is closed by the needle  15 . The spill chamber  11  is connected to the spill port  3  by means of spill channel  13 . The additional control valve  23  is de-energized and closed. 
     When electric current is supplied to the control valve  21  it connects the control chamber  5  to the spill port  3  and allows the hydraulic valve to move further towards the closed position. At a certain point, the hydraulic valve reaches a second intermediate position in which the edge  38  (See FIG. 3) begins to open the bypass channel  29  as the hydraulic valve moves on. Finally, the hydraulic valve closes the connection between the inlet port  2  and the poppet chamber  27 , as shown in FIG.  2 . The control valve  21  stays open and allows the fuel to flow from the working chamber  10  through the throttling slot  28  to poppet chamber  27 , further through bypass channel  29  to control chamber  5  and out through spill port  3 . The flow area of the throttling slot  28  is such that the flow through it causes the hydraulic force to act on the hydraulic valve  4  in the direction of the flow, which holds the hydraulic valve closed with the additional assistance of the force exerted by the spring  31 . When the pressure in the working chamber  10  has decreased to a certain level, piston  6  and plunger  7  move up under the pressure in the compression chamber  12 , the fuel pressure being transmitted through the non-return valve  19 . At a certain point in the travel of the plunger its groove  8  closes the connection between compression chamber  12  and the cut-off channel  20 , and at or beyond this point it isolates cut-off channel  20 , and thereby the locking chamber  17 , from the compression chamber  12 . In the point of further upward movement of the plunger when its groove  8  opens the connection between the cut-off channel  20  and the control channel  22  thereby connecting the locking chamber  17  with control channel  22 , and at or beyond this point it keeps locking chamber  17  and control channel  22  connected with each other (FIG.  2 ). In this manner, the pressure in the locking chamber  17  equalizes with the pressure in the control channel  22 . Also, at a certain point in the travel of the plunger its edge  9  closes off the connection between spill chamber  11  and spill channel  13 , and at or beyond this point the spill port  3  and spill chamber  11  remain disconnected from each other. The period of time during which piston  6  and plunger  7  move up is determined by the duration of opening of the control valve  21 , which is in turn determined by the duration of the current supplied by the engine management system. 
     The operation of the present invention will now be described with reference to so-called pilot injection and boot injection, which are types of injection which are previously known. The term “pilot injection” refers to a small separate injection preceding a main injection. Usually 1 to 10% of the total fuel delivered in a cycle may be injected during the pilot injection. The term “boot injection” refers to a single injection shaped like a front end of a boot, i.e. with a low “step” in the beginning of the injection and then a gradual rise of the injection rate and pressure from this low level. 
     If a pilot or a boot-shaped injection is required, while the plunger  7  has not yet started an injection stroke, the current is supplied to the additional control valve  23 , which opens. The flow areas of the open valve  23  and the link channel  24  are such that the pressure in the control channel  22 , and therefore in the locking chamber  17 , is reduced. The reduced pressure in the locking chamber allows the pressure in the outlet chamber  18  to lift the needle  15 , provide an initial opening of the nozzle  14 , and begin injection of fuel which is supplied to the outlet chamber  18  from the inlet port  2  through the non-return valve  19 . If a pilot injection is required, the additional control valve  23  is closed before a main injection is started, and the pressure in the control channel  22  and in the locking chamber  17  then equalize with the pressure in the inlet channel  2 , and the nozzle is closed by the spring  16 . If a boot-shaped injection is required, the additional control valve is closed at a later stage so that the nozzle does not close before main injection starts. FIG. 5 illustrates the instant when a boot injection is in progress while the piston  6  and the plunger  7  are still travelling up with the valve  21  open. 
     When the piston  6  and the plunger  7  have reached a required position which is determined by the fuel delivery required at that instant, the current supplied to the control valve  21  is switched off and the valve  21  closes, thereby isolating the control chamber  5  and spill port  3 . As a result, the fuel flow through the throttling slot  28  stops and the hydraulic force holding the hydraulic valve  4  closed ceases to act. The fuel pressure in the inlet port  2  acting on the differential spot in the hydraulic valve overcomes the force of spring  31  and provides an initial opening of the hydraulic valve. This allows fuel to flow through the inlet port  2  to the poppet chamber  27  and through the throttling slot  28  to working chamber  10 , and by means of the bypass channel  29  to the control chamber  5 . This fuel flow increases the pressure in poppet chamber  27  and control chamber  5  which forces hydraulic valve  4  to open. The pressure in the working chamber  10  rises and causes the piston  6  and the plunger  7  to move down thereby compressing the fuel in the compression chamber  12  and closing the non-return valve  19 . 
     As the fuel pressure in the compression chamber  12  increases, the pressure in the nozzle outlet chamber  18  also increases and opens the nozzle  14 , overcoming the force of spring  16  and pressure in the locking chamber  17 . In this manner, a main injection is started. The moment of nozzle opening, and correspondingly the pressure developed in the compression chamber  12  at the moment of nozzle opening, depend on the pressure in the locking chamber  17 , which is equal to the pressure in the control channel  22 . If a boot injection is already in progress, the increase in pressure in the compression chamber  12  resulting from the injecting stroke of the plunger completes the boot stage of the injection and starts the main injection. 
     When the opening hydraulic valve arrives at the second intermediate position as described above, the edge  38  (See FIG. 3) closes the bypass channel  29 . The part of the opening stroke of the hydraulic valve from the second intermediate position to the first intermediate position is characterized by a lower pressure in the control chamber  5  due to the increasing volume of the chamber and the fact that the bypass channels  29  and  30  are closed. The throttling slot  28  is designed such that the pressure differential between the poppet chamber  27  and the working chamber  10  provides an hydraulic force on the poppet  25  which is sufficient to open the hydraulic valve even if the pressure in the control chamber  5  falls below atmospheric pressure. However, the lower pressure in the control chamber  5  impedes faster opening of the hydraulic valve. A slower opening of the hydraulic valve, in turn, delays a pressure increase in the working chamber  10  during an injection stroke of the plunger  7 . This provides for a more gradual rise of injection pressure. 
     If a more rapid rate of rise of injection pressure is desired in the beginning of a main injection, then the engine management system sets the pressure in the hydraulic control system  35  (FIG. 1) to a higher level, which overcomes the force of spring  33  and lifts up the secondary valve  32 , opening the additional bypass channel  30 . A relatively large flow area between the poppet chamber  27  and the control chamber  5  in this case helps to maintain a higher pressure in the control chamber  5  during the entire opening stroke of the hydraulic valve, which increases its opening rate and therefore the rate of injection pressure rise in the beginning of an injection. 
     During an injection stroke of the piston  6  and the plunger  7  fuel is injected through opened nozzle  14 . At a final stage of an injection stroke the groove  8  disconnects the cut-off channel  20  from the control channel  22  and then opens the connection between the compression chamber  12  and the cut-off channel  20 . In addition, at a final stage of an injection stroke the edge  9  opens the connection between spill chamber  11  and spill port  3 . With the cut-off channel  20  and compression chamber  12  connected to each other the pressures in locking chamber  17  and compression chamber  12  equalize, and the needle  15  closes nozzle  14  and the piston  6  and the plunger  7  stay at the bottom of the stroke. When the piston is stationary there is no fuel flow through the hydraulic valve  4  and the pressures in the working chamber  10 , poppet chamber  27  and control chamber  5  equalize, with the pressure in the inlet port  2  and the spring  31  moving the hydraulic valve up. Thus, the system returns to the initial position as shown in FIG.  1 . 
     The main principle upon which the present invention is based relates to the fact that the hydraulic valve is designed so that it can completely close the connection between the poppet chamber  27  and the control chamber  5  during an initial part of the opening stroke of the hydraulic valve. This allows for a more significant reduction of the opening speed of the hydraulic valve during an initial part of its opening stroke. Furthermore, additional bypass channel  30  is arranged between the poppet and control chambers, and the secondary valve  32  is arranged in this additional bypass channel  30 . As a consequence, application of the secondary valve  32  in the additional bypass channel  30  provides for flexible electronic control and for a wider control range of the opening rate of the hydraulic valve (and hence the injection curve shape). 
     In an alternate form of the present invention shown in FIG. 2 the fuel injection system works in the same way. In the initial position, the spring  31  closes the hydraulic valve  4  completely, even when the secondary valve  32  is closed because there is the third bypass channel  39  which allows the fuel to escape from the control chamber  5  back to the poppet chamber  27  and the working chamber  10  during the closing of the hydraulic valve. The third bypass channel  39  is designed such that while the hydraulic valve is between the second and first intermediate positions during its opening stroke, the third bypass channel provides sufficient restriction to the flow from the poppet chamber  27  to the control chamber  5  to keep the pressure in this chamber low (provided that the additional bypass channel  30  is closed), thus reducing the rate of injection pressure rise in the beginning of a main injection as described above. By varying the flow area of the third bypass channel  39  it is possible to alter the degree of rate shaping of the main injection which can be activated or deactivated by closing or opening the secondary valve  32 . 
     In another alternate form of the present invention shown in FIG. 4 the fuel injection system works in the same way. The third bypass channel is absent, and the secondary valve  32  is designed such that it cannot completely close the additional bypass channel  30 . When the secondary valve  32  is pushed by the spring  33  against its stop as shown in FIG. 4, it leaves the poppet chamber  27  and the control chamber  5  still connected to each other and thus the function of a third bypass channel, as described above, is maintained. 
     The fact that the link channel  24  connects the control channel  22  to the hydraulic control system  35 , instead of the inlet port  2 , allows for an improvement in the controllability of the pilot injections, especially at low common rail pressures. This is because the pressure in the system  35  can be kept higher than in the inlet port when a low injection pressure is desired, so that the forces acting on the needle  15  to close the nozzle  14  and end a pilot injection will be higher and the closing period of the needle will be shorter. 
     In yet another alternate form of the present invention shown in FIG. 5 the fuel injection system works in the same way except that in this case the position of the secondary valve  32  is determined by the pressure in the control channel  22 . When the additional control valve  23  is closed, the pressure in the control channel  22  is high and the secondary valve  32  opens the additional bypass channel  30 . When the valve  23  opens and the pressure in the control channel  22  and therefore in the control chamber  34  falls down due to a relatively small flow area of the link channel  24 , the secondary valve  32  closes the additional bypass channel  30 . By this means, the control over the shape of the leading front of the main injection curve can be exercised without the need of a separate hydraulic control system. 
     Other embodiments of the present invention are also possible which incorporate the features of the present invention described above in different combinations, for example, the control channel  22  can be connected directly to the hydraulic control system  35  in FIG. 1 without the use of the additional control valve  23  and the link channel  24 , so that the nozzle opening pressure and the flow area of the additional bypass channel  30  can both be controlled through pressure modulation in the hydraulic control system. A lower pressure would provide for both a slower initial rise of injection pressure, as the nozzle would open at a lower pressure in the outlet chamber  18 , and for a slower injection pressure increase at the later stages of injection due to slower opening of the hydraulic valve  4 , and vice versa. Another possible embodiment would incorporate a resilient means biasing the needle  15  to close the nozzle  14 , which has a variable stiffness, such that an initial opening of the needle is possible at a lower pressure in the outlet chamber  18 , but at other positions of the needle when it is close to its maximum lift the stiffness of the resilient means increases. This will assist quicker closing of the nozzle during an injection cut-off. Such a variable stiffness can be achieved by the use of a well-known two-spring design of the resilient means. 
     The advantages of the present invention over known fuel injection systems are achieved mainly by the following means: 
     application of the hydraulic valve  4 , which is adapted to control the flow area of the bypass channel  29  such that the bypass channel is open when the hydraulic valve is in its closed and open positions or near these positions and closed during the other positions of the hydraulic valve; 
     application of the additional bypass channel  30  for connection of the poppet chamber  27  to the control chamber  5 ; 
     application of the secondary valve  32 , which is installed in the additional bypass channel  30  and which can control the flow area of this channel depending on the commands of the engine management system; 
     application of the third bypass channel  39  connecting the poppet chamber  27  to the control chamber  5 ; 
     application of the additional control valve  23  between the control channel  22  and the spill port  3 , whereby the plunger  7  is adapted to connect the control channel to the cut-off channel  20  at some positions of the plunger, other than its cut-off positions, and connect the cut-off channel  20  to the compression chamber  12  during the cut-off positions of the plunger, and application of the link channel  24  connecting the control channel  22  to the inlet port  2  or, alternatively, to the hydraulic control system  35 , wherein the flow areas of the link channel  24  and the open additional control valve  23  are such that when the additional control valve is open the pressure in the control channel is reduced. 
     Application of the hydraulic valve  4  which is adapted to control the flow area of the bypass channel  29  such that the bypass channel is open when the hydraulic valve is in the closed and open positions, or near these positions, and closed during its other positions, allows one to reduce the opening speed of the hydraulic valve on the first parts of its opening stroke, achieving a more gradual rise of the injection pressure, and at the same time reduce the maximum flow area of the control valve  21  which is required to hold the hydraulic valve in the closed position when the control valve  21  is open, because the pressure drop across the hydraulic valve in this case acts on the area of the poppet  27  which is larger than the area of the cylindrical sealing surface of the hydraulic valve. In the known fuel injection systems, such as the system disclosed in U.S. Pat. No. 5,785,021, the working chamber is in permanent and direct connection with the control chamber in order to facilitate transport of fuel from the working chamber to the spill port when the control valve is open and the HDV is closed, as the bypass channel in this position of the HDV is closed. Therefore, the pressure drop across the HDV in the case of prior art injection system acts on the area of the sealing cylindrical surface of the HDV, which is smaller than the area of the poppet, which requires a bigger pressure drop to hold the HDV closed and consequently a larger flow area for the control valve  21 . Moreover, such a permanent connection of the HDV control chamber to the working chamber prevents efficient reduction of the opening speed of the HDV during a part of its opening stroke taking place at the closed bypass channel. 
     Application of the additional bypass channel  30  for connection of the poppet chamber  27  to the control chamber  5  allows one to achieve the same objective of reducing the maximum flow area of the control valve  21  which is necessary to hold the hydraulic valve closed in case the bypass channel  29  is closed in this position of the hydraulic valve, as described above, but without the additional groove  37  on the hydraulic valve. 
     Application of the secondary valve  32 , which is installed in the additional bypass channel  30 , and which can control the flow area of this channel depending on the commands of the engine management system, allows for electronic control of the rate of injection pressure increase in the beginning of injection. If the secondary valve is open, the opening speed of the hydraulic valve is not reduced by a lower pressure in the control chamber  5  because it is connected to the poppet chamber  27  at all times, and if the secondary valve  32  is closed, the opening speed of the hydraulic valve is slower on the first parts of its opening stroke due to a lower pressure in the control chamber  5  as the bypass channel is closed when the hydraulic valve is between its second and first intermediate positions. Application of the third bypass channel connecting the poppet chamber to the control chamber allows one to adjust the opening speed of the hydraulic valve between its second and first intermediate positions when the additional bypass channel  30  is closed by the secondary valve  32  and therefore adjust the shape of the leading front of the main injection. This can be accomplished by optimizing the flow area of the third bypass channel  39 , the distances between the open, first, second and closed positions of the hydraulic valve, and the design of the throttling slot  28  of the hydraulic valve. 
     Application of the additional control valve  23  between the control channel  22  and the spill port  3 , wherein the plunger  7  is adapted to connect the control channel to the cut-off channel  20  at some positions of the plunger other than its cut-off positions, and connect the cut-off channel  20  to the compression chamber  12  during the cut-off positions of the plunger, and application of the link channel  24  connecting the control channel  22  to the inlet port  2  or, alternatively, to the hydraulic control system  35 , wherein the flow areas of the link channel  24  and the open additional control valve  23  are such that when the additional control valve is open the pressure in the control channel is reduced, allows one to achieve electronic control of pilot or boot injections and at the same time improve the shape of the rear front of an injection curve, simplify the injector design and increase its reliability. In the known fuel injection systems, such as the system disclosed in International Patent Application No. PCT/AU98/00073, the additional control valve which controls the pilot or boot injections is installed in the control channel which is connected to the cut-off channel of the injector at all times, so that during the cut-off of injection a high pressure is present in the control channel, and therefore the additional control valve must be able to seal against high pressure, which complicates the injector design. This also entails a larger volume to which the cut-off fuel is directed, which slows the needle closing and therefore deteriorates the shape of the injection curve. Application of the link channel  24  according to the present invention, as described above, allows one to install the additional control valve in the control channel which is disconnected from the cut-off channel during the cut-off positions of the plunger, so that the nozzle opening pressure can be controlled by the additional control valve but this valve is not subject to high pressure during a cut-off of injection. This is also beneficial in terms of equalizing the nozzle opening pressures of different injectors of an engine and in consecutive cycles of injection, as the pressure in the nozzle locking chamber  17  in case of incomplete sealing in the closed additional control valve will still be equal to the pressure in the inlet port (or the hydraulic control system). In case of the prior art injection system, a change in the leakage rate from the control channel is more likely to affect the nozzle opening pressure. 
     It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in these specific embodiments without department from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive. 
     For example, other types of valves can be used instead of the hydraulically controlled differential valve  4  described above, due to the fact that neither the control channel  22 , the additional control valve  23  nor the link channel  24  are related to the design of a hydraulically controlled differential valve. 
     Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.