Accumulator type fuel injection system

After an engine has been started, a controller 8 of a fuel injection system opens an injection rate switching change-over valve 5 for a period of time not less than a fuel injection period until the time a fuel pressure in a low pressure accumulator 4 connected to the portion of a fuel passage 10a which is on the downstream side of the change-over valve 5 has reached a set level, whereby a fuel injection starting delay ascribed to a fuel pressure decrease in the second accumulator can be prevented. After the engine has been started, the controller 8 of the fuel injection system sets the change-over valve opening starting time in each fuel injection cycle to the time earlier than the injector opening starting time, or the opening starting time of both an injector and change-over valve to the time earlier than those in a regular mode until the time the fuel pressure in the low-pressure accumulator 4 connected to the portion of the fuel passage 10a which is on the downstream side of the injection rate switching change-over valve 5 has reached the set level, whereby the fuel injection starting delay can be prevented. Therefore, the engine starting characteristics are improved, and the discharging of white smoke from the engine is prevented.

FIELD OF THE INVENTION
 This invention relates to an accumulator type fuel injection system.
 BACKGROUND OF THE INVENTION
 An accumulator type fuel injection system is known which is adapted to
 stably supply a high-pressure fuel stored in an accumulator into each
 cylinder of a diesel engine, and enable the engine performance to be
 improved in a wide operating range thereof. However, when a fuel injection
 rate immediately after the starting of a fuel injection operation is
 excessively high even in such a fuel injection system, a sudden explosion
 combustion is carried out in an initial stage of combustion, and not only
 operating noise of the engine but also the NOx content of an exhaust gas
 increases.
 To eliminate such inconveniences, an accumulator type fuel injection system
 adapted to inject a fuel at a lower injection rate in an initial stage of
 each fuel injection cycle has been proposed. This proposed system is
 provided with, for example, a low-pressure accumulator adapted to store a
 low-pressure fuel, a high-pressure accumulator adapted to accumulate a
 high-pressure fuel, a change-over valve adapted to selectively communicate
 the low-pressure accumulator or the high-pressure accumulator with an
 injector (fuel injection nozzle) and thereby switch an injection rate, and
 a switch valve adapted to communicate and shut off a control chamber of
 the injector with and from a fuel tank and thereby control the injection
 time.
 Regarding the formation of a fuel pressure in an accumulator, there are,
 for example, an accumulator adapted to obtain low-pressure and
 high-pressure fuels by using a low-pressure pump and a high-pressure pump
 which are driven by an engine, and an accumulator (Japanese Patent
 Laid-Open No. 93936/1994) adapted to obtain a high-pressure fuel by using
 a high-pressure pump, and a low-pressure fuel by regulating the pressure
 of the high-pressure fuel introduced into a low-pressure accumulator.
 In the accumulator type fuel injection system disclosed in International
 Patent Laid-Open No. WO98/09068, which is adapted to obtain a low-pressure
 fuel of a low-pressure accumulator from a high-pressure fuel of a
 high-pressure accumulator, for example, a fuel passage by which a
 change-over valve and a fuel chamber of an injector are connected together
 is filled with a low-pressure fuel by closing both an injection time
 control switch valve and an injection rate change-over valve, and the
 injector is maintained in a valve-closed state by supplying the
 low-pressure fuel to a control chamber of the injector which communicates
 with the fuel passage. At the arrival of the injection starting time, the
 switch valve is opened to discharge the low-pressure fuel in the control
 chamber to a fuel tank, whereby a valve of the injector is opened to carry
 out initial low-pressure injection (which will hereinafter be referred to
 as low-pressure injection). When the low-pressure injection period has
 elapsed, the change-over valve is opened to carry out main high-pressure
 injection (which will hereinafter be referred to as high-pressure
 injection) by injecting the high-pressure fuel in the high-pressure
 accumulator from a nozzle. At the arrival of the injection finishing time,
 the change-over valve is closed. In the low-pressure accumulator, a
 low-pressure fuel is obtained by regulating the pressure of the
 high-pressure fuel flowing from the fuel passage thereinto. In such an
 accumulator type fuel injection system, the fuel in the fuel passage
 gradually flows out when the engine is stopped, via a clearance around a
 plunger of a high-pressure pump and a clearance around the control chamber
 of the injector, and the fuel pressure in the fuel passage and
 low-pressure accumulator decreases to a level corresponding to that of the
 atmosphere.
 When the cranking is done at a subsequent engine starting time, a
 pressurized fuel is supplied from the high-pressure pump to the
 high-pressure accumulator, and the injection rate switching control
 change-over valve and injection time control switch valve are opened and
 closed so as to carry out low-pressure injection and high-pressure
 injection. When the pressurized fuel is discharged from the high-pressure
 accumulator during a high-pressure injection period, the fuel pressure
 therein decreases correspondingly, and the formation of a fuel pressure in
 the high-pressure accumulator tends to be delayed immediately after the
 starting of the engine. Consequently, the formation of a fuel pressure in
 the low-pressure accumulator is delayed.
 Therefore, it is difficult immediately after the starting of the engine to
 obtain a low-pressure fuel of a predetermined pressure exceeding a valve
 opening pressure of the injector. The low-pressure injection is not
 executed until a fuel pressure in the low-pressure accumulator has reached
 a predetermined level, and the fuel injection starting time is delayed, so
 that inconveniences, such as imperfect starting of the engine and the
 discharging of white smoke occur.
 SUMMARY OF THE INVENTION
 Therefore, the present invention aims at providing an accumulator type fuel
 injection system adapted to carry out early a fuel injection operation in
 a regular mode in which high-pressure injection is carried out immediately
 after low-pressure injection in each fuel injection cycle, by promoting
 the formation of a fuel pressure in a low-pressure accumulator at the
 engine starting time, or by rationalizing the fuel injection starting time
 at the engine starting time at which the formation of a fuel pressure in
 the low-pressure accumulator is insufficient.
 An accumulator type fuel injection system is characterized in that it
 comprises a first accumulator adapted to store a high-pressure fuel
 pressurized by a pump, a second accumulator connected to the portion of a
 fuel passage which is on the downstream side of a control valve for
 controlling the discharging of the high-pressure fuel in the first
 accumulator to a downstream side of the fuel passage, and adapted to store
 a low-pressure fuel, and a fuel control means for opening the control
 valve for a period of time longer than that, during which a fuel injection
 nozzle is opened, until a fuel pressure in the second accumulator has
 reached a set level after the start of the engine.
 According to this accumulator type fuel injection system, when the fuel
 pressure in the second accumulator decreases at the engine starting time,
 the control valve is opened not only throughout a period of time during
 which the fuel injection nozzle is opened but also during at least a part
 of other period of time. Since the control valve is opened at the starting
 time of the period of time during which the nozzle is opened, the
 injection of the fuel from the first accumulator is carried out from an
 initial stage of the period of time during which the nozzle is opened, so
 that a fuel injection starting delay ascribed to the decrease in the fuel
 pressure in the second accumulator does not occur. Since the fuel in the
 first accumulator is capable of flowing into the second accumulator while
 the control valve is opened during a period of time other than that during
 which the nozzle is opened, the fuel pressure in the second accumulator
 can be increased positively, and a set pressure at which the injection of
 the fuel can be carried out is attained in a short period of time.
 Preferably, the fuel control means continuously opens the control valve
 until the fuel pressure in the second accumulator has reached a set level.
 Owing to this operation, the control valve is kept open continuously
 irrespective of the opening and closure of the fuel injection nozzle, so
 that the fuel in the first accumulator is supplied continuously to the
 fuel passage. Therefore, the fuel pressure in the second accumulator can
 be increased positively during the whole of the period of time in which
 the fuel injection nozzle is opened, and the formation of a fuel pressure
 in the second accumulator is further promoted.
 Preferably, the fuel control means opens the control valve in accordance
 with the opening of the fuel injection nozzle until the fuel pressure in
 the second accumulator has reached a set level, and closes the control
 valve later than the closing time of the fuel injection nozzle. Owing to
 this operation, the fuel pressure in the second accumulator can be
 increased positively between the time at which the fuel injection nozzle
 is closed and the time at which the control valve is closed. When the fuel
 pressure in the second accumulator reaches a set level, the fuel injection
 in a subsequent fuel injection cycle is carried out in a regular mode. In
 this stage in a preferred mode, the fuel pressure in the fuel passage
 decreases between the time at which the control valve is closed after the
 completion of the fuel injection cycle started at the time of attainment
 of a set pressure and the time at which a subsequent fuel injection cycle
 is started, so that low-pressure injection is carried out smoothly even in
 a fuel injection cycle executed for the first time after the fuel
 injection mode has been transferred to a regular fuel injection mode.
 The fuel control means may set a fuel discharge pressure of the pump to a
 level higher than a set level of the fuel pressure in the first
 accumulator, whereby the pressure of the fuel discharged from the first
 accumulator to the fuel passage becomes high to enable the fuel pressure
 in the second accumulator to increase early.
 The fuel control means may also maintain the idling condition of the engine
 in preference to a driver's accelerator pedal stepping operation until the
 fuel pressure in the second accumulator has reached a set level. At the
 start of an engine quipped with the fuel injection system according to the
 present invention, a fuel of a comparatively high pressure supplied from a
 first accumulator is injected. Therefore, there is the possibility that a
 combustion sound and the deterioration of the exhaust gas characteristics
 occur as compared with a case where fuel injection of a regular mode, in
 which high-pressure injection following low-pressure injection is done, is
 carried out. However, when the idling condition of the engine is
 maintained, the number of times of carrying out combustion until the fuel
 pressure in the second accumulator has reached a set level can be reduced,
 and the occurrence of a combustion sound and the deterioration of the
 exhaust gas characteristics can be prevented.
 The accumulator type fuel injection system defined is characterized in that
 it is provided with a fuel control means adapted to open a control valve
 in agreement with or earlier than a target fuel injection starting time
 set in accordance with the operating condition of an engine, until the
 fuel pressure in a second accumulator, which is joined to the portion of a
 fuel passage which is on the downstream side of the control valve adapted
 to control the discharge of a high-pressure fuel in a first accumulator,
 and which stores a low-pressure fuel therein, has reached a predetermined
 level after the start of the engine.
 According to this accumulator type fuel injection system, when the engine
 is started, the control valve is opened in agreement with or earlier than
 a target fuel injection starting time. When the fuel injection system is
 formed so that both a fuel injection nozzle and control valve are put in
 an opened state at the arrival of the target fuel injection starting time,
 the pressurized fuel in the first accumulator is supplied to the fuel
 injection nozzle at the arrival of the target fuel injection starting time
 via the opened control valve and fuel passage, and injected from the fuel
 injection nozzle. Therefore, unlike the case of a fuel injection system in
 which a control operation for opening a control valve later than a target
 fuel injection starting time by a length of a period of time of
 low-pressure injection is carried out from the time immediately after the
 starting of the cranking of the engine, a delay of fuel injection starting
 time does not occur after the starting of the engine in the system
 according to the present invention in which the opening time of the
 control valve is set to earlier. Accordingly, the engine starting
 characteristics are improved, and the discharging of white smoke from the
 engine is prevented.
 Preferably, the fuel control means opens the fuel injection nozzle at the
 target fuel injection starting time, and the control valve at such time
 that is earlier than the target fuel injection starting time.
 Consequently, an opened condition of both the fuel injection nozzle and
 control valve is attained at the target fuel injection starting time.
 Therefore, when the engine is started, the fuel injection is started at
 planned time, and a delay of fuel injection starting time does not occur.
 In a fuel injection control operation at the starting of the engine, the
 controlling of the driving of the fuel injection nozzle can be carried out
 in the same manner as in a regular case. Accordingly, the controlling of
 the driving of the control valve only may be changed as compared with a
 control operation in a regular case, so that the contents of the control
 operation become simple. Since the time at which the control valve is
 opened is prior to that at which the fuel injection nozzle is opened, the
 supplying of the pressurized fuel from the first accumulator to the
 portion of the fuel passage which is on the downstream side of the control
 valve is done between the time at which the control valve is opened and
 that at which the fuel injection nozzle is opened, and the fuel injection
 at a sufficient fuel pressure is carried out simultaneously with the
 opening of the fuel injection nozzle.
 Even when the fuel injection nozzle is opened at the time earlier than
 target fuel injection starting time with the control valve opened at the
 target fuel injection starting time in contrast with the above-described
 cases, both the fuel injection nozzle and control valve can be put in an
 opened state at the target fuel injection starting time. In this case, it
 is preferable that the contents of the control operation is simplified by
 setting the fuel injection nozzle opening time earlier by a length of
 period of time during which the low-pressure fuel injection is carried
 out.
 Preferably, the predetermined pressure mentioned above is a pressure set in
 accordance with the operating condition of the engine. According to this
 preferred mode, the control valve is closed at a point in time at which
 the fuel injection nozzle is opened in each fuel injection cycle executed
 after the fuel pressure in the second accumulator has reached a
 predetermined level equal to a set level which suits the operating
 condition of the engine, so that the pressurized fuel of a set pressure
 already supplied from the second accumulator to the fuel injection nozzle
 is injected therefrom. Namely, low-pressure fuel injection is carried out.
 When a predetermined period of time has elapsed after the point in time at
 which the fuel injection nozzle was opened, with the control valve opened
 at the same time, the fuel in the first accumulator is injected from the
 fuel injection nozzle. Namely, high-pressure fuel injection is carried
 out. When a predetermined pressure, which constitutes a requirement for
 transferring the fuel injection mode to a regular fuel injection mode, is
 thus set equal to a set pressure, desired fuel injection comprising
 low-pressure injection and high-pressure injection can be carried out in
 each fuel injection cycle from the time immediately after the fuel
 pressure in the second accumulator has reached a predetermined level.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The preferred embodiments of the present invention will now be described
 with reference to the drawings.
 First, an accumulator type fuel injection system on which the present
 invention is based will be described.
 An accumulator type fuel injection system is mounted on, for example, a
 6-series-cylinder diesel engine (not shown), and provided with a
 high-pressure pump 1 as shown in FIGS. 1 and 2. The high-pressure pump 1
 is driven by the engine and adapted to draw up a fuel in a fuel tank 17
 and pressurized, and it comprises, for example, a displacement plunger
 pump, a fuel discharge pressure of which can be regulated by adjusting an
 effective section of its force feed stroke. The adjustment of the force
 feed stroke is made by regulating the closing time of, for example, an
 electromagnetic valve (not shown), and, while this electromagnetic valve
 is opened, a force feed operation becomes ineffective. In a 6-cylinder
 engine, a high-pressure pump is provided with, for example, two plungers.
 Each plunger has relation to three cylinders, and is adapted to make three
 force feed strokes while a high-pressure pump shaft revolves once.
 A controller 8 in the accumulator type fuel injection system is adapted to
 variably regulate the force feed stroke of the pump 1 in accordance with
 an engine speed Ne detected by an engine speed sensor 8a and an
 accelerator pedal stepping amount (degree of opening of accelerator)
 A.sub.CC detected by a relative sensor (not shown), and make the force
 feed stroke (fuel pressure) feedback control in accordance with an actual
 pressure P.sub.HP in a high-pressure accumulator (first accumulator) 3
 detected by a pressure sensor 3a (FIG. 2), whereby a high-pressure fuel
 which suits the operating condition of the engine is obtained.
 The fuel pressurized by the pump 1 is stored in the high-pressure
 accumulator 3. This high-pressure accumulator 3 is common to all
 cylinders, and communicates with a fuel passage 10a. In intermediate
 portions of the fuel passage 10a, fuel injection rate switching
 change-over valves (control valves) 5 comprising, for example, two-way
 electromagnetic valves are provided for the respective cylinders. A check
 valve 32 is also provided in the portion of each fuel passage 10a which is
 on the immediate downstream side of the change-over valve 5.
 A low-pressure accumulator (second accumulator) 4 common to all cylinders
 is connected to the fuel passages 10a via a fuel passage 10b branching
 from the portion of each fuel passage 10a which is on the downstream side
 of the check valves 32. A check valve 6 is provided in an intermediate
 portion of the fuel passage 10b, and a bypass fuel passage shunting the
 check valve 6 is added to the fuel passage 10b, an orifice 6a being
 provided in this bypass fuel passage. When the fuel pressure in the fuel
 passage 10a is higher than that in the fuel passage 10b, the fuel in the
 fuel passage 10a flows into the fuel passage 10b via the orifice 6a, and
 further into the low-pressure accumulator 4. Between the low-pressure
 accumulator 4 and fuel tank 17, a pressure control valve 34 is provided
 which is adapted to be operated under the control of a controller 8.
 Referring to FIG. 2, a reference numeral 4a denotes a pressure sensor for
 detecting a fuel pressure P.sub.LP in the low-pressure accumulator 4.
 The controller 8 is adapted to control the pressure control valve 34 on the
 basis of an actual pressure P.sub.LP detected by the pressure sensor 4a,
 in such a manner that the fuel pressure in the low-pressure accumulator 4
 attains a level which suits the operating condition of the engine
 represented by the engine speed Ne and stepping amount A.sub.CC of the
 accelerator pedal.
 The injector (fuel injection nozzle) 9 for each cylinder of the engine has
 a control chamber 11 and a fuel chamber 12 which are connected to the fuel
 passage 10a, and the control chamber 11 is connected to the fuel tank 17
 via a fuel return passage 10c. Reference numerals 15, 16 denote orifices.
 A reference numeral 7 denotes an injection time control switch valve
 provided in an intermediate portion of the fuel return passage 10c and
 comprising, for example, a two-way electromagnetic valve. This switch
 valve 7 may be incorporated in the injector.
 The injector 9 has a needle valve 13 adapted to open and close a nozzle
 port thereof, and a hydraulic piston 14 provided movably in the control
 chamber 11, the needle valve 13 being urged toward the nozzle port by a
 spring (not shown). When the fuel is supplied from the fuel passage 10a to
 the control chamber 11 and fuel chamber 12 with the injection time control
 switch valve 7 closed, a resultant force of a resilient force of the
 spring and a fuel pressure is imparted to the needle valve 13 via the
 hydraulic piston 14, and the needle valve 13 closes the nozzle port
 against the fuel pressure in the fuel chamber 12.
 When the switch valve 7 is opened to cause the fuel in the control chamber
 11 to be discharged toward the fuel tank 17, the needle valve 13 is moved
 toward the hydraulic piston 14 against the resilient force of the spring
 owing to the fuel pressure in the fuel chamber 12 to open the nozzle port,
 so that the fuel in the fuel chamber 12 is injected into a combustion
 chamber (not shown) of the engine.
 The operation in a regular mode of the fuel injection system of the
 above-described construction will now be described.
 The fuel pressure in the high-pressure accumulator 3 and that in the
 low-pressure accumulator 4 are controlled under the control of the
 controller 8 so that these fuel pressures suit the operating condition of
 the engine, and a fuel injection time (fuel injection starting and
 finishing time) and a low-pressure injection period are set in accordance
 with the operating condition (engine speed and accelerator pedal stepping
 amount) of the engine.
 Until the arrival of the fuel injection starting time, the change-over
 valve 5 and switch valve 7 are all closed as shown in FIG. 3, and a
 low-pressure fuel is supplied from the low-pressure accumulator 4 to the
 portion of the fuel passage 10a which is on the downstream side of the
 change-over valve 5, this low-pressure fuel being supplied to the control
 chamber 11 and fuel chamber 12. Since the switch valve 7 is closed, the
 fuel pressure supplied to the interior of the control chamber 11 is
 applied to the needle valve 13 via the hydraulic piston 14, and the nozzle
 port of the injector 9 is closed with the needle valve 13.
 When the fuel injection starting time has come, the switch valve 7 alone is
 opened, and the low-pressure fuel in the control chamber 11 drains away
 via the orifice 16 and fuel return passage 10c. When the resultant force
 of the fuel pressure imparted to the needle valve 13 via the hydraulic
 piston 14 and the resilient force of the spring has become smaller than
 the fuel pressure in the fuel chamber 12 which works so as to lift the
 needle valve 13, the needle valve 13 moves up to open the nozzle port, and
 the low-pressure fuel is injected from the injector 9. Namely,
 low-pressure initial injection at a comparatively low fuel injection rate
 (quantity of fuel injected per unit time) is carried out. Owing to this
 low-pressure injection, the quantity of fuel prior to ignition decreases
 to cause a premixed combustion rate to lower, so that the combustion in an
 initial stage of a fuel injection period is carried out comparatively
 slowly to contribute to the reduction of the NOx content of an exhaust
 gas.
 When a predetermined period of time has elapsed after the start of the
 low-pressure injection, the injection rate switching change-over valve 5
 is opened with the injection time control switch valve 7 kept open, and a
 high-pressure fuel is supplied to the fuel chamber 12 and injected from
 the injector 9. Namely, the fuel injection (high-pressure main injection)
 is executed at an injection rate higher than that at which the
 low-pressure injection is carried out.
 When the fuel injection finishing time has come, the injection time control
 switch valve 7 is closed, and the high-pressure fuel supplied to the
 control chamber 11 works on the needle valve 13 via the hydraulic piston
 14, so that the needle valve 13 closes the nozzle port of the injector 9.
 At the fuel injection finishing time, the fuel injection rate falls
 speedily to contribute to a decrease in the rates of discharge of black
 smoke and particulates (PM) from the engine. The injection rate switching
 change-over valve 5 is closed simultaneously with the closure of the
 switch valve 7 at the fuel injection finishing time Te, or at a point in
 time at which a predetermined time (shown by a symbol .DELTA.Te in FIG. 8)
 has elapsed after the fuel injection finishing time.
 As shown in FIG. 4, the fuel pressure in the portion of the fuel passage
 10a which is between the injector 9 and injection rate switching
 change-over valve 5 gradually decreases from a point in time at which the
 fuel injection in each fuel injection cycle finishes, to a level, which
 suits low-pressure injection, by the time when the fuel injection in a
 subsequent fuel injection cycle is started, and the injection rate in a
 subsequent low-pressure injection operation reaches a required level.
 When the operation of the engine is stopped, the fuel in the fuel passages
 10a, 10b gradually flows out via a clearance around the control chamber 11
 of the injector 9 and a clearance around the plunger of the high-pressure
 pump 1, and the fuel pressure in the fuel passages 10a, 10b and
 low-pressure accumulator 4 decreases to a level corresponding to that of
 the atmospheric pressure.
 As has already been described, when the fuel injection at the engine
 starting time, after the operation of the engine was once stopped, is
 executed in the same manner as that in the above-mentioned case of a
 regular mode, the formation of a fuel pressure in, especially, the
 low-pressure accumulator 4 is delayed, and it is difficult to obtain
 immediately a low-pressure fuel of a predetermined level exceeding the
 pressure for opening the injector 9. Therefore, the low-pressure injection
 is not executed, and the fuel injection starting time is delayed to cause
 inconveniences including imperfect starting of the engine to occur.
 Therefore, in the accumulator type fuel injection system according to the
 present invention, fuel injection in a starting mode different from the
 above-mentioned regular mode is executed at the engine starting time to
 promote the formation of a fuel pressure in the low-pressure accumulator,
 and enable the fuel injection in a regular mode to be executed early.
 A concrete example (first embodiment) in which this starting mode is
 effected by promoting the formation of a fuel pressure in the low-pressure
 accumulator at the engine starting time will now be described.
 When a power source is turned on at the engine starting time, the
 controller 8 executes the fuel injection mode judgement routine shown in
 FIG. 5 in a predetermined cycle.
 In this judgement routine, a target value (indicated value) P.sub.LPRAR of
 the low-pressure fuel which suits the operating condition of the engine is
 determined with reference to, for example, a map (not shown) of operating
 condition of an engine and indicated value of low-pressure fuel on the
 basis of, for example, engine speed Ne and accelerator pedal stepping
 amount A.sub.CC. An output P.sub.LP, which is representative of the fuel
 pressure in the low-pressure accumulator 4, from the pressure sensor 4a is
 read, and an actual pressure value of the low-pressure fuel is detected.
 This actual pressure value P.sub.LP is then judged (Step S1) as to whether
 it has reached the indicated level P.sub.LPTAR.
 When the result of the judgement in Step S1 is affirmative (Yes), the
 formation of a fuel pressure in the low-pressure accumulator 4 has been
 completed, the procedural action is transferred (Step S2) to the fuel
 injection in a regular mode, to finish the execution of the judgement
 routine of FIG. 5.
 When the result of the judgement in Step S1 is negative (No), the formation
 of a fuel pressure in the low-pressure accumulator 4 has not been
 completed. When fuel injection in a regular mode is executed, there is a
 fear of occurrence of inconveniences, such as imperfect start of the
 engine as mentioned above, fuel injection in a starting mode is executed
 (Step S3).
 In the starting mode, the high-pressure pump 1 is driven in a full force
 feed condition. Namely, the high-pressure pump 1 is driven with the
 effective section of a force feed stroke set maximal. The injection rate
 switching change-over valve 5 is always kept open. A value "zero" is set
 (FIG. 6) instead of an actual value (value of an output signal from the
 sensor) detected by the degree of opening of accelerator sensor (not
 shown), as the accelerator pedal stepping amount (degree of opening of the
 accelerator) A.sub.CC which is used with the engine speed Ne for setting a
 fuel injection rate, fuel injection time and target fuel pressures in the
 accumulators 3, 4.
 Therefore, when the cranking for the starting of the engine is started, the
 starting mode is selected in the judgement routine of FIG. 5 since the
 formation of a fuel pressure in the low-pressure accumulator 4 is
 insufficient at this point in time. Consequently, the driving of the pump
 in a full force feed condition is started simultaneously with the starting
 of the cranking operation, to start the supplying of a pressurized fuel
 from the high-pressure pump 1 to the high-pressure accumulator 3. A
 change-over valve driving signal supplied to the injection rate switching
 change-over valve 5 is always put in an ON-state, whereby the change-over
 valve 5 is always kept open. Accordingly, the pressurized fuel in the
 high-pressure accumulator 3 is supplied to the fuel passage 10a, and this
 pressurized fuel is capable of being supplied to the control chamber 11
 and fuel chamber 12 of the injector 9. Since the high-pressure pump 1 is
 driven in a full force feed condition, the formation of a fuel pressure in
 the high-pressure accumulator 3 is promoted.
 Since the accelerator pedal stepping amount A.sub.CC is set to a value
 "zero" in a starting mode, the injection time control switch valve 7 is
 driven by an injector driving signal at such valve opening and closing
 time that permits a fuel injection time and period which suit the idling
 of the engine to be attained. As a result, the pressurized fuel in the
 fuel chamber 12 supplied from the high-pressure accumulator 3 via the fuel
 passage 10a is injected from the nozzle port of the injector 9 while the
 switch valve 7 is opened. Namely, in the starting mode, the fuel injection
 is executed at substantially the same injection rate throughout the whole
 of the injection period. Accordingly, the fuel injection is started at a
 desired injection starting time, so that an injection delay, which is
 ascribed to the non-execution of low-pressure injection occurring when
 fuel injection is executed in a regular mode, is prevented despite the
 incompletion of the formation of a fuel pressure in the low-pressure
 accumulator 4.
 Since a fuel of a comparatively high pressure supplied from the
 high-pressure accumulator 3 is injected in the starting mode as mentioned
 above, there is the possibility that a combustion sound and the
 deterioration of the exhaust gas characteristics occur as compared with
 the case where fuel injection is carried out in a regular mode. However,
 when the idling condition is maintained by setting the accelerator pedal
 stepping amount A.sub.CC to a value "zero", the fuel consumption
 decreases, so that it becomes possible to reduce the number of times of
 combustion carried out until the fuel pressure in the low-pressure
 accumulator 4 has attained a set level, and prevent the occurrence of a
 combustion sound and the deterioration of the exhaust gas characteristics.
 The change-over valve 5 is always kept open in a starting mode unlike that
 in the regular mode, so that the pressurized fuel from the high-pressure
 accumulator 3 is always supplied to the low-pressure accumulator 4 via the
 fuel passage 10a and orifice 6a. Especially, in a period of time other
 than the fuel injection time, the supply of the pressurized fuel to the
 low-pressure accumulator 4 is done positively. Owing to this effect in the
 starting mode as well as the characteristics that the pressure of the
 pressurized fuel supplied via the fuel passage 10a and orifice 6a is
 increased by driving the high-pressure pump 1 in a full force feed
 condition and thereby increasing the pressure of the pressurized fuel
 supplied from the high-pressure accumulator 3 to the fuel passage 10a, the
 formation of a fuel pressure in the low-pressure accumulator 4 is
 promoted, and the fuel pressure in the low-pressure accumulator 4
 increases rapidly with the lapse of time as shown in FIG. 6.
 When the formation of a pressure in the low-pressure accumulator 4 is thus
 completed in a short period of time, a judgement that the actual pressure
 value P.sub.LP in the low-pressure accumulator 4 has exceeded the
 indicated value P.sub.LPTAR in Step S1 of the judgement routine of FIG. 5
 is given. In this case, the fuel injection in the starting mode is
 transferred to fuel injection in a regular mode.
 Another concrete example (second embodiment) which can be attained by
 promoting the formation of a pressure in the low-pressure accumulator at
 the engine starting time will now be described.
 The system of the second embodiment is identical with that of the first
 embodiment in that the promotion of the formation of a fuel pressure in
 the low-pressure accumulator at the engine starting time is intended, and
 different therefrom in that the injection rate switching change-over valve
 5, which is always kept open in the starting mode in the first embodiment,
 is opened intermittently.
 Regarding these characteristics, a judgement routine shown in FIG. 7
 instead of the fuel injection mode judgement routine of FIG. 5 related to
 the first embodiment is executed in the second embodiment. The remaining
 portions of the second embodiment are substantially identical with those
 of the first embodiment, and the descriptions thereof will be omitted.
 In the fuel injection mode judgement routine of FIG. 7, a target pressure
 value (indicated value) P.sub.LPTAR of a low-pressure fuel which suits the
 operating condition of the engine is determined in the same manner as in
 the case of the routine of FIG. 5, and an actual value P.sub.LP of the
 fuel pressure in the low-pressure accumulator 4 is judged (Step S11) as to
 whether it has reached the indicated level. When the result of this
 judgement is affirmative, fuel injection in a regular mode is executed
 (Step S12).
 When a judgement that the fuel pressure in the low-pressure accumulator 4
 has not reached the indicated level is given in Step S1, fuel injection in
 a starting mode is executed (Step S13). In the fuel injection in the
 starting mode, the length of the time during which the injection rate
 switching change-over valve 5 is opened is set long as compared with that
 in the case of the fuel injection in the regular mode. In the second
 embodiment, a predetermined period of time .DELTA.Te between the fuel
 injection finishing time (instant at which the injector driving signal
 falls) and the change-over valve opening time (instant at which the
 change-over valve driving signal falls) is extended by extension time
 .DELTA.Testa.
 When the cranking for the start of the engine is started, a starting mode
 is selected in the judgement routine of FIG. 7, and the driving of a
 high-pressure pump 1 is started. A pressurized fuel from the high-pressure
 pump 1 is supplied to a high-pressure accumulator 3, from which the
 pressurized fuel is further supplied to a fuel passage 10a and a
 low-pressure accumulator 4.
 In the starting mode, an injection time control switch valve 7 is driven
 with this valve opened and closed so that a fuel injection period suiting
 the operating condition of the engine is obtained, and, while the switch
 valve 7 is opened, the pressurized fuel in a fuel chamber 12 is injected
 from a nozzle port of an injector 9.
 In the starting mode in this embodiment, the opening of the injection rate
 switching change-over valve 5 is timed in the same manner as in a regular
 mode. In the starting mode, the change-over valve 5 may be opened
 simultaneously with the switch valve 7 for the purpose of preventing the
 injection delay ascribed to the imperfect formation of fuel pressure in
 the low-pressure accumulator 4.
 In the starting mode, the time, unlike that in a regular mode, at which the
 change-over valve 5 is opened is extended by .DELTA.Testa. Accordingly,
 the time during which the pressurized fuel is supplied from the
 high-pressure accumulator 3 to the low-pressure accumulator 4 via the fuel
 passage 10a and an orifice 6a is long, and, especially, in a part of a
 period other than the fuel injection period, the supplying of the
 pressurized fuel to the low-pressure accumulator 4 is done positively.
 Therefore, in the starting mode, the formation of fuel pressure in the
 low-pressure accumulator 4 is promoted, and a rate of increase of the fuel
 pressure in the low-pressure accumulator 4 becomes large as shown by a
 solid line in FIG. 8, as compared with a case (broken line) where the
 formation of fuel pressure in the starting mode is not executed.
 When a judgement that an actual value in the low-pressure accumulator 4 has
 exceeded the indicated level, i.e., the formation of fuel pressure in the
 low-pressure accumulator 4 has been completed is given in Step S11 in the
 judgement routine of FIG. 7, the fuel injection is transferred to fuel
 injection in a regular mode.
 When an actual value of the fuel pressure in the low-pressure accumulator 4
 has reached the indicated level in a certain fuel injection cycle as
 mentioned above, the fuel pressure in the fuel passage 10a decreases to a
 level, which corresponds to the fuel injection pressure in a low-pressure
 injection operation in a regular mode, between the time at which the
 change-over valve 5 is closed after the completion of this fuel injection
 cycle and the time at which a subsequent fuel injection cycle is started.
 Therefore, the low-pressure injection is carried out smoothly even in a
 fuel injection cycle executed for the first time after the fuel injection
 mode has been transferred to a regular mode.
 In this embodiment, the change-over valve 5 comprises a two-way
 electromagnetic valve, which is typically so formed that it is opened when
 an electric current is applied thereto. Namely, a normally-closed type
 electromagnetic valve is used. In this case, the time during which the
 change-over valve 5 is opened is extended, not kept open at all times, in
 the starting mode in this embodiment as compared with that in the regular
 mode, so that the electromagnetic valve constituting the change-over valve
 5 is closed intermittently. Accordingly, the application of an electric
 current to the electromagnetic valve is not continuously carried out, so
 that the durability of the electromagnetic valve is improved as compared
 with that of an electromagnetic valve to which an electric current is
 applied continuously. Since it is unnecessary to form the electronic valve
 so that it withstands the heat occurring due to the continuous application
 of electric current, i.e., since the thermal requirements for the
 electronic valve are lightened, the electromagnetic valve can be formed
 inexpensively.
 The present invention is not limited to the first and second embodiments,
 and can be modified variously.
 For example, in the second embodiment, the opening time of the injection
 rate switching change-over valve 5 is extended in the time region after
 the fuel injection finishing time. This opening time may also be extended
 in the time region before the fuel injection starting time.
 In the second embodiment, the accelerator pedal stepping amount A.sub.CC
 may be set to "zero" while the fuel injection is carried out in a starting
 mode, in the same manner as in the first embodiment.
 In the first and second embodiments, the actual value of the fuel pressure
 in the low-pressure accumulator 4 is judged as to whether it has reached
 an indicated level (set pressure) P.sub.LPTAR or not, on the basis of an
 output P.sub.LP from the pressure sensor 4a fixed to the low-pressure
 accumulator (second accumulator) 4. The attainment of the set pressure may
 also be judged on the basis of the length of time elapsed after the engine
 starting time or the engine speed.
 A concrete example (third embodiment) for rationalizing the fuel injection
 starting time at the engine starting time will now be described. In this
 embodiment, a starting mode is formed on the basis of a logic different
 from that on which the starting mode of the first and second embodiments
 is based. Since the remaining portions of the third embodiment are
 substantially identical with those of the first and second embodiments,
 the descriptions thereof will be omitted.
 When a power source is turned on at the engine starting time in this
 embodiment, a controller 8 executes in a predetermined cycle a fuel
 injection mode judgement routine shown in FIG. 9.
 In this judgement routine, a target value (indicated value) P.sub.LPTAR of
 a low-pressure fuel which suits the operating condition of the engine is
 determined with reference to, for example, an operating condition of
 engine and low-pressure fuel indicated value map (not shown) on the basis
 of, for example, an engine speed Ne and an accelerator pedal stepping
 amount A.sub.CC. An output P.sub.LP from a pressure sensor 4a which
 represents a fuel pressure in a low-pressure accumulator 4 is read, and an
 actual pressure value of the low-pressure fuel is detected. The actual
 pressure value P.sub.LP is judged (Step S21) as to whether it has reached
 the indicated level P.sub.LPTAR or not.
 When the result of the judgement in Step S21 is affirmative (Yes), the
 formation of fuel pressure in the low-pressure accumulator 4 has been
 completed, so that the procedural action is transferred (Step S22) to the
 fuel injection in the above-mentioned regular mode to finish the execution
 of the judgement routine of FIG. 9.
 When the result of the judgement in Step S21 is negative (No), the
 formation of fuel pressure in the low-pressure accumulator 4 has not been
 completed. When fuel injection in a regular mode is executed in this case,
 there is a fear of occurrence of inconveniences, such as imperfect engine
 starting, as mentioned above, and, therefore, fuel injection in a starting
 mode is executed (Step S23).
 In the starting mode, the injector opening period (opening starting and
 finishing time) is set in the same manner as that in a regular mode, and
 the opening starting time of an injection rate switching change-over valve
 5 is set to time earlier than that in a regular mode. In this embodiment,
 the opening starting time of the change-over valve 5 is set to time
 earlier than that (target fuel injection starting time) of the injector 9
 by a period of time .DELTA.Ts as shown in FIG. 10. Namely, the opening
 starting time of the change-over valve 5 is set to time earlier than that
 (shown by a broken line) in a regular mode by a period of time equal to
 the sum of .DELTA.Ts and a low-pressure injection period of time
 .DELTA.T.sub.LP.
 When a change-over valve driving signal is applied from the controller 8 to
 the change-over valve 5 on the arrival of the opening starting time
 thereof in each fuel injection cycle in a starting mode, the change-over 5
 is opened. As a result, the pressurized fuel in a high-pressure
 accumulator 3 is supplied to a control chamber 11 and a fuel chamber 12 of
 the injector 9 via a fuel passage 10a. When the target fuel injection
 starting time has come with the time .DELTA.Ts having elapsed after the
 opening starting time of the change-over valve 5, an injector driving
 signal is applied from the controller 8 to a switch valve 7, so that the
 switch valve 7 is opened to cause the pressurized fuel in the control
 chamber 11 to drain away, and a needle valve 13 of the injector 9 to be
 lifted. As a result, a nozzle port is opened, and the pressurized fuel
 supplied to the fuel chamber 12 is injected. As described above, even in a
 starting mode, the opening time (target fuel injection starting time) of
 the switch valve 7 is set so that it suits the operating condition of the
 engine. Therefore, when fuel injection is started simultaneously with the
 opening of the switch valve 7 as mentioned above, the fuel injection
 starting time comes to suit the operating condition of the engine.
 Accordingly, an injection delay ascribed to the non-execution of
 low-pressure injection which occurs when fuel injection is executed in a
 regular mode in spite of the insufficient formation of fuel pressure in
 the low-pressure accumulator 4 is prevented.
 When the formation of fuel pressure in the low-pressure accumulator 4 has
 been completed, a judgement that the actual pressure value P.sub.LP in the
 low-pressure accumulator 4 has exceeded the indicated level P.sub.LPTAR is
 given in Step S21 of the judgement routine of FIG. 9. In this case, the
 fuel injection in the starting mode is transferred to the fuel injection
 in a regular mode.
 When the actual value P.sub.LP of the fuel pressure in the low-pressure
 accumulator 4 has reached the indicated level P.sub.LPTAR in a certain
 fuel injection cycle as mentioned above, the fuel pressure in the fuel
 passage 10a decreases to a level corresponding to the fuel injection
 pressure in the low-pressure injection in the regular mode between the
 time at which the change-over valve 5 is closed after the completion of
 this fuel injection cycle and the time at which a subsequent fuel
 injection cycle is started. Therefore, the low-pressure injection is
 carried out smoothly even in a fuel injection cycle executed for the first
 time after the fuel injection in the starting mode has been transferred to
 the fuel injection in a regular mode.
 Another concrete example (fourth embodiment) in which the fuel injection
 starting time at the engine starting time is rationalized will now be
 described.
 The system of the fourth embodiment is identical with that of the third
 embodiment in that the prevention of a fuel injection starting delay at
 the start of the engine is intended, and different therefrom in that the
 opening periods of both an injector 9 and a change-over valve 5 are set
 equal to those in a regular mode with the opening starting times thereof
 set to time earlier than those in a regular mode, though, in the system of
 the third embodiment, the opening period (opening starting and finishing
 time) of the injector is set to the same level as in the case of a regular
 mode with the opening starting time of the change-over valve 5 set to
 early time.
 Regarding these characteristics, a judgement routine shown in FIG. 11 is
 executed in the fourth embodiment instead of the fuel injection mode
 judgement routine of FIG. 9 related to the third embodiment. The remaining
 portions of the fourth embodiment are substantially identical with those
 of the first to third embodiments, and the descriptions thereof will be
 omitted.
 In the fuel injection mode judgement routine of FIG. 11, an injector
 opening pressure (injection executable pressure) P.sub.INJ determined on
 the basis of a set load of a spring imparted to a hydraulic piston 14 of
 the injector 9 is read from a storage unit in a controller 8 to judge
 (Step S31) whether an actual value PLP of the fuel pressure in a
 low-pressure pressure accumulator 4 has reached the injector opening
 pressure P.sub.INJ or not. When the result of the judgement is
 affirmative, the fuel injection in a regular mode is executed (Step S32).
 When a judgement that the actual value of the fuel pressure in the
 low-pressure accumulator 4 has not reached the injector opening pressure
 P.sub.INJ is given in Step S31, the fuel injection in a starting mode is
 executed (Step S33). In the starting mode, the opening starting time of
 the injector 9 and change-over valve 5 in accordance with the operating
 condition of the engine and a low-pressure injection period
 .DELTA.T.sub.LP are determined with reference to a map (not shown) in the
 same manner as in the case of a regular mode. Both the injector opening
 time and change-over valve opening time are corrected to the side of early
 time by using the low-pressure injection period .DELTA.T.sub.LP.
 Accordingly, when the injector opening starting time corrected to the time
 earlier than the injector opening starting time in a regular mode by the
 low-pressure injection period .DELTA.T.sub.LP has come in a starting mode
 as shown in FIG. 12, an injection time control switch valve 7 is opened.
 The formation of fuel pressure in the low-pressure accumulator 4 is
 insufficient in this case, so that the fuel pressure supplied to a fuel
 chamber 12 of the injector 9 is smaller than the set load of a spring
 imparted to the hydraulic piston 14. Therefore, the injector 9 is kept
 closed.
 When the change-over valve opening starting time corrected to the time
 earlier than that in a regular mode by the length of the low-pressure
 injection period .DELTA.T.sub.LP has then come, the injection rate
 switching change-over valve 5 is opened. As a result, the pressurized fuel
 in a high-pressure accumulator 3 is supplied to the fuel chamber 12 via a
 fuel passage 10a, and the fuel pressure in the fuel chamber 12 exceeds the
 injector opening pressure to cause the pressurized fuel to be injected
 from a nozzle port. After all, in the starting mode, the fuel injection is
 started at such time that substantially agrees with the low-pressure
 injection starting time in a regular mode. In other words, when a fuel
 injection control operation in a starting mode in which the opening
 starting time of the injector and change-over valve is corrected to
 earlier time is carried out, an injection delay ascribed to the
 non-execution of low-pressure injection occurring in a case where fuel
 injection is executed in the regular mode is prevented in spite of the
 insufficient formation of fuel pressure in the low-pressure accumulator 4.
 When the formation of fuel pressure in the low-pressure accumulator 4 has
 sufficiently progressed, a judgement that the actual value in the
 low-pressure accumulator 4 has exceeded the injection executable pressure
 level is given in Step S31 of the judgement routine of FIG. 11, and the
 fuel injection is transferred to fuel injection in the regular mode.
 The present invention is not limited to the above-described third and
 fourth embodiments, and it can be modified variously.
 For example, in the third and fourth embodiments, the actual value of the
 fuel pressure in the low-pressure accumulator 4 is judged as to whether it
 has reached the indicated level (set pressure) P.sub.LPTAR or the
 injection executable pressure level P.sub.INJ, on the basis of an output
 P.sub.LP from a pressure sensor 4a fixed to the low-pressure accumulator
 (second accumulator) 4, and the attainment of the set level or injection
 executable pressure level may also be judged on the basis of the length of
 time elapsed after the point in time at which the engine was started or
 the engine speed.