Fuel-injection system for engine

A fuel-injection system for an engine is disclosed in which a standard conductive duration of the individual injectors required for a desired volume of fuel injected per cycle may be easily provided by correcting a standard conductive duration to a solenoid-operated valve, which has been found depending on a standard fuel-injection characteristic. A controller unit is stored with a standard fuel-injection characteristic A that is used for obtaining a standard actuating pulse width Pws for the standard conductive duration corresponding to a desired volume Qf of the injected fuel, which is required depending on the operating conditions of the engine. A specified operating point (Q1, Pw1) is a known data that has been previously observed for the individual injectors. The actuating pulse width Pw necessary for determining the desired volume Qf of injected fuel is given by multiplying the standard actuating pulse width Pw1 by a correction coefficient that is a ratio of the standard actuating pulse width Pws1 to the specified actuating pulse width Pw1. The correction coefficient is computed at plural selected pressure ranges of a hydraulically actuated fluid while the process of interpolation provides the correction coefficient at the residual pressure ranges. This makes it possible to eliminate the stoop change of the correction coefficient K with the result of the protection of the engine from torque-shock.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a fuel-injection system having injectors
 that may inject fuel in accordance with fuel-injection characteristics,
 which is dependent on operating conditions of an engine.
 2. Description of the Prior Art
 A fuel-injection system has been well known in which an injector is
 provided with a needle valve movable in an injector body in a
 reciprocating manner to open and close injection holes, and a
 solenoid-operated valve having an electromagnetic actuator that is applied
 with an actuating current so as to control a hydraulically actuated fluid
 for driving the needle valve upwards and downwards, whereby the fuel to be
 injected out of the injector is regulated in injection timing and volume
 of injected fuel per cycle by a controller unit in response to the
 operating conditions of the engine.
 There have been conventionally known two types of the injector used in the
 fuel-injection system, one of which is comprised of a solenoid-operated
 valve to control an ingress of the hydraulically actuated fluid, or
 hydraulic oil, into the injector body, and a boosting piston to pressurize
 the fuel in an intensified chamber, whereby the pressurized fuel makes the
 needle valve move so as to inject the pressurized fuel through the
 injection holes that have been free from the needle valve. Another type of
 the injector operates so as to regulate an ingress and egress of the
 highly pressurized fuel, which is accumulated in a common fuel supply
 rail, to a controlled pressure chamber in the injector body, whereby the
 pressurized fuel makes the needle valve move so as to inject the
 pressurized fuel through the injection holes that have been free from the
 needle valve.
 FIG. 7 shows a prior fuel-injection system in which is incorporated the
 former type of the injector. The multicylinder engines, for example,
 four-cylinder or six-cylinder engine, have been dominated in most modern
 engines to attain the high horsepower. The injectors are each assigned to
 each cylinder to inject the fuel into the combustion chamber. In the
 fuel-injection system in FIG. 7, the fuel may be fed from a fuel tank 52
 to a common fuel supply rail 51 through a fuel filter 54 by the driving of
 a fuel pump 53. The common fuel supply rail 51 is communicated with each
 of the injectors 1. It will be thus understood that the injectors 1 are
 constantly supplied with the fuel of the required pressure at their fuel
 inlets 11 and fuel outlets 12 through the common fuel supply rail 51. The
 unconsumed fuel remaining in each injector 1 may return to the fuel tank
 52 through a recovery line 55.
 The injectors 1 are supplied with the hydraulically actuating fluid, or
 high-pressurized oil, from a high-pressure fluid manifold 56 through a
 solenoid-operated valve 10. The high-pressure fluid manifold 56 is fed
 with the fluid in a fluid reservoir 57 through a fluid supply line 61 by
 the driving of a fluid pump 58. There are provided a fluid cooler 59 and a
 fluid filter 60 midway in the fluid supply line 61. Moreover the fluid
 supply line 61 is branched into a lubricant line 67 communicating with an
 oil gallery 62 and a hydraulic fluid line 66 communicated with pressure
 chambers 8, shown in FIG. 8, in the injectors 1. A hydraulic pump 63 is
 provided in the hydraulic fluid line 66 while a flow control valve 64
 regulates the fluid supply to the high-pressure fluid manifold 56 from the
 hydraulic pump 63. A controller unit 50 is to control both of the flow
 control valve 64 and solenoids 10 of the injectors 1. The controller unit
 50 is applied with data indicative of the operating conditions of an
 engine, that is, rotational frequencies detected by a rotational frequency
 sensor 68, throttle valve openings detected by a accelerometer 69 and
 crankshaft angles detected by a crank angle sensor 70. The controller unit
 50 is also input with a hydraulic pressure in the high-pressure manifold
 56, which is detected by a pressure sensor 71 in the high-pressure fluid
 manifold 56. The crank angles detected by the crank angle sensor 70 are
 available to control the beginning and duration of the electric conduction
 of the actuating current per cycle, in cooperation with signals from
 sensors indicative that a piston has reached the top dead center or the
 pre-determined position just before the top dead center of the compression
 phase at any standard cylinder or each cylinder.
 FIG. 8 is an axial cross-sectioned view showing an exemplary injector 1
 incorporated in the fuel-injection system in FIG. 7. The injector 1 is
 comprised of a nozzle body 2 formed at a distal end thereof with
 fuel-injection holes 13, a solenoid body 3 having mounted thereon a
 solenoid 15 serving as the electromagnetic actuator, an injector body 4
 and a fuel supply body 5. The injector 1 further includes an intensified
 chamber supplied with fuel from the common fuel supply rail 51, a pressure
 chamber 8 supplied with a hydraulically actuating fluid, a boosting piston
 9 actuated by the hydraulically actuated fluid from the pressure chamber 8
 to apply the pressure to the fuel in the intensified chamber 7, a return
 spring 17 for forcing the boosting piston 9 to return to its neutral
 position, and a casing 6 having a fuel inlet 11 and a fuel outlet 12,
 which are communicated with the common fuel supply rail 51 to thereby
 provide a fuel chamber in the casing 6. In the injector 1 described just
 above, a needle valve 23 may move upwards and downwards by the action of
 the fuel pressure from the intensified chamber 7 to thereby open and close
 the injection holes 13. A solenoid-operated valve 10 has a valve body 16
 that is actuated by the solenoid 15 to regulate the hydraulically actuated
 fluid supplied to the pressure chamber 8. The boosting piston 9 is
 composed of a radially-enlarged portion 25 and a radially-reduced portion
 24, the former portion 25 being arranged for reciprocating movement in a
 first concave 26 in the injector body 4 and provided with a bottom face to
 define partially the pressure chamber 8, and the latter portion 24 being
 arranged for reciprocating movement in a second concave 27 and provided
 with a bottom face to define partially the intensified chamber 7.
 FIG. 9 illustrates fuel-injection characteristics in the injectors, which
 are expressed as the coordinate relation of an actuating pulse width Pw
 versus an volume Q of fuel injected per cycle with taking a parameter of a
 hydraulic pressure in the high-pressure fluid manifold 56, or a rail
 pressure Pr. These characteristics may be obtained by the measurement of
 the volume Q of injected fuel per cycle with respect to the actuating
 pulse width Pw that is at least longer or equal to a pre-determined width.
 According to the characteristics, it will be seen that, as the actuating
 pulse width Pw increases, the duration when the injection holes are open
 becomes longer and then the volume Q of injected fuel per cycle increases.
 It will be further understood that the higher the rail pressure Pr is, the
 higher is the speed of opening the injection holes and the greater is the
 fuel-injection ratio so that the volume of injected fuel increases.
 Disclosed in Japanese Patent Laid-Open No. 49591/1996 is an exemplary
 fuel-injection system, likewise with the system described above with
 reference to FIG. 7, and an injector adapted to be used in the system. The
 injector in the above citation is composed of a control valve, an
 intensifier and a nozzle. Moreover, Published Japanese translations on PCT
 international publication No. 511527/1994 discloses a similar
 fuel-injection system and an injector therefor. In these prior
 fuel-injection systems, controlling the electric conduction timing and
 duration to the electromagnetic actuator makes the fuel-injection start at
 the desired beginning of the fuel-injection and continue for the desired
 duration with the desired fuel-injection pressure, whereby the desired
 volume of fuel per cycle may be injected into the engine.
 The prior injectors for the engines, as described above, are hard to be
 steady, but usually varied or scattered in the fuel-injection
 characteristic owing to the mechanical errors inevitably originating in
 working, assembly or the like of the components. For example, even if the
 solenoid-operated valve in the injector is kept at constant in the
 standard conductive duration thereto, the injectors each are uneven in
 their volumes of fuel injected per cycle. The Japanese Utility Model
 Publication No. 39037/1994 discloses, for example, a fuel supply system
 that has for its object to achieve the moderate fuel-injection control by
 compensating the uneven flow-rate characteristics in the fuel-injection
 valves, thereby preventing the deterioration in output and exhaust
 performances of the engine. In the prior fuel supply system in this
 citation, the fuel-injection valves are previously divided into plural
 subgroups in accordance with the levels in the flow-rate characteristic.
 The engine is provided with a fuel-injection valve matching with any one
 selected subgroup and further provided with resistors each having a
 resistance value corresponding to each subgroup of the flow-rate
 characteristic. There is provided compensating means that may discriminate
 the flow-rate characteristic, depending on the resistance values of the
 resistors, to thereby compensate the pulse width of the injection pulse
 signal in response to the correction value corresponding to the associated
 flow-rate characteristic. The compensating means are further designed such
 that the fuel-injection valve may match with the subgroup of the medium
 flow-rate characteristic when the resistance value is in infinity.
 To cope with the dispersion or scattering in fuel-injection characteristic
 of the injectors, although the improvement in working accuracy of the
 components in the injectors is any one of means for reducing the
 dispersion or scattering in the fuel-injection characteristic, it is very
 hard to completely eliminate such dispersion while improving the accuracy
 in working and assembly results in a steep rise in the production cost of
 the injector. It will be conceived to previously observe the data of the
 relation between the duration conductive to the solenoid-operated valve
 and the volume of the injected fuel at numerous plots for each of the
 individual injectors and store the resultant data into the controller
 unit. Nevertheless, this involves a major problem such that enormous
 efforts are required to take the data and the controller unit must carry
 out the vast steps of calculation, resulting in raising the production
 cost for not only the injector but also the fuel-injection system having
 incorporated the injector therein.
 Instead of previous observation of the fuel-injection characteristics at
 all plotting areas for the individual injectors, it will be conceivable
 that the required fuel-injection control may be realized inexpensively by
 correcting the fuel-injection characteristic in only the standard injector
 to regulate the fuel-injection of the individual injectors. That is, even
 if there is the dispersion or scattering for each injector in the
 fuel-injection characteristic regarding the relation between the standard
 conductive duration of the actuating current to the electromagnetic
 actuator and the volume of fuel injected out of the injection holes, the
 standard fuel-injection (reference fuel-injection) characteristic is
 assigned beforehand to the standard (reference) injector having, for
 example, the central value of dispersion or scattering in fuel-injection
 characteristic. The controller unit may be stored with only the standard
 fuel-injection characteristic in place of the individual fuel-injection
 characteristics in each injector. With attention to a definite correlation
 between the standard fuel-injection characteristic in the standard
 injector regarding the relation of the standard (reference) conductive
 duration of the actuating current versus the volume of injected fuel, and
 the fuel-injection characteristics in the individual injectors regarding
 the relation of the standard conductive duration of the actuating current
 versus the volume of injected fuel, for example, a proportional
 correlation of the standard conductive duration versus the volume of
 injected fuel, the definite correlation may be found out from the
 information relating to a specific point in the fuel-injection
 characteristic of the individual injectors. Hence, the standard conductive
 duration of the actuating current in the individual injectors may be
 determined by the correction of the standard fuel-injection
 characteristic, depending on the definite correlation.
 In general, when the operating load in the engine detected as the
 depression of an accelerator pedal undergoes a change, the pressure in the
 hydraulically actuated fluid forced out from the pump varies while the
 standard conductive duration of the actuating current to the
 solenoid-operated valve is made longer or shorter so that the volume of
 the injected fuel may increase or decrease. It is true that the correction
 of the standard conductive duration defined in a pressure range of the
 hydraulically actuated fluid is usually different from that in another
 pressure range of the fluid. With the hydraulically actuated fluid
 undergoing a pressure change at a pressure range between pressure ranges
 different from each other, the standard conductive duration varies
 stepwise and therefore the actual volume of injected fuel undergoes a
 steep change while the torque from the engine also varies suddenly to
 thereby cause what is known as torque-shock. It is thus preferred that the
 standard conductive duration of the actuating current is kept from its
 steep change even under the pressure variation in the hydraulically
 actuated fluid whereby the engine may be protected from the sudden changes
 in its output power.
 SUMMARY OF THE INVENTION
 A primary object of the present invention is to overcome the shortcomings
 in the prior art as having been described above, and to provide
 inexpensively a fuel-injection system for an engine, which has
 incorporated therein the injectors that are uneven in their fuel-injection
 characteristics. The fuel-injection system of the present invention may be
 provided without a steep rise in the production cost of the injector owing
 to the improvement in finishing accuracy of the components to eliminate
 the dispersion or scattering in the fuel-injection characteristic and also
 without enormous efforts to previously observe the data of the relation
 between the duration conductive to the solenoid-operated valve and the
 volume of the injected fuel at numerous plots for individual injectors.
 An object of the present invention is to provide injectors and a
 fuel-injection system having incorporated therein, which may be
 inexpensively constructed without enormous efforts to previously observe
 the data of the relation between the standard conductive duration to the
 solenoid-operated valve and the volume of the injected fuel at numerous
 plots at every variation of the pressure in the hydraulically actuated
 fluid, and also to provide a fuel-injection system for an engine, which
 may be protected from the torque-shock owing to the sudden change in the
 actual volume of injected fuel at the pressure changes in the
 hydraulically actuated fluid.
 This invention relates to a fuel-injection system for an engine, comprising
 injectors provided with injection holes through which fuel is injected
 into the engine and an electromagnetic actuator applied with an actuating
 current so as to control a hydraulically actuated fluid to open and close
 the injection holes, means for detecting operating conditions of the
 engine, and a controller unit for determining a desired volume of injected
 fuel correspondingly to the operating conditions obtained at the detecting
 means and further regulating a standard conductive duration of the
 actuating current to the electromagnetic actuator, depending on the
 desired volume of injected fuel, to thereby control a volume of fuel
 injected out of the injectors, the controller unit being stored with a
 standard fuel-injection characteristic that has been previously found in a
 relation between the volume of injected fuel versus a standard conductive
 duration whereby the standard conductive duration to the electro-magnetic
 actuator of the injector for determining the desired volume of injected
 fuel is provided by correcting the standard conductive duration, which is
 found depending on the standard fuel-injection characteristic, by using a
 correction quantity (a correction constant).
 Further, this invention relates to a fuel-injection system for an engine,
 comprising injectors each provided with injection holes through which fuel
 is injected into the engine and an electromagnetic actuator applied with
 an actuating current so as to control a hydraulically actuated fluid to
 open and close the injection holes, means for detecting operating
 conditions of the engine, and a controller unit for determining a desired
 volume of injected fuel correspondingly to the operating conditions
 obtained at the detecting means and further regulating the regulating the
 standard conductive duration of the actuating current to the
 electromagnetic actuator as well as the pressure of the hydraulically
 actuated fluid, depending on the desired volume of injected fuel, to
 thereby control a volume of fuel injected out of the injectors, the
 controller unit being stored with a standard fuel-injection characteristic
 that has been previously found in a relation between the volume of
 injected fuel and a standard conductive duration whereby the standard
 conductive duration to the electromagnetic actuator for determining the
 desired volume of injected fuel is provided by correcting the standard
 conductive duration, which is found correspondingly to the standard
 fuel-injection characteristic depending on the standard fuel-injection
 characteristic, by using a correction quantity, the correction quantity
 being obtained for each of plural selected pressure ranges while the
 correction quantity at residual pressure ranges between the selected
 pressure ranges being found by the interpolation of the correction
 quantity at the plural selected pressure ranges.
 In an aspect of the present invention, the correction quantity is a
 correction coefficient to be multiplied by the standard conductive
 duration. On the other hand, in case where the correction quantity is the
 correction coefficient, the controller unit is stored with at least a pair
 of previously observed inherent data consisting of a specified conductive
 duration in the injectors and a specified volume of injected fuel
 corresponding to the specified conductive duration, and the correction
 coefficient is computed in the form of a ratio of the specified conductive
 duration in the injectors to the standard conductive duration that is
 given correspondingly to the specified volume of injected fuel, depending
 on the standard fuel-injection characteristic.
 To find the correction quantity corresponding to each of the pressure
 ranges of the hydraulically actuated fluid, the controller unit is stored
 with previously observed inherent data consisting of pairs of a specified
 conductive duration to the electromagnetic actuator at each of the plural
 selected pressure ranges of the hydraulically actuated fluid and a
 specified volume of injected fuel corresponding to each the specified
 conductive duration, and the correction coefficient is computed
 correspondingly to each of the paired inherent data in the form of a ratio
 of the specified conductive duration to the electromagnetic actuator to
 the standard conductive duration.
 In another aspect of the present invention, the correction quantity may be
 a corrected standard conductive duration to be added with the standard
 conductive duration.
 The injectors are each provided with a solenoid-operated valve having a
 needle valve movable in a body upwards and downwards in a reciprocating
 manner so as to open and close the injection holes and the electromagnetic
 actuator applied with the actuating current to control a hydraulically
 actuated fluid to make the needle valve move upwards and downwards.
 Moreover the injectors are each comprised of an intensified chamber
 supplied with fuel from a common fuel supply rail, a pressure chamber
 supplied with the hydraulically actuated fluid, a boosting piston driven
 by the hydraulically actuated fluid to pressurize the fuel in the
 intensified chamber, a return spring for forcing the boosting piston
 towards its neutral position, and a casing formed with a fuel chamber and
 also a fuel inlet and a fuel outlet, both of which are communicated with
 the common fuel supply rail, the needle valve being made to move upwards
 and downwards dependently on the hydraulic pressure of the fuel from the
 intensified chamber to thereby open and close the injection holes through
 which is injected the fuel, and the solenoid-operated valve being provided
 with a valve body actuated by the electromagnetic actuator to regulate the
 supply of the hydraulically actuated fluid to the pressure chamber.
 In case where the correction quantity is found for each of the plural
 selected pressure ranges of the hydraulically actuated fluid, the
 correction quantity at the residual pressure range between the plural
 selected pressure ranges is given by the linear interpolation of the
 correction quantitys. Further the plural selected pressure ranges and the
 correction quantity for the pressure ranges are of a paired low-pressure
 range and low-pressure correction quantity for the low-pressure range and
 another paired high-pressure range and high-pressure correction quantity
 for the high-pressure range.
 The controller unit is stored with the standard fuel-injection
 characteristic that has been previously found as the relation between the
 standard conductive duration versus the volume of injected fuel and also
 calculates the desired volume of injected fuel depending on the output
 signals from the means that is to detect the operating conditions of the
 engine. No volume of injected fuel out of the injection holes usually
 reaches the desired volume of injected fuel by simply direct supply of the
 actuating current having the standard conductive duration that has been
 defined correspondingly to the standard fuel-injection characteristic. In
 contrast, the controller unit corrects the standard conductive duration
 that is obtained depending on the standard fuel-injection characteristic
 correspondingly to the desired volume of injected fuel. This makes it
 possible to attain the desired volume of fuel injected out of the
 injection holes of each of the individual injectors.
 In case where the correction quantity for compensating the standard
 conductive duration is found correspondingly to each of plural selected
 pressure ranges of the hydraulically actuated fluid, the correction
 quantity of the residual pressure ranges is given by the process of
 interpolating the correction quantity at the selected pressure ranges of
 the hydraulically actuated fluid. The introduction of interpolation
 results in the smooth transition of the correction quantity without sudden
 variation from the selected pressure ranges from the residual pressure
 ranges, so that the volume of fuel injected actually may be undergo no
 steep change.
 The controller unit is stored with at least a pair of previously observed
 inherent data at a specified operating point of each of the individual
 injectors, and the correction coefficient is computed by using the
 inherent data and the standard fuel-injection characteristic. The
 correction coefficient has experimentally been confirmed effectively
 adaptable for other operating points. Hence the standard conductive
 duration to the injectors enough to attain the desired volume of injected
 fuel may be given by multiplying the correction coefficient by the
 standard conductive duration that is obtained correspondingly to the
 desired volume of injected fuel, depending on the standard fuel-injection
 characteristic.
 Moreover, where the correction coefficient is found at plural selected
 pressure ranges of the hydraulically actuated fluid, the controller unit
 is stored with plural pairs of the inherent data at each of specified
 operating points of the individual injectors, and the correction
 coefficients are computed by using the inherent data and the standard
 fuel-injection characteristic. Therefore, the standard conductive duration
 to the injectors enough for attaining the desired volume of injected fuel
 correspondingly to the operating conditions of the engine may be given by
 multiplying the correction coefficient by the standard conductive duration
 that is obtained correspondingly to the desired volume of injected fuel,
 depending on the standard fuel-injection characteristic. In this case, the
 correction quantitys are of a low-pressure correction quantity at the
 low-pressure range and another high-pressure correction quantity at the
 high-pressure range, while the correction quantity at the residual
 pressure range between the selected pressure ranges is given by the linear
 interpolation of the correction quantitys. This procedure may provide the
 simple calculation to find the correction quantity that is effective to
 keep the engine from the torque-shock.
 The fuel-injection system described just above may be adapted to the type
 of injectors that are each provided with a solenoid-operated valve having
 a needle valve movable in a body upwards and downwards in a reciprocating
 manner so as to open and close the injection holes and the electromagnetic
 actuator applied with the actuating current to control a hydraulically
 actuated fluid to make the needle valve move upwards and downwards. In
 particular, the system of this invention is preferred to adapt for the
 injectors that are each comprised of an intensified chamber supplied with
 fuel from a common fuel supply rail, a pressure chamber supplied with the
 hydraulically actuated fluid, and a boosting piston driven by the
 hydraulically actuated fluid to pressurize the fuel in the intensified
 chamber.
 The controller unit provides the standard conductive duration of the
 actuating current, which is to be applied to the electromagnetic actuators
 in the individual injectors, by correcting the standard conductive
 duration corresponding to the desired volume of injected fuel that is
 given depending on the standard fuel-injection characteristic previously
 stored. This makes it possible to inject the desired volume of injected
 fuel with no measurement of the fuel-injection characteristic over the
 whole pressure range at the individual injectors.
 The standard conductive duration in the injectors may be provided by the
 multiplication of the correction coefficient by the standard conductive
 duration given depending on the standard fuel-injection characteristic.
 Hence, the controller unit may provide the standard conductive duration
 through a simple calculating process. In order to find the correction
 coefficient, it may be sufficient to simply store at least a pair the
 inherent data consisting of a specified conductive duration and a
 specified volume of injected fuel correspondingly to the standard
 conductive duration in the injectors with no necessity of troublesome
 effort for gathering the data of the injectors. Consequently, the
 injectors and the fuel-injection system incorporated with the injectors
 according to the present invention may be inexpensively provided
 irrespective of the dispersion or scattering in the fuel-injection
 characteristics of the injectors, because no rise in the production cost
 of the injectors may be necessary for improving the accuracy in finishing
 and assemblage and no huge effort may be necessary for gathering the data
 regarding to the fuel-injection characteristics.
 Moreover, the correction quantity for compensating the standard conductive
 duration to determine the standard conductive duration of the individual
 injectors is given by storing plural pairs of the inherent data consisting
 each of the specified conductive duration and the specified volume of
 injected fuel corresponding to the standard conductive duration of the
 injectors, depending on the plural selected pressure ranges of the
 hydraulically actuated fluid applied in the injectors. While the
 correction quantity at the residual pressure ranges between the selected
 pressure ranges is given by interpolating the correction coefficient.
 Hence, no variation in pressure of the hydraulically actuated fluid causes
 the steep change in the correction coefficient so that the volume of
 injected fuel is eliminated from the sudden change that might otherwise
 result in the torque-shock in the engine. According to the fuel-injection
 system for the engine of the present invention as described above, the
 fuel-injection characteristics of the individual injectors are given by
 using the correction quantity and the process of interpolation, depending
 on the standard fuel-injection characteristic, so that no troublesome
 effort may be necessary for gathering data with taking parameters of the
 standard conductive duration, volume of injected fuel and pressure of the
 hydraulically actuated fluid.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 The present invention will be described in detail with reference to the
 accompanying drawings. It is to be noted that the prior fuel-injection
 system and the injectors shown in FIGS. 7 and 8 are simply adapted to a
 fuel-injection system and injectors incorporated in the system according
 to the present invention. In other words, the fuel-injection system of the
 present invention includes injectors that are each provided with a needle
 valve movable in an injector body in a reciprocating manner to open and
 close injection holes, and a solenoid-operated valve having an
 electromagnetic actuator that is applied with an actuating current so as
 to control a hydraulically actuated fluid for driving the needle valve
 upwards and downwards in a reciprocating manner, whereby the fuel to be
 injected out of the injector is regulated in injection timing and volume
 of injected fuel per cycle by a controller unit in response to the
 operating conditions of the engine. In the following description, the same
 reference character identifies equivalent or same parts or components and
 the repetition of the same parts or components will be omitted.
 On the fuel-injection system for the engine, the controller unit 50 is to
 find a fundamental volume of injected fuel, depending on operating
 conditions of the engine, or a rotational frequency of the engine detected
 by the rotational frequency sensor 68 and a depression of the accelerator
 pedal detected by the accelerometer 55. The controller unit 50 is also
 stored beforehand with the standard fuel-injection characteristic
 representing the relation between the standard conductive duration of the
 actuating current and the volume of injected fuel. The standard
 fuel-injection characteristic is indicative of the data of the standard
 injector that is, for example, located at the central value of dispersion
 or scattering. The standard injector may be of an injector manufactured
 especially for the purpose or an injector having the average
 fuel-injection characteristic. It is to be noted that the actual
 fuel-injection characteristics of the individual injectors in the
 multicylinder engine usually differ from the standard fuel-injection
 characteristic of the standard injector.
 On assemblage of the engine, a correction coefficient obtained by a
 computing routine in FIG. 1 is stored in a memory to compensate or correct
 the individual cylinders. Moreover in operation of the engine, a standard
 conductive duration for the individual injectors, or an actuating pulse
 width that is the ordinary type of an actuating current, may be found by
 using the correction coefficient in the memory along a computing routine
 shown in FIG. 2.
 FIG. 1 is a flow chart of the computing routine for the correction
 coefficient that may be given by the steps described hereinafter. FIG. 3
 is a graphical representation of a standard fuel-injection characteristic
 and other fuel-injection characteristics of the individual injectors, in
 the relation of the actuating pulse width versus the volume of injected
 fuel per cycle. Comparing approximate lines of the slopes at a specified
 operating point, it has been experimentally found that the actual
 fuel-injection characteristics of the individual injectors are different
 from the standard fuel-injection characteristic of the standard injector
 by the dispersion, which is represented as straight lines crossing on the
 ordinate, or y-axis, under the same rail pressures (for example, Pr1,
 Pr2). The standard fuel-injection characteristics A, C and the
 fuel-injection characteristics of the individual injectors B, D in FIG. 3
 are the approximate lines of the slopes at the specified operating points
 under the rail pressures Pr1 and Pr2, whereas the actual data of the
 standard fuel-injection characteristics are mapped as shown in FIG. 9
 while the actual data of the individual fuel-injection characteristics are
 simply provided as the data of the specified operating points as will be
 described hereinafter. The data of the individual injectors may be
 appended, for example, in the form of bar-coded data, following the
 measurement at the production of the individual injectors.
 Step (S1)=The inherent data 1 of the individual injectors are stored. That
 is, if the volume Q1 of the injected fuel were computed when the solenoid
 15 for the electromagnetic actuator was applied with an actuating pulse of
 an actuating pulse width Pw1, which is any standard conductive duration of
 the actuating current, under the rail pressure Pr1 of the hydraulically
 actuated fluid in the high-pressure manifold, the controller unit 50 would
 be stored with a set of inherent data 1 consisting of the rail pressure
 Pr1, actuating pulse width Pw1 and the volume Q1 of injected fuel, all of
 which have been already observed. In this case, the rail pressure Pr1 and
 the actuating pulse width Pw1 are determined on a lower rail pressure Pr1
 and a smaller pulse width Pw1, respectively, corresponding to the low
 load.
 Step (S2)=The standard actuating pulse width Pws1 for the standard
 conductive duration corresponding to the volume Q1 of injected fuel is
 computed depending on the standard fuel-injection characteristic stored in
 the controller unit 50.
 Step (S3)=The correction coefficient K1 (or low pressure correction
 coefficient) corresponding to the inherent data 1 is given as
EQU K1=Pw1/Pws1
 and stored in a memory.
 Step (S4)=Likewise above S1, the inherent data 2 of the individual
 injectors are stored. That is, if the volume Q2 of the injected fuel were
 computed when the solenoid 15 for the electromagnetic actuator was applied
 with an actuating pulse of an actuating pulse width Pw2, which is any
 standard conductive duration of the actuating current, under the rail
 pressure Pr2 of the hydraulically actuated fluid in the high-pressure
 manifold, the controller unit 50 would be stored with another set of
 inherent data 2 consisting of the rail pressure Pr2, actuating pulse width
 Pw2 and the volume Q2 of injected fuel, all of which have been already
 observed. In this case, the rail pressure Pr2 and the actuating pulse
 width Pw2 are determined on a higher rail pressure Pr2 and a larger pulse
 width Pw2, respectively, corresponding to the high load.
 Step (S5)=Likewise S2, the standard actuating pulse width Pws2 for the
 standard conductive duration corresponding to the volume Q2 of injected
 fuel is computed depending on the standard fuel-injection characteristic.
 Step (S6)=Likewise S3, the second correction coefficient K2 (or high
 pressure correction coefficient) corresponding to the inherent data 2 is
 given as
EQU K2=Pw2/Pws2
 and stored in a memory.
 The routine described just above is executed on assemblage of the engine,
 more particular, on electric connection of the controller unit with the
 injectors.
 FIG. 2 is a flow diagram illustrating a computing routine of a standard
 conductive duration of an actuating current to be applied to the
 electromagnetic actuators of the individual injectors, or an actuating
 pulse width, by using the resultant correction coefficients obtained in
 the computing routine of the correction coefficient in FIG. 1. This
 computing routine is combined in the fuel-injection control routine during
 operation of the engine and the actuatingpulse width may be computed by
 the following steps.
 Step (S11)=The operating conditions of the engine are stored. In this step,
 periodically stored in the controller unit 50 are a rotational frequency
 Ne of the engine detected at the rotational frequency sensor 68, a
 depression Ac of the accelerator pedal detected at the accelerometer 69
 and a rail pressure Pr from a pressure sensor 71.
 Step (S12)=The desired volume Qf of fuel to be injected is computed by
 using a previously determined map, for example, a map illustrative of the
 relation of the engine rotational frequency Ne versus the desired volume
 Qf of the injected fuel, with a parameter being taken as the depression Ac
 of the accelerator pedal, depending on the actual engine rotational
 frequency Ne and the actual depression Ac of the accelerator pedal.
 Step (S13)=The standard actuating pulse width Pws for the standard
 conductive duration corresponding to the volume Qf of fuel to be injected
 is computed depending on the standard fuel-injection characteristic stored
 in the controller unit 50.
 Step (S14)=It is discriminated whether or not the rail pressure Pr is less
 than a rail pressure Pri corresponding to a small load such as when
 idling. It is to be noted that the rail pressure Pri is made larger than
 the rail pressure Pri.
 Step (S15)=When the decision (S14) is YES, the correction coefficient K1 in
 the memory is input as the correction coefficient K.
 Step (S16)=When the decision (S14) is NO, it is further discriminated that
 whether or not the rail pressure Pr is more than a rail pressure Prr
 corresponding to a large load such as when operating under a high load. It
 is to be noted that the rail pressure Prr is made smaller than the rail
 pressure Pr2.
 Step (S17)=When the decision (S16) is YES, the correction coefficient K2 in
 the memory is input as the correction coefficient K.
 Step (S18)=When the decision (S14) is NO, a correction coefficient obtained
 as a function f of the rail pressure Pr is input for correction
 coefficient K. The function f(Pr) is linearly interpolated, for example,
 as shown in FIG. 4, but any other suitable interpolation may be fairly
 allowed; and
 Step (S19)=The final actuating pulse width Pw is obtained by the
 multiplication of the standard actuating pulse width Pws calculated at the
 step (S13) by the correction coefficient K1 found at the step (S15), (S17)
 or (S18).
 Following the completion of the routine described just above, other main
 routine or sub-routine, not shown, is executed.
 FIG. 5 graphically represents the fuel-injection characteristics E of the
 individual injectors, after corrected in the actuating pulse width by
 using the standard fuel-injection characteristic A, the individual
 fuel-injection characteristics B and the correction coefficient K2. The
 corrected individual fuel-injection characteristics results from the
 correction executed at a range F corresponding to the higher load, so that
 no correction of the pulse width is available at ranges other than a range
 F where the correction coefficient K2 may function effectively. As
 apparent from the graph in FIG. 5, the fuel-injection characteristics of
 the individual injectors may closely approximate at the corrected range F
 to the standard fuel-injection characteristic of the standard injector.
 FIG. 6 is a graphical representation likewise FIG. 5, in which the
 correction at the ranges exclusive of the range F is also carried out by
 using the process of interpolation, shown in FIG. 4, of the correction
 coefficient. According to FIG. 5, it will be found that the volume Q of
 the injected fuel undergoes steep changes at the boundaries of the
 corrected range. In contrast, the process of the interpolation makes the
 corrected fuel-injection characteristics G approximate closely to the
 standard fuel-injection characteristic A, resulting in eliminating the
 steep change in the volume Q of the injected fuel whereby the engine may
 be protected from the torque-shock.
 Graphically shown in FIG. 10 are both the standard fuel-injection
 characteristic and the fuel-injection characteristics of the individual
 injectors, which are different from FIG. 3 in the scattering pattern. The
 scattering pattern in FIG. 10 is such that the fuel-injection
 characteristics may move in parallel with the standard injector, depending
 on the change of the actuating pulse width versus the volume of injected
 fuel. In this case, the pulse width Pw to be corrected for injecting the
 constant volume Q1 of fuel is given by the deviation .DELTA.Pw (=Pw1-Pws).
 That is, the pulse width to be corrected, or a correction quantity, is
 defined as the deviation of the actuating pulse width Pw1 in the
 individual injectors from the actuating pulse width Pws obtained in
 correspondence with the same volume Q1 of injected fuel for the specified
 operating point, depending on the standard fuel-injection characteristics.
 The actuating pulse width Pw of the individual injectors is obtained by
 adding the correction quantity, or the correction pulse width .DELTA.Pw,
 to the standard actuating pulse width Pws corresponding to the desired
 volume of injected fuel that is determined dependent on the operating
 conditions of the engine.
 It should be understood that the foregoing relates to only preferred
 embodiments of the present invention, and that is intended to cover all
 changes and modifications of the examples of the invention herein chosen
 for the purposes of the disclosure, which do not constitute departure from
 the spirit and scope of the invention.