Abstract:
A fuel pressure control apparatus for cylinder injection engine having a fuel injection valve for injecting fuel into a cylinder of an engine, a pipe for conducting the fuel to the fuel injection valve, a fuel pump for supplying the fuel from a fuel supply system to the pipe, a fuel pressure regulator for regulating the fuel pressure within the pipe by discharging the fuel from the pipe to the fuel supply system, and a control unit for making feed-back control on the fuel pressure within the pipe by applying a control signal determined on the basis of an engine operation parameter to the fuel pressure regulator, wherein the fuel pressure regulator is controlled in a feed-forward manner by a predetermined control value so as to make better fuel pressure control at the time of start and when the fuel pressure transiently changes.

Description:
BACKGROUND OF THE INVENTION 
     The present invention generally relates to fuel pressure control apparatus for cylinder injection engine, and particularly to a fuel pressure control apparatus for cylinder injection engine which is suited to control the fuel pressure in the range from the start of engine to the stationary state, the fuel pressure at the time of the transient from the stationary state, and the fuel pressure at the time when fuel is cut. 
     The conventional fuel pressure control apparatus for cylinder injection engine has a high-pressure fuel pump to be driven by engine, and a low-pressure fuel pump provided on the upper stream side of the high-pressure fuel pump in order that the high-pressure fuel can be directly injected into combustion chambers through high-pressure fuel injection valves that are provided in the respective combustion chambers of the engine. In addition, the fuel pressure in the pipes at the high-pressure fuel injection valves is controlled by fuel pressure adjusting means which makes feed back control so that the actual fuel pressure measured by fuel pressure detecting means can be coincident with a target value that is optimum to the engine speed. 
     In this conventional fuel pressure control apparatus for cylinder injection engine, however, since the fuel pressure is always under the feed-back control, the discharge pressure of the fuel pump driven by the engine is greatly affected by the cranking speed of engine and combustion state. Particularly, at the time of start, it is difficult to control the fuel pressure by feed back. Even if the feed back control is carried out, hunting or the like is caused to make the fuel pressure control the more unstable. 
     An example of the fuel pressure control apparatus with the above drawback removed is proposed as for example disclosed in JP-A-5-149168. In this proposed technique, different fuel pressure control systems are respectively used at the start when the discharge pressure of the high-pressure fuel pump driven by engine is unstable and at the normal driving condition. In this case, an actual fuel pressure signal is supplied to a fuel pressure regulator of the high-pressure fuel system, and a control system makes feed-back control in order to cause the actual fuel pressure value to coincide with a target fuel pressure value. This control system includes start discriminating means for deciding whether the engine starts, and fuel pressure control extent calculating means which supplies a fuel pressure signal for that control extent according to the target fuel pressure value when the engine starts. 
     In the above fuel pressure control apparatus, at the normal driving time, or only when the discharge pressure of the pump based on the engine rotation is stable, the fuel pressure is controlled in a feed-back manner, while at the start when the discharge pressure of the pump is unstable the fuel pressure is controlled in a feed-forward manner by a fuel pressure signal according to only the target fuel pressure value as a fixed amount of control, thereby stabilizing the fuel pressure control. 
     However, this conventional fuel pressure control apparatus is constructed to stop the feed back control when the fuel pressure in the pipes at around the fuel injection valves is most greatly changed, or at the start time. When the engine starts at a low temperature, the voltage of the battery as a driving power supply is reduced, sometimes making even the operation for driving the feed-forward control unstable. Thus, even if the fuel pressure in the pipes at around the fuel injection valves is controlled in a feed-forward manner by the fuel pressure signal according to only the target fuel pressure value as a fixed amount of control, it is not possible to assure the stabilization of the fuel pressure control at the time of start. In addition, at the time of start, it is necessary to raise the fuel pressure in the pipes at around the fuel injection valves as soon as possible, and make the following control operation. The above proposed technique does not consider these points. 
     Moreover, it is necessary that the fuel pressure feed-back control consider the change of the target fuel pressure of engine due to the abrupt change of driving conditions at the transient or the like. In other words, when the target value is abruptly changed, or when the fuel pressure is, for example, instantaneously increased and then decreased, a proper amount of control for feed back is not given, so that the actual fuel pressure sometimes overshoots or undershoots. These defects will be ascribed to the control ability in transients. The above proposed technique considers neither the detailed control to take when the target fuel pressure is suddenly changed, nor the detailed method of setting the amount of reference control as the amount of feed-forward control. 
     Moreover, in this type of fuel pressure control apparatus, foreign matter such as abrasion powder or dust enters into the pipes or the regulator of fuel pressure control means, causing troubles in the pipes and control equipment. The foreign matter must be removed from those places. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a fuel pressure control apparatus for cylinder injection engine of the type in which a fuel pressure signal is supplied to the fuel pressure control means of the high pressure fuel system, controlling the actual fuel pressure to be coincident with a target fuel pressure value by feed-back control, wherein the fuel pressure control at the time of start and the fuel pressure control at the time of transients such as sudden change of target fuel pressure can be improved, and alien substances can be prevented from entering into the fuel supply system and pipes. 
     According to the invention, to achieve the object, there is provided a fuel pressure control apparatus of cylinder injection engine having a fuel pump, fuel injection valves, a pipe system connecting the fuel pump and the fuel injection valves, and a fuel pressure regulator provided in the pipe system and which discharges fuel from the pipe system to thereby adjust the fuel pressure, the fuel pressure regulator being controlled to stop discharge of fuel from the pipe system for a predetermine time from the start of engine. 
     A more specific example of the above fuel pressure control apparatus has fundamental duty calculating means, target fuel pressure calculating means, actual fuel pressure calculating means, fuel pressure correction calculating means, fundamental duty correcting means, and output duty calculating means, wherein the output duty calculating means controls the output duty value to the fuel pressure regulator to be zero for a predetermined time from when the start of engine is detected. 
     Since the above construction of the invention controls the discharge of fuel from the pipe system to stop without supplying an amount of control to the fuel pressure regulator when the engine starts, the fuel fed into the fuel pipes from the fuel pump is not discharged so that the pressure within the pipes can be fast increased, with the result that fuel can be injected stably at the time of engine start. That is, at the time of engine start the fuel pipe system discharges only the necessary fuel to be injected from the fuel injection valves, and is closed at the other time. Since the closed space can be formed only by closing the pressure control opening of the fuel pressure regulator without electric energy, the effect of power supply voltage or the like can be avoided, and the pressure in the pipes can be increased fastest. 
     In addition, in the above fuel pressure control apparatus, the fundamental duty calculating means computes a fundamental duty value on the basis of a load signal to the engine and revolution rate of engine, and the fuel pressure correction calculating means calculates a fuel pressure control deviation and a duty correction value on the basis of the target fuel pressure and actual fuel pressure, and decides whether feed-back control is permitted. The fundamental duty value is corrected according to the duty correction value and supplied to the output duty calculating means. The output duty calculating means, at the time of start, selectively makes the closing control of the fuel pressure regulator, feed-forward control or feed-back control on the basis of the battery voltage or a signal for fuel cut or the like. 
     Also, according to another embodiment of the fuel pressure control apparatus for cylinder injection engine of the invention, the fuel pressure regulator is controlled to keep the fuel discharge opening in a certain state for a constant time after a certain time has elapsed from the start of engine, and then controlled in a feed-back manner so that the actual fuel pressure reaches the target fuel pressure. In other words, after a predetermined time has elapsed from when the start of engine is detected, the output duty value to the fuel pressure regulator is controlled to be constant for a constant time, and then the fuel pressure correction value calculating means makes feed-back control so that the actual fuel pressure can reach the target value. 
     According to another embodiment of the invention, when the target fuel pressure to be determined by driving conditions is changed by a predetermined amount, the feed-back control is stopped, and the fuel pressure is controlled on the basis of the amount of feed-back at that time and the reference amount of control that depends on that driving conditions, and then placed under the feed-back control after the target fuel pressure and actual fuel pressure continue to be maintained in certain ranges for a predetermined time under the reference amount of control. 
     Thus, according to the invention, when the target fuel pressure is suddenly changed, for example instantaneously increased and then decreased, the feed-back control is stopped, and the fuel pressure is controlled in a feed-forward manner on the basis of not only the reference duty value (reference amount of control) but also the amount of feed back indicated when the feed-back control is stopped in order to absorb the scattering, thereby improving the ability of control at the transient time. In addition, when the feed-back control resumes, the fuel in the pipes is stable, and thus the actual fuel pressure can be well converged. 
     Moreover, when the feed-back control is started, decision is made of whether the actual fuel pressure is in a given range or not. If it is out of the range, occurrence of some trouble is detected, and the feed-back control can be inhibited. Here, whether it is in a certain range or not is decided by the maximum deviation of scattering on the basis of the temperature and voltage of the fuel pressure regulator, and its deterioration with time. 
     According to still another embodiment of the fuel pressure control apparatus of the invention, the fuel pressure regulator is controlled to cause the pipe system to discharge the maximum fuel under the condition that the fuel injection valves are controlled to inject the minimum amount of fuel including no injection. In other words, the output calculating means controls the output duty to the fuel regulator to be 100% under the condition that the fuel injection valves are controlled to inject the minimum amount of fuel including no injection. 
     According to another preferred embodiment of the invention, when the injection valves are released from the minimum injection control including non-injection, a certain amount of control is added to the amount of control calculated on the basis of feed-back control. 
     Thus, according to the invention, when the fuel injection valves are placed under the minimum injection control including non-injection, for example, when the accelerator pedal is released, the area of the opening of the fuel pressure regulator can be maximized, thus making it possible to positively remove the foreign matter such as abrasion powder or dust collected within the pipe system and fuel pressure regulator. 
     When the area of the opening is the maximum under the driving state, the pipe system generally increases discharge gas to reduce the fuel pressure to a predetermined value or below, but under the fuel cut state, the reduction of the pressure within the pipe system has no effect. In addition, since at the time of recovery it is necessary to swiftly return to a preset pressure, a certain amount of control offset is given, and fuel control is made to fast stabilize the fuel pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a general view of a cylinder injection engine having a fuel pressure control apparatus of one embodiment of the invention. 
     FIG. 2 is a block diagram of an example of the control unit shown in FIG.  1 . 
     FIG. 3 is a longitudinal cross-sectional diagram of one example of the construction of the variable fuel pressure regulator shown in FIG.  1 . 
     FIGS. 4 and 5 are graphs showing characteristics of the variable fuel pressure regulator. 
     FIG. 6 is a block diagram of the control unit of FIG. 1 showing the concept of the fuel pressure control operation. 
     FIG. 7 is a target fuel pressure map diagram of the target fuel pressure control means shown in FIG.  6 . 
     FIG. 8 is a fundamental duty map diagram of the fundamental duty calculating means shown in FIG.  6 . 
     FIG. 9 is a map diagram of another example of those shown in FIGS. 7 and 8. 
     FIG. 10 is a graph showing a characteristic of the fuel pump shown in FIG.  1 . 
     FIG. 11 is a diagram showing one example of the control characteristics of the variable fuel regulator shown in FIG.  3 . 
     FIG. 12 is a timing chart to which reference is made in explaining the operation of the invention. 
     FIGS. 13 to  15  are flowcharts to which reference is made in explaining an example of the operation of the invention at the start time of engine. 
     FIGS. 16 and 17 are flowcharts to which reference is made in explaining the operation in the case where the target value is greatly changed in the fuel pressure control apparatus of the invention. 
     FIG. 18 is a flowchart showing one example of the operation of the invention in the case where the engine is stopped. 
     FIG. 19 is a flowchart to which reference is made in explaining one example of the operation for the compensation of battery voltage change. 
     FIG. 20 is a timing chart to which reference is made in explaining the operation at the time of fuel cut. 
     FIGS. 21 and 22 are flowcharts showing one example of the operation at the time of fuel cut. 
     FIG. 23 is a flowchart showing an example different from that shown in FIG.  22 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     One embodiment of the fuel pressure control apparatus for cylinder injection engine according to the invention will be described in detail with reference to the accompanying drawings. 
     FIG. 1 shows the whole construction of an engine system including this embodiment of the fuel pressure control apparatus for cylinder injection engine. Referring to FIG. 1, there is shown an engine  507  that is composed by each cylinder including a piston  507   a , a cylinder  507   b  and a combustion chamber  507   c  formed by the piston  507   a  and cylinder  507   b , and by an inlet pipe  501  and exhaust pipe  519  provided above the combustion chamber  507   c  to be connected to the chamber. 
     The air to be sucked into the engine  507  is introduced from an inlet  502   a  of an air cleaner  502 , and fed through an air flow sensor  503  and through a throttle body  505  having a throttle valve  505   a  housed for controlling the sucked air flow to a collector  506 . The air in the collector  506  is distributed into the inlet pipes  501  which are respectively connected to the cylinders  507   b  of the engine  507 , and fed to each cylinder  507   b . The throttle valve  505   a  can be opened and closed by a motor  522 . The exhaust gas after combustion from the combustion chamber  507   c  is discharged through the exhaust pipe  519  and a catalyst  520 . 
     On the other hand, fuel such as gasoline from a fuel tank  514  is subjected to a first pressure by a fuel pump  510  and supplied to a pipe  541 . In addition, the fuel in the pipe is subjected to a second pressure by a fuel pump  511 , and fed to a pipe  542 . This pipe  542  is composed by two up and down pipes between which an injector  509  is interposed to form a fuel pipe system. The fuel subjected to the first pressure by the fuel pump  510  and fed to the pipe  541  is adjusted to be kept at a constant pressure (for example, 3 kg/cm 2 ) by a fuel pressure regulator  512 , and it is secondly pressed by the fuel pump  511  to be raised to a higher pressure, and fed to the pipe  542 . The fuel fed to the pipe  542  is adjusted to be at a constant pressure (for example, 70 kg/cm 2  by a fuel pressure regulator  513 , and injected into the cylinder  507   b  from the injector  509  that is provided in each cylinder  507   b  of the engine  507 . 
     In addition, the fuel pressure within the pipe  542  between the fuel pump  511  and the injector  509  is fundamentally controlled by the fuel pressure regulator  513 . If an amount of control is not supplied to this fuel pressure regulator  513 , or if the control system is disabled, a mechanical regulator  540  is instead operated to adjust. 
     The fuel injected from the injector  509  is ignited by an ignition coil  508  in response to an ignition signal boosted to a high voltage by an ignition coil  522 . 
     The air flow sensor  503  generates a signal indicating a sucked air flow, and supplies it to a control unit  515 . The throttle body  505  has a throttle sensor  504  mounted to detect the degree of opening of the throttle valve  505   a . The output signal from the sensor is also supplied to the control unit  515 . 
     A crank angle sensor  516  mounted on a cam shaft (not shown) of the engine  507  generates a reference angle signal REF indicating the rotational position of the crankshaft and an angle signal POS for detecting a rotation signal (revolution rate) signal, and also supplies them to the control unit  515 . 
     On the upstream side of the catalyst  520  of the exhaust pipe  519 , there is disposed an A/F sensor  518 . An accelerator opening sensor  521  is also provided in the engine  507 . The signals from these sensors are also supplied to the control unit  515 . 
     The main portion of the control unit  515  is formed by an MPU, a ROM, a RAM, an I/O LSI including an A/D converter, and so on. The control unit receives the signals from the above-mentioned sensors which detect the driving conditions of the engine, executes predetermined computing processes, generates various different control signals as a result of the calculation, and supplies certain control signals to the injector  509  and ignition coil  522 , thus making fuel supply control and ignition timing control. 
     Particularly as to the pressure control, the fuel pressure sensor  523  detects the pressure within the pipe  542  and supplies the control signal to the fuel pressure regulator, or variable pressure/regulator (variable P/Reg)  513 . This control signal controls the time in which the valve of the variable P/Reg  513  is opened, or the duty value indicating the ratio of the valve-open time to the valve-closed time. FIG. 3 is a longitudinal cross-sectional view of the variable P/Reg  513 . The fuel within the pipe  542  enters into the variable P/Reg  513  from the IN side shown in FIG. 3, and discharged into the pipe  541   a  on the OUT side shown in FIG. 3, or fed back to the pipe  541 . When no control signal is supplied to the variable P/Reg  513  from the control unit  515 , a ball valve  701  of the variable P/Reg  513  is pressed against a valve seat  700  by a spring  702 , and the fuel from the IN side of the pipe  542  is not discharged to the pipe  541   a  side. Since the fuel is not discharged from the pipe  542 , the pressure within the pipe  542  is determined by the amount of fuel discharged from the fuel pump  511  and the amount of fuel injected by the injector  509 . However, the maximum pressure within the pipe  542  is limited by the mechanical regulator  540 . 
     The amount that the ball valve  701  is controlled by the variable P/Reg  513  is adjusted by the duty value supplied as the duty signal to an electromagnetic coil  703  from the control unit  515 . In other words, the sucking force of a plunger  704  which supports the ball valve  701  is controlled by the average current of the electromagnetic coil  703  so as to control the amount of fuel escaped from the valve seat  700 . Thus, the actual pressure within the pipe  542  can be controlled to reach the target fuel pressure. FIG. 4 is a graph showing a fundamental characteristic of the variable P/Reg  513 , in which the ordinate indicates the fuel pressure within the pipe  542 . If the duty value is 0%, or if the amount of control is zero, the pressure within the pipe  542  becomes high since there is no escape fuel. However, the upper limit of the pressure is suppressed by the mechanical regulator  540 . If the duty value increases, the escape fuel from the pipe  542  increases, thus reducing the pressure as shown in the graph. 
     FIG. 5 shows a typical example of the voltage characteristic of the variable P/Reg  513 . From FIG. 5, it will be seen that if the driving voltage, or battery voltage is changed, the driving current is changed even under constant duty so that the fuel pressure varies. Thus, it will be understood that the variation of the battery voltage can be compensated by changing the duty value. 
     FIG. 6 is a block diagram showing the control operation to be executed by the control unit  515 . 
     Fundamental duty calculating means  101  calculates an engine revolution rate Ne from the detected signal which it receives from the crank angle sensor  516 , and an engine load T from the detected signal which it receives from the accelerator opening sensor  521 , and determines the fundamental duty value for controlling the variable P/Reg  513  on the basis of those calculated values. Target fuel pressure calculating means  102  also determines the target fuel pressure from the engine revolution rate Ne and the engine load T. Actual fuel pressure calculating means  103  converts the received detected value from fuel pressure sensor  523  into an actual fuel pressure. 
     Fuel pressure correction calculating means  104  compares the target fuel pressure from the target fuel pressure calculating means  102  and the actual fuel pressure from the actual fuel pressure calculating mean  103  to produce a deviation between both, and calculates an amount of fuel pressure correction on the basis of that deviation. The fuel pressure correction calculating means  104  also confirms if the target fuel pressure value is in a predetermined range and decides if the feed-back control can be permitted. 
     Limiter process means  105  sets the upper and lower limits of the amount of fuel correction calculated by the fuel pressure correction calculating means  104 . Fundamental duty correction means  107  corrects the fundamental duty value from the fundamental duty calculating means  101  on the basis of the amount of fuel pressure correction from the limiter means  105 , and produces a corrected duty value. 
     Output duty calculating means  106  decides if the driving state of engine is starting or at the time of F/C (fuel cut). The output duty calculating means  106  corrects the amount of control (duty value) to be applied to the variable P/Reg  513  in accordance with the driving voltage, and produces the corrected amount of control, since the duty-fuel pressure characteristic is changed with the driving voltage fed to the variable P/Reg  513  as shown in FIG.  5 . 
     The operation of this embodiment will be further described in detail with reference to FIG.  6 . 
     FIG. 7 shows a target fuel pressure map for reading a target fuel pressure on the basis of the engine revolution rate Ne and the load torque T. The load torque is computed by the well-known method on the basis of the detected signals from the accelerator opening sensor  521 , air flow sensor  503 , throttle sensor  504  and air-fuel ratio sensor  518 . The accelerator opening sensor  521  of these sensors is typically shown in FIG.  6 . The target fuel pressure calculating means  102  reads out the target fuel pressure value with reference to FIG. 7 on the basis of the engine revolution rate Ne and load torque T. FIG. 8 is a fundamental duty map for reading out the fundamental duty value by the fundamental duty calculating means  102  on the basis of the engine revolution rate Ne and load torque T as in FIG.  7 . Although the load axis is represented by torque in FIGS. 7 and 8, the load axis in the target fuel pressure map and fundamental duty map may be shown as in FIG. 9 by the amount of fuel, q which has a close mutual relation with the degree of accelerator opening and which is injected from the injector  509 . 
     According to one embodiment of the invention, the fuel pump  511  is driven directly by the engine. As shown in FIG. 10, the amount of fuel discharged from the pump  511  is proportional to the revolution rate of engine, Ne, or the revolution rate of the pump. At the operating point indicated in FIG. 10, the amount of fuel discharged from the pump is represented by Q when the revolution rate of the engine is N3. 
     FIG. 11 is a graph showing a fuel pressure vs. fuel return amount (escaped amount of fuel) characteristic with a parameter of duty value in the variable P/Reg  513 . In FIG. 10, when the revolution rate of the engine is N3, the amount of fuel discharged from the pump is Q. In FIG. 9, at the operating point indicated by broken lines, when the revolution rate of the engine is N3, the torque is T2, or the amount of injected fuel is q2. The fuel return amount Qret in the variable P/Reg  513  is expressed by 
     
       
         Qret=(Amount of fuel discharged from pump)−(amount of injected fuel) 
       
     
     At the above operating point, since the amount of fuel discharged is Q, and the amount of injected fuel in all cylinders is q2, the fuel return amount Qret can be definitely calculated from the relation of Qret=Q−q2. In addition, since the fundamental duty value at the operating point N3, T2 is 60% with reference to FIG. 8, the fuel pressure Pset in the pipe  542  at the above operating point can be determined from the intersection point between the return amount Qret line and the fundamental duty 60% line with reference to FIG.  11 . Particularly, from FIG. 9 it will be understood that the fundamental duty value can be determined on the basis of the revolution rate of engine and the amount of injected fuel. 
     The operation of the control unit  515  will be described with respect to time from the start of engine. 
     Before the start of engine, since the control unit  515  supplies no control signal to the variable P/Reg  513 , the variable P/Reg  513  is the minimum, or zero in its opening area, and thus the amount of fuel to be returned to the pipe  541  from the pipe  542  through the pipe  541   a  is zero. 
     Under this condition, at time t0 in FIG. 12, when the ignition switch is turned on, the operation flows shown in FIGS. 13 and 14 are started predetermined times later. In the flow of FIG. 13, at step  5001  decision is made of whether the starter switch is turned from on-state to off-state. Since the starter switch is now in the off state, the decision is No, and thus the program goes to step  5002 . At step  5002 , decision is made of whether the engine speed Ne is larger than a revolution rate Nset. Since the engine is now not started yet, the decision is No, and thus the program goes to step  5003 , where the start flag is set. Then, the program ends this flow. 
     When the operation flow in FIG. 14 is started, at step  4001  decision is made of whether the start flag set at step  5003  in FIG. 3 is set or not. Since the start flag is now set, the decision is Yes, and thus the program goes to step  4002 , where the output from the output duty calculating means  106  is set at 0%. Then, the program ends this flow. 
     When the starter switch is turned on at time t1 in FIG. 12, at step  5001  in FIG. 13 the decision is Yes, and the start flag is set. Then, when the starter switch is turned off at time t2, at this step the decision is No, and the program goes to step  5002 . At step  5002 , the decision is No when the engine speed Ne is not larger than the predetermine value Nset, and thus the start flag is kept set. Therefore, as in the operation flow shown in FIG. 14, the output duty value of the variable P/Reg  513  is maintained to be 0%. If at step  5002  the engine speed Ne is decided to have reached the predetermined value or above (time t3), or if the engine is decided to have started, or be Yes, the program goes to step  5004 , where the start flag is cleared. Then, the program ends this flow. Thereafter, as long as the revolution rate Ne is maintained to be the predetermined value Nset or above, the start flag is continuously cleared at step  5004 . After the start flag is cleared, at step  4001  in FIG. 15 the decision is No, and the program ends this flow. Thus, at time t3, the control for the output duty value of 0% ends. 
     The operations shown in FIGS. 13 and 14 are useful for explaining the operation of the output duty calculating means  106  in FIG.  6 . Thus, since the fuel pressure within the pipe  542  is low when the engine is started, the output duty calculating means  106  controls the duty to be 0% so that the fuel discharged from the pump  511  is not again returned to the pipe  541  side by the variable P/Reg  513 , and the pressure within the pipe  542  to fast increase, thus swiftly bringing about the state in which normal fuel injection can be made. 
     In this way, the engine ends the start condition at time t3, but the actual fuel pressure is not stabilized yet. When the feed-back control is immediately performed from this state, the engine operation may become unstable. Thus, in this embodiment, the feed-back control is not executed until it is confirmed that the actual fuel pressure within the pipe  542  is kept stable over a predetermined time. 
     FIG. 15 shows the operation flow that is executed in parallel with the above operation. A fixed duty flag is raised for a certain time Tmax after the starting state end time t3, and the variable P/Reg  513  is controlled by the fixed duty. The fixed duty may be replaced by the fundamental duty. In this case, the fuel pressure correction calculating means  104  does not produce any amount of correction, and the fundamental duty itself is supplied to the output duty calculating means  106  so that the feed-forward control is performed. The operation flow of FIG. 15 is started every certain time. 
     With reference to FIGS. 16 and 17, a description will be made of the operation flow for confirming that the actual fuel pressure has been maintained stable within a certain range for a predetermined period of time. 
     FIG. 16 shows the operation flow which is started every certain time to detect that the target fuel pressure has been greatly changed to exceed a predetermined value. Referring to FIG. 16, when this operation flow starts, first at step  3001  a target fuel pressure value is searched for from the target fuel pressure calculating means  102 . At step  3002 , the searched target fuel pressure value is compared with the previous one so that decision is made of whether it is equal to or larger than that. Since the flow is now between time t3 and time t4, the target fuel pressure is not greatly changed. Here, the decision is No, and at step  3005  the feed-back start decision flag is set. Then, the program ends this flow. 
     FIG. 17 shows the operation flow which is started every predetermined time to confirm that the actual fuel pressure has been maintained stable for a certain time and execute the feed-back control. Referring to FIG. 17, when this flow is started, at step  1001  decision is made of whether the feed-back decision flag that is to be set at step  3005  in FIG. 16 is set or not. Since the feed-back start decision flag is now set, the decision at step  1001  is Yes. At step  1002 , decision is made of whether the actual fuel pressure is between the upper limit High and the lower limit Low, or whether the actual fuel pressure has been maintained stable within a predetermined range. Here, if the decision is No, at step  1005  the feed-back operation is not allowed, and the feed-back allowance flag is cleared. Then, the program ends this flow. If the decision at step  1002  is Yes, at step  1003  decision is made of whether the actual fuel pressure has been maintained stable for a predetermined time. Here, if the decision is No, a timer makes continuous counting, and the program goes back to step  1002 . Thus, the above operation is repeated. If the decision at step  1003  is Yes, the program goes to step  1004 , where the feed-back allowance flag is set, and the feed-back start decision flag is cleared. Then, the program ends this flow. 
     When the feed-back allowance flag is set, the feed-back control is performed under normal conditions, but at the start time the feed-back control is not started when the operation flow in FIG. 15 is before time t4. When the feed-back start decision flag is set at step  3005 , and when time t4 has passed, the feed-back control is executed. 
     While the feed-back control is being performed, the fuel pressure correction calculating means  104  calculates the deviation ΔP between the target fuel pressure obtained by the target fuel pressure calculating means  102  and the actual fuel pressure detected by the fuel pressure sensor  523 , and multiplies this deviation by a predetermined value to produce a proportional control amount Pc. In addition, when this deviation ΔP is larger than a predetermined value (for example, 5 Kg/m 2 ), a fixed value is added to the previous integral control amount to produce an integral control amount Ic. Then, the sum of the proportional control amount Pc and the integral control amount Ic is produced as an amount of feed-back control. The fundamental duty correction means  107  corrects the fundamental duty value from the fundamental duty calculating means  101  with the amount of feed-back control from the fuel pressure correction calculating means  104 , and supplies the corrected duty value to the output control means  513 . 
     When, during the execution of the feed-back control, it is detected that the target fuel pressure at step  3002  in FIG. 16 has been changed to be larger than a predetermined value as shown after time t5 in FIG. 12, the decision at this step is Yes, and the program goes to step  3003 . At this step, the amount of feed-back at that time, or the output from the fuel pressure correction calculating means  104  at that time is stored in a memory  110 , and the feed-back allowance flag is cleared. Then, the program ends this flow. When the feed-back allowance flag is cleared, the feed-back control is stopped, and the fuel pressure correction calculating means  104  does not supply the amount of correction to the fundamental duty correcting means  107 . The period of this feed-forward control corresponds to the period from time t5 to time t7 in FIG.  12 . During this period, the feed-forward control is performed on the basis of the duty value that is the sum of the fundamental duty value and the amount of feed-back stored at step  3003 . 
     Under this feed-forward control, when the target fuel pressure is decided to have changed less than or equal to a predetermined value at step  3002  in FIG. 16, the feed-back start decision flag is set at step  3005 . Thus, at step  1001  in FIG. 17, the decision is Yes. At step  1002  and the following steps, it is confirmed that the actual fuel pressure has been maintained stable within a certain range and the feed-back allowance flag is set as in the above description. As a result, the feed-back control is again started from time t7 in FIG.  12 . 
     It is desired that the fuel pressure within the pipe  542  be kept high considering the case in which the engine is stopped for some reason and again started. FIG. 18 shows the operation flow for maintaining the fuel pressure high when the engine is stopped. This operation is performed by the output duty calculating means  106 . 
     Referring to FIG. 18, first at step  6001  decision is made of whether the ignition switch is turned off from the on state. Here, if the decision is Yes, the output duty 0% flag is set first of all at step  6002 , considering the case where the engine is started immediately after the stop of the engine. Consequently, the output duty calculating means  106  controls to makes the following control duty 0%, and maintain the pressure within the pipe  542  to be left unchanged. If the ignition switch is not turned off, the decision at step  6001  is No. At step  6003 , decision is made of whether the engine is stopped. If the decision here is Yes, the program goes to step  6002 , where the output duty 0% flag is set considering the case in which the engine is immediately again started. The output duty calculating means  106  makes the following control duty 0%, and maintains the pressure within the pipe  542  left unchanged. If the decision at step  6003  is No, the program goes to step  6004 , where the output duty 0% flag is cleared. Then, the program ends this flow. 
     The battery voltage is changed at low temperatures or by its deterioration due to the change with lapse of time. The characteristic of the relation between control duty and fuel pressure is changed depending on the battery voltage as described with reference to FIG.  5 . Therefore, when the battery voltage is reduced to be smaller than the normal value, it is necessary to compensate for that change. FIG. 19 shows the flow for the compensation. This flow is started every predetermined time to be performed by the output duty calculating means  106 . At step  7001 , computation is made for the correction of the battery voltage. In this correction computation, since the fundamental duty is defined on the basis of the battery voltage, the output duty value is corrected by the actual battery voltage. In other words, the corrected duty Dout can be expressed by 
     
       
         Dout=Dc×(Vbase/Vb) 
       
     
     where Dc is the output duty, Vbase is the reference battery voltage, and Vb is the actual battery voltage. 
     When the minimum fuel injection condition corresponding to substantially zero fuel injection is satisfied, the F/C signal indicating fuel cut is generated, thus controlling the control duty value to be 100%, and the opening area of variable P/Reg  513  to be the maximum. Under this condition, the fuel pressure within the pipe is not under a particular control, and the injector  508  injects no fuel. Therefore, since the fuel discharge area is the maximum, the abrasion powder discharged from the pump  511  and dust flowing into the pipe  542  can be removed through the variable P/Reg  513  particularly from the valve seat  700  and ball valve. This operation is performed by the output duty calculating means  106 . The operation will be described with reference to FIGS. 20 to  22 . 
     When the F/C signal is received at time ta so that the duty value of 100% is brought about, the actual fuel pressure is decreased away from the target value after that time point as shown in FIG.  20 . Then, when the F/C signal falls off at time tb, the actual fuel pressure cannot be suddenly shifted from the lower value to the target value set in the feed-back control even if the same amount of control as when the F/C signal is generated is applied. As indicated at time tb and the following in FIG. 20, the fuel pressure gradually approaches to the target value with a time lag. Since it is desired that the actual fuel pressure swiftly follow the target value in the engine control, it is necessary to fast remove the time lag. The output duty calculating means  106  controls the lag in the fuel pressure restoration to be the minimum when the original state is recovered from the F/C state. That is, the final control duty value is given an offset to be small, accelerating the fuel pressure restoration. 
     FIG. 21 is the operation flow which is started every predetermined time to detect the F/C signal at time ta. When this operation is started, decision is made of whether the F/C signal is applied or not at step  8001 . If the decision here is Yes, the program goes to step  8002 , where the amount of feed-back, F/B produced from the fuel pressure correction calculating means  104  at that time is stored in the memory  110 , and the duty 100% flag is set. Then, the program ends this flow. If the decision at step  8001  is No, the program goes to step  8003 , where the duty 100% flag is cleared. Then, the program ends this flow. While the duty 100% flag is set and maintained to be left unchanged at step  8002 , the output duty calculating means  106  causes the variable P/Reg  513  to operate at duty 100%, and the abrasion powder and dust to be quickly removed from the pipe  542  and variable P/Reg  513 . 
     FIG. 22 shows the operation flow at tb at which the F/C signal falls off. When this flow is started, at step  8010  decision is made of whether the duty 100% flag to be set at step  8002  in FIG. 21 is once set and then cleared. If the decision here is No, the program ends this flow. If the decision is Yes, the F/C signal falls off at time tb, and thus at step  8011  the duty is set at the recovery time. The recovery-time duty is determined by adding the fundamental duty produced from the fundamental duty calculating means  101  at that time, the feed-back amount F/B stored in the memory at step  8002  in FIG. 21, and a predetermined offset for reducing the control duty value. Since the feed-back control on the fuel pressure is performed on the duty value determined in this way, the fuel pressure within the pipe  542  can be quickly restored by applying this offset. The offset is empirically determined. In addition, the offset can be released from when the fuel pressure is well recovered. 
     The offset can be gradually decreased with the lapse of time, and finally made zero. FIG. 23 shows the operation flow for making this operation. Before this operation flow is executed, the attenuation flag is set at step  8011  in FIG.  22 . Then, the flow of FIG. 23 is started. First at step  8021 , it is checked if the attenuation flag is set at step  8021 . If the decision here is No, this flow ends. If the decision is Yes, the program goes to step  8022 , where a minute amount of offset is subtracted from the current offset, and the remainder, or new offset is set. Then, at step  8023 , decision is made of whether the new offset has become zero or negative. If the decision here is No, this flow ends. If the decision is Yes, the offset can be considered to be substantially zero, the program goes to step  8024 , where the attenuation flag is cleared. Then, this flow ends. At step  8023  the offset may be compared with a certain small value in place of zero. Since the operation flow of FIG. 23 is repeatedly performed every predetermined time, the offset of the control duty is reduced stepwise as shown in FIG. 20 after time tb.