Fuel pressure control apparatus for fuel injection system of engine

Abstract: An apparatus for controlling fuel pressure in an engine. An accumulator, or a common rail (4), stores highly pressurized fuel sent from a pump (6). Injectors (2) inject fuel stored in the common rail (4) into combustion chambers of the engine. An ECU (50) controls a pressure control valve (10), which is provided in the pump (6), to adjust the fuel pressure in the common rail (4) according to the running state of the engine. When a key switch (52) is turned off to stop the engine, the ECU (50) adjusts the fuel pressure in the common rail (4) to a value that is suitable for subsequently restarting the engine. Therefore, starting of the engine is improved.

Description: BACKGROUND OF THE INVENTION The present invention relates to fuel injection systems for engines that temporarily store highly pressurized fuel in an accumulator and inject the fuel into combustion chambers. More particularly, the present invention pertains to a fuel pressure control apparatus that optimizes the fuel pressure in the accumulator. Japanese Unexamined Patent Publication No. 7-103095 discloses such a fuel injection system. The system includes a common conduit, or accumulator, connected to a high pressure pump. The common conduit is known as a "common rail" in the art. The common rail is connected to electromagnetic valve type injectors. The pressure in the common rail is detected by a pressure sensor. The high pressure pump and the injectors are controlled by an electronic control unit (ECU). When the engine is started, the high pressure pump supplies fuel to the common rail, which, in turn, temporarily stores the fuel at a high pressure. When the ECU opens the injectors, fuel having the same pressure as the fuel in the common rail is injected into the combustion chambers of the engine. The ECU optimizes the fuel pressure in the common rail, or the injection pressure of the injector, based on the running state of the engine such as the engine speed and the amount of fuel injection. Particularly, the ECU computes a target value of the fuel pressure in the common rail based on the engine running state. The ECU generally increases the target value as a greater load acts on the engine. When the fuel pressure in the common rail detected by the pressure sensor is lower than the target value, the ECU controls the high pressure pump to increase the amount of fuel supplied to the common rail. When the fuel pressure in the common rail is greater than the target value, the ECU controls the high pressure pump to decrease the amount of fuel supplied to the common rail. When the key switch is turned off, the high pressure pump simultaneously stops sending fuel to the common rail. Therefore, even if the injectors remain open (due to a malfunction), the fuel pressure in the common rail drops rapidly when the key switch is turned off and fuel injection from the injectors is stopped. Thus, unwanted fuel injection is prevented after the key switch is turned off. When key switch is turned off, the fuel pressure in the common rail is maintained during the consequent non-operational state of the engine as long as there is no malfunction in the injectors. When the engine is started again, fuel is injected under the same pressure that existed when the key switch was turned off. Therefore, the injection pressure available when the engine is started greatly varies, depending on the running state of the engine when the key switch was previously turned off. This causes the following drawbacks. For example, if the key switch is turned off immediately after the fuel pressure in the common rail has been lowered, the fuel pressure will be insufficient when the engine is started again. As a result, the fuel injection will not be started until the fuel pressure in the common rail is raised to a sufficient level for starting the engine. Even if the fuel injection is started, the fuel pressure may not be enough high to supply sufficient fuel to the combustion chambers. In this case, atomization of fuel will be insufficient. This delays the starting of the engine. On the other hand, there are cases where the fuel pressure in the common rail is maintained relatively high. For example, the fuel pressure in the common rail is maintained high when the engine is racing or immediately after the engine stops racing. "Racing" of an engine refers to high engine speed when substantially no load is acting on the engine. When an engine is racing, the fuel pressure in the common rail is increased as the amount of fuel injection is increased. Therefore, if the key switch is turned off while the engine is racing or immediately after the engine was raced, the fuel pressure in the common rail is maintained too high. Thus, the fuel pressure in the common rail is higher than the level suitable for starting the engine. If fuel injection is started at a pressure higher than the appropriate pressure when the engine is started, the fuel is overly atomized for starting the engine. This causes the firing pressure in the combustion chambers to be rapidly changed thereby causing noise. SUMMARY OF THE INVENTION Accordingly, the objective of the present invention is to provide a fuel pressure control apparatus for fuel injection systems that improves the starting of an engine. To achieve the above objective, the present invention provides an apparatus for controlling fuel pressure in an engine. The apparatus comprises an accumulator for storing highly pressurized fuel sent from a pump, an injector for injecting fuel stored in the accumulator into a combustion chamber of the engine, and an adjuster for adjusting the pressure of fuel in the accumulator according to the running state of the engine. When the running state of the engine reaches a certain state, the adjuster adjusts the fuel pressure in the accumulator to approach a desired value, which is suitable for subsequent restarting of the engine, regardless of the fuel pressure that corresponds to the certain state. Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. BRIEF DESCRIPTION OF THE DRAWINGS The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings. Fig. 1 is a cross-sectional view illustrating a fuel pressure control apparatus in a fuel injection system according to a first embodiment of the present invention; Fig. 2 is a graph showing changes in a basic fuel injection amount based on the relationship between engine speed and gas pedal depression degree; Fig. 3 is a graph showing changes in a target fuel pressure based on the relationship between engine speed and a basic fuel injection amount; Fig. 4 is a timing chart showing changes of the state of a pressure control valve (PCV), the amount of fuel discharge from a supply pump and valve lift of a plunger; Fig. 5 is a flowchart showing a routine for controlling fuel pressure according to the first embodiment; Fig. 6 is a graph showing the relationship between coolant temperature and a requested fuel pressure; Fig. 7 is a graph showing the relationship between the engine speed when the key switch is turned off and a feedback coefficient; Fig. 8 is a timing chart showing changes in fuel pressure when a key switch is turned off; Fig. 9 is flowchart showing a fuel pressure control routine according to a second embodiment of the present invention; Fig. 10 is a graph showing the relationship between the engine speed when the key switch is turned off and fuel injection continuation time in connection with a third embodiment; Fig. 11 is a flowchart showing a routine for controlling fuel pressure according to the third embodiment of the present invention; Fig. 12 is a timing chart showing changes in the engine speed when the key switch is turned off in connection with the third embodiment; Fig. 13 is a timing chart showing changes in fuel pressure when the key switch is turned off in connection with the third embodiment; Fig. 14 is a flowchart showing a routine for controlling fuel pressure according to a fourth embodiment of the present invention; Fig. 15 is a flowchart showing a routine for computing a target fuel pressure according to a fifth embodiment of the present invention; Fig. 16 is a graph showing the relationship between the engine speed and a maximum target fuel pressure according to the fifth embodiment; Fig. 17 is a timing chart showing changes in the fuel pressure when the engine is racing according to the fifth embodiment; Fig. 18 is a flowchart showing a routine for computing a target fuel pressure according to a sixth embodiment of the present invention; Fig. 19 is a graph showing the relationship between the engine speed and a maximum basic fuel injection amount according to the sixth embodiment; Fig. 20 is a timing chart showing changes in the fuel pressure when the engine is racing according to the sixth embodiment; Fig. 21 is a flowchart showing a routine for compensating the gas pedal depression degree according to a seventh embodiment of the present invention; Fig. 22 is a graph showing the relationship between the engine speed and a maximum gas pedal depression degree according to the seventh embodiment; Fig. 23 is a cross-sectional view illustrating a fuel pressure control apparatus in a fuel injection system according to an eighth embodiment of the present invention; Fig. 24 is a flowchart illustrating a routine for controlling the fuel pressure according to the eighth embodiment; Fig. 25 is a timing chart showing changes in the fuel pressure during a non-operational state of an engine according to the eighth embodiment; Fig. 26 is a cross-sectional view illustrating a fuel pressure control apparatus in a fuel injection system according to a ninth embodiment of the present invention; Fig. 27 is a flowchart showing a routine for controlling a relief valve according to the ninth embodiment; Fig. 28 is a timing chart showing changes in fuel pressure in relation to the state of a starter signal in the embodiment of Fig. 27; Fig. 29 is a flowchart showing a routine for controlling the fuel pressure according to a tenth embodiment of the present invention; Fig. 30 is a graph showing the relationship between coolant temperature and sustaining time in the embodiment of Fig. 29; and Fig. 31 is a timing chart showing changes in the fuel pressure in relation to the state of the starter signal in the embodiment of Fig. 29. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A first embodiment of a fuel pressure control apparatus according to the present invention will now be described with reference to Figs. 1-8. The apparatus is employed in a fuel injection system of a diesel engine 1. As shown in Fig. 1, the diesel engine 1 is employed in a vehicle and includes four cylinders #1-#4. The diesel engine 1 has injectors 2 each corresponding to the combustion chamber of one of the cylinders #1-#4. Each injector 2 injects fuel into the associated combustion chamber and includes an electromagnetic valve 3 for injecting fuel. The amount and timing of fuel injection from each injector 2 is controlled by opening and closing the corresponding valve 3. The injectors 2 are connected to an accumulator, or a common rail 4. The common rail 4 is connected to an outlet port 6a of a supply pump 6 by a supply pipe 5. A check valve 7 is located in the supply pipe 5. The check valve 7 prevents fuel from flowing back to the supply pump 6 from the common rail 4. The supply pump 6 has a suction port 6b and a return port 6c. The suction port 6b is connected to a fuel tank 8 by a filter 9 and the return port 6c is connected to the fuel tank 8 by a return pipe 11. Each injector 2 includes a return port 3a located in the vicinity of its electromagnetic valve 3. The return port 3a is connected with the fuel tank 8 by the return pipe 11. Part of fuel supplied to each injector 2 from the common rail 4 leaks into the injector 2 as the injector 2 opens and closes. The leaked fuel is returned to the fuel tank 8 from the return port 3a through the return pipe 11. The supply pump 6 has a plunger and a pressurizing chamber (both not shown). The plunger is reciprocated in synchronization with rotation of a crankshaft (not shown) of the engine 1. Fuel is supplied to the pressurizing chamber from the fuel tank 8. The pump 6 then pressurizes the fuel with the plunger. The pressurized fuel is sent to the common rail 4 through a discharge port 6a. A pressure control valve (PCV) 10 is located in the vicinity of the discharge port 6a. The PCV 10 regulates the pressure of fuel discharged from the discharge port 6a. The supply pump 6 includes a feed pump that supplies fuel to the pressurizing chamber from the fuel tank 8. The combustion chamber of each cylinder #1-#4 is connected to an intake passage 13 and a discharge passage 14. A throttle valve (not shown) is located in the intake passage 13. The throttle valve controls the opening of the intake passage 13 in accordance with the depression degree of the gas pedal 15 thereby controlling the amount of air supplied to the combustion chamber. The glow plug 16 is arranged such that its distal end is located in the combustion chamber. The glow plug 16 is heated by electric current supplied to a glow relay 16a immediately before the engine 1 is started. Fuel is sprayed from the injector 2 to the heated glow plug 16. This promotes ignition and combustion of fuel. The engine 1 also has various sensors to detect its running condition. A gas pedal sensor 20 is located in the vicinity of the gas pedal 15 to detect the depression degree (ACCP) of the pedal 15. A coolant temperature sensor 21 is located in the cylinder block of the diesel engine 1 to detect the temperature THW of coolant in the cylinder block. A fuel pressure sensor 22 is located in the common rail 4 to detect the pressure PC of fuel in the common rail 4. A fuel temperature sensor 23 is located in the return pipe 11 to detect the temperature THF of fuel. Further, an intake pressure sensor 24 is located in the intake passage 13 to detect the pressure PM of intake air in the passage 13. A crank sensor 25 is located in the vicinity of the crankshaft of the engine 1. A cam sensor 26 is located in the vicinity of a camshaft (not shown), which rotates in synchronization with the crankshaft. The crank sensor 25 and the cam sensor 26 detect the number of rotations of the crankshaft per unit of time (the engine speed NE) and the rotational angle of the crankshaft (crank angle CA). A transmission (not shown) is coupled to the crankshaft. A vehicle speed sensor 27 is located in the vicinity of the transmission to detect the vehicle speed (SPD). Signals from the sensors 20-27 are input to an electronic control unit (ECU) 50 of the engine 1. The ECU 50 includes a central processing unit (CPU), a memory, an input-output circuit and a driving circuit (none of which is separately shown). The ECU 50 is connected to a battery 53 by a main relay 51 and a key switch 52. The main relay 51 has a switch 51a and a coil 51b to open and close the switch 51a. The ECU 50 controls the main relay 51 based on the ON/OFF state of the key switch 52 to start and stop supplying current to the ECU 50. The ECU 50 excites the coil 51b of the main relay 51 when the key switch 52 is turned on. As a result, the switch 51a is closed and current is supplied to the ECU 50 from the battery 53. On the other hand, the ECU 50 de-excites the coil 51b when a predetermined time period has elapsed after the key switch 52 is turned off. As a result, the switch 51a is opened when the predetermined time period has elapsed after the key switch 52 is turned off. This stops current from the battery 53 to the ECU 50. Current is supplied to the ECU 50 for the predetermined time period after the key switch 52 is turned off to allow the ECU 50 to perform control programs for stopping the engine 1 and to write results of malfunction detection to its memory. The engine 1 has a starter 19 and a starter switch 19a. The starter switch 19a detects that the starter 19 is actuated. The key switch 52 is moved among the OFF position, ON position and a start position. When starting the engine 1, the key switch 52 is moved from the OFF position to the start position via the ON position. The starter switch 19a issues starter signal STA to the ECU 50 only when the starter 19 is actuated, that is, when the engine is being cranked. When cranking of the engine 8 is completed (when the engine 1 starts running), the key switch 52 is moved back to the ON position from the start position. The ECU 50 receives information representing the running state of the diesel engine 1 based on signals from the sensors 20-27 and controls the electromagnetic valve 3 and the PCV 10 thereby controlling fuel injection and fuel pressure. That is, the ECU 50 receives information as to the gas pedal depression degree ACCP, the coolant temperature THW, the fuel pressure PC, the fuel temperature THF, the intake pressure PM and the vehicle speed SPD based on signals from the gas pedal sensor 20, the coolant temperature sensor 21, the fuel pressure sensor 22, the fuel temperature sensor 23, the intake pressure sensor 24 and the vehicle speed sensor 27, respectively. Further, the ECU 50 computes the engine speed NE and the crank angle CA based on signals from the crank sensor 25 and the cam sensor 26. The ECU 50 controls the fuel injection based on the values representing the running state of the engine 1. Specifically, the ECU 50 computes a basic injection amount QBASE based on the gas pedal depression degree ACCP and the engine speed NE. The memory of the ECU 50 stores the function data shown in Fig. 2. The function data defines the value of the basic injection amount QBASE based on the engine speed NE and the gas pedal depression degree ACCP. The ECU 50 refers to the function data for computing the basic injection amount QBASE. As shown in Fig. 2, the value of the basic injection amount QBASE increases as the value of the gas pedal depression amount ACCP increases and as the value of the engine speed NE decreases. The ECU 50 also computes a maximum injection amount QMAX based on the engine speed NE, the intake pressure PM, the coolant temperature THW and the fuel temperature THF. The ECU 50 compares the maximum injection amount QMAX with the basic injection amount QBASE and sets the lower value as a final injection amount QFIN. For example, if the value of the engine speed NE is NE1 and the gas pedal depression degree ACCP is ACCP1, the computed basic injection amount QBASE is QBASE1. If the value of the computed maximum injection amount QMAX is QMAX1, the value QBASE1 is smaller than the value QMAX1. Therefore, the value QBASE1 of the basic injection amount QBASE is selected as the final injection amount QFIN. If the gas pedal depression degree ACCP is increased to the value ACCP2 with the engine speed NE remaining at the value NE1, the basic injection amount QBASE is QBASE2. In this case, the value of the maximum injection amount QMAX is QMAX1. Since the value QMAX1 is smaller than the value QBASE2, the maximum injection amount QMAX having the value QMAX1 is selected as the final injection amount QFIN. In this manner, the final injection amount QFIN is always equal to or smaller than the maximum fuel injection amount QMAX. Thus, the amount of fuel is always prevented from being excessive in relation to the amount of air introduced into the combustion chambers. This also limits the maximum value of the engine speed NE. If the key switch 52 is at the ON position, the ECU 50 performs various compensations to the final injection amount QFIN. The ECU 50 then controls the electromagnetic valve 3 based on the compensated final injection amount QFIN. As a result, the injector 2 injects an amount of fuel that is represented by the compensated final injection amount QFIN to the combustion chamber. In this manner, the fuel injection amount is controlled to be suitable for the running state of the engine 1. If the key switch 52 is moved to the OFF position, the ECU 50 gradually decreases the final injection amount QFIN and controls the fuel injection based on the decreased final fuel injection amount QFIN. Therefore, when the key switch is turned off, the engine speed NE is gradually lowered as the fuel injection amount is decreased, and the engine 1 is then finally stopped. This process, in which the engine is stopped by gradually decreasing the fuel injection amount, will hereafter be referred to as "engine stopping injection control". The engine stopping injection control is performed to prevent the engine from vibrating when the engine is being stopped. The engine stopping injection control also allows the crankshaft to continue rotating for a certain time period after the key switch 52 is turned off. Thus, the supply pump 6 is able to discharge fuel. Further, the ECU 50 controls the pressure in the common rail 4. That is, the ECU 50 computes a target fuel pressure PTRG of the fuel pressure PC in the common rail 4 based on the basic fuel injection amount QBASE and the engine speed NE. The memory of the ECU 50 stores the function data shown in Fig. 3. The function data of Fig. 3 defines the value of the target fuel pressure PTRG based on the basic injection amount QBASE and the engine speed NE. The ECU 50 refers to the function data of Fig. 3 for computing the target fuel pressure PTRG. As shown in Fig. 3, the value of the target fuel pressure PTRG increases as the value of the engine speed NE increases and as the value of the basic injection amount QBASE increases. This is because fuel atomization must be promoted by increasing the fuel pressure PC in the common rail 4 when the load acting on the engine 1 is great or when the engine speed NE is high. The ECU 50 controls the PCV10 such that the fuel pressure PC in the common rail 4 detected by the fuel pressure sensor 22 matches the target fuel pressure PTRG. The relationship between the state of the PCV 10 and the amount of fuel discharged from the supply pump 6 will now be described referring to a timing chart of Fig. 4. Fig. 4 shows changes in the state of the PCV 10 and the amount of fuel discharged from the supply pump 6 in relation to the crank angle CA. Specifically, the values (a), (c) and (e) represent various operation patterns of the PCV 10, the values (b), (d) and (f) represent fuel discharge patterns from the supply pump 6, which correspond with the PCV patterns (a), (c) and (e), respectively, and the value (g) represents the lift of the plunger in the supply pump 6. In Fig. 4, the horizontal axis represents the changes of the crank angle CA over time. The lift of the plunger increases during a period between a time t2 and a time t6. This period corresponds to the discharge stroke of the supply pump 6. The lift of the plunger decreases between the time 6 to a time 7. This period corresponds to the suction stroke of the pump 6. During a period between a time t0 and a time t1, which is prior to the discharging stroke, the PCV 10 is opened as shown in patterns (a), (c) and (e). Therefore, the pressurizing chamber of the supply pump 6 is connected to the return pipe 11 through the return port 6c, and fuel is not supplied to the common rail 4 from the pressurizing chamber. At the time t1, the PCV10 is closed as shown in (a), (c) and (e). When the PCV10 is closed, the pressurizing chamber is disconnected from the return port 6c. Therefore, fuel in the pressurizing chamber is ready to be discharged in accordance with the lift of the plunger. At the time t2, the plunger starts pressurizing the fuel in the pressurizing chamber and fuel in the pressurizing chamber starts moving to the common rail 4 through the discharge port 6a and the supply pipe 5. After the time t2, the amount of discharged fuel gradually increases as the lift of the plunger increases. If the PCV 10 is opened in this state, the discharge of fuel is stopped and fuel in the pressurizing chamber is returned to the fuel tank 8 through the return port 6c and the return pipe 11. The ECU 50 computes the time to open the PCV 10 for stopping the supply of fuel to the common rail 4 based on an equation (1). The time to open the PCV 10 will hereafter be referred to as "valve opening time TF". TF = TFBASE + K(PTRG - PC) In the equation (1), the value "K" is a coefficient of feedback control, or a gain. The value "K" is determined based on the position of the key switch 52 during a fuel pressure control routine, which will be described later. The value "TFBASE" in the equation (1) is a reference value of a valve opening time TF. If the valve opening time TF is matched with the reference time TFBASE, the fuel pressure PC in the common rail 4 is maintained at the current pressure. The reference time TFBASE, which has been experimentally determined, is a function of the final injection amount QFIN and the fuel pressure PC. The memory of the ECU 50 stores function data that defines the relationship between the reference time TFBASE and the final injection amount QFIN and the fuel pressure PC. For example, the reference time TFBASE is set to a time t4 in the chart of Fig. 4. When judging that the fuel pressure PC is smaller than the target fuel pressure PTRG (PTRG - PC > 0), the ECU 50 retards the valve opening time TF from the reference time TFBASE (the time t4) to a time t5 (see (a) of Fig. 4), which corresponds to a retarded crank angle CA. This lengthens the period (from the time t1 to t5) during which the PCV10 is closed. Accordingly, the amount of fuel supplied to the common rail 4 is increased compared to the case in which the valve opening time TF is the reference time TFBASE (see (b) of Fig. 4). As a result, the fuel pressure PC is increased and the difference between the fuel pressure PC and the target fuel pressure PTRG (PTRG - PC) is decreased. When judging that the fuel pressure PC is greater than the target fuel pressure PTRG (PTRG - PC < 0), the ECU 50 advances the valve opening time TF from the reference time FBASE (the time t4) to a time t3, (see (e) of Fig. 4), which corresponds to an advanced crank angle CA. This shortens the period (from the time t1 to t3) during which the PCV10 is closed. Accordingly, the amount of fuel supplied to the common rail 4 is decreased compared to the case in which the valve opening time TF is the reference time TFBASE (see (e) of Fig. 4). As a result, the fuel pressure PC is decreased and the difference between the fuel pressure PC and the target fuel pressure PTRG (PTRG - PC) is decreased. As is obvious in the equation (1), the greater the value of the difference between the fuel pressure PC and the target pressure (PTRG - PC) is, the greater the amount of advancing or retarding of the valve opening time TF becomes. Thus, the fuel pressure PC is converged with the target fuel pressure PTRG in a stable manner. As described above, a predetermined amount of fuel is supplied to the common rail 4 during the pressurizing stroke (the period t2 -t6) of the supply pump 6. The pump 6 then moves on to the suction stroke (the period t6-t7). In the suction stroke, fuel in the fuel tank 8 is introduced into the pressurizing chamber through the suction port 6b in preparation for the next discharge stroke. The fuel pressure control will now be described. Fig. 5 is a flowchart showing a routine for controlling fuel pressure. The ECU 50 executes the routine in an interrupting manner at predetermined crank angle increments. When entering the routine, the ECU 50 judges whether a key switch flag XIG is one at step 100. The key switch flag XIG is used to judge the position of the key switch 52. The key switch flag XIG is one when the key switch 52 is at the ON position and is zero when the key switch 52 is at the OFF position. If the flag XIG is one, the ECU 50 moves to step 106 and judges whether the engine speed NE is greater than zero, that is, the ECU 50 judges whether the supply pump 6 is capable of supplying fuel to the common rail 4. If the determination is negative at step 106, that is, if the supply pump 6 is not operating, the ECU 50 moves to step 110. If the determination is positive at step 106, on the other hand, the ECU 50 moves to step 107. At step 107, the ECU 50 sets the feedback coefficient K, which is used to compute the valve opening time TF, to a predetermined value K1. The ECU 50 then moves to step 108. At step 108, the ECU 50 computes the target fuel pressure PTRG based on the current basic injection amount QBASE and the current engine speed NE. If the determination is negative at step 100, that is, if the key switch 52 is at the OFF position, the ECU 50 moves to step 101. At step 101, the ECU 50 judges whether a main relay flag XMR is one. The main relay flag XMR is always one if the key switch flag XIG is one. The flag XMR is changed from one to zero when the writing of malfunction diagnosis results and various processes for stopping the engine 1 are completed. If the determination is negative at step 101, that is, if the main relay flag XMR is zero, the ECU 50 moves to step 110. If the main relay flag XMR is set to zero, the coil 51b is de-excited and the switch 51a is opened. As a result, current to the ECU 50 is stopped. If the determination is positive at step 101, the ECU performs steps 102-105. The steps 102-105 are designed to adjust the fuel pressure PC in the common rail 4 to a level suitable for starting the engine 1. At step 102, the ECU 50 sets a value NEOFF as the engine speed NE recorded when the switch 52 is moved from the ON position to the OFF position. Therefore, the engine speed NEOFF is the engine speed NE at the time of turning the key switch off. The switching of the key switch 52 is detected based on the fact that the key switch flag XIG, which was one in the previous cycle of the routine, is changed to zero in the current routine. At step 103, the ECU 50 computes a requested fuel pressure PTRGSTA based on the coolant temperature THW. The requested fuel pressure PTRGSTA is a requested, or desired, value of the fuel pressure PC when the engine is restarted. The memory of the ECU 50 stores the function data shown in Fig. 6. A solid line represents the relationship between the coolant temperature THW and the requested fuel pressure PTRGSTA. The ECU 50 uses this data for computing the requested fuel pressure PTRGSTA. As shown in Fig. 6, the lower the coolant temperature THW is, the greater the value of the requested fuel pressure PTRGSTA becomes. A lower coolant temperature THW, which represents a lower temperature of the engine 1, hinders atomization of injected fuel. Therefore, the fuel pressure PC, or the pressure of injected fuel, must be increased to promote atomization of injected fuel for facilitating starting of the engine 1. At step 104, the ECU 50 computes the feedback coefficient K based on the engine speed NEOFF when the switch 52 is moved to the OFF position. The memory of the ECU 50 stores the function data shown in Fig. 7. A solid line represents the relationship between the feedback coefficient K and the engine speed NEOFF when the switch 52 is moved to the OFF position. The ECU 50 uses this data for computing the feedback coefficient K. As shown in Fig. 7, the feedback coefficient K is always greater than the value K1, which is set at step 107 when the key switch flag XIG is one. Also, the smaller the value of the engine speed NEOFF is, the greater the value of the coefficient K becomes. The feedback coefficient K is varied for the following reasons: When the key switch 52 is moved to the OFF position, the engine speed NE is lowered and rotation of the crankshaft stops after a certain period. The supply pump 6 is capable of supplying oil to the common rail 4 only when the crankshaft is rotating. Therefore, the fuel pressure PC must be increased to the target fuel pressure PTRG as early as possible by increasing the feedback gain, or the coefficient K. Secondly, the lower the engine speed NEOFF is, the shorter the period becomes from the time of turning off the key switch 52 to the time the crankshaft stops. Therefore, the feedback gain, or the coefficient K, must be further increased if the engine speed NEOFF is relatively low. At a subsequent step 105, the ECU 50 substitutes the requested fuel pressure PTRGSTA for the target fuel pressure PTRG. After executing steps 105 or 108, or when the determination is negative at steps 101 or 106, the ECU 50 moves to step 110. At step 110, the ECU 50 computes the reference time TFBASE. The ECU 50 then moves to step 112 and computes the valve opening time TF based on the reference time TFBASE, the feedback coefficient K, the target fuel pressure PTRG and the fuel pressure PC. The ECU 50 controls the time to close the PCV 10 based on the valve opening time TF in another control routine. The control of the fuel pressure PC will now be described. Fig. 8 is a timing chart illustrating changes of the fuel pressure PC when the key switch 52 is turned off while the gas pedal 15 is not pressed at all and the engine speed NE is gradually decreasing. The gas pedal 15 is released at a time t0. Thereafter, the target fuel pressure PTRG is lowered as the basic injection amount QBASE and the engine speed NE are lowered. As illustrated by a solid line, the fuel pressure PC is gradually decreased to a value PC1, which is lower than the requested fuel pressure PTRGSTA, for starting the engine 1. The key switch 52 is turned off at a time t1, and the target fuel pressure PTRG is replaced with the requested fuel pressure PTRGSTA, which is suitable for starting the engine 1. The valve opening time TF is retarded when the fuel pressure PC is lower than the target fuel pressure PTRG (from the time t1 to t2). Therefore, the fuel pressure PC is increased to approach the target fuel pressure PTRG by increasing the amount of fuel supplied to the common rail 4. When the fuel pressure PC matches the target fuel pressure PTRG at the time t2, the valve opening time TF is changed to the reference valve opening time TFBASE. After the time t2, the valve opening time TF is maintained at the reference valve opening time TFBASE. Thus, the fuel pressure PC is maintained at the target fuel pressure PTRG, or the requested fuel pressure PTRGSTA, which is suitable for starting the engine 1. If the fuel injection and fuel transfer are stopped at the same time when the key switch 52 is turned off, as in the prior art, the fuel pressure PC is maintained at a level (PC1) shown by a dashed line. The pressure PC1 is lower than the fuel pressure when the key switch 52 is turned off, that is, lower than the requested fuel pressure PTRGSTA, which is suitable for starting the engine 1. For staring the engine 1 again, the fuel pressure PC needs to be increased to the requested fuel pressure PTRGSTA before starting fuel injection. This lengthens the time required to start the engine 1. In other cases, fuel that is not sufficiently atomized is injected into the combustion chambers. This makes the engine 1 harder to start. However, in this embodiment, even if the fuel pressure PC is lower than the requested fuel pressure PTRGSTA when the key switch 52 is turned off, the pressure PC is increased to the requested fuel pressure PTRGSTA. Therefore, fuel is injected at a pressure suitable for starting the engine 1 when the engine 1 is started again. As a result, engine starting is improved. When the gas pedal 15 is not pressed at all, that is, when the engine 1 is idling, the engine speed NE is low. A rapid increase in the external load acting on the engine 1 in this state lowers the engine speed NE. The decrease of the engine speed NE is likely to stall the engine. Therefore, in a typical diesel engine, the basic injection amount QBASE is increased (for example, from point A to point B in Fig. 2) if the engine speed NE is lowered while the engine is idling. Also, the target fuel pressure PTRG (for example, from point A to point B in Fig. 3) is increased as the basic injection amount QBASE is increased. Increasing the basic injection amount QBASE and the target fuel pressure PTRG increases the engine speed NE thereby preventing the engine 1 from stalling. However, in the diesel engine 1 illustrated in this embodiment, the fuel injection amount is gradually decreased for preventing the engine from vibrating when stopping the engine 1. In this type of engine, the target fuel pressure PTRG can be set higher than the requested fuel pressure PTRGSTA when the engine speed NE is lowered. For example, if the feedback control of the fuel pressure PC is simply continued after the key switch 52 is turned off, the fuel pressure PC can follow a two-dot chain line in Fig. 8. That is, the fuel pressure PC is increased as the engine speed NE is lowered and is maintained at a value PC2, which is higher than the requested fuel pressure PTRGSTA. As a result, fuel is injected at a pressure that is higher than the requested fuel pressure PTRGSTA when the engine 1 is started again. This excessively atomizes injected fuel thereby rapidly changing the firing pressure in the combustion chamber. The rapidly changing firing pressure causes noise. In this embodiment, the target fuel pressure PTRG is not computed based on the engine speed NE and the basic fuel injection amount QBASE after the key switch 52 is turned off. Instead, the target fuel pressure PTRG is changed to the requested fuel pressure PTRGSTA, which is determined based on the coolant temperature THW. Thus, the fuel pressure PC does not exceed the requested fuel pressure PTRGSTA when the engine 1 is stopped. As a result, the engine 1 is not vibrated when being stopped. Further, noise of the engine 1 is suppressed when starting the engine 1. Further, in this embodiment, the lower the coolant temperature THW is, the higher the requested fuel pressure PTRGSTA becomes. Therefore, even if the engine 1 is stopped before the engine 1 is warm, starting of the engine immediately thereafter does not hinder the atomization of fuel. Engine ignition is thus improved. On the other hand, if the engine 1 is stopped after being warmed up and is started immediately thereafter, atomization of fuel is optimally suppressed. This lowers noise caused by starting the engine 1. After the key switch 52 is turned off, the fuel pressure PC is increased by the supply pump 6. Therefore, this embodiment does not require an extra pump for increasing the fuel pressure PC. This simplifies the construction of the fuel pressure control apparatus. The feedback coefficient K (feedback gain) is increased after the key switch 52 is turned off compared to the case where the switch 52 is at the ON position. This allows the fuel pressure PC to quickly reach the target fuel pressure PTRG. Thus, when the crankshaft is stopped, the fuel pressure PC is brought to the target fuel pressure PTRG before the supply pump 6 is unable to increase the fuel pressure PC. For example, if the key switch 52 is turned off when the engine speed NE is low, the crankshaft will be stopped in a relatively short period. However, the feedback coefficient K has a greater value for a lower engine speed NEOFF when the switch 52 is turned off. Therefore, even if the crankshaft is stopped in a short period after the key switch 52 is turned off, the fuel pressure PC is positively increased to the target fuel pressure PTRG before the supply pump 6 stops operating. A second embodiment of the present invention will now be described. The differences from the embodiment of Figs. 1-8 will mainly be discussed below, and like or the same reference numerals are given to those components that are like or the same as the corresponding components of the embodiment of Figs 1-8. In this embodiment, the control of the fuel pressure PC differs from that of the embodiment of Figs 1-8. In the embodiment of Figs. 1-8, the target fuel pressure PTRG is changed to the requested fuel pressure PTRGSTA after the key switch 52 is turned off. The valve opening time TF of the PCV 10 is determined based on the difference (PTRG - PC) between the changed target fuel pressure PTRG and the fuel pressure PC. In this embodiment, the supply pump 6 is controlled to maximize its fuel discharge if the fuel pressure PC is lower than the target fuel pressure PTRG when the key switch 52 is turned off. If the fuel pressure PC is higher than the target fuel pressure PTRG, the supply pump 6 is controlled to stop discharging fuel. The control of the fuel pressure PC according to this embodiment will now be described. Fig. 9 is a flowchart showing a routine for controlling fuel pressure PC. This routine is an interrupt executed by the ECU 50 at predetermined crank angle increments. When entering the routine, the ECU 50 judges whether a key switch flag XIG is one at step 200. If the determination is positive, the ECU 50 moves to step 210 and judges whether the engine speed NE is greater than zero, that is, the ECU 50 judges whether the supply pump 6 is capable of supplying fuel to the common rail 4. If the determination is positive at step 210, the ECU 50 moves to step 212. At step 212, the ECU 50 computes the valve opening time TF using the equation (1) and temporarily suspends the subsequent processing. The feedback coefficient K is a fixed value and is always equal to the value K1. If the determination is negative at step 210, that is, if the crankshaft is not rotating and the supply pump 6 is not capable of discharging fuel, the ECU 50 temporarily suspends the subsequent processing. If the determination is negative at step 200, the ECU 50 moves to step 202 and judges whether the main relay flag XMR is one. If the determination is positive, the ECU 50 moves to step 203 and judges whether the fuel pressure PC is lower than the requested fuel pressure PTRGSTA. As in the embodiment of Figs. 1-8, the requested fuel pressure PTRGSTA is determined based on the coolant temperature THW. If the determination at step 203 is negative, that is, if the fuel pressure PC is equal to or higher than the requested fuel pressure PTRGSTA, the ECU 50 moves to step 206 and temporarily stops controlling the PCV 10. Therefore, the PCV 10 is held open and communicates the pressurizing chamber of the supply pump 6 with the return pipe 11 through the return port 6c. Furthermore, the pump 6 temporarily stops supplying fuel to the common rail 4. In this state, if fuel injection is continued after turning the key switch 52 off, the fuel pressure PC drops rapidly. If fuel injection has stopped, the fuel pressure PC is maintained at the current level. If the determination is positive at step 203, the ECU 50 moves to step 204 and changes the valve opening time TF to a most retarded time TFMAX. This maximizes the amount of fuel discharged from the supply pump 6. After executing steps 204 or 206 or when the determination is negative at step 202, the ECU 50 temporarily suspends the current routine. The embodiment of Fig. 9 has the following advantages. If the fuel pressure PC is judged to be lower than the requested fuel pressure PTRGSTA after the key switch 52 is turned off, the PCV 10 is controlled such that the amount of the fuel discharged from the supply pump 6 is maximized. This maximizes the speed at which the fuel pressure PC is brought to the requested fuel pressure PTRGSTA. Therefore, the fuel pressure PC is positively raised to the requested fuel pressure PTRGSTA after the crankshaft stops rotating and before the supply pump 6 is incapable of increasing the fuel pressure PC. If the fuel pressure PC is judged to be equal to or higher than the requested fuel pressure PTRGSTA after the key switch 52 is turned off, the supply pump 6 stops discharging fuel. Therefore, the fuel pressure PC is positively lowered to the requested fuel pressure PTRGSTA after the engine 1 is stopped and before fuel injection is finished. A third embodiment of the present invention will now be described. The differences from the embodiment of Figs. 1-8 will mainly be discussed below, and like or the same reference numerals are given to those components that are like or the same as the corresponding components of the embodiment of Figs 1-8. As in the embodiment of Figs. 1-8, the fuel pressure control routine of Fig. 5 is executed in this embodiment. Further, fuel injection control is performed when the engine 1 is stopped. The engine stopping injection control is not executed immediately after the key switch 52 is turned off. Instead, the normal fuel injection based on the gas pedal depression degree ACCP and the engine speed NE is continued until a predetermined period has elapsed. That is, if the fuel pressure PC is higher than the requested fuel pressure PTRGSTA when the key switch 52 is turned off, the fuel pressure PC is quickly lowered to the request fuel pressure PTRGSTA. The time during which the normal fuel injection is continued is significantly short. In other words, the fuel pressure PC is lowered to the request fuel pressure PTRGSTA within a very short time. Therefore, the continued normal fuel injection does not disturb the driver, who has turned the key switch 52 off. The fuel injection control when the engine 1 is stopped will now be described. Fig. 11 is a flowchart showing a routine for controlling fuel injection when stopping the engine 1. This routine is an interrupt executed by the ECU 50 at predetermined time intervals. When entering the routine, the ECU 50 judges whether a key switch flag XIG is one at step 300. If the determination is negative, the key switch 52 is at the OFF position. The ECU 50 then moves to step 302 and sets the engine speed NEOFF when the key switch 52 is turned off. At a subsequent step 304, the ECU 50 computes an fuel injection continuation time NECT based on the engine speed NEOFF when key switch 52 is turned off. The continuation time NECT is a period from when the key switch 52 is turned off to when the engine stopping injection control is started. The memory of the ECU 50 stores the function data shown in Fig. 10. A solid line represents the relationship between the engine speed NEOFF and the fuel injection continuation time NECT. The ECU 50 uses this data for computing the continuation time NECT. As shown in Fig. 10, the greater the engine speed NEOFF is, the longer the continuation time NECT becomes. This relationship between NEOFF and NECT is determined for the following reason. The higher the engine speed NEOFF is, the higher the fuel pressure PC when the key switch 52 is turned off becomes. Therefore, a longer period is needed for lowering the fuel pressure PC to the requested fuel pressure PTRGSTA. At step 306, the ECU 50 increments a time period CIGOFF by a time corresponding to the length of the routine of Fig 11. The time period CIGOFF represents time that has elapsed since the key switch 52 is turned off. At step 308, the ECU 50 judges whether the time period CIGOFF has exceeded the continuation time NECT, that is, whether a predetermined time (namely, NECT) has elapsed after the key switch 52 is turned off. If the determination is negative, the ECU 50 moves to step 310. If the determination is positive at step 300, the ECU 50 moves to step 320 and initializes the time period CIGOFF to zero. The ECU 50 then moves to step 310. At step 310, the ECU 50 sets a flag XSTOP to zero. The flag XSTOP is used to judge whether the engine stopping injection control has to be started. The ECU 50 judges the state of the flag XSTOP in another injection control routine. If the flag XSTOP is zero, the ECU 50 performs the normal fuel injection control. If the flag XSTOP is one, the ECU 50 switches the normal fuel injection control to the engine stopping injection control thereby stopping the engine 1. If the determination is positive at step 308, that is, if the predetermined time has elapsed since the key switch 52 has been turned off, the ECU 50 moves to step 312 and sets the flag XSTOP to one. After executing steps 310 or 312, the ECU 50 temporarily suspends the current routine. As described above, the engine stopping injection control is started after the predetermined period has elapsed since the key switch 52 is turned off. In other words, the normal fuel control is performed for the predetermined period. Therefore, the diesel engine 1 continues running normally for a certain time after the switch 52 is turned off. Thereafter, the engine speed NE is gradually decreased until the engine 1 is stopped. Changes of the engine speed NE and the fuel pressure PC will be described with reference to timing charts of Figs. 12 and 13. In Fig. 12, a solid line represents a case where the engine speed NEOFF is relatively low (NEOFF1) and a dashed line represents a case where the engine speed NEOFF is relatively high (NEOFF2). In either case, the key switch 52 is turned off at a time t1. In the case of the engine speed NEOFF1, the injection continuation time NECT is a period NECT1, which lasts from time t1, at which time the key switch 52 is turned off, until a time t2. During the period NECT1, the engine speed NE is maintained. At the time t2, the engine stopping injection control is started and the amount of injected fuel is gradually decreased. This gradually lowers the engine speed NE. At a time t3, fuel injection is stopped and combustion of fuel is stopped, accordingly. As a result, the engine speed NE rapidly drops. The diesel engine 1 stops running at a time t4. In the case of the engine speed NEOFF2, the injection continuation time NECT is a relatively long period NECT2 (NECT2 > NECT1). Therefore, the normal fuel injection is continued for a relatively long time (from the time t1 to a time t5). Thus, when NECT1 is used, the total amount of fuel injected between the time at which the key switch 52 is turned off and the time at which diesel engine 1 is stopped is greater than that injected during NECT1. In other words, the fuel pressure PC drops by a greater amount. The engine speed NEOFF is relatively high when the key switch 52 is turned off while the engine 1 is being raced, that is, while the gas pedal 15 is pressed with the selector lever in a neutral range. Also, immediately after the engine 1 has been raced, turning the key switch 52 off before the engine speed NE is lowered results in a relatively high engine speed NEOFF. In these cases, the target fuel pressure PTRG is set to a higher level based on the increased engine speed NE. Under these circumstances, the fuel pressure PC is higher than the requested fuel pressure PTRGSTA, which is suitable for starting the engine 1. Therefore, the fuel pressure PC may not be lowered to the requested fuel pressure PTRGSTA by performing the engine stopping injection control after the key switch 52 is turned off. As a result, fuel will be injected at a pressure higher than the request fuel pressure PTRGSTA when the engine 1 is started again. This results in a sudden change of the firing pressure and thus causes noise. However, in this embodiment, if the key switch 52 is turned of at the time t1, the target fuel pressure PTRG is changed to the requested fuel pressure PTRGSTA as in the first embodiment. This decreases the amount of fuel discharged from the supply pump 6. Also, the normal injection is continued and the fuel pressure PC is rapidly decreased. At the time t2, the normal injection control is switched to the engine stopping injection control. Then the amount of injected fuel is gradually decreased. However, the fuel pressure PC continues dropping. At the time t3, the fuel pressure PC reaches the requested fuel pressure PTRGSTA. At the time t4, the diesel engine 1 stops running. From the time t3 to the time t4, the fuel pressure PC is maintained at the requested fuel pressure PTRGSTA. That is, the fuel pressure PC is decreased by fuel injection. However, the supply pump 6 sends fuel to the common rail 4 thereby increasing the pressure in the common rail 4. Accordingly, the decrease of the fuel pressure PC is compensated. The embodiment of Figs. 12 and 13 has the following advantages. The normal fuel injection is continued after the key switch 52 is turned off. The fuel pressure PC is therefore rapidly dropped to the requested fuel pressure PTRGSTA. Therefore, the injection pressure will not be excessive when the engine 1 is started again. The noise caused by starting the engine 1 is reduced, accordingly. The higher the engine speed NEOFF when the key switch 52 is turned off is, that is, the higher the fuel pressure PC when the key switch 52 is turned off is, the longer the normal injection control continues. In other words, the injection continuation time NECT is lengthened. Therefore, even if the pressure PC is greatly different from the request fuel pressure PTRGSTA, the fuel pressure PC is positively decreased to the requested fuel pressure PTRGSTA. If the fuel pressure PC is only slightly different from the request fuel pressure PTRGSTA, the injection continuation time NECT is set short. Therefore, the engine 1 is readily stopped after the key switch 52 is turned off. The injectors 2 are used to decrease the fuel pressure PC after the key switch 52 is turned off. Therefore, this embodiment does not require an extra pressure controller such as a relief valve. This simplifies the construction of the fuel pressure control apparatus. A fourth embodiment of the present invention will now be described. The differences from the embodiment of Figs. 10-13 will mainly be discussed below, and like or the same reference numerals are given to those components that are like or the same as the corresponding components of the embodiment of Figs 1-8. In the embodiment of Figs. 10-13, the injection continuation time NECT is computed based on the engine speed NEOFF when the key switch 52 is turned off. The normal fuel injection is continued until the time NECT elapses from when the key switch 52 is turned off. However, in the fourth embodiment of Fig. 14, the normal fuel injection is continued until the fuel pressure PC reaches the requested fuel pressure PTRGSTA. The fuel injection control when the engine 1 is stopped will now be described with reference to Fig. 14. This routine is an interrupt executed by the ECU 50 at predetermined time intervals. When entering the routine, the ECU 50 judges whether the key switch flag XIG is one at step 400. If the determination is negative, the ECU 50 judges that the key switch 52 is turned off and moves to step 402. At step 402, the ECU 50 reads the current fuel pressure PC from the fuel pressure sensor 22. At a subsequent step 408, the ECU 50 judges whether the fuel pressure PC is lower than the requested fuel pressure PTRGSTA, which is computed in the routine of Fig. 5. If the determination is negative at step 408 or if the determination is positive at step 400, the ECU 50 moves to step 410. At step 410, the ECU 50 sets the flag XSTOP for stopping the engine 1 to zero. If the determination is positive at step 408, that is, if the fuel pressure PC is lower than the requested fuel pressure PTRGSTA, the ECU to moves to step 412 and sets the flag XSTOP to one. After either steps 412 or 410, the ECU 50 temporarily suspends the current routine. Even if the key switch 52 is turned off, the engine stopping injection control is not started until the fuel pressure PC is lower than the requested fuel pressure PTRGSTA. In other words, the normal injection control is continued. This embodiment therefore has the same advantages as the embodiment of Figs. 10-13. In the fourth embodiment of Fig. 4, the fuel injection is not stopped before the fuel pressure PC is lowered below the requested fuel pressure PTRGSTA. Therefore, the fuel pressure PC is positively lowered to the requested fuel pressure PTRGSTA. Further, if the fuel pressure PC is lower than the requested fuel pressure PTRGSTA when the key switch 52 is turned off, the engine stopping injection control is started when the key switch 52 is turned off. The diesel engine 1 is therefore readily stopped. A fifth embodiment of the present invention will now be described with reference to Fig. 15. The differences from the embodiment of Figs. 1-8 will mainly be discussed below, and like or the same reference numerals are given to those components that are like or the same as the corresponding components of the embodiment of Figs 1-8. In the embodiment of Figs. 1-8, when the key switch 52 is at the ON position and the engine speed NE is greater than zero (see step 108 in the routine of Fig 5), the target fuel pressure PTRG is determined based on the basic injection amount QBASE and the engine speed NE. A value of the target fuel pressure PTRG corresponds to one value of the basic fuel injection amount QBASE and to one value of the engine speed NE. However, in the fifth embodiment of Fig. 15, there is an upper limit of the target fuel pressure PTRG when the engine 1 is racing. The target fuel pressure PTRG is controlled to remain below the upper limit. The routine for computing the target fuel pressure PTRG will now be described with reference to Fig. 15. This routine is an interrupt executed by the ECU 50 at predetermined time intervals. When entering the routine, the ECU 50 reads the basic injection amount QBASE, the engine speed NE, the gas pedal depression degree ACCP and the vehicle speed SPD at step 500. At step 502, the ECU 50 computes the target fuel pressure PTRG based on basic injection amount QBASE and the engine speed NE. At a subsequent step 504, the ECU 50 judges whether the vehicle speed SPD is zero. If the determination is positive at step 504, the ECU 50 moves to step 506. At step 506, the ECU 50 judges whether the gas pedal depression degree ACCP is greater than a predetermined value ACCP1. The value ACCP1 is used to judge whether there is a possibility that the target fuel pressure PTRG is set higher than the requested fuel pressure PTRGSTA. If the gas pedal depression degree ACCP is greater than the predetermined value ACCP1, the ECU 50 judges that the target fuel pressure PTRG is set higher than the requested fuel pressure PTRGSTA due to the increased engine speed NE. If the determinations in steps 504 and 506 are both positive, the ECU 50 judges that the engine 1 is racing and moves to step 508. At step 508, the ECU 50 computes a maximum target fuel pressure PTRGMAX based on the engine speed NE. The maximum target fuel pressure PTRGMAX is an upper limit of the target fuel pressure PTRG. The memory of the ECU 50 stores the function data shown in Fig. 16. A solid line represents the relationship between the maximum target fuel pressure PTRGMAX and the engine speed NE. The ECU 50 refers to the function data for computing the maximum target fuel pressure PTRGMAX. As shown in Fig. 16, the higher the engine speed NE is, the greater the value of PTRGMAX becomes. The target fuel pressure PTRG is set to a greater value for a higher engine speed NE for promoting atomization of fuel. The value PTRGMAX must be determined, accordingly. At step 510, the ECU 50 judges whether the target fuel pressure PTRG is greater than the maximum target fuel pressure PTRGMAX. If the determination is positive, that is, if the target fuel pressure PTRG exceeds its maximum value PTRGMAX, the ECU 50 moves to step 512. At step 512, the ECU 50 substitutes the maximum value PTRGMAX for the target fuel pressure PTRG. After executing step 512 or when the determination in steps 504, 506 or 510 is negative, the ECU 50 temporarily suspends the current routine. The value of the target fuel pressure PTRG set in the current routine is temporarily stored in the memory of the ECU 50. At step 108 in the routine of Fig. 5, the ECU 50 reads the target fuel pressure PTRG and then executes the processes of step 110 and the following steps. As described above, if the engine 1 is racing, the target fuel pressure PTRG is controlled to remain below the maximum target fuel pressure PTRGMAX, which is determined based on the engine speed NE. Fig. 17 is a timing chart illustrating such fuel pressure control. From a time t0, the gas pedal 15 is gradually pressed and the engine 1 starts racing. Then, the fuel pressure PC starts increasing as the target fuel pressure PTRG increases. If the increase of the target fuel pressure PTRG is not limited, the fuel pressure PC increases as shown by a two-dot chain line as the gas pedal depression degree ACCP increases. If the key switch 52 is turned off while the fuel pressure PC is increasing, the fuel pressure PC is decreased by subsequent fuel injection (the engine stopping injection control). However, when the fuel injection is stopped, the fuel pressure PC may be higher than the requested fuel pressure PTRGSTA, which is suitable for starting the engine 1. However, in this embodiment, fuel pressure PC changes along the solid line of the graph of Fig. 17. In this graph, the gas pedal depression degree ACCP exceeds the predetermined value ACCP1 at a time t1. Then, the target fuel pressure PTRG is controlled to remain below the maximum target fuel pressure PTRGMAX. In other words, the increase of the fuel pressure PC is suppressed after the time t1. Therefore, even if the engine 1 is stopped while racing or immediately after racing, the fuel pressure PC is always lower than its maximum value PTRGMAX. Thus, the injection pressure will not be excessive when the engine 1 is started again. The noise caused by starting the engine 1 is reduced, accordingly. The target fuel pressure PTRG is not simply set to a lower level when the engine 1 is racing, but the maximum target fuel pressure PTRGMAX is employed. The target fuel pressure PTRG is lowered to match PTRGMAX only when PTRG exceeds the PTRGMAX. In other words, the target fuel pressure PTRG is not controlled when below its maximum value PTRGMAX. Thus, normal racing of the engine 1 can be performed as long as PTRG is lower than PTRGMAX. A sixth embodiment of the present invention will now be described. The differences from the embodiment of Figs. 15-17 will mainly be discussed below, and like or the same reference numerals are given to those components that are like or the same as the corresponding components of the embodiment of Figs 1-8. In the fifth embodiment of Figs. 15-17, the target fuel pressure PTRG is limited when the engine 1 is racing. However, in the sixth embodiment, the basic injection amount QBASE is decreased when the engine 1 is racing. The target fuel pressure PTRG is computed based on the decreased basic injection amount QBASE. In this manner, the value of the target fuel pressure PTRG is limited. The routine for controlling the basic injection amount QBASE will now be described with reference to Fig. 18. This routine is an interrupt executed by the ECU 50 at every predetermined time period. When entering the routine, the ECU 50 reads the engine speed NE, the gas pedal depression degree ACCP and the vehicle speed SPD at step 600. At step 602, the ECU 50 computes the basic injection amount QBASE based on the values ACCP and NE. At a subsequent step 604, the ECU 50 judges whether the vehicle speed SPD is zero. If the determination is positive, the ECU 50 moves to step 606. At step 606, the ECU 50 judges whether the gas pedal depression degree ACCP is greater than the reference value ACCP1 as in step 506 of Fig. 15. If the determinations in steps 604, 606 are positive, the ECU 50 judges the engine 1 is racing and moves to step 608. At step 608, the ECU 50 computes a maximum value QBASEMAX of the basic fuel injection amount QBASE based on the engine speed NE. The value QBASEMAX is the upper limit of the basic fuel injection amount QBASE. The memory of the ECU 50 stores a function data shown in Fig. 19. A solid line represents the relationship between the value QBASEMAX and the engine speed NE. The ECU 50 uses this data for computing the value QBASEMAX. As shown in Fig. 19, the greater the engine speed NE is, the greater the value QBASEMAX becomes. When the engine speed NE is relatively high, the fuel atomization must be enhanced. Thus, the value QBASEMAX is increased to increase the target fuel pressure PTRG when the engine speed NE is high. At step 610, the ECU 50 judges whether the basic injection value QBASE is greater than its maximum value QBASEMAX. If the determination is positive, the ECU 50 moves to step 612 and substitutes the value of the maximum value QBASEMAX for the basic injection amount QBASE. After step 612, or when the determinations in step 604, 606 or 610 are negative, the ECU 50 moves to step 614. At step 614, the ECU 50 compares the basic injection amount QBASE with the maximum injection amount QMAX, and substitutes the smaller value for the final injection amount QFIN. The ECU 50 then temporarily suspends the current routine. The basic injection amount QBASE computed in the current cycle of the routine is temporarily stored in the memory of the ECU 50. The ECU 50 computes the target fuel pressure PTRG based on the basic injection amount QBASE at step 108 of Fig. 5 and then executes step 110. In this manner, the maximum value QBASEMAX of the basic injection amount QBASE is computed based on the engine speed NE. The basic injection amount QBASE is controlled to remain smaller than the value QBASEMAX when the engine 1 is racing. Fig. 20 is a timing chart illustrating such a fuel injection control. From a time t0, the gas pedal 15 is gradually pressed and the engine 1 starts racing. Then, the basic injection amount QBASE starts increasing as the gas pedal depression degree ACCP increases. If the increase of the basic injection amount QBASE is not limited, the value of QBASE keeps increasing as shown by a two-dot chain line as the gas pedal depression degree ACCP increases. As the basic fuel injection amount QBASE increases, the target fuel pressure PTR increases. Thus, the fuel pressure PC significantly increases (see the two-dot chain line in Fig. 17). However, in this embodiment, if the gas pedal depression degree ACCP exceeds the reference value ACCP1 at the time t1, the basic fuel injection amount QBASE increases along the solid line in Fig. 20. That is, the basic injection amount QBASE is controlled to remain smaller than the maximum value QBASEMAX after the time t1. As a result, the increase of the basic injection amount QBASE and the fuel pressure PC are suppressed (see the solid line of Fig. 17). This embodiment therefore has the same advantages as the embodiment of Figs. 15-17. Particularly, the basic fuel injection amount QBASE is controlled to remain smaller than its maximum value QBASEMAX when the engine 1 is racing. This prevents the engine speed NE from racing excessively. Therefore, the amount of exhaust gas is reduced and the fuel economy when the engine 1 is racing is improved. The basic injection amount QBASE is not simply set to a lower level when the engine 1 is racing. That is, the maximum basic fuel injection value QBASEMAX is employed. The basic fuel injection amount QBASE is lowered to match QBASEMAX only when QBASE exceeds QBASEMAX. In other words, the basic fuel injection amount QBASE is not controlled when it is below its maximum value QBASEMAX. Thus, a normal racing of the engine 1 can be performed as long as QBASE is lower than QBASEMAX. A seventh the embodiment of the present invention will now be described with reference to Fig. 21. The differences from the embodiment of Figs. 18-20 will mainly be discussed below, and like or the same reference numerals are given to those components that are like or the same as the corresponding components of the embodiment of Figs 1-8. In the embodiment of Figs. 1-8, the basic injection amount QBASE is determined based on the gas pedal depression degree ACCP. The target fuel pressure PTRG and the final injection amount QFIN are computed based on the basic injection amount PTRG. Therefore, the target fuel pressure PTRG and the final injection amount QFIN are both functions having the gas pedal depression degree ACCP as a parameter. In other words, the values PTRG and QFIN change in accordance with the value ACCP. However, in the seventh embodiment, the gas pedal depression degree ACCP is adjusted when the engine 1 is racing. The basic injection amount QBASE is computed based on the adjusted gas pedal depression degree ACCP (hereinafter referred to as ACCPCON). The routine for adjusting the gas pedal depression degree ACCP will now be described with reference to Fig. 21. This routine is an interrupt executed by the ECU 50 at predetermined time intervals. When entering the routine, the ECU 50 reads the engine speed NE, the basic injection amount QBASE, the gas pedal depression degree ACCP and the vehicle speed SPD. At a subsequent step 704, the ECU 50 judges whether the vehicle speed SPD is zero. If the determination is positive, the ECU 50 moves to step 706 and judges whether the gas pedal depression degree ACCP is greater than the reference value ACCP1 as in step 606 of Fig. 18. If the determinations of steps 704, 706 are both positive, the ECU 50 judges that the engine 1 is racing and moves to step 708. At step 708, the ECU 50 computes a maximum gas pedal depression degree ACCPMAX based on the engine speed NE. The value ACCPMAX is an upper limit of the gas pedal depression degree ACCP. The memory of the ECU 50 stores the function data shown in Fig. 22. A solid line represents the relationship between the value ACCPMAX and the engine speed NE. The ECU 50 uses this data for computing the value ACCPMAX. The greater the engine speed NE is, the greater the value ACCPMAX becomes. The fuel atomization must be enhanced when the engine speed NE is relatively high. Therefore, the target fuel PTRG is increased by increasing the value ACCPMAX. At step 710, the ECU 50 judges whether the gas pedal depression degree ACCP is greater than its maximum value ACCPMAX. If the determination is positive, that is, if ACCP is greater than ACCPMAX, the ECU 50 moves to step 712 and substitutes the value of ACCPMAX for the value of ACCPCON, which is used to adjust ACCP. If the determination is negative in steps 704, 706 and 710, the ECU 50 moves to step 714. At step 714, the ECU 50 substitutes the gas pedal depression degree ACCP, which is detected by the gas pedal sensor 20, for the value ACCPCON. After steps 712 or 714, the ECU 50 temporarily suspends the current routine. The ECU 50 computes the basic injection amount QBASE based on the value ACCPCON. The ECU 50 then computes the final injection amount QFIN based on the computed basic injection amount QBASE and computes the target fuel pressure PTRG based on the basic fuel injection amount QBASE at step 108 of Fig. 5. Therefore, if the value ACCPCON is set to the maximum gas pedal depression degree ACCPMAX when the engine 1 is racing, the value of the basic injection amount QBASE is relatively small. Thus, the values of the basic injection amount QBASE and the final injection amount QFIN, which are computed based on QBASE, are relatively small. As a result, the same advantages as the embodiment of Figs. 18-20 are obtained by this embodiment. An eighth embodiment of the present invention will now be described with reference to Figs. 23-25. The differences from the embodiment of Figs. 1-8 will mainly be discussed below, and like or the same reference numerals are given to those components that are like or the same as the corresponding components of the embodiment of Figs 1-8. Fig. 23 is a cross-sectional view illustrating a fuel pressure control apparatus. The return port 3a of each injector is connected with a fuel tank 8 by a return pipe 11. The return pipe 11 is regulated by an electromagnetic valve 12. The valve 12 includes a spool (not shown) and a pair of solenoids (not shown). The solenoids are located at the ends of the spool. The ECU 50 changes the position of the spool by exciting and de-exciting the solenoids thereby opening and closing the return pipe 11. Once controlled by the ECU 50, the valve 12 remains closed or open until the next time the ECU 50 sends a signal to the valve 12. As described above, part of fuel supplied to each injector 2 leaks into the injector 2 as the injector 2 opens and closes. The leaked fuel is returned to the fuel tank 8 from the return port 3a through the return pipe 11. When the injector 2 is closed, fuel generally does not leak into the injector 2. However, the injector 2, which includes the electromagnetic valve 3, has a number of movable members. Repetitive sliding of the movable members on guide members creates narrow space therebetween. Therefore, even if the injector 2 is closed, a very slight amount of fuel from the common rail 4 gradually leaks into the injector 2. The leaked fuel is returned to the tank 8 through the return pipe 11. This gradually decreases the fuel pressure PC as shown in Fig. 25. In Fig. 25, the engine 1 is stopped at a time t1 and the fuel pressure PC is equal to the requested fuel pressure PTRGSTA, which is suitable for starting the engine 1, at the time t1. However, the pressure PC is lowered as shown by a two-dot chain line if fuel leaks into the injector 2. When the engine 1 is started again at a time t2, fuel is injected with a pressure PC that is lower than the requested fuel pressure PTRGSTA. In this embodiment, the electromagnetic valve 12 is controlled to avoid this drawback. Fig. 24 is a flowchart of a routine for controlling the electromagnetic valve 12. This routine is an interrupt executed by the ECU 50 at predetermined time intervals. When entering this routine, the ECU 50 judges whether the key switch flag XIG is one at step 800. If the determination is negative, that is, if the key switch 52 is at the OFF position, the ECU 50 moves to step 801. At step 801, the ECU 50 computes a lowest reference value PCLOW and a highest reference value PCHI of the fuel pressure PC. The reference values PCHI and PCLOW are determined in relation with the requested fuel pressure PTRGSTA. The highest reference value PCHI is set higher than the requested fuel pressure PTRGSTA, and the lowest reference value PCLOW is set lower than the requested fuel pressure PTRGSTA. At a subsequent step 802, the ECU 50 judges whether the main relay flag XMR is one. If the determination is positive, the ECU 50 moves to step 804. At step 804, the ECU 50 judges whether the fuel pressure PC is lower than the lowest reference value PCLOW. If the determination is negative, the ECU 50 moves to step 808. At step 808, the ECU 50 judges whether the fuel pressure PC is higher than the highest reference value PCHI. If the determination is positive at step 804, that is, if the fuel pressure PC is lower than the lowest reference value PCLOW, the ECU 50 moves to step 806. At step 806, the ECU 50 controls the valve 12 to close the return pipe 11. This stops flow of fuel from the injector 2 to the fuel tank 8 through the return pipe 11. If the determination is positive at step 808, that is, if the fuel pressure PC is higher than the highest reference value PCHI, the ECU 50 moves to step 812. At step 812, the ECU 50 controls the valve 12 to open the return pipe 11. This allows flow of fuel from the injector 2 to the fuel tank 8 through the return pipe 11. If the determinations at steps 804 and 808 are negative, that is, if the fuel pressure PC is between the lowest reference value PCLOW and the highest reference value PCHI, the ECU 50 does not control the valve 12 and temporarily suspends the current routine. In other words, the region including the requested fuel pressure PTRGSTA (PCLOW≦PC≦PCHI) is so called a dead zone. Setting a dead zone prevents hunting in the electromagnetic valve 12 even if the fuel pressure PC fluctuates in the vicinity of the requested fuel pressure PTRGSTA. If the determination is positive at step 800, on the other hand, the ECU 50 moves to step 810. At step 810, the ECU judges whether the engine speed NE is greater than a reference value NE1. The reference value NE1 is used to judge whether the engine 1 has entered the complete combustion state, or whether the engine speed NE is sufficiently increased and the supply pump 6 is discharging sufficient amount of fuel. For example, if the engine 1 has not entered the complete combustion state after the key switch 52 is turned on (NE≦NE1), the ECU 50 controls the valve 12 to close the return pipe 11 at step 806. Therefore, the fuel pressure PC, which has been maintained at the requested fuel pressure PTRGSTA during non-operation of the engine 1, is not rapidly decreased when the key switch 52 is turned on. On the other hand, when the diesel engine 1 has entered the complete combustion state and the engine speed NE is relatively high (NE > NE1), the ECU 50 moves to step 812. At step 812, the ECU 50 controls the valve 12 to open the pipe 11. This allows fuel that has leaked into the injector 2 to flow back to the fuel tank 8 through the return pipe 11. The leaked oil in the injector 2 therefore does not hinder the operation of the injector 2. If the determination is negative at step 802 or after executing steps 806 or 812, the ECU 50 temporarily suspends the current routine. As described above, the electromagnetic valve 12 is closed if the fuel pressure PC is lower than the lowest reference value PCLOW after the key switch 52 is turned off. This prevents fuel in the injector 2 from flowing to the fuel tank 8 through the return pipe 11. Therefore, even if fuel leaks into the injector 2 while engine 1 is running, the fuel leak does not lower the fuel pressure PC. As shown by a solid line in Fig. 25, the fuel pressure PC is maintained substantially equal to the requested fuel pressure PTRGSTA from the time t1, at which the engine 1 is stopped, to the time t2, at which the engine 1 is started again. After the key switch 52 is turned on, the return pipe 11 must be open by the valve 12 for preventing oil leaked in the injector 2 from hindering the operation of the injector 2. However, if the valve 12 is opened at the same time the key switch 52 is turned on, the supply pump 6 is not able to discharge sufficient amount of fuel. This causes the fuel pressure PC, which has been maintained to the requested fuel pressure PTRGSTA, to abruptly drop. However, in this embodiment, even if the key switch 52 is turned on, the valve 12 is remained closed until the amount of fuel discharged from the supply pump 6 reaches a sufficient level. This prevents the fuel pressure PC from abruptly dropping when starting the engine 1 thereby positively starting the engine 1. A ninth embodiment according to the present invention will now be described with reference to Figs. 26-28. Components that are like or the same as the corresponding components of the embodiment of Figs. 1-8 are denoted with the same reference numerals. In the embodiment of Figs. 1-8, the fuel pressure PC in the common rail 4 becomes equal with the requested fuel pressure PTRGSTA, which is the optimal pressure value for starting the engine 1. However, if the electromagnetic valve 12 of the eighth embodiment is not employed, the fuel pressure PC may decrease as time elapses. Furthermore, if the electromagnetic valve 12 is employed, a long time period between the stopping of the engine 1 and the subsequent starting of the engine would cause a decrease in the fuel pressure PC. When the engine is not operated over a long period of time, the engine becomes cool. In such case, it is difficult to sufficiently atomize the injected fuel when starting the engine 1. Accordingly, to guarantee satisfactory starting of the engine 1, it is preferable that the fuel pressure PC be increased for sufficient atomization of the injected fuel. In this embodiment, the fuel pressure PC in the common rail 4 is increased in a sudden manner. As shown in Fig. 28, if the starter 19 is actuated when starting the engine 1, the ECU 50 sets the target fuel pressure PTRG at a relatively high value A1 and controls the PCV 10 such that the fuel pressure PC in the common rail 4 is increased to the high target value A1. This enhances the atomization of the injected fuel when actuating the starter 19 in comparison to when the engine 1 commences normal operation. Thus, the engine 1 is started in a satisfactory manner. The starter 19 is de-actuated when the engine 1 is started. In this state, the fuel pressure PC need not be as high as when starting the engine 1. Thus, if the starter signal STA changes to a state indicating OFF, the ECU 50 sets the target fuel pressure PTRG at a value D1 that is lower than value A1 and controls the PCV 10 such that the fuel pressure PC in the common rail 4 becomes lower than the low target value D1. However, it is difficult to readily decrease the fuel pressure PC from the high target value A1 to the low target value D1. Therefore, the fuel pressure PC actually decreases gradually as shown by the dashed line in Fig. 28. The gradual decrease is caused by the check valve 7 (refer to Fig. 26), which is located in the supply pipe 5 between the supply pump 6 and the common rail 4 to prohibit reverse flow of fuel from the common rail 4 to the pump 6. However, the check valve 7 is necessary to maintain the fuel pressure PC in the common rail 4 at a desired value and thus cannot be eliminated. If the decrease of the fuel pressure PC is gradual, the atomization of the injected fuel may become excessive after the starting of the engine 1. In such case, the sudden combustion pressure change in the combustion chambers may produce noise. Accordingly, in this embodiment, the fuel pressure control apparatus incorporates a relief valve 17 in the common rail 4. The ECU 50 opens the relief valve 17 when certain conditions (described later) are satisfied. As the relief valve 17 opens, the fuel in the common rail 4 is returned to the fuel tank 8 through the return pipe 11. This decreases the fuel pressure PC in the common rail 4. Fig. 27 is a flowchart showing a routine for controlling the relief valve 17. The ECU 50 executes the routine in an interrupting manner at predetermined time intervals. When the ECU 50 enters the routine, the ECU 50 carries out step 901 to determine whether or not the starter signal STA, which is sent from the starter switch 19, indicates an ON state. If the starter signal STA indicates an ON state, the starter 19 is actuated. In this case, the ECU 50 proceeds to step 902 and sets the starter flag XSTA to one. The ECU 50 then proceeds to step 903 and closes the relief valve 17. Afterwards, the ECU 50 temporarily terminates subsequent processing. With reference to Fig. 28, if the starter 19 is in an actuated state, the ECU 50 sets the target fuel pressure PTRG to a relatively high value A1 and controls the PCV 10 such that the fuel pressure PC in the common rail 4 becomes equal to the high target value A1. That is, the ECU 50 controls the PCV 10 to increase the amount of pressurized fuel discharged from the supply pump 6. Accordingly, when the starter 19 is actuated, the relief valve 17 is closed. This readily increases the fuel pressure PC in the common rail 4. The target value A1 can be set in accordance with the operating state of the engine 1 when starting the engine 1. At step 901, if it is determined that the starter signal STA indicates an OFF state, the ECU 50 determines that the engine 1 has already been started and proceeds to step 904. At step 904, the ECU 50 determines whether the starter flag XSTA is set at one. If it is determined that the starter flag XSTA is set at one, the ECU 50 judges that the starter signal STA indicated an ON state before executing the present cycle of the routine. In this case, the ECU 50 proceeds to step 905. At step 905, the engine 1 has been started and the fuel pressure PC in the common rail 4 must thus be readily decreased. Therefore, the ECU 50 opens the relief valve 17. As the relief valve 17 opens, the fuel in the common rail 4 is returned to the fuel tank 8 through the return pipe 11. If the starter signal STA changes to an OFF state, the ECU 50 sets the target fuel pressure PTRG at a low value D1 and controls the PCV 10 such that the fuel pressure PC in the common rail 4 is varied to the low target value D1, as shown in Fig. 28. In other words, the ECU 50 controls the PCV 10 such that the amount of pressurized fuel discharged by the supply pump 6 decreases. Accordingly, the relief valve 17 is opened immediately after the start signal STA indicates an OFF state. This decreases the fuel pressure PC in the common rail 4 at a faster rate in comparison with the prior art. The target value D1 can be changed in accordance with the operating state of the engine 1. At step 906, the ECU 50 determines whether or not the fuel pressure PC detected by the fuel pressure sensor 22 has decreased to the low target value D1. If it is determined that the fuel pressure PC has not yet reached the target value D1, the ECU 50 temporarily terminates subsequent processing and repetitively carries out steps 901, 904, 905, 906. Accordingly, the relief valve 17 remains opened until the fuel pressure PC reaches the target value D1. If it is determined that the fuel pressure PC has reached the target value D1, the ECU 50 proceeds to step 907 and sets the starter flag XSTA to zero. Afterwards, the ECU 50 terminates subsequent processing. By setting the starter flag XSTA to zero, the ECU 50 proceeds to step 903 from step 904 when executing the next cycle of this routine. In this case, the ECU 50 closes the relief valve 17 in step 903 and prohibits the fuel in the common rail 4 from returning to the fuel tank 8 through the return pipe 11. As described above, in this embodiment, the relief valve 17 is opened immediately after the starter signal STA changes from an ON state to an OFF state. This readily decreases the fuel pressure PC in the common rail 4 to a value that optimally corresponds with the normal operating state of the engine 1. Accordingly, the injected fuel is atomized appropriately and noise is not produced immediately after the starting of the engine 1. In this embodiment, the relief valve 17 is arranged in the common rail 4. However, the relief valve 17 may be arranged at other positions as long as the valve 17 is located downstream of the check valve 7. Furthermore, the relief valve 17 may be replaced with any type of device that lowers the fuel pressure PC. For example, a device that performs invalid injection controlling such as that described in Japanese Unexamined Patent Publication No. 2-191865 may be employed in lieu of the relief valve 17. A tenth embodiment according to the present invention will now be described with reference to Figs. 29-31. The tenth embodiment is a modification of the embodiment illustrated in Figs. 26-28. Components that are like or the same as the corresponding components of the embodiment of Figs. 26-28 are denoted with the same reference numerals. In the embodiment of Figs. 26-28, the relief valve 17 is opened immediately after the starter signal STA changes from a state indicating ON to a state indicating OFF to decrease the pressure in the common rail 4. However, in this embodiment, the relief valve 17 is eliminated. Instead of the relief valve 17, the PCV 10 is controlled such that the fuel pressure PC decreases a certain value during the time period starting from when the starter signal STA changes to an ON state and ending when the starter signal STA changes to an OFF state. Fig. 29 is a flowchart showing a routine for controlling the PCV 10. The ECU 50 executes the routine in an interrupting manner for every predetermined time interval. When entering the routine, the ECU 50 first carries out step 121 and determines whether the starter signal STA, which is sent from the starter switch 19, indicates an ON state. If the starter signal STA indicates an ON state, the starter 19 is actuated. In this case, the ECU 50 proceeds to step 122 and adds one in an incremental manner to the previous count value SCTAON, which indicates the elapsed time from when the starter 19 had been actuated in the previous cycle, and renews the count value SCTAON with the obtained sum. The count value SCTAON indicates the present elapsed time from when the starter 19 had been actuated. At step 123, the ECU 50 determines whether or not the starter flag XSTA is set at zero. If the starter flag XSTA is set at zero, the starter signal STA would have indicated an OFF state before entering the present cycle of this routine. In this case, the ECU 50 proceeds to step 124 and sets the starter flag XSTA to one. At step 125, the ECU 50 sets the target fuel pressure PTRG to a relatively high value A1 (refer to Fig. 31). At step 126, the ECU 50 computes a sustaining time TPT based on the coolant temperature THW. The sustaining time TPT refers to the time period during which the target fuel pressure PTRG is required to be sustained at the high target value A1. The fuel pressure PC in the common rail 14 required for satisfactory starting of the engine 1 is guaranteed when the target fuel pressure PTRG is sustained at the high target value A1 over the sustaining time TPT. The ECU 50 includes a memory that stores functional data for defining the relationship between the coolant temperature THW and the sustaining time TPT as shown in Fig. 30. The ECU 50 refers to the functional data to compute the sustaining time TPT. As shown in the graph of Fig. 30, the sustaining time TPT is set such that it becomes longer as the coolant temperature THW becomes lower. When the coolant temperature THW is low, the engine temperature is also low. In such case, it is difficult to atomize the injected fuel in an appropriate manner. Therefore, the sustaining time TPT must be prolonged to enhance atomization of the injected fuel for satisfactory starting of the engine 1. At step 127, the ECU 50 determines whether or not the count value SCTAON has reached a value corresponding to the sustaining time TPT, which was obtained in step 126. If it is determined that the count value SCTAON has not yet reached the sustaining time TPT, the ECU 50 temporarily terminates subsequent processing. Although not shown in the flowchart of Fig. 29, steps similar to those of steps 110 and 112, which are illustrated in the flowchart of the first embodiment (Fig. 5), are executed after determining the target fuel pressure PTRG to compute the valve opening time TF. Accordingly, the ECU 50 controls the PCV 10 in accordance with the valve opening time TF such that the fuel pressure PC in the common rail 4 becomes equal to the target fuel pressure PTRG. At step 123, if it is determined that the starter flag XSTA is set at one, the ECU 50 proceeds to step 131. As described above, the starter flag XSTA is set at zero immediately after the starter signal STA changes from a state indicating OFF to a state indicating ON. In such state, the starter flag XSTA is set at one in subsequent step 124. Accordingly, once the processing of steps 124-126 is carried out, the ECU 50 proceeds from step 123 to step 131 until the starter signal STA indicates an OFF state. At step 131, the ECU 50 determines whether or not the present target fuel pressure PTRG is set at value C1. The target fuel pressure PTRG is replaced by value C1 after the sustaining time TPT elapses and before the starter signal STA changes to an OFF state. Furthermore, value C1 is lower than value A1 and higher than the value D1, which is set as the target fuel pressure PTRG when the starter signal STA changes to an OFF state (refer to Fig. 31). If the present target fuel pressure PTRG has not yet reached value C1, the ECU 50 proceeds to step 127. If it is determined that the count value SCTAON has reached a value corresponding with the sustaining time TPT in step 127, the ECU 50 proceeds to step 128. At step 128, the ECU 50 sets the target fuel pressure PTRG at a value B1. The value B1 is a variable that varies between values A1 and C1. Additionally, the value B1 is decreased by a predetermined amount of α each time the ECU 50 executes step 128. The ECU 50 then proceeds to step 129 and determines whether or not the present target fuel pressure PTRG has reached the minimum B1 value B1LG, or value C1. If it is determined that the target fuel pressure PTRG has not yet reached value B1LG, the ECU 50 temporarily terminates subsequent processing. Thus, after the count value SCTAON reaches a value corresponding with the sustaining time TPT, the target fuel pressure PTRG decreases gradually until reaching value B1LG (refer to Fig. 31). If it is determined that the present target fuel pressure PTRG has reached value B1LG in step 129, the ECU 50 proceeds to step 130 and sets the target fuel pressure PTRG to value C1, which corresponds to value B1LG. The ECU 50 temporarily terminates subsequent processing afterwards. Accordingly, after the target fuel pressure PTRG is set at value C1, the ECU 50 does not proceed from step 131 to step 127 unless the starter signal STA changes to an OFF state. In other words, once the target fuel pressure PTRG is set at value C1 in step 130, the target fuel pressure PTRG is maintained at value C1 until the starter signal STA changes to an OFF state (refer to Fig. 31). If it is determined that the starter signal STA indicates an OFF state in step 121, the ECU 50 proceeds to step 132. At step 132, the ECU 50 determines that the starting of the engine 1 has been completed and sets the target fuel pressure PTRG at the low value D1 (refer to Fig. 31). The ECU 50 then proceeds to step 133 and sets the starter flag XSTA to zero. At step 134, the ECU 50 clears and resets the count value SCTAON to zero. The ECU 50 then temporarily terminates subsequent processing. In other words, after the engine 1 starts and commences normal operation, the ECU 50 controls the PCV 10 in accordance with the target value D1, which is lower than the target value A1 used when starting the engine 1. As shown in Fig. 31, the ECU 50 sets the target fuel pressure PTRG at the high value A1 when actuating the starter 19. After maintaining the target fuel pressure PTRG at the high value A1 over a period corresponding with the sustaining time TPT, which is the minimum time required for satisfactory starting of the engine 1, the target fuel pressure PTRG is decreased gradually until reaching value C1. Accordingly, when the starter 19 is de-actuated, the fuel pressure PC in the common rail 4 is lowered to value C1, which is lower than the high target value A1. The time required to decrease the fuel pressure PC from value C1 to value D1 is shorter than the time required to decrease the fuel pressure PC from value A1 to value D1. Therefore, when the starter 19 is de-actuated, the fuel pressure PC is readily decreased to value D1, which is the optimal value for normal operation of the engine 1. Accordingly, the advantages of the embodiment illustrated in Figs. 26-28 are also obtained in this embodiment. If the fuel pressure PC is excessively decreased while the starter 19 is in an actuated state, the starting of the engine 1 may be unsatisfactory. In this embodiment, however, the fuel pressure does not become lower than value C1 when the starter 19 is actuated. Thus, the fuel pressure PC is prevented from becoming too low when starting the engine 1. Accordingly, satisfactory starting of the engine 1 is guaranteed. Unlike the embodiment of Figs. 26-28, the relief valve 17 need not be provided. This simplifies the structure of the fuel control apparatus and reduces costs. In this embodiment, the sustaining time TPT of the target value A1 is based on the coolant temperature THW. However, the sustaining time TPT may be based on other parameters that indicate the temperature of the engine 1. As another option, the sustaining time TPT may be a fixed value. The target fuel pressure PTRG may be maintained at the high value A1 until the engine speed NE reaches a predetermined value. If the engine speed NE reaches a predetermined value, satisfactory starting is guaranteed regardless of the fuel pressure PC being low. In this embodiment, the target fuel pressure PTRG is decreased in a linear manner when represented by a graph, as illustrated in Fig. 31. However, the target fuel pressure PTRG may also be decreased in a curved manner when represented by a graph. It should be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the scope of the invention as described into the claims. Particularly, it should be understood that the invention may be embodied in the following forms. In the embodiments of Figs. 1-20, the requested fuel pressure PTRGSTA is computed based on the coolant temperature THW. However, as shown by a two-dot chain line in Fig. 6, the value of PTRGSTA may be constant at any given coolant temperature THW. Further, PTRGSTA may be increased as the fuel temperature THF increases. This makes the fuel pressure PC more suitable for starting the engine 1. In the embodiments of Figs. 1-20, the feedback coefficient K is changed when the key switch 52 is turned off. However, the value of the feedback coefficient K may be constant regardless of turning off of the switch 52. Alternatively, as shown in Fig. 7, the feedback coefficient K may be a value K1 before the key switch 52 is turned off and may be switched to a value K2, which is bigger than the value K1, after the key switch 52 is turned off. In the embodiment of Figs. 10-13, the normal injection control is continued until the injection continuation time NECT, which is computed based on the engine speed NEOFF, has elapsed since the key switch 52 is turned off. However, the engine stopping injection control may be started when the key switch 52 is turned off. Further, the ratio of decrease of the fuel injection amount may be controlled to be smaller for a higher value of the engine speed NEOFF. This construction positively decreases the fuel pressure PC to the requested fuel pressure PTRGSTA if the fuel pressure PC is relatively high when the key switch 52 is turned off. In the embodiment of Figs. 10-13, the injection continuation time NECT is set longer for a higher value of the engine speed NEOFF. However, the injection continuation time NECT may be a constant value NECT3 shown by a two-dot line in Fig. 10. The time NECT3 is preferably set to a value that decreases the fuel pressure PC to the requested fuel pressure PTRGSTA at any engine speed NEOFF, that is, at any value of the fuel pressure PC at the time of turning the key switch 52 off. In the embodiment of Figs. 10-13, the injection continuation time NECT is computed based on the engine speed NEOFF. However, the value of the injection continuation time NECT may be changed to a value QCT by changing part of the routine of Fig. 11. At step 302 in the routine of Fig. 11, the basic injection amount QBASE when the key switch 52 is turned off is labeled as a value QBASEOFF. Thereafter, at step 304, ECU 50 computes the injection continuation time QCT based on the value QBASEOFF. A higher value of QBASEOFF results in a higher fuel pressure PC when the key switch 52 is turned off. Accordingly, it takes longer to decrease the fuel pressure PC to the requested fuel pressure PTRGSTA. Therefore, like the relationship between the engine speed NEOFF and the injection continuation time NECT in Fig. 10, the injection continuation time QCT is lengthened for a greater value of QBASEOFF. This construction has the same advantages as the embodiment of Figs. 10-13. In the embodiment of Fig. 14, the flag XSTOP for stopping the engine 1 is set to one when the fuel pressure PC is lower than the requested fuel pressure PTRGSTA, that is, when the determination of step 408 of Fig. 14 is positive. This stops the normal injection control. In other words, the requested fuel pressure PTRGSTA is used as a criterion for determining whether to stop the normal injection control. The engine stopping injection control is performed immediately after the normal injection control. Thus, the fuel pressure PC can be either increased or decreased during the engine stopping injection control. Therefore, values other than PTRGSTA may be used as the criterion. In the embodiments of Figs. 15-22, the maximum target fuel pressure PTRGMAX, the maximum basic injection amount QBASEMAX and the maximum gas pedal depression degree ACCPMAX are employed and the target fuel pressure PTRG, the basic injection amount QBASE and the gas pedal depression degree ACCP are controlled to remain lower than the values PTRGMAX, QBASEMAX and ACCPMAX. However, the values PTRG, QBASE and ACCP may be multiplied by a compensation coefficient that is smaller than one, for example, when the engine 1 is racing. This also limits the values PTRG, QBASE and ACCP. In the embodiments of Figs. 15-22, the maximum values PTRGMAX, QBASEMAX and ACCPMAX of the target fuel pressure PTRG, the basic injection amount QBASE and the gas pedal depression degree ACCP are increased for a higher engine speed NE. However, the values PTRGMAX, QBASEMAX and ACCPMAX may be constant as shown in two-dot chain lines in Figs. 16, 19 and 22 at any engine speed NE. In the embodiment of Figs. 15-22, the engine 1 is judged to be racing when the vehicle speed SPD is zero and the gas pedal depression degree ACCP is equal to or greater than ACCP1. However, the engine 1 may be judged to be racing when the selector lever of the transmission is at the neutral position or the parking position and the gas pedal depression degree ACCP or the engine speed NE is equal to or greater than a reference value. Alternatively, the values PTRG, QBASE and ACCP may be limited simply when the selector lever of the transmission is at the neutral position or at the parking position, that is, when the engine 1 can be racing. In the embodiments of Figs. 10-14, the fuel pressure PC in the common rail 4 is decreased to the requested fuel pressure PTRGSTA by causing the injectors 2 to inject fuel. Alternatively, a relief valve as illustrated in the embodiment of Figs. 26-28 may be provided in the common rail 4. In this case, the ECU 50 controls the relief valve to lower the fuel pressure PC. In the embodiments of Figs. 1-10, 15-17 and 23-25, the normal fuel injection is continued until a predetermined time has elapsed since the key switch 52 is turned off. However, the fuel injection may be stopped at the same time as the key switch 52 is turned off. In this case, the crankshaft coasts and keeps activating the supply pump 6 until the crankshaft completely stops. During this time, the supply pump 6 is capable of discharging fuel. The present invention may be embodied in a direct injection type gasoline engine, in which injectors directly inject fuel into the combustion chambers. A direct injection type gasoline engine includes a delivery pipe, which stores highly pressurized fuel. The delivery pipe corresponds to the common rail 4 in the diesel engine 1. In the illustrated embodiments, pressurized fuel in a common rail, or accumulator, is injected from injectors. However, the apparatus according to the present invention may be employed in an engine in which fluid such as engine oil is sent from a pump to an accumulator and the pressure of the fluid in the accumulator is used to inject fuel from injectors. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive and the invention is not to be limited to the details given herein, but may be modified within the scope and equivalence of the appended claims.

Claims: An apparatus for controlling fuel pressure in an engine, the apparatus comprising: an accumulator (4) for storing highly pressurized fuel sent from a pump (6); an injector (2) for injecting fuel stored in the accumulator (4) into a combustion chamber of the engine; and an adjuster (50, 10) for adjusting the pressure of fuel in the accumulator (4) according to the running state of the engine, the apparatus being characterized in that the adjuster (50, 10) adjusts the fuel pressure in the accumulator (4) to approach a predetermined desired value, suitable for subsequent starting of the engine, within a period from when an operation for stopping the engine is initiated to when the engine is completely stopped. The apparatus according to claim 1 characterized in that the adjuster (50, 10) computes the desired value based on the temperature of the engine when the engine stopping operation is initiated. The apparatus according to claim 2 characterized in that the higher the engine temperature is, the lower the computed desired value is. The apparatus according to any one of claims 1 to 3 characterized in that , during normal engine operation, the fuel pressure in the accumulator (4) is based on at least the engine speed. The apparatus according to any one of claims 1 to 4 characterized in that the adjuster (50, 10) regulates the amount of fuel sent to the accumulator (4) from the pump (6) to adjust the fuel pressure in the accumulator (4). The apparatus according to claim 5 characterized in that , when the engine stopping operation is initiated, the adjuster (50, 10) maximizes the amount of fuel sent from the pump (6) if the fuel pressure in the accumulator (4) is lower than the desired value, and the adjuster (50, 10) stops the pump (6) from sending fuel if the fuel pressure in the accumulator (4) is equal to or greater than the desired value. The apparatus according to any one of claims 1 to 6 characterized in that the adjuster (50, 10) causes the injector (2) to continue the fuel injection to lower the fuel pressure in the accumulator (4) toward the desired value until a predetermined period has elapsed from when the engine stopping operation is initiated. The apparatus according to claim 7 characterized in that the adjuster (50, 10) determines the length of the predetermined period in accordance with the fuel pressure in the accumulator (4) when the engine stopping operation is initiated. The apparatus according to claim 8 characterized in that the higher the fuel pressure in the accumulator (4) is, the longer the predetermined period is. The apparatus according to claims 8 or 9 characterized in that the adjuster (50, 10) detects the engine speed, which indicates the fuel pressure in the accumulator (4). The apparatus according to any one of claims 1 to 6 characterized in that the adjuster (50, 10) causes the injector (2) to continue the fuel injection until the fuel pressure in the accumulator (4) falls to the desired value from when the engine stopping operation is initiated. The apparatus according to claim 1 characterized in that , when the engine is racing, the adjuster (50, 10) limits the fuel pressure in the accumulator (4). The apparatus according to claim 12 characterized in that the adjuster (50, 10) computes a target value of the fuel pressure in the accumulator (4) based on at least the engine speed, and wherein the adjuster (50, 10) limits the target value when the engine is racing. The apparatus according to claim 13 characterized in that the adjuster (50, 10) limits the target value such that the target value does not exceed an upper limit value, which is determined in accordance with the engine speed or is predetermined. The apparatus according to claim 12 characterized by a computer for computing the amount of fuel to be injected from the injector (2) based on at least the engine speed, wherein the adjuster (50, 10) limits the computed injection amount when the engine is racing. The apparatus according to claim 15 characterized in that the adjuster (50, 10) limits the computed injection amount such that the computed injection amount does not exceed an upper limit value, which is determined in accordance with the engine speed or is predetermined. The apparatus according to claim 1 characterized by : a preventing device (7) for preventing fuel from flowing backward to the pump (6) from the accumulator (4); a determiner (50) for determining whether the running state of the engine has changed from a first state to a second state, wherein the first state requires that the fuel pressure in the accumulator (4) be relatively high, wherein the second state requires the fuel pressure in the accumulator (4) to be low compared with the first state, and wherein the adjuster (50, 10) regulates the amount of fuel sent to the accumulator (4) from the pump (6) to adjust the fuel pressure in the accumulator (4) to the required value; and a relief device (17) for releasing fuel, which exists downstream of the preventing device (7), to promptly lower the fuel pressure in the accumulator (4) when the running state of the engine changes from the first state to the second state. The apparatus according to claim 17 characterized in that the relief device is a relief valve (17) for releasing fuel from the accumulator (4). The apparatus according to claim 1 characterized by : a preventing device (7) for preventing fuel from flowing backward to the pump (6) from the accumulator (4); and a determiner (50) for determining whether the running state of the engine has changed from a first state to a second state, wherein the first state requires that the fuel pressure in the accumulator (4) be a relatively high value, wherein the second state requires that the fuel pressure in the accumulator (4) be a relatively low value, wherein the adjuster (50, 10) regulates the amount of fuel sent to the accumulator (4) from the pump (6) to adjust the fuel pressure in the accumulator (4) to the required value, and wherein the adjuster (50, 10) lowers the fuel pressure in the accumulator (4) from the high value to an intermediate value, which is between the high value and the low value, before the running state of the engine changes from the first state to the second state. The apparatus according to claim 19 characterized in that when the running state of the engine reaches the first state, the adjuster (50, 10) lowers gradually the fuel pressure in the accumulator (4) to the intermediate value after maintaining the fuel pressure in the accumulator (4) at the high value for a certain time period. The apparatus according to claim 20 characterized in that the lower the temperature of the engine is, the longer the certain time period is. The apparatus according to any one of claims 17 to 21 characterized in that the determiner (50) determines that the running state of the engine is the first state when a starter (19) is operating to crank the engine, and wherein the determiner (50) determines that the running state of the engine is the second state when the starter (19) is stopped.

Inventors: KOTANI AKIRA, KOTANI, AKIRA
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1998
Main classification: F02D41
IPC codes: F02D41