CONTROL DEVICE AND CONTROL METHOD OF FUEL PRESSURE OF ENGINE

Abstract

 A control device of an engine, the control device includes an ECU (27) that is configured to: control a fuel discharge amount of a fuel pump (14) such that an actual fuel pressure (PR) of a fuel to be supplied to an injector (24) is equal to a target fuel pressure (PRRQ); perform a predictive control when the ECU predicts that an engine load would decrease, the predictive control is a control to decrease the fuel discharge amount to be smaller than a value corresponds to a current value of the target fuel pressure; and control the fuel discharge amount to maintain the actual fuel pressure at a lower limit guard value (PRGD) when the actual fuel pressure is equal to or lower than the lower limit guard value and is lower than the target fuel pressure, during the execution of the predictive control.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to a control device and a control method of a fuel pressure of an engine, the control device and the control method being for controlling a pressure of a fuel to be supplied to the engine.

2. Description of Related Art

[0002] In an injector for injecting a fuel to be burnt in an engine, an injector period per injection has a lower limit. When a pressure (fuel pressure) of the fuel to be supplied to the injector is high, a minimum amount (minimum injection amount) of an injectable fuel increases. Accordingly, when the fuel pressure is high at the time of a low load operation with a small fuel injection amount, the fuel injection amount may become larger than a request. Particularly, in an engine with a high fuel pressure like a cylinder-injection gasoline engine, an influence of the fuel pressure with respect to the minimum injection amount of the injector becomes large. Accordingly, in such an engine, a target fuel pressure is set based on an engine rotation speed and an engine load, and a fuel discharge amount of a fuel pump is controlled so that an actual fuel pressure approaches the target fuel pressure. Hereby, the fuel pressure is controlled to have a moderate value suitable for an operating state of the engine.

[0003] Japanese Patent Application Publication No. 2009-209829 (JP 2009-209829 A) describes a fuel pressure control device that performs a feedback control on a fuel discharge amount of a fuel pump according to a deviation between an actual fuel pressure and a target fuel pressure. However, even if such a feedback control is optimized, a fuel consumption decreases according to a decrease of an engine load and a fuel pressure is hard to decrease at the time of a deceleration operation, thereby resulting in that the fuel pressure cannot follow the target fuel pressure. In view of this, in the fuel pressure control device described in JP 2009-209829 A, when it is determined that a deceleration operation is performed from an operation state of an accelerator pedal, the feedback control is stopped temporarily, so as to limit the fuel discharge amount of the fuel pump to a reduction command value set based on an engine rotation number and an accelerator pedal operation amount. This can achieve immediate pressure reduction.

SUMMARY OF THE INVENTION

[0004] When an engine load decreases, a fuel injection amount decreases, so that a fuel consumption decreases. Because of this, even if the fuel discharge amount of the fuel pump is limited, an immediate decrease of the fuel pressure may become difficult. On that account, in order to surely perform a decrease of the fuel pressure at the time of a deceleration operation, it is preferable to start limitation on the fuel discharge amount at an earlier stage by predicting, at an earlier stage, that the engine load decreases in the future. However, depending on an operation condition of the engine, a decrease of the actual engine load may become slower than the prediction. In such a case, the limitation on the fuel discharge amount is performed in a state where the engine load is high and the fuel consumption is large, which may result in that the fuel pressure decreases too much.

[0005] The present invention provides a control device and a control method of a fuel pressure of an engine, the control device and the control method being able to restrain an excessive decrease of the fuel pressure and to realize an immediate decrease of the fuel pressure at the time when an engine load decreases.

[0006] An example aspect of the disclosure provides a control device of a fuel pressure of an engine, the engine including an injector and a fuel pump, the control device includes an electronic control unit. The electronic control unit is configured to: control a fuel discharge amount of the fuel pump such that an actual fuel pressure is equal to a target fuel pressure, the actual fuel pressure is a current value of a pressure of a fuel to be supplied to the injector, the target fuel pressure is set based on an engine load; perform a predictive pressure reduction control when the electronic control unit predicts that the engine load would decrease, the predictive pressure reduction control is a control to decrease the fuel discharge amount of the fuel pump to be smaller than a value set according to a current value of the target fuel pressure; and perform a lower limit guard process when the actual fuel pressure is equal to or lower than a lower limit guard value and is lower than the target fuel pressure, during the execution of the predictive pressure reduction control, the lower limit guard process is a process of controlling the fuel discharge amount so as to maintain the actual fuel pressure at the lower limit guard value.

[0007] When it is predicted that the engine load would decrease in the future, the predictive pressure reduction control is executed so as to decrease the fuel discharge amount of the fuel pump before the engine load actually decreases and a fuel consumption decreases. On that account, it is possible to immediately decrease the actual fuel pressure at the time of the decrease of the engine load. When the engine load decreases as predicted after the start of the predictive pressure reduction control at this time, the target fuel pressure also decreases according to the decrease of the engine load. In the meantime, if the engine load does not decrease as predicted but remains at a high value after the start of the predictive pressure reduction control, the target fuel pressure is also maintained at a high value. Therefore, when the target fuel pressure does not decrease though the actual fuel pressure decreases during the execution of the predictive pressure reduction control, the engine load does not decrease as predicted, which may cause an excessive decrease of the actual fuel pressure.

[0008] In that respect, in the fuel pressure control device of the engine, when the actual fuel pressure decrease to a specific lower limit guard value or less and the actual fuel pressure becomes a value that is less than the target fuel pressure during the execution of the predictive pressure reduction control, the lower limit guard process is performed. By the lower limit guard process, a control on the fuel discharge amount is performed so as to maintain the actual fuel pressure at the lower limit guard value, so that a further decrease of the actual fuel pressure is restrained. Accordingly, according to the fuel pressure control device of the engine, it is possible to restrain an excessive decrease of the fuel pressure and to realize an immediate decrease of the fuel pressure at the time of the decrease of the engine load.

[0009] In the control device, the electronic control unit may be configured to finish the predictive pressure reduction control and restart a control on the fuel discharge amount such that the actual fuel pressure is close to the target fuel pressure, when a duration from the start of the predictive pressure reduction control is a predetermined value or more and when a value obtained by subtracting the target fuel pressure from the actual fuel pressure is equal to or lower than a predetermined value. According to the above configuration, it is possible to restrain such a situation that the predictive pressure reduction control is continued unnecessarily and the actual fuel pressure is decreased excessively.

[0010] In the control device, the electronic control unit may be configured to limit the fuel discharge amount in the predictive pressure reduction control by stopping a pressurizing operation of the fuel pump. According to the above configuration, it is possible to decrease the actual fuel pressure more immediately, by stopping the fuel discharge of the fuel pump during the execution of the predictive pressure reduction control.

[0011] In the control device, the electronic control unit may be configured to finish the lower limit guard process when any of the following conditions is satisfied during the execution of the lower limit guard process: a condition that the target fuel pressure is equal to or lower than the lower limit guard value; a condition that the duration from the start of the predictive pressure reduction control is equal to or higher than a predetermined value; and a condition that the electronic control unit predicts that an increasing rate of a future engine load would exceed a predetermined value. According to the above configuration, a fuel discharge amount control according to a normal target fuel pressure can be restarted by finishing the lower limit guard process at a suitable timing.

[0012] In the control device, the electronic control unit may be configured to predict the decrease of the engine load based on a requested torque, the requested torque is a requested value of an engine torque.

[0013] Another example of the disclosure provides a control method of a fuel pressure of an engine, the engine including an injector and a fuel pump, the control method includes: controlling a fuel discharge amount of the fuel pump such that an actual fuel pressure is equal to a target fuel pressure, the actual fuel pressure is a current value of a pressure of a fuel to be supplied to the injector, the target fuel pressure is set based on an engine load; decreasing the fuel discharge amount of the fuel pump to be smaller than a value set according to a current value of the target fuel pressure, when it is predicted that the engine load would decrease; and controlling the fuel discharge amount so as to maintain the actual fuel pressure at a lower limit guard value, when the actual fuel pressure is equal to or lower than the lower limit guard value and is lower than the target fuel pressure during execution of a predictive pressure reduction control.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Features, advantages, and technical and industrial significance of exemplary embodiments of the invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is a schematic drawing schematically illustrating a configuration of a fuel system of an engine to which one embodiment of a fuel pressure control device of an engine is applied;

FIG. 2 is a graph illustrating a relationship of a target fuel pressure PRRQ with an engine rotation number NE and an engine load KL, the target fuel pressure PRRQ being calculated in the fuel pressure control device of the embodiment;

FIG. 3 is a flowchart of a predictive pressure reduction/lower limit guard execution determination routine executed in the fuel pressure control device of the embodiment;

FIG. 4 is a flowchart of a fuel discharge amount control routine executed in the fuel pressure control device of the embodiment;

FIG. 5 is a time chart showing one example of an execution mode of a predictive pressure reduction control in a case where a lower limit guard process is not executed at the time when a decrease of an engine load delays; and

FIG. 6 is a time chart showing one example of an execution mode of a predictive pressure reduction control in a case where a lower limit guard process is not performed at the time when a decrease of an engine load delays.

DETAILED DESCRIPTION OF EMBODIMENTS

[0015] One embodiment of a fuel pressure control device of an engine is described below in detail with reference to FIGS. 1 to 6. The fuel pressure control device of the present embodiment is applied to a cylinder-injection gasoline engine to be provided in a vehicle.

[0016] As illustrated in FIG. 1, a fuel system of an engine to which the fuel pressure control device of the present embodiment is applied includes a fuel tank 10 for accumulating a fuel therein. A filter 11 and a feed pump 12 are provided inside the fuel tank 10. The filter 11 filters the fuel. The feed pump 12 pumps up the fuel inside the fuel tank 10 via the filter 11. The feed pump 12 is connected to a high-pressure fuel pump 14 via a low-pressure fuel pipe 13 provided with a check valve 25.

[0017] The high-pressure fuel pump 14 includes a cylinder 15 and a plunger 16 disposed in the cylinder 15. The plunger 16 is configured to slide in a reciprocating manner along the cylinder 15 by means of a cam 18 provided in a camshaft 17 of the engine. A pressurizing chamber 19 is formed in the cylinder 15 such that the pressurizing chamber 19 is sectioned by a tip of the plunger 16. The low-pressure fuel pipe 13 is connected to the pressurizing chamber 19 via a discharge valve 20, which is a normally opened solenoid configured to be closed due to current application. A high-pressure fuel pipe 22 is connected to the pressurizing chamber 19 via a check valve 21. The high-pressure fuel pump 14 is connected to a delivery pipe 23 via the high-pressure fuel pipe 22. A cylinder injector 24 provided in a combustion chamber of each cylinder of the engine is connected to the delivery pipe 23. The delivery pipe 23 is provided with a fuel pressure sensor 26. The fuel pressure sensor 26 detects a pressure (an actual fuel pressure PR) of the fuel that is accumulated inside the delivery pipe 23 and supplied to the cylinder injector 24 of each cylinder.

[0018] In the high-pressure fuel pump 14, when the plunger 16 moves down in a state where current application to the discharge valve 20 is stopped so as to open the discharge valve 20, the fuel is sucked up from the low-pressure fuel pipe 13 into the pressurizing chamber 19 according to an increase in a volume of the pressurizing chamber 19. When the plunger 16 moves up with the discharge valve 20 being opened, the fuel thus sucked up into the pressurizing chamber 19 is returned to the low-pressure fuel pipe 13 along with a decrease in the volume of the pressurizing chamber 19. Meanwhile, when current application is started to close the discharge valve 20 during the moving-up of the plunger 16, the fuel sealed in the pressurizing chamber 19 is pressurized along with the decrease in the volume of the pressurizing chamber 19. When the pressure of the fuel in the pressurizing chamber 19 increases to a valve opening pressure of the check valve 21, the check valve 21 is opened, so that the fuel thus pressurized in the pressurizing chamber 19 is discharged to the high-pressure fuel pipe 22. In the high-pressure fuel pump 14, a fuel discharge amount can be adjusted by changing a current application duty ratio DUTY, which is a ratio of a current application period of the discharge valve 20 with respect to a moving-up/down period of the plunger 16.

[0019] The engine including such a fuel system is controlled by an electronic control unit (ECU) 27. The electronic control unit 27 includes: a central processing unit (CPU) that performs various arithmetic processes for an engine control; a read only memory (ROM) in which programs and data for the engine control are stored; and a random access memory (RAM) that temporarily stores an operation result of the central processing unit, detection results of sensors, and so on. Detection signals from not only the fuel pressure sensor 26, but also a crank angle sensor 28, an air flow meter 29, an accelerator pedal sensor 30, and the like are input into the electronic control unit 27. The crank angle sensor 28 detects a rotational phase (a crank angle) of a crankshaft of the engine. The air flow meter 29 detects an intake-air amount GA of the engine. The accelerator pedal sensor 30 detects a stepping amount of an accelerator pedal (an accelerator opening degree ACCP).

[0020] The electronic control unit 27 calculates an engine rotation number NE from the crank angle sensor 28. The electronic control unit 27 calculates an engine load KL from the engine rotation number NE and a detection result of the air flow meter 29. Based on a detection result of the accelerator pedal sensor 30, and the like, the electronic control unit 27 calculates a requested torque TQ, which is a requested value of an engine torque, and a future torque TQRQ, which is a predicted value of the engine torque at the time when the requested torque TQ is realized within a range where an intake-air amount GA can response. The electronic control unit 27 controls a fuel discharge amount of the high-pressure fuel pump 14 via opening/closing driving of the discharge valve 20.

[0021] The control of the fuel discharge amount of the high-pressure fuel pump 14 is basically performed in the following manner. The electronic control unit 27 first calculates a target fuel pressure PRRQ based on the engine rotation number NE, the engine load KL, and the like. Based on a deviation between an actual fuel pressure PR detected by the fuel pressure sensor 26 and the target fuel pressure PRRQ, the electronic control unit 27 performs a feedback control on a fuel discharge amount of the high-pressure fuel pump 14 in order that the actual fuel pressure PR reaches the target fuel pressure PRRQ.

[0022] As illustrated in FIG. 2, the target fuel pressure PRRQ is set to have a larger value as the engine rotation number NE is higher or as the engine load KL is higher. The setting of such a target fuel pressure PRRQ is performed for the following purpose. A rate of fuel injection (an amount of fuel injected per unit time) of the cylinder injector 24 increases as the actual fuel pressure PR is higher. In view of this, at the time of a heavy load/high revolution operation with a large fuel injection amount, the target fuel pressure PRRQ is increased so as to inject a necessary amount of fuel within a shorter time. In the meantime, there is a lower limit for a nozzle opening time of the cylinder injector 24, and if the actual fuel pressure PR is high, a minimum injection amount of the cylinder injector 24 increases. In view of this, at the time of a low load/low revolution operation with a few fuel injection amount, the target fuel pressure PRRQ is lowered so as to enable highly precise small-amount fuel injection.

[0023] When the engine load KL decreases at the time of deceleration of the vehicle, the fuel injection amount of the cylinder injector 24 also decreases. In the meantime, the actual fuel pressure PR increases/decreases according to a deviation between a fuel consumption of the delivery pipe 23 according to the fuel injection of the cylinder injector 24 and a fuel discharge amount of the high-pressure fuel pump 14. Accordingly, in a case where the engine load KL suddenly decreases due to the deceleration of the vehicle, even if the fuel discharge amount of the high-pressure fuel pump 14 decreases, the actual fuel pressure PR may not be able to follow the decrease of the target fuel pressure PRRQ according to the decrease of the engine load KL because the fuel consumption also decreases. As a result, the minimum injection amount of the cylinder injector 24 becomes excessive, which may make it difficult to control the fuel injection amount as requested.

[0024] In the present embodiment, the electronic control unit 27 predicts a future transition of the engine load KL based on the future torque TQRQ. When it is predicted that the engine load KL would decrease in the future, the electronic control unit 27 performs a predictive pressure reduction control to limit the fuel discharge amount of the high-pressure fuel pump 14 so that the fuel discharge amount becomes smaller than a value set according to the target fuel pressure PRRQ. More specifically, at the time of the predictive pressure reduction control, the electronic control unit 27 maintains the discharge valve 20 to be opened, and stops a pressurizing operation of the high-pressure fuel pump 14. Hereby, a fuel discharge of the high-pressure fuel pump 14 stops. When a predetermined condition is satisfied during the execution of the predictive pressure reduction control, the electronic control unit 27 performs a lower limit guard process of controlling the fuel discharge amount of the high-pressure fuel pump 14 so as to maintain the actual fuel pressure PR at a predetermined lower limit guard value PRGD.

[0025] FIG. 3 illustrates a flowchart of a predictive pressure reduction/lower limit guard execution determination routine in an execution determination on the predictive pressure reduction control and the lower limit guard process. The process of this routine is executed by the electronic control unit 27 every predetermined control period during the operation of the engine.

[0026] When the process of this routine is started, it is first determined whether or not an end condition of the lower limit guard process is satisfied in step S100. The end condition of the lower limit guard process is satisfied at the time when any of the following (a1) to (a3) is established.

(a1) The target fuel pressure PRRQ is a lower limit guard value PRGD or less. A pressure that is slightly higher than a minimum fuel pressure that can maintain stable burning is set to a value of the lower limit guard value PRGD.

(a2) A predictive pressure reduction execution time CPRGD, which is an elapsed time from a start of the predictive pressure reduction control, is a predetermined end determination time TPRGD or more. An execution time of the predictive pressure reduction control that can surely decrease the actual fuel pressure PR to a lower limit of the target fuel pressure PRRQ is set to a value of the end determination time TPRGD.

(a3) It is predicted that an increasing rate of a future engine load KL would become a predetermined value or more. More specifically, an increasing rate ΔTQ of the future torque TQRQ exceeds a specific increase determination value DTQA. The increasing rate ΔTQ indicates an increasing amount of the future torque TQRQ during a period between the present and a time point after a predetermined time has passed. That is, the increasing rate ΔTQ indicates an increasing rate of a future engine torque predicted from the future torque TQRQ. A value of the increasing rate ΔTQ is an index value of a predicted value of an increasing rate of the future engine load KL. A value of the increasing rate ΔTQ of the future torque TQRQ at the time when it is determined that the decrease of the engine load KL highly possibly stops after that is set to a value of the increase determination value DTQA.

[0027] If any one of the above (a1) to (a3) is established and the end condition of the lower limit guard process is satisfied (S100: YES), the process is advanced to step S101. In step S101, a lower limit guard execution flag XPRGD is cleared (OFF), and then the process is advanced to step S104. In the meantime, if any of the above (a1) to (a3) is not established and the end condition of the lower limit guard process is not satisfied (S100: NO), the process is advanced to step S102.

[0028] When the process is advanced to step S102, it is determined whether or not a start condition of the lower limit guard process is satisfied in step S102. The start condition of the lower limit guard process is satisfied at the time when all of the following (b1) to (b3) are established.

(b1) A predictive pressure reduction execution flag XDPMPSTP is set (ON).

(b2) The actual fuel pressure PR is the lower limit guard value PRGD or less.

(b3) The actual fuel pressure PR is lower than the target fuel pressure PRRQ.

[0029] If at least any one of the above (b1) to (b3) is not established and the start condition of the lower limit guard process is not satisfied (S102: NO), the process is just advanced to step S104. In the meantime, if the above (b1) to (b3) are all established and the start condition of the lower limit guard process is satisfied (S102: YES), the process is advanced to step S103. In step S103, the lower limit guard execution flag XPRGD is set (ON), and then the process is advanced to step S104.

[0030] When the process is advanced to step S104, it is determined whether or not an end condition of the predictive pressure reduction control is satisfied in step S104. The end condition of the predictive pressure reduction control is satisfied at the time when any of the following (c1) to (c4) is established.

(c1) The predictive pressure reduction execution time CPRGD, which is an elapsed time from the start of the predictive pressure reduction control, is the predetermined end determination time TPRGD or more, and a deviation ΔPR between the actual fuel pressure PR and the target fuel pressure PRRQ, that is, a value obtained by subtracting the target fuel pressure PRRQ from the actual fuel pressure PR is a predetermined end determination value DPPL or less.

(c2) It is predicted that the increasing rate of the future engine load KL would become a predetermined value or more. More specifically, the increasing rate ΔTQ of the future torque TQRQ exceeds the increase determination value DTQA.

(c3) At the time of engine starting. This condition is provided to initialize a current state to a state where the predictive pressure reduction execution flag XDPMPSTP is cleared, at the time of engine starting.

(c4) The lower limit guard execution flag XPRGD is set (ON).

[0031] If at least any one of the above (c1) to (c4) is established and the end condition of the predictive pressure reduction control is satisfied (S104: YES), the process is advanced to step S105. Then, after the predictive pressure reduction execution flag XDPMPSTP is cleared (OFF) in step S105, the process of this routine is finished. In the meantime, if all the above (c1) to (c4) are not established and the end condition of the predictive pressure reduction control is not satisfied (S104: NO), the process is advanced to step S106.

[0032] When the process is advanced to step S106, it is determined whether or not a start condition of the predictive pressure reduction control is satisfied in step S106. The start condition of the predictive pressure reduction control is satisfied at the time when all of the following (d1) to (d3) are established.

(d1) The future torque TQRQ is a predictive pressure reduction execution upper limit TQ0 or less.

(d2) It is predicted that the engine load KL would decrease in the future. More specifically, the increasing rate ΔTQ of the future torque TQRQ is a decrease determination value DTQD or less. Note that a specific negative value that is smaller than the increase determination value DTQA is set to the decrease determination value DTQD.

(d3) The actual fuel pressure PR is a predictive pressure reduction execution lower limit fuel pressure PRH or more. If at least any one of the above (d1) to (d3) is not established and the start condition of the predictive pressure reduction control is not satisfied (S106: NO), the process of this routine is just finished. In the meantime, if the above (d1) to (d3) are all established and the start condition of the predictive pressure reduction control is satisfied (S106: YES), the process is advanced to step S107. After the predictive pressure reduction execution flag XDPMPSTP is set (ON) in step S107, the process of this routine is finished.

[0033] FIG. 4 is a flowchart of a discharge amount control routine to control the fuel discharge amount of the high-pressure fuel pump 14 according to a result of such a predictive pressure reduction/lower limit guard execution determination routine. The process of this routine is executed by the electronic control unit 27 subsequently to the process of the predictive pressure reduction/lower limit guard execution determination routine.

[0034] When the process of this routine is started, the target fuel pressure PRRQ is initially calculated based on the engine rotation number NE, the engine load KL, and so on, in step S200 in the above manner. Subsequently, in step S201, it is determined whether or not the predictive pressure reduction execution flag XDPMPSTP is set (ON). Here, if the predictive pressure reduction execution flag XDPMPSTP is set (YES), the process is advanced to step S202. In step S202, the electronic control unit 27 instructs to maintain the discharge valve 20 to be opened in order to stop the pressurizing operation of the high-pressure fuel pump 14, and then, the process of the routine at this time is finished. That is, a current application duty ratio DUTY of the discharge valve 20 at this time is set to "0."

[0035] In the meantime, if the predictive pressure reduction execution flag XDPMPSTP is cleared (OFF) (S201:NO), the process is advanced to step S203. In step S203, it is determined whether or not the lower limit guard execution flag XPRGD is set (ON). If the lower limit guard execution flag XPRGD is set (S203: YES), the process is advanced to step S204. In step S204, the electronic control unit 27 performs a feedback control on the fuel discharge amount of the high-pressure fuel pump 14 so as to maintain the actual fuel pressure PR at the lower limit guard value PRGD, and then, the process of the routine is finished. That is, the current application duty ratio DUTY of the discharge valve 20 at this time is set according to a deviation ΔPRGD between the actual fuel pressure PR and the lower limit guard value PRGD.

[0036] If the lower limit guard execution flag XPRGD is cleared (S203: NO), the process is advanced to step S205. In step S205, the electronic control unit 27 performs a feedback control on the fuel discharge amount of the high-pressure fuel pump 14 based on the deviation between the actual fuel pressure PR and the target fuel pressure PRRQ, and then, the process of the routine is finished. That is, the current application duty ratio DUTY of the discharge valve 20 at this time is set according to the deviation ΔPR between the actual fuel pressure PR and the target fuel pressure PRRQ.

[0037] Next will be described an operation of the present embodiment. FIG. 5 illustrates one example of an execution mode of the predictive pressure reduction control in a case where the lower limit guard process is not executed at the time when a decrease of the engine load KL delays.

[0038] As illustrated in FIG. 5, the future torque TQRQ is calculated as indicated by an alternate long and short dash line relative to a decrease of the requested torque TQ from a time t1. Herein, the predictive pressure reduction control is started after that, and the fuel discharge amount of the high-pressure fuel pump 14 is set to "0."

[0039] In the example of FIG. 5, a decrease of the actual engine torque (actual torque) delays relative to a decrease of the future torque TQRQ. Since the engine load KL changes in a mode following the actual torque, the predictive pressure reduction control is executed in a state where the engine load KL is high and the fuel consumption is large at this time. Accordingly, the decrease of the actual fuel pressure PR occurs earlier, so that the actual fuel pressure PR largely undershoots the target fuel pressure PRRQ.

[0040] FIG. 6 illustrates one example of an execution mode of the predictive pressure reduction control in a case where the lower limit guard process is executed at the time when the decrease of the engine load KL delays, that is, an execution mode of the predictive pressure reduction control of the fuel control device of the present embodiment. Herein, when the actual fuel pressure PR decreases to the lower limit guard value PRGD at a time t2 after the predictive pressure reduction control is started at a time t1, the lower limit guard process is started, so that a pressurizing operation of the high-pressure fuel pump 14 is restarted. Then, the fuel discharge amount of the high-pressure fuel pump 14 is controlled so that the actual fuel pressure PR is maintained at the lower limit guard value PRGD. This can restrain an excessive decrease of the actual fuel pressure PR.

[0041] The lower limit guard process at this time is continued until any of the following conditions is satisfied: a condition that the target fuel pressure PRRQ decreases to the lower limit guard value PRGD; a condition that an elapsed time (the predictive pressure reduction execution time CPRGD) from the start of the predictive pressure reduction control reaches the end determination time TPRGD; and a condition that it is predicted that the increasing rate of the future engine load KL would become a predetermined value or more. Then, when the lower limit guard process is finished, a feedback control of a normal fuel discharge amount according to the target fuel pressure PRRQ is restarted.

[0042] According to the fuel pressure control device of the engine according to the present embodiment, it is possible to yield the following effects. In the present embodiment, when it is predicted that the engine load KL would decrease in the future, the predictive pressure reduction control is executed, and the fuel discharge amount of the high-pressure fuel pump 14 is limited before the engine load KL actually decreases and the fuel consumption decreases. Accordingly, the actual fuel pressure PR can be decreased before the fuel consumption decreases and becomes hard to decrease, thereby making it possible to immediately decrease the actual fuel pressure PR on the occasion of the decrease of the engine load KL at the time of the deceleration of the vehicle.

[0043] When the actual fuel pressure PR becomes a value that is the lower limit guard value PRGD or less and is also less than the target fuel pressure PRRQ during the execution of the predictive pressure reduction control, the lower limit guard process of controlling the fuel discharge amount so as to maintain the actual fuel pressure PR at the predetermined lower limit guard value PRGD. On that account, even if the decrease of the engine load KL occurs later than the prediction, the decrease of the actual fuel pressure PR stops in the vicinity of the lower limit guard value PRGD. Accordingly, it is possible to restrain an excessive decrease of the actual fuel pressure PR and to realize an immediate decrease of the actual fuel pressure PR at the time of the decrease of the engine load KL.

[0044] During the execution of the predictive pressure reduction control, if a duration (the predictive pressure reduction execution time CPRGD) from its start is the end determination time TPRGD or more and the value (the deviation ΔPR) obtained by subtracting the target fuel pressure PRRQ from the actual fuel pressure PR is the end determination value DPPL or less, the predictive pressure reduction control is finished at that point. After the predictive pressure reduction control is finished, the feedback control of the fuel discharge amount based on the target fuel pressure PRRQ is restarted. This accordingly makes it possible to restrain such a situation that the predictive pressure reduction control is continued unnecessarily and the actual fuel pressure PR is decreased excessively.

[0045] Since the limitation on the fuel discharge amount in the predictive pressure reduction control is performed by stopping the pressurizing operation of the high-pressure fuel pump 14, it is possible to decrease the actual fuel pressure PR more immediately than during the execution of the predictive pressure reduction control.

[0046] The lower limit guard process is finished when any of the following conditions is satisfied during the execution of the lower limit guard process: a condition that the target fuel pressure PRRQ decreases to the lower limit guard value PRGD; a condition that the elapsed time (the predictive pressure reduction execution time CPRGD) from the start of the predictive pressure reduction control reaches the end determination time TPRGD; and a condition that it is predicted that the increasing rate of the future engine load KL would become a predetermined value or more. Accordingly, the lower limit guard process is finished at a suitable timing and the feedback control of the fuel discharge amount according to the normal target fuel pressure PRRQ can be restarted.

[0047] The decrease of the engine load KL is predicted based on the future torque TQRQ calculated according to the requested torque TQ, which is a requested value of the engine torque. On that account, it is possible to more precisely predict a future decrease of the engine load.

[0048] The above embodiment can be modified as follows. In the above embodiment, the target fuel pressure PRRQ is set based on the engine rotation number NE and the engine load KL. Note that, depending on the engine, even if the actual fuel pressure PR is not changed according to the engine rotation number NE, a large influence may not occur in a burning state. When the present embodiment is applied to such an engine, the target fuel pressure PRRQ may be set only based on the engine load KL.

[0049] In the above embodiment, the predictive pressure reduction control is finished when the following condition is satisfied during the execution of the predictive pressure reduction control: a condition that the predictive pressure reduction execution time CPRGD is the end determination time TPRGD or more and the value obtained by subtracting the target fuel pressure PRRQ from the actual fuel pressure PR is the end determination value DPPL or less. However, such an end requirement (c1) may be omitted. That is, only when it is predicted that the engine load KL would increase, the predictive pressure reduction control may be finished.

[0050] The start condition of the predictive pressure reduction control may be changed. For example, when a decrease amount of the accelerator pedal opening degree ACCP reaches a prescribed value or more, the predictive pressure reduction control may be started, or a future decrease of the engine load KL may be predicted so as to start the predictive pressure reduction control.

[0051] For example, the end condition of the lower limit guard process may be changed by omitting one or two of the above (a1) to (a3). In the above embodiment, the pressurizing operation of high-pressure fuel pump 14 is stopped during the execution of the predictive pressure reduction control, but the predictive pressure reduction control may be performed by limiting the fuel discharge amount with the pressurizing operation being maintained. In any case, when the fuel discharge amount of the high-pressure fuel pump 14 is limited during the execution of the predictive pressure reduction control so that the fuel discharge amount becomes smaller than a value set according to the target fuel pressure PRRQ, it is possible to realize a more immediate decrease of the actual fuel pressure PR at the time of the decrease of the engine load KL.

[0052] In the above embodiment, by performing the feedback control on the fuel discharge amount of the high-pressure fuel pump 14 based on the deviation ΔPR between the actual fuel pressure PR and the target fuel pressure PRRQ, the control of the fuel discharge amount is performed so that the actual fuel pressure PR reaches the target fuel pressure PRRQ. The control may be performed not by the feedback control, but by a feedforward control of the fuel discharge amount based on the target fuel pressure PRRQ.

[0053] In the above embodiment, in the predictive pressure reduction control, the pressurizing operation of the high-pressure fuel pump 14 is stopped, that is, the fuel discharge amount of the high-pressure fuel pump 14 is set to "0." Here, even in a case where the pressurizing operation of the high-pressure fuel pump 14 is not stopped, if the fuel discharge amount is decreased to be less than a value to be set according to a current value of the target fuel pressure PRRQ, it is possible to decrease the actual fuel pressure PR earlier. The predictive pressure reduction control may be performed by setting the fuel discharge amount of the high-pressure fuel pump 14 to be less than a value to be set according to a current value of the target fuel pressure PRRQ. For example, even in a case where the predictive pressure reduction control is performed by correcting the current application duty ratio DUTY of the discharge valve 20 of the high-pressure fuel pump 14 so as to become smaller than a value set according to the deviation ΔPR between the actual fuel pressure PR and the target fuel pressure PRRQ, it is possible to decrease the actual fuel pressure PR earlier at the time of the decrease of the engine load KL.

Claims

1. A control device of a fuel pressure of an engine, the engine including an injector (24) and a fuel pump (14), the control device characterized by comprising:

an electronic control unit (27) configured to:

control a fuel discharge amount of the fuel pump (14) such that an actual fuel pressure (PR) is equal to a target fuel pressure (PRRQ), the actual fuel pressure (PR) being a current value of a pressure of a fuel to be supplied to the injector (24), the target fuel pressure (PRRQ) being set based on an engine load (KL);

perform a predictive pressure reduction control when the electronic control unit predicts that the engine load (KL) would decrease, the predictive pressure reduction control being a control to decrease the fuel discharge amount of the fuel pump (14) to be smaller than a value set according to a current value of the target fuel pressure (PRRQ); and

perform a lower limit guard process when the actual fuel pressure (PR) is equal to or lower than a lower limit guard value (PRGD) and is lower than the target fuel pressure (PRRQ), during the execution of the predictive pressure reduction control,

the lower limit guard process being a process of controlling the fuel discharge amount so as to maintain the actual fuel pressure (PR) at the lower limit guard value (PRGD).

2. The control device according to claim 1, wherein
the electronic control unit (27) is configured to finish the predictive pressure reduction control and restart a control on the fuel discharge amount such that the actual fuel pressure (PR) is close to the target fuel pressure (PRRQ), when a duration from the start of the predictive pressure reduction control is a predetermined value or more and when a value obtained by subtracting the target fuel pressure (PRRQ) from the actual fuel pressure (PR) is equal to or lower than a predetermined value.

3. The control device according to claim 1 or 2, wherein
the electronic control unit (27) is configured to limit the fuel discharge amount in the predictive pressure reduction control by stopping a pressurizing operation of the fuel pump (14).

4. The control device according to any one of claims 1 to 3, wherein
the electronic control unit (27) is configured to finish the lower limit guard process when any of the following conditions is satisfied during the execution of the lower limit guard process:

a condition that the target fuel pressure (PRRQ) is equal to or lower than the lower limit guard value (PRGD);

a condition that the duration from the start of the predictive pressure reduction control is equal to or higher than a predetermined value; and

a condition that the electronic control unit predicts that an increasing rate of a future engine load would exceed a predetermined value.

5. The control device according to any one of claims 1 to 4, wherein
the electronic control unit (27) is configured to predict the decrease of the engine load (KL) based on a requested torque (TQ), the requested torque (TQ) is a requested value of an engine torque.

6. A control method of a fuel pressure of an engine, the engine including an injector (24) and a fuel pump (14), the control method characterized by comprising:

controlling a fuel discharge amount of the fuel pump (14) such that an actual fuel pressure (PR) is equal to a target fuel pressure (PRRQ), the actual fuel pressure (PR) being a current value of a pressure of a fuel to be supplied to the injector (24), the target fuel pressure (PRRQ) being set based on an engine load (KL);

decreasing the fuel discharge amount of the fuel pump (14) to be smaller than a value set according to a current value of the target fuel pressure (PRRQ), when it is predicted that the engine load (KL) would decrease; and

controlling the fuel discharge amount so as to maintain the actual fuel pressure (PR) at a lower limit guard value, when the actual fuel pressure (PR) is equal to or lower than the lower limit guard value (PRGD) and is lower than the target fuel pressure (PRRQ) during execution of a predictive pressure reduction control.

Drawing