JP2004052610A - Engine control method and its device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an engine control method and its device for simplifying a system preventing the possible increase of an engine speed due to erroneous operation and preventing the jump-up of an actuator. <P>SOLUTION: A main pump 41 is driven by an engine 31 with a speed governing mechanism 32. An operating oil discharged from the main pump 41 is controlled by a control valve 44 and supplied to the actuator 43. When all remote control valves 45 for operating the control valve 44 are in non-operation positions, discharge pressure drop occurs in the main pump 41 and the drop is detected by a pressure switch 54. A governor controller 35 controls the speed governing mechanism 32 of the engine 31 in accordance with a detection signal from the pressure switch 54 so that the engine speed is reduced into a low-idle condition. A low-speed release switch 56 connected to the governor controller 35 is used for increasing and resetting the speed to a speed as set by an engine speed governing dial 36. <P>COPYRIGHT: (C)2004,JPO

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an engine control method and a device for driving a pump by an engine.
[0002]
[Prior art]
As shown in FIG. 4, in the hydraulic excavator, an upper revolving unit 13 is provided to be rotatable with respect to a lower traveling unit 11 via a revolving bearing unit 12. A swing bracket 16 is pivotally supported via a shaft 15 so as to freely swing in the lateral direction, and a working device 17 is mounted on the swing bracket 16.
[0003]
In the working device 17, the base end of a boom 19 is rotatably supported by a swing bracket 16 via a boom shaft 18 in a vertically rotatable manner, and the base of an arm 21 is connected to the distal end of the boom 19 via an arm shaft 20. An end is rotatably supported by a shaft, and a bucket 23 is rotatably supported by a tip end of the arm 21 via a bucket shaft 22.
[0004]
The swing bracket 16 is swung about the longitudinal axis 15 by the expansion and contraction operation of a swing cylinder provided on a side (not shown) by the front protrusion 14, and a boom 19 is provided on a boom cylinder 24 provided on an abdominal surface of the boom 19. The arm 21 is rotated about the arm shaft 20 by the expansion and contraction operation of an arm cylinder 25 provided on the back side of the boom 19, and the bucket 23 is moved by the arm 21. Is rotated about the bucket shaft 22 via the linkage 27 by the expansion and contraction operation of the bucket cylinder 26 provided on the back side of the bucket.
[0005]
An operator's seat 28 is provided on the upper revolving unit 13, and an engine (not shown) and a hydraulic pump (not shown) driven by the engine are mounted below the seat 28. A hydraulic motor (hereinafter, these hydraulic cylinders and hydraulic motors are referred to as “actuators”) for turning the upper revolving unit 13 with respect to each hydraulic cylinder of the working device 17 and the lower traveling unit 11 on both left and right sides. Operation levers 29 are provided, respectively.
[0006]
FIG. 5 shows a conventional engine control device. A governor lever 33, which is a part of a speed control mechanism 32, is provided on an engine 31. The governor lever 33 is controlled by a governor motor 34 that operates in response to an external command signal. Driven.
[0007]
The governor motor 34 is controlled by a control signal from a governor controller 35. The governor controller 35 is connected to an engine rotation adjustment dial 36 for setting the rotation speed of the engine 31 and a pressure switch 37 for detecting the generation of pressure in the hydraulic circuit.
[0008]
The hydraulic circuit includes a main circuit using a main pump 41 driven by the engine 31 as a hydraulic source, and a pilot circuit using a pilot pump 42 driven by the engine 31 together with the main pump 41 as a hydraulic source.
[0009]
The main circuit includes a control valve 44 having a built-in spool for controlling the direction and flow rate of the hydraulic oil discharged from the main pump 41 and supplying the control oil to the actuator 43. A pilot valve 45 (hereinafter, this pilot valve is referred to as a “remote control valve”) 45 is provided for supplying the discharged pilot oil to the spool end of the control valve 44 by performing direction control and pressure reduction control.
[0010]
The remote control valve 45 is manually operated by the operation lever 29. A pilot line 46 communicated from the remote control valve 45 to one end and the other end of the spool of the control valve 44 is branched from the middle, and a shuttle valve is provided. It is also connected to the pressure switch 37 via a pilot pressure detection line 48 drawn out via 47.
[0011]
Then, when the operator moves the operation lever 29, the pilot primary pressure supplied from the pilot pump 42 is controlled by the remote control valve 45 to generate a pilot secondary pressure corresponding to the lever operation amount. Is guided to one end or the other end of the spool of the control valve 44 to displace the spool of the control valve 44. The spool of the control valve 44 supplies the hydraulic fluid discharged from the main pump 41 to the actuator 43 by controlling the flow direction according to the direction control and the displacement amount.
[0012]
At the same time, the pilot secondary pressure of the remote control valve 45 is also supplied to the pressure switch 37 via the shuttle valve 47 and the pilot pressure detection line 48.
[0013]
Therefore, the state in which the remote control valve 45 is operated and the actuator 43 is operated is detected by the pressure switch 37 and input to the governor controller 35.
[0014]
Here, a plurality of spools of the control valve 44 and a plurality of remote control valves 45 are provided corresponding to the number of actuators 43 such as hydraulic cylinders. The governor controller 35 determines whether or not pilot pressure has been output from all the remote control valves 45. It is a complicated system because it is necessary to detect the
[0015]
FIG. 6 is a flowchart showing an engine control method in the conventional engine control device shown in FIG. Note that circled numbers in this flowchart indicate step numbers.
[0016]
(Step 1)
The governor controller 35 controls the rotation speed of the engine 31 to a rotation speed set by an engine rotation adjustment dial 36 via a governor motor 34 and a governor lever 33.
[0017]
(Step 2)
When all the operating levers 29 are in the neutral position in the non-operating state, the pilot secondary pressures of all the remote control valves 45 decrease, and this state can be detected by the pressure switch 37. Through 37, monitor whether all pilot secondary pressures have dropped.
[0018]
(Step 3)
If all pilot secondary pressures have decreased (YES in step 2), the governor controller 35 operates the engine 31 through the governor motor 34 and the governor lever 33 at a low speed at a predetermined low speed.
[0019]
(Step 4)
During this low idle operation, the governor controller 35 constantly monitors through the pressure switch 37 whether or not the pilot secondary pressure of the pilot line 46 has increased.
[0020]
If the increase in the pilot secondary pressure is detected by the pressure switch 37 (YES in step 4), the process returns to step 1 and the governor controller 35 sets the rotation speed of the engine 31 with the engine rotation adjustment dial 36. Control the rotation speed.
[0021]
[Problems to be solved by the invention]
As described above, conventionally, when all the remote control valves 45 are in the non-operation state, the governor controller 35 that detects the decrease in the pilot secondary pressure from these remote control valves 45 with the pressure switch 37 controls the engine 31. The governor controller 35 that automatically performs low idle operation, starts operation of at least one of the remote control valves 45, and detects the generated pilot secondary pressure by the pressure switch 37, operates the engine 31 automatically at the set high speed. Since the rotation is restored, it is necessary to always detect the secondary pilot pressure output from all the remote control valves 45, which complicates the system and raises the cost.
[0022]
In addition, when the operator has inadvertently moved the operation lever 29 while all the operation levers 29 are in the non-operation state at the neutral position and the operation is paused, the rotation speed of the engine 31 increases, and There is a possibility that the control valve 44 is operated by the remote control valve 45 and the actuator 43 is operated.
[0023]
Furthermore, operating the remote control valve 45 with the operation lever 29 to increase and return the rotation speed of the engine 31 as in the conventional method is equivalent to increasing and returning only the rotation speed of the engine 31 while maintaining the remote control valve 45 at the neutral position. Therefore, when the rotation speed of the engine 31 necessary for starting the actuator 43 is obtained, the remote control valve 45 is also displaced to the non-neutral position, and a jumping operation in which the actuator 43 starts suddenly occurs. There is also.
[0024]
SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and aims at simplifying a system, preventing a possibility of increasing the rotation speed of an engine due to an erroneous operation, and preventing an actuator from jumping out. The purpose is to provide.
[0025]
[Means for Solving the Problems]
The invention described in claim 1 operates the engine at a set rotation speed, detects a decrease in the discharge pressure of a pump driven by the engine, and reduces the rotation speed of the engine to a predetermined low speed by this detection. This is an engine control method that raises and returns the rotation speed of the engine in the low speed state to the set rotation speed by an operation of releasing the low speed, and detects a decrease in the discharge pressure of the pump to reduce the rotation speed of the engine to a predetermined low speed. The engine is operated at low idle by detecting all of the multiple pilot pressure drops, and the engine speed is raised and returned by detecting the increase in pilot pressure, as in the conventional case. The system can be simplified, and the engine speed in the low-speed state can be returned to the set speed by operating the switch. Runode, by increasing the pilot pressure by conventional such lever erroneous operation can be prevented a possibility that the rotational speed of the engine also rises.
[0026]
According to a second aspect of the present invention, in the engine control method according to the first aspect, the engine speed is reduced to a predetermined low speed after a lapse of a predetermined time after detecting a decrease in the discharge pressure of the pump. This is a control method, and even if the discharge pressure of the pump temporarily decreases due to fluctuations in the load pressure or the like during the operation of the actuator, the rotation speed of the engine can be maintained for a predetermined time without immediately lowering the rotation speed of the pump. Immediate response to recovery of discharge pressure.
[0027]
According to a third aspect of the present invention, there is provided an engine that maintains a rotation speed set by a governing mechanism, a pump driven by the engine, and a control valve that controls a working fluid discharged from the pump and supplies the working fluid to an actuator. An operating device that operates the control valve, a detector that detects a decrease in the pump discharge pressure that occurs when the operating device is in the non-operating position, and an engine that detects a decrease in the pump discharge pressure from the detector. And a low speed release switch connected to the controller for controlling the speed control mechanism to reduce the rotation speed of the engine to a predetermined low speed and for raising and returning the rotation speed of the low speed engine to the set rotation speed. An engine control unit that detects a drop in pump discharge pressure when the operating device that operates the control valve is in the non-operating position. The controller detects the engine rotation speed to a predetermined low speed and returns it by operating the low speed release switch. It is possible to simplify the system compared to the case of driving and detecting the rise in pilot pressure to raise the engine speed, and to set the engine speed controlled to the low speed state by operating the low speed release switch. Since the rotation speed is returned to the increased rotation speed, it is possible to prevent the possibility that the rotation speed of the engine will also increase due to the increase of the pilot pressure due to the lever erroneous operation as in the related art. Separate the low-speed release switch operation from the low-speed release switch operation. Only the engine rotation speed can be raised and returned by the switch, so after securing the engine rotation speed necessary for the operation of the actuator, operate the actuator from the neutral position to reliably prevent the actuator from jumping out. And operability can be improved.
[0028]
According to a fourth aspect of the present invention, in the engine control device according to the third aspect, the operation device has an operation lever, and the low-speed release switch is an engine control device provided on the operation lever, and the operation lever is held. Since the low-speed release switch provided on the operation lever can be operated while being held, operability can also be improved in this respect.
[0029]
According to a fifth aspect of the present invention, there is provided the engine control device according to the third or fourth aspect, wherein the control valve is operated by a pilot from a signal from an operating device and controls the working fluid supplied from the pump to the plurality of actuators. An engine control device comprising: a spool, and a center bypass passage that discharges a working fluid discharged from a pump to a tank when all spools are at a neutral position in correspondence with a non-operating position of all operating devices, When all the spools are in the neutral position in correspondence with the non-operating positions of all the actuators, the working fluid discharged from the pump is discharged to the tank through the center bypass passage, so that the discharge pressure of the pump is reduced. By detecting, it can be reliably detected that all the operating devices are in the non-operation position.
[0030]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, the present invention will be described in detail with reference to one embodiment shown in FIGS. Note that the engine control device according to the present invention is mounted on the hydraulic excavator shown in FIG.
[0031]
FIGS. 1 and 2 show an engine control device. An engine 31 is provided with a governor lever 33 which is a part of a speed control mechanism 32 for maintaining a set rotational speed. The governor lever 33 is provided with an external command signal. , And is driven by a governor motor 34 that operates according to.
[0032]
The governor motor 34 is controlled by a control signal from a governor controller 35 as a controller. The governor controller 35 is connected to an engine rotation adjustment dial 36 for setting the rotation speed of the engine 31.
[0033]
A main circuit using a main pump 41 as a pump driven by the engine 31 as a hydraulic source, and a pilot circuit using a pilot pump 42 driven by the engine 31 together with the main pump 41 as a hydraulic source are provided.
[0034]
The main circuit includes a control valve 44 including a plurality of spools 44a for controlling the direction and flow of the working oil as the working fluid discharged from the main pump 41 to supply the working oil to the plurality of actuators 43. The pilot circuit controls the direction and pressure of the pilot oil discharged from the pilot pump 42, supplies the pilot oil to the end of each spool 44a of the control valve 44, and pilots each spool 44a as a pilot valve (hereinafter referred to as an operating device). , This pilot valve is referred to as a “remote control valve”) 45.
[0035]
The plurality of spools 44 a are pilot-operated by pilot signals from the corresponding remote control valves 45, and control the hydraulic oil supplied from the main pump 41 to the plurality of actuators 43.
[0036]
The remote control valve 45 has the operation lever 29 shown in FIG. 4 and is a pressure reducing valve that is manually operated by the operation lever 29. Although only one is shown in FIG. A plurality of pilot lines 46 are provided corresponding to one end and the other end of each spool 44a from each remote control valve 45.
[0037]
When the remote control valve 45 is in the non-operation position, that is, the neutral position, the corresponding spools 44a of the control valve 44 are also in the neutral position. At this time, the pump discharge line 51 of the main pump 41 connects the spools 44a in the neutral position. Since it communicates with the tank 53 through the center bypass passage 52 that penetrates, the discharge pressure of the main pump 41 decreases.
[0038]
The center bypass passage 52 discharges the hydraulic oil discharged from the main pump 41 to the tank 53 only when all the spools 44a are at the neutral position corresponding to the non-operating positions of all the remote control valves 45.
[0039]
The pump discharge line 51 of the main pump 41 has a pressure sensor or pressure switch 54 as a detector for detecting a decrease in the discharge pressure of the main pump 41 that occurs when all the remote control valves 45 are in the non-operation position, that is, the neutral position. Is provided.
[0040]
The governor controller 35 has a function of controlling the speed control mechanism 32 of the engine 31 based on a detection signal of a decrease in pump discharge pressure from the pressure switch 54 to reduce the rotation speed of the engine 31 to a predetermined low speed, that is, a low idle state. Have. The rotation speed of the engine 31 is detected by the engine rotation speed sensor 55 and is fed back to the governor controller 35.
[0041]
A low-speed release switch 56 for releasing the low idle state of the engine 31 is provided, for example, at the upper end of the operation lever 29. The low-speed release switch 56 is connected to the governor controller 35 and has a function of returning the rotation speed of the engine 31 in the low-speed state, that is, the low idle state, to the rotation speed set by the engine rotation adjustment dial 36.
[0042]
Then, when the operator moves the operation lever 29, the pilot primary pressure supplied from the pilot pump 42 is controlled by the remote control valve 45 to generate a pilot secondary pressure corresponding to the lever operation amount. Is guided to one end or the other end of the spool 44a constituting the control valve 44 via one pilot line 46 to displace the spool 44a.
[0043]
The spool 44a of the control valve 44 controls the flow rate of the hydraulic oil discharged from the main pump 41 in accordance with the direction control and the displacement amount, and supplies the hydraulic oil to the actuator 43 via the output line 57.
[0044]
As described above, when the remote control valve 45 is operated with the lever, the spool 44a is displaced, so that the opening area communicating with the center bypass passage 52 decreases and the opening area communicating with the output line 57 to the actuator 43 increases. Therefore, the pump discharge pressure also increases in accordance with the load pressure of the actuator 43. This increase in the pump discharge pressure is detected by the pressure switch 54 and input to the governor controller 35. Therefore, the governor controller 35 can detect that the at least one remote control valve 45 has been operated by detecting an increase in the pump discharge pressure by the pressure switch 54.
[0045]
On the other hand, when all the remote control valves 45 are in the non-operating state, that is, in the neutral position, all the spools 44a are returned to the neutral position, and the pump discharge line 51 communicates with the tank 53 through the center bypass passage 52. No pressure is generated in the line 51, and this state is detected by the pressure switch 54. Therefore, the governor controller 35 can detect that all the remote control valves 45 are in the non-operating state by detecting the decrease in the pump discharge pressure by the pressure switch 54.
[0046]
Next, a control method of the engine 31 will be described with reference to a flowchart shown in FIG. Note that the circled numbers in the figure indicate step numbers.
[0047]
(Step 11)
The governor controller 35 controls the governor motor 34 and the governor lever 33 to control the rotation speed of the engine 31 to the rotation speed set by the engine rotation adjustment dial 36. At this time, the discharge pressure of the main pump 41 driven by the engine 31 is detected by the pressure switch 54.
[0048]
(Step 12)
The governor controller 35 determines whether the pump discharge pressure of the hydraulic oil discharged from the main pump 41 to the pump discharge line 51 has decreased to a low pressure generated when all the spools 44a of the control valve 44 are returned to the neutral position. Is monitored by a pressure sensor or pressure switch 54.
[0049]
(Step 13)
If a decrease in the pump discharge pressure is detected in step 12 (YES in step 12), the governor controller 35 monitors whether the state of the decrease in the pump discharge pressure continues for a predetermined time.
[0050]
At this time, even if the pump discharge pressure of the main pump 41 temporarily decreases (NO in step 13), the rotation speed set by the engine rotation adjustment dial 36 is maintained for a predetermined time without reducing the rotation speed of the engine 31. By looking at the above, a case is prepared in which the pump discharge pressure of the main pump 41 rises and returns in a short time.
[0051]
(Step 14)
When all the operation levers 29 are neutral, the pump discharge pressure decreases, and after a predetermined time has elapsed after the pump discharge pressure has decreased, a low idle signal is sent from the governor controller 35 to the governor motor 34. The governor motor 34 controls the governor lever 33 to reduce the rotation speed of the engine 31 to a predetermined low-speed state, that is, a low-idle state, and performs a low-idle operation.
[0052]
(Step 15)
When the operator starts an operation (operation), the low-idle release low-speed release switch 56 provided at the upper end of the operation lever 29 is pressed to be turned on, so that the governor controller 35 turns the low-speed release switch 56 on. It is always monitored whether or not the engine has been turned on. If the engine is turned on (YES), the rotation speed of the engine 31 in the low idle state is set to the rotation speed indicated by the engine rotation adjustment dial 36 before the low idle state. The ascent is returned (step 11).
[0053]
In this way, the fuel consumption becomes large when the engine speed such as when the vehicle is idle or waiting is kept at the high speed during the work, so the low idle when the lever is in neutral reduces fuel consumption. In addition, by automating this, the trouble of adjusting the engine speed can be reduced.
[0054]
There is a similar system of the related art that detects the pilot pressure of the remote control valve 45 to automatically make the low idle and automatically returns to the high-speed rotation at the same time as the operation is started. However, in such a system, all the pilot pressures are detected. Need to be a complex system.
[0055]
On the other hand, in the present system, when all of the remote control valves 45 that operate the control valve 44 are in the non-operation position, the governor controller 35 that detects the state in which the discharge pressure of the main pump 41 has decreased by the pressure switch 54 is The rotation speed of the engine 31 is automatically reduced to a low idling state, which is a predetermined low speed, and when returning to work, the low speed release switch 56 provided at the upper end of the operation lever 29 of the remote control valve 45 is pressed. Since the rotation speed of the engine 31 is increased and returned, the engine 31 is operated at a low idling speed by detecting all of the plurality of pilot pressure drops, and the rotation speed of the engine 31 is increased by detecting the increase in the pilot pressure as in the conventional case. The system can be simplified as compared with the case of returning.
[0056]
In addition, since the rotation speed of the engine 31 controlled to the low speed state is returned to the set rotation speed by operating the low speed release switch 56, the rotation of the engine 31 is increased by the pilot pressure increase due to the lever erroneous operation as in the related art. It is possible to prevent the speed from increasing.
[0057]
Further, the operation of the remote control valve 45 by the operation lever 29 and the switch operation of the low speed release switch 56 for increasing and returning the rotation speed of the engine 31 are separated, and the low speed release switch 56 is maintained while the remote control valve 45 is maintained at the neutral position. By operating the remote control valve 45 from the neutral position after securing the engine rotation speed necessary for the operation of the actuator 43, the jumping operation of the actuator 43 can be performed. Prevention can be reliably prevented, and operability can be improved.
[0058]
In addition, since the low-speed release switch 56 provided at the upper end of the operation lever 29 can be operated while holding the operation lever 29, the operability can also be improved in this respect.
[0059]
Further, when all the spools 44a are in the neutral position corresponding to the non-operation positions of all the remote control valves 45, the hydraulic oil discharged from the main pump 41 is discharged to the tank 53 through the center bypass passage 52. By detecting a decrease in the discharge pressure of the main pump 41, it is possible to reliably detect that all the remote control valves 45 are at the non-operation positions.
[0060]
The present invention is not limited to a hydraulic excavator, but can be widely applied to a working machine that drives a pump of a fluid pressure circuit by an engine.
[0061]
【The invention's effect】
According to the first aspect of the present invention, a decrease in the discharge pressure of the pump is detected, the rotational speed of the engine is reduced to a predetermined low speed, and the return is raised by operating a switch. The system can be simplified as compared with the case where the engine is operated at low idle by detecting a decrease in engine speed and the increase in the pilot pressure is detected by detecting an increase in the pilot pressure. Since the rotational speed is returned to the set rotational speed by the operation, it is possible to prevent the rotational speed of the engine from increasing due to an increase in pilot pressure due to a lever misoperation as in the related art.
[0062]
According to the second aspect of the present invention, even if the discharge pressure of the pump is temporarily reduced due to the fluctuation of the load pressure or the like during the operation of the actuator, the engine speed is maintained for a predetermined time without immediately decreasing. Therefore, it is possible to immediately cope with the recovery of the discharge pressure of the pump.
[0063]
According to the third aspect of the present invention, when the operating device for operating the control valve is in the non-operating position, the controller that detects the state where the discharge pressure of the pump has decreased by the detector reduces the rotational speed of the engine to the predetermined low speed. The engine speed is reduced by operating the low-speed release switch, and the engine is returned to the high speed by operating the low-speed release switch. The system can be simplified as compared with the case of returning to a higher speed, and the rotation speed of the engine controlled to a low speed state can be returned to the set rotation speed by operating the low speed release switch. It is possible to prevent the engine speed from increasing due to an increase in pilot pressure due to erroneous operation. The operation of the low-speed release switch that raises and returns the rotation speed of the gin is separated from the operation of the low-speed release switch.Only the engine rotation speed can be raised and returned by the low-speed release switch while the operating device is maintained at the neutral position. By operating the operating device from the neutral position after securing the engine rotation speed required for the operation, it is possible to reliably prevent the actuator from jumping out and improve operability.
[0064]
According to the fourth aspect of the present invention, since the low-speed release switch provided on the operation lever can be operated while holding the operation lever, the operability can be improved also in this respect.
[0065]
According to the fifth aspect of the present invention, when all the spools are at the neutral position corresponding to the non-operation positions of all the operation devices, the working fluid discharged from the pump is discharged to the tank via the center bypass passage. Therefore, by detecting a decrease in the discharge pressure of the pump, it is possible to reliably detect that all the operating devices are at the non-operation positions.
[Brief description of the drawings]
FIG. 1 is a circuit diagram showing an embodiment of an engine control device according to the present invention.
FIG. 2 is a block diagram of the engine control device.
FIG. 3 is a flowchart showing one embodiment of an engine control method according to the present invention.
FIG. 4 is a side view of a hydraulic shovel related to the present invention.
FIG. 5 is a block diagram showing a conventional engine control device.
FIG. 6 is a flowchart showing a conventional engine control method.
[Explanation of symbols]
29 Operation lever
31 engine
32 Governing mechanism
35 Governor controller as controller
41 Main pump as a pump
43 Actuator
44 Control valve
44a spool
45 Pilot valve as an actuator (remote control valve)
52 Center bypass passage
53 tank
54 Pressure switch as detector
56 Low speed release switch

Claims (5)

Runs the engine at the set speed,
Detects a drop in the discharge pressure of the pump driven by the engine,
By this detection, the rotation speed of the engine is reduced to a predetermined low speed,
An engine control method, wherein the rotation speed of an engine in a low-speed state is raised and returned to a set rotation speed by an operation of releasing the low speed. 2. The engine control method according to claim 1, wherein the rotation speed of the engine is reduced to a predetermined low speed after a lapse of a predetermined time after detecting a decrease in the discharge pressure of the pump. An engine that maintains the rotation speed set by the governing mechanism,
A pump driven by the engine,
A control valve for controlling the working fluid discharged from the pump and supplying it to the actuator,
An operating device for operating the control valve;
A detector that detects a decrease in pump discharge pressure that occurs when the operating device is in the non-operating position,
A controller for controlling a speed control mechanism of the engine based on a detection signal of a decrease in pump discharge pressure from the detector to reduce the rotation speed of the engine to a predetermined low speed;
An engine control device, comprising: a low-speed release switch connected to a controller for increasing and returning the rotation speed of the low-speed engine to a set rotation speed. The operating device has an operating lever,
The engine control device according to claim 3, wherein the low-speed release switch is provided on the operation lever. The control valve is
A plurality of spools each of which controls a working fluid supplied to a plurality of actuators from a pump by a pilot operation by a signal from an actuator,
5. A center bypass passage for discharging a working fluid discharged from a pump to a tank when all spools are at a neutral position corresponding to a non-operating position of all operating devices. An engine control device as described in the above.

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