JPH10274068A - Engine control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent a condition from switching into a low speed rotating condition against the intention of an operator by setting an engine speed when the work as a construction machine is rested, and setting a delay time when the rotating speed is switched from a command rotating speed into a low speed rotating speed by a throttle lever, on the basis of a lapsed time of a past low speed rotating condition. SOLUTION: A construction machine is provided with a controller 24 for reducing engine speed to a predetermined low rotating speed for saving a fuel consumption by a throttle command rotating speed which is set by a throttle lever after a delay time which is set beforehand is lapsed from a time point when all operating levers 14 to 19 become neutral condition. In this case, when at least one of an operating lever is operated from a neutral position to an operating position after a time exceeds a delay time, measuring of a lapsed time is cleared, and engine speed is held to a throttle command rotating speed. The delay time is set on the basis of a low rotating speed condition lapsed time after exceeding a delay time in a condition of a low rotating speed in a plural times in the past.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an engine control device for controlling the number of rotations of an engine mounted on a hydraulic construction machine or a work vehicle such as a hydraulic excavator to a deceleration number for fuel saving.

[0002]

2. Description of the Related Art As a hydraulic construction machine of this type, for example, a hydraulic shovel includes a hydraulic pump driven by an engine,
A plurality of actuators, such as a hydraulic cylinder and a hydraulic motor, which are operated by the hydraulic pressure oil discharged from the hydraulic pump, and a plurality of operating levers for operating the actuators, respectively. The work is performed by operating each actuator by supplying the actuator to each actuator via a direction switching valve driven in accordance with the operation of the lever. In general, a throttle lever for setting the engine speed is provided so that the operator can appropriately change the discharge flow rate of the hydraulic pump according to the working mode or the like.

[0003] In this type of hydraulic construction machine, as disclosed in Japanese Patent Publication No. 60-38661, in order to reduce the fuel consumption of the engine when the work is suspended, the engine is required to have a low fuel consumption. A switching valve switch for an operator to instruct a control device of the working machine whether to operate at a predetermined low rotation speed or to operate at a rotation speed indicated by a throttle lever. The engine is operated at the low speed when all the operation levers are held at the neutral position in a state in which the operation is instructed. In this case, in the case of the publication, when all the operation levers are accidentally operated to the neutral position during the operation in the state where the operation at the low speed is instructed by the changeover switch, the engine is operated. In order to prevent the rotation speed of the engine from being switched to the low speed, the engine speed is changed to the low speed after a predetermined delay time has elapsed from when all the operation levers are operated to the neutral position. I try to switch.

[0004] However, according to the publication, in order to operate the engine at a low speed when the work is stopped, the operator must operate the changeover switch, which is inconvenient. In addition, if the operator forgets to operate the changeover switch when the work is stopped, the engine runs forever at the high-speed rotation indicated by the throttle lever, and there is a disadvantage that unnecessary fuel consumption occurs. Further, in the publication, when at least one operation lever is operated from the neutral position to the work position while the engine is operating at the low speed, the engine speed is immediately indicated by the throttle lever. In particular, because it is to return to the rotation speed
When the speed indicated by the throttle lever is set to the high speed, the engine speed rapidly increases to the high speed even if the operation amount of the operation lever is small, and the hydraulic pump discharges. There is a disadvantage that the amount of the actuator suddenly increases and the operation speed of the actuator suddenly changes against the intention of the operator.

As described above, in the hydraulic construction machine, after a predetermined delay time has elapsed from the time when all of the plurality of operating levers have reached the neutral (position) state, the engine speed is calculated from the throttle command speed. So-called automatic deceleration control is performed to reduce the rotation speed to a low speed (deceleration speed). However, the delay time in the prior art is a fixed value and is not modulated with respect to the working state. Therefore, in the work in which the lever operation is performed again immediately after the lowering of the low speed, not only the original effect of the auto deceleration control is not exhibited, but also a meaningless fluctuation of the engine speed is brought about. This has a negative effect on fuel economy. In addition, there is a meaningless vertical fluctuation of the engine sound, which has a bad influence on noise. In addition, because the delay time is a fixed value, even in a work cycle in which a short pause time between lever operations is long, the engine speed does not immediately decrease, so that the auto deceleration effect has not been sufficiently exhibited. .

[0006]

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned disadvantages, and the operation of switching the engine speed from a speed indicated by a throttle lever to a low speed when the work is stopped is performed by a special operation such as a switch operation. And it is possible to avoid a situation in which the engine speed is switched to the low-speed speed contrary to the intention of the operator during the work, and
In addition, when the work is resumed while the engine speed is controlled to low speed by suspension of work, the engine speed is smoothly returned to the speed indicated by the throttle lever according to the intention of the operator. It is another object of the present invention to provide an engine control apparatus capable of more appropriately improving fuel efficiency by learning control of the delay time from a past work state in view of the inconvenient actual situation.

[0007]

According to an engine control apparatus of the present invention, an engine, a hydraulic pump driven by the engine, a plurality of actuators operated by hydraulic pressure discharged from the hydraulic pump, and a plurality of actuators are provided. A plurality of operation levers for operating each operation, and a throttle lever for setting the number of revolutions of the engine are provided, and a predetermined delay time is set from a point in time when the plurality of operation levers are all in a neutral state. After a lapse of time, in a construction machine having a control for reducing the engine speed to a predetermined low-speed speed for fuel saving from a throttle command speed set by the throttle lever, all the operation levers are in a neutral position. After being operated at the neutral position, when the engine is held at the neutral position, the engine is not operated until the delay time elapses. The elapsed time is measured while controlling the rotation speed to the throttle instruction rotation speed, and before the elapsed time reaches the delay time, when at least one of the operation levers is operated from the neutral position to the working position, the elapsed time is counted. A step of clearing timekeeping and maintaining the engine speed at the throttle-instructed speed, and setting a delay time after the current time comprises a plurality of past low-speed speeds performed immediately before the current time. In this state, a new setting is made based on the elapsed time of the low-speed rotation state after the preset delay time has been reached. In the above case, corresponding to the case where the elapsed time of each of the low-speed rotation states performed a plurality of times in the past is a predetermined short time or less, the time value of the preset delay time is added to the plurality of low-speed rotation speeds. The sum of the maximum time value of the state elapsed times is set as the time value of the new delay time after the current time. In this case, the time value of the new delay time is set to the predetermined upper limit time. The value is not exceeded. In addition, in response to a situation in which each time value of the low speed state elapsed time exceeds a predetermined upper limit time value, the time value of the new delay time is set so as not to exceed the time value of the preset delay time. Or set to be smaller than the sum time value.

According to the present invention, when the operation is stopped, all the operation levers are operated to the neutral position, and thereafter are continuously held at the neutral position. Therefore, in this case, the state in which all the operation levers are held at the neutral position continues for the delay time set at the time of operation to the neutral position, and therefore, the number of revolutions of the engine becomes less than the delay time. After the elapse, the rotation speed is controlled to the predetermined low speed from the throttle instruction speed by the throttle lever. On the other hand, for example, in the operation of a hydraulic excavator, a case where all the operation levers are held at the neutral position for a relatively short time during the operation may be continuously performed a plurality of times in the past.

The time value t ₀ of the delay time set as an initial value in the prior art is a fixed value to the last. Therefore, when the operation of the operation lever is performed again immediately after the engine has decelerated to the deceleration speed, the engine speed immediately increases from the deceleration speed to the throttle instruction speed, and the engine speed is reduced. The meaningless vertical fluctuation of the rotation speed adversely affects fuel economy. However, in the engine control device according to the embodiment of the present invention, in the operation in which at least one operation of the operation lever is performed again immediately after the engine drops to the deceleration speed, the past operation performed immediately before the current time is performed. The plurality of low-speed rotation state elapsed times T ₁ , T ₂ , and T ₃ are set to a predetermined small time, for example, 5 times.
If not more than _SEC , the plurality of elapsed times T ₁ , T
_2, the maximum time value of T ₃ (e.g., T ₃₎ was added sum time (although new delay time t _n, the time does not exceed a predetermined upper limit time value t _max.) By the throttle instruction rotational Be kept in numbers. That is, in the operation in which the operation is performed again after a short pause, the operation can be continued with the throttle rotation speed without decreasing the engine speed to a low speed, so that the work can be performed smoothly and the workability can be enhanced. be able to. Further, if the sum time is set to a new delay time t _n and a short pause time between lever operations is long, the sum time remains at the predetermined upper limit time value t.
Since the engine speed is set so as not to exceed _max , the engine speed can be reduced to the deceleration speed after the relatively short time of the sum to save fuel consumption.

In the case where the operation has been performed a plurality of times (for example, three times) in the past and each of the elapsed times T ₁ , T ₂ , and T ₃ is a predetermined small time, for example, 15 _seconds or more, a new delay time t 'time value of _n (t' _n), the initial setting of the preset delay time value t ₀ ,, or initial set value t ₀
Below, or alternatively, it is set to be smaller than the sum time value (t ₀ + T ₃ ). Therefore, even if the operation is performed again after a short pause but a relatively short pause, the operation can be continued without changing the engine speed to the low-speed speed and at the throttle-instructed speed. To improve the workability. Also even if there is a situation, such as a long pause time in between lever operation at the time of the work, it dropped immediately to the deceleration rotational speed after passing the new delay time t _'n, it is possible to measure the fuel saving.

[0011]

Embodiments of the present invention will be described below in detail with reference to the drawings. FIG. 1 is a system configuration diagram of a hydraulic construction machine (a hydraulic shovel in the present embodiment) equipped with an engine control device of the present invention. FIG. 2 is a control by a controller (described later) of an engine control device in the hydraulic shovel of FIG. FIG. 3 shows a delay time t from the time when all of the plurality of operation levers are in the neutral state.
FIG. 7 is a diagram for explaining the relationship between the engine speed corresponding to an elapsed time T in which the engine speed is reduced to a predetermined low speed of fuel consumption and maintained after a delay time t has elapsed. is there.

Referring to FIG. 1, reference numeral 1 denotes an engine, reference numerals 2 and 3 denote a pair of variable displacement hydraulic pumps driven by the engine 1, and reference numerals 4 and 5 adjust the capacities of the hydraulic pumps 2 and 3, respectively. And 6 to 11 are actuators for arms (hydraulic cylinders), actuators for turning (hydraulic motors), actuators for left traveling (hydraulic motors), actuators for booms (hydraulic cylinders), actuators for buckets (hydraulic cylinders) and This is a right-hand running actuator (hydraulic motor).

In the hydraulic shovel of this embodiment, the actuators 6 to 11 are classified into a group of actuators 6 to 8 using the hydraulic pump 2 as a driving source and a group of actuators 9 to 11 using the hydraulic pump 3 as a driving source. Have been. And in the group of actuators 6 to 8,
While adjusting the capacity of the hydraulic pump 2 through the regulator 4 in accordance with an instruction from a controller described later, the hydraulic pump 2
By operating the direction switching valve 12 between the actuator 6 and the actuators 6 to 8, the hydraulic oil is supplied from the hydraulic pump 2 to the actuators 6 to 8, and the actuators 6 to 8 are operated. Similarly, in the group of actuators 9 to 11,
By controlling the capacity of the hydraulic pump 3 via the regulator 5 and operating the directional control valve 13 provided corresponding to this group according to the instruction of the controller described later, the hydraulic pump 3 The actuators 9 to 11 are operated by supplying pressure oil. FIG.
For convenience, the direction switching valves 12 and 13 are described as being one for each group. However, actually, the direction switching valves 12 and 13 are provided for each of the actuators 6 to 8 and 9 to 11 of each group. These directional control valves are driven by a pilot pressure (oil pressure) applied through a pilot pipe (not shown) in response to the operation of an operation lever described later provided corresponding to each of the actuators 9 to 11. It has become.

In FIG. 1, reference numerals 14 to 19 denote operating levers for boom, arm, bucket, right-hand running, left-hand running and turning, respectively. Mode switching for switching between three modes: a mode (hereinafter, referred to as H mode), a medium speed mode for normal work (hereinafter, referred to as S mode), and a low speed mode for low speed work (hereinafter, referred to as FC mode). A switch 21, a throttle lever 21 for adjusting and setting the rotation speed of the engine 1 in each of the above modes, a rotation speed sensor 22 for detecting the rotation speed of the engine 1, a throttle motor 23 for driving a throttle of the engine 1, Is a controller including a microcomputer and the like. Each of the plurality of operation levers 14 to 19 outputs a signal corresponding to the operation direction and operation amount to the controller 24 via a pressure sensor or the like (not shown), and the mode changeover switch 20 sets the operation mode set by the operation. Is output to the controller 24. Further, the throttle lever 21 outputs a signal corresponding to the operation amount to the controller 24 as a signal indicating the throttle instruction rotation speed of the engine 1, and the rotation speed sensor 22 outputs a signal corresponding to the detected rotation speed of the engine 1 to the controller 24. 24.

The controller 24 includes operating levers 14 to
A signal input unit 25 for receiving signals from the signal input unit 25; a storage circuit unit 26 for storing programs and various data; and a signal data and storage circuit unit 26 provided to the signal input unit 25.
And a control output unit 28 for controlling the regulators 4 and 5
And a throttle drive unit 29 that drives the throttle motor 23. In this case, the arithmetic processing unit 27 generates a control signal for causing the operation of each of the actuators 6 to 11 according to the operation of each of the operation levers 14 to 19, and instructs the control signal to the control output unit 28. Control output unit 28 controls regulators 4 and 5 in accordance with Further, the arithmetic processing unit 27 generates an operation of the engine 1 in accordance with the operation of each of the operation levers 14 to 19, the operation mode of the engine 1 by the mode changeover switch 20, and the throttle rotation speed of the engine 1 by the throttle lever 21. The target rotation speed data of the engine 1 is generated and instructed to the throttle drive unit 29, and the throttle drive unit 29 drives the throttle motor 23 according to the instruction. still,
In the present embodiment, the control of the direction switching valves 12 and 13 is as follows.
Although the control is performed by hydraulic pressure, the control may be performed electrically by the control output unit 28 of the controller 24.

Next, a control mode of the engine control apparatus according to the embodiment will be described with reference to FIGS. In the engine control apparatus according to the embodiment of the present invention, all the operation levers 14 to 1 are used.
After a predetermined delay time t ( _sec ) has elapsed from the time point when the engine 9 becomes neutral, the rotation speed of the engine 1 is increased by the throttle command rotation speed N set by the throttle lever 21 (shown in FIG. 1). _{From TH} (shown in FIG. 3), a predetermined low-speed rotation speed (also referred to as deceleration speed) N for fuel saving cost
_D , the engine is maintained at the neutral position after all of the operation levers 14 to 19 are operated to the neutral position, and the engine is operated until the delay time t elapses. before first rotation speed measures the elapsed time T _x while controlling the throttle command rotational speed N _TH, the elapsed time T _x reaches the delay time t, the operating lever 1
When at least one of 4 to 19 is operated to the working position from the neutral position comprises a step of maintaining the rotational speed of the engine 1 by clearing the counting of the elapsed time T _x to throttle instruction rotational speed N _TH, from the present time to set the delay time t _n after the step, after reaching the preset delay time t in the immediate state of the low-speed rotational speed n _D plurality conducted the last prior time eg three previous present time Elapsed time (hereinafter referred to as “elapsed time”) of low-speed rotation speed state, for example, T ₁ , T ₂ , T ₃
Newly set based on. In the above case, the elapsed times T ₁ , T ₂ , and T ₃ performed in the past a plurality of times (for example, three times) are each a predetermined short time, for example, 5 _sec.
(5 seconds) or less, the time value (t ₀ ) of the preset delay time t is added to the plurality of elapsed times T
_The sum of the maximum time value (eg, T ₃ ) of ₁ , T ₂ , and T ₃ is set as the time value (t _n ) of the new delay time t _n after the current time, and in this case, The time value (t _n ) of the new delay time t _n does not exceed a predetermined upper limit time value t _max. (For example, 15 _sec ). Therefore, when the equation for calculating the sum is expressed by an equation, the equation 1 is as follows.

[0017]

[Formula 1]

Each of the elapsed time values T ₁ , T ₂ ,
In response to T ₃ exceeding the predetermined upper limit time value t _max. , The time value (t ′ _n ) of the new delay time t ′ _n is calculated as follows:
The delay time t is set so as not to exceed the preset time value (t ₀ ), or is set to be smaller than the sum time value (t ₀ + T ₃ ). Therefore, in this case, the equation for setting the time value (t ′ _n ) of the new delay time t ′ _n to the time value (t ₀ ) of the preset delay time t is expressed by the following Equation 2. It is.

[0019]

[Formula 2]

Alternatively, the equation for setting the time value (t ' _n ) of the new delay time t' _n to be smaller than the sum time value (t ₀ + T ₃ ) is expressed by the following equation ( ₃ ).

[0021]

[Equation 3]

Next, the operation of the engine control device of the present invention will be described. The time value t ₀ of the delay time preset as an initial value in the prior art is a fixed value to the last. Therefore, in an operation in which the operation of the operation lever is performed again immediately after the engine speed has decreased to the deceleration speed, the engine speed immediately increases from the deceleration speed to the throttle instruction speed, and the engine speed increases. The meaningless vertical fluctuation of the number adversely affects fuel economy. However, in Example engine control apparatus of the present invention, operations such as one operation is performed again the engine 1 is at least the operating lever 14 to 19 immediately after even down to the deceleration rotational speed N _D (shown in FIG. 3) In the case where the elapsed time T performed immediately before the current time is a predetermined small time, for example, 5 _SEC or less, that is, if the determination result in step A of the flowchart shown in FIG. setting the plus the elapsed time T of the delay time t ₀ which is set in advance as a new delay time t _n as being in the (step B shown in _a) as. This case will be described in detail. If the determination result of step A is YES, step C _a
As shown by, the elapsed time T is stored in the storage circuit unit 26 of the controller 24 (shown in FIG. 1) at each time. Then, as shown in step D _a , the elapsed times T ₁ , T ₂ , and T ₃ performed a plurality of times in the past (for example, three times as shown in FIG. 3) are each a predetermined short time, for example, 5 _seconds or less. Is determined, and the result of the determination is Y
When the ES is the time value is the initial value of the preset delay time t as indicated in step B _b (t _0),
A value obtained by adding the maximum time value (for example, T ₃ ) of the plurality of elapsed times T ₁ , T ₂ , and T ₃ is set as a new delay time t _n (t _n ). Then, as shown in step E, it is determined whether the time value (t _n ) of the new delay time t _n does not exceed a predetermined upper limit time value t _max . As a result, the judgment result of step E becomes YE
In the case of S is, in the case or return currently working in the neutral position or move a multiple of the operating lever, it is possible to learn control a new delay time t _n from just before the last of the operation state of the present time . That is, when the engine 1 has the deceleration speed N
_In a work in which at least one operation of the operation levers 14 to 19 is performed again immediately after descending to _D , the rotation speed of the engine 1 is set to the initial set value t ₀ of the delay time (the previously set initial value The sum of the sum of the maximum time value (for example, T ₃ ) of the plurality of elapsed times T ₁ , T ₂ , and T ₃ to the time value t ₀ of the delay time set as (new delay time) is a t _n, the time does not exceed a predetermined upper limit time value t _max.) is held by the throttle instructed rotational speed.
In other words, when the operation is performed again after a short short pause, the operation can be continued at the throttle instruction speed without the engine speed dropping to the low speed speed. I can be sure.
Also, even if a situation occurs in which the small pause time between lever operations is long when the sum time is set to the new delay time t _n , the sum time exceeds the predetermined upper limit time value t _max. Since the engine speed is not set, the engine speed is reduced to the deceleration speed after the relatively short time, so that fuel saving can be achieved.

Further, in the case where the operation has been performed a plurality of times (for example, three times) in the past and each of the elapsed times T ₁ , T ₂ , and T ₃ is a predetermined short time, for example, 15 _seconds or more, that is, in the flowchart shown in FIG. If the decision result in the step A is NO,
If the determination result of the next step C _b is YES, 'the time value of _n (t' new delay time t _n), the initial set value t ₀ of the preset delay time, or the initial setting value It is set to t ₀ or less (shown in step _Bc ). Alternatively, although not described in the flowchart, the new time value (t ′ _n ) of the delay time t ′ _n is calculated by using the time value (t
₀ + T ₃ ). Therefore, even if the operation is performed again after a short pause but a relatively short pause, the operation can be continued without changing the engine speed to the low-speed speed and at the throttle-instructed speed. To improve the workability. Also even if there is a situation, such as a long pause time in between lever operation at the time of the work, it dropped immediately to the deceleration rotational speed after passing the new delay time t _'n, it is possible to measure the fuel saving.

[0024]

Conventionally, in a hydraulic construction machine, after a predetermined delay time has elapsed from the time when a plurality of operating levers have all reached the neutral state, the engine speed is reduced to a speed lower than the throttle command speed. (Deceleration speed), that is, so-called auto deceleration control. However, the time value t ₀ of the delay time previously set as an initial value in the above-described prior art is a fixed value to the last. Therefore, in an operation in which the operation of the operation lever is performed again immediately after the engine speed has decreased to the deceleration speed, the engine speed immediately increases from the deceleration speed to the throttle instruction speed, and the engine speed increases. The meaningless vertical fluctuation of the number adversely affects fuel economy. However, in the engine control device according to the embodiment of the present invention, in the operation in which at least one operation of the operation lever is performed again immediately after the engine drops to the deceleration speed, the past operation performed immediately before the current time is performed. The plurality of low-speed rotation state elapsed times T ₁ , T ₂ , and T ₃ are set to a predetermined small time, for example, 5 times.
If not more than _SEC , the plurality of elapsed times T ₁ , T
_2, the maximum time value of T ₃ (e.g., T ₃₎ was added sum time (although new delay time t _n, the time does not exceed a predetermined upper limit time value t _max.) By the throttle instruction rotational Be kept in numbers. In other words, when the operation is performed again after a short short pause, the operation can be continued at the throttle instruction speed without the engine speed dropping to the low speed speed. You can fight. The sum time is a new delay time t _n
Is set so that the sum time does not exceed a predetermined upper limit time value t _max. Can be reduced to the deceleration speed after the sum time, which is a relatively short time, to save fuel consumption. Further, in the case of the work in which the elapsed times T ₁ , T ₂ , and T ₃ performed in the past plural times (for example, three times) are each a predetermined short time, for example, 15 _sec or more, a new delay time t ′ _n time value (t _'n), the preset delay time of the initial set value t _0, or default value t ₀ less, or alternatively to be set smaller than the time value of the sum (t ₀ + T ₃₎ I have to. Therefore, even if the operation is performed again after a short pause, which is a short time, but a little longer, the operation can be continued at the throttle instruction speed without lowering the engine speed to the low speed speed. Work can be performed to enhance the workability. Also even if there is a situation, such as a long pause time in between lever operation at the time of the work, it dropped immediately to the deceleration rotational speed after passing the new delay time t _'n, it is possible to measure the fuel saving. Therefore, in the engine control device according to the embodiment of the present invention,
By learning control of the delay time from the past work state immediately before the present time, fuel efficiency can be more appropriately improved and work operability can be enhanced.

[Brief description of the drawings]

FIG. 1 is a system configuration diagram of a hydraulic shovel equipped with an engine control device of the present invention.

FIG. 2 is a flowchart showing control by a controller of an engine control device in the hydraulic shovel of FIG. 1;

FIG. 3 is a diagram for explaining a relationship between a delay time t and an engine speed corresponding to an elapsed time T.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine 14, ..., 19 Operation lever 21 Throttle lever 22 Speed sensor 23 Throttle motor 24 Controller

(Equation 1)

(Equation 2)

(Equation 3)

Claims (5)

[Claims] 1. An engine, a hydraulic pump driven by the engine, a plurality of actuators operated by hydraulic pressure discharged from the hydraulic pump, and a plurality of operating levers for operating the plurality of actuators, respectively. A throttle lever for setting the number of revolutions of the engine, and after elapse of a preset delay time from the time when all of the plurality of operation levers are in the neutral state, the number of revolutions of the engine In a construction machine having a control to reduce a fuel-saving cost to a predetermined low-speed rotation speed from a throttle-instruction rotation speed set by a lever, after all of the operation levers are operated to a neutral position, they are held at the neutral position. The engine speed is controlled to the throttle instruction speed until the delay time elapses. When at least one of the operation levers is operated from the neutral position to the work position before the elapsed time reaches the delay time, the elapsed time is cleared and the engine speed is throttled. A step of setting the delay time after the current time, wherein the step of setting the delay time after the current time reaches the preset delay time in the past plural times of the low-speed rotations performed immediately before the current time. An engine control device, wherein a new setting is made based on the elapsed time of the low-speed rotation state after the setting. 2. The method according to claim 1, wherein the plurality of low-speed rotation speed states performed in the past a plurality of times are respectively equal to or less than a predetermined short time, and the plurality of low-speed rotation speed values are added to the time value of the preset delay time. 2. The engine control device according to claim 1, wherein a sum of a maximum time value of the several elapsed times is set as a time value of a new delay time after the current time. 3. The engine control device according to claim 1, wherein a time value of the new delay time does not exceed a predetermined upper limit time value. 4. The time value of the new delay time does not exceed the time value of the preset delay time in response to a situation where each time value of the elapsed time of the low-speed rotation state exceeds a predetermined upper limit time. The engine control device according to claim 1, wherein the setting is made as follows. 5. The time value of the new delay time is set smaller than the time value of the sum according to the situation where each time value of the elapsed time of the low-speed rotation state exceeds a predetermined upper limit time value. 4. The engine control device according to claim 1, wherein: