JPH0893521A - Idle rotation speed controller - Google Patents

Abstract

PURPOSE: To quickly stabilize the idle rotation speed to a set value by issuing a command for prohibiting the decrease of field current to a voltage regulator. CONSTITUTION: An ECU 10 instructs a regulator 8 to preferentially prohibit the increase of field current when the idle rotational speed is less than the specific set value in discriminating of the idle state, and to prohibit the decrease of field current when the idle rotational speed is more than the set value, thereby, the idle rotational speed can be prevented from being deviated from the set value caused by the change of the generator output by immediately responding to the fluctuation of the idle rotational speed. Accordingly, adverse effect to be generated by the effect of fluctuation of the electric load of a generator 1 can be avoided, thereby, the fluctuation restraining effect of the idle rotational speed and the response can be remarkably improved in comparison with the conventional ones.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an idle rotation speed control device, and more particularly to an idle rotation speed control device using a vehicle battery power generator.

[0002]

2. Description of the Related Art Japanese Patent Laid-Open No. 63-314346 suppresses fluctuations in idle speed by feedback-controlling a generator output based on a difference between a detected idle speed and an average value of idle speeds. I suggest you do. Japanese Patent Laid-Open No. 63-297744 discloses that when the idle speed of an engine is controlled, the idle speed is set to an average generated power command value calculated based on an input parameter for controlling a battery voltage when an idle state is detected. It is proposed that the generator be controlled by adding a rotation speed correction command value for setting a predetermined value, thereby suppressing fluctuations in the idle rotation speed.

Also, Japanese Patent Application Laid-Open No. 5-3 filed by the present applicant.
Japanese Patent No. 28799 proposes to suppress the fluctuation of the idle rotation by suppressing the changing speed (change rate) of the generator output when the changing speed (change rate) of the engine rotation speed exceeds a predetermined value. are doing.

[0004]

However, in the above-mentioned Japanese Patent Laid-Open No. 63-314346, the generator output is feedback-controlled so that the idle rotation speed converges to the average value of the idle rotation speed. Inherently, there is a problem that fluctuation control is not sufficient.

That is, the idle rotation speed must be controlled so as to have a constant set value. However, in the feedback control in which the idle rotation speed converges to the average value of the idle rotation speed, the idle rotation speed is It is not easy to converge the speed to a predetermined set value, and even if it converges, the response speed is slow. For example, if the average value of the past idle rotation speed is high, even if the detected value of the idle rotation speed exceeds the above set value, it is not possible to instruct a sufficient increase in the generator output, and in some cases a decrease in the generator output is required. This may also occur when a command is issued to separate the idle rotation speed from the set value.

Further, the above-mentioned Japanese Patent Laid-Open No. 63-314346.
According to the technique disclosed in the publication, when controlling the setting value of the idle rotation speed by changing the mode of the AT vehicle, it is not easy to converge the idle rotation speed to a new setting value, and it also converges. Even in this case, the response speed is slow, and in some cases, the idle rotation speed may be separated from the set value by instructing the decrease of the generator output.

Next, the above-mentioned Japanese Patent Laid-Open No. 63-297744.
The gazette discloses a rotation speed for setting an idle rotation speed to a predetermined value for an average generated power command value calculated based on an input parameter (for example, a difference between a battery voltage and a set value thereof) as in the normal generator output control. The generator output is controlled by the control parameter to which the speed correction command value is added. Therefore,
Although only the ratio of the added rotation speed correction command value can suppress the fluctuation of the idle rotation speed by the generator output, the average generated power command value (for example, the change of the battery voltage) which is the main component of the control parameter of the generator output is For example, if the load fluctuates greatly due to the application of a large load, the generator output fluctuates largely independently of the idle rotation speed as in the conventional case, and as a result, the idle rotation speed greatly fluctuates.

Further, according to the above-mentioned Japanese Patent Laid-Open No. 5-328799, when the changing speed (rate of change) of the engine rotation speed exceeds a predetermined value, the changing speed (rate of change) of the generator output.
Therefore, when the change speed (change rate) of the engine rotation speed is large, it is possible to prevent the change speed (change rate) of the engine rotation speed from being further multiplied by the change in the generator output. It was not possible to sufficiently suppress fluctuations in the engine idle rotation speed, and it was also impossible to sufficiently suppress fluctuations in the engine idle rotation speed even when the speed of change (rate of change) in the rotation speed was small.

The present invention has been made in view of the above problems, and an object thereof is to provide an idle rotation speed control device capable of quickly stabilizing the idle rotation speed to a set value.

[0010]

A first structure of an idle speed control device of the present invention is a generator driven by an engine to supply power to a vehicle load, and a field current of the generator of the generator. Control based on a predetermined control parameter related to the output voltage (for example, the output voltage of the generator or the terminal voltage of the battery fed from the generator) to control the predetermined control parameter related to the output voltage to a predetermined level. A voltage regulator, a rotation speed detection unit that detects the rotation speed of the engine, and a determination whether the engine is in an idle state, and when the detection value of the rotation speed is below a predetermined set value when the idle state is determined. To the voltage regulator to prohibit the increase of the field current, and when the detected value of the rotation speed exceeds a predetermined set value when the idle state is determined. It is characterized in that it comprises a command means for commanding the prohibition of reduction of the field current to the voltage regulator.

According to a second aspect of the present invention, in addition to the above-mentioned first aspect, the command means further includes a voltage regulator when the detected value of the rotation speed is below a predetermined set value at the time of determining the idle state. A command is given to reduce the field current, and when the detected value of the rotation speed exceeds a predetermined set value at the time of determining the idle state, the voltage regulator is instructed to increase the field current.

A third structure of the present invention is the above-mentioned first or second structure.
In the above configuration, the command means further commands the engine control device for controlling the engine to increase the air flow rate or the fuel injection amount when the output voltage of the generator is smaller than a predetermined voltage when the idle state is determined. When the output voltage of the generator is higher than a predetermined voltage, the engine controller is instructed to reduce the air flow rate or the fuel injection amount.

According to a fourth aspect of the present invention, in addition to the first or second aspect, the command means is such that the detected value of the rotational speed is below a predetermined set value when the idle state is determined, and When the output voltage of the generator is smaller than a predetermined voltage, the engine control device that controls the engine is instructed to increase the air flow rate or the fuel injection amount, and when the idle state is determined, the detected value of the rotation speed is a predetermined value. When the output voltage of the generator exceeds a set value and is larger than a predetermined voltage, the engine control device for controlling the engine is instructed to reduce the air flow rate or the fuel injection amount. .

[0014]

According to the first structure of the present invention,
When determining the idle state, when the idle speed is below the specified value, the increase of the field current is preferentially prohibited, and when it is higher, the decrease of the field current is preferentially prohibited. The idle rotation speed can be prevented from fluctuating in the direction deviating from the set value due to the change in the generator output immediately in response to, and the adverse effect due to the fluctuation of the electric load of the generator can be avoided. As a result, the effect of suppressing fluctuations in the idle rotation speed and its response can be significantly improved as compared with the prior art.

According to the second structure of the present invention, in the first structure, the field current is further reduced when the idle speed is lower than a predetermined set value, and the field current is decreased when the idle speed is higher than the predetermined set value. Since the magnetic current is increased, it is possible to further improve the idle rotation speed fluctuation suppression. According to the third configuration of the present invention, further, in the first or second configuration, when the output voltage of the generator (which may be the battery voltage) is smaller than a predetermined voltage, the intake air flow rate or fuel injection of the engine is further increased. When the output voltage of the generator (which may be the battery voltage) is higher than a predetermined voltage, a command is issued to reduce the intake air flow rate or the fuel injection amount of the engine.

That is, if the intake air flow rate or the fuel injection amount of the engine is increased when the generator output voltage (which may be the battery voltage) is lower than a predetermined voltage, the engine torque is increased and the engine speed is set to the idle set speed. Since the number is equal to or more than the number, the field current is increased by the field current control of the first or second configuration, and the output current of the generator can be increased. As a result, the output voltage of the generator (which may be the battery voltage) can be increased to a predetermined voltage without causing the engine speed to drop.

On the contrary, when the output voltage of the generator (which may be the battery voltage) is higher than the predetermined voltage, if the intake air flow rate or the fuel injection amount of the engine is reduced,
Since the engine torque decreases and the engine speed becomes equal to or lower than the idling set speed, the field current is reduced by the field current control of the first or second configuration, and the output current of the generator can be decreased. As a result, the output voltage (may be the battery voltage) of the generator can be reduced to a predetermined voltage without causing an increase in engine rotation.

Further, in the case of suppressing the fluctuation of the idle rotation speed by the field current control of the second structure, if the idle rotation speed falls below the set value, the field current will be reduced. The output voltage of the generator (which may be the battery voltage) becomes lower than a predetermined set voltage. Therefore, by detecting this, the intake air flow rate of the engine or the fuel injection amount increases, so that the engine generated torque increases and the idle rotation speed approaches its set value, and the generator output voltage (output current) -rotation Due to the speed characteristic, the output voltage (output current) of the generator also increases, and the effect of two birds with one stone can be obtained.

On the contrary, in the case of suppressing the fluctuation of the idle rotation speed by the field current control of the second structure, if the idle rotation speed exceeds the set value, the field current is increased. The output voltage of the generator (which may be the battery voltage) becomes higher than the predetermined set voltage. Therefore, by detecting this, the intake air flow rate or fuel injection amount of the engine decreases, so the engine generated torque decreases and the idle rotation speed approaches its set value, and the generator output voltage (output current) -rotation Due to the speed characteristic, the output voltage (output current) of the generator is also reduced, and the effect of two birds with one stone is obtained.

The fourth structure of the present invention further increases the torque generated by the engine when the engine speed and the output voltage are small at the time of determining the idle state in the above-mentioned first or second structure, thereby increasing the torque of the generator. The output of the generator is automatically increased with increasing the rotation speed, and conversely, the torque generated by the engine is reduced when the engine rotation speed and the output voltage are large at the time of determining the idle state. And automatically reduce the output of the generator.

With this configuration, the output voltage of the generator, that is, the state of charge of the battery can be stably maintained despite the idle speed control by the generator described in the configurations 1 and 2 of the present invention.

[0022]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the idle speed control device of the present invention will be described with reference to the circuit diagram shown in FIG. 1
Is an automotive alternator driven by an in-vehicle engine 2, 3 is a field coil thereof, 4 is a three-phase armature coil thereof, 5 is a three-phase full-wave rectifier thereof, and a three-phase armature coil is shown. The voltage of each phase 4 is rectified by the three-phase full-wave rectifier 5 and supplied to the battery 6 and the electric load 7 as the output voltage of the generator 1. The output voltage of the generator 1 is applied to the collector of the field current control transistor 14 through the field coil 3 or the flywheel diode D, and the emitter thereof is grounded.

Reference numeral 8 indicates the generator 1 so that the detected value (here, the battery voltage divided voltage) Vb of the output voltage (here, the battery voltage) of the generator 1 becomes equal to a predetermined reference value Vr in the non-idle state. It is an exciting current control device (regulator) that controls the exciting current of the exciting coil 3. Reference numeral 9 is a well-known ISC actuator for idle speed control that controls the intake air amount of the engine during idling, and is composed of, for example, an electric motor that drives and controls a valve (not shown) disposed in the intake path. .

Reference numeral 10 denotes an engine control unit (ECU) having a microcomputer for controlling the operation of the engine 2.
And CPU 20, ROM 21, RAM 22, I / O
It has a port 18. Reference numeral 16 is a pickup (rotational speed detecting means in the present invention) for detecting the rotational speed of the engine 2.

Reference numeral 19 is a throttle valve (not shown) of the engine 2 for determining the idle state of the engine 2.
Is a switch for detecting the fully closed state of. The regulator 8 is a comparator 1 that compares the voltage Vb with the voltage Vr.
1 and the output signal of the comparator 11 to the transistor 14
AND circuit 12 for determining whether or not to transmit the signal, and an OR circuit for transmitting a predetermined control signal S2 output from the ECU 10 to the transistor 14 when the AND circuit 12 prohibits transmission of the output signal of the comparator 11 instead. And circuit.

The operation of this circuit will be described below with reference to the flowchart of FIG. After starting the engine 2, the routine is started and it is checked in steps 101 and 103 whether the engine is in the idle state. The throttle switch 19 is off (throttle valve is fully closed), and the engine speed is 1200 rp according to the rotation speed signal from the pickup 16.
It is determined whether or not it is equal to or less than m, and if so, the process proceeds to step 105 as an idle state, and if not, it is determined to be a non-idle state and a high signal is sent to the AND circuit 12,
The Lo signal is output to the OR circuit 13 (111), and the process returns to step 101. Therefore, in the non-idle state, the exciting transistor 14 is connected to the comparator 11
The detection voltage Vb of the battery 6 is controlled by the set value V
matches r.

On the other hand, if the idle state is determined, the Lo signal is output to the AND circuit 12 as the control signal S1 (1
05), and then pick-up 1 in a step not shown
It is determined whether the engine speed Ne read from No. 6 is less than or equal to a predetermined idle set speed (107), and if it is low, the Lo signal is output to the OR circuit 13 as the control signal S2 (109), and step 115 is executed. move on. As a result, the transistor 14 is cut off, the exciting current of the generator 1 is reduced, and the mechanical load of the generator 1 on the engine 2 is reduced.

On the contrary, when the engine speed read by the ECU 10 in step 107 is higher than the predetermined idle set speed, a Hi signal is output to the OR circuit 13 (113) to force the field current. And then check whether the output of the comparator 11 is Hi (11
5) If the output of the comparator 11 is Hi, it is determined that the battery 6 requires charging, and the ISC actuator 9 is operated in the engine output increasing direction to increase the output torque of the engine (117).
If the output of 1 is Lo, the ISC actuator 9 is operated in the engine output down direction to reduce the output torque of the engine (119), and the process returns to step 101. In this embodiment, the ISC actuator 9 controls the intake flow rate of the engine 1, but it goes without saying that it may control the fuel injection amount.

FIG. 3 shows the rotation speed-generated torque characteristic of the generator 1 and the rotation speed-generated torque characteristic of the engine 2 when the exciting current (also called field current) is constant and when the generated current is constant. When the battery voltage is controlled to be constant even in the idle state as in the conventional case, the output current (generated current) of the generator 1 in the steady state in which the electric load 7 does not change.
Is naturally constant. Therefore, conventionally, the exciting current is increased to compensate for the decrease in the output voltage of the generator 1 as the rotation speed of the generator 1 decreases, and the torque of the generator (engine 2
The load torque as seen from above was increasing. When this rate of change (inclination) becomes larger than the engine torque characteristic, idle speed drift occurs. Therefore, when an attempt is made to stabilize the rotation of the engine 2 by controlling the amount of air supplied from the ECU 10 to the engine by the ISC actuator 9, there is a delay in the response of the engine torque (generally, there is a delay of several seconds). (Idle hunting) was occurring.

In this embodiment, as described above, the value of the exciting current is reduced when the engine speed Ne becomes less than the idle set speed (step 10).
5, 107), the slope of the torque characteristic of the generator is changed.
As a result, the engine speed increases. Conversely, when the engine speed Ne becomes equal to or higher than the idle set speed, the value of the exciting current is forcibly increased (step 11
3) Change the inclination of the torque characteristic of the generator. As a result, engine rotation is reduced. With the above operation, the engine speed Ne at the time of idling can be controlled to match the idling set speed. The response delay of the generator torque is determined by the time constant of the field coil 3, and is generally 0.
1 to 0.2 seconds. Therefore, the idling speed control which is much more responsive than the conventional one is realized.

Further, according to the flowchart of FIG. 2, the following operation is also performed. That is, the detected value Vb of the battery voltage
Is less than the reference value Vr, the output of the comparator 11 is H
i becomes i, and the amount of air supplied to the engine 2 is increased by the ISC actuator 9 (step 117). As a result, the engine speed Ne becomes equal to or higher than the idle set speed, the exciting current is increased to stabilize the speed as described above (step 113), and as a result, the generated current increases and the battery voltage increases. . On the contrary, when the detected value Vb of the battery voltage becomes higher than the reference value Vr, the amount of air supplied to the engine 2 decreases (step 11
9), the engine speed Ne decreases, the exciting current is decreased to stabilize the speed (steps 105 and 107), and as a result, the battery voltage decreases. According to the present embodiment, the above operation is repeated so that the detected value Vb of the battery voltage is controlled to be equal to the set value Vr. If you do this,
Even when the electric load 7 is turned on, the reduction in engine speed is sufficiently suppressed. Further, according to this embodiment, for example, the idle speeds are 700 rpm and 600 r between the automatic parking mode and the drive mode, respectively.
Stable control is possible even when the idle set speed changes, such as setting to pm.

However, in the control of the above embodiment, the torque of the generator 1 is used to control the engine speed in the idle state, so that the response speed is fast but the torque control range is relatively small. Become. Therefore, in this embodiment,
It is desirable to be used together with the conventional ISC control, that is, the control that determines the operation amount of the ISC actuator 19 based on the difference between the engine speed and the idle speed setting. In this case, the engine water temperature and the intake air temperature change relatively slowly. The engine torque can be stabilized by the conventional ISC control, and the idle hunting can be suppressed in this embodiment.

(Other Embodiments) FIG. 5 shows the engine speed Ne and the battery voltage (here, its partial pressure) Vb in the idle speed range, which are 4 by the idle speed and the voltage set value (reference value) Vr. It represents a state divided into two quadrants. In the case where the electric load is steady and the same exciting current is applied, if the engine speed Ne increases, the generator output increases correspondingly, and the battery voltage also increases, moving to the first quadrant of FIG. 5 and vice versa. As the engine speed Ne decreases, the generator output decreases, and the battery voltage also decreases and the third quadrant of FIG. 5 is entered. That is, even if the exciting current is constant, the output of the generator fluctuates due to the fluctuation of the engine speed Ne. Therefore, it is understood that the hunting of the engine speed during idling can be suppressed by controlling the exciting current changes in the first and third quadrants and suppressing the output of the generator 1.

When the electric load is turned on, the engine speed Ne and the voltage setting value Vr are the third quadrant and the fourth quadrant.
While reciprocating between the quadrant and the idle rotation speed in the vicinity of the set rotation speed, the battery voltage can be increased without a significant decrease in rotation speed. When the electric load is cut off, the same control can be performed between the first quadrant and the second quadrant to stabilize the battery voltage.

FIG. 4 shows a circuit example of the exciting current control device 8 for realizing the above-described operation mode of FIG. 5, and the operation of the ECU 10 at this time will be described with reference to the flowchart of FIG. During non-idle, the control signal S1 output from the ECU 10 becomes Hi, so the knot circuit 107 outputs Lo to the AND circuit 105 and the transistor 104
Turn off.

When the battery voltage is low, the comparator 11 outputs a Hi signal to the U / D counter 101,
The U / D counter 101 counts up with the clock signal output from the oscillator 103 while the Hi signal is input from the comparator 11, and the PWM circuit 102 increases the duty ratio of the excitation voltage according to the count value. Output to the transistor 14, and the on-duty ratio of the transistor 14 increases by this count up,
The battery voltage is restored by the increase in the exciting current. vice versa,
When the battery voltage is high, the comparator 11 outputs the Lo signal to the U / D counter 101, and the U / D counter 101
Counts down with the clock signal output from the oscillator 103 while the Lo signal is input from the comparator 11, and the PWM circuit 102 according to the count value.
Turns down the duty ratio of the excitation voltage
It outputs to 4, and the transistor 1 for this countdown
The on-duty ratio of No. 4 decreases, and the excitation voltage decreases, so that the battery voltage decreases.

At the time of idling, the control signal S1 output from the ECU 10 becomes Lo, so the knot circuit 107 outputs Hi to the AND circuit 105, and as a result, EX-O.
When the R circuit 106 outputs Hi, the increase / decrease of the duty ratio during the non-idle state is prohibited, and the duty ratio is fixed. That is, the EX-OR circuit 106
Is when the battery voltage is high and the comparator 11 outputs Lo, and the control signal S2 is Hi (when the engine speed Ne is equal to or higher than the idle set speed), and when the battery voltage is low and the comparator 11 outputs Hi. In the state, when the control signal S2 is Lo (when the engine speed Ne is less than the idling set speed), the duty ratio is fixed during idling.

That is, the duty ratio of the exciting voltage is fixed in the first and third quadrants of FIG. For example, during idling and when the engine speed Ne is equal to or higher than the idling set speed, the control signal S2 becomes Hi by a routine described later. In this state, when Vb> Vr, the output of the comparator 11 is Lo,
The EX-OR circuit 106 outputs a Hi signal. At the time of idling, the control signal S1 is Lo, and the knot circuit 107 outputs Hi to the AND circuit 105, so that the transistor 104 is turned on, the U / D counter 101 stops counting, and the PWM circuit 102 causes the transistor 14 to turn off.
Is controlled with a constant duty ratio, and the average field voltage has a constant duty ratio. At this time, as will be described later, I
The SC actuator 9 operates to reduce the intake air amount of the engine 2. As a result, the torque generated by the engine 2 decreases, and when the engine speed Ne decreases, the battery voltage also decreases. That is, the operation is the first quadrant in FIG.

Further, when the engine is idling and the engine speed N
When e is less than the idling set speed, the control signal S2 becomes Lo by the routine described later. In this state, when Vr> Vb, the output of the comparator 11 becomes H
Since it is i, the EX-OR circuit 106 outputs a Hi signal. At the time of idling, the control signal S1 is Lo and the knot circuit 107 outputs Hi to the AND circuit 105, so that the transistor 104 is turned on, the U / D counter 101 stops counting, and the PWM circuit 102 turns on the transistor 14. Control is performed with a constant duty ratio, and the average field voltage has a constant duty ratio. Also at this time,
As will be described later, the ISC actuator 9 operates to increase the intake air amount of the engine 2. As a result, the torque generated by the engine 2 increases and the battery voltage also increases as the engine speed Ne increases. That is, the operation of the third quadrant in FIG. 5 is performed.

After all, in the first quadrant during idling (that is, when both the engine speed Ne and the battery voltage are large), the duty ratio of the exciting current is not reduced as in the ordinary exciting current control, and it is fixed. Thus, the torque of the load generated by the generator 1 is kept large to avoid the reduction of the engine load, and the battery voltage is reduced by reducing the output current due to the reduction of the intake air amount.

On the other hand, in the third quadrant during idling (that is, when both the engine speed Ne and the battery voltage are small), the duty ratio of the exciting current is not increased like the usual exciting current control, and is fixed. As a result, the torque of the load generated by the generator 1 is kept small to prevent the engine load from increasing, and the battery voltage is increased by increasing the output current due to the increase in the intake air amount.

Next, the control operation of the ECU 10 for outputting the control signals S1 and S2 to the circuit of FIG. 4 is shown in the flowchart of FIG. This flowchart operates essentially the same as that of FIG. 3, and the control signal S1 is set to Lo in the idle state (204), and the engine speed Ne is large and the battery voltage is large in the idle state. In the case (214), that is, in the first quadrant, the intake air amount is increased (216), in the idle state, the engine speed Ne is small, and the battery voltage is small (218), that is, the intake air amount is decreased in the third quadrant. It is characterized by the point (220).

[Brief description of drawings]

FIG. 1 is a circuit diagram showing an embodiment of an idle speed control device of the present invention.

2 is a flow chart showing a control operation of an ECU 10 of FIG.

FIG. 3 is a characteristic diagram showing a relationship between torque and engine speed of the conventional engine and generator of FIG.

FIG. 4 is a circuit diagram of essential parts showing a modified example of the idle speed control device of FIG. 1.

FIG. 5 is a quadrant diagram showing each control mode state divided by the engine speed and the output voltage of the generator in the modified example of FIG.

FIG. 6 is a flowchart showing an operation of the ECU 10 in the modified example of FIG.

[Explanation of symbols]

Reference numeral 1 is a generator, 2 is an engine, 8 is a voltage regulator, 16 is a pickup (rotation speed detecting means), and 10 is an ECU (command means).

Continuation of front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location H02P 9/04 M 9/30 D

Claims (4)

[Claims] 1. A generator driven by an engine to supply power to a vehicle load, and a field current of the generator controlled to a predetermined output voltage based on a predetermined control parameter related to an output voltage of the generator. A voltage regulator that controls a related predetermined control parameter to a predetermined level, a rotation speed detection unit that detects a rotation speed of the engine, a determination whether the engine is in an idle state, and the rotation speed when the idle state is determined. When the detected speed value is lower than a predetermined set value, the voltage regulator is instructed to prohibit the field current from increasing, and when the idle state is determined, the detected rotation speed value exceeds the predetermined set value. In this case, an instruction means for instructing the voltage regulator to prohibit the decrease of the field current, and an idle rotation speed control device. 2. The command means commands the voltage regulator to reduce the field current when the detected value of the rotation speed falls below a predetermined set value at the time of determining the idle state, and
2. The idle rotation speed control device according to claim 1, wherein when the detected value of the rotation speed exceeds a predetermined set value when the idle state is determined, the voltage regulator is instructed to increase the field current. 3. The command means commands an engine control device for controlling the engine to increase an air flow rate or a fuel injection amount when the output voltage of the generator is smaller than a predetermined voltage when the idle state is determined. 3. The idle rotation speed control device according to claim 1, wherein the engine control device is instructed to reduce the air flow rate or the fuel injection amount when the output voltage of the generator is higher than a predetermined voltage. 4. The command means controls the engine when the detected value of the rotation speed is below a predetermined set value when the idle state is determined and the output voltage of the generator is smaller than a predetermined voltage. An engine control device for controlling is instructed to increase the air flow rate or the fuel injection amount, the detected value of the rotation speed exceeds a predetermined set value when the idle state is determined, and the output voltage of the generator is a predetermined voltage. 3. The idle rotation speed control device according to claim 1, wherein the engine control device that controls the engine is instructed to reduce the air flow rate or the fuel injection amount when larger than the above.