JPH0611035A - Look up control device for automatic transmission - Google Patents

Abstract

PURPOSE:To avoid generation of a shock at the time of changing set air-fuel ratio of an engine. CONSTITUTION:The changed quantity of air-fuel ratio of intake mixture of an engine detected by an air-fuel ratio sensor is compared with a set level SLAF (S2). When a change of air-fuel ratio over a set level is detected, it is discriminated whether the lock up clutch provided in a torque converter of an automatic transmission is in the lock up condition or not (S3). When it is in the lock up condition, the lock up clutch is released, or shifted to the slip condition, and hence the coupling force of the lock up clutch is forcedly weakened.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lock-up control device for an automatic transmission, and more particularly, to a lock-up clutch that mechanically directly connects the input and output shafts of a torque converter to control a set air-fuel ratio of an engine. A technique for avoiding a shock.

[0002]

2. Description of the Related Art As a conventional automatic transmission for a vehicle, a hydraulic lock-up clutch capable of mechanically directly connecting an input shaft and an output shaft thereof is provided in a torque converter, and the lock-up is performed under a constant operating condition. The efficiency of the torque converter is improved by engaging the clutch (see Japanese Patent Application Laid-Open No. 2-38574).

On the other hand, for the purpose of improving fuel economy, a lean air-fuel ratio (for example, 20 to 20) which is much higher than the theoretical air-fuel ratio (14.7) is used.
It has been proposed that a lean-burn engine that burns in 25) be used. In such a lean combustion engine, for example, when operating at low rotation speed and low load, fuel efficiency is improved by burning at the lean air-fuel ratio, and torque performance is emphasized during acceleration and high load to give a theoretical air-fuel ratio. As the air-fuel ratio on the slightly rich side (for example, about 13), both improvement of fuel consumption and securing of output torque are compatible (Japanese Patent Laid-Open No. 18733/1989).
No. 8, etc.).

[0004]

By the way, in the lean combustion engine as described above, if the target air-fuel ratio is gradually changed from the lean combustion region to the stoichiometric air-fuel ratio region on the high load side, the air-fuel ratio in the middle of the air-fuel ratio is gradually changed. The NOx emission amount increases in the fuel ratio state. Therefore, from the viewpoint of exhaust properties, as the setting of the target air-fuel ratio, it is not preferable to set the intermediate air-fuel ratio, it is necessary to rapidly change from the lean air-fuel ratio to near the stoichiometric air-fuel ratio, thereby, At the time of switching from the lean air-fuel ratio to near the stoichiometric air-fuel ratio,
The output torque of the engine will change suddenly.

If the lock-up clutch is in the engaged state when the setting is switched from the lean air-fuel ratio to near the stoichiometric air-fuel ratio, the damper effect of the torque converter cannot be exerted and the engine output torque suddenly changes. Was transmitted directly to the vehicle body, causing a shock. The present invention has been made in view of the above problems, and when the set air-fuel ratio of the engine is switched and the engine output torque changes abruptly, the damper effect of the torque converter is reliably exhibited, and the output torque changes suddenly. It is intended to be able to avoid being directly transmitted to the vehicle body.

[0006]

Therefore, in the lock-up control device for an automatic transmission according to the present invention, the internal combustion engine combined with the automatic transmission can switch the set air-fuel ratio of the intake air-fuel mixture according to the engine operating conditions. A lock of an automatic transmission including an engine, and a lock-up clutch mechanically directly connecting an input shaft and an output shaft of a torque converter interposed between an output shaft of the engine and an input shaft of a gear type transmission. The up controller is configured as shown in FIG.

In FIG. 1, air-fuel ratio switching detection means detects switching of the set air-fuel ratio in the internal combustion engine. On the other hand, the engagement force reduction control means forcibly reduces the engagement force of the lockup clutch when the air-fuel ratio switching detection means detects switching of the set air-fuel ratio during engagement of the lockup clutch. Here, the engagement force reduction control means sets a target of the rotation difference of the input / output shaft of the lockup clutch according to the switching step of the set air-fuel ratio, and engages the lockup clutch so that the target rotation difference is achieved. The force may be feedback controlled.

[0008]

According to this structure, when the set air-fuel ratio of the engine intake air-fuel mixture is switched and the engine output torque suddenly changes, if the lockup clutch is in the engaged state, the engaging force is controlled to be reduced. . Therefore, it becomes possible to prevent the change in the engine output torque due to the switching of the set air-fuel ratio from being directly transmitted, and it is possible to prevent the occurrence of shock due to the switching of the set air-fuel ratio.

Further, when the engagement force of the lockup clutch is reduced when switching the set air-fuel ratio, the target of the rotational difference between the input and output shafts of the lockup clutch, that is, the engagement force of the engagement force set a goal. Then, if the engagement force of the lock-up clutch is feedback-controlled so that the target rotation difference is achieved, it is possible to surely prevent the occurrence of a shock without reducing the engagement force more than necessary to alleviate the shock. .

[0010]

EXAMPLES Examples of the present invention will be described below. In FIG. 2 showing the system configuration of this embodiment, an automatic transmission 2 is connected to the output side of an internal combustion engine 1 mounted on a vehicle (not shown). The automatic transmission 2 is connected to a torque converter 3 interposed on the output side of the engine 1 via the torque converter 3, and an internal combustion engine output torque is transmitted via the torque converter 3 to a gear transmission 4. For gear shift control, lockup control, line pressure control, internal combustion engine brake control, etc., which engages and disengages various friction elements (front clutch, rear clutch, brake band, overrun clutch, lockup clutch, etc.) And a solenoid valve group 5. The solenoid valve group 5 includes a lockup solenoid, a shift solenoid A, a shift solenoid B, an overrun clutch solenoid, a line pressure solenoid, and the like.

Signals from various sensors are input to the automatic transmission control unit 6 which controls the solenoid valve group 5. As the various sensors, there is provided a potentiometer-type throttle sensor 8 for detecting an opening TVO of a throttle valve 7 which is interposed in an intake system of the internal combustion engine 1 and works in conjunction with an accelerator pedal (not shown).

Further, the output shaft of the automatic transmission 2 is provided with a vehicle speed sensor 9 which emits a pulse signal at every predetermined rotation angle of the output shaft. Control unit 6 for automatic transmission
Is based on the operation position signal of the select lever operated by the driver, for example, when the select lever is in the drive range (D range), with reference to a map of preset shift patterns, the throttle valve opening TVO and the vehicle speed VSP According to the above, the shift positions of the 1st speed to the 4th speed are automatically set, and automatic shift control is performed to control the gear type transmission 4 to the shift position via the solenoid valve group 5.

In addition to the automatic shift control as described above, in this embodiment, the torque converter 3 is provided with a lockup clutch 40 as shown in FIG. 3, and the lockup clutch 40 allows the input shaft of the torque converter 3 to operate. And the output shaft can be directly connected mechanically. Similar to the automatic transmission control, the automatic transmission control unit 6 refers to a lockup control map preset according to the throttle valve opening TVO and the vehicle speed VSP, and controls the lockup solenoid via the lockup control map. It also controls the engagement state of the lockup clutch 40.

In FIG. 3, a lock-up plate 49 (hydraulic clutch) having a clutch facing 48 is disposed integrally with the torsion damper 50 so as to face the inner wall 42a of the drive shaft 41 of the case 42. The damper 50 is spline-fitted to the clutch hub 51, and the clutch hub 51 is spline-fitted to the driven shaft 44.

As a result, the lockup plate 49 becomes movable in the axial direction of the driven shaft 54, and the pressures P1 and P of the pressure chambers 52 and 53 formed on both sides of the lockup plate 49.
Move according to 2. The pressure chamber 52 has a pressure passage 54b.
The converter oil pressure (operating oil pressure) is supplied via the pressure chamber 53, and the converter oil pressure is supplied to the pressure chamber 53 via the pressure passage 54a.

Here, when P1> P2, the lock-up plate 49 moves to the left in the figure, and the inner wall 42 of the case 42 is moved.
When it is pressed against a, the drive shaft 41 and the driven shaft 54 are mechanically connected to each other in a lockup state (clutch direct connection state). Conversely, when P2> P1, the lockup plate 49 moves to the right in the figure. Then, the case 42 separates from the inner wall 42a of the case 42 and enters a non-lockup state (torque converter state). here,
Supply of converter hydraulic pressure (operating hydraulic pressure) to the pressure chambers 52, 53 via the hydraulic passages 54b, 54a is controlled by a lockup solenoid 55 in the solenoid valve group 5.

That is, by controlling the lockup solenoid 55, the operation of the lockup control valve 56 is controlled, and the converter hydraulic circuit connected to the lockup control valve 56 is connected to the lockup plate 49.
It is to switch between the open side and the fastening side. Here, the lock-up solenoid 55 is duty-controlled by the control unit 6,
When the OFF time is long, the converter hydraulic pressure supplied from the oil pump 57 acts on the pressure chamber 53, and further the pressure chamber 53
Since the oil flows into the pressure chamber 52 from P2> P1, P2> P1 and the lockup is released (released), and vice versa.
When the OFF time is short, the converter oil pressure is in the pressure chamber 52.
P1> P2, and the lockup plate 49 is pressed against the inner wall 42a of the case 42 to be in a tightened state.
Further, the converter hydraulic pressure P2 acting on the pressure chamber 53 can be appropriately reduced based on the OFF time ratio to bring the clutch into a half-clutch state (slip state).

The drive shaft 41 is connected to the output shaft of the internal combustion engine 1, and the driven shaft 44 is connected to the input shaft of the gear type transmission 4. On the other hand, the internal combustion engine 1 in this embodiment is
An electronically controlled fuel injection device is provided, the fuel injection amount Ti is calculated by the engine control unit 11, and an injection pulse signal having a pulse width corresponding to the fuel injection amount Ti is provided for each cylinder. The fuel supply to the engine is electronically controlled by outputting the valves 12a to 12d at predetermined timings.

The engine control unit 11 includes
In order to calculate the fuel injection amount Ti, detection signals from various sensors that detect the engine operating state are input. As the various sensors, an air flow meter 13 for detecting the intake air flow rate Q of the engine, a crankshaft of the engine 1 or a shaft that rotates in synchronization with the crankshaft are provided, and a pulse signal is output for each predetermined rotation angle of the crankshaft. Generated crank angle sensor 14,
Further, there is provided a known air-fuel ratio sensor 15 that outputs a detection signal corresponding to the air-fuel ratio by detecting the oxygen concentration in the exhaust gas that is closely related to the air-fuel ratio of the engine intake air-fuel mixture.

The engine speed Ne is calculated based on the period of the pulse signal output from the crank angle sensor 14 or the number of pulses generated within a predetermined time. Further, the throttle valve opening TVO detected by the throttle sensor 8
Is also input to the control unit 11, while the detection signals of the air-fuel ratio sensor 15 and the air flow meter 13 are also output to the automatic transmission control unit 6.

Further, the engine control unit 11 and the automatic transmission control unit 6 can communicate with each other. Here, the engine control unit 11 refers to a target air-fuel ratio (set air-fuel ratio) set in advance for each operating region divided based on the engine load and the engine rotation speed Ne (see FIG. 9). , The basic fuel injection amount Tp corresponding to the target air-fuel ratio is calculated based on the engine speed Ne and the intake air flow rate Q, while the actual air-fuel ratio detected by the air-fuel ratio sensor 15 and the target air-fuel ratio are calculated. By comparison, an air-fuel ratio feedback correction coefficient LMD for correcting the basic fuel injection amount Tp is set so that the actual air-fuel ratio approaches the target air-fuel ratio. Then, the basic fuel injection amount Tp is set to the air-fuel ratio feedback correction coefficient LMD.
The air-fuel ratio of the engine intake air-fuel mixture is feedback-controlled to the target air-fuel ratio by driving and controlling the fuel injection valve in accordance with the final fuel injection amount Ti obtained by the correction.

The target air-fuel ratio map is, as shown in FIG. 9, a region in which the lean air-fuel ratio is significantly larger than the theoretical air-fuel ratio, a region in which the theoretical air-fuel ratio is set as the target air-fuel ratio,
It is divided into an area where the target air-fuel ratio is slightly richer than the theoretical air-fuel ratio, and in particular, the target air-fuel ratio changes with a large step at the boundary between the lean air-fuel ratio area and the theoretical air-fuel ratio area. Has become.

Therefore, there is a characteristic that the engine output torque changes abruptly at the boundary, and when the lockup clutch 40 of the torque converter 3 is in the direct connection state when the engine output torque changes abruptly, the output torque of the output torque changes. Sudden changes are directly transmitted to the car body. Therefore, in this embodiment, the control unit 6 as shown in FIGS.
By the control function of the lock-up clutch 40 according to the above, it is possible to avoid the occurrence of shock due to the sudden change in the torque.

In the flow chart of FIG. 4, first, in step 1 (denoted as S1 in the figure. The same applies hereinafter),
The detection signal of the air-fuel ratio (oxygen concentration in exhaust gas) by the air-fuel ratio sensor 15 is read. Then, in the next step 2, by comparing the difference between the previous value and the current value of the air-fuel ratio detected by the air-fuel ratio sensor 15 and the slice level SLAF, the set air-fuel ratio of the engine intake air-fuel mixture has a large step. It is determined whether or not it has changed.

Here, at the boundary between the lean air-fuel ratio region and the stoichiometric air-fuel ratio region in the target air-fuel ratio map shown in FIG. 9, the target air-fuel ratio changes from the lean air-fuel ratio to the stoichiometric air-fuel ratio, or from the stoichiometric air-fuel ratio to the lean air-fuel ratio. The air-fuel ratio step detected by the air-fuel ratio sensor 15 is set to exceed the slice level SLAF in step 1 when the air-fuel ratio is switched to the air-fuel ratio.

That is, the fact that the target air-fuel ratio (set air-fuel ratio) is switched with a large step is detected based on the change in the oxygen concentration in the exhaust gas as a result of combustion.
This portion corresponds to the air-fuel ratio switching detection means in this embodiment. When it is detected in step 2 that the target air-fuel ratio is switched with a large step, the process proceeds to step 3, and it is determined whether or not the lockup clutch 40 is engaged (lockup state). To do.

Here, when the lockup clutch 40 is in the completely engaged state (directly coupled state), if the fully engaged state is maintained as it is, the torque converter 3 does not slip,
A sudden change in the engine output torque due to the switching of the target air-fuel ratio is directly transmitted to the vehicle body, which causes a shock. Therefore, when the lock-up clutch 40 is in the engaged state, the routine proceeds to step 4, where the engaging force of the lock-up clutch 40 is forcibly weakened to shift to the slip state, or the engaged state is forcibly completely released. Makes the fastening force zero. The shift to the slip state or the release of the engagement is performed by the duty control of the lockup solenoid 55.

By forcibly weakening the engagement force of the lock-up clutch 40 in this way, a sudden change in the engine output torque is buffered in the torque converter 3 and the shock is not directly transmitted to the vehicle body. It can be suppressed. The above steps 3 and 4 correspond to the fastening force reduction control means in this embodiment.

When the control for weakening the engagement force of the lockup clutch 40 is performed as described above when the target air-fuel ratio is switched, the process proceeds to step 5 and 1 is set in the flag Flag1 indicating that the control is executed. . On the other hand, the engagement force control of the lockup clutch 40 accompanying the target air-fuel ratio switching control is canceled according to the flowchart of FIG.

In the flowchart of FIG. 5, first, at step 11, it is judged that the flag Flag1 is set to 1, and control for weakening the engagement force of the lock-up clutch 40 is executed to set the flag Flag1 to 1. Go to step 12 only when set. In step 12, it is determined based on the output signal of the air-fuel ratio sensor 15 whether or not the change amount of the set air-fuel ratio has settled below a predetermined level.

When the change in the air-fuel ratio detected by the air-fuel ratio sensor 15 falls below a predetermined level, it is considered that the target air-fuel ratio switching control has been completed, and the routine proceeds to step 13, where the normal throttle valve opening degree is set. It returns to the lockup control based on the TVO and the vehicle speed VSP. When returning to the normal control, the flag Flag1 and a flag F described later
Reset each lag2 to zero.

By the way, in the flow chart of FIG. 4, the switching of the target air-fuel ratio (set air-fuel ratio) is configured to be detected based on the detection result of the air-fuel ratio sensor 15, but as shown in the flow chart of FIG. Fuel injection amount Ti
The change of the target air-fuel ratio may be detected from the change of In the flowchart of FIG. 6, the fuel injection amount Ti calculated by the engine control unit 11 in step 21 is calculated.
Read the data. Then, in the next step 22, the change amount of the fuel injection amount Ti is changed to the predetermined slice level S.
It is determined whether the LTI is exceeded.

Here, when the amount of change in the fuel injection amount Ti exceeds the slice level SLTI, the target air-fuel ratio is switched with a large step, and is therefore calculated following the change in the target air-fuel ratio. It is assumed that the fuel injection amount Ti is significantly changed. Incidentally, the steps 21 and 22
The portion of corresponds to the air-fuel ratio switching detection means.

Then, in step 23, the air-fuel ratio sensor 15
Similarly to the case of detecting the switching of the target air-fuel ratio from the detection signal of, if the lockup clutch 40 is in the completely engaged state, the process proceeds to step 24, and the lockup clutch 40 is forcibly shifted to the slip state or the open state. Then, in the next step 25, 1 is set in the flag Flag1.
In addition, as described above, when the switching of the target air-fuel ratio is detected based on the change amount of the fuel injection amount Ti, it is preferable to be configured to be able to distinguish from the change of the fuel injection amount Ti according to the acceleration / deceleration of the engine. , For example, the cylinder intake air amount Q / N
It is advisable to variably set the slice level SLTI according to the amount of change in e.

Further, as a method of detecting the switching of the target air-fuel ratio (set air-fuel ratio), the target air-fuel ratio map is also stored in the control unit 6 side as shown in the flowchart of FIG. 7, and this map is stored. There is a way to refer to. In the flowchart of FIG. 7, first, in step 31, data of the intake air flow rate Q and the engine speed Ne are read in order to retrieve the target air-fuel ratio from the target air-fuel ratio map.

In the next step 32, the same map as the target air-fuel ratio map stored in the engine control unit 11 is referred to on the basis of the engine load (Q / Ne) and the engine speed Ne, and the current engine operation is performed. The target air-fuel ratio corresponding to the conditions is searched and obtained. Then, in step 33, the target air-fuel ratio change amount obtained by searching from the map is compared with a predetermined slice level SLMR to determine whether the target air-fuel ratio has been set to be largely switched. Steps above
The part from 31 to step 33 corresponds to the air-fuel ratio switching detection means.

Here, when the switching of the target air-fuel ratio is detected, as in the above-described embodiment, if the lock-up clutch 40 is in the completely engaged state, the control for reducing the engaging force (complete opening or (Transition to slip state) is executed (step 34 to step 36). The embodiments of the switch detection means for the target air-fuel ratio (set air-fuel ratio) of the engine intake air-fuel mixture have been described above with reference to the respective flowcharts.
The present invention is not limited to the above embodiment, for example, detecting the boundary operating condition of the target air-fuel ratio switching on the target air-fuel ratio map, or automatically executing the target air-fuel ratio switching execution signal from the engine control unit 11 side. You may make it output to the transmission control unit 6.

Next, a preferred embodiment in which the lockup clutch 40 is in the completely engaged state and the lockup clutch 40 is shifted to the slip state during the target air-fuel ratio switching control will be described with reference to the flowchart of FIG.
The control shown in the flowchart of FIG. 8 corresponds to the fastening force reduction control means. In the flowchart of FIG. 8, first, at step 41, the engine rotation speed Ne, the output shaft rotation speed No of the transmission (vehicle speed VSP), and the transmission gear ratio G _R are read.

In the next step 42, the slip lock-up control is initially performed in which the hydraulic pressure is feedback-controlled so that the rotation difference between the input and output shafts of the lock-up clutch 40 becomes a predetermined value to bring the lock-up clutch 40 into a slip state. The flag Flag2 indicating whether or not it is in the state is determined. If it is determined that the flag Flag2 is 0 and the shift control to the slip lockup control is the first time, step 43
Then, the target TSL of the input / output shaft rotation difference of the lockup clutch 40 is set according to the target air-fuel ratio switching step ΔAF.

The step ΔAF of the target air-fuel ratio may be the step of the actual target air-fuel ratio obtained from the target air-fuel ratio map, or the step of the air-fuel ratio detected by the air-fuel ratio sensor 15, and further, It may be a step of the fuel injection amount Ti that is increased / decreased according to the switching of the target air-fuel ratio. The target TSL of the rotation difference in step 43 is set to a larger value as the step difference ΔAF of the target air-fuel ratio is larger, so that when the step difference of the target air-fuel ratio is large and the change range of the engine output torque is large. The more the fastening force of the lock-up clutch 40 is weakened and the fastening force of the lock-up clutch 40 is not unnecessarily weakened, the sudden change in the output torque can be surely buffered.

In the next step 44, the target T of the rotational difference is
The initial value of the duty given to the lockup solenoid 55 is set according to SL. That is, the initial value becomes a reference value when the duty ratio is feedback-controlled so that the actual rotation difference approaches the target TSL. Step 45
Then, the output shaft rotation speed of the lockup clutch 40 is calculated from the output shaft rotation speed No (vehicle speed VSP) of the transmission and the gear ratio G _R, and the output shaft rotation speed and the input shaft rotation speed of the lockup clutch 40 are calculated. Engine speed N corresponding to
An input / output shaft rotation difference of the lockup clutch 40 is calculated as a difference with e. Then, the difference NS between the actual rotation difference and the target rotation difference TSL set according to the air-fuel ratio step difference.
Find the LIP.

In step 46, a correction value ΔDUTY for the basic duty obtained from the target rotation difference TSL is calculated by proportional / integral / derivative control based on the deviation NSLIP between the target rotation difference TSL and the actual rotation difference. Then, the basic duty is corrected by the correction value ΔDUTY and given to the lockup solenoid 55, so that the actual rotation difference approaches the target rotation difference TSL.

In step 47, the flag Flag2 is set so that it is determined that the lockup clutch 40 is controlled in the slipping state in response to the switching of the target air-fuel ratio.
Set 1 to. The transition to the slip state, which is performed to weaken the engagement force of the lockup clutch 40 at the time of switching the target air-fuel ratio, must be feedback-controlled so that the actual rotation difference matches the target rotation difference, as described above. However, the feed forward control may be applied to give a basic duty according to the target rotation difference. Further, the configuration may be such that the duty output in the engaged state is changed by a predetermined value in the opening direction to shift to the slip state.

Further, an automatic transmission having a specification for controlling the lock-up clutch 40 to a slip state as a normal control may be used. In this case, when the target air-fuel ratio is switched in the slip state, the slip occurs. It is advisable to shift the state to the fully open state or switch the target of the rotation difference to the target rotation difference TSL corresponding to the step of the target air-fuel ratio and perform feedback control.

[0045]

As described above, according to the present invention, the set air-fuel ratio in the engine combined with the automatic transmission is switched according to the operating conditions, and when the engine output torque suddenly changes, the torque converter in the engaged state is obtained. Since the lockup clutch engaging force is forcibly weakened, a sudden change in engine output torque can be buffered in the torque converter, and a shock can be suppressed when switching the set air-fuel ratio. There is.

[Brief description of drawings]

FIG. 1 is a block diagram showing the basic configuration of the present invention.

FIG. 2 is a system schematic diagram showing an embodiment of the present invention.

FIG. 3 is a cross-sectional view showing a lock-up mechanism provided in the torque converter.

FIG. 4 is a flowchart showing a first embodiment of engagement force reduction control.

FIG. 5 is a flowchart showing a return from the engagement force reduction control to the normal control.

FIG. 6 is a flowchart showing a second embodiment of engagement force reduction control.

FIG. 7 is a flowchart showing a third embodiment of engagement force reduction control.

FIG. 8 is a flowchart showing slip control when switching the target air-fuel ratio.

FIG. 9 is a diagram showing an example of a target air-fuel ratio map.

[Explanation of symbols]

 1 Internal Combustion Engine 3 Torque Converter 4 Gear Type Transmission 5 Solenoid Valve Group 6 Automatic Transmission Control Unit 8 Throttle Sensor 9 Vehicle Speed Sensor 11 Engine Control Unit 12a-12d Fuel Injection Valve 13 Air Flow Meter 14 Crank Angle Sensor 15 Air-Fuel Ratio Sensor

Claims (2)

[Claims] 1. An internal combustion engine combined with an automatic transmission,
An engine in which the set air-fuel ratio of the intake air-fuel mixture is switched according to engine operating conditions, and an input shaft and an output shaft of a torque converter interposed between the output shaft of the engine and the input shaft of the gear type transmission. Is a lockup control device for an automatic transmission including a lockup clutch mechanically directly connected to the air-fuel ratio switching detection means for detecting switching of the set air-fuel ratio, and the air-fuel ratio during engagement of the lockup clutch. An automatic transmission including: an engagement force reduction control means for forcibly reducing the engagement force of the lock-up clutch when the setting air-fuel ratio is detected to be switched by the switching detection means. Lockup controller. 2. The engagement force reduction control means sets a target of the rotation difference of the input / output shafts of the lockup clutch according to the switching step of the set air-fuel ratio, and the lockup clutch is set to the target rotation difference. The lock-up control device for an automatic transmission according to claim 1, wherein the engagement force of (1) is feedback-controlled.