Fluid pressure control device for lock-up mechanism

A fluid pressure control device including a torque converter that is placed between a output shaft of an engine and an input shaft of a transmission, a mechanical oil pump that is driven by the output shaft, a clutch that directly engages the output shaft and the input shaft by employing an engagement pressure based on a fluid pressure that is generated by the mechanical oil pump, an electric oil pump that can supply a fluid pressure to the clutch, and a control unit for controlling the electric oil pump, wherein when the engagement pressure based on the fluid pressure that is generated by the mechanical oil pump is an amount below a necessary engagement pressure that is necessary to engage the clutch, the control unit drives the electric oil pump so as to supply a fluid pressure by at least the amount to the clutch.

INCORPORATION BY REFERENCE

The disclosure of Japanese Patent Application No. 2002-382546 filed on Dec. 27, 2002 including the specification, drawings and abstract are incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of Invention

The invention relates to a fluid pressure control device for a lock-up mechanism so as to perform a lock-up control of a torque converter that is mounted on a vehicle such as an automobile.

2. Description of Related Art

There exists torque converters that are mounted on a vehicle that include a lock-up clutch. When the lock-up clutch operates, a revolution of an output shaft of an engine that was transmitted to an input shaft of a transmission via fluid in the torque converter is directly transmitted to the input shaft of the transmission via the lock-up clutch. Therefore, fuel efficiency is improved.

The lock-up clutch is operated normally by an engagement pressure based on a line pressure that is generated by a mechanical oil pump. The maximum fluid pressure (supply limit fluid pressure) that is generated by the mechanical oil pump is determined by the rpm of the engine. That is, the supply limit fluid pressure is high when the rpm is high while the supply limit fluid pressure is low when the rpm is low.

SUMMARY OF THE INVENTION

In the lock-up clutch described as above, a sufficient engagement pressure cannot be achieved in order to maintain an engaged state when an rpm of the engine decreases. In this case, the engagement of the lock-up clutch is released, and a fuel consumption amount increases by an equivalent amount.

The invention thus provides a fluid pressure control device for a lock-up mechanism that uses an electric oil pump so as to supplement an insufficient engagement pressure of the lock-up clutch that is insufficient due to inadequate fluid pressure from a mechanical oil pump.

The invention, according to a first exemplary aspect, includes a torque converter that is placed between a output shaft of an engine and an input shaft of a transmission, a mechanical oil pump that is driven by the output shaft, a clutch that directly engages the output shaft and the input shaft by employing an engagement pressure based on a fluid pressure that is generated by the mechanical oil pump, an electric oil pump that can supply a fluid pressure to the clutch, and a control unit for controlling the electric oil pump, wherein when the engagement pressure based on the fluid pressure that is generated by the mechanical oil pump is an amount below a necessary engagement pressure that is necessary to engage the clutch, the control unit drives the electric oil pump so as to supply a fluid pressure by at least the amount to the clutch.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1schematically shows an example of an automatic transmission (A/T)10to which a fluid pressure control device for a lock-up mechanism according to the invention can be applied. The automatic transmission10shown inFIG. 1is provided with a torque converter11, an automatic transmission device12and a differential device13that are stored in a case15(However, a part of the case15is shown inFIG. 1).

The torque converter11includes a pump impeller20that is connected to an output shaft17on the engine (E/G)16via a front cover19, a turbine runner22that is connected to an input shaft21on the automatic transmission device12(on the wheel side), and a stator25that is supported by the case15via a one-way clutch23.

A lock-up clutch26is located in the torque converter11. The lock-up clutch26is connected to the input shaft21on the automatic transmission device12in the same manner as the turbine runner22is connected to the input shaft21. The lock-up clutch26, for example, has a substantially disc-like clutch plate27, and is movably located along the input shaft21through a spline connection of a boss portion (not shown) that is secured inside of the clutch plate to the input shaft21. In addition, a clutch facing29is installed in a front face on the outer peripheral side of the clutch plate27(a face opposite to an inner face of the front cover19). The lock-up clutch26causes the clutch facing29to contact and be separated from the front cover19using a differential pressure ΔP (to be described later) between a front side A of the clutch plate27and a back face side B (the right side ofFIG. 1).

When the lock-up clutch26is operated, that is, when the clutch facing29is pressed against the front cover19, the revolution of the engine16is directly transmitted to the input shaft21via the output shaft17, the front cover19, and the clutch plate27. This is referred to as a direct connection state. On the other hand, when the lock-up clutch26is released, that is, when the clutch facing29is separated from the front cover19, the revolution of the engine16is transmitted to the input shaft21via the output shaft17, the front cover19, and fluid (oil) in the torque converter11. The operation of the lock-up clutch26will be detailed later.

The revolution that is transmitted to the input shaft21is shifted by the automatic transmission device12depending on a driving state, or reversely revolved and transmitted to the differential device13, and then transmitted to right and left axles30,31, and driving wheels (i.e. wheels: not shown). In addition, common four to six speed automatic transmissions or a belt-type continuously variable transmission (CVT) or the like may be selected for the automatic transmission device12in the invention.

The aforementioned automatic transmission10is provided with a mechanical oil pump32and an electric oil pump33. The mechanical oil pump32is connected to the pump impeller20as shown inFIG. 1. Therefore, the revolution of the engine16is directly transmitted to the mechanical oil pump32via the output shaft17, the front cover19, and the pump impeller20. That is, the mechanical oil pump32is capable of outputting a high fluid pressure (a large discharge amount) when the rpm of the engine16is high; while it is only capable of outputting a low fluid pressure (a small discharge amount) when the rpm is low. On the other hand, the electric oil pump33is driven by a motor (not shown) and is controlled by a control device100(a control unit) independent of the mechanical oil pump32. In addition, various types of information about the vehicle such as a vehicle speed, a throttle angle, a gear speed, an oil temperature or the like is input from time to time to the control device100for controlling the electric oil pump33.

FIG. 2shows a fluid pressure circuit for controlling the operation of the lock-up clutch26. The reference numerals ofFIG. 2that are the same as those ofFIG. 1show the same members or the like. Of the fluid pressure circuit for a five forward speed automatic transmission10,FIG. 2shows a valve or the like which directly influences a control of the lock-up clutch26, and appropriately omits other valves.

As stated above, inFIG. 2, the reference numeral11denotes the torque converter,26denotes the lock-up clutch,32denotes the mechanical oil pump, and33denotes the electric oil pump.

In addition, the reference numeral40denotes a primary regulator valve (pressure regulating unit) that regulates a line pressure PLthat is generated by the mechanical oil pump32or a line pressure PLthat is generated by the mechanical oil pump32and the electric oil pump33. Reference numeral41denotes a liner solenoid valve for controlling a line pressure,42denotes a lock-up relay valve for switching ON/OFF of the lock-up clutch26,43denotes a lock-up control valve for controlling a fluid pressure of the lock-up clutch26. Reference numeral45denotes a linear solenoid valve for electrically controlling the operation of the lock-up relay valve42and the lock-up control valve43, and46denotes a secondary regulator valve for generating a secondary pressure Psec.

In this embodiment, as shown inFIG. 2, the line pressure PLthat is generated by the mechanical oil pump32and electric oil pump33is controlled as the secondary pressure Psec by the secondary regulator valve46, and is supplied to the torque converter11so as to operate as hydraulic oil for the torque converter11, as well as an engagement pressure of the lock-up clutch26. Hereafter, an operation of the embodiment will be described in detail.

First, a case will be explained where the lock-up clutch26is controlled by the engagement pressure based on the fluid pressure that is generated by the mechanical oil pump32. The mechanical oil pump32is driven by the revolution of the output shaft17, which in turn is driven by the revolution of the engine16, and a fluid pressure is output from a discharge port32a. In this case, the fluid pressure that is output from the discharge port32ais generated depending on the rpm of the output shaft17, that is, the rpm of the engine16. A high fluid pressure is generated when the revolution of the engine16is high, and a low fluid pressure is generated when the revolution of the engine16is low.

The fluid pressure that is output from the discharge port32ais input to an input port40aof the primary regulator valve40via a fluid passage a1. In addition, the fluid pressure of the fluid passing through the fluid passage a1is also input to an input port40bas a control pressure. A control pressure that is output from an output port41aof the linear solenoid valve41is input to an input port40dof the primary regulator valve40via a fluid passage f1. The primary regulator valve40performs a line pressure PLcontrol for a fluid pressure of the mechanical oil pump32by a control pressure that is input to the input port40d, and the fluid pressure is output from an output port40c.

The fluid pressure that is output from the output port40cof the primary regulator valve40is input to an input port46aof the secondary regulator valve46, is output from the output port41aof the linear solenoid valve41, and regulated by a control pressure that is input to an input port46bof the secondary regulator valve46via a fluid passage f2, and extra oil is output from the output ports46c,46d. As a result, the fluid pressure that is discharged from the output port40cof the primary regulator valve40is regulated as the secondary pressure Psec, and is input to an input port42aof the lock-up relay valve42via a fluid passage b1, a check valve47and a fluid passage b2.

Note that the fluid pressure that is discharged from the output port46dof the secondary regulator valve46is returned to the mechanical oil pump32and the electric oil pump33via a fluid passage g1. Moreover, the fluid pressure that is output from the output port47cis supplied to a cooler (not shown) via a fluid passage h1and the lock-up relay valve42.

When the lock-up relay valve42is positioned in the right half position (ON position), the lock-up clutch26is in the locked state. That is, in order to place the lock-up clutch26into the locked state, a signal pressure is output from an output port45aby the operation of the linear solenoid valve45, is input to an input port42bof the lock-up relay valve42via fluid passages c1, c2to press down a spool. As a result, the input port42aand the output port42cof the lock-up relay valve42are communicated, the secondary pressure Psec that is input to the input port42ais output from the output port42c, and is then supplied to the torque converter11from a lock-up clutch ON port11avia an oil passage d1.

In this case, based on the fact that the lock-up relay valve42is positioned in the right half position, the fluid pressure between the front face side A of the lock-up clutch26and the front cover19is input to an input port42dof the lock-up relay valve42via a lock-up OFF port11band a fluid passage e1, and is then output from an output port42e. Further, the fluid pressure is input to an input port43aof the lock-up control valve43via a fluid passage e2and is discharged from a drain port d. The fluid pressure on the front face side A of the lock-up clutch26is discharged, therefore, a differential pressure ΔP (=P2−P1) is generated for the fluid pressure P1on the front side A and the fluid pressure P2on the back face side B (P2>P1). The lock-up clutch26is engaged with the front cover19by this differential pressure ΔP. As a result, the revolution of the engine16(refer toFIG. 1) is directly transmitted to the input shaft21via the output shaft17, the front cover19, and the lock-up clutch26and not via fluid (oil) in the torque converter11.

A slip control is performed to reduce shock when the lock-up clutch26is engaged or when the engagement thereof is released. That is, the fluid pressure that is output from the output port45aof the linear solenoid valve45is input to the input port42bof the lock-up relay valve42as a signal pressure via the fluid passages c1, c2as stated above so that ON/OFF of the lock-up relay valve42is switched. In addition, the fluid pressure is input to an input port43bof the lock-up control valve43as a control pressure via a fluid passage c3in order to adjust an oil amount that is discharged from the drain port d. As a result, the fluid pressure P1on the front side A of the lock-up clutch26is controlled, and then the differential pressure ΔP (=P2−P1) between the front side A and the back face side B enables a slip control.

So far, the case where the lock-up relay valve42is in the locked state (the right half position) has been explained. Next, a case where the lock-up relay valve42is in an unlocked state (the left half position) will be explained.

The lock-up relay valve42is in the locked state when a signal pressure that is output from the output port45aof the linear solenoid valve45, and that is then input to the input port42bof the lock-up relay valve42via the fluid passages c1, c2, is smaller than a predetermined value. In this case, the secondary pressure Psec that is output from the output port40cof the primary regulator valve40is input to the input port42aof the lock-up relay valve42via the fluid passage b1, the check valve47, and the fluid passage b2. The secondary pressure Psec that has been input is output from the input port42dbased on the left half position of the lock-up relay valve42, and is supplied to the torque converter11from the lock-up OFF port11bvia the fluid passage e1. The supplied secondary pressure Psec increases the fluid pressure P1on the front side A of the lock-up clutch26. Then, the differential pressure ΔP (P2−P1) between the fluid pressure P1on the front side A and the fluid pressure P2on the back face side B of the lock-up clutch26is reduced to a necessary engagement pressure (necessary engagement fluid pressure) or less that can maintain the engaged state of the lock-up clutch26, and the engaged state is released.

In the meantime, even when the lock-up relay valve42is in the locked state, if the secondary pressure Psec that is generated by the mechanical oil pump32, and is supplied to the lock-up clutch26via the primary regulator valve40, the lock-up relay valve42or the like is not sufficient to maintain the engaged state of the lock-up clutch26, that is, if the fluid pressure does not meet the necessary engagement pressure of the lock-up clutch26, the engaged state of the lock-up clutch26is released.

At this point, the lock-up clutch26is defined to be in the locked state when the lock-up relay valve42is in the locked state. In this definition, namely, the locked state of the lock-up clutch26is classified into two cases where the lock-up clutch26is in an engaged state and where the engaged state is released. The distinction between the engaged state and the engagement release state is determined based on if the engagement pressure of the lock-up clutch26is equal to the necessary engagement pressure or more.

In the invention, in the case where the lock-up relay valve42is in the locked state, and the engagement pressure based on the fluid pressure output from the mechanical oil pump32does not meet the necessary engagement pressure that is necessary to maintain the engaged state of the lock-up clutch26, the electric oil pump33is driven and fluid pressure that supplements the insufficient engagement pressure or more is supplied to the lock-up clutch.

In this embodiment, as shown inFIG. 2, the electric oil pump33is located upstream of the primary regulator valve40along the fluid pressure flow. In addition, the fluid pressure that is output from a discharge port33aof the electric oil pump33is input to the input ports40a,40bof the primary regulator valve40via the fluid passage a1in the same manner as that of the fluid pressure that is output from the discharge port32aof the mechanical oil pump32. That is, the oil that is discharged from the mechanical oil pump32and the electric oil pump33is joined so as to be supplied to the primary regulator valve40as a line pressure PL.

For this reason, a final engagement pressure can be easily controlled by controlling an oil amount that is output from the mechanical oil pump32and electric oil pump33. Moreover, there is no need for another valve for controlling the fluid pressure that is discharged from the electric oil pump33.

Hereafter, the control of the electric oil pump33for ensuring the necessary engagement pressure of the lock-up clutch26will be explained. For simplification of the explanation, a case will be explained where the line pressure PLthat is generated by the mechanical oil pump32or the line pressure PLthat is generated by the mechanical oil pump32and the electric oil pump33is regulated as the secondary pressure Psec by the primary regulator valve40, the secondary regulator valve46or the like, is supplied as the engagement pressure of the lock-up clutch26.

That is, the secondary pressure Psec is supplied to the lock-up clutch26as the maximum engagement pressure. Since both the primary regulator valve40and the secondary regulator valve46are controlled by the control pressure PSLTfrom the linear solenoid valve41, the relationship between the secondary pressure Psec and the line pressure PLis expressed in the formula (3) based on the following (1) and (2).
PL=A*PSLT+B(1)
Psec=C*PSLT+D(2)
Psec=C/A*(PL−B)  (3)

That is, since all of the maximum engagement pressure of the lock-up clutch26, the secondary pressure Psec and the line pressure PLare in a direct proportional relationship, a necessary amount of the engagement pressure of the lock-up clutch26can be ensured if the line pressure PLis controlled to a predetermined value.

Therefore, the necessary line pressure can be obtained after the necessary engagement fluid pressure is calculated using the formula (3) because the necessary engagement fluid pressure is equal to the necessary secondary pressure.

A and B in the above formulas (1), (2), and (3) are values showing the characteristic of the primary regulator valve40. C and D are values showing the characteristic of the secondary regulator valve46, and PSLT is a valve showing the output fluid pressure of the linear solenoid valve41.

FIG. 3shows a leakage flow rate characteristic of the automatic transmission (A/T)10and a performance curve of the mechanical oil pump32. The horizontal axis indicates a line pressure PL(kPa) that is generated by the mechanical oil pump32and the vertical axis indicates an oil supply flow rate Q (L/min) that is discharged from the mechanical oil pump32. The leakage flow rate characteristic, that is specific to the automatic transmission, is determined by clearance or the like of each valve of a valve body (not shown) and shows the supply flow rate Q that is necessary to generate a certain line pressure PL. The curve is shown as an upward-sloping curve in the figure.

Moreover, the performance curve of the mechanical oil pump32shows a relationship between the supply flow rate and the generated fluid pressure in the performance of the mechanical oil pump32, that is, the rpm of the engine (3000 rpm, 2000 rpm, and R1rpm in the figure). The curve is shown as a downward-sloping curve in the figure.

In the figure, when the rpm of the engine is R1, and the necessary engagement pressure of the lock-up clutch26is P0, the supply flow rate that is discharged from the mechanical oil pump32is Qm. In this case, the amount of the supply flow rate is insufficient by an insufficient amount Qe, which is supplemented by driving the electric oil pump33.

Next,FIG. 4shows a performance curve of the electric oil pump32. LikeFIG. 3, the horizontal axis and the vertical axis indicate the line pressure PL(kPa) that is generated by the mechanical oil pump32and the oil supply flow rate Q (L/min) that is discharged from the mechanical oil pump32. In the figure, the horizontal axis indicates the necessary engagement pressure P0, the vertical axis indicates the insufficient amount Qe of the supply flow rate, and a current value I (A) that passes through the intersection of P0and Qe is a current command value. That is, in the aforementioned example, the insufficient amount of the mechanical oil pump32is supplemented by applying the current command value I (A) to the electric oil pump33. Note that the performance curve of the mechanical oil pump shown inFIG. 3and the performance curve of the electric oil pump shown inFIG. 4change depending on the oil temperature, respectively. Thus it is desirable to provide an oil temperature detection sensor (not shown) so as to determine the current command value based on the performance curve that is corrected according to the output of the sensor.

Hereafter, control of the electric oil pump33for ensuring the necessary engagement pressure P0of the lock-up clutch26will be explained. First, a flow of control will be explained referring to flow charts inFIGS. 5 and 6.

An ON/OFF determination of the lock-up (Lup) by a control device (not shown) (S1inFIG. 5) is first made. The control device makes an optimal ON/OFF determination of the lock-up mainly based on the vehicle speed, the throttle angle, a gear speed or the like. Next, the lock-up control is performed (S2). In the lock-up control, the linear solenoid valve45shown inFIG. 2performs a slip control of the lock-up clutch26so as to reduce shock during the engagement and the engagement release, engage or release of the lock-up clutch. Next, the line pressure control during a normal state is performed (S3). The line pressure PLthat is necessary during the normal state is calculated based on the vehicle speed, the throttle angle, the gear speed or the like to control each valve.

Then, an ON/OFF determination of the lock-up (S4) is made. In the case of OFF (“No” in S4), that is, when the lock-up relay valve is in an unlocked state, the control is complete without driving the electric oil pump33. On the other hand, in step S4, when the lock-up relay valve42is in the locked state (“Yes” in S4), the step proceeds to control the electric oil pump (electric OP)33(S5).

The control of the electric oil pump33is performed according to the flow chart shown inFIG. 6. When the control of the electric oil pump33is started (S11), the necessary engagement pressure P0(lock-up necessary fluid pressure) of the lock-up clutch26is calculated (S12). This calculation is performed so as to calculate the necessary engagement pressure P0after load torque to the lock-up clutch26is detected. In the present embodiment, as stated above, the secondary pressure Psec is supplied as the engagement pressure of the lock-up clutch26, thus the necessary engagement pressure P0is equal to the necessary line pressure.

As a result of the calculation in step S12, a determination is made as to whether the necessary engagement pressure P0is larger than the supply limit fluid pressure of the mechanical oil pump (S13). In this case, the necessary engagement pressure P0and the supply limit fluid pressure of the mechanical oil pump32fluctuates depending on the rpm of the engine16. If the result of the determination in step S13is No, the control is complete without operating the electric oil pump33.

On the other hand, if the result is Yes in step S13, the insufficient flow rate is calculated based on the limit performance of the mechanical oil pump32and the necessary line pressure of the mechanical oil pump that are obtained from the performance curve of the mechanical oil pump32shown inFIG. 3as mentioned above (S14). The insufficient flow rate corresponds to the insufficient amount Qe of the supply flow rate in the figure.

After the insufficient flow rate is calculated, the current command value is calculated based on the performance of the electric oil pump33and the insufficient flow rate using the performance curve of the electric oil pump33(S15). The current command value corresponds to1(A) inFIG. 4.

After the current command value is determined, the electric oil pump33is operated (S16) by applying the current command value1(A). As a result, the insufficient amount of the fluid pressure generated by the mechanical oil pump32, which is inadequate due to a low rpm of the engine16, is supplemented by the electric oil pump33so as to ensure the necessary line pressure, that is, the necessary engagement pressure P0of the lock-up clutch26. That is, in comparison with a conventional fluid pressure control device where there is no electric oil pump33, and the necessary engagement pressure P0is generated by the mechanical oil pump32only, the engaged state of the lock-up clutch26can be maintained for a longer period. Therefore, as the engaged state of the lock-up clutch26can be maintained for a longer period, the fuel consumption amount is greatly reduced.

Hereafter, this effect will be specifically explained by taking an example where the vehicle is in a coasting state. For simplification of the explanation as stated above, a case will be explained where the line pressure PLthat is generated by the mechanical oil pump32is regulated as the secondary pressure Psec by the primary regulator valve40, the secondary regulator valve46or the like, is supplied to the lock-up clutch26as the engagement pressure. That is, the case where the engagement pressure almost equals the secondary pressure Psec will be explained.

FIG. 7shows the limit fluid pressure (which is substantially equal to the maximum line pressure) Pm that is generated by the mechanical oil pump32. The horizontal axis indicates the R (rpm) of the engine (E/G)16, and the vertical axis indicates the line pressure PL. The upward-sloping curve in the figure that passes through the origin indicates the limit fluid pressure Pm that is generated by the mechanical oil pump32. As the rpm of the engine16increases, the limit fluid pressure Pm that is generated increases; while as the rpm of the engine16decreases, the limit fluid pressure Pm that is generated decreases.

FIG. 8shows the torque that is generated by the engine16, or the necessary engagement pressure of the lock-up clutch26that is necessary to receive drag torque when the engine is not driven such as during coasting or the like. The horizontal axis indicates the line pressure PL(kPa) and the vertical axis indicates the torque Trq (Nm). By substituting the relationship between the engagement pressure of the lock-up clutch26and the capacity of the lock-up torque in the formula (3), the necessary line pressure can be obtained from the lock-up input torque, that is shown as a graph inFIG. 8. This graph may be stored as a map in the control device100.

Now, assuming that the vehicle is in a coasting state, the necessary engagement pressure P0of the lock-up clutch26for generating predetermined drag torque T0by the relationship between the engagement pressure and the torque inFIG. 8is uniquely determined. The engagement pressure of the lock-up clutch26is generated by the mechanical oil pump32, and the limit fluid pressure Pm that is generated by the mechanical oil pump32decreases as the rpm of the engine16decreases as shown inFIG. 7. As shown in the figure, when the rpm of the engine16is equivalent to R0or more (for example, R2), the limit fluid pressure Pm that is generated by the mechanical oil pump32is equivalent to the necessary engagement pressure P0or more, thus the engaged state of the lock-up clutch26can be maintained. On the other hand, when the rpm of the engine16does not reach R0(for example, R1), the necessary engagement pressure P exceeds the generated limit fluid pressure Pm, and the engaged state of the lock-up clutch26cannot be maintained. Actually, in the coasting state, the rpm of the engine16gradually decreases, therefore, it is highly presumable that the engaged state of the lock-up clutch26cannot be maintained.

Therefore, in this embodiment, as stated above, the electric oil pump33is operated so as to supplement the engagement pressure, thus the lock-up state of the lock-up clutch26can be maintained for a longer period. As a result, the fuel cut (fuel stop) range is increased, enabling reduction in the fuel consumption amount.

Hereafter, the reduction in the fuel consumption amount will be specifically explained referring toFIG. 9.FIG. 9A,FIG. 9B, andFIG. 9Care time charts showing the vehicle speed V (km), the rpm of the engine (rpm) and the engagement pressure P (kPa) of the lock-up clutch26while the vehicle is coasting, respectively. In all time charts, horizontal axes indicate time t.

Time t0to t1along the time axis indicates a period where the vehicle is driven at a constant speed. In this period, the vehicle speed V0, and the rpm R2(corresponding to R2inFIG. 7as stated above) of the engine16are constant. In this case, the lock-up relay valve42as shown inFIG. 2is in the locked state, the limit fluid pressure Pm that is generated by the mechanical oil pump32is constant, and exceeds the necessary engagement pressure P0of the lock-up clutch26. Therefore, the engaged state of the lock-up clutch26is maintained.

In the time t1, when the accelerator pedal is turned off, the vehicle is in a coasting state, and the supply of the fuel to the engine16is stopped. That is, the fuel cut is started. In this coasting state, the vehicle speed V, the rpm of the engine R, the limit fluid pressure P that is generated by the mechanical oil pump gradually decreases. Then, in the time t3, when the rpm of the engine R becomes R0, the limit fluid pressure that is generated by the mechanical oil pump32reaches the necessary engagement pressure P0as shown inFIG. 7, thus the engaged state of the lock-up clutch26cannot be maintained. For this reason, conventionally, the rpm of the engine rapidly decreases as shown by a dashed line between the time t3and t4, and the fuel supply is started again so as to maintain the rpm of the engine in the idling state. That is, the fuel cut is complete. In addition, conventionally, the idling state is continued, and the time t1to t3(or to t4) is the fuel cut range.

On the other hand, in the present embodiment, as stated above, the electric oil pump33is driven by a predetermined current command value (I (A) shown inFIG. 4) at a predetermined timing (time t2to be described later) so as to extend the fuel cut range during coasting.

After the vehicle enters a coasting state on and after t1, before the engaged state of the lock-up clutch26becomes unable to be maintained because the rpm of the engine is reduced to R0, that is, when the limit fluid pressure Pm that is generated by the mechanical oil pump32reaches the first threshold value P2that is slightly larger than the necessary engagement pressure P0, the engagement pressure of the lock-up clutch26is increased to P3by driving the electric oil pump33. The timing for driving the electric oil pump33is the time t2. From the t2to t5, the maximum engagement pressure Pm of the mechanical oil pump32and the engagement pressure (P3−P2) that is generated by the electric oil pump33are supplied to the lock-up clutch26. If the engagement pressure that comes from both the mechanical oil pump32and the electric oil pump33exceeds the necessary engagement pressure P0until the rpm of the engine reaches the idling state, the engaged state of the lock-up clutch26can be maintained. Therefore, the rpm of the engine can be gradually reduced as shown by a solid line between the time t1to t5inFIG. 9. During this period, since the engaged state of the lock-up clutch26is maintained, the fuel supply to the engine16can be stopped. That is, in the present embodiment, the fuel cut range can extend from time t1to t5by appropriately employing the electric oil pump33, thus extending the fuel cut range in comparison with the conventional fuel cut range of time t1to t3. As a result, the fuel consumption amount can be reduced by the increase amount in the fuel cut range.

In addition, inFIG. 9, if the value of the engagement pressure (P3−P2) that is generated by the electric oil pump33is appropriately set, it is possible to set both the engagement pressure of both mechanical oil pump32and electric oil pump33to be lower than the necessary engagement pressure P0before the time t5when the engine16is in an idling state. Alternatively, it is possible to set the engagement pressure of both mechanical oil pump32and electric oil pump33to exceed the necessary engagement pressure P0after the time t5. In the latter case, even when the engine16is in an idling state, the engaged state of the lock-up clutch26is maintained, thus knocking might occur.

Therefore, in the latter case, in order to prevent such a problem, the second threshold value is set around the idling revolution of the engine16. Then, when the rpm of the engine16is reduced to the second threshold value, it is desirable to release the engaged state of the lock-up clutch26by turning off the lock-up relay valve42by the linear solenoid valve45as shown inFIG. 2.

According to the first exemplary aspect of the invention, even if the engaged state of the lock-up clutch cannot be maintained by only the mechanical oil pump, the engaged state of the lock-up clutch can be maintained by driving the electric oil pump when necessary. As a result, a reduction in the fuel consumption amount is possible. The reduction in the fuel consumption amount results from extending a fuel cut range mainly during the period while the vehicle is coasting.

According to the second exemplary aspect of the invention, the electric oil pump is driven only when the lock-up clutch is in the locked state and an engagement pressure of the lock-up clutch does not reach a necessary engagement pressure level by only using the mechanical oil pump.

According to the third exemplary aspect of the invention, both the mechanical oil pump and electric oil pump are located upstream of a pressure regulating unit. Therefore, a fluid pressure that is output from the mechanical oil pump and the electric oil pump is jointly regulated by the pressure regulating unit. As a result, a special member such as a pressure regulating valve for regulating the fluid pressure that is output from the mechanical oil pump is not necessary. Thus, the configuration is simplified, and a control of the engagement pressure is easy.

According to the fourth exemplary aspect of the invention, when the engagement pressure of the lock-up clutch is reduced to a first threshold value that is higher than the necessary engagement pressure, the electric oil pump is driven. Therefore, a driving timing can be precisely set. In addition, a timing is delayed at which the engagement pressure that is supplied to the lock-up clutch is reduced to the necessary engagement pressure. As a result, the fuel consumption amount can be reduced by extending the fuel cut range while the vehicle is coating by an equivalent amount.

According to the fifth exemplary aspect of the invention, the engagement of the lock-up clutch is released when the rpm of the engine is reduced to a second threshold value close to the rpm of idling revolution. Therefore, a knocking can be prevented that results from the engagement of the lock-up clutch during idling revolution of the engine.

According to the sixth exemplary aspect of the invention, the engagement pressure of the lock-up clutch is controlled by a current command value with respect to the electric oil pump. As a result, responsivity is high and control with high precision.