Vehicle control system

Abstract

To provide a vehicle control device capable of suppressing increase in oil temperature while suppressing generation of engine stall.SOLUTION: There is provided an electronic control device 90 (control device) of a vehicle 10 comprising a torque converter 14 with a lock-up clutch LU provided between an engine 12 and an automatic transmission 16 in a power transmission path PT. When an oil temperature THoil of oil OIL, which is fluid that transmits power in the torque converter 14 when the lock-up clutch LU is in the disengaged state, is equal to or higher than a predetermined judgment oil temperature THoil_jdg, the electronic control device sets a minimum turbine rotation speed Nt_min, which is a lower limit of a turbine rotation speed Nt (vehicle speed related value) at which the lock-up clutch LU is engaged when the automatic transmission 16 is in the first gear shift stage "1st", higher than when the automatic transmission 16 is in the second gear shift stage "2nd" or higher.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a control device for a vehicle including an automatic transmission provided in a power transmission path between an engine and a pair of drive wheels, and a torque converter with a lock-up clutch provided between the engine and the automatic transmission.

Background Art

[0002] A control device for a vehicle including an automatic transmission provided in a power transmission path between an engine and a pair of drive wheels, and a torque converter with a lock-up clutch provided between the engine and the automatic transmission is known. For example, the one described in Patent Document 1 is such a device. In the control device for the vehicle described in Patent Document 1, when the lock-up clutch is in a released state, disengagement / engagement control of the lock-up clutch is executed according to the oil temperature of oil, which is a fluid that transmits power in the torque converter. Specifically, when the oil is at a high temperature, the rotational speed range in which the lock-up clutch is in an engaged state is shifted to the low-speed side compared to when the oil is at a low temperature. Thereby, since the lock-up clutch is likely to be in an engaged state at a high temperature, a further increase in the oil temperature is suppressed.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0004] Incidentally, in the vehicle control device described in Patent Document 1, there is concern that engaging the lock-up clutch at high temperatures, for example on slopes or sandy roads, may cause engine stalling. Therefore, it is conceivable to limit the engagement of the lock-up clutch to only when the automatic transmission is in the second gear or higher. In other words, it is conceivable that the lock-up clutch is not engaged when the automatic transmission is in the first gear. However, even if the lock-up clutch is engaged when the automatic transmission is in the second gear, the automatic transmission may downshift from the second gear to the first gear due to insufficient torque. In this case, the lock-up clutch is released when the transmission is downshifted to the first gear, and there is a risk that the amount of heat generated in the torque converter will not be suppressed, causing the oil temperature to rise.

[0005] The present invention was made against the above circumstances, and its objective is to provide a vehicle control device that suppresses the occurrence of engine stalls while suppressing the rise in oil temperature. [Means for solving the problem]

[0006] The gist of the present invention is a control device for a vehicle comprising an engine, an automatic transmission provided in a power transmission path between the engine and a pair of drive wheels, and a torque converter with a lock-up clutch provided between the engine and the automatic transmission in the power transmission path, wherein when the oil temperature of the oil, which is the fluid that transmits power in the torque converter, is above a predetermined determination oil temperature when the lock-up clutch is in a disengaged state, the lower limit of the vehicle speed-related value for engaging the lock-up clutch when the automatic transmission is in the first gear is set higher than when the automatic transmission is in the second gear or higher. [Effects of the Invention]

[0007] According to the vehicle control device of the present invention, when the lock-up clutch is in the disengaged state and the oil temperature of the fluid that transmits power in the torque converter is above a predetermined determination oil temperature, the lower limit of the vehicle speed-related value for engaging the lock-up clutch when the automatic transmission is in the first gear is set higher compared to when the automatic transmission is in the second gear or higher. This suppresses the occurrence of engine stall and reduces the amount of heat generated in the torque converter, thereby suppressing the rise in oil temperature. [Brief explanation of the drawing]

[0008] [Figure 1] This diagram shows a schematic configuration of a vehicle equipped with an electronic control device according to an embodiment of the present invention, as well as a functional block diagram representing the main parts of the control functions for various control functions in the vehicle. [Figure 2] This diagram illustrates an example of the relationship between the throttle valve opening and the lower limit of the turbine rotational speed at which the lock-up clutch is engaged in each gear of the automatic transmission. [Figure 3] Figure 1 shows an example of a flowchart illustrating the control operation of the electronic control unit. [Modes for carrying out the invention]

[0009] Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. Note that in the embodiments, the drawings have been simplified or modified as appropriate, and the dimensional ratios and shapes of each part are not necessarily depicted accurately. [Examples]

[0010] Figure 1 is a schematic diagram of a vehicle 10 equipped with an electronic control device 90 according to an embodiment of the present invention, as well as a functional block diagram showing the main parts of the control functions for various controls in the vehicle 10.

[0011] The vehicle 10 comprises an engine 12, a torque converter 14, an automatic transmission 16, a differential gear 18, a pair of drive wheels 20, a hydraulic control circuit 50, an oil pump 60, and an electronic control unit 90. For example, the torque converter 14, automatic transmission 16, differential gear 18, and hydraulic control circuit 50 are housed in a case 40.

[0012] The engine 12 is the power source for driving the vehicle 10, and is a well-known internal combustion engine such as a gasoline engine or a diesel engine.

[0013] The torque converter 14 is a well-known fluid-type power transmission device. The input pump impeller 14p of the torque converter 14 is connected to the engine 12 via the crankshaft 30. The output turbine impeller 14t of the torque converter 14 is connected to the turbine shaft 32. A lock-up clutch LU is provided between the pump impeller 14p and the turbine impeller 14t. The torque converter 14 is a torque converter with a lock-up clutch LU. When the lock-up clutch LU is engaged (=direct connection), the pump impeller 14p and the turbine impeller 14t are directly connected and rotated as a single unit. When the lock-up clutch LU is disengaged (=disconnected), the pump impeller 14p and the turbine impeller 14t are not directly connected, and power is transmitted via fluid. Hereinafter, the state in which the lock-up clutch LU is engaged will be referred to as the "LU on state," and the state in which the lock-up clutch LU is disengaged will be referred to as the "LU off state." A mechanical oil pump 60, for example, is connected to the pump impeller 14p of the torque converter 14 in a power-transmitting manner. The oil pump 60 is rotationally driven by the engine 12 to pump oil to the hydraulic control circuit 50.

[0014] The oil is the fluid that transmits power between the pump impeller 14p and the turbine impeller 14t when the torque converter 14 is in the LU-off state. In this embodiment, the oil is also the hydraulic fluid supplied to the hydraulic actuator, which is provided to control the engagement and disengagement of the lock-up clutch LU and the shifting of the automatic transmission 16. The oil is, for example, ATF (Automatic Transmission Fluid). As is well known, the torque converter 14 generates more heat in the LU-off state than in the LU-on state. This is because the oil is vigorously agitated within the torque converter 14 in the LU-off state.

[0015] The automatic transmission 16 is a well-known automatic transmission, such as a stepped transmission including planetary gears or a normally meshing parallel shaft. The input shaft 34, which is the input rotating member of the automatic transmission 16, is connected to the turbine shaft 32 in a manner that prevents relative rotation. The output shaft 36, which is the output rotating member of the automatic transmission 16, is connected to the differential gear 18. The automatic transmission 16 is controlled by a hydraulic control circuit 50 controlled by an electronic control device 90 (described later) so that a desired gear ratio γat is formed from among different gear ratios γat. The gear ratio γat is the rotational speed ratio (=Nin / Nout) of the input rotational speed Nin and the output rotational speed Nout [rpm]. The input rotational speed Nin is the rotational speed of the input shaft 34, and the output rotational speed Nout is the rotational speed of the output shaft 36. For example, the automatic transmission 16 has eight gears for forward driving, from the first gear "1st" to the eighth gear "8th". The first gear "1st" is the low-speed gear, and the eighth gear "8th" is the high-speed gear. In other words, among the eight gears of the automatic transmission 16, the gear ratio γat of the first gear "1st" is the largest, and the gear ratio γat of the eighth gear "8th" is the smallest.

[0016] The differential gear 18 is a well-known differential gear that receives the power for driving transmitted from the output shaft 36 of the automatic transmission 16 and transmits equal driving torque to a pair of drive wheels 20 via a pair of drive shafts 38, while allowing an appropriate difference in rotational speed.

[0017] The power for driving output from the engine 12 is transmitted to a pair of drive wheels 20 via a torque converter 14, an automatic transmission 16, and a differential gear 18. The power transmission path PT is the transmission path through which the power for driving is transmitted between the engine 12 and the pair of drive wheels 20. The automatic transmission 16 is located in the power transmission path PT, and the torque converter 14 is located between the engine 12 and the automatic transmission 16 within the power transmission path PT.

[0018] The hydraulic control circuit 50 uses the hydraulic pressure of the oil pumped from the oil pump 60 as its base pressure to supply oil necessary for lubrication, cooling, and operational control to various parts within the case 40. For example, the oil is supplied to the torque converter 14 as a fluid that transmits power between the pump impeller 14p and the turbine impeller 14t. For example, oil is supplied to hydraulic actuators provided for tasks such as controlling the engagement and disengagement of the lock-up clutch LU and controlling the shifting of the automatic transmission 16.

[0019] The electronic control unit 90 is configured to include a so-called microcomputer, for example, equipped with a CPU, RAM, ROM, input / output interface, etc. The CPU performs various controls of the vehicle 10 by performing signal processing according to a program pre-stored in ROM while utilizing the temporary storage function of RAM. The electronic control unit 90 is configured to include computers for engine control, hydraulic control, etc., as needed. The electronic control unit 90 corresponds to the "control device" in this invention.

[0020] Based on the detection values from various sensors and the like (for example, engine speed sensor 92, turbine speed sensor 94, oil temperature sensor 96, and accelerator pedal opening sensor 98, etc.) provided in the vehicle 10, various signals (for example, engine speed Ne [rpm] which is the rotational speed of the engine 12, turbine speed Nt [rpm] which is the rotational speed of the turbine shaft 32, oil temperature THoil [°C] which is the temperature of the oil OIL, and accelerator pedal opening θacc [%] which is the accelerator operation amount of the driver representing the magnitude of the driver's acceleration operation, etc.) are respectively input to the electronic control unit 90.

[0021] From the electronic control unit 90, various command signals (for example, engine control signal Se for controlling the engine 12, shift control signal Sat for performing shift control of the automatic transmission 16 via the hydraulic control circuit 50, and LU control signal Slu for performing connection / disconnection control of the lock-up clutch LU, etc.) are respectively output to each device (for example, engine 12 and hydraulic control circuit 50, etc.) provided in the vehicle 10.

[0022] The electronic control unit 90 functionally includes an engine control section 90a, a shift control section 90b, a LU control section 90c, an oil temperature determination section 90d, a condition setting section 90e, and a condition determination section 90f.

[0023] During vehicle running, the engine control section 90a calculates the driving demand amount for the vehicle 10 by the driver, for example, by applying the accelerator pedal opening θacc and the vehicle speed V [km / h] to a driving demand amount map. The driving demand amount map is a relationship for obtaining the driving demand amount that has been experimentally or designedly obtained and stored in advance. The engine control section 90a controls the engine torque Te [Nm] so as to realize the driving demand amount for the vehicle 10. In the driving demand amount map, instead of the vehicle speed V, the output rotational speed Nout which is the rotational speed of the output shaft 36 or the turbine speed Nt, etc. may be used, or instead of the accelerator pedal opening θacc, the throttle valve opening θth [%] which is the opening of the electronic throttle valve, etc. may be used.

[0024] The gear shift control unit 90b performs gear shifting decisions for the automatic transmission 16 using, for example, a gear shift map, and performs gear shifting control as necessary. The gear shift map is a predetermined relationship that has a gear shift line on a two-dimensional coordinate system where, for example, the accelerator opening θacc and the vehicle speed V are variables, allowing for the determination of gear shifting for the automatic transmission 16. In the gear shift map, instead of the vehicle speed V, the output rotational speed Nout, which is the rotational speed of the output shaft 36, may be used, or instead of the accelerator opening θacc, the required drive torque Trdem [Nm] or the throttle valve opening θth may be used.

[0025] The LU control unit 90c controls the engagement and disengagement state of the lock-up clutch LU of the torque converter 14. The LU control unit 90c controls the LU to be in the off state when the vehicle speed is relatively low, such as when the vehicle is starting, and to be in the on state when the vehicle speed is relatively high. The conditions for switching the engagement and disengagement state of the lock-up clutch LU will be described later.

[0026] The oil temperature determination unit 90d determines whether the oil temperature THoil is above a predetermined determination oil temperature THoil_jdg [°C]. The predetermined determination oil temperature THoil_jdg is a determination temperature for oil temperature THoil that is experimentally or design-wise predetermined so that each part of the case 40 cooled by the oil does not become overheated. Hereinafter, the state in which "oil temperature THoil is below the predetermined determination oil temperature THoil_jdg" will be referred to as the "normal temperature state," and the state in which "oil temperature THoil is above the predetermined determination oil temperature THoil_jdg" will be referred to as the "high temperature state."

[0027] The condition setting unit 90e sets the switching conditions for the open / closed state of the lock-up clutch LU. For example, the condition setting unit 90e sets the switching conditions for the open / closed state of the lock-up clutch LU as follows.

[0028] Here, the turbine rotation speed Nt that switches between the LU-off state and the LU-on state will be referred to as the "minimum turbine rotation speed Nt_min". The "minimum turbine rotation speed Nt_min" is the "lower limit of the rotational speed range of turbine rotation speed Nt for which the conditions for controlling to be in the LU-on state are met". In this embodiment, when the turbine rotation speed Nt is equal to or greater than the minimum turbine rotation speed Nt_min, it is controlled to be in the LU-on state, and when the turbine rotation speed Nt is less than the minimum turbine rotation speed Nt_min, it is controlled to be in the LU-off state. That is, the minimum turbine rotation speed Nt_min is the boundary value between the rotational speed range of turbine rotation speed Nt for which the LU-on state is controlled and the rotational speed range of turbine rotation speed Nt for which the conditions for controlling to be in the LU-off state are met. To prevent engine stalls, it is better to set the minimum turbine rotation speed Nt_min high. On the other hand, to suppress the amount of heat generated in the torque converter 14, it is better to set the minimum turbine rotation speed Nt_min low. Therefore, the minimum turbine rotation speed Nt_min is set based on the viewpoint of suppressing engine stall and suppressing the amount of heat generated in the torque converter 14. Note that the turbine rotation speed Nt is a rotational speed that increases or decreases in conjunction with the increase or decrease of the vehicle speed V, and corresponds to the "vehicle speed-related value" in this invention. The minimum turbine rotation speed Nt_min corresponds to the "lower limit value" in this invention.

[0029] If the oil temperature determination unit 90d determines that the oil is at a normal temperature, the condition setting unit 90e sets the minimum turbine rotation speed Nt_min according to the gear position of the automatic transmission 16. If the oil temperature determination unit 90d determines that the oil is at a high temperature, the condition setting unit 90e sets the minimum turbine rotation speed Nt_min according to the gear position of the automatic transmission 16. The specific setting of the minimum turbine rotation speed Nt_min will be described later.

[0030] The condition determination unit 90f determines whether the turbine rotation speed Nt is equal to or greater than a predetermined minimum turbine rotation speed Nt_min. When the condition determination unit 90f determines that the turbine rotation speed Nt is equal to or greater than the predetermined minimum turbine rotation speed Nt_min, it is equivalent to the condition determination unit 90f determining that the conditions for controlling the LU to the ON state are met. When the condition determination unit 90f determines that the turbine rotation speed Nt is less than the predetermined minimum turbine rotation speed Nt_min, it is equivalent to the condition determination unit 90f determining that the conditions for controlling the LU to the ON state are not met. In other words, it is equivalent to the condition determination unit 90f determining that the conditions for controlling the LU to the OFF state are met.

[0031] The LU control unit 90c controls the engagement and disengagement state of the lock-up clutch LU based on the conditions determined to be met by the condition determination unit 90f. That is, if the conditions for controlling the LU to the ON state are met, the LU control unit 90c controls the LU to the ON state. If the conditions for controlling the LU to the OFF state are met, the LU control unit 90c controls the LU to the OFF state. For example, when the gear shift of the automatic transmission 16 is switched from the second gear "2nd" to the first gear "1st" by downshifting, the gear shift control unit 90b performs gear shift control, and the LU control unit 90c controls the engagement and disengagement state of the lock-up clutch LU based on the conditions determined to be met by the condition determination unit 90f.

[0032] Figure 2 illustrates an example of the relationship between the throttle valve opening θth and the minimum turbine rotational speed Nt_min at each gear stage of the automatic transmission 16. Note that Figure 2 shows the case where the gear stages of the automatic transmission 16 are 1st gear ("1st") to 3rd gear ("3rd"), while 4th gear ("4th") to 8th gear ("8th") are not shown.

[0033] For example, when the oil is at a high temperature, if the automatic transmission 16 is in the second gear ("2nd") or higher, the minimum turbine rotational speed Nt_min is set to 1300 [rpm] regardless of the throttle valve opening θth. This 1300 [rpm] is the lower limit of the rotational speed range for the turbine rotational speed Nt that controls the LU-on state experimentally or by design when the oil is at a high temperature, and is a rotational speed at which engine stall does not occur even when the LU-on state is reached.

[0034] For example, when the oil is at a high temperature and the automatic transmission 16 is in the first gear ("1st"), the minimum turbine rotational speed Nt_min is set to 2000 [rpm] regardless of the throttle valve opening θth. This 2000 [rpm] is the lower limit of the rotational speed range for the turbine rotational speed Nt that controls the LU-on state experimentally or by design when the oil is at a high temperature, and is a rotational speed at which engine stall does not occur even when the LU-on state is reached. Thus, when the oil is at a high temperature and the automatic transmission 16 is in the first gear ("1st"), the minimum turbine rotational speed Nt_min is set higher than when the automatic transmission 16 is in the second gear ("2nd") or higher. Preferably, when the vehicle speed V does not change due to downshifting, if the automatic transmission 16 is in the second gear "2nd" and the LU is on, the minimum turbine rotation speed Nt_min in the first gear "1st" and the second gear "2nd" are set such that the LU is maintained even when the automatic transmission 16 is in the first gear "1st".

[0035] For example, when the oil is at a normal temperature and the automatic transmission 16 is in the second gear ("2nd"), the minimum turbine rotational speed Nt_min is set to 2000 [rpm] regardless of the throttle valve opening θth. This 2000 [rpm] is the lower limit of the rotational speed range for the turbine rotational speed Nt that controls the LU-on state, which is experimentally or design-defined in advance, when the oil is at a normal temperature, and is a rotational speed at which engine stall does not occur even when the LU-on state is reached.

[0036] For example, when the oil is at a normal temperature and the automatic transmission 16 is in the 3rd gear or higher, the minimum turbine rotational speed Nt_min is set to 2000 rpm when the throttle valve opening θth is 50% or less, and when the throttle valve opening θth is 60% or more, the minimum turbine rotational speed Nt_min is set to 2300 rpm. This 2300 rpm is the lower limit of the rotational speed range for the turbine rotational speed Nt that controls the LU-on state experimentally or by design, when the oil is at a normal temperature, and is a rotational speed at which engine stall does not occur even when the LU-on state is reached. When the throttle valve opening θth is between 50% and 60%, the minimum turbine rotational speed Nt_min is set to increase linearly from 2000 rpm to 2300 rpm in accordance with the increase in the throttle valve opening θth. Thus, when the oil is at a normal temperature and the automatic transmission 16 is in the third gear or higher, the minimum turbine rotational speed Nt_min is set higher when the throttle valve opening θth is larger compared to when the throttle valve opening θth is small. The reason for this is that when the load on the engine 12 is large and the engine rotational speed Ne is low, the NV (Noise Vibration) performance deteriorates, so the LU is turned off to suppress torque fluctuations transmitted from the torque converter 14 to the pair of drive wheels 20.

[0037] For example, when the oil is at a normal temperature and the automatic transmission 16 is in the first gear ("1st"), the minimum turbine rotational speed Nt is set to a value greater than the practically expected range of rotational speeds, regardless of the throttle valve opening θth. In other words, the minimum turbine rotational speed Nt_min is set so that the LU is always in the off state.

[0038] As shown in Figure 2, when the gear ratio of the automatic transmission 16 is the same, the minimum turbine rotation speed Nt_min is set lower when the oil is at a high temperature compared to when the oil is at a normal temperature. For example, when the gear ratio of the automatic transmission 16 is the first gear "1st", the minimum turbine rotation speed Nt_min is set to a value greater than the rotation speed range of the turbine rotation speed Nt that is practically expected when the oil is at a high temperature, and the minimum turbine rotation speed Nt_min is set to 2000 [rpm] when the oil is at a high temperature. For example, when the gear ratio of the automatic transmission 16 is the second gear "2nd", the minimum turbine rotation speed Nt_min is set to 2000 [rpm] when the oil is at a normal temperature, and the minimum turbine rotation speed Nt_min is set to 1300 [rpm] when the oil is at a high temperature. For example, when the automatic transmission 16 is in the third gear position ("3rd"), the minimum turbine rotational speed Nt_min is set to a rotational speed range between 2000 [rpm] and 2300 [rpm] when the oil is at a normal temperature, and to 1300 [rpm] when the oil is at a high temperature.

[0039] Figure 3 is an example of a flowchart illustrating the control operation of the electronic control device 90 shown in Figure 1. The flowchart in Figure 3 is executed repeatedly, for example, while the vehicle is in motion.

[0040] First, in step S10 (hereinafter, "step" will be omitted), which corresponds to the function of the oil temperature determination unit 90d, it is determined whether the oil temperature THoil is equal to or greater than a predetermined determination oil temperature THoil_jdg. If the determination in S10 is YES, S20 is executed. If the determination in S10 is NO, S40 is executed.

[0041] In S20, which corresponds to the function of the condition setting unit 90e, the minimum turbine rotation speed Nt_min is set when the gear position of the automatic transmission 16 is the second gear position "2nd" or higher. For example, regardless of the throttle valve opening θth, the minimum turbine rotation speed Nt_min is set to 1300 [rpm]. After the execution of S20, in S30, which corresponds to the function of the condition setting unit 90e, the minimum turbine rotation speed Nt_min is set when the gear position of the automatic transmission 16 is the first gear position "1st". For example, regardless of the throttle valve opening θth, the minimum turbine rotation speed Nt_min is set to 2000 [rpm].

[0042] In S40, which corresponds to the function of the condition setting unit 90e, the minimum turbine rotation speed Nt_min is set when the gear position of the automatic transmission 16 is the second gear position "2nd" or higher. For example, when the gear position of the automatic transmission 16 is the second gear position "2nd", the minimum turbine rotation speed Nt_min is set to 2000 [rpm] regardless of the throttle valve opening θth. For example, when the gear position of the automatic transmission 16 is the third gear position "3rd" or higher, the minimum turbine rotation speed Nt_min is set to 2000 [rpm] when the throttle valve opening θth is 50 [%] or less, and when the throttle valve opening θth is 60 [%] or more, the minimum turbine rotation speed Nt_min is set to 2300 [rpm]. When the throttle valve opening θth is 50-60%, the minimum turbine rotational speed Nt_min is set to a rotational speed range between 2000 rpm and 2300 rpm, depending on the throttle valve opening θth. After the execution of S40, in S50, which corresponds to the function of the condition setting unit 90e, the minimum turbine rotational speed Nt_min when the gear stage of the automatic transmission 16 is the first gear stage "1st" is set to a value greater than the rotational speed range of the turbine rotational speed Nt that is practically expected.

[0043] After the execution of S30 and S50, in S60, which corresponds to the function of the condition determination unit 90f, it is determined whether the turbine rotation speed Nt is equal to or greater than the minimum turbine rotation speed Nt_min. In other words, it is determined whether the condition for controlling to the LU ON state is met.

[0044] If the result of S60 is YES, S70, which corresponds to the function of the LU control unit 90c, is controlled to turn the LU ON. If the result of S60 is NO, S80, which corresponds to the function of the LU control unit 90c, is controlled to turn the LU OFF. After the execution of S70 and S80, the program returns.

[0045] According to the electronic control device 90 of this embodiment, when the oil temperature THoil is high, i.e., when the oil temperature THoil is above a predetermined determination oil temperature THoil_jdg, the minimum turbine rotation speed Nt_min is set higher when the automatic transmission 16 is in the first gear ("1st") compared to when the automatic transmission 16 is in the second gear ("2nd") or higher. This suppresses the occurrence of engine stalls and reduces the amount of heat generated in the torque converter 14, thereby suppressing the rise in the oil temperature THoil.

[0046] According to the electronic control device 90 of this embodiment, when the automatic transmission 16 is in the second gear position "2nd" or higher, the minimum turbine rotation speed Nt_min is set lower when the oil temperature THoil is above a predetermined determination oil temperature THoil_jdg compared to when the oil temperature THoil is at its normal temperature, i.e., when the oil temperature THoil is below a predetermined determination oil temperature THoil_jdg. When the oil temperature THoil is high, the minimum turbine rotation speed Nt_min is set lower compared to when the oil temperature THoil is low, making it easier to control the system to the LU-on state. As a result, the amount of heat generated in the torque converter 14 is suppressed, and the rise in the oil temperature THoil of the oil is suppressed.

[0047] According to the electronic control device 90 of this embodiment, when the oil is at a normal temperature, i.e., the oil temperature THoil is below a predetermined determination oil temperature THoil_jdg, and the gear position of the automatic transmission 16 is the first gear "1st", the LU (Low Energy Control) is turned off. When the oil temperature THoil is not at a high temperature, an increase in the oil temperature THoil is permitted, so the LU is turned off even when the gear position of the automatic transmission 16 is the first gear "1st". As a result, when the gear position of the automatic transmission 16 is the first gear "1st", the occurrence of engine stall is suppressed, and a state of insufficient torque is less likely to occur.

[0048] Although embodiments of the present invention have been described in detail above with reference to the drawings, the present invention is also applicable to other embodiments.

[0049] In the previously described embodiment, the execution of the flowchart in Figure 3 was explained for the case while the vehicle was in motion. However, the present invention is not limited to this and can also be applied when the vehicle is starting or stopped.

[0050] In the above-described embodiment, the minimum turbine rotational speed Nt_min was set according to the throttle valve opening θth when the oil was at a normal temperature and the automatic transmission 16 was in the third gear or higher. However, the present invention is not limited to this. For example, when the oil was at a normal temperature and the automatic transmission 16 was in the third gear or higher, the minimum turbine rotational speed Nt_min may be set to a fixed value of 2000 [rpm] regardless of the throttle valve opening θth.

[0051] In the above-described embodiment, the minimum turbine rotational speed Nt_min was set to the same 2000 rpm when the oil was at a high temperature and the automatic transmission 16 was in the first gear ("1st"), and when the oil was at a normal temperature and the automatic transmission 16 was in the second gear ("2nd") and third gear ("3rd") (provided that the throttle valve opening θth was 50% or less). However, the present invention is not limited to these cases. For example, the minimum turbine rotational speed Nt_min may be set to a different value in these cases.

[0052] In the above-described embodiments, the "minimum turbine rotation speed Nt_min" has been described as the "turbine rotation speed Nt for switching between the LU off state and the LU on state". However, the present invention is not limited to this aspect. For example, the "minimum turbine rotation speed Nt_min" may be set as the lower limit value of the rotation speed range of the turbine rotation speed Nt for switching from the LU off state to the LU on state, and the "maximum turbine rotation speed Nt_max" may be set as the upper limit value of the rotation speed range of the turbine rotation speed Nt for switching from the LU on state to the LU off state. In this aspect, when the maximum turbine rotation speed Nt_max is set to a value lower than the minimum turbine rotation speed Nt_min, it is possible to prevent the magnitude of the driving torque transmitted from the engine 12 to the pair of drive wheels 20 from becoming unstable due to the repeated switching between the LU on state and the LU off state in a short period. The difference between the minimum turbine rotation speed Nt_min and the maximum turbine rotation speed Nt_max (<Nt_min) is determined in advance experimentally or by design so that the LU on state and the LU off state are not repeatedly switched in a short period.

[0053] In the above-described embodiments, when the gear stage of the automatic transmission 16 is the second gear stage "2nd" or higher, the minimum turbine rotation speed Nt_min is set lower when the oil OIL is in a high-temperature state compared to when the oil OIL is in a normal-temperature state. However, the present invention is not necessarily limited to this aspect. For example, even when the oil OIL is in a high-temperature state, when the gear ratio γat of the automatic transmission 16 is at a high-speed gear stage side with a low gear ratio (e.g., the eighth gear stage "8th"), compared to when it is at a low-speed gear stage, the oil OIL is not agitated violently in the torque converter 14 and the heat generation amount of the torque converter 14 is low. Therefore, in such a case, the minimum turbine rotation speed Nt_min when the oil OIL is in a high-temperature state may not be set lower compared to when the oil OIL is in a normal-temperature state.

[0054] In the above-described embodiment, when the oil was at a normal temperature and the gear of the automatic transmission 16 was in the first gear ("1st"), the system was controlled to always be in the LU off state. However, the present invention is not limited to this embodiment. For example, even in such a case, if there is no torque shortage and no engine stall occurs, a minimum turbine rotation speed Nt_min may be set. In the above-described embodiment, when the oil was at a normal temperature and the gear of the automatic transmission 16 was in the second gear ("2nd") or higher, the system was either in the LU off state or the LU on state. However, the present invention is not limited to this embodiment. For example, even in such a case, the system may be controlled to always be in the LU off state, similar to the first gear ("1st").

[0055] In the above-described embodiment, the turbine rotational speed Nt was a "vehicle speed-related value," but the present invention is not limited to this embodiment. The "vehicle speed-related value" can be any rotational speed that increases or decreases in conjunction with the increase or decrease of the vehicle speed V, instead of the turbine rotational speed Nt. For example, the "vehicle speed-related value" may be the average value of the wheel speeds, which are the rotational speeds of a pair of drive wheels 20, the output rotational speed Nout, the rotational speed of the pump impeller 14p, the engine rotational speed Ne, or the vehicle speed V itself.

[0056] It should be noted that the above-described examples are merely embodiments of the present invention, and the present invention can be implemented in various modified and improved forms based on the knowledge of those skilled in the art, without departing from its spirit. [Explanation of symbols]

[0057] 10: Vehicle, 12: Engine, 14: Torque converter, 16: Automatic transmission, 20: Pair of drive wheels, 90: Electronic control unit (control unit), LU: Lock-up clutch, Nt: Turbine rotational speed (vehicle speed related value), Nt_min: Minimum turbine rotational speed (lower limit), OIL: Oil, PT: Power transmission path, THoil: Oil temperature, THoil_jdg: Predetermined judgment oil temperature

Claims

1. A control device for a vehicle comprising an engine, an automatic transmission provided in the power transmission path between the engine and a pair of drive wheels, and a torque converter with a lock-up clutch provided in the power transmission path between the engine and the automatic transmission, When the lock-up clutch is in the disengaged state, if the oil temperature of the oil that transmits power in the torque converter is above a predetermined determination oil temperature, the lower limit of the vehicle speed-related value that engages the lock-up clutch is set higher when the automatic transmission is in the first gear compared to when the automatic transmission is in the second gear or higher. A vehicle control device characterized by the following features.

2. When the gear of the automatic transmission is the second gear or higher, the lower limit of the vehicle speed-related value for engaging the lock-up clutch is set lower when the oil temperature is equal to or greater than the predetermined determination oil temperature, compared to when the oil temperature is below the predetermined determination oil temperature. The vehicle control device according to feature 1.

3. If the oil temperature is below the predetermined determination oil temperature and the gear position of the automatic transmission is the first gear position, the lock-up clutch is released. A vehicle control device according to claim 1 or 2.

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