Hybrid vehicle control method and control system

Abstract

To appropriately perform hydraulic control through learning on the basis of actual hydraulic pressure detected by a hydraulic sensor when performing the hydraulic control for shifting a friction fastening element from a release state to a fastening state.SOLUTION: In a hybrid vehicle 1, when performing hydraulic control for shifting a first clutch CL1 (friction fastening element) from a release state to a fastening state, a controller 20 determines an estimation hydraulic pressure applied to the first clutch, determines estimation timing at which the shift from the release state to the fastening state is started in the first clutch on the basis of the estimation hydraulic pressure, determines a dispersion value of an actual hydraulic pressure detected by a hydraulic sensor SN4, determines actual timing at which the shift from the release state to the fastening state in the first clutch is started on the basis of the dispersion value, and performs learning for correcting the hydraulic pressure to be applied to the first clutch during the hydraulic control on the basis of a difference between the estimation timing and the actual timing.SELECTED DRAWING: Figure 5

Description

[Technical Field]

[0001] The present invention relates to a control method and control system for a hybrid vehicle having an engine and a motor as power sources, and a frictional engagement element (clutch) that switches between transmitting and cutting off torque between the engine and the motor. [Background technology]

[0002] Conventionally, hybrid vehicles have been known that include an engine (internal combustion engine), a motor (electric motor) located downstream of the engine in a power transmission path to the wheels, and a clutch (frictional engagement element) located between the engine and the motor in an intermittent manner. This hybrid vehicle is configured to be switchable between a first driving mode (EV driving mode) in which the hybrid vehicle is propelled using torque from the motor without using torque from the engine, and a second driving mode (engine driving mode or hybrid driving mode) in which the hybrid vehicle is propelled using at least torque from the engine. In particular, when switching from the first driving mode to the second driving mode, this hybrid vehicle performs control such that, in order to start a stopped engine, the clutch is shifted from a disengaged state to an engaged state and motor torque is transmitted to the engine via the clutch (in other words, the engine is cranked by the motor).

[0003] A technology related to such hybrid vehicles is described, for example, in Patent Document 1. Specifically, Patent Document 1 describes a technology that optimizes the clutch engagement pressure by correcting the clutch engagement pressure to be applied to the clutch through learning in accordance with the time difference between the predicted start time and the actual start time when starting the engine to switch from a first driving mode to a second driving mode. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-31005 Summary of the Invention [Problem to be solved by the invention]

[0005] In the hybrid vehicle described above, when switching from the first driving mode to the second driving mode, hydraulic pressure control is performed to apply hydraulic pressure to the clutch to transition the clutch from a released state to an engaged state. In this case, it is desirable to suppress shock that occurs when the clutch starts to transition from a released state to an engaged state (basically, when this transition begins, the clutch begins to transmit torque). For example, the clutch includes a clutch piston, clutch plates, and the like, and is configured to be engaged when the clutch piston is in contact with the clutch plate and released when the clutch piston is separated from the clutch plate. The shock described above can occur when the clutch piston comes into contact with the clutch plate. One method for suppressing this shock is to perform hydraulic pressure control so as to ensure a sufficient time (hereinafter simply referred to as the "transition time") until the clutch starts to transition from a released state to an engaged state. However, if this transition time is too long, engine start responsiveness will be impaired, causing the driver discomfort (such as an inability to accelerate in response to an acceleration request). Therefore, it is preferable to perform hydraulic pressure control by setting an appropriate time (hereinafter referred to as the "target transition time").

[0006] In hydraulic control, the hydraulic pressure applied to the clutch (i.e., the profile of the command hydraulic pressure, etc.) is set to achieve this target transition time. However, even with such hydraulic control, the target transition time may not be achieved due to factors such as component assembly accuracy, aging, and component variations. To address this issue, a method can be considered in which the actual hydraulic pressure applied to the clutch is detected by a hydraulic pressure sensor, the actual transition time (hereinafter referred to as the "actual transition time") is calculated based on this actual hydraulic pressure, and the hydraulic pressure applied to the clutch in the hydraulic control is learned based on the relationship between the actual transition time and the target transition time. In this case, the timing at which the clutch starts to transition from a released state to an engaged state can be determined based on the actual hydraulic pressure, and the actual transition time can be calculated. However, hydraulic systems that supply oil to the clutch have complex oil passages and other components, so the hydraulic system is prone to so-called oil vibrations (caused by oil pressure pulsations within the hydraulic system). As a result, the actual hydraulic pressure detected by the hydraulic pressure sensor contains relatively large noise, making it difficult to accurately determine the timing at which the clutch starts to transition from a released state to an engaged state. Therefore, it can be said that it is difficult to accurately learn about clutch hydraulic control based on the actual hydraulic pressure.

[0007] The present invention has been made to solve the problems of the conventional technology described above, and aims to provide a control method and control system for a hybrid vehicle that performs hydraulic control to transition a frictional engagement element provided between an engine and a motor from a released state to an engaged state, by learning based on the actual hydraulic pressure detected by a hydraulic sensor, thereby enabling accurate hydraulic control of the frictional engagement element. [Means for solving the problem]

[0008] In order to achieve the above-mentioned object, the present invention provides a control method for a hybrid vehicle having an engine, a motor, a frictional engagement element that can be in one of two states: an engaged state in which torque is transmitted between the engine and the motor, and a disengaged state in which torque transmission between the engine and the motor is interrupted, depending on the hydraulic pressure applied thereto, and a hydraulic sensor that detects the actual hydraulic pressure applied to the frictional engagement element, characterized by having the following steps: a first step of performing hydraulic control to apply hydraulic pressure to the frictional engagement element so as to transition the frictional engagement element from the disengaged state to the engaged state; a second step of determining an estimated hydraulic pressure applied to the frictional engagement element during hydraulic control; a third step of determining a variance value of the actual hydraulic pressure detected by the hydraulic sensor during hydraulic control; a fourth step of determining an estimated timing at which the frictional engagement element will begin to transition from the disengaged state to the engaged state based on the estimated hydraulic pressure; a fifth step of determining the actual timing at which the frictional engagement element will begin to transition from the disengaged state to the engaged state based on the variance value; and a sixth step of performing learning to correct the hydraulic pressure applied to the frictional engagement element during hydraulic control based on the difference between the estimated timing and the actual timing.

[0009] In the present invention configured as described above, when hydraulic control is performed to transition a frictional engagement element from a disengaged state to an engaged state, an estimated hydraulic pressure applied to the frictional engagement element is calculated and an estimated timing at which the frictional engagement element will begin to transition from a disengaged state to an engaged state is calculated based on this estimated hydraulic pressure, while a variance of the actual hydraulic pressure detected by the hydraulic pressure sensor is calculated and an actual timing at which the frictional engagement element will begin to transition from a disengaged state to an engaged state is calculated based on this variance, and learning is performed to correct the hydraulic pressure applied to the frictional engagement element during hydraulic control based on the difference between the estimated timing and the actual timing.In other words, in the present invention, rather than using the actual hydraulic pressure detected by the hydraulic pressure sensor itself, the actual timing is calculated based on the variance of the actual hydraulic pressure and learning is performed to correct the command hydraulic pressure for the frictional engagement element based on this actual timing. Here, the actual oil pressure detected by the oil pressure sensor contains relatively large noise due to the influence of oil pressure vibrations as described above, which tends to increase the variance of the actual oil pressure during hydraulic control. However, at the timing when the frictional engagement element starts to transition from the disengaged state to the engaged state, the influence of oil pressure vibrations decreases, temporarily suppressing the variance of the actual oil pressure and decreasing the variance. Therefore, by taking into account this tendency in the variance of the actual oil pressure, it is possible to accurately determine the actual timing when the frictional engagement element starts to transition from the disengaged state to the engaged state from this variance. Therefore, according to the present invention, based on this actual timing and estimated timing, it is possible to accurately learn the oil pressure to be applied to the frictional engagement element during hydraulic control so as to achieve the target transition time. As a result, by transitioning the frictional engagement element from the disengaged state to the engaged state using this learned hydraulic control, it is possible to reliably suppress the shock that occurs at the frictional engagement element during the transition and to suppress the discomfort felt by the driver due to a delay in the transition.

[0010] In the present invention, preferably, in the fifth step, the timing at which an inflection point appears in the variance value is determined as the actual timing. According to the present invention configured in this way, the actual timing can be determined with higher accuracy.

[0011] In the present invention, preferably, the frictional engagement element has a hydraulic chamber into which oil is introduced, and is configured to be in either an engaged state or a released state depending on the oil introduced into this hydraulic chamber, and in a first step, at the start of hydraulic control, a predetermined hydraulic pressure is applied to the frictional engagement element for a predetermined time so as to fill the hydraulic chamber of the frictional engagement element with oil, and in a sixth step, if the actual timing is earlier than the estimated timing, a correction is made to shorten the predetermined time for applying the predetermined hydraulic pressure during hydraulic control, and if the actual timing is later than the estimated timing, a correction is made to lengthen the predetermined time for applying the predetermined hydraulic pressure during hydraulic control. In the present invention configured as described above, learning is performed to correct the predetermined time for applying a predetermined oil pressure to fill the oil chamber of the frictional engagement element with oil, depending on the magnitude relationship between the actual timing and the estimated timing. As a result, the oil pressure applied to the frictional engagement element can be learned to accurately achieve the target transition time.

[0012] In the present invention, preferably, the fourth step further acquires an estimated oil pressure applied to the frictional engagement element at the estimated timing, the fifth step further acquires an actual oil pressure detected by the oil pressure sensor at the actual timing, and the sixth step further corrects the oil pressure applied to the frictional engagement element when the frictional engagement element starts to transition from a released state to an engaged state during oil pressure control based on the difference between the estimated oil pressure acquired in the fourth step and the actual oil pressure acquired in the fifth step. In the present invention configured as described above, the hydraulic pressure applied to the frictional engagement element when it starts to transition from a disengaged state to an engaged state is corrected based on the difference between the estimated hydraulic pressure at the estimated timing and the actual hydraulic pressure at the actual timing. This allows the hydraulic pressure to be changed at an appropriate rate during the period from when the frictional engagement element starts to transition from a disengaged state to an engaged state. This more reliably reduces shocks that occur in the frictional engagement element during the transition and the discomfort felt by the driver due to a delay in the transition.

[0013] In the present invention, preferably, in the sixth step, when hydraulic control is performed, learning is prohibited in at least one of the following cases: (i) when the driver requests a predetermined acceleration, (ii) when the hydraulic pressure (line pressure) in the hydraulic system that supplies hydraulic pressure to at least the frictional engagement elements is equal to or greater than a predetermined value, (iii) when the hybrid vehicle is started, or (iv) when the engine is started by a starter separate from the motor because the charge level of the battery that supplies power to the motor is less than a predetermined value. According to the present invention configured in this manner, it is possible to prohibit learning in a situation where the behavior of the frictional engagement element is not stable, thereby ensuring the accuracy of learning.

[0014] In the present invention, preferably, in the sixth step, learning is terminated when the number of times that the frictional engagement element has been transitioned from a released state to an engaged state by hydraulic control reaches or exceeds a predetermined value, or when hydraulic control in which the difference between the estimated timing and the actual timing is less than a predetermined value has been performed a predetermined number of times in succession. According to the present invention configured in this manner, learning can be terminated when learning is no longer necessary, and thereafter, the load of processing and control related to learning can be reduced.

[0015] In the present invention, preferably, in the sixth step, learning is resumed when the total distance traveled by the hybrid vehicle since the end of learning reaches or exceeds a predetermined value. According to the present invention configured in this way, deviations that occur after learning has ended can be accurately corrected by restarting learning.

[0016] In a preferred example of the present invention, the frictional engagement element has a hydraulic chamber into which oil is introduced, a clutch piston that operates in response to the oil supplied to this hydraulic chamber, and a clutch plate with which this clutch piston comes into contact, and is configured to be in an engaged state when the clutch piston is in contact with the clutch plate and to be in a released state when the clutch piston is separated from the clutch plate, and in the fourth and fifth steps, the timing at which the separated clutch piston comes into contact with the clutch plate during hydraulic control is determined as the estimated timing and the actual timing, respectively.

[0017] In another aspect, to achieve the above object, the present invention provides a control system for a hybrid vehicle, the control system comprising: an engine and a motor; a frictional engagement element that can be in one of two states: an engaged state in which torque is transmitted between the engine and the motor; and a disengaged state in which torque transmission between the engine and the motor is interrupted, depending on the hydraulic pressure applied; a hydraulic sensor that detects an actual hydraulic pressure applied to the frictional engagement element; and a control device configured to control the engine, the motor, and the frictional engagement element, wherein the control device performs hydraulic control to apply hydraulic pressure to the frictional engagement element to transition the frictional engagement element from the released state to the engaged state, and determines an estimated hydraulic pressure applied to the frictional engagement element during the hydraulic control, determines a variance of the actual hydraulic pressure detected by the hydraulic control sensor, determines an estimated timing at which the frictional engagement element will begin to transition from the released state to the engaged state based on the estimated hydraulic pressure, determines an actual timing at which the frictional engagement element will begin to transition from the released state to the engaged state based on the variance, and performs learning to correct the hydraulic pressure applied to the frictional engagement element during the hydraulic control, based on the difference between the estimated timing and the actual timing. With the present invention configured in this manner, the actual timing at which the frictional engagement element begins to transition from a released state to an engaged state can be accurately determined based on the variance value of the actual oil pressure, and based on this actual timing and the estimated timing, it is possible to accurately learn the oil pressure to be applied to the frictional engagement element during oil pressure control so as to achieve the above-mentioned target transition time. [Effects of the Invention]

[0018] According to the present invention, in a control method and control system for a hybrid vehicle that performs hydraulic control to transition a clutch provided between an engine and a motor from a released state to an engaged state, hydraulic control of the clutch can be performed accurately by learning based on the actual hydraulic pressure detected by a hydraulic sensor. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a schematic configuration diagram of a hybrid vehicle according to an embodiment of the present invention. [Figure 2] FIG. 2 is a schematic diagram of a first clutch according to an embodiment of the present invention. [Figure 3] 1 is a block diagram showing an electrical configuration of a hybrid vehicle according to an embodiment of the present invention. [Figure 4] 4 is a time chart for explaining the basic concept of hydraulic control according to an embodiment of the present invention. [Figure 5] 4 is a time chart for explaining hydraulic pressure learning control according to an embodiment of the present invention. [Figure 6] 4 is a flowchart showing hydraulic pressure learning control according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0020] Hereinafter, a control method and a control system for a hybrid vehicle according to an embodiment of the present invention will be described with reference to the accompanying drawings.

[0021] [Device configuration] FIG. 1 is a schematic diagram of a hybrid vehicle to which a control method and control system for a hybrid vehicle according to an embodiment of the present invention is applied.

[0022] As shown in FIG. 1, the hybrid vehicle 1 mainly comprises an engine 2 (e.g., a gasoline engine) that generates torque for driving the hybrid vehicle 1, a starter 3 that is a motor for starting the engine 2, a motor 4 that is located downstream of the engine 2 on the power transmission path of the hybrid vehicle 1 and generates torque for driving the hybrid vehicle 1, a battery 5 that exchanges power with the motor 4 via an inverter or the like (not shown), a transmission 6 that is located downstream of the motor 4 on the power transmission path of the hybrid vehicle 1 and changes the rotational speed of the engine 2 and / or the motor 4, a power transmission system 8 that transmits torque from the transmission 6 downstream, a drive shaft 10 that drives wheels 12 with torque from the power transmission system 8, and the wheels (drive wheels) 12.

[0023] The output shaft of the engine 2 and the rotating shaft of the motor 4 are coaxially connected by an axis AX1 via a first clutch CL1 that can be connected and disconnected. This first clutch CL1 allows for switching between transmission and disconnection of torque between the engine 2 and the motor 4. For example, the first clutch CL1 is configured as a dry multi-plate clutch or a wet multi-plate clutch that can change the transmission torque capacity by continuously or stepwise controlling the clutch hydraulic oil flow rate and / or clutch hydraulic pressure using a motor or solenoid (not shown).

[0024] Here, a specific configuration of the first clutch CL1 will be described with reference to Fig. 2. Fig. 2 is a schematic diagram showing an example of the first clutch CL1. As shown in Fig. 2, the first clutch CL1 includes a hydraulic chamber 15a into which oil is introduced, an oil passage 15b that supplies oil to the hydraulic chamber 15a (see arrow A1), a clutch piston 15c that operates in accordance with the oil (i.e., the hydraulic pressure) supplied to the hydraulic chamber 15a (see arrow A2), a first clutch plate 15d that comes into contact with the clutch piston 15c, a second clutch plate 15e that transmits torque between the first clutch plate 15d and the clutch piston 15c when the clutch piston 15c comes into contact with the first clutch plate 15d, a solenoid 15f that is provided on the oil passage 15b and that can adjust the hydraulic pressure supplied to the hydraulic chamber 15a, and a hydraulic sensor SN4 that detects the hydraulic pressure (actual hydraulic pressure) supplied to the hydraulic chamber 15a.

[0025] By controlling the applied hydraulic pressure, the first clutch CL1 can be switched between a released state in which the clutch piston 15c is separated from the first clutch plates 15d and an engaged state in which the clutch piston 15c is in contact with the first clutch plates 15d. When the first clutch CL1 is in the released state, torque transmission between the engine 2 and the motor 4 is interrupted, and when the first clutch CL1 is in the engaged state, torque is transmitted between the engine 2 and the motor 4. This engaged state is a state in which the clutch piston 15c is in contact with the first clutch plate 15d as described above, but this state also includes a slip state in which the first clutch plate 15d and the second clutch plate 15e slip (typically a state in which the first clutch plate 15d and the second clutch plate 15e are separated and torque is transmitted via oil between them), and a fully engaged state in which torque is completely transmitted between the first clutch plate 15d and the second clutch plate 15e (basically a state in which the first clutch plate 15d and the second clutch plate 15e are firmly in contact). Note that this type of first clutch CL1 is an example of a "frictional engagement element" in this invention.

[0026] Returning to FIG. 1 , the rotating shaft of the motor 4 and the rotating shaft of the transmission 6 are coaxially connected by an axis AX2. The transmission 6 is typically an automatic transmission equipped with one or more planetary gear sets including a sun gear S1, a ring gear R1, a pinion gear P1 (planetary gear), and a carrier C1, as well as frictional fastening elements such as clutches and brakes, and capable of automatically switching gears (gear ratios) according to vehicle speed, engine speed, and other factors. The ring gear R1 is disposed concentrically with the sun gear S1, and the pinion gear P1 is disposed between the sun gear S1 and the ring gear R1 so as to mesh with the sun gear S1 and the ring gear R1. The carrier C1 holds the pinion gear P1 so that it can rotate on its own axis and revolve around the sun gear S1.

[0027] The transmission 6 also includes an internal second clutch CL2 that is connectable and disconnectable, and this second clutch CL2 allows for switching between transmitting and disconnecting torque between the upstream side of the transmission 6 (the engine 2 and the motor 4) and the downstream side of the transmission 6 (the wheels 12, etc.). For example, the second clutch CL2 is configured as a dry multi-plate clutch or a wet multi-plate clutch that can change the transmission torque capacity by continuously or stepwise controlling the clutch hydraulic oil flow rate and / or the clutch hydraulic pressure using a motor or solenoid (not shown). The second clutch CL2 can also be switched between a released state and an engaged state (a slip state or a fully engaged state) by controlling the hydraulic pressure applied to it. In reality, the second clutch CL2 is made up of multiple clutches used to change between various gear stages in the transmission 6. For simplicity's sake, only one planetary gear set is shown in Fig. 1, but in reality the transmission 6 is equipped with multiple planetary gear sets. By selectively engaging frictional engagement elements such as multiple clutches typified by the second clutch CL2 and multiple brakes (not shown) to switch the power transmission path that passes through each planetary gear set, it is possible to achieve, for example, multiple forward gear stages and one reverse gear stage.

[0028] Torque is input to the power transmission system 8 via the output shaft AX3 of the transmission 6. The power transmission system 8 is configured to include a differential gear that distributes driving force to a pair of left and right wheels 12, a final gear, and the like.

[0029] The hybrid vehicle 1 can switch between driving modes by switching between engagement and disengagement of the first clutch CL1. That is, the hybrid vehicle 1 has a first driving mode in which the first clutch CL1 is set to a disengaged state and the hybrid vehicle 1 is driven using the torque of the motor 4 without using the torque of the engine 2, and a second driving mode in which the first clutch CL1 is set to an engaged state and the hybrid vehicle 1 is driven using at least the torque of the engine 2. The first driving mode is a so-called EV driving mode, and the second driving mode includes an engine driving mode in which the hybrid vehicle 1 is driven using only the torque of the engine 2, and a hybrid driving mode in which the hybrid vehicle 1 is driven using the torque of both the engine 2 and the motor 4.

[0030] Next, FIG. 3 is a block diagram showing the electrical configuration of the hybrid vehicle according to the embodiment of the present invention.

[0031] As shown in Figure 3, the controller 20 receives as inputs a signal from an engine speed sensor SN1 that detects the speed of the engine 2, a signal from a motor speed sensor SN2 that detects the speed of the motor 4, a signal from an accelerator opening sensor SN3 that detects the accelerator opening corresponding to the amount of depression of the accelerator pedal by the driver, a signal from a hydraulic sensor SN4 (see Figure 2) that detects the actual hydraulic pressure supplied to the hydraulic chamber 15a, a signal from a start switch SN5 that starts the hybrid vehicle 1, and a signal from an SOC sensor SN6 that detects the SOC (State of Charge) that indicates the amount of charge of the battery 5.

[0032] The controller 20 is configured by a computer having one or more processors 20a (typically a CPU) and a memory 20b such as a ROM or RAM for storing various programs (including basic control programs such as an OS and application programs that are run on the OS and realize specific functions) interpreted and executed by the processors and various data. The controller 20 corresponds to the "control device" of the present invention and executes the "hybrid vehicle control method" of the present invention.

[0033] Specifically, the controller 20 controls the engine 2, the starter 3, the motor 4, the first clutch CL1, and the second clutch CL2 by outputting control signals to these components based on signals from the above-mentioned sensors (including switches) SN1 to SN6. For example, the controller 20 performs control to adjust the ignition timing, fuel injection timing, and fuel injection amount of the engine 2, control to adjust the rotation speed and torque of the motor 4, and hydraulic control to switch the states (released state, slip state, fully engaged state) of the first and second clutches CL1 and CL2. In practice, the controller 20 controls the ignition plugs, fuel injection valves, throttle valves, etc. of the engine 2, controls the motor 4 via an inverter, and controls the first and second clutches CL1 and CL2 via hydraulic control circuits (such as the motor and solenoid 15f).

[0034] [Control content] Next, the control contents performed by the controller 20 in this embodiment will be described. In this embodiment, the controller 20 mainly performs hydraulic control to apply hydraulic pressure (hereinafter referred to as "CL1 hydraulic pressure" as appropriate) to the first clutch CL1 in order to start the stopped engine 2 in order to switch from a first traveling mode in which the hybrid vehicle 1 is traveled using the torque of the motor 4 without using the torque of the engine 2 to a second traveling mode in which the hybrid vehicle 1 is traveled using at least the torque of the engine 2, thereby transitioning the first clutch CL1 from a released state to an engaged state, and performing control over the motor 4 and the engine 2 to start the engine 2 by cranking the motor 4. In order to realize such hydraulic control, the controller 20 controls a hydraulic control circuit (for example, the solenoid 15f) of the first clutch CL1.

[0035] First, the basic concept of hydraulic control according to this embodiment will be described with reference to Fig. 4. Fig. 4 is a time chart showing hydraulic control according to this embodiment. In Fig. 4, graph G11 shows the command value (command hydraulic pressure) of the CL1 hydraulic pressure applied in hydraulic control, graph G12 shows the estimated value (estimated hydraulic pressure) of the CL1 hydraulic pressure estimated when hydraulic control is performed using this command hydraulic pressure, and graph G2 shows the integrated flow rate (also an estimated value) of oil flowing into the hydraulic chamber 15a of the first clutch CL1 during this hydraulic control. These estimated hydraulic pressures and integrated flow rates are calculated by the controller 20 from a predetermined model.

[0036] At time t1, the controller 20 starts hydraulic control to transition the first clutch CL1 from a released state to an engaged state in order to switch from the first traveling mode to the second traveling mode. Specifically, from time t1, the controller 20 sets the command hydraulic pressure to a relatively high hydraulic pressure (period T1) to fill the hydraulic chamber 15a of the first clutch CL1 with oil (pre-charge). Hereinafter, this period T1 will be referred to as the "pre-charge period." Then, from time t2, the controller 20 significantly reduces the command hydraulic pressure from the pre-charge period T1 and then gradually increases it, thereby moving the clutch piston 15c, which is separated from the first clutch plate 15d in the first clutch CL1, and bringing the clutch piston 15c into contact with the first clutch plate 15d (period T2). Hereinafter, this period T2 will be referred to as the "stroke period." Regarding the length of this stroke period T2, an optimal time (hereinafter referred to as the "target stroke time") to be applied in hydraulic control is determined in advance from the viewpoint of suppressing the shock when the clutch piston 15c contacts the first clutch plate 15d while ensuring responsiveness in engine start-up.

[0037] After time t3 when the clutch piston 15c contacts the first clutch plate 15d, the controller 20 increases the command oil pressure by a value greater than that during the stroke period T2, maintaining the first clutch plate 15d and the second clutch plate 15e in a slipped state (slip state), and then fully engaging them (period T3). In this slip state of the first clutch CL1, torque is transmitted from the motor 4 to the engine 2 via the first clutch CL1. In this state, the controller 20 controls the motor 4 and the engine 2 so that the engine speed is gradually increased by cranking the motor 4 to start the engine 2. Then, when the engine speed and the motor speed are synchronized, the controller 20 maintains the command oil pressure constant to maintain the first clutch CL1 in a fully engaged state (time t4).

[0038] On the other hand, during the hydraulic control described above, the controller 20 uses a predetermined model to calculate an estimated value of the CL1 hydraulic pressure (estimated hydraulic pressure) and the cumulative flow rate of oil flowing into the hydraulic chamber 15a (see graphs G12 and G2). Specifically, the controller 20 calculates the estimated hydraulic pressure and cumulative flow rate based on the flow rate of oil flowing into the hydraulic chamber 15a (corresponding to the commanded hydraulic pressure) and the previously known volume of the hydraulic chamber 15a. The controller 20 then calculates the timing at which the calculated cumulative flow rate reaches a flow rate Q1 (hereinafter referred to as the "upper limit flow rate") corresponding to the volume of the hydraulic chamber 15a as the estimated timing at which the clutch piston 15c contacts the first clutch plate 15d (hereinafter referred to as the "estimated touch timing"). This estimated touch timing basically coincides with the end time t3 of the stroke period T2 applied at the commanded hydraulic pressure, as described above. This is because the estimated hydraulic pressure is calculated based on the commanded hydraulic pressure for hydraulic control. The controller 20 also determines the estimated oil pressure P1 at this estimated touch timing t3 (hereinafter referred to as "estimated touch oil pressure").

[0039] As described above, hydraulic control sets the command hydraulic pressure to achieve the target stroke time. However, even with such hydraulic control, the target stroke time may not be achieved due to factors such as component assembly accuracy, aging, and component variations. To address this issue, a method is considered in which the actual hydraulic pressure applied to the first clutch CL1 is detected by the hydraulic pressure sensor SN4, and the actual timing at which the clutch piston 15c contacts the first clutch plate 15d (hereinafter referred to as the "actual touch timing") is calculated based on this actual hydraulic pressure. The command hydraulic pressure applied in the hydraulic control is then corrected and learned based on the difference between this actual touch timing and the estimated touch timing. However, the hydraulic system supplying oil to the first clutch CL1 has a complex oil passageway and other components, which supply oil to many other components in addition to the first clutch CL1. This hydraulic system is prone to oil vibrations (caused by oil pressure pulsations within the hydraulic system). Therefore, the actual hydraulic pressure detected by the hydraulic pressure sensor SN4 contains relatively large noise, making it difficult to accurately determine the actual touch timing. Therefore, it can be said that it is difficult to accurately learn the command oil pressure based on the actual touch timing corresponding to the actual oil pressure.

[0040] Therefore, in this embodiment, the actual touch timing is determined with high accuracy from the actual oil pressure detected by the oil pressure sensor SN4, and based on this actual touch timing, the command oil pressure to be applied in the oil pressure control of the first clutch CL1 can be accurately learned so as to accurately realize the target stroke time. To achieve this, in this embodiment, the controller 20 does not use the actual oil pressure detected by the oil pressure sensor SN4 itself, but determines the actual touch timing based on the variance value of this actual oil pressure, and performs learning (hereinafter referred to as "hydraulic pressure learning control") to correct the command oil pressure to be applied in the oil pressure control based on the difference between this actual touch timing and the estimated touch timing.

[0041] Specifically, the controller 20 determines the timing at which an inflection point appears in the variance of the actual oil pressure as the actual touch timing. As described above, the actual oil pressure detected by the oil pressure sensor SN4 contains relatively large noise due to the influence of oil pressure vibrations. This causes a tendency for the variance of the actual oil pressure to increase during hydraulic control, i.e., for the variance to increase. However, at the moment when the clutch piston 15c contacts the first clutch plate 15d, the influence of oil pressure vibrations decreases, temporarily suppressing the variance of the actual oil pressure and causing the variance to decrease. Therefore, in this embodiment, the controller 20 uses the timing at which the variance is minimized and the inflection point of the variance appears as the timing at which the clutch piston 15c contacts the first clutch plate 15d (the actual touch timing). This allows the actual touch timing to be accurately determined from the actual oil pressure, enabling the command oil pressure to be applied in the hydraulic control of the first clutch CL1 to be accurately learned.

[0042] Next, hydraulic pressure learning control according to this embodiment will be described in detail with reference to FIG. 5. FIG. 5 is a time chart for explaining hydraulic pressure learning control according to this embodiment. In FIG. 5, graph G11 shows the command hydraulic pressure applied in hydraulic pressure control, graph G12 shows the estimated hydraulic pressure when this hydraulic pressure control is performed, graph G13 shows the actual hydraulic pressure detected by hydraulic pressure sensor SN4 during this hydraulic pressure control, and graph G3 shows the variance value of this actual hydraulic pressure (hereinafter referred to as the "hydraulic pressure variance value"). Note that the actual hydraulic pressure detected by hydraulic pressure sensor SN4 actually contains noise and repeats small fluctuations, but for convenience of explanation, FIG. 5 shows a simplified actual hydraulic pressure with such noise removed (graph G13). Furthermore, in FIG. 5, elements with the same symbols as those in FIG. 4 have the same meanings as those in FIG. 4, and their explanations will be omitted as appropriate.

[0043] In the example shown in Figure 5, when hydraulic control is performed using the command hydraulic pressure shown in graph G11, the hydraulic pressure sensor SN4 detects an actual hydraulic pressure as shown in graph G13, which is far from the estimated hydraulic pressure shown in graph G12. The controller 20 calculates a hydraulic pressure variance value as shown in graph G3 from this actual hydraulic pressure. A specific method for calculating this hydraulic pressure variance value will be explained below. Here, the method for calculating the hydraulic pressure variance value at time tx will be used as an example. Note that, as a premise, the controller 20 acquires the actual hydraulic pressure at predetermined time intervals in response to a signal sent from the hydraulic pressure sensor SN4, that is, it collects a plurality of discrete actual hydraulic pressures during hydraulic control.

[0044] First, the controller 20 calculates the sum P_sum of the multiple actual oil pressures acquired between t1 and tx, then calculates a value (average value) P_ave_sum by dividing this sum P_sum by the time between t1 and tx, and calculates the square of this average value P_ave_sum P_sqr_ave_sum. At the same time, the controller 20 calculates the square P_sqr of each of the multiple actual oil pressures acquired between t1 and tx, calculates a sum P_sum_sqr of these square values ​​P_sqr, and calculates a value (average value) P_ave_sum_sqr by dividing this sum P_sum_sqr by the time between t1 and tx. The controller 20 then uses the value obtained by subtracting the square value P_sqr_ave_sum from the average value P_ave_sum_sqr as the oil pressure variance at time tx.

[0045] In this case, an inflection point of the hydraulic variance value appears at time t3a (specifically, the sign of the second-order differential of the hydraulic variance value changes from negative to positive), in other words, the hydraulic variance value reaches its minimum value after time t2. Therefore, the controller 20 determines this time t3a as the actual touch timing. The controller 20 then corrects the command hydraulic pressure to be applied in hydraulic control according to the difference dt between this actual touch timing t3a and the above-mentioned estimated touch timing t3 (hereinafter referred to as the "touch timing difference"). Specifically, the controller 20 corrects the time tp of the pre-charge period T1 (hereinafter referred to as the "pre-charge time") included in the command hydraulic pressure during hydraulic control based on the touch timing difference dt so that the target stroke time is realized in the actual hydraulic control. In this case, the controller 20 increases the correction amount of the pre-charge time tp as the touch timing difference dt increases. Furthermore, when the actual touch timing t3a is earlier than the estimated touch timing t3 (as shown in the example of FIG. 5), the controller 20 corrects the precharge time tp to be shorter, whereas when the actual touch timing t3a is later than the estimated touch timing t3, the controller 20 corrects the precharge time tp to be longer.

[0046] Furthermore, when the controller 20 determines the actual touch timing t3a as described above, it acquires the actual oil pressure P2 (hereinafter referred to as the "actual touch oil pressure") detected by the oil pressure sensor SN4 at this actual touch timing t3a. The controller 20 then further corrects the command oil pressure applied in the oil pressure control in accordance with the difference dP (hereinafter referred to as the "touch oil pressure difference") between this actual touch oil pressure P2 and the above-mentioned estimated touch oil pressure P1. Specifically, the controller 20 corrects the command oil pressure P3 (hereinafter referred to as the "touch command oil pressure") applied at the timing t3 when the clutch piston 15c contacts the first clutch plate 15d based on the touch oil pressure difference dP so as to change the oil pressure at an appropriate gradient during the stroke period. More specifically, the controller 20 increases the correction amount of the touch command oil pressure P3 as the touch oil pressure difference dP increases. In addition, when the actual touch oil pressure P2 is greater than the estimated touch oil pressure P1, the controller 20 makes a correction to decrease the command oil pressure P3 at the time of touch, whereas when the actual touch oil pressure P2 is less than the estimated touch oil pressure P1 (example shown in Figure 5), the controller 20 makes a correction to increase the command oil pressure P3 at the time of touch.

[0047] Next, the overall flow of hydraulic pressure learning control according to this embodiment will be described with reference to Fig. 6. Fig. 6 is a flowchart showing hydraulic pressure learning control executed by the controller 20 in this embodiment.

[0048] The hydraulic learning control according to this flowchart is initiated when a request to execute hydraulic control for transitioning the first clutch CL1 from a released state to an engaged state is issued. In particular, the request to execute hydraulic control corresponds to a request to start the engine 2 for switching from the first driving mode to the second driving mode. This is because, when switching from the first driving mode to the second driving mode, the first clutch CL1 is transitioned from a released state to an engaged state, and the engine 2 is started by cranking the motor 4. For example, a request to start the engine 2 is issued when the driver requests relatively high acceleration in the EV mode (i.e., when the driver requests acceleration that requires switching the driving mode from the EV mode to the HV mode). In addition to such a driver request, a start request is also issued from a control system including a powertrain, etc. (hereinafter, this start request is referred to as a "system request"). This system request is issued when the driving mode of the hybrid vehicle 1 should be switched from the EV mode to the HV mode depending on the vehicle speed, load, battery state, engine temperature, etc. For example, a system request is issued when the driving force of the motor 4 alone is insufficient to achieve the target driving force, when the battery 5 needs to be charged (when the SOC of the battery 5 is less than a predetermined value), or when engine braking by the engine 2 needs to be applied during deceleration.

[0049] When hydraulic learning control is started, first, in step S101, the controller 20 acquires various information. Specifically, the controller 20 acquires signals from at least the above-mentioned sensors (including switches) SN1 to SN6.

[0050] Next, in step S102, the controller 20 determines whether or not the conditions (learning conditions) for learning the command hydraulic pressure for hydraulic control are met. Specifically, the controller 20 uses the following conditions as the learning conditions. (i) There is no predetermined acceleration request (corresponding to a request for quick engine start) from the driver, that is, the accelerator opening detected by the accelerator opening sensor SN3 is less than a predetermined value. (ii) The hydraulic pressure in the hydraulic system that supplies hydraulic pressure to the first clutch CL1, etc. (detected by a hydraulic pressure sensor provided in the hydraulic system) is less than a predetermined value, that is, the line pressure in the hydraulic system is not high. (iii) Hybrid vehicle 1 does not start, i.e., the signal of start switch SN5 is not switched on. (iv) The engine 2 is not started by the starter 3 when the SOC of the battery 5 (detected by the SOC sensor SN6) is below a predetermined value.

[0051] When performing hydraulic control, if all of the conditions (i) to (iv) are satisfied, the controller 20 determines that the learning conditions are satisfied (step S102: Yes) and proceeds to step S103. In contrast, when performing hydraulic control, if at least one of the conditions (i) to (iv) is not satisfied, that is, if (i) there is a predetermined acceleration request from the driver, i.e., the accelerator opening detected by the accelerator opening sensor SN3 is equal to or greater than a predetermined value, (ii) the line pressure in the hydraulic system is equal to or greater than a predetermined value, (iii) the hybrid vehicle 1 is started, i.e., the signal from the start switch SN5 is switched on, or (iv) the engine 2 is started by the starter 3 because the SOC of the battery 5 is less than a predetermined value, the controller 20 determines that the learning conditions are not satisfied (step S102: No). In this case, the controller 20 terminates hydraulic learning control to prohibit learning of the command hydraulic pressure in a situation where the behavior of the first clutch CL1 is unlikely to stabilize.

[0052] Next, in step S103, the controller 20 determines whether learning of the command hydraulic pressure for hydraulic control is unnecessary. Specifically, if the number of times that the first clutch CL1 has been shifted from a released state to an engaged state by hydraulic control (i.e., the number of times that the first driving mode has been switched to the second driving mode) is equal to or greater than a predetermined value, or if hydraulic control in which the touch timing difference between the estimated touch timing and the actual touch timing is less than a predetermined value has been performed a predetermined number of times (e.g., two times) consecutively, the controller 20 determines that learning of the command hydraulic pressure for hydraulic control is unnecessary in order to end the learning (step S103: Yes). In this case, the controller 20 proceeds to step S104. On the other hand, if the number of times that the first clutch CL1 has been shifted from a released state to an engaged state by hydraulic control is less than the predetermined value and hydraulic control in which the touch timing difference between the estimated touch timing and the actual touch timing is less than a predetermined value has not been performed a predetermined number of times consecutively, the controller 20 determines that learning of the command hydraulic pressure for hydraulic control is necessary (step S103: No). In this case, the controller 20 proceeds to steps S105 and S108 to perform learning. When determining whether learning is unnecessary in step S103, a condition may be added that hydraulic control in which the touch hydraulic pressure difference between the actual touch hydraulic pressure and the estimated touch hydraulic pressure becomes less than a predetermined value has been performed a predetermined number of times (for example, twice) in succession.

[0053] Next, in step S104, controller 20 determines whether to resume learning, which was terminated in step S103 after it was determined that learning was unnecessary. Specifically, controller 20 determines to resume learning if the total traveling distance of hybrid vehicle 1 since learning was terminated is equal to or greater than a predetermined value (e.g., 5000 km) (step S104: Yes). In this case, controller 20 proceeds to steps S105 and S108 to perform learning. On the other hand, if the total traveling distance of hybrid vehicle 1 since learning was terminated is less than the predetermined value, controller 20 determines not to resume learning (step S104: No) and terminates hydraulic learning control.

[0054] If the controller 20 determines in step S103 that learning is necessary (step S103: No), or if the controller 20 determines in step S104 that learning should be resumed (step S104: Yes), the controller 20 proceeds to steps S105 and S108. In this case, the controller 20 performs the processes of steps S105 to S107 and steps S108 to S110 in parallel.

[0055] First, the processing of steps S105 to S107 will be described. In step S105, the controller 20 uses a predetermined model to calculate an estimated value of the CL1 oil pressure (estimated oil pressure) and an integrated flow rate of oil flowing into the oil pressure chamber 15a based on the flow rate of oil flowing into the oil pressure chamber 15a of the first clutch CL1 (corresponding to the command oil pressure) and the previously known volume of the oil pressure chamber 15a. Next, in step S106, the controller 20 determines whether the integrated flow rate calculated in step S105 has reached an upper limit flow rate corresponding to the volume of the oil pressure chamber 15a. As a result, if the integrated flow rate has not reached the upper limit flow rate (step S106: No), the controller 20 returns to step S105. In this case, the controller 20 repeats steps S105 and S106 until the integrated flow rate reaches the upper limit flow rate. On the other hand, if the integrated flow rate reaches the upper limit flow rate (step S106: Yes), the controller 20 proceeds to step S107, determines the timing when the integrated flow rate reaches the upper limit flow rate as the estimated touch timing, and acquires the estimated oil pressure at this timing as the estimated touch oil pressure.Then, the controller 20 proceeds to step S111.

[0056] Next, the processing of steps S108 to S110, which is performed in parallel with the above steps S105 to S107, will be described. In step S108, the controller 20 calculates the hydraulic variance value from the actual hydraulic pressure detected by the hydraulic pressure sensor SN4. The method for calculating the hydraulic variance value is as described above (see FIG. 5). Next, in step S109, the controller 20 determines whether an inflection point has appeared in the hydraulic pressure variance value calculated in step S108. As a result, if the controller 20 determines that an inflection point has not appeared in the hydraulic pressure variance value (step S109: No), the controller 20 returns to step S108. In this case, the controller 20 repeats steps S108 and S109 until an inflection point appears. On the other hand, if an inflection point has appeared in the hydraulic pressure variance value (step S109: Yes), the controller 20 proceeds to step S110, where it determines the timing at which the inflection point of the hydraulic pressure variance value appeared as the actual touch timing, and acquires the actual hydraulic pressure detected by the hydraulic pressure sensor SN4 at this timing as the actual touch hydraulic pressure. Then, the controller 20 proceeds to step S111.

[0057] Next, in step S111, the controller 20 calculates the touch timing difference (absolute value) from the estimated touch timing and actual touch timing acquired as described above, and also calculates the touch oil pressure difference (absolute value) from the actual touch oil pressure and estimated touch oil pressure acquired as described above. The controller 20 then determines whether the touch timing difference is equal to or greater than a predetermined value or whether the touch oil pressure difference is equal to or greater than a predetermined value. Here, it is determined whether a touch timing difference or touch oil pressure difference that makes learning desirable has occurred. As a result, if the touch timing difference is less than the predetermined value and the touch oil pressure difference is less than the predetermined value (step S111: No), the controller 20 determines that learning is unnecessary and terminates the oil pressure learning control.

[0058] On the other hand, if the touch timing difference is equal to or greater than a predetermined value or the touch oil pressure difference is equal to or greater than a predetermined value (step S111: Yes), the controller 20 proceeds to step S112 to perform learning. In step S112, the controller 20 corrects the pre-charge time applied in the hydraulic control based on the touch timing difference between the actual touch timing and the estimated touch timing so that the target stroke time is achieved in the actual hydraulic control. Specifically, if the actual touch timing is earlier than the estimated touch timing, the controller 20 corrects the pre-charge time to be shorter in accordance with the magnitude of the touch timing difference, whereas if the actual touch timing is later than the estimated touch timing, the controller 20 corrects the pre-charge time to be longer in accordance with the magnitude of the touch timing difference.

[0059] Next, in step S113, the controller 20 corrects the touch command oil pressure applied in the oil pressure control based on the touch oil pressure difference between the actual touch oil pressure and the estimated touch oil pressure so as to change the oil pressure at an appropriate gradient during the stroke period. Specifically, when the actual touch oil pressure is greater than the estimated touch oil pressure, the controller 20 corrects the touch command oil pressure to be smaller in accordance with the magnitude of the touch oil pressure difference, while when the actual touch oil pressure is smaller than the estimated touch oil pressure, the controller 20 corrects the touch command oil pressure to be larger in accordance with the magnitude of the touch oil pressure difference. After this, the controller 20 terminates the oil pressure learning control.

[0060] [Action and effect] Next, the operation and effects of the hybrid vehicle control method and control system according to the embodiment of the present invention will be described.

[0061] In this embodiment, when performing hydraulic control to apply hydraulic pressure to first clutch CL1 to transition first clutch CL1 from a released state to an engaged state, controller 20 calculates an estimated hydraulic pressure applied to first clutch CL1 and, based on this estimated hydraulic pressure, calculates an estimated timing (estimated touch timing) at which first clutch CL1 will start transitioning from a released state to an engaged state. Controller 20 also calculates a variance value of actual hydraulic pressure detected by hydraulic pressure sensor SN4 and, based on this variance value, calculates an actual timing (actual touch timing) at which first clutch CL1 will start transitioning from a released state to an engaged state. Controller 20 performs learning to correct the hydraulic pressure applied to first clutch CL1 during hydraulic control based on the difference between the estimated timing and the actual timing (touch timing difference). In other words, in this embodiment, controller 20 does not use the actual hydraulic pressure detected by hydraulic pressure sensor SN4 itself, but calculates the actual timing based on the variance value of the actual hydraulic pressure, and performs learning to correct the command hydraulic pressure of first clutch CL1 based on this actual timing.

[0062] Here, the actual oil pressure detected by the oil pressure sensor SN4 contains relatively large noise due to the influence of oil pressure vibrations as described above, which tends to increase the variance of the actual oil pressure during hydraulic control. However, at the timing when the first clutch CL1 starts to transition from the released state to the engaged state, the influence of oil pressure vibrations decreases, temporarily suppressing the variance of the actual oil pressure and decreasing the variance. Therefore, by taking into account this tendency in the variance of the actual oil pressure, the actual timing when the first clutch CL1 starts to transition from the released state to the engaged state can be accurately determined from this variance. Therefore, according to this embodiment, the oil pressure applied to the first clutch CL1 during hydraulic control can be accurately learned based on this actual timing and estimated timing so as to achieve the target stroke time. As a result, by transitioning the first clutch CL1 from the released state to the engaged state using hydraulic control learned in this manner, it is possible to reliably suppress the shock occurring in the first clutch CL1 during the transition and to suppress the discomfort felt by the driver due to a delay in the transition.

[0063] In this embodiment, the controller 20 determines the actual timing as the timing at which an inflection point appears in the variance value of the actual oil pressure, thereby enabling the actual timing to be determined with higher accuracy.

[0064] In this embodiment, the controller 20 shortens the pre-charge time for filling the oil into the hydraulic chamber 15a of the first clutch CL1 when the actual timing is earlier than the estimated timing, and lengthens the pre-charge time when the actual timing is later than the estimated timing. This makes it possible to accurately achieve the target stroke time by learning the hydraulic pressure applied to the first clutch CL1.

[0065] Furthermore, in this embodiment, the controller 20 corrects the hydraulic pressure applied when the first clutch CL1 starts to transition from a released state to an engaged state during hydraulic control, based on the difference between the estimated hydraulic pressure (estimated touch hydraulic pressure) at the estimated timing and the actual hydraulic pressure (actual touch hydraulic pressure) at the actual timing. This makes it possible to change the hydraulic pressure at an appropriate gradient during the period (stroke period) until the first clutch CL1, which is in a released state, starts to transition to an engaged state. This makes it possible to reliably suppress shocks that occur in the first clutch CL1 during the transition and to suppress any discomfort felt by the driver due to a delay in the transition.

[0066] Furthermore, in this embodiment, when performing hydraulic control, the controller 20 prohibits learning in at least one of the following cases: (i) when there is a predetermined acceleration request from the driver, (ii) when the hydraulic pressure (line pressure) in the hydraulic system that supplies hydraulic pressure to the first clutch CL1 is equal to or greater than a predetermined value, (iii) when the hybrid vehicle 1 is started, or (iv) when the engine 2 is started by the starter 3 because the SOC of the battery 5 is less than a predetermined value. This makes it possible to prohibit learning in situations where the behavior of the first clutch CL1 is unlikely to be stable, thereby ensuring the accuracy of learning.

[0067] Furthermore, in this embodiment, the controller 20 terminates learning when the number of times that the first clutch CL1 has been transitioned from the released state to the engaged state by hydraulic control reaches a predetermined value or when hydraulic control that causes the difference between the estimated timing and the actual timing to be less than a predetermined value has been performed a predetermined number of times in succession. By terminating learning when it is no longer necessary, it is possible to reduce the load of processing and control related to learning thereafter.

[0068] In this embodiment, the controller 20 resumes learning when the total distance traveled by the hybrid vehicle 1 since the end of learning reaches or exceeds a predetermined value. This allows the resumed learning to accurately correct any deviation that occurs after the end of learning. [Explanation of symbols]

[0069] 1 Hybrid vehicle 2 engines 3 Starter 4 motors 5 Battery 6-speed 8 Power transmission system 12 wheels 15a Hydraulic chamber 15c clutch piston 15d First clutch plate 15e Second clutch plate 15f solenoid 20 Controller (control device) CL1 First clutch (friction engagement element) CL2 Second clutch SN4 Oil Pressure Sensor

Claims

1. A control method for a hybrid vehicle having an engine, a motor, a frictional engagement element that can be in one of two states: an engaged state in which torque is transmitted between the engine and the motor, and a disengaged state in which torque transmission between the engine and the motor is interrupted, depending on an applied hydraulic pressure, and a hydraulic pressure sensor that detects an actual hydraulic pressure applied to the frictional engagement element, a first step of performing hydraulic control to apply hydraulic pressure to the frictional engagement element so as to transition the frictional engagement element from the released state to the engaged state; a second step of calculating an estimated oil pressure applied to the frictional engagement element during the oil pressure control; a third step of determining a variance value of the actual oil pressure detected by the oil pressure sensor during the oil pressure control; a fourth step of determining an estimated timing at which the frictional engagement element will start to transition from the released state to the engaged state based on the estimated hydraulic pressure; a fifth step of determining an actual timing at which the frictional engagement element starts to transition from the released state to the engaged state based on the variance value; a sixth step of performing learning to correct the hydraulic pressure applied to the frictional engagement element during the hydraulic control based on the difference between the estimated timing and the actual timing; A control method for a hybrid vehicle, comprising:

2. 2. The method for controlling a hybrid vehicle according to claim 1, wherein in the fifth step, a timing at which an inflection point appears in the variance value is determined as the actual timing.

3. the frictional engagement element has a hydraulic chamber into which oil is introduced, and is configured to assume either the engaged state or the released state in response to the oil introduced into the hydraulic chamber; In the first step, at the start of the hydraulic control, a predetermined hydraulic pressure is applied to the frictional engagement element for a predetermined time so as to fill the hydraulic chamber of the frictional engagement element with oil; In the sixth step, when the actual timing is earlier than the estimated timing, a correction is made to shorten the predetermined time for applying the predetermined hydraulic pressure during the hydraulic control, and when the actual timing is later than the estimated timing, a correction is made to lengthen the predetermined time for applying the predetermined hydraulic pressure during the hydraulic control. The method for controlling a hybrid vehicle according to claim 1 .

4. The fourth step further includes acquiring the estimated oil pressure applied to the frictional engagement element at the estimated timing; In the fifth step, the actual oil pressure detected by the oil pressure sensor at the actual timing is further acquired, The sixth step further comprises correcting the hydraulic pressure applied to the frictional engagement element when the frictional engagement element starts to transition from the released state to the engaged state during the hydraulic control, based on the difference between the estimated hydraulic pressure obtained in the fourth step and the actual hydraulic pressure obtained in the fifth step. The method for controlling a hybrid vehicle according to claim 1 .

5. 2. The hybrid vehicle control method of claim 1, wherein the sixth step prohibits the learning when the hydraulic control is performed in at least one of the following cases: (i) a driver requests a predetermined acceleration; (ii) the hydraulic pressure in a hydraulic system that supplies hydraulic pressure to at least the frictional engagement elements is equal to or greater than a predetermined value; (iii) the hybrid vehicle is started; or (iv) the engine is started by a starter separate from the motor because the charge amount of a battery that supplies power to the motor is less than a predetermined value.

6. 2. The hybrid vehicle control method according to claim 1, wherein in the sixth step, the learning is terminated when the number of times that the frictional engagement element has been transitioned from the released state to the engaged state by the hydraulic control is equal to or greater than a predetermined value, or when the hydraulic control in which the difference between the estimated timing and the actual timing is less than a predetermined value has been performed a predetermined number of times in succession.

7. 7. The hybrid vehicle control method according to claim 6, wherein in the sixth step, the learning is resumed when a total travel distance of the hybrid vehicle since the learning was terminated reaches or exceeds a predetermined value.

8. The frictional engagement element includes a hydraulic chamber into which oil is introduced, a clutch piston that operates in response to the oil supplied to the hydraulic chamber, and a clutch plate that comes into contact with the clutch piston, and is configured to be in the engaged state when the clutch piston is in contact with the clutch plate, and to be in the released state when the clutch piston is separated from the clutch plate, In the fourth step and the fifth step, timings at which the separated clutch piston contacts the clutch plate during the hydraulic control are determined as the estimated timing and the actual timing, respectively. The method for controlling a hybrid vehicle according to claim 1 .

9. A control system for a hybrid vehicle, comprising: an engine and a motor; a frictional engagement element that can be in either an engaged state in which torque is transmitted between the engine and the motor, or a released state in which torque transmission between the engine and the motor is interrupted, depending on the applied hydraulic pressure; a hydraulic pressure sensor for detecting an actual hydraulic pressure applied to the frictional engagement element; a control device configured to control the engine, the motor, and the frictional engagement element; The control device performing hydraulic control to apply hydraulic pressure to the frictional engagement element so as to transition the frictional engagement element from the released state to the engaged state; During the hydraulic control, an estimated hydraulic pressure applied to the frictional engagement element is calculated; During the hydraulic control, a variance value of the actual hydraulic pressure detected by the hydraulic pressure sensor is calculated. determining an estimated timing at which the frictional engagement element will start to transition from the released state to the engaged state based on the estimated hydraulic pressure; determining an actual timing at which the frictional engagement element starts to transition from the released state to the engaged state based on the variance value; learning is performed to correct the hydraulic pressure applied to the frictional engagement element during the hydraulic control based on the difference between the estimated timing and the actual timing; A control system for a hybrid vehicle, characterized in that the control system is configured as follows.

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