SYNCHRONIZATION CONTROL METHOD FOR ENGINE CLUTCH LOCK-UP IN HYBRID ELECTRIC VEHICLE

Abstract

A control method for synchronization of engine speed and motor speed when lock-up of an engine clutch of a hybrid electric vehicle is carried out. The synchronization control method includes increasing engine speed by controlling toward motor speed as a target speed when synchronization starts according to a demand for the lock-up of an engine clutch during travelling of the hybrid electric vehicle using motor driving power while the engine clutch is opened; estimating an engine speed at a time of speed change when the motor speed increases and reaches a preset changed speed at which a speed change is carried out; calculating motor speed after the speed change using the changed speed and information on gear ratio before and after the changed speed when the estimated engine speed at the time of speed change is less than the preset changed speed; and controlling the engine speed by changing the target speed into the motor speed after the speed change.

Description

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit under 35 U.S.C. §119(a) of Korean Patent Application No. 10-201 4-0147856 filed on Oct. 29, 2014, the entire contents of which are incorporated herein by reference.

BACKGROUND

(a) Technical Field

The present invention relates generally to a control method for a hybrid electric vehicle, and more particularly to synchronization of engine speed (RPM) and motor speed (RPM) upon lock-up of an engine clutch of a hybrid electric vehicle.

(b) Background Art

In general, a hybrid electric vehicle travels using an engine and an electric motor as a driving power source. Since, in the hybrid electric vehicle, fossil fuel energy and electric energy are used together for travel, the hybrid electric vehicle is environmentally friendly to achieve reduction of exhaust gas and enhancement of fuel ratio.

FIG. 1 schematically shows a hybrid powertrain including an engine and a motor as a power source for travel in a hybrid electric vehicle, and an engine clutch and a transmission to transmit a driving force.

As shown in FIG. 1, the powertrain including a driving system such as an engine, a motor, and a power transmission in a hybrid electric vehicle, includes an engine 1 and a motor 3 disposed serially as a power source for travel, an engine clutch 2 disposed between the engine 1 and the driving motor 3 to transmit or cut off driving power, an inverter 5 for driving and controlling the motor 3, a transmission 4 for changing the driving power of the engine 1 and the motor 3 so as to transmit the changed driving power to a driving shaft, and a hybrid starter and generator (HSG) 7 connected to the engine 1 for transmission of the driving power to start the engine and generate electricity from the driving power.

A battery 6 as a power source (an electric power source) of the motor 3 is connected to the motor through the inverter 5 to charge and discharge.

The engine clutch 2 transmits and cuts off selectively the driving power between the engine 1 and the motor 3 by locking up and opening as needed using hydraulic pressure.

The transmission 4 is connected to an output side of the motor 3 to transmit driving power of the engine and the motor to a driving shaft of the vehicle. Transmission 4 may be a manual transmission (MT), an automatic transmission (AT), an automatic manual transmission (AMT), or a dual clutch transmission (DCT).

Moreover, the inverter 5 changes direct current of the battery 6 to three-phase alternating current, and feeds the changed current to the motor to drive the motor 3.

In a general hybrid electric vehicle, a travel mode is selected according to travelling conditions, such as a pure electric vehicle mode using only the driving power of the motor 3, that is, electric vehicle (EV) mode and a hybrid electric vehicle (HEV) mode using driving powers of the engine 1 and the driving motor 3 together.

Moreover, upon braking of a vehicle or coasting under inertia, a regeneration mode in which the braking energy and the inertia energy are collected by generation of the motor to charge the battery 6 is performed.

In addition, the HSG 7 serves as a generator due to the driving power of the engine 1 or due to a driving power transmitted through the engine under regeneration conditions to charge the battery 6.

Meanwhile, in the HEV mode, the engine clutch is locked up so that the vehicle travels with the sum of output torque from the engine and the motor, while in the EV mode, the engine clutch is opened so that the vehicle travels only with the output torque of the motor.

In addition, when the engine clutch is locked up in during transition from the EV mode to the HEV mode, synchronization between speeds at both ends of the engine clutch after cranking of the engine by the HSG, that is, synchronization between the engine speed (RPM) and the motor speed (RPM) is made. The engine clutch is locked up, and transition of the travel mode from the HEV mode is carried out.

As such, for easy lock-up of the engine clutch in the transition from the EV mode to the HEV mode, speed at an input end of the engine clutch connected to the output side of the engine must be synchronized with speed at the output end of the engine clutch connected to the input side of the motor. That is, the engine speed must be synchronized with the motor speed. This is necessary to prevent slip of the engine clutch speed difference between the engine and the motor when the engine clutch is locked up.

Synchronization is necessary to lock-up and separate between the engine and the motor in the hybrid system. A synchronization control strategy in the transition is important because power performance and fuel ratio of a vehicle are influenced by synchronization.

FIG. 2 is a view showing a problem of an existing synchronization control method, and shows motor speed at early starting of a vehicle and engine speed (RPM) controlled during the synchronization for the lock-up of the engine clutch.

Moreover, FIG. 2 shows an accelerator pedal position sensor (APS) signal to detect a position of the accelerator pedal when a driver steps on the accelerator pedal at early starting.

As shown, when a driver steps the accelerator pedal, early starting of the vehicle is performed by the driving power of a motor (an output torque) in a state in which the engine clutch is released (opened). After that, needed is a process of locking up the engine clutch to transmit the driving power (output torque) of an engine when demand torque of a vehicle increases as the driver steps on the accelerator pedal (lock-up of the engine clutch for switch into the HEV mode).

Additionally needed is a process of synchronizing engine speed with motor speed for lock-up of the engine clutch, and at this time, engine speed follows motor speed as a target speed.

Moreover, when speed change is carried out as the engine follows the motor speed (a target speed), that is as engine speed is controlled to follow motor speed, the motor speed decreases sharply. At this time, since the target speed followed by the engine has sharply changed, performance thereafter is inferior.

That is, a zone arises where the engine speed increases toward the target speed (the motor speed) and sharply decreases (See a portion ‘A’ in FIG. 2). At this time, since response of the engine is not fast, delay of synchronization occurs.

When delay of synchronization occurs as described above, clutch lock-up and mode transition are delayed so that transmission of power is also late. As a result, power performance of a vehicle deteriorates.

SUMMARY OF THE DISCLOSURE

Accordingly, the present invention has been made in an effort to solve the above-mentioned problems, and it is an object of the present invention to provide a synchronization control method of a hybrid electric vehicle for improving synchronization delay that may occur during the speed change, lock-up and mode transition delay of an engine clutch, and poor vehicle driving power performance due to the delays in locking-up the engine clutch to transmit the engine driving power (in transition to a HEV mode) when demand torque increases upon vehicle acceleration such as upon early starting while a vehicle travels using the driving power of the motor and the engine clutch is separated.

In accordance with an aspect of the present invention, there is provided a synchronization control method for lock-up of an engine clutch of a hybrid electric vehicle, including: increasing engine speed by controlling motor speed toward a target speed when synchronization starts according to a demand for lock-up of an engine clutch during travelling of the hybrid electric vehicle using the driving power of a motor while the engine clutch is opened; estimating engine speed at a time of speed change when the motor speed increases and reaches a preset speed where a speed change is carried out; calculating motor speed after the speed change using the changed speed and information on gear ratio before and after the changed speed when the estimated engine speed at the time of speed change is less than the preset changed speed; and controlling the engine speed by changing the target speed to be equal to the motor speed after the speed change.

According to the synchronization control method of the present invention, change of motor speed after speed change is estimated during synchronization for the clutch lock-up, and the engine speed is controlled by controlling the motor speed after the estimated speed change. Fast lock-up of the engine clutch is carried out so that power performance of the vehicle thereby may be achieved.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now be described in detail with reference to certain exemplary embodiments thereof illustrated the accompanying drawings which are given hereinbelow by way of illustration only, and thus are not limitative of the present invention, and wherein:

FIG. 1 is a schematic view showing a powertrain of a hybrid electric vehicle;

FIG. 2 is a view showing a problem occurring during an existing synchronization control;

FIG. 3 is a flowchart illustrating a synchronization control according to an embodiment of the present invention; and

FIG. 4 is a view showing a state in which an engine speed is controlled during synchronization control according to the embodiment of the present invention.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various preferred features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Hereinafter, the present invention will be described in detail so that those skilled in the art to which the present invention pertains can easily carry out the present invention.

The present invention provides a synchronization control method for a hybrid electric vehicle by improving delay of synchronization that may occur during speed change, delay of lock-up and mode transition of an engine clutch, and inferior vehicle driving power performance due to delays in locking-up the engine clutch to transmit the engine driving power (upon transition to a HEV mode) when demand torque increases upon accelerated vehicle speed as may occur during early starting while a vehicle travels using the driving power of the motor with the engine clutch separated.

To this end, change of a target speed (a motor speed) is estimated in determination of whether an engine clutch is locked up, and the target speed of the engine is changed to reduce synchronization following time so that delay of clutch lock-up is improved and cut-off of a driving power is removed; thereby, power performance is enhanced.

FIG. 3 is a flowchart illustrating a synchronization control according to an embodiment of the present invention. FIG. 4 is a view showing a state when engine speed is controlled during synchronization control according to the embodiment of the present invention.

First, when it is necessary to accelerate a vehicle by a driver stepping on an accelerator pedal such as during an early starting as a vehicle travels with motor driving power with an engine clutch opened. If a demand torque, by which a driver steps on an accelerator pedal, is greater than the torque of a motor, a controller determines that lock-up of the engine clutch is needed, outputs an engine clutch lock-up command (S11 and S12) and starts synchronization between engine speed and motor speed.

In this case, the controller increases the engine speed assigning the motor speed as a target speed after starting of synchronization and calculates an estimated engine speed (RPM) at a speed changing point where the motor speed increases and reaches at a speed (RPM) set to change speed.

In order to calculate the estimated engine speed at the speed changing point, first, the rate of increase of motor speed at the time of determining demand of clutch lock-up and of starting of synchronization (speed change per unit time, that is, speed slope (RPM/s)), is calculated.

In other words, the rate of increase of motor speed that occurs by the stepping on an accelerator pedal by a driver is obtained. The rate of increase of motor speed may be calculated using motor speed (RPM) at two times as motor speed increases.

In more detail, as shown in FIG. 4, the rate of increase may be obtained from motor speed RPM_A at time A before synchronization, motor speed RPM_B after synchronization, and time between time A and time B. An equation for the rate of increase of motor speed is following formula (1):

increase rate of a motor speed=(RPM_B−RPM_A)/Δt  (1),

where time B may be time of determining demand for clutch lock-up, that is, time of starting synchronization.

In addition, an estimate time s when the motor speed RPM_B at time of determining demand for clutch lock-up reaches the preset change speed according to the rate of increase of a motor speed RPM/s, is described by the following formula (2):

time for reaching at change speed=(change speed−RPM_B)/increase rate of a motor speed  (2),

where RPM_B is a motor speed RPM at time B, that is, motor speed at a time of synchronization.

At this time, the estimated engine speed RPM at the speed change is estimated by calculating engine speed at a time reaching at the speed change (time of change in speed) with consideration of response characteristics RPM/s (engine properties) using the following formula (3),

engine speed at speed change=response rate of an engine×time of reaching changed speed  (3).

When engine speed at the speed change, as shown in FIG. 3, occurs, the engine speed at the speed change is compared with the changed speed (S13).

In this case, when the engine speed at the speed change is less than the changed speed, the engine speed changes during the increase and the target speed (the motor speed) is sharply changed. Thus, when engine speed is controlled toward the motor speed as a target speed, it may be determined that a delay of synchronization occurs as in an existing synchronization control method.

When determining synchronization delay, the target speed for the control of the engine speed is altered using the changed speed and information of gear ratio before and after the speed change.

At this time, the motor speed decreased after the speed change, that is, the motor speed RPM at a target change gear step, is calculated. Here, the calculated motor speed is the speed decreasing when speed change is carried out. After this speed change, the motor speed is modified to the target speed to control engine speed (S14).

In this case, the motor speed RPM after the speed change may be calculated by the following formula (4), and engine speed is controlled with the calculated motor speed after the speed change to synchronize the motor speed with the engine speed, and then the clutch lock-up is completed (S16).

Motor speed after speed change=changed speed×(gear ratio at target change gear step/gear ratio at current change gear step)  (4)

As such, after completion of the clutch lock-up, engine speed control is released (S17).

In the step S13, when engine speed at the speed changing time is greater than the changed speed, engine speed is controlled to follow the motor speed following the motor speed itself as a target speed (S15), and after that engine speed control is released when clutch lock-up is completed (S16 and S170).

By doing so, in the synchronization control method according to the embodiment of the present invention, change of a motor speed after speed change during synchronization for clutch lock-up is estimated and engine speed is controlled by following the estimated motor speed after the speed change so that speed of the clutch lock-up may be improved.

Although the present invention has been described in detail until now, the scope of the present invention is not limited to the description but various modifications made by those skilled in the art using the basic concept of the present invention defined by the claims also fall within the scope of the present invention.

Claims

1. A synchronization control method for lock-up of an engine clutch of a hybrid electric vehicle, the synchronization control method comprising: increasing an engine speed by controlling a motor speed toward a target speed when synchronization starts according to a demand for the lock-up of an engine clutch while the hybrid electric vehicle is travelling under motor driving power while the engine clutch is opened;estimating an engine speed at a time of speed change when the motor speed increases and reaches a preset changed speed where a speed change is carried out;calculating a motor speed after the speed change using the changed speed and information on a gear ratio before and after the changed speed when the estimated engine speed at the time of speed change is less than the preset changed speed; andcontrolling the engine speed by changing the target speed to the motor speed after the speed change.

2. The synchronization control method of claim 1, wherein the estimating an engine speed at the time of speed change comprises: calculating a rate of increase of the motor speed;calculating an estimated time when a motor speed at a starting time of synchronization reaches at the preset changed speed according to the estimated rate of increase of the motor speed; andcalculating an engine speed at the time of speed change from the calculated estimated time and a response rate of an engine.

3. The synchronization control method of claim 2, wherein the increase rate of a motor speed (RPM/s) is calculated by the following formula 1 from motor speeds (RPM/s) at two times during the increase of the motor speed, increase rate of a motor speed=(RPM_B−RPM_A)/Δt  (1)where RPM_A and RPM_B are motor speeds at times A and B and Δt is time duration between the times A and B.

4. The synchronization control method of claim 3, wherein the motor speeds at the two times are motor speeds as the motor speed increases when a driver steps on an accelerator pedal.

5. The synchronization control method of claim 3, wherein the time A is a time before the synchronization starts and the time B is a time after the synchronization starts.

6. The synchronization control method of claim 2, wherein a time of reaching a changed speed (s) as the estimated time when the motor speed reaches the preset change speed is calculated from a motor speed (RPM) at the starting time of synchronization by the following formula 2: time for reaching change speed=(change speed−motor speed at starting time of synchronization)/rate of increase of a motor speed   (2).

7. The synchronization control method of claim 2, wherein the engine speed (RPM) at a time of speed change is obtained by a formula: ‘engine speed at change speed=response rate of an engine (RPM/s)×time (s) of reaching at changed speed.’

8. The synchronization control method of claim 1, wherein the motor speed (RPM) after the speed change is obtained by the following formula: ‘motor speed after speed change=changed speed×(gear ratio at target change gear step/gear ratio at a current change gear step).

9. The synchronization control method of claim 1, wherein, when the engine speed at the speed changing time is greater than the changed speed, the engine speed is controlled to follow the motor speed designating the motor speed itself as a target speed without change of the target speed.