METHOD FOR STARTING AN INTERNAL COMBUSTION ENGINE OF A HYBRID VEHICLE

Abstract

The invention relates to a method for starting an internal combustion engine of a hybrid vehicle, where an electric motor of the hybrid vehicle is accelerated to a predetermined engine speed and a hybrid disconnect clutch, which is arranged between the internal combustion engine and the electric motor, is moved in the closing direction depending on the set-point clutch torque. In a method which safeguards a high reproducibility of the restart operation, the set-point clutch torque for restarting the internal combustion engine is determined depending on an engine switch-off position of the internal combustion engine in a first phase in which the internal combustion engine is not running.

Claims

1. A method for starting an internal combustion engine in a hybrid vehicle, comprising accelerating an electric motor of the hybrid vehicle to a predetermined motor speed and moving a hybrid disconnect clutch arranged between the internal combustion engine and the electric motor depending on a set-point clutch torque to be transmitted in a closing direction, and in a first phase in which the internal combustion engine is switched off, determining the set-point clutch torque for restarting the internal combustion engine depending on a motor switch-off position of the internal combustion engine.

2. The method according to claim 1, wherein the set-point clutch torque at least one of includes friction or compression torque due to the motor switch-off position of the internal combustion engine.

3. The method according to claim 2, wherein the friction and compression torques are influenced by temperature.

4. The method according to claim 1, further comprising superimposing the set-point clutch torque by a portion in the first phase, which considers an inertia and a set-point acceleration of the internal combustion engine.

5. The method according to claim 1, further comprising in a second phase, in which the internal combustion engine starts in motion, determining the set-point clutch torque based on a target acceleration of the internal combustion engine deducted from a dynamic torque.

6. The method according to claim 5, wherein the set-point clutch torque is controlled depending on the target acceleration of the internal combustion engine.

7. The method according to claim 5, further comprising in a third phase in which a rotational speed difference of the internal combustion engine and the electric motor is below a predetermined speed threshold, completely closing the hybrid disconnect clutch.

8. The method according to claim 7, wherein a ramp function or a slip-control is used for a complete closing of the hybrid disconnect clutch.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The invention allows numerous embodiments. One of them shall be explained in greater detail with the figures shown in the drawing.

[0013] Shown are:

[0014] FIG. 1 an exemplary embodiment of a drivetrain of a hybrid vehicle,

[0015] FIG. 2 compression torques of the internal combustion engine as a function of the crankshaft angle, and

[0016] FIG. 3 an exemplary embodiment of the method according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0017] FIG. 1 shows an exemplary embodiment of a drivetrain 1 of a hybrid vehicle with an internal combustion engine 2, which comprises a crankshaft 3. An electric motor 4 comprises a rotor 5, with a hybrid disconnect clutch 6 being arranged between the electric motor 4 and the internal combustion engine 2. Another clutch, embodied in the exemplary embodiment shown as a torque converter 7, which additionally may include a converter lockup clutch, is arranged between the transmission 8 and the electric motor 4. The transmission 8 transfers the drive torque generated by the drive units (internal combustion engine 2 and electric motor 4) individually or jointly to the drive wheels 9. If the internal combustion engine 2 exclusively transmits torque when the hybrid disconnect clutch 6 is closed, the electric motor 4 is set current-less and the rotor 5 serves here as a flywheel. Upon electrification of the electric motor 4 and when the hybrid disconnect clutch 6 is closed both drive units 2, 4 transmit torque to the transmission 8. If only the electric motor 4 shall act as the drive, the hybrid disconnect clutch 6 is opened. If braking is to occur with the electric motor 4, the hybrid disconnect clutch 4 is opened and the electric motor 4 operates as a generator. Additionally, in order to generate a greater braking effect, the drag torque of the internal combustion engine 2 is used by closing the hybrid disconnect clutch 6.

[0018] In hybrid vehicles it frequently occurs that after electric driving operation, with the hybrid vehicle being in operation, the switched-off internal combustion engine 2 shall be started in order to this way perform a hybrid drive operation. FIG. 3 shows a diagram illustrating the process of restarting the internal combustion engine 2. In such a restarting process the internal combustion engine 2 is initially switched off, and the hybrid vehicle is operated by an electric motor 4, while the hybrid disconnect clutch 6 is opened. The restarting phase of the internal combustion engine 2 is divided into three phases. In the first phase the electric motor 4 is accelerated to a predetermined engine speed. In order to ensure a secure start of the internal combustion engine 2, starting at zero, an appropriate set-point clutch torque must be generated. The set-point clutch torque to be generated depends here essentially on two components. The first component MkuppPart1 comprises specific features of the internal combustion engine 2, such as the friction and compression behavior. This friction and compression behavior is discernible from the given motor switch-off position of the internal combustion engine 2 when restarting the internal combustion engine 2. The switch-off position of the internal combustion engine 2 is here for example referenced to an absolute angular position, i.e. a clutch torque portion is defined which is dependent on the switch-off position of the motor. The background is here that the closer the piston to the upper dead end, the higher the required clutch torque in order to overcome the compression.

[0019] The various motor switch-off positions of the pistons of the internal combustion engine 2 are divided over the crankshaft angle φ into four conditions of the internal combustion engine. FIG. 2 shows the compression torques of the internal combustion engine depending on the crankshaft angle φ. The conditions of the internal combustion engine 2 are here defined as follows: [0020] Suction phase: 0<φ<180° [0021] Compression phase: 180°<φ<360° [0022] Combustion/expansion phase: 360°<φ<540° [0023] Exhaust phase: 540° C.<φ<720°

[0024] Depending at what angular crankshaft position the internal combustion engine 2 is set at the time of restart, different friction and compression forces develop, which must be overcome by the set-point clutch torque. Furthermore it is considered that the friction and compression torques are subject to temperature influences.

MkuppPart1=Mstart_eng (φ, temp)

[0025] In this phase 1, another second component MkuppPart2 of the set-point clutch torque is considered, which can be called dynamic torque. This dynamic torque determines the dynamic acceleration of the internal combustion engine 2, with this dynamic torque perhaps also being subject to temperature influences and typically being determined based on the following equation

Mkupp2=Jmot*wTgt,

[0026] with

[0027] Jmot=weight inertia of the internal combustion engine

[0028] wTgt=target acceleration of the internal combustion engine in rad/sec.

[0029] It can be deducted therefrom that in the first phase the control torque Mkupp of the hybrid disconnect clutch 6 is determined as

Mkupp=Mstart_eng (φ, temp)+Jmot*wTgt,

[0030] in which the dynamic torque being superimposed the set-point clutch torque and being provided by the electric motor 3.

[0031] In the second phase of the restarting process the internal combustion engine 2 begins to rotate. Here the set-point clutch torque is limited to the dynamic torque, while the friction and compression torque is reduced or completely set to zero. The set-point clutch torque includes only portions to adjust the desired target acceleration of the internal combustion engine 2.

[0032] As discernible from FIG. 3, the torque N_Emot of the electric motor 4 increases in phase 2 in order to entrain the internal combustion engine 2. The adjustment of the target acceleration of the internal combustion engine 2 can here be supported by the control unit. It must be observed here that the overall clutch torque towards the end of phase 2, when the torque of the internal combustion engine N_ICE approaches that of the electric motor N_Emot, is as low as possible in order to avoid unnecessary coupling pressures of the hybrid vehicle.

[0033] The abbreviations required in FIG. 3 are as follows: [0034] Trq_Cl_Tgt Set-point clutch torque [0035] N_Emot Motor speed of the electric motor [0036] N_ICE Motor speed of the internal combustion engine [0037] Trq_Start_ICE Start set-point clutch torque of the internal combustion engine

[0038] At the end of the restarting process in phase 3, with the rotational speed difference between the internal combustion engine N_ICE and the electric motor N_Emot is below a predetermined threshold, the hybrid disconnect clutch 6 is completely closed.

[0039] This can be realized, on the one hand in that a ramp function is used in the control system. Alternatively a slip-control is possible, as well.

[0040] Based on the suggested solution the restart functionality is optimized in a hybrid vehicle by rendering in the first phase of the restart process the control of the internal combustion engine 2 dependent on its switch-off position. This is advantageous in that this way a secure restart is possible and simultaneously the subsequent acceleration behavior is clearly more reproducible.

LIST OF REFERENCE CHARACTERS

[0041] 1 drivetrain

[0042] 2 internal combustion engine

[0043] 3 crankshaft

[0044] 4 electric motor

[0045] 5 rotor

[0046] 6 hybrid disconnect clutch

[0047] 7 torque converter

[0048] 8 transmission

[0049] 9 drive wheels