GB2413998A

GB2413998A - Hybrid electric vehicle flywheel assisted engine start

Info

Publication number: GB2413998A
Application number: GB0508262A
Authority: GB
Inventors: Rainer Hofmann; Georg Kruse; Uwe Hinrichsen
Original assignee: Volkswagen AG
Current assignee: Volkswagen AG
Priority date: 2004-05-13
Filing date: 2005-04-25
Publication date: 2005-11-16
Anticipated expiration: 2025-04-25
Also published as: GB0508262D0; DE102004023673B4; GB2413998B; DE102004023673A1

Abstract

In a method of controlling the drive train of a hybrid vehicle, an engine VM is started by an electric motor generator EM connected to a flywheel S. The vehicle has a parallel hybrid drive with a series arrangement of the engine VM, the motor generator EM and a drive gear G connected to a drive axle A. Controllable friction clutches K1, K2 are provided at the input and output sides of the motor generator EM respectively. In an exclusively electric mode, the flywheel S is accelerated by the motor generator EM to build up surplus momentum, the output clutch K2 being controlled in a slip mode. Closure of the motor input side clutch K1 then starts the engine VM.

Description

24 1 3998

DESCRIPTION

Method of controlling the drive train of a hybrid vehicle The invention relates to a method of controlling the drive train of a hybrid vehicle which has a parallel hybrid drive with a series arrangement of an internal combustion engine, an electric motor constructed as a motor starter generator and provided with a flywheel, and a drive gear connected on the take-off side to an axle drive, in which a first controllable friction clutch is disposed between the internal combustion engine and the electric motor and a second controllable friction clutch is disposed between the electric motor and the drive gear, wherein the internal combustion engine is started out of the exclusively electric mode by means of the electric motor.

A hybrid vehicle is understood to mean a vehicle with two different drive motors, usually an internal combustion engine and an electric motor. Depending upon the arrangement and the mechanical connection of the drive motors, a distinction is made between a series hybrid drive and a parallel hybrid drive. In a series hybrid drive the entire power of the internal combustion engine is converted by means of a generator into electrical power, and the vehicle is driven exclusively by the electric motor. flowever, this type of drive has hitherto been used only in heavy commercial vehicles and locomotives. On the other hand, in known research and mass-produced ears with hybrid drive the parallel hybrid drive is used in which both drive motors are connected to the axle drive or can be connected thereto, so that with appropriate construction and arrangement ol the drive train the vehicle in question can be driven separately only by the electric motor (exclusively electric mode), separately only by the internal combustion engine (exclusively internal combustion mode) or jointly by both drive motors (mixed mode).

Therefore motor vehicles with a parallel hybrid drive offer the possibility in electric mode of utilising the high driving torque of the electric motor for starting and for emission-free driving at low speed for example in inner city areas and possibly for conservation areas which are closed to motor vehicles with internal combustion engines or in internal combustion mode. Then outside closed locations, in particular in order to achieve a greater acceleration and a higher travelling speed, the internal combustion engine can be started and can be connected on the driving side to the drive gear and the vehicle can be changed over to exclusively internal combustion mode possibly by subsequent shutdown of the electric motor or by switching the electric motor over to generator mode.

When hybrid drives, particularly parallel hybrid drives are used in passenger cars, the principal aim is to conserve fuel for the operation of the internal combustion engine and thus to reduce the emissions of pollutants, particularly the CO2 emissions.

In such a parallel hybrid drive the possibilities for starting the internal combustion engine out of the exclusively electric mode are largely determined by the specific arrangement and installation of the two drive motors within the drive train.

Equally the construction of the electric motor or of an electric machine which carries out the function of the electric motor is important. When the internal i; 3 combustion engine is started out of the exclusively electric mode, whilst avoiding an interruption of traction what is basically sought is a starting operation which involves as little jolting as possible and thus puts little load on the components of the drive train and is more comfortable Nor the occupants of the vehicle. On the other hand the drive train should also be as light as possible and should not occupy much space.

By way of example, DE 44 44 545 A1 and DE 198 38 853 A1 each disclose a parallel hybrid drive with a series arrangement of an internal combustion engine, an electric motor and a drive gear which is connected on the takeoff side to an axle drive, in which a first friction clutch is disposed between the internal combustion engine and the electric motor and a second friction clutch is disposed between the electric motor and the drive gear. However, in these hybrid drives tor starting the internal combustion engine a separate starter motor is provided in each case by which the internal combustion engine can be started out of the exclusively electric mode with the first clutch open without disruption of the driving torque on the take-ot'J' side. However, by comparison with the use of one single electric motor constructed as a motor starter generator these hybrid drives have the disadvantage that they are heavier and require more space.

On the other hand, DE 199 01 470 A1 and DE 101 50 990 A1 each disclose a parallel hybrid drive with a series arrangement of an internal combustion engine, an electric motor constructed as a motor starter generator and a drive gear which is connected on the take-off side to an axle drive, in which a friction clutch is disposed between the internal combustion engine and the electric motor and a rigid connection is disposed between the electric motor and the drive gear. In this case the internal combustion engine is started out of the exclusively electric mode by the closure of the clutch by means of the electric motor which for this purpose has to be appropriately high-powered and thus has the disadvantage of being large and heavy. Moreover, the starting of the internal combustion engine has the disadvantageous effect of fluctuations in the torque and the speed, which the occupants of the vehicle are aware of as reducing their comfort.

In contrast to this, in the present invention the starting point is a parallel hybrid drive which has a series arrangement of an internal combustion engine, an electric motor constructed as a motor starter generator and provided with a flywheel, and a drive gear connected on the take-off side to an axle drive, and in which a first friction clutch is disposed between the internal combustion engine and the electric motor and a second friction clutch is disposed between the electric motor and the drive gear. Hitherto the internal combustion engine has been started out of the exclusively electric mode by closure of the first clutch either with the second clutch open and accepting a brief interruption of traction of the vehicle drive or with the second clutch closed and accepting fluctuations in the torque and speed on the take-off side.

Therefore the problem addressed by the present invention is to provide a method of controlling the aforementioned drive train of a hybrid vehicle by which the starting ol the internal combustion engine out of the exclusively electric mode can be carried out particularly smoothly and comfortably without an interruption of the traction of the vehicle drive.

According to the invention this object is achieved in connection with the preamble to Claim 1 in that the second clutch is controlled in slip mode, that the flywheel is then accelerated by means of the electric motor in order to build up a surplus momentum and that thereupon the internal combustion engine is started by closure of the first clutch.

Due to the slip mode of the second clutch a fixed transmittable torque is set there so that an interruption of traction and a severe drop in torque of the vehicle drive is avoided. Furthermore, due to the slip mode any fluctuations in torque and speed occurring on the driving side are partially compensated and their trend over time is smoothed and thus their effect is reduced. Then in the present case, as distinct from that described for example in DE 100 31 438 Al, where there is a rigid connection between the internal combustion engine and the electric motor and the internal combustion engine is started by means of the driving torque of the electric motor, first of all the flywheel is accelerated by means of the electric motor in order to build up a surplus momentum. For this purpose, by comparison with a direct start of the internal combustion engine, as substantially lower driving torque of the electric motor is necessary, so that with a constant driving torque of the electric motor the reduction in torque of the vehicle drive is less or for an increase in the driving torque a less powerful and thus more compact and lighter electric motor is necessary. The actual starting of the internal combustion engine then takes place basically as a so-called impulse start by closure of the first clutch, whereby the surplus momentum previously built up is reduced again, i.e. the flywheel is appropriately decelerated. Thus by the control procedure according to the invention a particularly smooth and comfortable starting operation of the internal combustion engine is provided which avoids an interruption ol the traction of the vehicle drive.

Advantageous embodiments of the method according to the invention are set out in the sub-clams 2 to 11.

When the second clutch is controlled in slip mode the procedure may be such that this clutch is opened clearly below the slip threshold and that thereupon the flywheel is accelerated by the resulting surplus momentum of the electric motor.

For this purpose the electric motor advantageously does not need to have any power reserve or any significant power reserve, since the acceleration of the flywheel in this case takes place largely due to the driving torque (surplus momentum) taken from the vehicle drive. In this case the driving torque transmitted at the second clutch to the drive gear for the vehicle drive is lowered slightly, but a complete interruption of the traction is avoided.

As an alternative to this the second clutch can also be opened just up to the slip threshold and kept at the corresponding transmittable torque. The flywheel is then accelerated by an increase in the torque delivered by the electric motor, which does however presuppose a corresponding power reserve of the electric motor.

However, in this case a lowering of the driving torque transmitted at the second clutch to the drive gear for the vehicle drive is advantageously avoided.

The starting of the internal combustion engine by the impulse start, i.e. by the reduction in the surplus momentum of the flywheel, is advantageously assisted by a brief increase in the torque delivered by the electric motor when the electric motor has a power reserve.

The surplus momentum of the flywheel which is built up by means of the electric motor is advantageously adapted as precisely as possible to the necessary starting momentum of the internal combustion engine. For this purpose a distinction is advantageously made for example between a cold start of the internal combustion engine with a higher necessary starting momentum and a warm start of the internal combustion engine with a lower necessary starting momentum. In practice this can be achieved in that the operating temperature of the internal combustion engine is ascertained, the ascertained operating temperature is compared with a previously fixed threshold temperature, in the event that the operating temperature is below the threshold temperature a higher surplus momentum is set and when the operating temperature reaches or exceeds the threshold temperature a lower surplus momentum is set by means of the electric motor before the first clutch is closed for starting of the internal combustion engine.

When the transmission is constructed as an automatic change-speed gearbox, such as an automatic gearbox or a double clutch gearbox, in which the second clutch is formed by two clutches which can overlap in operation, the starting of the internal combustion engine is advantageously carried out utilising the slip phase of the second clutch in conjunction with a changing operation of the drive gear. For this purpose when it is intended to start the internal combustion engine an imminent changing of the drive gear can be given priority in time or an intended starting of the internal combustion engine can be delayed until an imminent changing of the drive gear. In order after starting of the internal combustion engine to be able to utilise a higher driving torque of the internal combustion engine which is available at a higher engine speed, it is particularly advantageous if the starting of the internal combustion engine is combined with a change-back operation of the drive gear.

Further details of the invention are apparent from the following details description and the appended drawing which serves by way of example for explanation of the invention.

In the drawing: Figure 1 shows a drive train of a hybrid vehicle in schematic form.

The drive train of a hybrid vehicle which is the basis of the method according to the invention has in the power flow direction a series arrangement of an internal combustion engine VM, an electric motor EM which is constructed as a motor starter generator and is provided with a flywheel S. and a drive gear G connected on the take-oft side to an axle drive A. A first controllable friction clutch K1 which is provided as starter clutch is disposed between the internal combustion engine VM and the electric motor EM. A second controllable friction clutch Ks which serves primarily as a starting and gear-change clutch is located between the electric motor EM and the drive gear G. in exclusively electric mode, i.e. with the first cutch K I open, the internal combustion engine VM stationary, the second clutch K2 closed and power flowing only from the electric motor EM into the drive gear G or into the axle drive A, the internal combustion engine should be started under certain operating conditions, e.g. when the driver wishes for greater acceleration, when a previously fixed threshold speed is exceeded, or when an electric energy store is almost empty. For this purpose it is provided according to the invention that first of all the second clutch is brought into slip mode or in the case of a continuous-slip clutch it is kept in slip mode. In this case in a first variant of the method the second clutch can be opened so far that by comparison with the previously transmitted torque a lower driving torque is now transmitted to the drive gear G and to the axle drive A. As a result in the electric motor EM a surplus momentum is produced by which the flywheel S firmly connected to the rotor of the electric motor EM is accelerated. As a result a surplus momentum J5*An is built up on the flywheel S (with Js = mass moment of inertia of the flywheel S. An = n2 - n', nil = initial speed of EM and S at the beginning of the acceleration, n2 = final speed of EM and S at the end of the acceleration), which is subsequently reduced again by closure of the first clutch Kl and used in a so-called impulse start for starting the internal combustion engine VM. In a second variant of the method the second clutch K2 is opened just up to the slip threshold, i.e. the torque which can be transmitted by the clutch K2 corresponds approximately to the previously transmitting driving torque. In this way by comparison with the first variant of the method a lowering of the driving torque which drives the vehicle is advantageously avoided. However, in this casein order to build up a surplus momentum on the flywheel S. i.e. in order to increase the speed of the electric motor EM, an increase in the generated torque of the electric motor EM is necessary, and for this purpose the electric motor must have an adequate power reserve and must be controlled appropriately.

By comparison with known methods of controlling a corresponding starting operation ol the internal combustion engine VM, according to the invention both variants of the method in each case avoid an interruption of the flow of power to the axle drive A and at least a more severe drop in the driving torque transmitted on the second clutch K2. Due to the slipping operation of the second clutch K2, any fluctuations in the speed and torque occurring on the drive side are also compensated or at least greatly reduced. Likewise when the present method is used it is possible to dispense with oversizing ol the electric motor EM, such as is necessary for starting of the internal combustion engine with exclusively electric power, and thus a saving is made on space and weight. ! 11

By the adaptation of the surplus momentum built up by the electric motor EM on the flywheel S as precisely as possible to the operating state of the internal combustion engine VM, in particular by the distinction between a cold start with a higher necessary starting momentum and a warm start with a lower necessary starting momentum, the entire starting operation can be accelerated and the driving comfort can be increased. With the same object, when a change-speed gearbox is used as drive gear G the starting of the internal combustion engine VM can advantageously be combined with a changing operation of the drive gear G utilising the appropriate slip phase of the second clutch K2, but for this purpose the drive gear G should be automatically controllable. Furthermore, a higher driving torque of the started internal combustion engine VM can be used if the preceding starting of the internal combustion engine VM takes place in conjunction with a change-back operation of the drive gear G. Flywheel should be understood here to mean both the gyrating mass of an additional component connected to the rotor of the electric motor EM including the rotating mass of rotating parts and also the gyrating mass of the rotating parts alone. This means, in particular when a larger electric motor EM is used, the rotating parts of the electric motor EM alone plus the clutch parts connected thereto can have a sufficient mass moment of inertia Is for starting the internal combustion engine so that no further gyrating mass or flywheel is required as an additional component. )

List of references A axle drive EM electric motor G drive gear Js mass moment of inertia of S Kl first clutch K2 second clutch nil initial speed of S n2 final speed of S S flywheel Tr; threshold temperature of VM JOB operating temperature of VM VM internal combustion engine An speed differential of S (An = n2 - nil)

Claims

{74 3 Boil /-

I

Claims 1. Method of controlling the drive train of a hybrid vehicle which has a parallel hybrid drive with a series arrangement of an internal combustion engine, an electric motor constructed as a motor starter generator and provided with a flywheel, and a drive gear connected on the take-off side to an axle drive, in which a first controllable friction clutch is disposed between the internal combustion engine and the electric motor and a second controllable friction clutch is disposed between the electric motor and the drive gear, wherein the internal combustion engine is started out of the exclusively electric mode by means of the electric motor, characterized in that the second clutch is controlled in slip mode, that the flywheel is then accelerated by means of the electric motor in order to build up a surplus momentum J5*An and that thereupon the internal combustion engine is started by closure of the first clutch.
2. Method according to Claim 1, characterized in that the second clutch is opened clearly below the slip threshold and that the flywheel is accelerated by the resulting surplus momentum of the electric motor.
3. Method according to Claim], characterized in that the second clutch is also opened just up to the slip threshold and kept at the corresponding transmittable torque and that the flywheel is then accelerated by an increase in the torque delivered by the electric motor. At)
4. Method according to any one of Claims 1 to 3, characterized in that the starting of the internal combustion engine is assisted by an increase in the torque delivered by the electric motor.
5. Method according to any one Claims 2 lo 4, characterized in that the surplus momentum J5*An of the flywheel which is built up by means of the electric motor is adapted to the necessary starting momentum of the internal combustion engine.
6. Method according to Claim 5, characterized in that a distinction is made between a cold start of the internal combustion engine with a higher necessary starting momentum and a warm start of the internal combustion engine with a lower necessary starting momentum.
7. Method according to Claim 6, characterized in that an operating temperature TB of the internal combustion engine is ascertained, that the ascertained operating temperature TB is compared with a previously fixed threshold temperature TG. and that in the event that the operating temperature TH is below the threshold temperature TG a higher surplus momentum Js*An is set and when the operating temperature al B reaches or exceeds the threshold temperature TO a lower surplus momentum Js*An is set by means of the electric motor before the first clutch is closed for starting of the internal combustion engine.
8. Method according to any one of Claims I to 7, characterised in that when the drive gear is constructed as an automatic change-speed gearbox the starting of the internal combustion engine is advantageously carried out utilising the slip phase of the second clutch in conjunction with a changing operation of the drive gear.
9. Method according to Claim 8, characterised in that when it is intended to start the internal combustion engine an imminent changing of the drive gear can be given priority in time.
10. Method according to Claim 8, characterized in that an intended starting of the internal combustion engine can be delayed until an imminent changing of the drive gear.

Method according to any one of Claims 8 to 10, characterized in that for subsequent utilization of a higher driving torque of the internal combustion engine the starting of the internal combustion engine is combined with a change-back operation of the drive gear.