BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention relates to a method to brake a vehicle with a hybrid powertrain according to the preamble of claim 1 . The invention also relates to a hybrid power- train according to the preamble of claim 1 1 and a vehicle according to the preamble of claim 12, comprising such a hybrid powertrain.

A hybrid-driven vehicle is operated by a combustion engine and an electric machine, which interact to deliver the desired output and to, among others, achieve a good fuel economy in the vehicle. The electric machine may also be used to brake the vehicle, wherein the electric machine functions as a generator and thus returns energy to an electric accumulator in the vehicle. The vehicle is also equipped with a transmission to distribute power from the combustion engine and the electric machine, and to achieve a suitable gearing for the vehicle's driving wheels. In the vehicle's powertrain there are gaps between the components interacting in the transmission, arising e.g. as a result of backlash among cogwheels in engagement. There is also a torsion spring effect in the rotatable shafts in the powertrain. The gap and the torsion spring effect become obviously noticeable to passengers in the vehicle, when the electric machine is used to decelerate the vehicle when braking and when the electric machine is used as a drive engine after the braking has ceased.

At braking and subsequent cessation of braking, the electric machine's torque will, because of said gaps and resilient shafts, achieve a pulse and shock wave in the hybrid powertrain, which will be perceived as a disturbance by the driver and pas- sengers in the vehicle. The pulse and the shock wave may also elicit a swinging in the hybrid powertrain because of the torsion spring effect of the shafts. This swinging will also be perceived as uncomfortable by the vehicle's driver and passengers. Document DE10201 1 101487 shows a vehicle equipped with a hybrid powertrain, which comprises a combustion engine and an electric machine. At braking, the electric machine is used as a generator to brake the vehicle. SUMMARY OF THE INVENTION

Despite prior art solutions there is a need to further develop a hybrid powertrain, which provides good driving comfort in the vehicle during braking and when braking ceases.

The objective of the present invention is thus to provide a hybrid powertrain, which provides good driving comfort in the vehicle during braking and when braking ceases.

This objective is achieved with a method for braking a vehicle with a hybrid power- train of the type specified above, which is characterised by the features specified in claim 1 .

Such a hybrid powertrain will provide good driving comfort for the driver and passengers during braking and when braking ceases. By controlling the electric machine with a predetermined torque limitation during braking and when braking ceases, the gap and torsion spring effects in the powertrain may be taken care of and evened out, entailing that the pulse and shock wave in the hybrid powertrain is avoided, entailing that the driving comfort in the vehicle increases. According to another embodiment, the torque of the electric machine is increased step by step during the torque limitation. This means that any gap and torsion spring effects in the powertrain are taken care of and evened out, entailing that the pulse and shock wave in the hybrid powertrain are avoided, so that the driving comfort in the vehicle increases.

The above objectives are also achieved with a hybrid powertrain of the type specified above, which is characterised by the features specified in claim 1 1 , and by a vehicle of the type specified above, which is characterised by the features specified in claim Other advantages of the invention are set out in the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

Below is a description, as an example, of preferred embodiments of the invention with reference to the enclosed drawings, in which:

Fig. 1 shows a schematic side view of a vehicle with a powertrain according to the present invention,

Fig. 2 shows a schematic side view of a powertrain according to the present invention, Fig. 3 shows a diagram of the brake pedal position and torque in the method to brake the vehicle with the hybrid powertrain according to the present invention, and

Fig. 4 shows a flow chart of the method to brake the vehicle with the hybrid power- train according to the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Fig. 1 shows a schematic side view of a vehicle 1 , comprising a hybrid powertrain 2 with a combustion engine 3 and an electric machine 4, which are connected to a gearbox 6. The gearbox 6 is also connected to the driving wheels 8 of the vehicle 1 . The driving wheels 8 are equipped with wheel brakes 9.

Fig. 2 shows a schematic view of a hybrid powertrain 2, comprising a combustion engine 3 and an electric machine 4, which are connected to an input shaft 10 of the gearbox 6. The combustion engine 3 may be connected to and disconnected from the input shaft 10 via a coupling device 12, which may be manually and/or automatically manoeuvrable. The gearbox 6 may be any gearbox 6, but preferably it is a combined manual and automatic gearbox 6 of split type and comprises a main shaft 14, a lay shaft 16, and an output shaft 18 on which one or several cogwheels 20 are arranged. Between the main shaft 14 and the output shaft 18, a retarder 22 is arranged. The output shaft 18 is connected to a final gear 24, which in turn is connected to the driving wheels 8 of the vehicle 1 , which are equipped with wheel brakes 9. An electronic control device 26 is connected to the combustion engine 3, the cou- pling device 12, the electric machine 4, the gearbox 6 and the wheel brakes 9 via electrical conductors 28. A brake pedal 29 is connected to the control device to impact the electric machine 4 and the wheel brakes 9. Instead of transmitting signals through the electrical conductors 28, signals between the electronic control device 26 and the combustion engine 3, the coupling device 12, the electric machine 4, the gearbox 6, the wheel brakes 9 and the brake pedal 29 may be transmitted wirelessly. The electronic control device 26 may comprise a memory M and a computer program P. It is also possible to connect a computer 30 to the control device 26.

Fig. 3 shows a diagram of the brake pedal position and torque in the method to con- trol the hybrid powertrain 2 according to the present invention. The top solid curve represents the position of the brake pedal 29. The bottom solid curve represents the torque of the electric machine 4 when braking the vehicle 1 .

At the point in time t1 , the driver of the vehicle 1 presses the brake pedal 29, which is thus moved from a first position D1 towards a second position D2. According to Fig. 3, the brake pedal reaches the position D2 at the point in time t2. When the brake pedal 29 is activated by being pressed down and moved from the position D1 , the combustion engine 3 is disconnected from the input shaft 10 of the gearbox 6 via the coupling device 12, which takes place at the point in time t1 . In the event the brake pedal position in the second position D2, or the brake pedal's acceleration when it leaves the position D1 , corresponds to a very sharp brake, the wheel brakes 9 of the vehicle 1 are activated. When the brake pedal 29 is pressed, and thus activated, at the point in time t1 , the electric machine 4 is also switched to generator operation to initiate braking and to, jointly with the wheel brakes 9, further contribute to the brak- ing of the vehicle 1 via a braking torque from the electric machine 4 acting on the powertrain 2. At the same time as the brake pedal 29 is activated at the point in time t1 , a signal is sent from the control device 26 to the electric machine 4. When it is clear that the brake pedal 29 is activated with an acceleration and/or a force exceeding a predetermined threshold, or when the brake pedal 29 is moved to a certain po- sition corresponding to the position D2, the wheel brakes 9 of the vehicle 1 will also be activated.

In the event the brake pedal 29 is activated with an acceleration and/or a force which falls below a predetermined threshold, or if the brake pedal position in the second position D2 corresponds to soft braking, only the electric machine 4 will be activated by switching to generator operation to brake the vehicle 1 via a braking torque acting on the powertrain 2. In the event the braking torque from the electric machine 4 is sufficient to brake the vehicle 1 , the wheel brakes 9 of the vehicle 1 are not activated. At the position D1 of the brake pedal 29, the braking torque TO from the electric machine 4 is substantially absent or zero.

In the gearbox 6 and the final gear 24, there are gaps between the components interacting in the gearbox 6 and the final gear 24, e.g. backlash between cogwheels 20 in engagement. There is also a torsion spring effect in the rotatable shafts in the gearbox 6 and the final gear 24, and in the shafts that connect the gearbox 6, the final gear 24 and the driving wheels 8, such as propeller and driving shafts. In the event the electric machine 4 exerts a powerful braking torque at the switch to generator operation, said gap and resilient shafts will cause a pulse and a shock wave in the hybrid powertrain 2, which will be experienced as a disturbance by the driver and passengers in the vehicle 1 . The pulse and the shock wave will also elicit a swinging in the hybrid powertrain 2, due to the torsion spring effect of the shafts. This swinging will also be perceived as uncomfortable by driver and passengers of the vehicle 1 . By controlling the electric machine 4 with a predetermined torque limitation, and/or gradually increasing the torque of the electric machine 4 during the time period t1-t3, to avoid that a pulse and a shock wave in the hybrid powertrain 2 arises, the negative effects described above may be eliminated. Thus, the electric machine 4 is torque- limited under controlled forms via the control device 26, after the braking has been initiated at t1 . This entails that during the time period between t1 and t3, the electric machine's 4 torque will increase without any significant torque being transmitted to the driving wheels 8 of the vehicle 1 , because the gap and torsion spring effect in the powertrain 2 is absorbed by the limited torque of the electric machine 4. At t3, a torque T1 is achieved, where the gaps and torsion spring effect in the powertrain 2 has been absorbed by the torque of the electric machine 4, so that the torque limitation of the electric machine 4 ceases. The torque of the electric machine 4 will thus increase drastically from the torque T1 during a period between t3 and t4, in order to subsequently be limited again at an achieved torque T2, which arises just before a torque T3 is achieved, corresponding to the second position D2 of the brake pedal 29. When the point in time t5 has been reached, the vehicle is slowed down with the torque requested by the driver when the brake pedal 29 is pressed down.

The torque limitation and time period between t1 and t3 is controlled by the electronic control device 26. The point in time t4, occurring just before a torque T3 is achieved, is detected and controlled by the control device 26. A number of speed sensors and elements to determine the torque (not displayed) are arranged in the hybrid power- train 2 to provide information about the speed and torque of the combustion engine 3, the coupling device 12, the electric machine 4 and the rotating components in the gearbox 6.

The gap and torsion spring effect in the powertrain 2, controlled by the limited torque from the electric machine 4, may be described with the following equation:

T _e req = Je ^x + T _e - x ⁽ i ⁾ _e ± Offset [ 1 ]

T _e req relates to the requested torque for the electric machine 4 J _e represents the moment of inertia of the electric machine 4. 60 _w represents acceleration of the driving wheels 8 of the vehicle 1 . TQ represents torque from the electric machine 4.

( e represents the acceleration of the electric machine 4.

Offset represents a predetermined value for the acceleration of the electric machine. The lower the offset value, the longer the time to absorb the gap in the powertrain. The larger the gap and torsion spring effect in the hybrid powertrain 2, the longer the time period between t1 and t3, and between t4 and t5. The time period between t1 and t3 is impacted by the Offset value, which may be predetermined or determined instantly via a feedback function to the control device 26. In the event the Offset value is predetermined, the predetermination is determined by empirical values for the size of the gap and the torsion spring effect, and the time it takes to even out the gap and the torsion spring effect.

At the point in time t7, the driver manoeuvres the brake pedal 29 from the second to the first position D1 , in order to thus complete the braking of the vehicle 1 . In order to avoid that a pulse or a shock arises when the torque of the electric machine 4 ceases, the electric machine 4 is controlled so that the torque is reduced by a predetermined torque limitation. During a time period corresponding to t6-t7, the torque is reduced from T3 to T2. Subsequently, the electric machine 4 is controlled without torque limitation, towards a torque which represents the first position D1 of the brake pedal 29. In order to avoid a pulse or a shock in the powertrain 2 when the torque of the electric machine 4 ceases entirely, the electric machine 4 is controlled to a state with a predetermined torque limitation at the point in time t8, before the torque which represents the first position D1 of the brake pedal 29 is achieved at the point in time t9. The torque of the electric machine 4 may be reduced step by step during the torque limitation, when the braking of the vehicle 1 has been completed. Preferably the torque limitation is carried out during a predetermined time period, which is based on empirical values.

Fig. 4 shows a flow chart of the method to control the hybrid powertrain 2 according to the present invention. The method comprises the following steps:

a) to switch the electric machine 4 to generator operation, when the brake pedal 29 is manoeuvred by the vehicle's driver from a first position D1 to a second position D2; b) to control the electric machine 4 with a predetermined torque limitation, to avoid that a pulse and shock wave arises in the hybrid powertrain 2;

c) to control the electric machine 4 without torque limitation towards a torque T3, re- quested by the driver and representing the second position D2 of the brake pedal 29; and

d) to control the electric machine 4 from a state without torque limitation to a state with a predetermined torque limitation, before the torque T3, representing the second position D2 of the brake pedal 29, is achieved. Preferably the torque is gradually increased in the electric machine 4 during the torque limitation in step b) and/or step d). The torque limitation in step b) and/or step d) is preferably carried out during a predetermined time period t1 -t3 and t4-t5, which torque limitation and time period are determined by a control signal to the electric machine 4.

The method also comprises the additional step, when the brake pedal 29 is manoeu- vred from the second position D2 to the first position D1 :

e) to control the electric machine 4 without torque limitation towards a torque TO, representing the first position D1 of the brake pedal 29.

Before step e), the method also comprises the additional step:

f) to control the electric machine 4, so that the torque is reduced by a predetermined torque limitation.

After step e), the method also comprises the additional step:

g) to control the electric machine 4 from a state without torque limitation to a state with a predetermined torque limitation, before the torque TO, representing the first position D1 of the brake pedal 29, is achieved.

Preferably, the torque is gradually reduced in the electric machine 4 during the torque limitation in step f) and/or step g).

The torque limitation in step f) or step g) is preferably carried out during a predetermined time period t6-t7 and t8-t9.

Between the step a) and the step b), the method also comprises the additional step: h) to disconnect the combustion engine 3 from the input shaft 10 via a coupling device 12.

The hybrid powertrain 2 is preferably controlled via an electronic control device 26. According to the invention, a computer program P is provided, which may comprise procedures to control the hybrid powertrain 2 according to the present invention.

The computer program P may comprise procedures to brake a vehicle 1 with a hybrid powertrain 2, according to the method steps specified above.

The program P may be stored in an executable manner, or in a compressed manner, in a memory M and/or a read/write memory R. The invention also relates to a computer program product comprising program code stored in a medium readable by a computer 30, to perform the method steps specified above, when said program code is executed in the electronic control device 26 or a computer 30 connected to the control device 26. Said program code may be stored in non-volatile storage on said medium readable by a computer 30.

The components and features specified above may, within the framework of the invention, be combined between different embodiments specified.