WO2019151918A1

WO2019151918A1 - A method and an apparatus for controlling shifting of a transmission in a motor vehicle

Info

Publication number: WO2019151918A1
Application number: PCT/SE2019/050020
Authority: WO
Inventors: Mikael ÖGREN; Martin Jakobsson; Roger HÄLLEBERG
Original assignee: Scania Cv Ab
Priority date: 2018-01-30
Filing date: 2019-01-15
Publication date: 2019-08-08
Also published as: DE112019000280T5; SE541422C2; SE1850098A1

Abstract

A method and an apparatus for controlling shifting of a transmission in a motor vehicle, the method comprising: - determining (101 ) that a downhill condition applies, wherein the downhill condition applies if the motor vehicle is travelling in a downhill slope or is approaching a downhill slope, - determining (102) that a braking condition applies, wherein the braking condition applies if a travelling speed of the motor vehicle is being limited by a cruise control controlling a gap to a lead vehicle travelling ahead of the motor vehicle, wherein the travelling speed in the downhill slope is or will be limited to a value requiring braking of the motor vehicle, - initiating (103) a shift to a lower gear based on at least the determined downhill condition and the determined braking condition. The invention also relates to a computer program for carrying out the method in a computer, a computer readable medium and a motor vehicle comprising the apparatus.

Description

A method and an apparatus for controlling shifting of a transmission in a motor vehicle

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a method and an apparatus for controlling shifting of a transmission in a motor vehicle. The invention also relates to a computer program, a computer-readable medium and a motor vehicle.

BACKGROUND AND PRIOR ART

Modern motor vehicles are commonly provided with cruise controls for controlling vehicle speed. Usually, a driver of the vehicle selects a set speed, which is thereafter automatically maintained. Fuel-economising cruise controls, aiming to reduce fuel consumption by adjusting the driving to the characteristics of the road ahead, are also becoming increasingly common. For example, by taking topographic information about the road section ahead of the vehicle into account, the speed may be temporarily increased before e.g. an uphill slope, so that downshifting to a lower transmission mode can be avoided or delayed. In this way, a total energy consumption can be reduced. Also information about road curvature and legal speed limits along the road section ahead of the vehicle can be taken into account.

Some cruise controls offer the possibility to select not only a desired set speed, but also a braking speed, i.e. a defined threshold speed above which the motor vehicle is braked when travelling downhill. Normally, the braking speed is set to a value slightly higher than a set speed of a cruise control regulating the travelling speed. For example, if the set speed of the cruise control is 80 km/h, the braking speed may be set to 83 km/h . If the motor vehicle is travelling downhill and accelerates so that the braking speed is exceeded, a shift to a lower gear is carried out and an auxiliary brake in the form of an engine brake, such as a compression release brake or an exhaust brake, is engaged. The engine brake functions better at the higher engine speed resulting from the gear shift, and can thereby be used to brake the motor vehicle more efficiently.

One of the main factors affecting the energy consumption of a vehicle, in particular at high speeds and for large motor vehicles having a large front area, is air resistance. A way to reduce the air resistance is to drive behind a lead vehicle, i.e. another vehicle travelling ahead of the present vehicle, and exploit the so-called slipstream effect. Modern motor vehicles may be equipped with radar technology and/or other sensors such as cameras or lidar technology to measure a gap to a lead vehicle. Some vehicles can also be equipped with a control system to automatically maintain a specified gap to a lead vehicle. Such a control system is usually referred to as an Adaptive Cruise Control (ACC), a Radar Cruise Control, or an Autonomous Cruise Control system. On a downhill road stretch, maintaining the specified gap may involve braking of the vehicle. SUMMARY OF THE INVENTION

It would be desirable to achieve an in at least some aspect improved method and apparatus for controlling shifting of a transmission in a motor vehicle. In particular, it would be advantageous to provide such a method and apparatus by means of which a motor vehicle can be more efficiently braked when travelling downhill behind a lead vehicle. To better address these concerns, a method and an apparatus as defined in the independent claims are provided.

According to one aspect, a method for controlling shifting of a transmission in a motor vehicle is provided. The method comprises determining that a downhill condition applies, wherein the downhill condition applies if the motor vehicle is travelling in a downhill slope or is approaching a downhill slope. The method further comrpsies determining that a braking condition applies, wherein the braking condition applies if a travelling speed of the motor vehicle is being limited by a cruise control controlling a gap to a lead vehicle travelling ahead of the motor vehicle, wherein the travelling speed in the downhill slope is or will be limited to a value requiring braking of the motor vehicle. Still further, the method comprises initiating a shift to a lower gear based on at least the determined downhill condition and the determined braking condition.

According to another aspect, an apparatus for controlling shifting of an automatic transmission in a motor vehicle is provided. The apparatus is configured to determine that a downhill condition applies, wherein the downhill condition applies if the motor vehicle is travelling in a downhill slope or is approaching a downhill slope, and to determine that a braking condition applies, wherein the braking condition applies if a travelling speed of the motor vehicle is being limited by a cruise control controlling a gap to a lead vehicle travelling ahead of the motor vehicle, wherein the travelling speed in the downhill slope is or will be limited to a value requiring braking of the motor vehicle. The apparatus is further configured to initiate a shift to a lower gear based on at least the determined downhill condition and the determined braking condition.

The method and the apparatus according to the invention have several advantages. An advantage is that the method and apparatus enable a more efficient braking of the motor vehicle using auxiliary brakes, in particular using an engine brake, when the motor vehicle is travelling behind a lead vehicle. A shift to a lower gear while travelling in a downhill slope results in a higher engine speed compared to travelling with a higher gear engaged , and thereby to increased braking power of the engine brake. Wheel brakes of the motor vehicle may thereby be relieved . Furthermore, the higher engine speed results in a higher flow rate of cooling fluid in a cooling system of the motor vehicle, whereby the cooling of the engine becomes more efficient. This is also beneficial for an auxiliary brake such as a retarder, which is more efficiently cooled and thereby less likely to overheat.

The method and apparatus according to the invention are particularly useful when the motor vehicle is equipped with a downhill speed control, i.e. a speed control configured to automatically engage an auxiliary brake and initiate a shift to a lower gear when the travelling speed exceeds a set braking speed in a downhill slope. When travelling behind a lead vehicle with an adaptive cruise control controlling the gap to the lead vehicle, it is common that the motor vehicle has to be braked even though the braking speed is not exceeded. In this case, the downhill speed control will not initiate the shift to the lower gear. Engine braking will therefore be insufficient and other brakes, e.g. a retarder and wheel brakes, will primarily brake the motor vehicle. The method and apparatus according to the invention ensure that a gear shift to a lower gear is initiated even though the braking speed is not exceeded, and engine braking can thereby be used more efficiently.

In particular, but not exclusively, the method and the apparatus according to the invention are suitable for use in a heavy goods vehicle, such as a truck or a bus. The method and the apparatus according to the invention are also particularly suitable for motor vehicles equipped with an automatic transmission.

Preferred embodiments of the invention are disclosed in the dependent claims.

It will be appreciated that all the embodiments described with reference to the method aspect of the present invention are applicable also for the apparatus aspect of the present invention. The apparatus may thus be configured to perform the method according to any one of the described embodiments. Other advantageous features as well as advantages of embodiments of the present invention will appear from the following description. DEFINITIONS

By a motor vehicle is herein intended a motor vehicle which is powered by an internal combustion engine and/or by an electric motor.

By a gap is herein intended a gap measured in terms of either time or in terms of distance.

By a braking speed is herein intended a defined threshold speed used to control braking of a motor vehicle when travelling downhill. If the braking speed is exceeded, auxiliary brakes of the motor vehicle are automatically engaged and/or a shift to a lower gear is initiated. Thus, the braking speed is a set speed of a downhill speed control.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will in the following be further described by means of example with reference to the appended drawings, wherein

Fig. 1 schematically shows a simplified motor vehicle according to an embodiment of the invention, Fig. 2 is a flow chart showing a method according to an embodiment of the invention ,

Fig. 3 is a flow chart showing the method according to another embodiment, and

Fig. 4 schematically shows a control unit for carrying out the method according to an embodiment of the invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

A motor vehicle 1 in which a method according to an embodiment of the invention may be carried out is schematically illustrated in fig. 1 . The motor vehicle 1 comprises an internal combustion engine 3 and a transmission 4 that form part of a powertrain 5 configured to drive driving wheels 6, 7 of the motor vehicle 1 . The transmission 4 may preferably be an automatic transmission , but it may also be an automated manual transmission. A control device 2, which may be distributed on several control units configured to control different parts of the motor vehicle 1 , is provided. The method may be performed e.g. by the control device 2.

The control device 2 is configured to control the powertrain 5 of the motor vehicle 1 . The control device 2 is also configured to control a gap to a lead vehicle travelling ahead of the motor vehicle 1 by automatically maintaining a specified gap to the lead vehicle. The control device 2 may for this purpose include a cruise control, e.g. an adaptive cruise control (ACC), or be configured to control a separately provided cruise control. A current gap to the lead vehicle may be determined e.g. using e.g. radar technology, lidar technology, camera information, map data in combination with GPS (global positioning system) technology, or some other known technique. The specified gap to the lead vehicle may e.g. be preset by a driver of the motor vehicle. The specified gap can also be set automatically in some applications, for instance in platooning where the gap is set to a value which is as small as possible to reduce the air resistance. The control device 2 may be configured to control the gap to a value equal to the specified gap, or to a value that does not differ from the specified gap by more than a certain amount.

The control device 2 may also be configured to control a downhill travelling speed by automatically engaging at least one auxiliary brake when a travelling speed of the motor vehicle 1 exceeds a set braking speed in a downhill slope. The braking speed can either be set automatically by a system, such as a distance controller or a predictive controller aiming at minimising fuel consumption, or manually by a driver of the motor vehicle via some interface. The braking speed is usually set by its own dedicated buttons or switches. The braking speed can also be set as an offset to a set speed of a cruise control. In this case, if the set speed of the cruise control is changed, the absolute braking speed is automatically changed to maintain the offset.

A method according to an embodiment of the invention, carried out e.g. by the control device 2 in the motor vehicle 1 illustrated in fig. 1 , is schematically illustrated in the flow chart of fig. 2. A method according to another embodiment is illustrated in fig. 3. In a step 101 , 202, it is determined that a downhill condition applies, wherein the downhill condition applies if the motor vehicle is travelling in a downhill slope or is approaching a downhill slope. That a downhill condition applies may be determined in different ways. It may e.g . be determined that the downhill condition applies if the downhill slope has a steepness and/or length exceeding a preset threshold. Determining that the downhill condition applies may e.g . involve determining a road gradient of the road on which the motor vehicle is travelling. That the downhill condition applies may also be determined based on vehicle variable values, e.g. vehicle inclination, acceleration, an indication that an auxiliary brake is engaged , etc. The road gradient of the downhill slope may be obtained in various different ways. It may be determined on the basis of map data, e.g. from digital maps containing topographical information , in combination with positioning information , e.g. GPS (global positioning system) information . The positioning information may be used to determine the location of the motor vehicle relative to the map data so that the road gradient can be extracted from the map data. Various present-day cruise control systems use map data and positioning information. Such systems may then provide the map data and positioning information required for the method according to the present invention, thereby minimising the additional complexity involved in determining the road gradient. The road gradient may also be obtained based on radar information, camera information, or information from another vehicle. It may also be determined from positioning information and road gradient information stored previously on board, or from information obtained from traffic systems related to the expected travelling route. In systems where there is information exchange between vehicles, road gradients estimated by one vehicle may also be made available to other vehicles, either directly or via an intermediate unit such as a data base or the like.

In a step 102, 205, it is determined that a braking condition applies, wherein the braking condition applies if a travelling speed of the motor vehicle is being limited by a cruise control controlling a gap to a lead vehicle travelling ahead of the motor vehicle, wherein the travelling speed in the downhill slope is or will be limited to a value requiring braking of the motor vehicle. When the braking condition applies, the travelling speed is on one hand limited by a cruise control such as an ACC, e.g. included in the control device 2, controlling the gap to the lead vehicle based on a specified gap which is to be maintained. The specified gap may e.g. be preset by the driver of the motor vehicle. On the other hand, it is determined that the travelling speed in the downhill slope is or will be limited to a value requiring braking if the specified gap is to be maintained. Thus, in order to maintain the specified gap, the motor vehicle must be braked using e.g . a wheel brake and/or one or more auxiliary brakes in the form of a retarder, a compression release brake or an exhaust brake.

The present travelling speed is typically given by e.g. a road speed sensor. The expected travelling speed in a downhill slope which the motor vehicle is approaching may be modelled or simulated as will be further described below. From the travelling speed, also an acceleration of the motor vehicle may be determined. That the braking condition applies may be determined in various ways, such as from vehicle data and road gradient data. For example, it may be checked whether an auxiliary brake is engaged, thus indicating that the braking condition applies. Further examples of how it may be assessed that the braking condition applies will also be given further below.

In a step 103, 207, a shift to a lower gear is initiated based on at least the determined downhill condition and the determined braking condition. In other words, the downhill condition and the braking condition must apply in order for the shift to the lower gear to be initiated, but, optionally, also other conditions that must be fulfilled in order to initiate the gear shift may be specified. The shift to the lower gear is carried out automatically without any action from the driver of the motor vehicle. Initiating the shift to the lower gear also comprises selecting a suitable lower gear.

At least one gear shifting condition may be specified, wherein the method may further comprise, in a step 206, determining that the at least one defined gear shifting condition is fulfilled, wherein the initiation of the shift to the lower gear is further based on the at least one gear shifting condition being fulfilled. This is illustrated in fig. 3. That is, if the at least one gear shifting condition is not fulfilled, no shift to the lower gear is initiated in this embodiment. It is thereby possible to specify a gear shifting condition so that the shift to a lower gear is only initiated when this is considered advantageous from e.g. an energy efficiency and/or a ride comfort point of view. This improves the control of the gear shifting.

The at least one gear shifting condition may be at least one of: - a first condition relating to an expected duration of the downhill slope, wherein the first condition is considered to be fulfilled if the expected duration exceeds a defined threshold,

- a second condition relating to an expected braking power of the shift to the lower gear, wherein the second condition is considered to be fulfilled if the expected braking power exceeds a defined threshold, and

- a third condition relating to an expected duration of an engagement of the lower gear, wherein the third condition is considered to be fulfilled if the expected duration of the engagement of the lower gear exceeds a defined threshold.

One or more of the first, second and third gear shifting conditions may be applied. The gear shifting conditions are selected so that shifting to a lower gear is avoided if this is not considered advantageous from, primarily, a ride comfort point of view. Excessive gear shifts, which may be experienced as disturbing for the driver and possible passengers of the motor vehicle, and which furthermore have a limited contribution on the braking power, may hereby be avoided.

The method may further comprise:

- in a step 201 , retrieving road gradient data relating to a road gradient along an expected travelling route ahead of the motor vehicle,

- in the step 201 , retrieving vehicle data relating to at least one of the travelling speed, a mass of the motor vehicle, and a driving resistance of the motor vehicle,

- based on at least the retrieved road gradient data and the retrieved vehicle data, determining that the at least one gear shifting condition and/or the at least one braking condition is/are fulfilled .

Retrieved road gradient data and vehicle data are here used to determine if a shift to a lower gear is to be carried out. Various systems of modern motor vehicle already use such data for different purposes, and may thereby provide the data needed for carrying out the method according to the present embodiment. Using road gradient data and vehicle data, the acceleration of the motor vehicle in the downhill slope, and its resulting velocity profile, can be accurately estimated. It can thereby also be well determined whether the gear shifting condition(s) and/or the braking condition is/are fulfilled.

Road gradient data may be retrieved as explained above. Vehicle data may be retrieved from various sensors and systems within the motor vehicle. The mass of the motor vehicle may typically be determined by a mass estimation algorithm based on either information from a suspension of the vehicle or on a measured or estimated moment of inertia of the motor vehicle. The driving resistance may be estimated based on a force equation describing the forces acting on the motor vehicle, e.g. driving force, rolling resistance, friction arising in the powertrain, gravitational force, and air resistance. The driving force is usually well known since it may, according to prior art, be calculated straightforwardly with the use of the torque emitted by the engine. This torque is usually specified in a control system of the motor vehicle. With the use of gear ratio and wheel diameter, the torque can be converted into a driving force acting on the driving wheels. Models for estimating a driving resistance of a motor vehicle are already commonly used in modern vehicles for different purposes. These purposes include, but are not limited to, predicting a behaviour of the motor vehicle as a road gradient changes, estimating a mass of the motor vehicle, and ensuring an adequate function of cruise controls, gear shifting systems and other systems used in the motor vehicle.

The method may further comprise, in a step 203:

- based on at least the retrieved road gradient data and the retrieved vehicle data, simulating an expected velocity profile of the motor vehicle, wherein the expected velocity profile is used in determining that the at least one gear shifting condition and/or the at least one braking condition is/are fulfilled .

The velocity profile describes the travelling speed during an upcoming time period or over an upcoming road section. Such an expected velocity profile may already be simulated for other purposes, in which case this step 203 adds no additional complexity to the inventive method. The velocity profile is useful for accurately determining whether the gear shifting condition(s), and/or the braking condition , is/are fulfilled . The expected velocity profile may describe the expected travelling speed as it would be if the adaptive cruise control did not control the gap to the lead vehicle, or it may be an expected velocity profile set to maintain the gap to the lead vehicle.

The method may further comprise, in a step 204, obtaining an expected velocity profile of the lead vehicle, wherein the expected velocity profile of the lead vehicle is used in determining that the at least one gear shifting condition and/or the at least one braking condition is/are fulfilled . Using the expected velocity profile of the lead vehicle may improve the precision in the assessment of whether the gear shifting condition(s) and/or the braking condition is/are fulfilled. The velocity profile of the lead vehicle may e.g. be used to determine the expected velocity profile of a present motor vehicle, i.e the motor vehicle in which the method is being performed , under the assumption that the travelling speed of the present motor vehicle is expected to follow the travelling speed of the lead vehicle, or follow a velocity profile resulting in a constant or essentially constant gap to the lead vehicle. However, the expected velocity profile of the present motor vehicle may also be determined in the step 203 independently of the velocity profile of the lead vehicle as described above. The velocity profile of the lead vehicle may be communicated to the motor vehicle using e.g . a vehicle-to-vehicle communication system, or may be calculated or estimated on-board the motor vehicle. For example, it may be assumed that the lead vehicle will continue to travel at its present travelling speed, i.e. the velocity profile of the lead vehicle is estimated to a constant value equal to the present travelling speed .

The method may further comprise, in a step 208, braking the motor vehicle using an engine brake, wherein the engine brake is an exhaust brake or a compression release brake. The braking power of the engine brake is increased by the higher engine speed resulting from the shift to a lower gear. The motor vehicle can thereby be more efficiently braked using the engine brake, thus relieving other brakes of the motor vehicle. The method may further comprise, in a step 209, braking the motor vehicle using a retarder. The retarder benefits from the higher flow rate of cooling fluid resulting from the higher engine speed after downshifting and is less likely to overheat. The engine brake and the retarder may be used simultaneously.

The lower gear may be selected such that, at the lower gear, full exhaust braking or full compression braking will cover at least 50 % of a total braking power needed to maintain the gap to the lead vehicle. Efficient braking using an engine brake is thereby ensured. The available braking power of the engine brake at different gear exchange ratios, and the total braking power needed, may e.g. be modelled based on vehicle data and road gradient data.

When the motor vehicle is equipped with a downhill speed control configured to automatically engage at least one auxiliary brake when the travelling speed of the motor vehicle exceeds a set braking speed in a downhill slope, the initiation of the shift to the lower gear may further be based on that the cruise control limits the travelling speed to a value below the set braking speed. In this case, it can be ensured that benefits resulting from a higher engine speed are achieved also below the set braking speed, for example that an engine brake is efficiently used for braking the motor vehicle.

In an example, the method according to an embodiment of the invention is carried out in a motor vehicle travelling along a road section behind a lead vehicle. In a present mode of operation, the powertrain of the motor vehicle is controlled by an ACC. The travelling speed v of the motor vehicle is therefore automatically adjusted to control a gap d to the lead vehicle by maintaining a specified gap d_set, wherein the specified gap d_set has been set either by the driver or automatically. The motor vehicle is also equipped with a downhill speed control specifying a braking speed V_brake, above which, during downhill travel, the motor vehicle is automatically braked using auxiliary brakes, and a shift to a lower gear is initiated. A simulated expected velocity profile of the motor vehicle is continuously updated in step 203 based on vehicle data and road gradient data obtained in step 201 .

A downhill condition, a braking condition and a gear shifting condition have been defined. The downhill condition is defined so that it is considered fulfilled if the motor vehicle enters a downhill slope having a road gradient a exceeding a defined value O_{dh .} The braking condition is defined so that it is considered fulfilled if it is determined that, without braking, the gap d to the lead vehicle will become smaller than the specified gap d_set. The gear shifting condition is defined so that it is considered fulfilled if the expected total duration Ttot of the downhill slope is larger than a defined value T_{gs .}

As the motor vehicle enters a downhill slope, the simulated velocity profile determined in step 203 reveals that braking of the motor vehicle will be required to maintain the specified gap d_set. However, the travelling speed v is below the set braking speed V_brake of the downhill speed control. Therefore, the downhill speed control does not initiate a shift to a lower gear. It is in the step 202 checked whether the downhill condition applies. Based on the road gradient data received in step 201 , it is determined that the road gradient exceeds the defined value O_dh and the downhill condition therefore applies.

In the step 205, it is checked whether the braking condition applies. Based on the velocity profile simulated in step 203, it is determined that, without braking, the gap d will become smaller than the specified gap d_set and the braking condition therefore applies.

In the step 206, it is checked whether the gear shifting condition applies. The road gradient data of the road section ahead of the vehicle retrieved in step 201 and the velocity profile simulated in step 203 indicate that the expected total duration Ttot of the downhill slope is larger than the defined value T_gs and the gear shifting condition therefore applies.

In the step 207, a suitable lower gear is selected and a shift to the selected lower gear is carried out. The engine brake may thereby be used in step 208 to efficiently brake the motor vehicle in the downhill slope, even though the braking speed is not exceeded.

If it had instead been found that the expected total duration Ttot of the downhill slope was smaller than the defined value T_{g s} , no gear shift would have been initiated, since the time at the lower gear would become too short.

One skilled in the art will appreciate that a method according to embodiments of the present invention may be implemented in a computer program which, when executed in a computer, causes the computer to perform the method. The computer program usually takes the form of a computer program product which comprises a suitable digital storage medium on which the computer program is stored. Said computer-readable digital storage medium comprises a suitable memory, e.g. ROM (read only memory), PROM (programmable read-only memory), EPROM (erasable PROM), flash memory, EEPROM (electrically erasable PROM), a hard disc unit, etc.

An apparatus, e.g. the above described control device 2, according to the invention, adapted to carry out the above described steps of the method according to the invention, may comprise one electronic control unit or two or more cooperating electronic control units. Fig. 4 illustrates very schematically such an electronic control unit 30 forming part of an apparatus according to an embodiment of the invention. The control unit 30 comprises an execution means 31 , such as a central processor unit (CPU), for executing a computer program. The execution means 31 communicates with a memory 32, for example of the type RAM, through a data bus 33. The control unit 30 comprises also a non- transitory data storing medium 34, for example in the form of a Flash memory or a memory of the type ROM, PROM, EPROM or EEPROM. The execution means 31 communicates with the data storing medium 34 through the data bus 33. A computer program comprising computer program code for implementing a method according to an embodiment of the invention is stored on the data storing medium 34. The invention is of course not in any way restricted to the em bodiments described above, but many possibilities to modifica tions thereof would be apparent to a person with skill in the art without departing from the scope of the invention as defined in the appended claims.

Claims

1 . A method for controlling shifting of a transmission (4) in a motor vehicle (1 ), the method comprising:

- determining (101 , 202) that a downhill condition applies, wherein the downhill condition applies if the motor vehicle (1 ) is travelling in a downhill slope or is approaching a downhill slope,

- determining (102, 205) that a braking condition applies, wherein the braking condition applies if a travelling speed of the motor vehicle (1 ) is being limited by a cruise control controlling a gap to a lead vehicle travelling ahead of the motor vehicle (1 ), wherein the travelling speed in the downhill slope is or will be limited to a value requiring braking of the motor vehicle (1 ),

- initiating (103, 207) a shift to a lower gear based on at least the determined downhill condition and the determined braking condition.

2. The method according to claim 1 , further comprising:

- determining (206) that at least one defined gear shifting condition is fulfilled, wherein the initiation of the shift to the lower gear is further based on the at least one gear shifting condition being fulfilled .

3. The method according to claim 2, wherein the at least one gear shifting condition is at least one of:

- a first condition relating to an expected duration of the downhill slope, wherein the first condition is considered to be fulfilled if the expected duration exceeds a defined threshold, - a second condition relating to an expected braking power of the shift to the lower gear, wherein the second condition is considered to be fulfilled if the expected braking power exceeds a defined threshold, and

4 The method according to claim 2 or 3, further comprising :

- retrieving (201 ) road gradient data relating to a road gradient along an expected travelling route ahead of the motor vehicle ( 1 ) ,

- retrieving (201 ) vehicle data relating to at least one of the travelling speed, a mass of the motor vehicle (1 ), and a driving resistance of the motor vehicle (1 ),

5 The method according to claim 4, further comprising:

- based on at least the retrieved road gradient data and the retrieved vehicle data, simulating (203) an expected velocity profile of the motor vehicle (1 ), wherein the expected velocity profile is used in determining that the at least one gear shifting condition and/or the at least one braking condition is/are fulfilled .

6. The method according to any one of claims 2-5, further comprising :

- obtaining (204) an expected velocity profile of the lead vehicle, wherein the expected velocity profile of the lead vehicle is used in determining that the at least one gear shifting condition and/or the at least one braking condition is/are fulfilled .

7. The method according to any one of the preceding claims, further comprising:

- braking (208) the motor vehicle (1 ) using an engine brake, wherein the engine brake is an exhaust brake or a compression release brake.

8. The method according to any one of the preceding claims, further comprising:

- braking (209) the motor vehicle (1 ) using a retarder.

9. The method according to any one of the preceding claims, wherein the lower gear is selected such that, at the lower gear, full exhaust braking or full compression braking will cover at least 50 % of a total braking power needed to maintain the gap to the lead vehicle.

10. The method according to any one of the preceding claims, wherein the motor vehicle (1 ) is equipped with a downhill speed control configured to automatically engage at least one auxiliary brake when the travelling speed of the motor vehicle (1 ) exceeds a set braking speed in a downhill slope, wherein the initiation of the shift to the lower gear is further based on that the cruise control limits the travelling speed to a value below the set braking speed.

1 1 . A computer program comprising instructions which, when the computer program is executed by a computer, causes the computer to carry out the method according to any one of the preceding claims.

12. A computer-readable medium comprising instructions which, when executed by a computer, cause the computer to carry out the method according to any one of claims 1 -10.

13. An apparatus for controlling shifting of a transmission (4) in a motor vehicle (1 ), the apparatus being configured to:

- determine that a downhill condition applies, wherein the downhill condition applies if the motor vehicle (1 ) is travelling in a downhill slope or is approaching a downhill slope,

- determine that a braking condition applies, wherein the braking condition applies if a travelling speed of the motor vehicle is being limited by a cruise control controlling a gap to a lead vehicle travelling ahead of the motor vehicle (1 ), wherein the travelling speed in the downhill slope is or will be limited to a value requiring braking of the motor vehicle,

- initiate a shift to a lower gear based on at least the determined downhill condition and the determined braking condition .

14. A motor vehicle (1 ) comprising an apparatus according to claim 13.