Patent Publication Number: US-2023158891-A1

Title: Trailer vehicle unit for a towing vehicle

Description:
RELATED APPLICATIONS 
     The present application claims priority to European Patent Application No. 21210505.0, filed on Nov. 25, 2021, and entitled “TRAILER VEHICLE UNIT FOR A TOWING VEHICLE,” which is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a trailer vehicle unit for a towing vehicle. The present invention also relates to a vehicle combination comprising such trailer vehicle unit. Although the invention will mainly be directed to a towing vehicle in the form of a truck using an electric traction motor propelling the vehicle combination, the invention may also be applicable for other types of vehicles and vehicle combinations at least partially propelled by an electric traction motor, such as e.g. an electric vehicle, a hybrid vehicle comprising an electric machine as well as an internal combustion engine for propulsion. In addition, the present invention may be used in a vehicle combination comprising a number of trailer vehicle units. 
     BACKGROUND 
     The propulsion systems of vehicles are continuously developed to meet the demands from the market. A particular aspect relates to the emission of environmentally harmful exhaust gas. Therefore, vehicles propelled by electric machines and/or electric machine receiving electric power from hydrogen fuel cells have been increasingly popular, in particular for trucks and other heavy-duty vehicles. 
     In comparison to a vehicle propelled solely by an internal combustion engine (ICE), a vehicle propelled by an electric machine conventionally struggles with obtaining the desired functionality of auxiliary braking. For an ICE operated vehicle, the auxiliary braking can be achieved by means of a retarder or by so called engine braking. However, for an electric vehicle, the auxiliary braking functionality can be a dimensioning factor for the cooling system since the cooling capacity of e.g., a fuel cell electric vehicle (FCEV) as well as a battery electric vehicle (BEV) is a limiting factor. The reason is that for such type of vehicles, the auxiliary braking places a lot of energy in the cooling system. 
     There is thus a desire to provide an improved energy management system for managing excessive energy generated e.g., during regenerative braking of an electric vehicle which is at least partially propelled by an electric traction motor. 
     SUMMARY 
     It is thus an object of the present invention to at least partially overcome the above-described deficiencies. This object is achieved by a trailer vehicle unit according to claim  1 . The objective is also achieved by the other independent claims. The dependent claims are directed to advantageous embodiments of the disclosure. 
     According to a first aspect, there is provided a trailer vehicle unit for a towing vehicle. The trailer vehicle unit comprises a coupling configured to couple the trailer vehicle unit to the towing vehicle. The trailer vehicle unit further comprises an electrical energy dissipating system at least partly disposed in a fluid medium conduit for receiving a fluid medium. The electrical energy dissipating system is configured to receive electric energy and to dissipate received electric energy by utilizing a flow of fluid medium. Moreover, the trailer vehicle unit comprises a slave control unit having a processing circuitry configured to receive a control signal from the towing vehicle indicative of a request for dissipating energy from the electrical energy dissipating system. Further, the processing circuitry is configured to control the electrical energy dissipating system in response to the received control signal so as to dissipate energy from the electrical energy dissipating system. 
     Hereby, the electrical energy dissipating system of the trailer vehicle unit is capable of dissipating excessive energy generated from regenerative braking of the towing vehicle and/or the vehicle combination. 
     The proposed trailer vehicle unit provides for individual control of a trailer of a vehicle combination with respect to regenerative braking by using and controlling an electrical energy dissipating system arranged on the trailer. In this manner, the proposed trailer vehicle unit is configured to improve the driving situation of a vehicle combination during a regenerative braking event. In other words, by using the proposed trailer vehicle unit in e.g., a vehicle combination in the form of a tractor-trailer combination, it becomes possible to improve driving situations where the towing vehicle is operated in a regenerative braking mode and its electric storage system is unable to receive the electric power generated during the regenerative braking operation. In such situations, the towing vehicle can send a request to the slave control unit of the trailer vehicle unit, where the request contains data indicating a need for using and controlling the electrical energy dissipating system of the trailer vehicle unit to receive electric energy generated from the regenerative braking event and to dissipate the received electric energy by utilizing the flow of fluid medium through one or more components of the electrical energy dissipating system. 
     In addition, or alternatively, the arrangement of the electrical energy dissipating system can be used by the towing vehicle if there is a desire not to regenerate more than a certain electric power level in the electric power system of the towing vehicle. In such situation, the electric power system of the towing vehicle can instead supply at least a portion of the generated electric power to the electrical energy dissipating system of the trailer vehicle unit which in turn dissipates the received electric energy by utilizing a flow of fluid medium. By way of example, the generated electric power is supplied to an electric machine of the electrical energy dissipating system which in turn, by means of the received electric power, operates an air compressor to force a flow of fluid medium in the form of air through the fluid medium conduit in the form of an air conduit. Other examples of suitable electrical energy dissipating systems are described in the following description. An advantage is thus that the electrical energy dissipating system is able to dissipate electric power which cannot be handled by the electric storage system of the towing vehicle. 
     To this end, by the arrangement of the electrical energy dissipating system in the trailer vehicle unit, it becomes possible to improve the energy management of the vehicle combination by allowing the electrical energy dissipating system of the trailer vehicle unit to dissipate an additional amount of energy during braking with e.g., a heavier-duty vehicle combination. This may further allow for an improved balancing of the available brake capability of the vehicle combination without adding extra equipment or weight on the towing vehicle itself. 
     In addition, by the proposed trailer vehicle unit, the auxiliary braking capability provided from the trailer may be more efficiently matched with the needs from the towing vehicle. Further, it may also have a positive impact on the required components and weight of the towing vehicle because the towing vehicle may not need to be equipped with an overly over-dimension braking system for both the towing vehicle and any trailers coupled thereto. 
     To this end, the proposed trailer vehicle unit when coupled to the towing vehicle, is capable of contributing to the general braking strategy of the coupled vehicle combination in a positive and efficient manner. Such arrangement and configuration of the vehicle combination may also at least partly reduce any potential problems between the towing vehicle and the trailer during driving. One example of a potential problem may refer to a situation with potential jack knifing between the towing vehicle and the trailer. 
     Favourably, the control of the electrical energy dissipating system may be controlled and coordinated with any braking system of the towing vehicle so as to provide an even more efficient vehicle combination energy management system for handling excessive energy from regenerative braking events. 
     It should be noted that the provision of the slave control unit having the processing circuitry configured to receive a control signal from the towing vehicle refers to driving situation when the trailer vehicle unit is in a coupled state with the towing vehicle. Hence, the processing circuitry of the slave control unit is configured to receive a control signal from the towing vehicle indicative of a request for dissipating energy from the electrical energy dissipating system when the trailer vehicle unit is in the coupled state with the towing vehicle. The coupled state here refers to a state between the towing vehicle and the trailer vehicle unit wherein the trailer vehicle unit is mechanically coupled to the towing vehicle, and wherein there is an electrical connection established between the trailer vehicle unit and the towing vehicle. In addition, in the coupled state, there is also generally a communication established between the slave control unit of the trailer vehicle unit and the towing vehicle. 
     The slave control unit may generally be arranged in communication with the electrical energy dissipating system. In addition, or alternatively, the slave control unit may be an integral part of the electrical energy dissipating system. 
     The electrical energy dissipating system can be provided in several different manners, as further described herein. 
     The fluid medium may typically be air. However, in some example embodiments, the fluid medium may be a liquid, such as a coolant. One example of a liquid coolant is a liquid containing glycol. As such, the term “fluid medium”, as used herein, generally encompasses any one of a gaseous fluid medium, such as air and a liquid fluid medium, such as liquid coolant. In particular, the term “fluid medium” refers to any one of air and liquid coolant. 
     Hence, according to example embodiments, the fluid medium is air and the fluid medium conduit is an air conduit for receiving air. In such example embodiments, when the fluid medium is air, there is provided a trailer vehicle unit for a towing vehicle, wherein the trailer vehicle unit comprises a coupling configured to couple the trailer vehicle unit to the towing vehicle. The trailer vehicle unit further comprises an electrical energy dissipating system at least partly disposed in an air conduit for receiving air. The electrical energy dissipating system is configured to receive electric energy and to dissipate received electric energy by utilizing a flow of air. Moreover, the trailer vehicle unit comprises a slave control unit having a processing circuitry configured to receive a control signal from the towing vehicle indicative of a request for dissipating energy from the electrical energy dissipating system. Further, the processing circuitry is configured to control the electrical energy dissipating system in response to the received control signal so as to dissipate energy from the electrical energy dissipating system. 
     According to one example embodiment, wherein the fluid medium is air, the electrical energy dissipating system comprises an electric machine operable from the received electric energy and an air flow producing unit connected to an output shaft of the electric machine. The air flow producing unit is operable by the electric machine by rotation of the output shaft. Further, the air flow producing unit is arranged in the fluid medium conduit in the form of an air conduit. This configuration of the electrical energy dissipating system provides for a less complex system with few components. Another advantage is that such auxiliary braking system of the trailer vehicle unit provides for an increased controllability at all speeds. 
     The term “air flow producing unit”, as used herein, generally refers to an arrangement configured to generate and convey a flow of air through the air conduit. The air flow producing unit may thus be any one of an air fan operated by the electric machine, an air compressor operated by the electric machine and an air blower device operated by the electric machine. Typically, the air flow producing unit may increase the temperature level of the air flowing therethrough. 
     Also, the air flowing through the air flow producing unit may be heated, and the heated air can thus be used for heating components of the vehicle in need of thermal management. 
     According to one example embodiment, the electrical energy dissipating system comprises an air heating arrangement. The air heating arrangement comprises at least one electric air heater operable from the received electric energy. The air heating arrangement is arranged in the fluid medium conduit in the form of an air conduit. This configuration of the electrical energy dissipating system provides for a less complex system with few components. Another advantage is that such auxiliary braking system of the trailer vehicle unit provides for an increased performance at high speeds where the system receives air from the ambient environment to cool the electric air heater. To this end, the system is able to make use of the air velocity at higher vehicle speeds. This system can favourable be arranged or positioned at the roof of the trailer vehicle unit. 
     The air heating arrangement provides for dissipating electric power during regenerative braking. The air heating arrangement may be an electric brake resistor arrangement, whereby the at least one electric air heater in such a case is an electric brake resistor. The air heating arrangement may advantageously be incorporated in the electrical energy dissipating system for heating the air in the air conduit. 
     According to one example embodiment, the electrical energy dissipating system comprises the electric machine operable from the received electric energy, the air flow producing unit connected to the output shaft of the electric machine, and the air heating arrangement comprising the at least one electric air heater operable from the received electric energy. In this example embodiment, the at least one electric air heater is arranged in the air conduit in downstream fluid communication with the air flow producing unit. An advantage is thus that when the electric machine is unable to receive all electric power that needs to be dissipated, the electric power system can supply electric power also to the at least one electric air heater. Thus, an improved electric energy dissipation capacity is provided. In addition, by using an electrical energy dissipating system having both the air flow producing unit and the air heating arrangement, it becomes possible to benefit from transferring air with a high flow from the air flow producing unit to the electric air heater. Thereby, it becomes possible to maximize the temperature level of the components making up the system, thus also increasing the power dissipation for a certain air mass flow. This arrangement may further provide an increased usage flexibility and an even further increased power dissipation capability. 
     By way of example, the air flow producing unit is arranged to compress the received air and transmit the compressed air to the at least one electric air heater. According to one example embodiment, the air flow producing unit is an air compressor arranged to compress the received air and transmit the compressed air to the at least one electric air heater. 
     Optionally, the electrical energy dissipating system may further comprise an air flow restriction arrangement positioned downstream the air heating arrangement. 
     According to one example embodiment, wherein the fluid medium is a liquid coolant and the fluid medium conduit is a liquid coolant conduit, the electrical energy dissipating system comprises a liquid coolant heating arrangement. The liquid coolant heating arrangement comprises at least one electric brake resistor arrangement operable from the received electric energy. The liquid coolant heating arrangement is arranged in the liquid coolant conduit in the form of a liquid coolant conduit. As mentioned herein for the air heating arrangement, the electric brake resistor arrangement may generally be provided by at least one electric brake resistor. Accordingly, the electrical energy dissipating system comprises a liquid coolant heating arrangement, the liquid coolant heating arrangement being disposed in the liquid coolant conduit and comprising at least one electric brake resistor arrangement operable from the received electric energy. 
     The electrical energy dissipating system can be installed in the trailer vehicle unit in various ways depending on the type of trailer vehicle unit, and may even be designed to be retrofitted to one or many standard trailers. 
     According to one example embodiment, the trailer vehicle unit further comprises a mounting structure for arrangement of the electrical energy dissipating system to the trailer vehicle unit. The mounting structure may be any one of a housing for accommodating the electrical energy dissipating system, a frame structure for holding the electrical energy dissipating system and a bracket assembly comprising one or more brackets for fixating the electrical energy dissipating system to the trailer vehicle unit. 
     By way of example, the mounting structure is arranged vertically below a supporting frame structure of the trailer vehicle unit. 
     Optionally, the electrical energy dissipating system may be releasably attached to the mounting structure. This type of arrangement provides for a temporarily mounting of the electrical energy dissipating system to the trailer vehicle unit. 
     According to one example embodiment, the electrical energy dissipating system is configured to be retrofitted into the trailer vehicle unit. 
     According to one example embodiment, the trailer vehicle unit further comprises a conventional service brake system. By way of example, the trailer vehicle unit further comprises one or more wheel brake devices. 
     According to one example embodiment, the trailer vehicle unit further comprises an electric drive axle arrangement. By way of example, the electric drive axle arrangement comprises one or more electric hub motors for providing traction to one or more wheels of the trailer vehicle unit, wherein the electrical energy dissipating system is configured to receive electric energy from the electric hub motors. The combination of the electrical energy dissipating system in the trailer vehicle unit and an electric drive axle arrangement may be particularly suitable for avoiding, or at least reducing the risk of jack knifing between the tractor vehicle and the trailer vehicle unit. The combination of the electrical energy dissipating system in the trailer vehicle unit and the electric drive axle arrangement provides for enhance energy dissipation so as to allow braking with the trailer vehicle unit and any traction motors on the trailer vehicle unit. 
     The slave control unit can be configured in several different manners in view of the type of trailer vehicle unit and the type of towing vehicle. Typically, the slave control unit may be configured to control the supply of electric energy from the towing vehicle to the electrical energy dissipating system in response to the received signal from the towing vehicle so as to permit the electrical energy dissipating system to dissipate received electrical energy generated from a regenerative braking event of the towing vehicle. 
     According to a second aspect, there is provided a vehicle combination formed by a towing vehicle and one or more trailer vehicle units according to the first aspect, including any one of the features, examples and embodiments relating to the first aspect. 
     Effects and features of the second aspect are largely analogous to those described above in relation to the first aspect. 
     According to one example embodiment, the vehicle combination is any one of an on-the-road vehicle combination, an off-the road vehicle combination and a combination thereof. 
     The vehicle combination may generally comprise more than one trailer vehicle unit. In one example embodiment, the vehicle combination comprises a towing vehicle in the form of a tractor unit, a trailer vehicle unit in the form of a semitrailer comprising an electrical energy dissipating system, and further a trailer vehicle unit in the form of a trailer comprising a corresponding electrical energy dissipating system. 
     Typically, the towing vehicle comprises a master control unit having a processing circuitry configured to control the electrical energy dissipating system of the trailer vehicle unit. By way of example, the master control unit is configured to control the electrical energy dissipating system of the trailer vehicle unit via the slave control unit of the trailer vehicle unit. As such, the electrical energy dissipating system is arranged to operate in a slave mode with regard to the master control unit arranged in the towing vehicle. In addition, or alternatively, the master control unit may be configured to directly control the electrical energy dissipating system of the trailer vehicle unit. 
     The master control unit may be arranged in communication with an electric power system of the towing vehicle. The master control unit may be configured to: receive a signal indicative of a regenerative braking request for the vehicle combination, determine a level of electric power generated by an electric traction motor during the regenerative braking of the vehicle combination, receive a signal indicative of a current electric charging capacity of an electric storage system, compare the level of electric power generated during the regenerative braking with the current electric charging capacity of the electric storage system, and control the electric power system to supply electric power to the electrical energy dissipating system during the regenerative braking when the level of electric power generated during the regenerative braking is higher than the current charging capacity of the electric storage system. 
     The towing vehicle may further comprise the electric traction motor configured to propel the towing vehicle and to controllably regenerate electric power during regenerative braking of the vehicle combination, the electric machine and the electric power system electrically connected to the electric machine. The electric power system may comprise the electric storage system configured to receive and supply electric power, wherein the electric machine is operated by electric power received from the electric power system. In addition, the electric power system may further be electrically connected to the electric traction motor and configured to receive electric power during regenerative braking. 
     In addition, or alternatively, the master control unit may be configured to selectively control the electric power system to supply electric power to the electrical energy dissipating system or to the electric machine during the regenerative braking when the level of electric power generated during the regenerative braking is higher than the current charging capacity of the electric storage system. 
     According to one example embodiment, the towing vehicle comprises a braking system for a vehicle. The braking system comprises the electric traction motor configured to propel the vehicle and to controllably regenerate electric power during regenerative braking of the vehicle, the electric machine comprising an output shaft, an air flow producing unit connected to the output shaft of the electric machine, the air flow producing unit being operable by the electric machine by rotation of the output shaft, wherein the air flow producing unit is arranged in an air conduit, an electric power system electrically connected to the electric machine, the electric power system comprising an electric storage system configured to receive and supply electric power, wherein the electric machine is operated by electric power received from the electric power system, the electric power system being further electrically connected to the electric traction motor and configured to receive electric power during regenerative braking, and the master control unit connected to the electric power system, the master control unit comprising processing circuitry configured to receive a signal indicative of a regenerative braking request for the towing vehicle, determine a level of electric power generated by the electric traction motor during the regenerative braking of the towing vehicle, receive a signal indicative of a current electric charging capacity of the electric storage system, compare the level of electric power generated during the regenerative braking with the current electric charging capacity of the electric storage system, and control the electric power system to supply electric power to the electric machine during the regenerative braking when the level of electric power generated during the regenerative braking is higher than the current charging capacity of the electric storage system. 
     Moreover, the electric storage system should be construed, as also indicated above, as an arrangement configured to receive and supply electric power. According to a preferred embodiment, the electric storage system is a battery which can be charged by electric power during regenerative braking, or to supply electric power to e.g., the electric machine for operation of the air flow producing unit. The electric storage system thus has a charging capacity, which corresponds to the available electric power that the electric storage system can receive before being filled. The charging capacity may also be based on e.g., the temperature of the electric storage system, state of charge (SOC), etc. Accordingly, when the electric storage system has reached its maximum allowed state of charge level it cannot receive any further electric power. The braking system may thus be referred to as an electric braking system. 
     Further features of, and advantages will become apparent when studying the appended claims and the following description. The skilled person will realize that different features may be combined to create embodiments other than those described in the following, without departing from the scope of the present disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above, as well as additional objects, features, and advantages, will be better understood through the following illustrative and non-limiting detailed description of exemplary embodiments, wherein: 
         FIG.  1    is a lateral side view illustrating an example embodiment of a vehicle combination in the form of a truck and a trailer vehicle unit; 
         FIGS.  2 A to  2 D  are schematic illustrations of systems for dissipating excessive energy from the vehicle combination in  FIG.  1    according to various example embodiments, 
         FIG.  3    is a detailed schematic illustration of one system for dissipating excessive energy from the vehicle combination in  FIG.  1   , according to another example embodiment, and 
         FIG.  4    is a flow chart of a method of controlling the system according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments are shown. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness. Like reference character refer to like elements throughout the description. 
     Referring now to the drawings and to  FIG.  1    in particular, there is depicted an exemplary vehicle combination  1  in the form of a towing vehicle  10  configured to be coupled to a trailer vehicle unit  20 . The vehicle combination is particularly suitable for cargo transport where the herein disclosed techniques can be applied with advantage. 
     As illustrated in  FIG.  1   , the vehicle combination  1  comprises the towing vehicle in the form of the truck  10 , a first trailer vehicle unit in the form of a semi-trailer  20 , and a second trailer vehicle unit in the form of a conventional trailer  20 . However, the truck may likewise be operated without a semi-trailer. Alternatively, the truck may likewise be operated only with the semi-trailer and not the second trailer. The trailer is here a conventional non-powered trailer, but may likewise be an electrified trailer. Further, any one of the semi-trailer and the trailer illustrated in  FIG.  1    is generally a trailer vehicle unit  20 , as further described herein. 
     Generally, each one of the towing vehicle and the trailer vehicle units of the vehicle combination comprises corresponding control units forming parts of a control system  98 , as will be further described in relation to  FIGS.  2 A to  4   . The control system is here a wireless control system. The wireless control system may of course also be implemented in other types of vehicle combinations. 
     While the propulsion systems of the towing vehicle  10  and the trailer vehicle units  20  of the vehicle combination may be provided in several different ways, the primary propulsion system of at least the towing vehicle  10  is an electric propulsion system. However, in some examples, the trailer vehicle unit  20  of the second trailer may comprises a corresponding electric propulsion system. 
     The towing vehicle  10  is illustrated in the form of a truck. The towing vehicle  10  comprises a traction motor  101  for propelling the wheels of the towing vehicle  10 . The traction motor  101  is in the example embodiment an electric traction motor  101  in the form of an electric machine, which is arranged to receive electric power from an electric power system ( 104  in  FIG.  2 A ) and/or directly from e.g., a fuel cell system. 
     If any one of the trailer vehicle units  20  is an electrified powered trailer, the trailer may comprise a corresponding electric machine. 
     Moreover, each one of the towing vehicle  10  and the trailer vehicle units  20  comprises one or more pair of wheels, and may often include a number of additional pair of wheels. At least some of the pair of wheels of the towing vehicle are driven by means of the electric machines. It should be readily appreciated that several different configurations may be conceivable depending on type of vehicle combination. By way of example, the truck may likewise be a hybrid truck including an internal combustion engine and the electric machine. 
     The towing vehicle  10  and trailer vehicle units  20  of the vehicle combination  1  can be mechanically coupled to each other in several different ways, e.g., by an articulated coupling. In  FIG.  1   , each one of the trailer vehicle units  20  comprises a coupling  22  configured to couple the corresponding trailer vehicle unit to another unit. By way of example, the first trailer vehicle unit  20  (semi-trailer) comprises the coupling  22  configured to couple the first trailer vehicle unit to the towing vehicle. By way of example, the truck  10  comprises a coupling part in the form of a fifth wheel configuration for the coupling  22  of the semi-trailer. Similarly, the semi-trailer comprises a rear coupling part  22  in the form of a rear drawbar for connecting with a drawbar of the second trailer. Accordingly, each one of the trailer vehicle units  20  in  FIG.  1    is configured to couple with a towing vehicle and/or another trailer vehicle unit. In this manner the trailer units  20  are coupled to each other such that they move with respect to each other when the vehicle combination is in motion. 
     As illustrated in  FIG.  1   , each one of the trailer vehicle units  20  also comprises one or more electrical energy dissipating systems  40 . Further details of some examples of electrical energy dissipating systems  40  will be described herein in relation to  FIGS.  2 A to  2 D ,  FIG.  3    and  FIG.  4   . 
     In this example embodiment, each one of the electrical energy dissipating systems  40  is detachably arranged to the corresponding trailer vehicle units  20 . By way of example, the trailer vehicle unit  20  comprises a mounting structure  60  for arrangement of the electrical energy dissipating system  40  to the trailer vehicle unit  20 . The mounting structure is here an integral part of the trailer vehicle unit  20 . In other examples, the mounting structure may an integral part of the electrical energy dissipating system  40 . The mounting structure  60  is here further provided in the form of brackets configured to arrange the electrical energy dissipating system  40  to the trailer vehicle unit  20 . 
     Moreover, in  FIG.  1   , the mounting structure  60  is arranged vertically below a supporting frame structure  24  of the trailer vehicle unit  20 . Hence, each one of trailer vehicle units  20  comprises a supporting frame structure  24  for arrangement of the mounting structure  60 . The supporting frame structure  24  may be a part of the chassis of the trailer vehicle unit  20 . 
     In addition, each one of the electrical energy dissipating systems  40  is releasably attached to the mounting structure  60 . By way of example, each one of the electrical energy dissipating system  40  is releasably attached to the mounting structure  60  by a number of fasteners, such as bolts and nuts. 
     In some designs, the electrical energy dissipating system  40  is configured to be retrofitted into already manufactured trailer vehicle unit. Alternatively, the trailer vehicle unit may be manufactured with the mounting structure  60  and the electrical energy dissipating system  40  releasably attached to the mounting structure. 
     Turning now to  FIG.  2 A , there is illustrated further details of one example embodiment of a trailer vehicle unit  20  coupled to the towing vehicle  10  so as to form the vehicle combination  1  in  FIG.  1   . 
     As illustrated in  FIG.  2 A , the towing vehicle  10  comprises a control unit  90  for controlling various operations as will also be described in further detail below, including at least an operation of performing an auxiliary braking action for the vehicle combination  1 . In this example, the control unit  90  of the towing vehicle is a master control unit. 
     Analogously, the trailer vehicle unit  20 , e.g., the semi-trailer or the second trailer in  FIG.  1   , comprises a slave control unit  90  for controlling various operations as will also be described in further detail below, but including at least an operation of performing an auxiliary braking action for the trailer vehicle unit  20 . In this example, the slave control unit  94  of the trailer vehicle unit  20  is at least partly controllable by the master control unit  90  of the towing vehicle. The master control unit  90  and the slave control unit  94  here forms part of the control system of the vehicle combination  1 . 
     Each one of the master control unit  90  and the slave control unit  94  generally comprises a corresponding processing circuitry  92 ,  95 , respectively. The processing circuitry of each one of the master control unit  90  and the slave control unit  94  is generally a processor. Each one of the processors may each include a microprocessor, microcontroller, programmable digital signal processor or another programmable device. Each one of the processors may also, or instead, each include an application specific integrated circuit, a programmable gate array or programmable array logic, a programmable logic device, or a digital signal processor. Where each one of the processors includes a programmable device such as the microprocessor, microcontroller or programmable digital signal processor mentioned above, the processor may further include computer executable code that controls operation of the programmable device. 
     The master control unit  90  and the slave control unit  94  are in communication with each other via a communication interface  98 . Typically, each one of the towing vehicle  10  and the trailer vehicle unit  20  comprises respective transceivers (although not shown) for receiving a request from each other. 
     Each transceiver may comprise a transmitter side and a receiver side or may constitute a combined device. Each transceiver may comprise or be connected to at least one antenna, which may be mounted on top of the corresponding vehicle and trailer vehicle unit, or underneath, or in a different suitable position. The antenna may comprise multiple antenna elements, especially on the receiver side. 
     In other words, the communication between the towing vehicle  10  and the trailer vehicle unit  20  is performed via the communication interface  98 . The communication interface is here performed by a wireless link, such as a communications network defined by the third-generation partnership program, 3GPP. Examples of such networks include 4G, 5G and 6G communication networks, as well as networks in the 802.11 family, in particular 802.11p. Alternatively, or in addition, the communication interface is performed by wire, as is commonly known in the art. 
     Thus, the communication interface  98  provides for transmitting data between the truck  10  and the trailer vehicle unit  20 . 
     It should also be noted that the towing vehicle and trailer vehicle unit are each configured to transfer and receive electric energy between each other in a coupled configuration forming the vehicle combination. By way of example, the transfer of surplus energy from the truck  10  to the trailer vehicle unit  20  may be handled via an electrical connection, here in the form of a conventional conductive coupling  96 , as exemplified by the line  96  in  FIG.  2 A . The conductive coupling is here provided in the form of a pair of conventional connectors arranged on the respective vehicles  10  and  20 . In other examples, the transfer of surplus energy from the truck  10  to the trailer vehicle unit  20  may be handled via an inductive coupling. Such electric energy transfer coupling can thus be provided in several different manners. The actual transfer of surplus energy from the towing vehicle  10  to the trailer vehicle unit  20  is generally controlled by the master control unit  90 . 
     In general, as described herein, when the towing vehicle  10  and the trailer vehicle unit  20  form the vehicle combination  1 , the control unit  90  of the truck  20  is generally the “master”, while the control unit of the trailer vehicle unit  20  is configured to act in a slave mode. Hence, as long as the truck  20  is connected to the trailer vehicle unit  20 , the truck  20  acts as master, while the one or more trailer vehicle units  20  operate in a slave mode. The communication between the master and slave mode control units  90  and  94  is either via wireless connection such as a unit to unit (U2U) communication or via some form of wired communication such as Ethernet communication between units. 
     Turning again to the components of the towing vehicle  10  and a driving operation involving a regenerative braking event, reference is made to  FIG.  2 A  which is a schematic illustration of vehicle combination  1  comprising a trailer vehicle unit  20  according to an example embodiment. As can be seen in  FIG.  2 A , the towing vehicle  10  comprises an electric traction motor  101 , in  FIG.  2 A  illustrated as a pair of electric traction motors  101 . The towing vehicle  10  further comprises an electric power system  104  which is connected to the electric traction motor(s)  101  for supply of electric power to the electric traction motor(s)  101  when the electric traction motor(s)  101  is/are propelling vehicle  10 , and to receive electric power from the electric traction motor(s)  101  when the electric traction motor(s)  101  operates in a regenerative braking mode. Thus, the towing vehicle  10  comprises an auxiliary braking system. The towing vehicle  10  may of course also comprise conventional braking systems for the wheels. 
     The electric power system  104  further comprises an electric storage system  160 . The electric storage system  160  is preferably arranged in the form of a vehicle battery and will in the following be referred to as a battery  162 . The battery is generally a high voltage battery. The battery  162  is configured to receive electric power generated by the electric traction motor(s)  101  when the electric traction motor(s)  101  operates in the regenerative braking mode. The battery  162  is also arranged to supply electric power to the electric traction motor(s)  101  when the electric traction motor(s)  101  propel the vehicle  10 . Although not depicted in  FIG.  2 A , the electric power system  104  may comprise various components, such as traction inverters, brake inverters, a junction box, etc. 
     The above-described master control unit  90  is connected to the electric power system  104 . The master control unit  90  comprises the processing circuitry  92  for controlling operation of the electric power system. The master control unit  90  thus receives data from the electric power system  104 , such as e.g., a state-of- (SOC) of the battery  162 , etc, and transmits control signals to the electric power system  104 . As will be evident from the below disclosure, the control signals from the master control unit  90  to the electric power system  104  may, for example, comprise instructions to which device the electric power system  104  should supply electric power during regenerative braking. 
     The towing vehicle  10  further comprises an electric machine  102  connected to the electric power system  104 . The electric machine  102  is thus operated by receiving electric power from the electric power system  104 . The electric machine  102  is thus arranged as an electric power consumer. The towing vehicle  10  also comprises an air blower  106 . The air blower is an air flow producing unit. The air blower  106  is preferably an air compressor  106  and will in the following be referred to as such. The air compressor  106  is arranged in an air conduit  111  and configured to receive air  113 . The received air  113  is pressurized by the air compressor  106  and supplied further through the air conduit  111  downstream the air compressor  106 . The air compressor  106  is connected to, and operable by, the electric machine  102 . As illustrated in  FIG.  2 A , the air compressor  106  is mechanically connected to the electric machine  102  by an output shaft  107  of the electric machine  102 . In further detail, the air compressor  106  is operated by rotation of the output shaft  107 , which rotation is generated by operating the electric machine  102 . 
     According to the exemplified embodiment in  FIG.  2 A , the towing vehicle  10  further comprises a flow restriction arrangement  103  in the air conduit  111 . The flow restriction arrangement  103  is arranged in downstream fluid communication with the air compressor  106  and configured to increase the pressure level of the flow of air exhausted by the air compressor  106 . The towing vehicle  10  also comprises an air heating arrangement  108  in the air conduit  111 . The air heating arrangement  108  comprises at least one electric air heater  108   a . In the following, the air heating arrangement  108  will also be referred to as an electric brake resistor arrangement  108 , and the electric air heater(s) will be referred to as electric brake resistors  108   a.    
     The electric brake resistor arrangement  108  is arranged in the air conduit  111  in downstream fluid communication with the air compressor  106 . The electric brake resistor arrangement  108  is also electrically connected to, and operable by, the electric power system  104 . Thus, also the electric brake resistor arrangement  108  is arranged as an electric power consumer. When the electric brake resistor arrangement  108  receives electric power from the electric power system  104 , the pressurized air from the air compressor is heated by the at least one brake resistor  108   a . The pressurized and heated air is thereafter directed towards the ambient environment or other components in need of thermal management. The air from the electric brake resistor arrangement  108  is preferably directed into a muffler  150  of the braking system  100 . The muffler  150  reduces noise and can also provide a pressure drop of the air. 
     The electric brake resistor arrangement  108  is preferably, although not depicted, connected to a cooling system in order to avoid excess heating. The cooling system may either be a liquid cooling system or an air-cooling system. Thus, the electric brake resistor arrangement  108  may be a liquid cooled electric brake resistor arrangement  108  or an air-cooled electric brake resistor arrangement  108 . 
     Although not depicted in  FIG.  2 A , it should be readily understood that the master control unit  90  can be connected to other components in addition to the connection to the electric power system  104 . For example, the master control unit  90  may be connected to the electric traction motor(s)  101 , the battery  162 , the electric machine  102 , the air heating arrangement  108 , as well as connected to an upper layer vehicle control system (not shown). The master control unit  90  is also connected to the slave control unit  94  when the trailer vehicle unit  20  is coupled to the towing vehicle  10 . 
     During operation of the towing vehicle  10 , i.e., when the electric traction motor  101  operates as generators to control the vehicle speed, i.e., the towing vehicle  10  operates in the regenerative braking mode, electric power is transmitted from the electric traction motor  101  to the electric power system  104 . If the battery  162  is not able to receive all, or parts of the electric power generated by the electric traction motor  101 , for example because of a current electric charging capacity, i.e. the level of electric power the battery is able to receive until being fully charged or has reached its maximum allowed state of charge level, the excess electric power should preferably be dissipated. In the present case, the electric power system  104  is controlled to supply electric power to the electric machine  102 . The electric machine  102  is hereby, by the received electric power from the electric power system  104 , rotating the output shaft  107  to operate the air compressor  106 . The air compressor  106  in turn pressurize air  117  and supply the pressurized air further through the air conduit  111 . 
     Turning now to the trailer vehicle unit  20  in  FIG.  2 A , there is depicted one example embodiment of a trailer vehicle unit  20  configured to transport cargo etc. when coupled to the towing vehicle  10 . Similar to the components described above in relation to the towing vehicle  10 , the trailer vehicle unit  20  also includes a functionality of dissipating excessive electric energy. Hence, as briefly mentioned above, and as illustrated in  FIG.  2 A , the trailer vehicle unit  20  comprises the electrical energy dissipating system  40 . When the trailer vehicle unit  20  is coupled to the towing vehicle  10 , as illustrated in  FIG.  1   , one or more components of the electrical energy dissipating system  40  is arranged in a connected state with one or more components of the towing vehicle, as will be described hereinafter. 
     In  FIG.  2 A , the electrical energy dissipating system  40  comprises an electric machine  52 . The electric machine  52  is connected to the electric power system  104  of the towing vehicle by an electric connection  96 . In this manner, electric energy from the electric power system  104  can be transferred to the electrical energy dissipating system  40 . 
     As such, the electric machine  52  is configured to be operated by receiving electric power from the electric power system  104 . In other words, the electric machine  52  is configured to be powered by the electric power system  104  of the towing vehicle  10 . The electric machine  52  is thus arranged as an electric power consumer in the trailer vehicle unit  20 . 
     In addition, the electrical energy dissipating system  40  is at least partly disposed in an air conduit  70  for receiving air. The air conduit  70  is generally similar in its configuration as the air conduit  111  of the towing vehicle  10 . The air conduit  70  is here an integral part of the electrical energy dissipating system  40 . 
     The electrical energy dissipating system  40  also comprises an air flow producing unit  42 . The air flow producing unit  42  is preferably an air compressor and will in the following be referred to as such. The air compressor  42  is arranged in the air conduit  70  and configured to receive air  72 . The received air  72  is generally air supplied from the ambient environment. Hence, the received air  72  is here ambient air. The received air  72  is pressurized by the air compressor  42  and supplied further through the air conduit  70  downstream the air compressor  42 . The air compressor  42  is connected to, and operable by, the electric machine  52 . As illustrated in  FIG.  2 A , the air compressor  42  is mechanically connected to the electric machine  52  by an output shaft  57  of the electric machine  52 . In further detail, the air compressor  42  is operated by rotation of the output shaft  57 , which rotation is generated by operating the electric machine  52 . The output shaft  57  is here an integral part of the electric machine  42 . 
     As further illustrated in  FIG.  2 A , the electrical energy dissipating system  40  comprises an air heating arrangement  44  in the air conduit  70 . The air heating arrangement  44  comprises at least one electric air heater  44   a . The electric air heater  44   a  is operable from the received electric energy. In the following, the air heating arrangement  44  will also be referred to as an electric brake resistor arrangement, and the electric air heater  44   a  will be referred to as an electric brake resistor. 
     The electric brake resistor arrangement  44  is arranged in the air conduit  70  in downstream fluid communication with the air compressor  42 . The electric brake resistor arrangement  44  is also electrically connected to, and operable by, the electric machine  52 . Thus, also the electric brake resistor arrangement  44  is arranged as an electric power consumer. When the electric brake resistor arrangement  44  receives electric power from the electric power system  104 , the pressurized air from the air compressor  42  is heated by the at least one brake resistor  44   a . The pressurized and heated air is thereafter directed towards the ambient environment or other components of the trailer vehicle unit  20  in need of thermal management. The air from the electric brake resistor arrangement  44  is preferably directed into a muffler (not illustrated). A muffler is configured to reduce noise and can also provide a pressure drop of the air. 
     The electric brake resistor arrangement  44  is possibly, although not depicted, connected to a cooling system of the trailer vehicle unit  20  in order to avoid excess heating. The cooling system may either be a liquid cooling system or an air-cooling system. Thus, the electric brake resistor arrangement  44  may be a liquid cooled electric brake resistor arrangement or an air-cooled electric brake resistor arrangement. 
     As mentioned above, the trailer vehicle unit  20  further comprises the slave control unit  94  having the processing circuitry  95 . The processing circuitry  95  is configured to receive a control signal from the towing vehicle  10 . In particular, the processing circuitry  95  is configured to receive a control signal from the corresponding processing circuitry  92  of the master control unit  90 . The control signal generally contains data indicative of a request for dissipating energy from the electrical energy dissipating system  40 , as further described herein. In  FIG.  2 A , a dashed line such as the line  98  generally refers to a communication line for transferring control signals of various types between the components. 
     In addition, the slave control unit  94  is in communication with the electrical energy dissipating system  40 . More specifically, the slave control unit  94  is in communication with the electric machine  52 , the air flow producing unit  42  and the air heating arrangement  44 . Although not depicted in  FIG.  2 A , it should be readily understood that the slave control unit  94  can be connected to and in communication with other components in addition to the connection to the electrical energy dissipating system  40 . For example, the slave control unit  94  may be connected to any electric traction motor(s) and battery of the trailer vehicle unit  20 . 
     In particular, the slave control unit  94  is connected to the master control unit  90  when the trailer vehicle unit  20  is coupled to the towing vehicle  10 . Accordingly, the slave control unit  94  is a so-called slave system to the higher-level brake dissipation demand from the towing vehicle truck  10 . 
     In other words, the processing circuitry  95  of the slave control unit  94  is configured to receive a control signal from the towing vehicle  10  when the trailer vehicle unit  20  is in the coupled state with the towing vehicle  10 . Hence, the processing circuitry  95  of the slave control unit  94  is configured to receive the control signal from the towing vehicle  10 . The control signal is indicative of a request for dissipating energy from the electrical energy dissipating system  40  when the trailer vehicle unit  20  is in the coupled state with the towing vehicle  10 . The coupled state thus generally refers to a state between the towing vehicle  10  and the trailer vehicle unit  20  wherein the trailer vehicle unit  20  is mechanically coupled to the towing vehicle  10 , and wherein there is an electrical connection established between the trailer vehicle unit  20  and the towing vehicle  10 . In addition, in the coupled state, there is a communication established between the slave control unit  94  of the trailer vehicle unit  20  and the master control unit  90  of the towing vehicle  10 . 
     In operation of the vehicle combination  1 , the processing circuitry  92  of the master control unit  90  determines a level of electric power dissipation for the electric power system  104 , i.e., a level of electric power that should be dissipated since it is not suitable to supply such power to the battery  162 . The level of electric power dissipation is hence a difference between the level of electric power generated during the regenerative braking and the current electric charging capacity of the battery  162 . If the electric machine  102  is able to handle, i.e., receive and be operated by, electric power corresponding to the level of electric power dissipation, all excess electric power, i.e., the generated power not being supplied to the battery  162  for charging, is supplied to the electric machine  102 . 
     However, the electric machine  102  may have a motor dissipation threshold. In further detail, the motor dissipation threshold is a maximum capacity of how much electric power the electric machine  102  can receive. Another limiting factor could be a temperature level of the air compressor  106 , as well as a temperature level of the electric machine  102 , e.g., at high ambient temperature conditions. If the electric machine  102  receives too much electric power, the rotational speed of the output shaft  107  is at a risk of being too high, or the temperature level of the electric machine or the air compressor  106  could be too high. 
     As such, the processing circuitry  92  of the master control unit  90  preferably compares the level of electric power generated during regenerative braking with the motor dissipation threshold. When the level of electric power generated during regenerative braking is higher than the motor dissipation threshold, the electric power system  104  is controlled to supply electric power to any one of the electric brake resistors  108   a , the air compressor  106  and the electrical energy dissipating system  40  of the trailer vehicle unit  20 . 
     If the master control unit  90  determines to supply electric energy to the electrical energy dissipating system  40  of the trailer vehicle unit  20 , the master control unit  90  sends control signal and a request to the slave control unit  94  of the trailer vehicle unit  20 . As mentioned above, the control signal generally contains a request for dissipating energy from the electrical energy dissipating system  40 . When the slave control unit  94  receives the control signal from the towing vehicle  10 , the processing circuitry  95  controls the electrical energy dissipating system to dissipate energy from the electrical energy dissipating system  40 . 
     By way of example, the electrical energy dissipating system  40  may be controlled to receive the excess electric power from the electric power system  104  and further controlled to supply electric power to the electric machine  52 . The electric machine  52  is hereby, by the received electric power from the electric power system  104 , rotating the output shaft  57  to operate the air compressor  42 . The air compressor  42  in turn pressurize air and supply the pressurized air further through the air conduit  70 . In particular, the air compressor  42  is arranged to compress the received air and transmit the compressed air to the brake resistor arrangement  44 . 
     The operation is thus generally similar to the operation as described above with regards to dissipating energy from the towing vehicle  10 . 
     In addition, or alternatively, the electrical energy dissipating system  40  may be controlled to receive the excess electric power from the electric power system  104  and further controlled to supply electric power to the electric brake resistors  44   a.    
     The request from the master control unit  90  to the slave control unit  94  is generally based on the current energy dissipation request for the towing vehicle  10 , That is, the request from the master control unit  90  to the slave control unit  94  is generally based on a desired energy level of the electric storage system  160 ,  162  at an upcoming driving position for the vehicle combination  1 . 
     Prior to the vehicle combination  1  arrives at the upcoming driving position, any one of the master control unit  90  and slave control unit  94  is configured to control the electrical energy dissipating system  40  to control the electric machine  52  to propel the air flow producing unit  42  for producing a flow of air to the brake resistor arrangement  44 ; and control the electrical energy dissipating system  40  to supply electric power to the brake resistor arrangement  44  for heating the air received by the brake resistor  44   a.    
     Further, the split of electric power supply to the electric machine  42  and the electric brake resistor  44   a  can also, for example, be controlled to provide a desired brake performance, a low outlet temperature and/or to reduce wear of components of the trailer vehicle unit  20 , etc. In particular, the temperature level of the electric brake resistor may be used as an input parameter when determining how much electric power to supply to the electric machine  52 . 
     Any one of the master control unit  90  and slave control unit  94  are further configured to: determine, based on the current energy dissipation request, an amount of energy to dissipate from the electrical energy dissipating system  40 ; determine a first maximum operational power level of the air flow producing unit  42 ; determine a second maximum operational power level of the brake resistor arrangement  44 ; and control the electric machine  52  and the brake resistor arrangement  44  to dissipate the amount of energy without exceeding the first and second maximum operational power levels. 
     By way of example, the slave control unit  94  is further configured to:
         receive a signal indicative of a braking request for the vehicle combination  1 , and   control the electrical energy dissipating system  40  to dissipate energy, e.g., by controlling the electric machine  52  to propel the air flow producing unit  42  and/or the electric machine  52  to supply electric power to the brake resistor arrangement  44  when the vehicle combination requests braking.       

     Further control functionalities of the electrical energy dissipating system  40  will be given below in relation to the description of  FIG.  4   . 
       FIG.  2 B  depicts another example embodiment of an electrical energy dissipating system  40 . The functional operation of the towing vehicle  10  and the trailer vehicle unit  20  in  FIG.  2 B  is generally the same as described above in relation to  FIG.  2 A . The electrical energy dissipating system  40  in  FIG.  2 B  is, however, not provided with the air heating arrangement  44 , as described in relation to  FIG.  2 A . Rather, the electrical energy dissipating system  40  in  FIG.  2 B  here comprises the electric machine  52  and further the air flow producing unit  42  connected to the output shaft  57  of the electric machine  52 . The air flow producing unit  42  is here an air compressor operable by the electric machine  42  by rotation of the output shaft  57 . Further, as illustrated in  FIG.  2 B , the air compressor  42  is arranged in the air conduit  70 . The electrical energy dissipating system  40  also includes the output shaft  57  connecting the electric machine  52  with the air compressor  42 . The electrical energy dissipating system  40  in  FIG.  2 B  is also controlled in similar vein as the system in  FIG.  2 A , i.e., by the slave control unit  94  upon a request from the towing vehicle  10 . 
       FIG.  2 C  depicts another example embodiment of an electrical energy dissipating system  40 . The functional operation of the towing vehicle  10  and the trailer vehicle unit  20  in  FIG.  2 C  is generally the same as described above in relation to  FIG.  2 A . The electrical energy dissipating system  40  in  FIG.  2 C  is, however, not provided with the electric machine  52  and the air flow producing unit  42 , as described in relation to  FIG.  2 A . Rather, the electrical energy dissipating system  40  in  FIG.  2 C  here comprises the air heating arrangement  44 . The air heating arrangement  44  comprises the electric air heater  44   a  operable by electric energy. The electric air heater  44   a  is here an electric brake resistor. Further, as illustrated in  FIG.  2 C , the electric air heater  44   a  is arranged in the air conduit  70 . The electric air heater  44   a  of the electrical energy dissipating system  40  is operable by electric energy from the towing vehicle  10 , as described above. The electrical energy dissipating system  40  in  FIG.  2 C  is also controlled in a similar vein as the system in  FIG.  2 A , i.e., by the slave control unit  94  upon a request from the towing vehicle  10 . 
     In another example embodiment of the electrical energy dissipating system  40 , the air conduit is replaced with a liquid coolant conduit. Further, the air heating arrangement with the electric air heater is here a liquid coolant heating arrangement having an electric liquid coolant heater operable by electric energy. The electric liquid coolant heater is an electric brake resistor. Hence, this example arrangement resembles the example embodiment in  FIG.  2 C  in terms of the arrangement of the component. That is, the electric brake resistor is still operable by electric energy supplied from the electric power system  140 , but it is disposed in another type of conduit, namely a fluid medium conduit in the form of a liquid coolant fluid medium. The liquid coolant is e.g., a coolant based on glycol. Hence, to sum up, this example embodiment provides a trailer vehicle unit  20  having an electrical energy dissipating system  40  comprising a liquid coolant heating arrangement. 
     The liquid coolant heating arrangement comprises at least one electric brake resistor arrangement operable from the received electric energy. The liquid coolant heating arrangement is further arranged in the liquid coolant conduit. In an example with a liquid coolant conduit, the liquid coolant conduit is a recirculation conduit, i.e., a closed coolant circuit loop (not illustrated in  FIG.  2 C ). Typically, there is also disclosed a cooler in the liquid coolant conduit. The cooler is configured to regulate the temperature of the coolant. The cooler can be a heat exchanger, a condenser, or the like, and may further include a fan (not illustrated). Optionally, there is also disposed a coolant pump (not illustrated) in the liquid coolant conduit for pumping the coolant therethrough. As such, this example embodiment with a liquid coolant works by passing liquid continually through or around the electric brake resistor arrangement. Powered by the coolant pump, the coolant is circulated through the conduit and absorbs heat from the electric brake resistor. 
       FIG.  2 D  depicts another example embodiment of an electrical energy dissipating system  40 . The functional operation of the towing vehicle  10  and the trailer vehicle unit  20  in  FIG.  2 D  is generally the same as described above in relation to  FIG.  2 A . The embodiment in  FIG.  2 D  only describes the components arranged in direct connection with the air conduit  70 . Hence, the electrical energy dissipating system  40  in  FIG.  2 D  comprises the components as described in relation to  FIG.  2 A . 
     As illustrated in  FIG.  2 D , the electrical energy dissipating system  40  further comprises a flow restriction arrangement  47  in the air conduit  70 . The flow restriction arrangement  47  is arranged in downstream fluid communication with the air compressor  42  and configured to increase the pressure level of the flow of air exhausted by the air compressor  42 . 
     Moreover, the electrical energy dissipating system  40  here comprises an air flow restriction arrangement  48  positioned downstream the air heating arrangement  44 . 
     In addition, the electrical energy dissipating system  40  here comprises an air heating arrangement in the form of a heat exchanger  46 . The heat exchanger  46  is arranged upstream the air compressor  42  in air conduit  70 . 
     Reference is now made to  FIG.  3    in order to describe another example embodiment of the towing vehicle  10  that can be combined with the electrical energy dissipating system  40  of the trailer vehicle unit  20 . The  FIG.  3    example is a further detailed illustration of components that may form part of the towing vehicle  10 . The embodiment in  FIG.  3    only describes the components arranged in direct connection with the air conduit  111 . The functional operation of the towing vehicle  10  and the trailer vehicle unit  20  in  FIG.  3    is thus the same as described above in relation to  FIG.  2 A . The master control unit  90  is omitted from  FIG.  3    and the following description but should be construed as also being incorporated in this example embodiment. 
     As can be seen in  FIG.  3   , the towing vehicle  10  comprises the electric machine  102  arranged to receive electric power from the electric power system  104  described above in relation to  FIG.  2 A . Moreover, the electric machine  102  can also be connected to a cooling system  105  of the vehicle  10 . The cooling system  105  may either be a liquid cooling system or an air-cooling system. 
     The towing vehicle  10  further comprises the above-described air compressor  106 , which here is mechanically connected to, and operated by, the electric machine  102 . Preferably, the air compressor  106  is mechanically connected to the electric machine  102  by the output shaft  107 . 
     The towing vehicle  10  further comprises the above-described air heating arrangement  108 ,  110 . In  FIG.  3   , one of the air heating arrangements  108 ,  110  is arranged upstream the air compressor  106  in the form of a heat exchanger  110 . The air heating arrangement  108  arranged downstream the air compressor  106  is preferably of the same type as described above, i.e., an electric brake resistor arrangement  108 . 
     Similar to the embodiment described in relation to  FIG.  2 A , the electric air heating arrangement  108  may be arranged in the air conduit  111  at a position downstream the air compressor  106 , i.e., for receiving pressurized air from the air compressor  106 . The electric air heating arrangement  108  is connected to the electric power system  104 . 
     As also described above, the electric air heating arrangement  108  is preferably implemented in the form of an electric brake resistor arrangement  108  comprising at least one electric brake resistor  108 . The electric air heating arrangement  108  thus receives the pressurized air from the air compressor  106 , whereby the air is heated in the electric air heating arrangement by the electric power received from the electric power system  104 . The air is thereafter preferably supplied towards the muffler  150  and further to the ambient environment or a component/structure in need of thermal management. 
     Further, the heat exchanger  110  is arranged in upstream fluid communication with the air compressor  106  in the air conduit  111 . The heat exchanger  110  can instead be arranged further downstream in the air conduit  111 , and the illustration in  FIG.  3    is merely for illustrative purposes. The heat exchanger  110  is in  FIG.  3    arranged as a heat exchanger connected to the cooling system  105  of the towing vehicle  10 . Thus, the heat exchanger receives liquid fluid from the cooling system  105  and pre-heats the air before it is delivered to the air compressor  106 . The heat exchanger  110  is thus preferably an air-to-liquid heat exchanger but may, as an alternative, be an air-to-air heat exchanger which uses relatively warm air to heat the air that is supplied to the air compressor  106 . As a not depicted alternative, the heat exchanger  110  may be replaced by the electric machine  102 . In such a case, the electric machine receives the air, and pre-heats the air before the air is supplied to the air compressor  106 . The heat exchanger  110  may also be arranged at other positions in the material transportation system of the vehicle combination  1  than what is depicted in  FIG.  3   . For example, the heat exchanger  110  may be in the air conduit  111  downstream the air compressor  106 . The arrangement described in  FIG.  3    may also be used in other vehicle combinations, e.g., in combination with a sand blasting equipment, steamer, etc. 
     Furthermore, the exemplified towing vehicle  10  of  FIG.  3    comprises a flow injecting arrangement  402  positioned in the air conduit  111 . The flow injecting arrangement  402  is arranged in downstream fluid communication with the air compressor  106 , i.e., the flow injecting arrangement  402  receives the pressurized air exhausted from the air compressor  106 . Although the flow injecting arrangement  402  is depicted as being positioned downstream the air compressor  106 , it could instead, or in addition, be arranged upstream the air compressor  106 . Although not depicted in detail in  FIG.  3   , the flow injecting arrangement  402  comprises a portion configured to admit a flow of fluid into the flow of air exhausted from the air compressor  106 . As can be seen in  FIG.  3   , the flow injecting arrangement  402  is arranged in the form of a venturi arrangement comprising a constricted portion  115 . The flow injecting arrangement may as an alternative be formed by e.g., a pump injecting a flow through a nozzle, etc. The constricted portion  115  is arranged as a reduced diameter of the venturi arrangement in which the flow velocity of the flow of air from the air compressor  106  will increase. The portion configured to admit the flow of fluid into the venturi arrangement is preferably arranged at the constricted portion  115  of the venturi arrangement. As can be seen in  FIG.  3   , the portion is arranged as an orifice  404  in which a flow of fluid  109  can enter the constricted portion  115 . According to the example embodiment depicted in  FIG.  3   , the venturi arrangement  402  is arranged in downstream fluid communication with the electric air heating arrangement  108 . It should however be readily understood that the venturi arrangement  402  can be arranged in upstream fluid communication with the electric air heating arrangement  108 , i.e., between the air compressor  106  and the electric air heating arrangement  108 . 
     Turning now to an example of a braking force strategy for the vehicle combination  1 , including the operation of the electrical energy dissipating system  40  so as to dissipate excessive energy generated from a regenerative braking event of the vehicle combination. The electrical energy dissipating system  40  can be provided according to any one of the systems  40  described in relation to  FIGS.  2 A to  2 D  and  FIG.  3   . 
     In regard to the master control unit  90  of the towing vehicle  10 , the processing circuitry  92  is arranged in communication with the electric power system  104  of the towing vehicle  10 . Moreover, the processing circuitry  92  is configured to control the electrical energy dissipating system  40  of the trailer vehicle unit  20 . Typically, the processing circuitry  92  is configured to control the electrical energy dissipating system  40  of the trailer vehicle unit  20  by communicating one or more control signals to the slave control unit  94 . The slave control unit  94  is here configured to control the operation of receiving electric energy from the towing vehicle  10  to the electrical energy dissipating system  40  in response to the received signal from the master control unit  90  of the towing vehicle  10 . In this manner, the slave control unit  94  is configured to control and permit the electrical energy dissipating system  40  to receive electrical energy and dissipate the received electrical energy generated from the regenerative braking operation of the towing vehicle  10 . 
     In particular, the master control unit  92  is configured to receive the signal indicative of a regenerative braking request for the vehicle combination  1 , determine a level of electric power generated by the electric traction motor  101  during the regenerative braking of the vehicle combination  1 , receive a signal indicative of a current electric charging capacity of the electric storage system  160 , compare the level of electric power generated during the regenerative braking with the current electric charging capacity of the electric storage system  160 , and control the electric power system  104  to supply electric power to the electrical energy dissipating system  40  during the regenerative braking when the level of electric power generated during the regenerative braking is higher than the current charging capacity of the electric storage system  160 . 
     The master control unit  90  is further configured to selectively control the electric power system  104  to supply electric power to any one of the electrical energy dissipating system  40  and the electric machine  102  during the regenerative braking when the level of electric power generated during the regenerative braking is higher than the current charging capacity of the electric storage system  160 . 
     It should also be noted that the slave control unit  94  is configured to control the supply of electric energy from a traction electric motor of the trailer vehicle unit  20  to the electrical energy dissipating system  40  in response to control signal from the master control unit  90  of the towing vehicle  10 . In this manner, the slave control unit is configured to control and permit the electrical energy dissipating system  40  to dissipate the received electrical energy generated from a regenerative braking operation of the trailer vehicle unit  20 , when connected to the towing vehicle  10 . Such situation may e.g., arise when the trailer vehicle unit  20  comprises its own electric drive axle arrangement. Electric drive axle arrangement for trailers can be provided in several different ways, but may generally include one or more electric hub motors for providing traction to one or more wheels. To this end, the electrical energy dissipating system  40  is configured to receive electric energy from the electric hub motors of the trailer vehicle unit  20 . The combination of the electrical energy dissipating system  40  in the trailer vehicle unit  20  and an electric drive axle arrangement may be particularly suitable for avoiding, or at least reducing the risk of jack knifing between the tractor vehicle  10  and the trailer vehicle unit  20 . The combination of the electrical energy dissipating system  40  in the trailer vehicle unit  20  and the electric drive axle arrangement provides for enhance energy dissipation so as to allow braking with the trailer vehicle unit  20  and any traction motors on the trailer vehicle unit. 
     In order to sum up, reference is made to  FIG.  4    which is a flow chart of a method  200  of controlling the electrical energy dissipating system  40  of the trailer vehicle unit  20  when coupled to the towing vehicle  10 . During operation of the vehicle combination  1 , i.e., when the trailer vehicle unit  20  is coupled to the towing vehicle  10 , and when there is a desire to control the vehicle speed, the electric traction motor  101  is controlled S 1  to perform regenerative braking of the towing vehicle  10 . A level of electric power generated by the electric traction motor  101  during the regenerative braking is determined S 2 , and a current electric charging capacity of the battery  162  is determined S 3 . These steps, S 1  to S 3 , are generally performed by the master control unit  90 . Optionally, these steps may be performed by support from the slave control unit  94 . 
     Subsequently, the level of generated electric power is compared S 4  with the charging capacity. This step is generally performed by the master control unit  90 . Optionally, this step may also be performed by support from the slave control unit  94 . 
     When the level of electric power generated during the regenerative braking is higher than the current charging capacity of the battery  162 , the electric power system  104  is controlled to supply electric power to the electrical energy dissipating system  40 , wherein the electrical energy dissipating system  40  is controlled to receive the electric power from the towing vehicle  10  and further controlled to dissipate the received electric power by utilizing the flow of air, as described above. The electric power system  104  is generally controlled by the master control unit  90 , as mentioned above. In addition, the electrical energy dissipating system  40  is generally controlled by the slave control unit  94 , as mentioned above. However, it may be noted that the master control unit may indirectly control the operation of the electrical energy dissipating system  40  by the control signal to the slave control unit  94 . 
     By way of example, the slave control unit  94  is typically configured to control the distribution of excessive electric energy to any one of the electric consumers of the electrical energy dissipating system  40 . By way of example, the slave control unit  94  is configured to control the electrical energy dissipating system  40  so that excessive electric energy is distributed to the electric machine  52  and to the electric brake resistors  44  in response to the received control signal from the master control unit  90 . The distribution of electric energy to the electric machine  52  and to the electric brake resistors  44  can either be performed in a simultaneous manner or in a selective manner. 
     Accordingly, as described herein, the control of the electrical energy dissipating system  40  relates not only to the need for braking of the vehicle combination, but also the available capacity of the braking capacity of the vehicle combination  1 . By way of example, when the master control unit  90  has received information of a connected slave control unit  94  and data indicating information about the trailer vehicle unit&#39;s braking capacity, the control of the vehicle combination in regard to braking and regenerative braking can be optimized for this particular vehicle combination. So, if the trailer vehicle unit  20  provides for a high auxiliary brake power capacity to the vehicle combination, the control in regard to charging and braking can be adjusted accordingly, e.g., permitting a later operation of dissipating energy from the system  40  if the braking capacity of the trailer vehicle unit  20  is higher compared to a trailer without such system  40 . This will lead to an overall improved energy management for the vehicle combination during operating. 
     It should be noted that the trailer vehicle unit  20  may of course also comprise a conventional braking system for the wheels. Such braking system may e.g., be a conventional disc brake or the like. 
     In addition, the trailer vehicle unit  20  may generally comprise additional components such as power electronics and electrical connections to connect and control the components of the electrical energy dissipating system. 
     The invention also relates to the vehicle combination  1  formed by the towing vehicle  10  and one or more trailer vehicle units  20 , as described herein. It should be noted that the trailer vehicle unit  20  may be coupled to any type of towing vehicle with a control unit and an energy management system for dissipating excessive energy from a regenerative braking event, and thus not necessarily a towing vehicle as described in relation to  FIG.  1   . Hence, one conceivable example is a system where there only are the controls at high level in the truck, but no actual component configured to dissipate energy within the towing vehicle  10 , such as the compressor and/or braking resistor in the towing vehicle  10 . 
     It is to be understood that the present disclosure is not limited to the embodiments described above and illustrated in the drawings; rather, the skilled person will recognize that many changes and modifications may be made within the scope of the appended claims.