The present invention relates to a semi-trailer, particularly a semi-trailer which is configured to be pulled by a tractor to form an articulated heavy goods vehicle, and an electric drive module suitable for mounting on such a semi-trailer.

Such semi-trailers are provided with a chassis which supports a trailer body into which a load may be placed. The chassis is supported at its rear end by an axle carrying at least a first pair of ground engaging wheels, whilst further axles and wheels may be provided further along the chassis towards the front end thereof. In use, the front end of the semi-trailer is supported by a tractor unit. The tractor unit has, at its rear, a generally horizontal platform on which is provided a hitch part generally known as a fifth wheel. A corresponding coupling part known generally as a kingpin is located on the underside of the chassis close to the front end thereof, so that the semi-trailer may be connected to a tractor unit with the front end of the semi trailer resting on the platform and the kingpin engaged with the fifth wheel.

As a large proportion of the weight of the semi-trailer is supported by the tractor unit, the semi-trailer is provided with landing gear, such as support or landing legs, which may be lowered to support the front end of the semi-trailer when decoupled from a tractor unit.

As they are intended to be pulled by a tractor unit, such semi-trailers are not provided with a drive engine or motor. Refrigerated semi-trailers may, however, be provided with an auxiliary diesel engine to power refrigeration equipment.

Commercial semi-trailers are often required to be moved around distribution centres and loading yards, and conventionally this is achieved by using a pedestrian operated trailer mover, yard tug, or simply a conventional tractor unit of the sort described above and normally used to pull the semi-trailer. The power to drive conventional tractor units and yard tugs is provided by a diesel engine, which can be noisy and produce exhaust gas emissions.

It is an object of the present invention to provide an improved configuration of semi trailer. According to first aspect of the invention we provide a semi-trailer comprising a chassis, a first pair of ground-engaging wheels located at and supporting a rear end of the chassis, a coupling part which is located underneath the chassis at the front end of the semi-trailer and which is configured to couple the semi-trailer to a tractor unit, and a second pair of ground engaging wheels which are located towards the front end of the semi-trailer relative to the first pair of wheels, wherein the semi-trailer is further provided with an electrically powered motor which is connected to one of the first or second pair of wheels and operable to drive rotation of the first or second pair of wheels respectively.

As such, the motor may be used to move the semi-trailer around a distribution centre or loading yard either whilst coupled to a tractor unit or yard tug or whilst uncoupled from either a tractor unit, yard tug or trailer mover, without the noise and exhaust gas emissions associated with use of the diesel engine of the tractor unit or yard tug.

The semi-trailer preferably further comprises an electrical energy storage apparatus, such as a battery, which is connected to and provides electrical power to the motor.

The coupling part may be configured to engage with a hitch on a tractor unit in such a way that the coupling part can pivot relative to the hitch about an axis of the coupling part so that the semi-trailer can articulate relative to the tractor unit.

The coupling part may comprise a kingpin which is configured to engage with a fifth wheel hitch on a tractor unit.

The semi-trailer may further comprise a generator which converts rotational movement of the motor driven pair of wheels to electrical power.

In this case, the generator may be connected to the electrical storage apparatus such that the electrical power it generates charges the electrical storage apparatus.

The semi-trailer may comprise an auxiliary electrically powered apparatus to which the generator is connected such that the electrical power generator by the generator can be used to power the auxiliary electrically powered apparatus. The semi-trailer may further comprise an electrical controller which is configured to control the operation of the motor, the controller having a processor which is programmed to drive the semi-trailer autonomously.

The motor may be connected to the second pairs of wheels and operable to drive rotation of the second pair of wheels.

The chassis may comprise a first chassis part which is located at the rear end of the semi-trailer and a second chassis part which is located at the front end of the semi trailer, the first pair of wheels being mounted on the first chassis part, and the second pair of wheels and coupling part being mounted on the second chassis part, and the first and second chassis parts being connected together by means of a pivotable connector such as a turntable which allows the second chassis part to articulate relative to the first chassis part.

In this case, where the coupling part is configured to engage with a hitch on a tractor unit in such a way that the coupling part can pivot relative to the hitch about an axis of the coupling part so that the semi-trailer can articulate relative to the tractor unit, semi-trailer may further be provided with a blocking device which prevents the coupling part from pivoting relative to the hitch.

The second chassis part may comprise a platform which is configured to be generally parallel to the ground, the coupling part being mounted on a lower face of the platform and the pivotable connector being mounted on an upper face of the platform.

The pivotable connector may be located further towards the rear end of the semi trailer than the coupling part.

Alternatively, the pivotable connector may be located generally above the coupling part. In this case, the pivotable connector may facilitate rotation of the first chassis part relative to the second chassis part about an axis which is coincident with the axis of the coupling part.

The semi-trailer may further be provided with a steering mechanism which can be operated to pivot each wheel of whichever of the first or second pair of wheels is driven by the motor about an axis which is generally perpendicular to the axis of rotation of each wheel.

The steering mechanism may comprise an electrically controlled steering actuator which is configured to drive pivoting of the respective first or second pair of wheels about the steering axis. In this case, the semi-trailer may further comprise an electrical steering controller which is operable to control the operation of the electrically controlled steering actuator.

The steering controller may also be connected to the processor, the processor being programmed to control operation of the motor and steering actuator to drive the semi-trailer autonomously.

Whichever of the first or second pair of wheels is connected to the motor may be pivotally connected to the chassis such that they can be moved towards the chassis to an inoperative position in which they do not engage with the ground on which the semi-trailer is standing.

The second set of wheels is mounted on the chassis generally in the location where landing legs are typically provided, the second set of wheels being provided instead of landing legs.

Where the motor is connector to the second pair of wheels, the semi-trailer may be provided with a further electrically powered motor which is connected to the first pair of wheels and operable to drive rotation of the first pair of wheels.

According to a second aspect of the invention we provide a vehicle comprising a tractor coupled to a semi-trailer according to the first aspect of the invention via the coupling part of the semi-trailer.

According to a third aspect of the invention we provide a vehicle comprising a tractor coupled to a semi-trailer the semi-trailer comprising a chassis, a pair of ground engaging wheels, an electrically powered motor which is connected to and operable to drive rotation of the pair of ground engaging wheels, and a coupling part which is located underneath the chassis at the front end of the semi-trailer by means of which the semi-trailer is coupled to a hitch provided on the tractor, the coupling part and hitch being configured to allow the coupling part to pivot relative to the hitch so that the semi-trailer can articulate relative to the tractor, wherein the semi-trailer is further provided with a pivotable connector by means of which the semi-trailer can be coupled to a further semi-trailer whilst allowing the semi-trailer to articulate relative to the further semi-trailer.

The semi-trailer may further be provided with a blocking device which prevents the coupling part from pivoting relative to the hitch.

The coupling part may comprise a kingpin which is engaged with a fifth wheel hitch on the tractor unit.

The semi-trailer preferably further comprises an electrical energy storage apparatus, such as a battery, which is connected to and provides electrical power to the motor.

The semi-trailer may further comprise a generator which converts rotational movement of the motor driven pair of wheels to electrical power.

In this case, the generator is connected to the electrical storage apparatus such that the electrical power it generates charges the electrical storage apparatus.

The semi-trailer may comprise an auxiliary electrically powered apparatus to which the generator is connected such that the electrical power generator by the generator can be used to power the auxiliary electrically powered apparatus.

The semi-trailer may further comprise an electrical controller which is configured to control the operation of the motor, the controller having a processor which is programmed to drive the semi-trailer autonomously.

The chassis may comprise a platform which is configured to be generally parallel to the ground, the coupling part being mounted on a lower face of the platform and the pivotable connector being mounted on an upper face of the platform.

The pivotable connector may be located further towards the rear end of the semi trailer than the coupling part.

Alternatively, the pivotable connector may be located generally above the coupling part. In this case, the pivotable connector may facilitate rotation of the first chassis part relative to the second chassis part about an axis which is coincident with the axis of the coupling part.

The tractor may comprise a pair of ground engaging tractor wheels and an engine which is operable to drive said tractor wheels.

According to a fourth aspect of the invention we provide an electric drive module suitable for securing to a the chassis of a semi-trailer, the drive module comprising a pair of wheels each of which is mounted for rotation relative to the chassis, each wheel having an electric motor which is operable to drive rotation of the wheel about an axis of rotation, the drive module further comprising an electrically operable steering mechanism which is operable to pivot each of the wheels about a steering axis, an electrical power storage apparatus which is configured to supply electrical power to operate the motors and the steering mechanism, a suspension system by means of which the wheels may be secured to the chassis of a semi-trailer, the suspension system being operable to allow for controlled translational movement of the wheels relative to the chassis, and an electronic control unit for controlling operation of the motors and steering mechanism.

The module may be provided with a housing which at least partially encloses the suspension mechanism, the electronic control unit, and electrical energy storage apparatus, and which is shaped to provide an aerodynamic baffle.

Each wheel may be connected to the housing via an independent suspension system.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, of which

FIGURE 1 is an illustration of a side view of the front end of a semi-trailer according to a first embodiment of the first aspect of the invention,

FIGURE 2 is an illustration of a side view of the front end of an alternative version of semi-trailer according to first embodiment of the first aspect of the invention, FIGURE 3 is an illustration of a top view of a semi-trailer according to the embodiment of the first aspect of invention illustrated in Figure 2 whilst coupled to a tractor unit,

FIGURE 4 is an illustration of a side view of a semi-trailer according to a second embodiment of the first aspect of the invention,

FIGURE 5 is an illustration of a front view of a portion of a semi-trailer according to the second embodiment of the first aspect of the invention,

FIGURE 6 is an illustration of a top view of one of the wheels of the semi-trailer illustrated in Figure 5 with a portion of the associated axle and suspension system,

FIGURE 7 is an illustration of a front view of a portion of an alternative configuration of semi-trailer according to the second embodiment of the first aspect of the invention,

FIGURE 8 is an illustration of a bottom view of one of the wheels of the semi-trailer illustrated in Figure 7 along with a portion of the associated axle and suspension,

FIGURE 9 is an illustration of a side view of a semi-trailer according to a third embodiment of the first aspect of the invention,

FIGURE 10 is an illustration of a front view of a part of the semi-trailer illustrated in Figure 9,

FIGURE 1 1 is an illustration of a top view of an embodiment of drive module according to the second aspect of the invention suitable for use in the semi-trailer illustrated in Figure 9 and 10,

FIGURE 12 is an illustration of a side view of the drive module illustrated in Figure 1 1 along the line labelled X in Figure 1 1 , shown mounted on the chassis of the semi trailer illustrated in Figure 9, and

FIGURE 13 is an illustration of a side view of an alternative configuration of semi trailer according to the third embodiment of the first aspect of the invention. Referring now to Figures 1 and 2 there is shown the front end of a semi-trailer 31 comprising a chassis 33. The semi-trailer 31 has a longitudinal axis which extends from the front end 31 a to a rear end 31 bof the semi-trailer 31 generally centrally between its sides.

As shown in Figure 3, the semi-trailer 31 has a first pair of ground-engaging wheels 23 located at and supporting the chassis 33 at the rear end 31 b of the semi-trailer 31. In this embodiment, a further pair of ground-engaging wheels 24 is located adjacent to the first pair of wheels 23. It will be appreciated, however, that the provision of such a further pair of wheels 24 is not essential.

The semi-trailer further comprises a coupling part 21 which is located underneath the chassis 33 at the front end 31 a of the semi-trailer 31. The coupling part 21 is configured to be used to couple the semi-trailer 31 to a tractor unit 80 a rear end of which is illustrated in dashed lines in Figures 1 and 2. As is conventional, the tractor unit 80 has ground engaging wheels and an engine, typically powered by a fossil fuel such as a diesel engine, which is operable to drive the tractor unit 80.

The coupling part 21 is configured to engage with a hitch 82 on the tractor unit 80 in such a way that the coupling part 21 can pivot relative to the hitch 82 about an axis A of the coupling part 21 , so that the semi-trailer 31 can articulate relative to the tractor unit 80. In this embodiment of the invention, the coupling part 21 is a conventional kingpin which is configured to engage with a conventional fifth wheel hitch 82 on the tractor unit 80.

A trailer body 34, suitable for containing loads is mounted on the chassis 33.

The semi-trailer 31 is further provided with a second pair of ground engaging wheels 25 which are located towards the front end 31 a of the semi-trailer 31 relative to the first pair of wheels. The second pair of ground engaging wheels 25 are not located right at the front end of the semi-trailer 31 , however. They are located in the position where landing legs are typically located on a conventional semi-trailer 31 , so that the coupling part 21 is located between the very front of the semi-trailer 31 and the second pair of wheels 25. The second pair of wheels 25 can therefore replace the landing legs. The second pair of wheels 25 are mounted on an axle and provided with independent suspension using an axle beam or any other means of providing longitudinal and lateral constraint to the installation, with vertical control being provided using a conventional spring and damper arrangement.

In this embodiment of the invention, the chassis 33 comprises a first chassis part 33a which, in this embodiment, extends from the rear end of the semi-trailer 31 to its front end 31 a. The trailer body 34 is mounted on the first chassis part 33a.

The chassis 33 further comprises a second chassis part 33b which is located at the front end 31 a of the semi-trailer 31 , underneath a front end portion of the first chassis part 33a. The first and further pair of wheels 23, 24 are mounted on the first chassis part 33a, and the second pair of wheels 25 and coupling part 21 are mounted on the second chassis part 33b. The first and second chassis parts 33a, 33b are connected together by means of a pivotable connector, which in this example is a turntable 29, which allows the second chassis part 33b to articulate relative to the first chassis part 33b about a turntable pivot axis B, as illustrated in Figure 3.

Rubbing blocks 35 are provided between the first and second chassis parts 33a,

33b, in order to control the pitching motion (fore and aft vertical rotation) of the semi trailer 31 .

In this example, the second chassis part 33b comprises a platform which is configured to be generally parallel to the ground, the coupling part 21 being mounted on a lower face of the platform and the turntable 29 being mounted on an upper face of the platform.

The kingpin 21 is configured to pivot in the fifth wheel hitch 82, so, conventionally, a vehicle formed from such a semi-trailer 31 and tractor unit 80 would articulate about the axis A of the kingpin 12. In this embodiment, the semi-trailer 31 is further provided with a blocking device 27 which prevents the kingpin 21 from pivoting relative to the hitch 82. In this example, the blocking device 27 is a wedge which is secured to the underside of the second chassis part 33 adjacent the coupling part 21 , but closer to the rear end of the semi-trailer 31 relative to the coupling part 21 , and which engages with the hitch 82 to prevent the hitch 82 from rotating about axis A relative to the coupling part 21 . This may be a conventional blocking wedge as used in tractor and semi-trailer combinations in which the semi-trailer is provided with a rear steering axle.

In the embodiment illustrated in Figure 1 , the turntable 29 is located further towards the rear end of the semi-trailer 31 than the coupling part 21 , with the turntable pivot axis B lying on the longitudinal axis of the semi-trailer 31. In this embodiment, one or more rubbing blocks 35 are provided between the first and second chassis parts 33a, 33b at the front end 31 a of the semi-trailer 31.

In the embodiment illustrated in Figure 2, the turntable 29 is located generally above the coupling part 21 , in this example so that the turntable 29 is concentric with the kingpin 21 , i.e. the turntable pivot axis B coincides with the axis A of the kingpin 21.

In this embodiment, two sets of rubbing blocks 35 are provided - a first set at the front end 31a of the semi-trailer 31 , and a second set between the first and second chassis parts 33a, 33b at the rear of the second chassis part 33b.

The semi-trailer 31 is further provided with an electrically powered motor (not shown) which is connected to the second pair of wheels 25 and operable to drive rotation of the second pair of wheels 25. The motor is powered by a source of stored electrical energy such as a battery which is located on the chassis 33 of the semi-trailer, for example in the trailer body 34, and is configured to be capable of driving the second pair of wheels 25 in two opposite rotational directions about an axis of rotation R.

The motor may be connected to both wheels 25 of the pair using a conventional power train arrangement. Alternatively, the motor may comprise two separate conventional hub motors, one hub motor being mounted on the hub of each wheel 25.

The motor or motors is/are configured to be controlled remotely by means of a controller (not shown), which may, for example, be located in a driver’s cab 80a of the tractor unit. The connection between the controller and the motor(s) may be wireless, or it may be wired, for example using a communications line which extends between the tractor unit 80 and the semi-trailer 31. Where the connection is wireless, the controller could be used in the driver’s cab 80a, or at a location outside the tractor and semi-trailer combination. The motor(s) may be used to move the semi-trailer 31 around a distribution centre or loading yard either whilst coupled to a tractor unit or yard tug or whilst uncoupled from either a tractor unit, yard tug or trailer mover, without the noise and exhaust gas emissions associated with use of the diesel engine of the tractor unit or yard tug.

The controller could be configured to be operated manually by an operator.

Additionally or alternatively, it could be provided with or connected to for receipt of control signals from a processor programmed to facilitate autonomous driving of the semi-trailer 31 without operator intervention. For example, the processor may be programmed to drive the semi-trailer into a designated parking or loading bay without any human intervention, or with human intervention purely to control the steering via the conventional steering mechanism in then driver’s cab 80a of the tractor unit 80 as described above.

When the semi-trailer 3T is pulled by a tractor unit, the second pair of wheels 25’ may free-wheel. Advantageously, however, a generator is provided to convert the rotational movement of the wheels 25 electrical energy. In this embodiment, the motor/s is/are configured to act as a generator when the second pair of wheels 25 are rotating as a result of the semi-trailer 31 being pulled by the tractor unit 80 in the conventional manner, i.e. using the engine of the tractor unit 80, and is connected to the battery in such a way that the generated electrical energy is transmitted to the battery, to recharge the battery. Alternatively, or additionally, the generated electrical energy could be used to power refrigeration or any other electrical equipment on the semi-trailer 31 , or to charge an alternative battery used to power such equipment.

The second set of wheels 25 could be pivotally mounted on the second chassis part 33b, so that they can be pivoted upwards towards the chassis 33 so that they no longer contact the ground 10, when the semi-trailer 3T is coupled to a tractor unit 80, in the same way as conventional landing legs are folded away when not in use. In this case, it will be appreciated, however, that the second set of wheels 25 cannot be used to generate electrical power when stowed in this manner.

When a tractor unit 80 is coupled to the semi-trailer 31 , the normal steering mechanism of the tractor unit 80 can be used to steer the semi-trailer 31 , in exactly the same way if the normal diesel engine of the tractor unit were being used to move the semi-trailer 31. This is illustrated in Figure 3, which shows a front pair of steerable wheels 84 of the tractor unit 80 being used to steer the vehicle, the first chassis part 33a of the semi-trailer 31 pivoting about the turntable pivot axis B to allow the semi-trailer 31 to travel along a curved path. It will be appreciated that, by virtue of the blocking wedge 27 preventing pivoting of the kingpin 21 in the fifth wheel hitch 82, the first chassis part 33a does not pivot relative to the tractor unit 80, and hence the second pair of wheels 25 effectively acts as a third pair of wheels for the tractor unit 80.

Where the controller is connected to a processor programmed to provide for autonomous operation of the motor as described above, where the semi-trailer 13 is coupled to a tractor unit 80, the semi-trailer 31 may be driven semi-autonomously, for example into a parking space or loading bay, with the controller controlling operation of the motor, but with human intervention to control the steering of the semi-trailer 31 via the conventional steering mechanism in the driver’s cab 80a of the tractor unit 80 as described above.

It will be appreciated that, as mentioned above, in this embodiment, the second pair of wheels 25 are not steerable. As such, movement of the semi-trailer 31 driven by the motor when a tractor unit 80 is not coupled to the semi-trailer 31 is limited to straight-line, forward and back movement. The second set of wheels 25 could, however, be provided with an electronic differential to facilitate some movement of the semi-trailer 31 along a curved path.

A semi-trailer with a steerable electrically driven set of wheels is provided in a second embodiment of the invention, which is illustrated in Figures 4 - 9.

Referring now to Figure 4, there is shown a semi-trailer 3T comprising a chassis 33’. The semi-trailer 3T has a longitudinal axis which extends from the front end 31 a’ to a rear end 31 b’ of the semi-trailer 31 generally centrally between its sides.

The semi-trailer 3T has a first pair of ground-engaging wheels 23’ located at and supporting the chassis 33’ at the rear end 31 b’ of the semi-trailer 3T. In this embodiment, a further pair of ground-engaging wheels 24’ is located adjacent to the first pair of wheels 23’. It will be appreciated, however, that the provision of such a further pair of wheels 24’ is not essential.

The semi-trailer 3T further comprises a coupling part 2T which is located

underneath the chassis 33’ at the front end 31 a’ of the semi-trailer 3T. The coupling part 2T is configured to be used to couple the semi-trailer 3T to a tractor unit (not shown).

The coupling part 2T is configured to engage with a hitch on the tractor unit in such a way that the coupling part can pivot relative to the hitch so that, when towing by the tractor unit, the semi-trailer 31’ can articulate relative to the tractor unit. In this embodiment of the invention, the coupling part 2T is a conventional kingpin which is configured to engage with a conventional fifth wheel hitch on the tractor unit.

A trailer body 34’, suitable for containing loads is mounted on the chassis 33’.

The semi-trailer 3T is further provided with a second pair of ground engaging wheels 25’ which are located towards the front end 31 a’ of the semi-trailer 3T relative to the first pair of wheels 23’. The second pair of ground engaging wheels 25’ are not located right at the front end of the semi-trailer 3T, however. They are located in the position where landing legs are typically located on a conventional semi-trailer, so that the coupling part 2T is located between the very front of the semi-trailer 3T and the second pair of wheels 25’. The second pair of wheels 25’ can therefore replace the landing legs.

The semi-trailer 3T is further provided with an electrically powered motor which, in this example, is connected to the second pair of wheels 25’ and operable to drive rotation of the second pair of wheels 25’. It will be appreciated, however, that the motor could, alternatively be connected to either the first 23’ or further 24’ pair of wheels, and operable to drive these wheels 23’, 24’ rather than the second pair of wheels 25’.

The motor is powered by a source of stored electrical energy such as a battery, which is located on the chassis 33’ of the semi-trailer, for example in the trailer body 34’, and is configured to be capable of driving the second pair of wheels 25’ in two opposite rotational directions about a wheel axis R’. In this embodiment, the motor comprises two conventional hub motors 71 , one hub motor 71 being mounted on the hub of each wheel 25’ of the second pair.

The motors 71 are configured to be controlled remotely by means of a controller (not shown). The connection between the controller and the motors 71 may be wireless, or it may be wired.

The motors 71 may be used to move the semi-trailer 31 around a distribution centre or loading yard either whilst coupled to a tractor unit or yard tug or whilst uncoupled from either a tractor unit, yard tug or trailer mover, without the noise and exhaust gas emissions associated with use of the diesel engine of the tractor unit or yard tug.

When the semi-trailer 3T is pulled by a tractor unit, the second pair of wheels 25’ may free-wheel. Advantageously, however, a generator is provided to convert the rotational movement of the wheels 25’ electrical energy. In this embodiment, the motors are configured to act as a generator when the second pair of wheels 25’ are rotating as a result of the semi-trailer 31 being pulled by the tractor unit or yard tug in the conventional manner, i.e. using the engine of the tractor unit, and is connected to the battery in such a way that the generated electrical energy is transmitted to the battery, to recharge the battery. Alternatively, or additionally, the generated electrical energy could be used to power refrigeration or any other electrical equipment on the semi-trailer 3T, or to charge an alternative battery used to power such equipment.

The second set of wheels 25’ could be pivotally mounted on the chassis 33’, so that they can be pivoted upwards towards the chassis 33’ so that they no longer contact the ground 10, when the semi-trailer 3T is coupled to a tractor unit 80, in the same way as conventional landing legs are folded away when not in use. It will be appreciated, however, that the second set of wheels 25’ cannot be used to generate electrical power when stowed in this manner.

In this embodiment, the second pair of wheels 25’ are steerable, and, as such a conventional steering mechanism is provided to pivot each wheel 25’ of the pair about a steering axis S which is perpendicular to their axis of rotation R’. The steering mechanism comprises a steering actuator 61 which is advantageously configured to be controlled electrically using the same controller as the motors 71. It may, for example, comprise an electrically operated actuator, or a hydraulically or pneumatically operated actuator in which control of the flow of hydraulic or pneumatic fluid to the actuator is achieved using electrically operated valves.

The controller could be configured to be operated manually by an operator.

Additionally or alternatively, it could be provided with, or connected to for receipt of control signals from, a processor programmed to facilitate autonomous driving of the semi-trailer 31 without operator intervention. For example, the processor may be programmed to drive the semi-trailer 3T, through control of operation of both the hub motors 71 and the steering actuator 61 , into a designated parking or loading bay without any human intervention.

The wheels 25’ of the second pair may be mounted on the chassis 33 with a double wishbone independent suspension system, as illustrated in Figures 5 & 6. The hub motor 71 is mounted on a king post 65, and the king post 65 is held between a lower wishbone 51 and an upper wishbone 53 such that it and the associated wheel 25’ can pivot about steering axis S. Pivoting of the king post 65 can be driven by force applied from the ground 10 on which the wheels 25’ are travelling, or a force applied by the steering mechanism. In this example, the steering mechanism comprises a steering actuator 61 which is linked to the king post 65 by a steering arm 63 in a conventional manner.

A suspension system provides for vertical movement of the wheel 25’ relative to the chassis 33’. This is controlled by a spring and damper assembly 55 fitted between one or other of the wishbones 51 , 53 and the chassis 33’. The inboard ends of the wishbones 51 , 53 are connected to the chassis 33 such that they can pivot on their longitudinal, generally horizontal axes at the position 37 conventionally occupied by landing legs.

An alternative suspension system is illustrated in Figures 7 and 8. In this, the second pair of wheels 25’ are mounted on an axle beam 91 with a leading arm 93 suspension. Each hub motor 71 is mounted to a king post 65 which is pivotally connected to an end of the axle beam 81 such that the king post 65 and associated wheel 25’ can pivot about the steering axis S when force is applied either from the ground 10 or by steering actuator 61 which is linked to king post 65 by the steering arm 63. Vertical movement of the wheels 25’ relative to the chassis 33’ is controlled by a spring and damper assembly 55 fitted between leading arm 93 and the chassis 33’. The rearward end of the leading arm 93 is attached to a horizontal pivot 85 at the position 37 conventionally occupied by landing legs.

In this embodiment of the invention, the assembly comprising the second pair of wheels 25’ and associated axle 91 , motors 71 , steering mechanism and suspension system could be pivotally connected to the chassis 33 so that they could be folded up towards the chassis 33 when not in use, in the same way as conventional landing legs.

A third embodiment of the invention is illustrated in Figures 9 - 13.

Referring now to Figure 9, there is shown a semi-trailer 31” comprising a chassis 33”. The semi-trailer 31” has a longitudinal axis which extends from the front end 31 a” to a rear end 31 b” of the semi-trailer 31” generally centrally between its sides.

The semi-trailer 31” has a first pair of ground-engaging wheels 23” located at and supporting the chassis 33” at the rear end 31 b” of the semi-trailer 31”. In this embodiment, a further pair of ground-engaging wheels 24” is located adjacent to the first pair of wheels 23”. It will be appreciated, however, that the provision of such a further pair of wheels 24’ is not essential.

The semi-trailer 31” further comprises a coupling part 21” which is located

underneath the chassis 33” at the front end 31 a” of the semi-trailer 31”. The coupling part 21” is configured to be used to couple the semi-trailer 31” to a tractor unit (not shown).

The coupling part 21” is configured to engage with a hitch on the tractor unit in such a way that the coupling part 21” can pivot relative to the hitch so that, when towing by the tractor unit, the semi-trailer 31” can articulate relative to the tractor unit. In this embodiment of the invention, the coupling part 21” is a conventional kingpin which is configured to engage with a conventional fifth wheel hitch on the tractor unit.

A trailer body 34”, suitable for containing loads is mounted on the chassis 33”. The semi-trailer 31” is further provided with a second pair of ground engaging wheels 25” which are located towards the front end 31 a” of the semi-trailer 31” relative to the first pair of wheels 23”. The second pair of ground engaging wheels 25” are not located right at the front end of the semi-trailer 31”, however. They are located in the position where landing legs are typically located on a conventional semi-trailer, so that the coupling part 21” is located between the very front of the semi-trailer 31” and the second pair of wheels 25”. The second pair of wheels 25” can therefore replace the landing legs.

The semi-trailer 31” is further provided with an electrically powered motor which is connected to the second pair of wheels 25” and operable to drive rotation of the second pair of wheels 25”. The motor is powered by a source of stored electrical energy such as a battery, and is configured to be capable of driving the second pair of wheels 25’ in two opposite rotational directions about a wheel axis R.

In this embodiment, the motor comprises two conventional hub motors 7T, one hub motor 71’ being mounted on the hub of each wheel 25’ of the second pair.

The motors 7T are configured to be controlled remotely by means of a controller (not shown). The connection between the controller and the motors 7T may be wireless, or it may be wired.

The motors 7T may be used to move the semi-trailer 31” around a distribution centre or loading yard either whilst coupled to a tractor unit or yard tug or whilst uncoupled from either a tractor unit, yard tug or trailer mover, without the noise and exhaust gas emissions associated with use of the diesel engine of the tractor unit or yard tug.

When the semi-trailer 31” is pulled by a tractor unit 80, the second pair of wheels 25” may simply free-wheel. Advantageously, however, a generator is provided to convert the rotational movement of the wheels 25” electrical energy. In this embodiment, the motors 7T are each configured to act as a generator when the second pair of wheels 25” are rotating as a result of the semi-trailer 31” being pulled by the tractor unit or yard tug in the conventional manner, i.e. using the engine of the tractor unit, and is connected to the battery in such a way that the generated electrical energy is transmitted to the battery, to recharge the battery. Alternatively, or additionally, the generated electrical energy could be used to power refrigeration or any other electrical equipment on the semi-trailer 31”, or to charge an alternative battery used to power such equipment.

In this embodiment, the second pair of wheels 25” are steerable, and, as such a steering mechanism is provided to pivot each wheel 25” of the pair about a steering axis S’ which is perpendicular to their axis of rotation R”. The steering mechanism comprises a steering actuator 6T which is advantageously configured to be controlled electrically using the same controller as the motors 7T. It may, for example, comprise an electrically operated actuator, or a hydraulically or pneumatically operated actuator in which control of the flow of hydraulic or pneumatic fluid to the actuator is achieved using electrically operated valves.

As for the above embodiments of the invention, the controller could be configured to be operated manually by an operator. Additionally or alternatively, it could be provided with or connected to for receipt of control signals from a processor programmed to facilitate autonomous driving of the semi-trailer 31 without operator intervention. For example, the processor may be programmed to drive the semi trailer 3T, through control of operation of both the hub motors 71 and the steering actuator 61 , into a designated parking or loading bay without any human

intervention.

In this embodiment, the second pair of wheels 25” are mounted on a self-contained drive module 41 comprising the battery for powering the hub motor 7T, steering mechanism, suspension system and control electronics for facilitating control of the motor and steering mechanism via the controller. As such, a conventional semi trailer may be converted to a semi-trailer according to the invention simply by replacing each of the landing legs on either side of the semi-trailer 31” with such a drive module 41.

The battery, steering actuator and control electronics are contained within a module housing 41 a which is shaped to provide an aerodynamic baffle to the underside of the semi-trailer 31. This may assist in reducing aerodynamic drag, improving zero emissions driving range and reducing exhaust gas emissions when the semi-trailer 31” is towed behind a conventional tractor unit. Figure 10 shows a front view of the drive module 41 installed on the chassis 33” of the semi-trailer 31”. The chassis 33” is supported on the wheels 25” via an independent suspension system, which in this example is a multi-link suspension 9T. Any other appropriate form of independent suspension, such as a wishbone suspension, could equally be used, however. The module housing 41 a is secured to the chassis 33” and supports the multi-link suspension 9T on which the hub motors 7T and wheels 25” are mounted. A steering actuator 6T is provided for the wheels 25” and may be located inside or outside the module housing.

A top view of one example of drive module 41 is illustrated in Figure 11. The drive module 41 is provided with two mounting pads 42, located at a front end of the housing 41 a, and these are the parts which are secured to the chassis 33”. Also at the front end of the housing 41 , there is a compartment 47 which, in this example, contains a power electronics module which provides the necessary electronic interface for communicated of control signals from the controller to the hub motors 7T and steering actuator 61 (or its associated control valves). The independent suspension system 9T is connected to two suspension reaction structures 43 located on either side of the housing 41 a adjacent the wheels 25”. The energy storage apparatus, i.e. the battery, is located in a central rear compartment 49 of the housing 41a.

A sectional view of the drive module 41 illustrated in Figure 11 along line X, is shown in Figure 12. This shows the aerodynamic baffle profile 101 of the front end of the housing 41 a.

As with the other embodiments of the invention described above, the second set of wheels 25” could be pivotally mounted on the chassis 33”, so that they can be pivoted upwards towards the chassis 33” so that they no longer contact the ground 10, when the semi-trailer 3T is coupled to a tractor unit 80, in the same way as conventional landing legs are folded away when not in use. It will be appreciated, however, that the second set of wheels 25” cannot be used to generate electrical power when stowed in this manner. It should be appreciated that an electrical drive module could be provided to replace the first or further pair of wheels 23”, 24” with the second pair of wheels 25’ being non-driven.

It is possible to connect the motor to one or more of the other pairs of wheels on the semi-trailer so that the motor drives the rotation of two or more pairs wheels on the semi-trailer. Alternatively an additional motor or motors may be provided to drive one of more of the other pairs of wheels. For example, means may be provided to electrically drive the first pair of wheels 23, 23’, 23”, in addition to the second pair of wheels 25, 25’, 25”. For example, in the embodiment illustrated in Figure 13, the first pair of wheels 23”, located at the rear end 31 b” of the semi-trailer 31” are also provided in an electric drive module 41. Similarly, the one of more of the sets of wheels at the rear end 31 b’ of the semi-trailer 31’ could comprises a steerable electrically driven set of wheels of the kind illustrated in Figures 4 to 8. As a result, the semi-trailer 31’, 31” could be moved around a distribution centre or loading yard in zero emissions mode purely by using the electric motors 71 , 71’, but in this case, in four wheel drive mode, which may improve the manoeuvrability of the semi-trailer 31’, 31”.