"FOUR-WHEEL-DRIVE VEHICLE WITH HYBRID PROPULSION"

TECHNICAL FIELD

The present invention relates to a four-wheel-drive vehicle with hybrid propulsion.

The present invention finds advantageous application in an automobile, to which the ensuing treatment will make explicit reference without this implying any loss of generality.

BACKGROUND ART

Currently, a high-performance sports car has a rear drive and is provided with a self-locking differential in an attempt to maximize in any condition of the torque transmitted by the rear wheels to the road surface.

The rear drive with self-locking differential proves advantageous for a sporting driving style in optimal conditions of adherence (dry road) or in any case in not particularly poor conditions (wet road) ; however, said solution, in combination with a high torque and wide tyres leads to an extremely problematical and potentially dangerous drive when the conditions of adherence are poor (flooded or icy road) . To improve the drivability of a sports car in conditions of poor grip it has been proposed to use the permanent four-wheel drive or the four-wheel drive that can be engaged .

The permanent four-wheel drive enables a considerable improvement of the behaviour of the automobile in poor conditions of adherence, but presents the drawbacks of increasing, in each situation, the torque losses of the transmission system and of conferring upon the automobile a behaviour in optimal conditions of adherence that is not always appreciated by drivers.

The four-wheel drive that can be engaged enables the driver of the automobile to decide whether to use the rear drive or the four-wheel drive; in this way, the driver can use the rear drive in optimal conditions of adherence and can use the four- wheel drive in poor conditions of adherence.

To obtain an automobile with the four-wheel-drive that can be engaged, it has been proposed to use a hybrid system of propulsion provided both with an internal -combustion engine and with at least one electric motor. However, the configurations so far proposed to obtain a hybrid four-wheel- drive vehicle are complex to produce, present large overall dimensions, and in general are not suited to the characteristics of a high-performance sports car.

The patent application No. US2005116680A1 describes a hybrid four-wheel-drive vehicle, in which two wheels are driven just by the internal -combustion engine and the other two wheels are driven just by the electric motor.

The patent application No. US2003054910A1 describes a hybrid four-wheel-drive vehicle comprising an internal-combustion engine set in a rear position and provided with a gearchange that transfers the motion both to the rear driving wheels by means of a rear differential and to the front driving wheels by means of a front differential. The gearchange transfers the motion to the front differential by means of a transmission shaft, a drive unit, and a further transmission shaft. The drive unit comprises an epicyclic gear, which in turn comprises a first rotating element (planetary carrier) fixed with respect to the transmission shaft and to a stator of an electric motor, a second rotating element (ring gear) fixed with respect to the transmission shaft, and a third rotating element (sun wheel) fixed with respect to a rotor of the electric motor.

The patent application No. US2002061801Al describes a hybrid four-wheel-drive vehicle comprising an internal-combustion engine set in a front position, which transfers the motion to the front driving wheels, and an electric motor set in a rear position, which transfers the motion to the rear driving wheels .

The patent No. US6549840B1 describes a hybrid four-wheel-drive vehicle comprising a main drive for transmitting the motion to the front driving wheels and a secondary drive for transmitting the motion to the rear driving wheels . The main drive comprises an internal -combustion engine, a first reversible electrical machine, and an epicyclic gear having a first rotating element (sun wheel) fixed with respect to the engine shaft of the internal-combustion engine, a second rotating element (planetary carrier) fixed with respect to the rotor of the first reversible electrical machine, and a third rotating element (ring gear) , connected to a continuously variable transmission (CVT) , in turn connected to the front driving wheels. The secondary drive comprises a second reversible electrical machine, connected to the rear driving wheels by means of a cascade of gears and a rear differential.

DISCLOSURE OF INVENTION The aim of the present invention is to provide a four-wheel- drive vehicle with hybrid propulsion, said vehicle being free from the drawbacks described above, and being at the same time easy and inexpensive to produce.

According to the present invention, a four-wheel-drive vehicle with hybrid propulsion is provided in accordance with what is claimed in the annexed claims .

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described with reference to the annexed drawings, which illustrate a non-limiting example

of embodiment thereof, and in which:

• Figure 1 illustrates a schematic plan view of a four- wheel-drive automobile with hybrid propulsion built in accordance with the present invention; • Figure 2 illustrates a schematic plan view of a different four-wheel-drive automobile with hybrid propulsion;

• Figure 3 illustrates a schematic plan view of a further four-wheel-drive automobile with hybrid propulsion;

• Figure 4 illustrates a schematic plan view of a variant of the automobile of Figure 2;

• Figure 5 illustrates a schematic plan view of a further variant of the automobile of Figure 2 ; and

• Figure 6 illustrates a schematic plan view of a rear- wheel-drive automobile with hybrid propulsion.

BEST MODES FOR CARRYING OUT THE INVENTION

In Figure 1, designated by the reference number 1 is an automobile provided with two front driving or driven wheels 2, set on one and the same axis, and two rear driving or driven wheels 3, set on one and the same axis. The automobile 1 comprises an internal-combustion engine 4, which is set in a front position and is provided with a drive shaft 5, which in use is set in rotation. The drive shaft 5 of the internal- combustion engine 4 transfers the motion to the rear wheels 3 by means of a transmission line 6, which originates from a rear drive of the drive shaft 5, and transfers the motion to the front wheels 2 by means of a transmission line 1, which originates from a front drive of the drive shaft 5 opposite to the rear drive .

The transmission line 6 comprises a dry or oil-bath clutch 8, which is housed in a bell that is fixed with respect to the internal-combustion engine 4 and is designed to connect the drive shaft 5 of the internal-combustion engine 4 to a rear transmission shaft 9 terminating in a mechanical gearchange 10 set on the rear axle; connected in cascaded fashion to the

gearchange 10 is a self-locking rear differential 11, from which there depart a pair of axle shafts 12, each of which is fixed with respect to a rear driving wheel 3. In particular, a primary shaft of the gearchange 10 is fixed with respect to the transmission shaft 9, whilst a secondary shaft of the gearchange 10 is coupled to the self-locking rear differential 11.

The transmission line 7 comprises a front transmission shaft 13, which is connected to a front differential 14, from which there depart a pair of axle shafts 15, each of which is fixed with respect to a front driving wheel 3.

Furthermore, the transmission line 7 comprises at least one battery 16 for storing electrical energy, a reversible electrical machine 17 (i.e., one that can function both as motor by absorbing electrical energy and generating mechanical work and as generator by absorbing mechanical work and generating electrical energy) driven by a drive 18, electrically connected to the battery 16, and a reversible electrical machine 19, driven by a drive 20, electrically connected to the battery 16.

Finally, the transmission line 7 comprises an epicyclic gear 21, which comprises a rotating element 22, mechanically connected to the drive shaft 5, a rotating element 23, mechanically connected to the front differential 14 by means of the front transmission shaft 13, and a rotating element 24, mechanically connected to the reversible electrical machine 17.

In the embodiment illustrated in Figure 1, the rotating element 22 of the epicyclic gear 21 is a sun wheel, the rotating element 23 of the epicyclic gear 21 is a planetary carrier carrying satellites, and the rotating element 24 of the epicyclic gear 21 is a ring gear.

According to a different embodiment (not illustrated) , the rotating element 22 of the epicyclic gear 21, mechanically connected to the drive shaft 5 is a planetary carrier carrying satellites, the rotating element 23 of the epicyclic gear 21, mechanically connected to the front differential 14 is a sun wheel, and the rotating element 24 of the epicyclic gear 21, mechanically connected to the reversible electrical machine 17 is a ring gear.

According to a preferred embodiment (not illustrated) , the rotating element 22 of the epicyclic gear 21, mechanically connected to the drive shaft 5, is a planetary carrier carrying satellites, the rotating element 23 of the epicyclic gear 21, mechanically connected to the front differential 14, is a ring gear, and the rotating element 24 of the epicyclic gear 21, mechanically connected to the reversible electrical machine 17, is a sun wheel.

The reversible electrical machine 19 is mechanically connected to the front transmission shaft 13 and is thus mechanically connected both to the rotating element 23 of the epicyclic gear 21 and to the front differential 14. In the embodiment illustrated in Figure 1, the reversible electrical machine 19 is mechanically connected to the front transmission shaft 13 by means of a gear transmission 25, which has a gear 26, fitted to the front transmission shaft 13, and a gear 27, fitted to a shaft 28 of the reversible electrical machine 19. Preferably, the gear 27 has a diameter smaller than that of the gear 26 in such a way that the gear transmission 25 will have a transmission ratio that demultiplies the motion. According to a different embodiment (not illustrated) , the front transmission shaft 13 also performs the function of shaft of the reversible electrical machine 19 and hence a rotor of the reversible electrical machine 19 is directly fitted to the transmission shaft 13.

Also the reversible electrical machine 17 is mechanically connected to the rotating element 23 of the epicyclic gear 21 by means of a gear transmission 29, which comprises a gear 30 that meshes with the rotating element 23 of the epicyclic gear 21 and is fitted to a shaft 31 of the reversible electrical machine 17.

In the embodiment illustrated in Figure 1, the internal- combustion engine 4 is connected by means of the gearchange 10 to the rear wheels 3 and is connected by means of the epicyclic gear 21 to the front wheels 2. According to a different embodiment (not illustrated) , the internal - combustion engine 4 is connected by means of the gearchange 10 to the front wheels 2 and is connected by means of the epicyclic gear 21 to the rear wheels 3.

In the embodiment illustrated in Figure 1, the internal- combustion engine 4 and the clutch 8 are set in a front position, and the gearchange 10 is set on the rear axle. According to a different embodiment (not illustrated) , also the gearchange 10 is set in a front position. According to a further embodiment (not illustrated) , the internal-combustion engine 4 is set in a central position, and the clutch 8 and the gearchange 10 are set on the rear axle.

Normally, the epicyclic gear 21 behaves as a power-split and in the case of redundancy of torque (power) to the rear axle (for example, in the case of skidding of the rear wheels 3 or in the case of opening of the clutch 8) divides symmetrically (i.e., fifty, fifty) the torque (power) in excess between the front wheels 2 and the reversible electrical machine 17. By appropriately actuating the reversible electrical machine 17 (i.e., by causing the reversible electrical machine 17 to function as generator for absorbing mechanical power or as engine for supplying mechanical power) , it is possible to vary the distribution of the torque (power) between the front

wheels 2 and the reversible electrical machine 17.

The epicyclic gear 21 carries out an adaptation of speed between the speed of rotation of the drive shaft 5 and the speed of rotation of the front wheels 2. In fact, it should be noted that the ratio between the speed of rotation of the drive shaft 5 and the speed of rotation of the front wheels 2

(which is rigidly linked to the speed of rotation of the rear wheels 3) can vary as a function of the gear engaged in the gearchange 10.

According to a preferred embodiment, the transmission ratios of the two transmission lines 6 and 7 are sized in such a way that, when one of the gears of the gearchange 10 is engaged, the speed of rotation of the rotating element 22 of the epicyclic gear 21 is identical to the speed of rotation of the rotating element 23 of the epicyclic gear 21.

According to a further embodiment (not illustrated) , the front transmission line 7 does not envisage the reversible electrical machine 19.

It should be noted that, using the two reversible electrical machines 17 and 19, it is possible to start the internal- combustion engine 4; in particular, to start the internal- combustion engine 4 the clutch 8 is opened, the reversible electrical machine 19 functions as static brake and continues to keep the rotating element 23 of the epicyclic gear 21 stationary (alternatively, to keep the rotating element 23 of the epicyclic gear 21 stationary, the brakes of the front wheels 2 and/or of the rear wheels 3 could be used) , and the reversible electrical machine 17 functions as motor to set in rotation the rotating element 24 of the epicyclic gear 21 and hence (given that the rotating element 23 of the epicyclic gear 21 is blocked) the rotating element 21 of the epicyclic gear 21 fixed with respect to the drive shaft 5, which is thus

- S -

set in rotation.

From what has been described above, it emerges clearly that the four-wheel drive is engaged when desired, for example, when the rear wheels 3, which are normally driving wheels, lose grip. Furthermore, the reversible electrical machines 17 and 19 can be used to carry out regenerative braking so as to obtain an electrical boost, or else for an exclusively electrical propulsion (obviously, for short stretches, the length of which depends upon the capacity of the battery 16) .

According to a further embodiment, the gearchange 10 is a servo gearchange, i.e., it is structurally similar to a manual gearchange of a traditional type except for the fact that the clutch pedal and the lever for selection of the gears that can be actuated by the user are replaced by corresponding electrical or hydraulic servos.

An order for a change of gear can be generated manually, i.e., following upon a command imparted by the driver, or else automatically, i.e., independently of the driver's action. When the order to perform a change of gear is generated in the gearchange, the control unit of the transmission drives the servo associated to the clutch 8 for opening the clutch 8 so as to separate the primary shaft of the gearchange 10 mechanically from the drive shaft 5. Simultaneously, the control unit of the transmission acts on the control unit of the internal-combustion engine 4 so as to reduce temporarily, if necessary, the torque delivered by the internal-combustion engine 4 itself.

Once the control unit of the transmission has verified opening the clutch 8, it drives the servos associated to the gearchange 10 to disengage the gear that is currently engaged. When the control unit of the transmission has verified disengagement of the gear, it drives the servos associated to

the gearchange 10 for engagement the new gear. Finally, when the control unit of the transmission has verified that the new gear has been engaged, it drives the servo associated to the clutch 8 for closing the clutch 8 so as to render angularly fixed with respect to one another the primary shaft of the gearchange 10 and the drive shaft 5. Simultaneously, the control unit of the transmission acts on the control unit of the internal-combustion engine 4 for restoring, if necessary, the torque supplied by the internal -combustion engine 4 itself.

It is evident that, as long as the clutch 8 is open, the drive shaft 5 is disconnected from the primary shaft of the gearchange 10 and hence from the rear driving wheels 3. Consequently, as long as the clutch 8 is open, the torque generated by the internal-combustion engine 4 is not transmitted to the rear driving wheels 3, or, in other words, there occurs a torque drop on the rear driving wheels 3. Last- generation servo gearchanges currently available on the market that operate according to the modalities described above require a time generally ranging between 150 and 300 ms to perform a change of gear; the time effectively employed depends both upon the dynamic performance of the components of the gearchange 10 and upon the degree of comfort required. Consequently, the torque drop on the rear driving wheels 3 caused by a change of gear is clearly perceived by the occupants of the automobile 1, in so far as it results in a non-negligible discontinuity in the longitudinal acceleration of the automobile 1.

It should be noted that the torque drop on the rear driving wheels 3 caused by a change of gear is clearly perceptible by the occupants of the automobile 1 when the automobile 1 is accelerating, i.e., during gearing-up (i.e., change of gear from a lower gear to a higher gear) or else during gearing- down in acceleration (i.e., change of gear from a higher gear

to a lower gear similar to the "kick-down" of a traditional automatic transmission) , in so far as, during acceleration of the automobile 1, the internal-combustion engine 4 is under load to accelerate the automobile 1 itself. Instead, when the automobile 1 is slowing-down, i.e., during gearing-down in deceleration (change of gear from a higher gear to a lower gear) , the torque drop on the rear driving wheels 3 caused by a change 10 of gear is not normally perceived by the occupants of the automobile 1, in so far as the deceleration of the automobile 1 is substantially carried out by the braking system, and the internal-combustion engine 4 does not play an essential role in the dynamics of the automobile 1.

According to a possible embodiment, when performing a change of gear in the gearchange 10, i.e., as long as the clutch 8 remains open, the transmission line 7 can be used for transmitting torque to the front wheels 2 so as to compensate, at least in part, for the drop in torque generated on the rear wheels 3 by opening the clutch 8.

Obviously, to compensate, at least in part, for the torque drop that occurs on the rear wheels 3 as a result of opening of the clutch 8, it could be possible to activate simultaneously both of the reversible electrical machines 17 and 19.

The automobile 1 described above presents numerous advantages, in so far as it is simple and inexpensive to produce, presents small overall dimensions and adapts very well to the characteristics of a high-performance sports car.

In Figure 2, designated by the number 1 is an automobile provided with two front driving or driven wheels 2 set on one and the same axis and two rear driving or driven wheels 3 set on one and the same axis. The automobile 1 comprises an internal-combustion engine 4, which is set in a front position

and is provided with a drive shaft 5, which in use is set in rotation. The drive shaft 5 of the internal-combustion engine

4 transfers the motion to the rear wheels 3 by means of a transmission line 6, which originates from a rear drive of the drive shaft 5, and transfers the motion to the front wheels 2 by means of a transmission line 7 mechanically independent of the internal-combustion engine 4.

The transmission line 6 comprises a dry or oil-bath clutch S, which is housed in a bell fixed with respect to the internal- combustion engine 4 and is designed to connect the drive shaft

5 of the internal-combustion engine 4 to a rear transmission shaft 9 terminating in a mechanical gearchange 10 set on the rear axle; connected in cascaded fashion to the gearchange 10 is a self-locking rear differential 11, from which there depart a pair of axle shafts 12, each of which is fixed with respect to a rear driving wheel 3. In particular, a primary shaft of the gearchange 10 is fixed with respect to the transmission shaft 9, whilst a secondary shaft of the gearchange 10 is coupled to the self-locking rear differential 11.

The automobile comprises at least one battery 16 for storing electrical energy and a reversible electrical machine 32, which is fitted to the drive shaft 5 by means of a belt or gear transmission 33 and is driven by a drive 34, electrically connected to the battery 16. According to a different embodiment, the reversible electrical machine 32 could be coaxial to the drive shaft 5 and fitted directly to the drive shaft 5 itself without interposition of the transmission 33. The transmission line 7 comprises a pair of reversible electrical machines 35, which are fitted to respective front axle shafts 15 fixed with respect to the front wheels 2 and are driven by respective drives 36, electrically connected to the battery 16. Alternatively, the reversible electrical machines 35 could be integrated directly in the respective

front wheels 2, or else a single reversible electrical machine 35 could be provided, connected to the front wheels 2 by means of a front differential.

If two reversible electrical machines 35 are provided, these can:

• hang internally from the supporting structure of the front suspension with concentric meshing;

• hang internally from the supporting structure of the front suspension with external meshing;

• hang internally from the supporting structure of the front suspension with epicyclic gear to obtain coaxiality;

• hang internally from the supporting structure of the front suspension with direct-drive coaxial meshing;

• hang from the front cross member with concentric meshing;

• hang from the front cross member with external meshing;

• hang from the front cross member with epicyclic gear to obtain coaxiality; • hang from the front cross member with coaxial meshing in direct drive;

• hang from the front cross member but be set in a single housing;

• be integrated in the wheels coaxially to the hub; • be integrated in the wheels with epicyclic gear to obtain coaxiality; and

• be integrated in the wheels with concentric gear between the rotor and the rim.

If a single reversible electrical machine 35 is provided, this can:

• be set upstream of a differential and hang from the front cross member; and

• be set upstream of a differential and hang from the front part of the engine (with possibility of integration in a single housing with the reversible electrical machine 32) .

Illustrated in Figures 4 and 5 are two variants of the automobile 1 illustrated in Figure 1. In the variants illustrated in Figures 4 and 5, the mechanical gearchange 10 is set behind and underneath the differential 11 (and hence not in front of the differential 11 as illustrated in Figure 2) . In addition, the clutch 8 is set on the rear axle (and hence not in a bell fixed with respect to the internal- combustion engine 4 as illustrated in Figure 2) .

In the variant illustrated in Figure 5, the reversible electrical machine 32 is coaxial and fitted directly to the drive shaft 5 without the interposition of the transmission 33 and is set behind the internal-combustion engine 4, instead of being set in front of the internal-combustion engine 4 as illustrated in Figure 2.

In the variant illustrated in Figure 4, the reversible electrical machine 32 is set on the rear axle and is preferably fitted to a primary shaft of the mechanical gearchange 10. Consequently, in the variant illustrated in Figure 4, the reversible electrical machine 32 is set downstream of the clutch 8. The reversible electrical machine 32 could be set alongside a case of the mechanical gearchange 10 and fitted to the primary shaft of the mechanical gearchange 10 by means of a transmission similar to the belt or gear transmission 33 described above. Alternatively, the reversible electrical machine 32 could be set in front of the case of the mechanical gearchange 10 and coaxially to the primary shaft of the mechanical gearchange 10 so as to be fitted directly to the primary shaft thereof. In a further alternative, the reversible electrical machine 32 could be set in front of the case of the mechanical gearchange 10 and fitted to the primary shaft of the mechanical gearchange 10 by means of a transmission similar to the belt or gear transmission 33 described above. Finally, the reversible electrical machine 32 could be set within the case of the

mechanical gearchange 10 and coaxialIy to the primary shaft of the mechanical gearchange 10 so as to be fitted directly to the primary shaft thereof .

In Figure 6, designated by the number 1 is a hybrid-propulsion automobile with a rear-wheel drive (i.e., provided with two front wheels 2 that are always driven wheels and two rear wheels 3 that are always driving wheels) . The automobile 1 illustrated in Figure 6 repeats exactly the mechanical arrangement of the automobile 1 illustrated in Figure 4 with elimination of the transmission line 7 (i.e., of the reversible electrical machines 35 and of the respective drives 36) . Further variants (not illustrated) of the automobile 1 illustrated in Figure 6 envisage repeating, once again with elimination of the transmission line 7, the mechanical arrangement of the automobiles 1 illustrated in Figures 2 and 5.

Summarizing all the possible variants, when the reversible electrical machine 32 is set in series (i.e., in front or behind) with respect to the internal-combustion engine 4, said machine 32 can be :

• set in front of the internal-combustion engine 4 with concentric gear; • set in front of the internal-combustion engine 4 with external gear;

• set in front of the internal-combustion engine 4 with epicyclic gear to obtain coaxiality;

• set in front of the internal-combustion engine 4 coaxially to the drive shaft 5 in direct drive,-

• set behind the internal-combustion engine 4 with external meshing on a flywheel;

• set behind the internal-combustion engine 4 with epicyclic gear; • set behind the internal-combustion engine 4 coaxially to the drive shaft 5 in direct drive;

• set behind the clutch 8 on the primary shaft of the gearchange 10 with coaxial meshing in direct drive;

• set behind the primary shaft of the gearchange 10 with coaxial meshing in direct drive; • set behind the primary shaft of the gearchange 10 with concentric gear,-

• set behind the primary shaft of the gearchange 10 with external gear; and

• set behind the primary shaft of the gearchange 10 with epicyclic gear to obtain coaxiality.

Summarizing all the possible variants, when the reversible electrical machine 32 is set in a parallel position (i.e., laterally, above, or below) with respect to the internal- combustion engine 4, the reversible electrical machine 32 itself can be:

• set on the right-hand side/left-hand side of the internal-combustion engine 4 with drive taken from the front part of the drive shaft 5 via belt/gears; • set on the right-hand side/left-hand side of the internal-combustion engine 4 with drive taken from the rear part of the drive shaft 5 (via belt/gears) ;

• set on the right-hand side/left-hand side of the internal -combustion engine 4 with drive taken from the drive shaft 5 (via gears) ;

• set in the middle of the V of the cylinder banks of the internal-combustion engine 4 with drive taken from the front part of the drive shaft 5 (via belt/gears) ;

• set in the middle of the V of the cylinder banks of the internal-combustion engine 4 with drive taken from the rear part of the drive shaft 5 (via belt/gears) ;

• set on the right-hand side/left-hand side of the gearchange 10 with drive taken from the primary shaft of the gearchange 10 downstream of the clutch 8 (via gears) ; • set on the right-hand side/left-hand side of the gearchange 10 with drive taken from the primary shaft of

the gearchange 10 and downstream of the primary shaft of the gearchange 10 itself (via gears) ; and

• set on the left-hand side of the gearchange 10 with drive taken from the primary shaft of the gearchange 10 through the idle gear of the reverse (driven via gears) or else through the control gear of the oil pump of the gearchange 10.

In Figure 3, designated by the reference number 1 is an automobile provided with two front driving or driven wheels 2 set on one and the same axis and two rear driving or driven wheels 3 set on one and the same axis . The automobile 1 comprises an internal-combustion engine 4, which is set in a front position and is provided with a drive shaft 5, which in use is set in rotation. The drive shaft 5 of the internal - combustion engine 4 transfers the motion to the rear wheels 3 by means of a transmission line 6, which originates from a rear drive of the drive shaft 5, and transfers the motion to the front wheels 2 by means of a transmission line 7, which originates from a front drive of the drive shaft 5 opposite to the rear drive .

The transmission line 6 comprises a dry or oil-bath clutch 8, which is housed in a bell fixed with respect to the internal- combustion engine 4 and is designed to connect the drive shaft 5 of the internal-combustion engine 4 to a rear transmission shaft 9 terminating in a mechanical gearchange 10 set on the rear axle. Connected in cascaded fashion to the gearchange 10 is a self-locking rear differential 11, from which there depart a pair of axle shafts 12, each of which is fixed with respect to a rear driving wheel 3. In particular, a primary shaft of the gearchange 10 is fixed with respect to the transmission shaft 9, whilst a secondary shaft of the gearchange 10 is coupled to the self-locking rear differential 11.

The transmission line 7 comprises at least one battery 16 for storing electrical energy, and a reversible electrical machine 37, which constitutes an electromagnetic joint and connects the drive shaft 5 to the front transmission shaft 13, which transfers the motion to the front wheels 2 by means of a front differential 14. The reversible electrical machine 37 comprises an armature 38, which is fitted to the drive shaft 5 and is driven by a drive 39, electrically connected to the battery 16, and an inductor 40, which is magnetically coupled to the armature 38, is fitted to the front transmission shaft 13, and is driven by a drive 41, electrically connected to the battery 16. According to a different embodiment, the armature 38 is fitted to the front transmission shaft 13, and the inductor 40 is fitted to the drive shaft 5.

In use, by appropriately exciting the armature 38 and/or the inductor 40, it is possible to transmit a torque from the drive shaft 5 to the front transmission shaft 13 and then to the normally driven front wheels 2. For short periods, the reversible electrical machine 37 can also be used as electric motor for generating a further torque (electrical boost) to be applied to the front wheels 2 in the case of need. Furthermore, the reversible electrical machine 37 can be used as generator to carry out regenerative braking (which acts on the front wheels 2, which discharge to the ground the majority of the braking torque) in the case of deceleration of the vehicle.

Finally, the reversible electrical machine 37 can be used to start the internal-combustion engine 4. In this case, it is necessary for the automobile 1 to be stationary and for the front brakes to be applied to determine blocking of the front wheels 2, hence of the front differential 14, of the front transmission shaft 13, and of the part of the reversible electrical machine 37 fixed with respect to the front transmission shaft 13.