A method for determining a torque distribution between a front axle and a rear axle in a vehicle having an electric powertrain

Abstract

There is described a method, applicable to a vehicle (V) having an electric powertrain, for controlling the torque delivered by each electric motor (M3; M3, M4) of at least one drive unit associated with a front axle (FA) of the vehicle and the torque delivered by each electric motor (M1, M2; M3) of the at least one drive unit associated with the rear axle (RA) of the vehicle (V), in such a way that a torque distribution ratio (m), corresponding to the ratio of a torque (TFA) delivered overall by each electric motor (M3; M3, M4) of the at least one drive unit of the front axle to a torque (TRA) delivered overall by each electric motor (M1, M2; M3) of the at least one drive unit of the rear axle (RA) may be maintained at values comprised between a minimum torque distribution ratio (m.MIN) and a maximum torque distribution ratio ( mMAX), which are determined as a function of a thermal control of the drive units.

Description

[0001] "A method for determining a torque distribution between a front axle and a rear axle in a vehicle having an electric powertrain"

[0002] ★★★★

[0003] TEXT OF THE DESCRIPTION

[0004] Field of the Invention

[0005] The present invention relates to vehicles having an electric powertrain, specifically to BEV vehicles. In more detail, the invention was developed with reference to vehicles having an electric powertrain comprising at least one electric motor associated to each of the front and the rear axles.

[0006] Prior art

[0007] In a vehicle having an electric powertrain comprising at least one drive unit associated with the front axle and at least one drive unit associated with the rear axle, wherein each drive unit includes an electric motor and an operational connection of the electric motor to a respective axle or to a respective axle wheel, there are control logics which instantaneously determine the torque delivery of the drive units, thereby controlling the torque of the electric motors. If one axle is used more than the other, e.g. in the case of a very unbalanced torque distribution in favour of that axle, the thermal power rejected by the (at least one) corresponding drive unit will be higher in proportion, leading to an uncontrolled increase of the temperatures of the components of that drive unit (electric motors, lubricants, inverters, etc.). The only available solution in the prior art for avoiding overheating and the corresponding failures consists in imposing a reduction of the delivered torque, which however will be perceived by the driver since it is not consistent with the targets of the vehicle dynamics which he has requested. Object of the Invention

[0008] The invention aims at solving the technical problem outlined in the foregoing. Specifically, the object of the invention consists in providing a method for determining a torque distribution between a front axle and a rear axle in a vehicle having an electric powertrain, wherein each of the front and rear axles comprises at least one drive unit including an electric motor and an operational connection of said electric motor to a respective axle or to a respective axle wheel, which at the same time is effective in limiting the thermal excursion of the drive units towards values which may be dangerous for the integrity of the drive units themselves and is unperceivable by the driver.

[0009] Summary of the Invention

[0010] The object of the invention is achieved by means of a method having the features set forth in the claims that follow, which form an integral part of the technical disclosure provided herein in relation to the invention.

[0011] Brief Description of the Figures

[0012] As a general premise, the method according to the invention may be applied to vehicles V having an electric powertrain comprising a front axle and a rear axle, wherein each of the front and rear axles comprises at least one drive unit including an electric motor and an operational connection of the electric motor to the respective axle or to a respective axle wheel. Figures 1 to 4 exemplify configurations of the vehicle V which may implement the method according to the invention and which are different from one another as regards the configuration of the powertrain. Throughout the Figures, the reference BT generally denotes a battery of the vehicle V, which may be either a single battery or a battery pack with distributed arrangement and architecture. The reference P followed by a subscript denotes a power flow delivered (solid-line arrow) or absorbed (dashed-line arrow) by the electric motors of the drive units, the subscript being equal to the reference adopted for the corresponding motor.

[0013] Figure 1 shows a vehicle V having an electric powertrain including a first and a second drive units, which are associated with the rear axle RA, and a third drive unit associated with the front axle FA.

[0014] The first drive unit comprises a first electric motor Ml associated (in the same way as the first drive unit as a whole) with a rear left wheel RL of the rear axle, the second drive unit comprises a second electric motor M2 associated (in the same way as the second drive unit as a whole) with a rear right wheel RR of the rear axle, and the third drive unit comprises a third electric motor M3 associated (in the same way as the third drive unit as a whole) with the front axle. The mechanical connection between the motors Ml, M2 of the respective drive units and the wheels RL, RR is implemented by means of a direct drive or by means of a reduction gear, whereas the mechanical connection between the motor M3 of the third drive unit and the front axle is implemented by means of a differential gear.

[0015] Figure 2 shows a vehicle V having an electric powertrain with a configuration which is essentially mirror-like with respect to the vehicle of Figure 1. The powertrain of the vehicle V in Figure 2 includes a first and a second drive units which are associated with the front axle FA and a third drive unit associated with the rear axle RA.

[0016] The first drive unit comprises a first electric motor Ml associated (in the same way as the first drive unit as a whole) with a front left wheel FL of the front axle, the second drive unit comprises a second electric motor M2 associated (in the same way as the second drive unit as a whole) with a front right wheel FR of the front axle, and the third drive unit comprises a third electric motor M3 associated (in the same way as the third drive unit as a whole) with the rear axle. The mechanical connection between the motors Ml, M2 of the respective drive units and the wheels FL, FR is implemented by means of a direct drive or by means of a reduction gear, whereas the mechanical connection between the motor M3 of the third drive unit and the front axle is implemented by means of a differential gear.

[0017] Figure 3 shows a vehicle V having an electric powertrain with a single drive unit on each axle. The powertrain of the vehicle V in Figure 3 comprises a first drive unit associated with the rear axle RA and a second drive unit associated with the front axle FA.

[0018] The first drive unit comprises a first electric motor Ml associated (in the same way as the first drive unit as a whole) with the rear axle RA, whereas the second drive unit comprises a second electric motor M3 associated (in the same way as the second drive unit as a whole) with the front axle FA. The mechanical connection between the motors Ml, M3 of the respective drive units and the axles RA, FA is implemented by means of respective differential gears.

[0019] Finally, Figure 4 shows a vehicle V having an electric powertrain including a first and a second drive units, which are associated with the rear axle RA, and a third and a fourth drive units associated with the front axle FA.

[0020] The first drive unit comprises a first electric motor Ml associated (in the same way as the first drive unit as a whole) with a rear left wheel RL of the rear axle, the second drive unit comprises a second electric motor M2 associated (in the same way as the second drive unit as a whole) with a rear right wheel RR of the rear axle. The third drive unit comprises a third electric motor M3 associated (in the same way as the third drive unit as a whole) with a front left wheel FL of the front axle, the fourth drive unit comprises a fourth electric motor M4 associated (in the same way as the fourth drive unit as a whole) with a front right wheel FR of the front axle.

[0021] The mechanical connection between the motors Ml, M2, M3, M4 of the respective drive units and the wheels RL, RR, FL, FR is implemented by means of a direct drive or by means of a reduction gear.

[0022] In various embodiments of the invention, with reference to figure 5, irrespective of the arrangement of the powertrain, the method - schematically represented by a flow diagram associated with reference 1 - comprises: determining (block 2), for each drive unit associated with the front axle FA, a limit value of the rejectable thermal power EHR,LIM,R equal to a sum of a cooling power ECOOL,AVL,n available for the drive unit and a ratio of a limit thermal energy AELIM,R to a reference time interval tREF,n, wherein said limit thermal energy AELIM,R depends on a difference between a limit temperature TLIM,R for the drive unit and a current temperature TACT,R of the drive unit; the subscript "n" of the above quantities identifies a corresponding drive unit (for example, n = 3 for Figure 1 and Figure 3, n = 1, 2 for Figure 2, n = 3, 4 for Figure 4), determining (block 4), for each drive unit associated with the rear axle, a limit value of the rejectable thermal power equal to a sum of a cooling power ECOOL,AVL,R available for the drive unit and a ratio of a limit thermal energy AELIM,R to a reference time interval tREF,n, wherein the limit thermal energy depends on a difference between a limit temperature TLIM,R for the drive unit and a current temperature TACT,n of the drive unit; the subscript "n" of the above quantities identifies a corresponding drive unit (for example, n = 1, 2 for Figure 1 and Figure 4, n = 3 for Figure 2, n = 1 for Figure 3), determining (block 4), for each drive unit associated with the front axle FA, a limit mechanical power PMEc,LiM,n deliverable by the drive unit as a function of the limit value of the thermal power EnR,LiM,n rejectable by the drive unit itself, and a maximum torque Tc,MAx,n deliverable by the drive unit as a function of the value of the limit mechanical power PMEc,LiM,n of the drive unit itself and of the rotational speed nc of the corresponding front axle FA or front axle wheel; as stated in the foregoing, the subscript "n" of the above quantities identifies a corresponding drive unit (for example, n = 3 for Figure 1 and Figure 3, n = 1, 2 for Figure 2, n = 3, 4 for Figure 4), determining (block 6), for each drive unit associated with the rear axle RA, a limit mechanical power PMEc,LiM,n deliverable by the drive unit as a function of the limit value EHR,LIM,R of the rejectable thermal power of the drive unit itself, and a maximum torque Tc,MAx,n deliverable by the drive unit as a function of the value of the limit mechanical power PMEC,LIM,II of the drive unit itself and the rotational speed nc of the corresponding rear axle RA or rear axle wheel RL or RR,

[0023] - determining (block 8) a target torque TT as a sum of a torque TEA deliverable overall by each drive unit associated with the front axle FA and a torque TRA deliverable overall by each drive unit associated with the rear axle RA (thus, TT= TEA + TRA). The torque TTis a so-called "barycentric" torque, and therefore it corresponds to the torque to be delivered based on the request by the vehicle driver; determining (block 10) a minimum torque distribution ratio IHMIN as the ratio of a minimum torque TFA,MIN deliverable overall to the front axle FA by the at least one drive unit associated therewith to a maximum torque TRA,MAX deliverable overall to the rear axle RA by the at least one drive unit associated therewith, wherein the minimum torque TFA,MIN deliverable overall to the front axle FA comprises a difference between the target torque TT and the maximum torque TRA,MAX deliverable overall to the rear axle, determining (block 10) a maximum torque distribution ratio IHMAX as a ratio of a maximum torque deliverable overall to the front axle TFA,MAX by the at least one drive unit associated therewith to a minimum torque deliverable overall to the rear axle TRA,MINby the at least one drive unit associated therewith, wherein the minimum torque deliverable overall to the rear axle TRA,MIN includes a difference between the target torque TTand the maximum torque TFA,MAX deliverable overall to the front axle FA.

[0024] The determinations performed in the blocks 2-10 will now be described in detail with reference to a preferred embodiment of the invention.

[0025] As regards block 2, the limit thermal energy AELiM,n may be calculated as AELiM,n = CcoMP,n*(TEMPLiM,n - ATEMPSAFE,II - TEMPACT,n), wherein:

[0026] - CcoMP,n is a thermal capacity of the drive unit (to be understood as an equivalent overall thermal capacity, thus determined on the basis of the thermal capacities of the electric motor and of the mechanical connection between the latter and the axle or the wheel),

[0027] - TEMPLiM,n is a limit temperature of the drive unit,

[0028] - ATEMPsAFE,n is a safety margin expressed as a temperature range; ATEMPSAFE may in itself be optional in the calculation, but it is preferable to take it into account,

[0029] - TEMPACT,R is the current temperature of the drive unit, the subscript "n" of the above quantities identifies a corresponding drive unit.

[0030] In this way, the expression of the limit thermal power EHR,LIM,R rejectable by the drive unit becomes:

[0031] EHR,LIM,R = AELIM,R / tREF,n + ECOOL,AVL,n from which

[0032] EHR,LIM,R = CcOMP* (TEMPLIM - ATEMPSAFE “ TEMPACT) / tREF,n + ECOOL,AVL,R

[0033] The time tREF,n is a time interval of duration equal to that of a manoeuvre whose torque delivery to the front axle or to the rear axle by the corresponding drive unit is to be controlled. The subscript n denotes the drive unit, and generally the reference time tREF,n may vary as a function of the drive unit and of the manoeuvre itself.

[0034] The available cooling power ECOOL,AVL,R is an item of data known per se, and it depends on the characteristics of the cooling circuit associated with the drive unit.

[0035] As regards block 4, the limit mechanical power PMEC,LIM,R deliverable by each drive unit is calculated starting from the definition of a mechanical efficiency r|c,nof the drive unit, the definition whereof is r|C,n = PMEC,LIM,n / PMEC,MAX,n

[0036] (the subscript "n" of the above quantities identifies a corresponding drive unit)

[0037] PMEC,MAX,R is an ideal maximum mechanical power of the drive unit, corresponding to (ideal) conditions of unitary efficiency. The power PMEC,MAX,R may moreover be expressed as a sum of the limit power PMEC,LIM,n, which corresponds to the power which is actually delivered, and of the thermal power EHR,LIM,R, which corresponds to the power which is dissipated in the form of heat.

[0038] PMEC,MAX,n = PMEC,LIM,n + EHR,LIM,n

[0039] Therefore, the mechanical efficiency pc,n may be written as

[0040] I)C,n = PMEC,LIM,n / PMEC,MAX,n = PMEC,LIM,n / (PMEC,LIM,n + EHR,LIM,R), SO that :

[0041] (PMEC,LIM,n + EHR,LIM,FI)*r|c,n = PMEC,LIM,n from which

[0042] EnR,LiM,n*r|c,n = PMEc,LiM,n* (I- i]c,n), and finally

[0043] PMEC,LIM,n = EHR,LIM,n*Pc / (1-pc)

[0044] From the calculation of the power PMEC,LIM,n it is possible to obtain a maximum torque TC,MAX,R deliverable by each drive unit as:

[0045] Tc,MAx,n = PMEC,LIM / nc,n if nc,nis expressed in rad / s or as Tc,MAx,n = (60*PMEC,LIM) / (2*n* nc,n) if nc,nis expressed in RPM wherein nc,nis a rotational speed of the axle or the axle wheel associated with the drive unit (the subscript "n" of the above quantities identifies the drive unit itself).

[0046] The calculation of TC,MAX,R enables determining the maximum torques TRA,MAX, TEA,MAX deliverable overall to the front axle FA and to the rear axle RA. If a single drive unit is associated with the axle, the maximum torque TRA,MAX or TEA,MAX equals the torque Tc,MAx,n of the drive unit on the axle (this applies to the front axle in Figure 1, to the rear axle in Figure 2, and to both axles in Figure

[0047] 3), whereas, if at least one of the front axle FA and rear axle RA comprises a first drive unit including a first electric motor associated with a right wheel and a second drive unit including a second electric motor associated with a left wheel, and wherein - according to the dynamic settings of the vehicle - the delivery of a torque difference AT is provided between the right wheel and the left wheel, then the maximum torque TX,MAX deliverable overall to the axle X, with X = FA, RA, is calculated in such a way as to be limited by the more critical component in terms of cooling, and therefore may be expressed as:

[0048] Tx,MAx,n = min (Tx,R,MAx,nZ Tx,L,MAx,n + AT) + min (Tx,L,MAX,n,' Tx,R,MAX,n - AT)

[0049] (assuming that the torque TX,R delivered to the right wheel is equal to the torque TX,L delivered to the left wheel, with the addition of the difference AT) wherein:

[0050] - TX,L,MAX,II is the maximum torque deliverable to the left wheel of the axle X, with X = FA, RA; in the calculation, this corresponds to the torque TC,MAX,II with n being equal - depending on the embodiment shown in the Figures - to the subscript of the drive unit associated with the left wheel of the axle (n = 1 for the embodiment of Figures 1, 2, n = 1, 3 for the embodiment of Figure

[0051] 4),

[0052] - TX,R,MAX,II is the maximum torque deliverable to the right wheel of the axle X, with X = FA, RA; in the calculation, it corresponds to the torque TC, MAX, H with n being equal ■ depending on the embodiment shown in the Figures to the subscript of the drive unit associated with the right wheel of the axle (n 2 for the embodiment of Figures 1 , 2 , n = 22 ,, 4 for the embodiment of Figure 4 ) , and

[0053] - the subscript n of the above quantities identi fies the drive unit .

[0054] Referring to Figure 6 , it corresponds to a diagram with the torque TRA on the X-axis and the torque TFA on the Y-axis , which graphically shows the set of the geometric loci corresponding to the definitions of IHMIN and m.MAx and to the definition of TT , the target torque determined according to tthhee driver' s request ( and, in the second place , on the basis of the dynamic setting of the vehicle ) .

[0055] The geometric locus corresponding to the definition of TTcorresponds , oonn the plane TRA- TFA, to a straight line having a negative angular coef ficient ( T FA = T T - TRA) , whereas mMiN and IHMAX aarree the angular coef ficients (which aarree positive and in increasing order ) of two straight lines which intersect the origin of the axes T RA- T FA .

[0056] The intersection between the straight lines with angular coef ficient IHMIN and IHMAX with the straight line identi fying the geometric locus of the torque TT defines a segment S which contains , according to the invention, the maximum overall torques TFA, MAX and TRA, MAX delivered to the front axle FA and to the rear axle RA, so as to remain within the limits of the thermal control of the drive units and, at the same time , ssoo aass to satis fy the driver' s request corresponding to the torque TT . In other words , once the total target torque TT has been fixed for the vehicle V, and once the definition has been made

[0057] - by means of the thermal control considerations detailed in the foregoing - of the limits TEA,MAX and TRA,MAX, and therewith of the limits IHMIN and IHMAX, the target torque TT may be delivered by pairs of values TEA, TRA which are contained within the segment S, having end points (TRA,MIN, TEA,MAX) and (TRA,MAX, TRA,MIN)•

[0058] In other words, the method according to the invention comprises controlling the torque delivered overall by each electric motor of the at least one drive unit of the front axle FA and the torque delivered overall by each electric motor of the at least one drive unit of the rear axle RA, in such a way that a torque distribution ratio m, corresponding to the ratio of an overall torque TEA delivered by each electric motor of the at least one drive unit of the front axle FA to an overall torque TRA delivered by each electric motor of the at least one drive unit of the rear axle RA, is maintained at values comprised between the minimum torque distribution ratio IHMIN and the maximum torque distribution ratio IHMAX, SO that, while respecting the target torque TT corresponding to the driver's request, and even in conditions of maximum torque delivery to the axles (TEA,MAX, TRA,MAX), the limits of thermal control of the drive units are always respected.

[0059] Of course, the implementation details and the embodiments may amply vary with respect to what has been described and illustrated herein without departing from the extent of the present invention, as defined by the annexed claims.

Claims

CLAIMS1. A method for determining a torque distribution between a front axle (FA) and a rear axle (RA) in a vehicle (V) having an electric powertrain, wherein each of said front axle (FA) and said rear axle (RA) comprises at least one drive unit including an electric motor (Ml, M2, M3; Ml, M3; Ml, M2, M3, M4) and an operational connection of said electric motor (Ml, M2, M3; Ml, M3; Ml, M2, M3, M4) to a respective axle (FA, RA) or to a respective wheel of said axle (FA, RA), the method comprising:- determining, for each drive unit associated with the front axle (FA), a limit value of the rejectable thermal power (EHR,LIM,II) equal to a sum of a cooling power (ECOOL_AVL,F) available for the drive unit and a ratio of a limit thermal energy (AELIM,R) to a reference time interval (IREF), wherein said limit thermal energy depends on a difference between a limit temperature (TLiM,n) for the drive unit and a current temperature (TACT,II) of the drive unit- determining, for each drive unit associated with the rear axle, a limit value of the rejectable thermal power (EHR,LiM,n) equal to a sum of a cooling power (ECOOL_AVL,n) available to the drive unit and a ratio of a limit thermal energy (AELIM,R) to a reference time interval (tREF), wherein said limit thermal energy depends on a difference between a limit temperature (TLiM,n) for the drive unit and a current temperature (TACT,R) of the drive unit,- determining, for each drive unit associated with the front axle (FA), a limit mechanical power (PMEC,LIM,II) deliverable by the drive unit as a function of the limit value of the rejectable thermal power (EHR,LIM,R) of the drive unit itself, and a maximum torque (Tc,MAx,n) deliverable by the drive unit as a function of the valueof the limit mechanical power (PMEC,LIM,II) of the drive unit itself and the rotational speed (nc,n) of the corresponding front axle or front axle wheel,- determining, for each drive unit associated with the rear axle (RA), a limit mechanical power (PMEC,LIM,II) deliverable by the drive unit as a function of the limit value of the rejectable thermal power (EHR,LIM,R) of the drive unit itself, and a maximum torque (TC,MAX,II) deliverable by the drive unit as a function of the value of the limit mechanical power (PMEC,LIM,R) of the drive unit itself and the rotational speed (nc,n) of the corresponding rear axle (RA) or rear axle wheel,- determining a target torque (TT) as a sum of a torque (TEA) deliverable overall by each drive unit associated with the front axle (FA) and a torque (TRA) deliverable overall by each drive unit associated with the rear axle (RA),- determining a minimum torque distribution ratio (HIMIN) as the ratio between a minimum torque (TEA,MIN) deliverable overall to the front axle (FA) by the at least one drive unit associated therewith and a maximum torque (TRA,MAX) deliverable overall to the rear axle (RA) by the at least one drive unit associated therewith, wherein the minimum torque (TEA,MIN) deliverable overall to the front axle (FA) comprises a difference between the target torque (TT) and a maximum torque (TRA,MAX) deliverable overall to the rear axle (RA),- determining a maximum torque distribution ratio (mMAx) as the ratio of a maximum torque deliverable overall to the front axle (TEA,MAX) by the at least one drive unit associated therewith to a minimum torque deliverable overall to the rear axle (TRA,MIN) by the at least one drive unit associated therewith, wherein the minimum torque deliverable overall to the rear axle(TRA,MIN) includes a difference between the target torque(TT) and the maximum torque (TEA,MAX) deliverable overall to the front axle (FA).

2. The method according to claim 1, comprising controlling the torque delivered by each electric motor (M3; M3, M4) of the at least one drive unit of the front axle (FA) and the torque delivered by each electric motor (Ml, M2; M3) of the at least one drive unit of the rear axle (RA) such that a torque distribution ratio (m), corresponding to the ratio between a torque (TEA) delivered overall by each electric motor (M3; M3, M4) of the at least one drive unit associated with the front axle and a torque (TRA) delivered overall by each electric motor (Ml, M2; M3) of the at least one drive unit associated with the rear axle (RA), is maintained at values between said minimum torque distribution ratio (HIMIN) and said maximum torque distribution ratio (IHMAX), and such that the sum of the torque (TEA) delivered overall by each electric motor (M3; M3, M4) of the at least one drive unit associated with the front axle and the torque (TEA) delivered overall by each electric motor (Ml, M2; M3) of the at least one drive unit associated with the rear axle (RA) is equal to said target torque (TT).

3. The method according to claim 1 or claim 2, wherein at least one of said front axle (FA) and said rear axle (RA) comprises a first drive unit including a first electric motor (Ml) associated with a left wheel and a second drive unit including a second electric motor (M2) associated with a right wheel, and wherein a torque difference AT between the right wheel and the left wheel is provided, and wherein the maximum torque deliverable overall to the axle is calculated as:TX,MAX — min (Tx,R,MAx,nZ TX,L,MAX,R + AT) + min (TX,L,MAX,R;Tx,R,MAX,n - AT)wherein:- TX,MAX,R is the maximum torque deliverable to axle X- Tx,L,MAx,n is the maximum torque deliverable by the first electric motor to the left wheel of axle X- Tx,R,MAx,n is the maximum torque deliverable by the second electric motor to the right wheel of the axle X- X designates the front axle or the rear axle,- the subscript n of the above quantities identifies the drive unit.

4. The method according to any one of the preceding claims, wherein said limit thermal energy (AELIM,R,R) is calculated as CCOMP* ( TEMPLIM,R_ATEMPSAFE,FI—TEMPACT,R), wherein:- CCOMP,n is a thermal capacity of the drive unit,- TEMPLIM,R is a limit temperature of the drive unit- ATEMPsAFE,n is a safety margin expressed as a temperature range,- TEMPACT,R is the current temperature of the drive unit,- the subscript n of the above quantities identifies the drive unit.

5. The method according to claim 4, wherein said limit mechanical power deliverable by each drive unit is calculated asPMEC,LIM,n — EHR,LIM,n*^C,n / (l_r|c,n) wherein:- PMEC,LIM,R is the mechanical power limit deliverable by the drive unit,- EHR,LIM,R is the limit rejectable thermal power value of the drive unit,- PMEC,LIM,R is the limit mechanical power,r|c,n is the mechanical efficiency of the drive unit,- the subscript n of the above quantities identifies the drive unit.

6. The method according to claim 5, comprising calculating a maximum torque TC,MAX deliverable by each drive unit as:Tc,MAX,n = PMEC, LIM, n / nc, n wherein :- nc,nis a rotational speed of the axle or axle wheel associated with the drive unit,- the subscript n of the above quantities identifies the drive unit.

7. The method according to claim 4, wherein said limit value EHR,LIM,R of thermal power that rejectable by the drive unit is calculated as:EHR,LIM,R = CcOMP* ( TEMPLIM,n - ATEMPsAFE,n - TEMPACT, n) / tREF,n + ECOOL, AVL,n wherein :- ECOOL, AVL,n is the cooling power available for the drive unit,- the subscript n of the above quantities identifies the drive unit.

8. The method according to any one of the preceding claims, wherein said reference time (tREF,n) is an interval of duration equal to that of a maneuvre whose torque delivery to said front axle or said rear axle by the corresponding at least one drive unit is to be controlled .

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