Method for controlling a hybrid drive train of a motor vehicle

Abstract

A method of controlling a hybrid drive-train of a vehicle having a combustion engine with a driveshaft, a transmission with an input shaft and an output shaft that drives a transfer box. An electric machine has a rotor that is connected to the transmission input shaft. A separator clutch is arranged between the engine driveshaft and the transmission input shaft. For coupling of the engine, the engine is first accelerated, with a rotational speed regulation, toward a target speed until a reference speed is reached or exceeded while the clutch remains disengaged, then, with continuous speed regulation, the engine is adjusted to a corrected target speed while the clutch is engaged until a target torque is obtained, then, with torque control, the engine is adjusted to the target torque while the clutch, with differential rotational speed regulation, is regulated to a target speed difference, and finally the clutch is engaged.

Claims

1. A method of controlling a hybrid drive-train of a motor vehicle, which comprises an internal combustion engine (VM) with a driveshaft (2), an adjustable-ratio transmission (5) with a transmission input shaft (4), and a transmission output shaft (6) in driving connection with either an axle transmission or a transfer box (7), an electric machine (EM) that is operable at least as a motor having a rotor (3) that is drive-connected to the transmission input shaft (4), and an automated friction separator clutch (K) is arranged between the driveshaft (2) of the internal combustion engine (VM) and the transmission input shaft (4) such that, starting from electrically powered driving with the separator clutch disengaged (x.sub.K=0; x.sub.K=x.sub.A), when a power increase is demanded and following starting, of the internal combustion engine, the internal combustion engine (VM) is accelerated to a transmission input speed (n.sub.GE) and is coupled to the transmission input shaft (4) by engaging the separator clutch (x.sub.K=1; x.sub.A<x.sub.K<x.sub.H), the method comprising the steps of: accelerating the internal combustion engine, during a first phase (t=t1 to t2), toward a target rotational speed (n.sub.VM.sub._.sub.Z1)which is higher than the transmission input speed (n.sub.GE) (n.sub.VM.sub._.sub.Z1>n.sub.GE), until a reference engine speed (n.sub.Ref), that is lower than the target rotational speed (n.sub.VM.sub._.sub.Z1) is either reached or exceeded (n.sub.VMn.sub.Ref, n.sub.Ref<n.sub.VM.sub._.sub.Z1), while the separator clutch (K) remains disengaged; adjusting the internal combustion engine (VM), during a second phase (t =t2 to t3), to a corrected target rotational speed (n.sub.VM.sub._.sub.Z2, n.sub.VM.sub._.sub.Z3) which is above the transmission input speed (n.sub.GE) (n.sub.VM.sub._.sub.Z2>n.sub.GE, n.sub.VM.sub._.sub.z3>n.sub.GE) while the separator clutch (K) is engaged until a target torque (M.sub.VM.sub._.sub.Ziel) of the internal combustion engine (VM) is reached (M.sub.K=M.sub.VM.sub._.sub.Ziel); adjusting the internal combustion engine (VM), during a third phase (t=t3 to t4), to the target torque (M.sub.VM.sub._.sub.Ziel) while adjusting the separator clutch (K) to a target rotational speed difference (n.sub.K.sub._.sub.Z1) (n.sub.K=n.sub.K.sub._.sub.Z1); and adjusting the separator clutch (K), during a fourth phase (t=t4 to t5), to an engaged condition for sustained operation thereof, while the speed of the internal combustion engine (VM) is controlled (x.sub.K=1; n.sub.K=n.sub.K.sub._.sub.Z2).

2. The method according to claim 1, further comprising the step of, during the first phase (t=t1 to t2), either holding the separator clutch (K) at a touch-point (x.sub.K=x.sub.A) when the separator clutch is already engaged as far as the touch-point at a beginning of the first phase (t=t1) (x.sub.K(t1)=x.sub.K(t2)=x.sub.A), or engaging the separator clutch as far as the touch-point (x.sub.K=x.sub.A) when, at the beginning of the first phase (t=1), the separator clutch is fully disengaged (x.sub.K(t1) =0; x.sub.K(t2)=x.sub.A).

3. The method according to claim 1, further comprising the step of, at a beginning of the first phase (t=t1) (n.sub.VM.sub._.sub.Z1=n.sub.GE(t1) +n.sub.OS1), setting the target rotational speed (n.sub.VM.sub._.sub.Z1) of the internal combustion engine (VM) at a value higher by a speed offset (n.sub.OS1) of 50 to 200 revolutions per minute than the transmission input speed (n.sub.GE).

4. The method according to claim 1, further comprising the step of, at a beginning of the first phase (t=t1), increasing a desired speed (n.sub.VM.sub._.sub.soll) of the internal combustion engine (VM) from a current engine speed (n.sub.VM(t1)=n.sub.VM.sub._.sub.idle) to the target rotational speed (n.sub.VM.sub._.sub.Z1).

5. The method according to claim 1, further comprising the step of ending the first phase (t=t2) when the reference engine speed (n.sub.Ref) is one of; equal to the transmission input speed (n.sub.GE(t2)) above the transmission input speed by a set speed offset (n.sub.OS2); and below the transmission input speed (n.sub.GE(t2)) by the set speed offset (n.sub.OS2) (n.sub.Ref=n.sub.GE(t2); n.sub.Ref=n.sub.GE(t2)n.sub.OS2).

6. The method according to claim 1, further comprising the step of, at a beginning of the second phase (t=t2), keeping the corrected target rotational speed (n.sub.VM.sub._.sub.Z2) of the internal combustion engine (VM) at a constant speed difference (n.sub.OS3) away from the transmission input speed (n.sub.GE) (n.sub.VM.sub._.sub.Z2=n.sub.GE=n.sub.OS3).

7. The method according to claim 1, further comprising the step of keeping the corrected target rotational speed of the internal combustion engine (VM) at a lower level lying between the target rotational speed (n.sub.VM.sub._.sub.Z1) of the first phase and the transmission input speed (n.sub.GE(t2)) (n.sub.GE(t2)<n.sub.VM.sub._.sub.Z3<n.sub.VM.sub._.sub.Z1) in the second phase (t=t2 to t3).

8. The method according to claim 7, further comprising the step of, at a beginning of the second phase (t=t2), reducing a desired speed (n.sub.VM.sub._.sub.soll) of the internal combustion engine (VM) along a gradient from the target rotational speed of the first phase (n.sub.VM.sub._.sub.Z1) to the corrected target rotational speed (n.sub.VM.sub._.sub.Z3).

9. The method according to claim 1, further comprising the step of determining the target torque (M.sub.VM.sub._.sub.Ziel) of the internal combustion engine (VM) either by: subtracting a current torque (M.sub.EM) of the electric machine (M.sub.VM.sub._.sub.Ziel=M.sub.FWM.sub.EM) from a desired torque (M.sub.FW) which is requested by a driver; or subtracting the current torque (M.sub.EM) of the electric machine (EM) and a set offset torque (.sub.MOS) from the desired torque (M.sub.FW) (M.sub.VM.sub._.sub.Ziel=M.sub.FWM.sub.EM.sub.MOS).

10. The method according to claim 1, further comprising the step of increasing a desired torque (M.sub.K.sub._.sub.soll)of the separator clutch (K) that is set by engaging the separator clutch (x.sub.K>x.sub.A), at a beginning of the second phase (t=t2), to the target torque (M.sub.VM.sub._.sub.Ziel) of the internal combustion engine (VM), the desired torque (MK_soll) of the separator clutch (K) being increased along a gradient.

11. The method according to claim 1, further comprising the step of limiting an engaging rate (dx.sub.K/dt) of the separator clutch (K) by at least one of maintaining a minimum speed difference (n.sub.K.sub._.sub.min) at the separator clutch and maintaining a maximum differential speed gradient ((dn.sub.K/dt).sub.max) at the separator clutch (n.sub.Kn.sub.K.sub.min; |dn.sub.K/dt|(dn.sub.K/dt).sub.max>0).

12. The method according to claim 1, further comprising the step of utilizing, during the second phase (t=t2 to t3), torque (MK) that is transmittable by the separator clutch (K), which is determined by a current engaging degree of the separator clutch (x.sub.K>x.sub.A), as either a pilot torque or part of a pilot torque for regulating speed of the internal combustion engine (VM).

13. The method according to claim 1, further comprising the step of ending the second phase (t=t3) once the internal combustion engine (VM) reaches the target rotational speed (n.sub.VM.sub._.sub.Z2; n.sub.VM.sub._.sub.Z3) to within a specified differential speed tolerance ((n.sub.T1)(n.sub.VM(t3)=n.sub.VM.sub._.sub.Z2n.sub.T1; n.sub.VM(t3)=n.sub.VM.sub._.sub.Z3n.sub.T1), and when either: a size of a differential speed gradient (dn.sub.K/dt) at the separator clutch has either reached or fallen below a specified minimum differential speed gradient ((dn.sub.K/dt).sub.min) (|dn.sub.K/dt|(dn.sub.K/dt).sub.min>0)or a size of a difference between torque (MK) produced by the internal combustion engine (VM) and torque (MK) that is transmittable via the separator clutch (K) either reaches or falls below a specified minimum torque difference (M.sub.min)(|M.sub.VMM.sub.K|M.sub.min>0).

14. The method according to claim 1, further comprising the step of, at a beginning of the third phase (t=t3)(n.sub.K.sub._.sub.Z1<n.sub.K(t3)), setting the target rotational speed difference (n.sub.K.sub._.sub.Z1) of the separator clutch (K) smaller than an initial speed difference (n.sub.K) of the separator clutch.

15. The method according to claim 14, further comprising the step of steadily reducing a desired speed difference (n.sub.K.sub._.sub.soll) of the separator clutch (K), at the beginning of the third phase (t=t3), from a current speed difference (n.sub.K(t3)) to the target rotational speed difference (n.sub.K.sub._.sub.Z1).

16. The method according to claim 1, further comprising the step of utilizing a current torque (M.sub.VM) of the internal combustion engine (VM), during the third phase (t=t3 to t4), as either a pilot torque or part of a pilot torque for regulating a differential speed of the separator clutch (K).

17. The method according to claim 1, further comprising the step of ending the third phase (t=t4) when a speed difference (n.sub.K) at the separator clutch (K) reaches the target rotational speed difference (n.sub.K.sub._.sub.Z1) to within a specified differential speed tolerance (n.sub.T2) (n.sub.K(t4)=n.sub.k.sub._.sub.Z1n.sub.T2).

18. The method according to claim 1, further comprising the steps of: fully engaging (X.sub.K=1) the separator clutch (K), during the fourth phase (t =t4 to t5), when the separator clutch (K) is not provided with continuous slip regulation, and continuing differential speed regulation, to regulate the separator clutch to a lower target speed difference (n.sub.K.sub._.sub.Z2) for sustained slipping operation (n.sub.K.sub._.sub.Z2<n.sub.K.sub._.sub.Z1), when continuous slip regulation of the separator clutch (K) is provided.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) To clarify the invention the description of drawings with example embodiments are attached. The drawings show:

(2) FIG. 1a: Relevant rotational speed variations in a parallel-action hybrid drive-train according to FIG. 3 during a first control sequence according to the invention for coupling an internal combustion engine,

(3) FIG. 1b: Relevant rotational speed variations in the hybrid drive-train of FIG. 3 during the first control sequence according to the invention for coupling the internal combustion engine,

(4) FIG. 1c: The closing degree of a separator clutch of the hybrid drive-train in FIG. 3 during the first control sequence according to the invention for coupling the internal combustion engine,

(5) FIG. 2a: Relevant rotational speed variations in the parallel-action hybrid drive-train of FIG. 3 during the second control sequence according to the invention for coupling the internal combustion engine,

(6) FIG. 2b: Relevant rotational speed variations in the hybrid drive-train of FIG. 3 during the second control sequence according to the invention for coupling the internal combustion engine,

(7) FIG. 2c: The closing degree of a separator clutch of the hybrid drive-train in FIG. 3 during the second control sequence according to the invention for coupling the internal combustion engine, and

(8) FIG. 3: A schematic view of a parallel-action hybrid drive-train for applying the control method to be described further with the help of FIGS. 1a to 1c and 2a to 2c.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(9) FIG. 3 illustrates, in schematic form, a parallel-action hybrid drive-train 1 of a motor vehicle, with which the control method according to the invention described below can be applied.

(10) The hybrid drive-train 1 has an internal combustion engine VM with a driveshaft 2, an electric machine EM that can be operated at least as a motor with a rotor 3, and a transmission 5 with a transmission input shaft 4 and a transmission output shaft 6. The electric machine EM is arranged coaxially over the transmission input shaft 4 and in this case, as a example, the rotor 3 of the electric machine EM is connected in a rotationally fixed manner directly to the transmission input shaft 4. Alternatively, the rotor 3 of the electric machine EM could be drive-connected to the transmission input shaft 4 by way of a step-down transmission stage, so enabling a coaxial or axis-parallel arrangement of the electric machine EM relative to the transmission input shaft.

(11) With the direct connection of the rotor 3 of the electric machine EM to the transmission input shaft 4 assumed in the present case, the rotational speeds of the electric machine n.sub.EM and of the transmission input shaft n.sub.GE are thus identical (n.sub.EM=n.sub.GE). Between the driveshaft 2 of the internal combustion engine VM and the transmission input shaft 4 is arranged a separator clutch K in the form of an automated friction clutch. The transmission 5 is preferably an automated multi-stage variable-speed transmission, i.e. an automated variable-speed transmission or an automatic planetary transmission, but it can also be a manually shifted transmission or a continuously adjustable transmission. The transmission output shaft 6 is drive-connected by way of an axle differential 7 to the drive wheels 8a, 8b of a drive axle of the motor vehicle. In an all-wheel vehicle the transmission output shaft 6 would be in driving connection with a transfer box.

(12) If a motor vehicle with a hybrid drive-train 1 of such type is being driven in traction operation purely under electric power, i.e. with the internal combustion engine VM switched off and the separator clutch K open, then owing to a demand from the driver or from a regulation unit for more power it can become necessary to start the internal combustion engine VM and couple it drive-effectively to the transmission input shaft 4. For this, the previously started internal combustion engine VM must be accelerated approximately to the transmission input speed n.sub.GE and the separator clutch K must be substantially closed.

(13) Below, with reference to the rotational speed variations n.sub.VM(t), n.sub.GE(t) of the internal combustion engine VM and the transmission input shaft 4, shown in FIG. 1a, the torque variations M.sub.VM(t) of the internal combustion engine VM, M.sub.K(t) of the separator clutch K, and M.sub.EM(t) of the electric machine EM and also the torque M.sub.FW(t) desired by the driver, shown in FIG. 1b, and the closing degree variation x.sub.K(t) of the separator clutch K, shown in FIG. 1c, it will be explained how, in a first method variant, after having been started the internal combustion engine VM is coupled to the transmission input shaft 4.

(14) When a demand for more power is received, first of all the internal combustion engine is started between times t0 and t1. In the present case this is done for example by a drag start in which, while operating with slip, the separator clutch K is briefly partially closed and the internal combustion engine VM is entrained into rotation until it has reached a minimum starting speed sufficient for it to start. Since the drag torque needed for this is applied by virtue of a simultaneous, brief increase of the torque M.sub.EM delivered by the electric machine EM, the starting process of the internal combustion engine VM advantageously has no effect on the transmission input speed n.sub.GE. Once the internal combustion engine VM has been started, at time t1 it runs at its idling speed n.sub.VM.sub._.sub.idle. After the drag starting of the internal combustion engine VM, the separator clutch K has been opened not completely (x.sub.K=0), but only as far as its touch-point (x.sub.K=x.sub.A), so it can thereafter be closed again without any delay.

(15) The actual coupling of the internal combustion engine VM to the transmission input shaft 4 takes place between times t1 and t5 and begins when, in a first phase (t=t1 to t2), the internal combustion engine VM is accelerated with rotational speed regulation in the direction toward a target speed n.sub.VM.sub._.sub.Z1 which is higher than the current transmission input speed n.sub.GE (n.sub.VM.sub._.sub.Z1>n.sub.GE). At the beginning of the first phase (t=t1) the target speed n.sub.VM.sub._.sub.Z1 of the internal combustion engine VM is set at a value higher than the transmission input speed n.sub.GE by a rotational speed offset n.sub.OS1 of about 50 to 200 revolutions per minute (n.sub.VM.sub._.sub.Z1=n.sub.GE(t1)+n.sub.OS1). The desired speed n.sub.VM.sub._.sub.soll of the speed regulation of the internal combustion engine VM is increased abruptly at the beginning of the first phase (t=t1) from the idling speed n.sub.VM.sub._.sub.idle to the target speed n.sub.VM.sub._.sub.Z1, which results in a relatively steep speed increase dn.sub.VM/dt of the internal combustion engine VM. When a reference speed n.sub.Ref lower than the target speed n.sub.VM.sub._.sub.Z1, which in the present case is defined as equal to the current transmission input speed n.sub.GE (n.sub.Ref=n.sub.GE(t2)), has been reached or exceeded by the engine speed n.sub.VM (n.sub.VMn.sub.Ref), the first phase ends (t=t2) and a second phase (t=t2 to t3) begins.

(16) In the second phase the speed regulation of the internal combustion engine VM is continued. At the same time, however, to produce a target torque M.sub.VM.sub._.sub.Ziel of the internal combustion engine VM that can be deduced from the torque M.sub.FW desired by the driver, the separator clutch K is closed with a steady increase of the torque M.sub.K that it can transmit. During this the internal combustion engine VM is regulated to a corrected target speed n.sub.VM.sub._.sub.Z2, with which in the present case the speed difference n.sub.OS3 from the transmission input speed n.sub.GE at time t2 is kept constant (n.sub.VM.sub._.sub.Z2=n.sub.GE+n.sub.OS3).

(17) The second phase ends (t=t3) when the internal combustion engine VM has reached its target speed n.sub.VM.sub._.sub.Z2 to within a specified differential speed tolerance and when the size of the differential speed gradient dn.sub.K/dt at the separator clutch K has reached or fallen below a defined minimum differential speed gradient (dn.sub.K/dt).sub.min (|dn.sub.K/dt|(dn.sub.K/dt).sub.min>0), or the size of the difference between the torque M.sub.VM produced by the internal combustion engine VM and the torque M.sub.K that can be transmitted by the separator clutch K has reached or fallen below a specified minimum torque difference M.sub.min(|M.sub.VMM.sub.K|M.sub.min>0).

(18) In the subsequent, third phase (t=t3 to t4) the internal combustion engine VM is adjusted in a controlled manner to the target torque M.sub.VM.sub._.sub.Ziel and at the same time, by differential speed regulation, the separator clutch K is adjusted to a target rotational speed difference n.sub.K.sub._.sub.Z1. For this purpose, for example, the target torque M.sub.VM.sub._.sub.Ziel of the internal combustion engine VM is increased steadily and the torque M.sub.EM delivered by the electric machine EM is correspondingly reduced. The target rotational speed difference n.sub.K.sub._.sub.Z1 of the differential speed regulation of the separator clutch K is set smaller than the speed difference n.sub.K at the beginning of the third phase (t=t3) (n.sub.K.sub._.sub.Z1<n.sub.K(t3)). The third phase ends (t=t4) when the rotational speed difference n.sub.K at the separator clutch K has reached the target speed difference n.sub.K.sub._.sub.Z1 to within a specified differential speed tolerance.

(19) In the fourth phase (t=t4 to t5) the separator clutch K is completely closed in a steady manner (x.sub.K=1), since in this case for example no continuous slip regulation of the separator clutch K is provided.

(20) In the rotational speed variations n.sub.VM(t), n.sub.GE(t) of the internal combustion engine VM and the transmission input shaft 4, shown in FIG. 2a, the torque variations M.sub.VM(t), M.sub.K(t), M.sub.EM(t) of the internal combustion engine VM, the separator clutch K and the electric machine EM and also that of the torque M.sub.FW(t) desired by the driver, shown in FIG. 2b, and the closing degree variation x.sub.K(t) of the separator clutch K, shown in FIG. 2c, a second method variant for coupling the internal combustion engine VM is described, which however, differs only in some details from the first method variant according to FIGS. 1a to 1c.

(21) In this case the internal combustion engine VM is started, for example, by means of an associated electric starter motor, so that at time t1 the separator clutch K is fully open (x.sub.K(t1)=0). To achieve rapid response behavior for the subsequent closing process, during the first phase (t=t1 to t2) the separator clutch is closed as far as its touch-point (x.sub.K=x.sub.A). Furthermore, at the beginning of the first phase (t=t1) the desired speed n.sub.VM.sub._.sub.soll of the speed regulation of the internal combustion engine VM is now increased along a gradient and thus steadily from the idling speed n.sub.VM.sub._.sub.idle to the target speed n.sub.VM.sub._.sub.Z1, which results in a less steep speed increase dn.sub.VM/dt of the internal combustion engine VM. The first phase again ends (t=t2) when the engine speed n.sub.VM has reached or exceeded the reference speed n.sub.Ref lower than the target speed n.sub.VM.sub._.sub.Z1 (v.sub.NMn.sub.Ref). In this case, however, the reference speed n.sub.Ref is for example defined lower than the current transmission input speed n.sub.GE by a speed offset n.sub.OS2 (n.sub.Ref=n.sub.GE(t2)n.sub.OS2).

(22) A further difference from the first method variant according to FIGS. 1a to 1c is that in the second phase (t=t2 to t3) the corrected target speed n.sub.VM.sub._.sub.Z3 of the speed regulation of the internal combustion engine VM is now kept at a lower level, lying between the target speed n.sub.VM.sub._.sub.Z1 in the first phase and the current transmission input speed n.sub.GE(t2) (n.sub.GE(t2)<n.sub.VM.sub._.sub.Z3<n.sub.VM.sub._.sub.Z1). For this, the desired speed n.sub.VM.sub._.sub.soll of the speed regulation of the internal combustion engine VM at the beginning of the second phase (t=t2) is reduced steadily, in particular along a gradient, from the target speed n.sub.VM.sub._.sub.Z1 of the first phase to the corrected target speed n.sub.VM.sub._.sub.Z3. By reducing the speed difference n.sub.K in this manner the thermal loading of the separator clutch K is decreased and its useful life is thereby extended.

(23) The speed difference n.sub.K.sub._.sub.Z1 regulated at the separator clutch K during the third phase (t=t3 to t4) is in this case correspondingly smaller than was the case in the first method variant according to FIGS. 1a to 1c. In the fourth phase (t=t4 to t5), with continued differential speed regulation, the separator clutch K is now regulated to a lower target speed difference n.sub.K.sub._.sub.Z2 suitable for sustained slipping operation (n.sub.K.sub._.sub.Z2<n.sub.K.sub._.sub.Z1), since in this case continuous slip regulation of the separator clutch K is provided in order to damp the rotation fluctuations of the internal combustion engine VM. In this mode of operation the separator clutch K is closed as far as its respective gripping point x.sub.H.

INDEXES

(24) 1 Hybrid drive-train 2 Driveshaft, crankshaft 3 Rotor of the electric machine 4 Transmission input shaft 5 Transmission 6 Transmission output shaft 7 Axle differential 8a, 8b Drive wheels EM Electric machine K Separator clutch, friction clutch M Torque M.sub.EM Torque from the electric machine M.sub.FW Torque desired by the driver M.sub.K Torque that can be transmitted by the separator clutch M.sub.K.sub._.sub.soll Nominal torque of the separator clutch M.sub.VM Torque from the internal combustion engine M.sub.VM.sub._.sub.Ziel Target torque of the internal combustion engine n Rotational speed n.sub.EM Rotational speed of the electric machine n.sub.GE Transmission input speed n.sub.Ref Reference speed n.sub.VM Rotational speed of the internal combustion engine n.sub.VM.sub._.sub.idle Idling speed of the internal combustion engine n.sub.VM.sub._.sub.soll Desired speed of the internal combustion engine n.sub.VM.sub._.sub.Z1 Target speed of the internal combustion engine n.sub.VM.sub._.sub.Z2 Corrected target speed of the internal combustion engine n.sub.VM.sub._.sub.Z3 Corrected target speed of the internal combustion engine t Time t0 to t5 Time points VM Internal combustion engine x.sub.A Touch-point of the separator clutch x.sub.H Gripping point of the separator clutch x.sub.K Degree of closure of the separator clutch M Torque difference, differential torque M.sub.min Minimum torque difference M.sub.OS Offset torque n Rotational speed difference n.sub.K Rotational speed difference at the separator clutch n.sub.K.sub._.sub.min Minimum rotational speed difference at the separator clutch n_.sub.Z1 First target speed difference at the separator clutch n_.sub.Z2 Second target speed difference at the separator clutch n.sub.OS1 Speed offset n.sub.OS2 Speed offset n.sub.OS3 Speed difference n.sub.T1 Differential speed tolerance n.sub.T2 Differential speed tolerance dn.sub.VM/dt Speed gradient of the internal combustion engine dx.sub.K/dt Closing rate of the separator clutch dn.sub.K/dt Differential speed gradient at the separator clutch (dn.sub.K/dt).sub.max Maximum differential speed gradient at the separator clutch (dn.sub.K/dt).sub.min Minimum differential speed gradient at the separator clutch