Method for operating a parallel hybrid drive train of a vehicle with at least one internal combustion engine and at least one electric machine

Abstract

Method for operating a parallel hybrid drive train (1) of a vehicle with at least one internal combustion engine (2) and at least one electric machine (3), wherein the electric machine (3) is arranged in the power train between an output and the at least one internal combustion engine (2) and a friction-fit switching element (5) is provided between the internal combustion engine (2) and the electric machine (3), while a starting element (6) comprising a hydrodynamic torque converter (6A) and a converter lock-up clutch (6B) is arranged between the electric machine (3) and the output, and wherein a target output torque (m_fahr_soll) to be applied to the output is dependent on a slip of the starting element (6), characterized in thatthat during an electric motor-side start-up process of the internal combustion engine (2), a drive torque generated by the electric machine (3) is partly transmitted via the torque converter (6A) and partly via the converter lock-up clutch (6B), wherein the converter lock-up clutch (6B) is kept in a slipping operation at least approximately during the entire start-up process of the internal combustion engine (2) by a speed control of the electric machine (3).

Description

The invention relates to a method for operating a parallel hybrid drive train of a vehicle with at least one internal combustion engine and at least one electric machine according to the type defined in more detail in the preamble of claim 1. Vehicles with parallel hybrid drive systems known from practical experience are typically equipped with a friction clutch-type starting element between the internal combustion engine and the vehicle's output shaft. This allows the vehicle to be started via the internal combustion engine in a known manner, with the starting element initially operating in a slipping mode during the starting process. If such a vehicle is additionally equipped with an electric drive unit, the controlled and regulated slipping operation of the starting element is not necessary when the vehicle is started solely via the electric motor, since electric motors, unlike conventional internal combustion engines, do not have a minimum rotational speed. DE 101 58 536 A1 describes a motor vehicle drive system with an electric motor and a clutch between the electric motor and the drive wheels. The motor vehicle drive system may include an automatic transmission with a torque converter, wherein the clutch is formed by a lock-up clutch of the torque converter. A method for controlling and regulating a drivetrain of a hybrid vehicle and a drivetrain of a hybrid vehicle, which is designed with a friction-fit starting element, is known from DE 10 2004 002 061 A1. According to the invention, a drivetrain of a hybrid vehicle can be operated in such a way as follows: The method for controlling and regulating a drivetrain of a hybrid vehicle, comprising an internal combustion engine, an electric machine, a switching element arranged between the electric machine and an output in a power flow of the drivetrain and designed with continuously variable transmission capacity, and a coupling device arranged between the electric machine and the internal combustion engine, via which the electric machine and the internal combustion engine can be brought into operative connection, comprises a drivetrain of a hybrid vehicle.that a transition from a drive of the hybrid vehicle by the electric machine to a parallel drive of the hybrid vehicle via the electric machine and the combustion engine or to a purely combustion engine-side drive of the hybrid vehicle, as well as a starting process of the combustion engine via the electric machine, is feasible without any drivetrain-side reaction torques perceptible to a driver of the hybrid vehicle. For this purpose, during operation of the drive train, the transmission capability of the switching element is adjusted during a start-up process of the internal combustion engine in such a way that a torque independent of a start-up process of the internal combustion engine is applied to the output of the drive train, whereby torque changes at the output that occur due to the start-up of the internal combustion engine are preferably avoided by a slip operation of the switching element. In this process, the speed of the electric motor is increased during the start-up phase of the internal combustion engine to a speed value that ensures the switching element between the electric motor and the drivetrain output remains in slip mode throughout the entire start-up process. This speed value is calculated by an algorithm implemented in the engine control unit and / or the transmission control unit and / or a higher-level torque manager. In addition, drive trains of vehicles with internal combustion engines exhibiting minimum operating speeds are known from practice, which are designed with a starting element comprising a hydrodynamic torque converter and a corresponding converter lock-up clutch in order to realize a combustion engine-side starting process of the vehicle, wherein the torque converter and the converter lock-up clutch are controlled and regulated in a manner known per se depending on the operating state of the vehicle's drive train. The present invention is based on the objective of providing a method for operating a parallel hybrid drive train of a vehicle, which is designed with a starting element comprising a hydrodynamic torque converter and a corresponding converter lock-up clutch, by the application of which the influence of the torque converter on a target output torque to be represented at the output during an electric motor-side starting process of an internal combustion engine is as low as possible. According to the invention, this problem is solved by a method having the features of claim 1. In the inventive method for operating a parallel hybrid drive train of a vehicle with at least one internal combustion engine and at least one electric machine, wherein the electric machine is arranged in the power train between an output and the at least one internal combustion engine and a friction-fit switching element is provided between the internal combustion engine and the electric machine, while a starting element comprising a hydrodynamic torque converter and a converter lock-up clutch is arranged between the electric machine and the output, and wherein a target output torque to be applied to the output is dependent on a slip of the starting element,During an electric start-up process of the combustion engine, the drive torque generated by the electric machine is at least partially transmitted via the torque converter and partly via the converter lock-up clutch. By means of the inventive method, the influence of the hydrodynamic torque converter on the representation of the requested target output torque at the output of the vehicle during an electric machine-side start-up process of the combustion engine is reduced, since the portion of the drive torque of the electric machine guided via the torque converter can be adjusted to a value at which torque fluctuations resulting from the start-up process of the combustion engine in the part of the parallel hybrid drive train on the combustion engine side with respect to the starting element can be damped in the area of ​​the converter lock-up clutch. In an advantageous embodiment of the method according to the invention, the converter lock-up clutch is kept in a slipping mode at least approximately during the entire starting process of the combustion engine by a speed control of the electric machine, during which the drive torque generated by the electric machine is essentially directed towards the output via the converter lock-up clutch. This minimizes the influence of the hydrodynamic torque converter during an electric-driven start-up of the combustion engine, as the portion of the electric machine's drive torque transmitted via the torque converter is set to a value at which the converter lock-up clutch is in a slip operation required for high driving comfort, while still dampening torque fluctuations in the area of ​​the converter lock-up clutch resulting from the combustion engine's start-up process. Furthermore, the portion of the drive torque of the electric machine that is routed via the torque converter is preferably so small that, due to the torque conversion between a pump wheel and a turbine wheel of the torque converter at the output, no reaction torques perceptible to a driver and resulting from the torque fluctuations occur. In a further advantageous variant of the inventive method for representing the target output torque to be applied to the output, a target drive speed of the electric machine and a target transmission capacity of the converter lock-up clutch are determined as a function of a minimum differential speed between an input speed and an output speed of the starting element, which depends on operating state parameters of the parallel hybrid drive train, wherein the starting element is preferably kept in a slipping mode at least approximately during the entire starting process of the combustion engine by specifying the minimum differential speed, during which the drive torque generated by the electric machine is essentially transmitted via the converter lock-up clutch in the direction of the output. By specifying the minimum differential speed, an undesirable "closing" of the torque converter lock-up clutch, i.e., synchronous operation of the torque converter lock-up clutch, is avoided in a simple way, while damping of torque fluctuations in the area of ​​the torque converter lock-up clutch in the combustion engine-side part of the parallel hybrid drive train during an electric machine-side start-up process of the combustion engine is not feasible to the desired extent. Further advantages and beneficial developments of the invention result from the patent claims and the exemplary embodiment described in principle with reference to the drawing. Figure 1 shows a highly schematic representation of a parallel hybrid drive train with a torque converter and a converter lock-up clutch arranged parallel to it in the parallel hybrid drive train; and Figure 2 shows a block diagram of the method according to the invention, by means of which a target drive torque and a target drive speed of the electric machine as well as a target transmission capacity of the converter lock-up clutch of the parallel hybrid drive train according to Figure 1 can be specified during an electric machine-side start-up process of the combustion engine. Fig. 1 shows a highly schematic block diagram representation of a parallel hybrid drivetrain 1 of a vehicle. The parallel hybrid drivetrain 1 comprises an internal combustion engine 2, an electric motor 3, a transmission 4, and a friction-fit switching element 5 arranged between the internal combustion engine 2 and the electric motor 3, which in this case is designed as a friction-fit multi-plate clutch. By means of the switching element 5, an active connection between the combustion engine 2 and the electric machine 3 can be established in order to be able to carry out various operating states of the parallel hybrid drive train 1 of the vehicle, such as a drive solely via the electric machine 3, a parallel drive via the combustion engine 2 and the electric machine 3 or a drive solely via the combustion engine 2. Furthermore, the arrangement of the switching element 5 between the combustion engine 2 and the electric machine 3 makes it possible to couple the combustion engine 2 to the electric machine 3 via the switching element 5 only when the rotational energy of the electric machine 3 required for a starting process of the combustion engine 2 is available, in such a way that the combustion engine 2 is started by the electric machine 3. Additionally, between the electric machine 3 and the transmission 4, which is arranged on the side of the electric machine 3 facing away from the combustion engine 2, a starting element 6 is provided, comprising a torque converter 6A designed as a so-called Trilok converter and a converter lock-up clutch 6B arranged parallel to it in the parallel hybrid drive train 1, with continuously variable transmission capacity, through which the electric machine 3 is operatively connected to the transmission 4. The transmission 4 is designed here as a conventional automatic transmission, through which various gear ratios can be represented, whereby the transmission can be any transmission known from practice. On the side opposite the starting element 6, or on the transmission output side, the transmission 4 is operatively connected to wheels of a vehicle drive axle of the parallel hybrid drive train 1 via an axle differential in a manner not shown in detail.Fig. 2 shows the inventive procedure for setting a driver-requested target output torque or a requested transmission input torque m_GE_target during a start-up process of the combustion engine 2 of the parallel hybrid drive train 1 according to Fig. 1 in the form of a highly schematic block diagram.The driver-requested target transmission input torque m_GE_soll, which is equivalent to a requested target output torque m_fahr_soll, along with a metrologically determined actual turbine speed n_t_ist and a minimum differential speed n_diff_min between an input speed and an output speed of the starting element 6, which depends on operating state parameters of the parallel hybrid drive train, represents an input variable for a determination routine to determine a target drive torque m_3_soll to be generated by the electric machine 3, a target drive speed n_3_soll of the electric machine 3 and a target transmission capacity m_WK_soll of the converter lock-up clutch 6B. In this process, the starting element 6 is operated in a slipping manner by specifying the minimum differential speed n_diff_min during an electric machine-side start-up process of the combustion engine 2, and a significant portion of the drive torque generated by the electric machine 3 during the start-up process is directed via the converter lock-up clutch 6B towards the transmission 4 or the output of the parallel hybrid drive train 1 downstream of the transmission. From the measured actual turbine speed n_t_ist of the torque converter 6A and the minimum differential speed n_diff_min, the target drive speed n_3_soll of the electric machine 3 is determined. This determined target drive speed n_3_soll, along with the actual turbine speed n_t_ist of the torque converter 6A, is fed as an input to a function block 7. In function block 7, a theoretical turbine torque m_t_theo and a theoretical pump torque m_p_theo of the torque converter 6A are calculated as intermediate values, taking converter characteristic maps into account. The theoretical turbine torque m_t_theo determined in function block 7 represents a portion of the requested target transmission input torque m_GE_soll or the requested target output torque m_fahr_soll. Therefore, the target transmission capacity m_WK_soll of the torque converter lock-up clutch 6B can be determined at a node 8 from the difference between the target transmission input torque m_GE_soll and the theoretical turbine torque m_t_theo, and supplied as a target value to the actual process 9 to be actuated, i.e., to the control unit of the torque converter lock-up clutch 6B. At the same time, the target transmission capacity m_WK_soll of the converter lock-up clutch 6B is added in a further node 10 to the theoretical pump torque m_p_theo of the torque converter 6A calculated in function block 7, whereby the sum represents a disturbance torque m_3_stoer of the control of the speed of the electric machine 3. A control deviation is determined between the calculated target drive speed n_3_target of the electric machine 3 and a measured actual speed n_3_actual of the electric machine 3, which is fed to a control device that can be implemented as a proportional-integral controller or as a PID controller, whose output value represents a control component m_3_position of a target drive torque m_3_target of the electric machine 3 to be determined. The control component m_3_stell of the target drive torque m_3_soll of the electric machine 3 is added at a node 12 to the disturbance torque m_3_stoer, which represents a control input for the target drive torque m_3_soll of the electric machine 3. The sum of the control input m_3_stoer and the control component m_3_stell essentially represents the target drive torque m_3_soll to be generated by the electric machine 3 of the parallel hybrid drive train 1 according to Fig. 1, which is required to represent the target output torque m_fahr_soll to be applied at the output 5. With the procedure described above, the drive torque of the electric machine 3 in the area of ​​the starting element 6 is divided between the torque converter 6A and the converter lock-up clutch 6B, and a portion of the drive torque of the electric machine 3 is transmitted via the hydrodynamic clutch or the torque converter 6A and the other portion of the drive torque of the electric machine 3 is transmitted via the slipping converter lock-up clutch 6B towards the transmission 4. Depending on the specified minimum differential speed n_diff_min between the input speed and the output speed of the starting element 6, which is adjustable by controlling the speed of the electric machine 3, it is possible in a simple way to direct as large a proportion as possible of the drive torque of the electric machine 3 via the converter lock-up clutch 6B towards the transmission input of the transmission 4 and to dampen torque fluctuations caused by the engagement of the combustion engine 2 in the part of the parallel hybrid drive train 1 on the combustion engine side with respect to the starting element 6 in the area of ​​the slipping starting element 6 and to prevent changes in the output torque m_GE_soll at the transmission input or the output torque at the output by starting the combustion engine 2. This requirement is implemented all the more the smaller the proportion of the drive torque of the electric machine 3 is that is directed towards the transmission 4 via the torque converter 6A, since in the parallel hybrid drive train during the start-up process of the combustion engine 2, speed disturbances occurring, in contrast to the slipping converter lock-up clutch 6B, have a detrimental effect on the course of the output torque of a vehicle in the area of ​​the torque converter 6A due to the hydrodynamic behavior of the torque converter and impair driving comfort. This utilizes the knowledge that the torque which can in principle be transmitted via a torque converter decreases as the speed difference decreases and increases as the speed difference increases, following a converter detection. In principle, when specifying the minimum differential speed between the input speed and the output speed of the starting element 6, care must be taken to ensure that the target value specification for the target drive speed n_3_target of the electric machine 3 outputs such values ​​during the entire starting process of the combustion engine that a minimum differential speed or a target slip is not undercut in the area of ​​the torque converter, in order to reliably prevent unintentional engagement of the converter lock-up clutch in the event of speed disturbances in the parallel hybrid drive train, i.e., to ensure synchronous or slip-free operation of the converter lock-up clutch. Furthermore, when determining the minimum differential speed n_diff_min of the starting element 6, it is also possible to take into account the vibration and humming behavior of the starting element 6 and the parallel hybrid drive train 1 in order to achieve the highest possible driving comfort. In principle, the minimum differential speed n_diff_min is a quantity that is applied depending on the requested target output torque m_fahr_soll and the actual turbine speed n_t_ist of the torque converter 6A, whereby it is of course at the discretion of the person skilled in the art to determine the minimum differential speed n_diff_min depending on the respective application case on the basis of a characteristic curve calculated as a function of the output torque applied to the output and / or the turbine speed of the torque converter or a characteristic map determined as a function of both quantities. Ideally, when the combustion engine 2 is switched off, i.e., when the vehicle equipped with the parallel hybrid drivetrain 1 according to Fig. 1 is driven solely by the electric motor, the switching element 5 is fully open, so that the electric motor 3 does not have to drag the rotating masses of the combustion engine 2. If a driving strategy requests the engagement of the combustion engine 2 when the switching element 5 has a transmission capacity where essentially no torque can be transmitted through the switching element 5, the transmission capacity of the switching element 5 is set to the value required for engaging the combustion engine 2. The switched-off combustion engine 2 is increasingly driven by the electric machine 3 as the transmission capacity of the switching element 5 increases, whereby the resulting drag torque, which opposes the drive torque of the electric machine 3, represents a disturbance variable for the control of the target drive speed n_3_target of the electric machine 3. During the starting process, the combustion engine 2 is operated in a controlled manner using a target drive torque profile or a target drive speed profile, thus facilitating the transition of the switching element 5 to a synchronous operating state when the combustion engine 2 is engaged. Additionally, when the combustion engine 2 is engaged and in a synchronous operating state (i.e., essentially at the end of the combustion engine 2's starting process), the transmission capacity of the switching element 5 is set to a value at which a torque applied to the switching element 5 is transmitted at least without slippage, provided the drive speed of the electric machine 3 is greater than or equal to the idle speed of the combustion engine 2. In the event that the drive speed or the target drive speed n_3_target of the electric machine is less than the idle speed of the internal combustion engine, the transmission capability of the switching element 5 is set after the starting process to a value at which the switching element 5 enters a slip operation and the internal combustion engine is operated without disturbance at the level of the idle speed, i.e. without causing the internal combustion engine 2 to stall. When the parallel hybrid drivetrain 1 operates in a state where, depending on the actual turbine speed n_t_ist and the minimum differential speed n_diff_min, a theoretical turbine torque m_t_theo is determined which is greater than the transmission input torque m_GE_soll requested by the driver, a suitable creep function is activated and the combustion engine 2 is started by the electric machine 3 with the torque converter lock-up clutch 6B fully open. This allows the combustion engine 2 to be started even when the vehicle is stationary, whereby, due to the activated creep function, the vehicle will at least begin to roll, depending on the drive torque of the electric machine and the torque converter detection, even if the vehicle brake is not applied. If rolling of the vehicle is not desired due to a higher-level driving strategy or due to driver activation of the brake pedal or a parking brake, and starting the combustion engine 2 is requested, for example, due to a low charge level of an electrical storage device assigned to the parallel hybrid drive train 1 or the electric machine 3, the power flow of the parallel hybrid drive train 1 between the electric machine 3 and the vehicle's output (not shown in detail) in the area of ​​the transmission 4 is reduced or completely interrupted, for example, by switching a friction clutch into a slip mode or by fully opening a switching element of the transmission 4, such that the output torque applied to the output during the starting process of the combustion engine 2 is essentially zero. Basically, with the aforementioned method according to the invention, during a starting process of an internal combustion engine, the proportion of the drive torque of the electric machine which is to be guided via the converter lock-up clutch is brought to a maximum in order to set the proportion of the drive torque of the electric machine which is guided via the torque converter of the starting element arranged between the electric machine and the transmission as low as possible and to keep the influences of the converter detection on the target output torque to be applied at the output as low as possible. The inventive method thus represents a method for speed control of the pump speed of the hydrodynamic torque converter using the converter lock-up clutch, wherein the turbine torque of the hydrodynamic torque converter is adjusted as a function of the differential speed at the torque converter. Reference sign 1 Parallel hybrid drivetrain 2 Internal combustion engine 3 Electric machine 4 Transmission 5 Friction-fit switching element 6 Starting element 6A Torque converter 6B Converter lock-up clutch 7 Functional block 8 Junction 9 Real process 10 Junction 12 Junction n_diff_min Minimum differential speed m_GE_set Target transmission input torque n_t_actual Actual turbine speed n_3_set Target input speed of the electric machine m_fahr_set Target output torque m_p_theo Theoretical pump torque of the torque converter m_t_theo Theoretical turbine torque of the torque converter m_WK_set Target transmission capacity of the converter lock-up clutch m_3_set Target input torque of the electric machine m_3_stell Control variable of the input torque of the electric machine m_3_stoer Disturbance torque

Claims

Method for operating a parallel hybrid drive train (1) of a vehicle with at least one internal combustion engine (2) and at least one electric machine (3), wherein the electric machine (3) is arranged in the power train between an output and the at least one internal combustion engine (2) and a friction-fit switching element (5) is provided between the internal combustion engine (2) and the electric machine (3), while a starting element (6) comprising a hydrodynamic torque converter (6A) and a converter lock-up clutch (6B) is arranged between the electric machine (3) and the output, and wherein a target output torque (m_fahr_soll) to be applied to the output is dependent on a slip of the starting element (6), characterized in thatthat during an electric motor-side start-up process of the internal combustion engine (2), a drive torque generated by the electric machine (3) is partly transmitted via the torque converter (6A) and partly via the converter lock-up clutch (6B), wherein the converter lock-up clutch (6B) is kept in a slipping operation at least approximately during the entire start-up process of the internal combustion engine (2) by a speed control of the electric machine (3). Method according to claim 1, characterized in that the drive torque generated by the electric machine (3) is directed to a predominant extent via the converter lock-up clutch (6B) in the direction of the output. Method according to claim 1 or 2, characterized in that, in order to represent the target output torque (m_fahr_soll) to be applied to the output, a target drive speed (n_3_soll) of the electric machine (3) and a target transmission capacity (m_WK_soll) of the converter lock-up clutch (6B) are determined as a function of a minimum differential speed (n_diff_min) between an input speed and an output speed of the starting element (6), which depends on operating state parameters of the parallel hybrid drive train (1). Method according to claim 3, characterized in that the starting element (6) is kept in slip operation at least approximately during the entire starting process of the combustion engine (2) by specifying the minimum differential speed (n_diff_min), during which the drive torque generated by the electric machine (3) is predominantly directed towards the output via the converter lock-up clutch (6B). Method according to claim 3 or 4, characterized in that the minimum differential speed (n_diff_min) is in each case a quantity determined as a function of the requested target output torque (m_fahr_soll) or is determined on the basis of a characteristic curve calculated as a function of the output torque. Method according to one of claims 3 to 5, characterized in that the minimum differential speed (n_diff_min) is an applied quantity determined as a function of an actual turbine speed (n_t_ist) of the torque converter (6A) or is determined on the basis of a characteristic curve calculated as a function of the turbine speed of the torque converter. Method according to one of claims 3 to 5, characterized in that the target drive speed (n_3_target) of the electric machine (3) is determined as a function of an actual turbine speed (n_t_actual) of the torque converter (6A) and the minimum differential speed (n_diff_min). Method according to claim 7, characterized in that a theoretical turbine torque (m_t_theo) of the torque converter (6A) is determined as a function of the actual turbine speed (n_t_ist) of the torque converter (6A) and the target drive speed (n_3_soll) of the electric machine (3) taking into account converter characteristic maps of the torque converter (6A). Method according to claim 8, characterized in that the target transmission capacity (m_WK_soll) of the converter lock-up clutch (6B) is determined from the difference between the theoretical turbine torque (m_t_theo) of the torque converter (6A) and the requested target output torque (m_fahr_soll). Method according to one of claims 7 to 9, characterized in that a theoretical pump torque (m_p_theo) of the torque converter (6A) is determined as a function of the actual turbine speed (n_t_ist) of the torque converter (6A) and the target drive speed (n_3_soll) of the electric machine (3) taking into account converter characteristic maps of the torque converter (6A). Method according to claim 10, characterized in that a disturbance torque (m_3_stoer) influencing the speed control of the electric machine (3) is calculated from the sum of the target transmission capacity (m_WK_soll) of the converter lock-up clutch (6B) and the theoretical pump torque (m_p_theo) of the torque converter (6A), which represents a control component of a target drive torque (m_3_soll) of the electric machine (3). Method according to one of claims 3 to 11, characterized in that a control deviation is determined between the actual speed (n_3_actual) of the electric machine (3) and the target speed (n_3_target) of the electric machine (3). Method according to claim 12, characterized in that the control deviation of the drive speed of the electric machine (3) is supplied as an input value to a control device, the output value of which represents the control component of the target drive torque (m_3_target) of the electric machine (3). Method according to claim 13, characterized in that the target drive torque (m_3_target) of the electric machine (3) represents a sum of the disturbance torque (m_3_disturb) and the control component (m_3_position). Method according to one of claims 1 to 14, characterized in that the target output torque (m_fahr_soll) is generated by the electric machine (3) when the switching element (5) arranged between the electric machine (3) and the combustion engine (2) has a transmission capacity in which no torque can be transmitted via the switching element (5), and / or when the combustion engine (2) is switched off, wherein the combustion engine (2) is connected to the parallel hybrid drive train (1) when a requirement to generate a drive torque on the combustion engine side is present by changing the transmission capacity of the switching element (5). Method according to one of claims 1 to 15, characterized in that the transmission capability of the switching element (5) is adjusted to the value required for switching on the combustion engine (2) when a request is made, wherein the switched-off combustion engine (2) is increasingly driven by the electric machine (3) with increasing transmission capability of the switching element (5) and the resulting drag torque, which opposes the drive torque (m_3) of the electric machine (3), represents a disturbance variable for determining the target drive speed (n_3_target) of the electric machine (3). Method according to one of claims 1 to 16, characterized in that the combustion engine (2) is operated in a controlled manner during the starting process by means of a target drive torque profile or a target drive speed profile and the switching element (5) is brought into a synchronous state when the combustion engine (2) is switched on. Method according to claim 17, characterized in that the transmission capability of the switching element (5) is set to a value when the combustion engine (2) is switched on and the switching element (5) is in a synchronous operating state, such that a torque applied to the switching element (5) is transmitted at least without slippage. Method according to one of claims 1 to 18, characterized in that the transmission capability of the switching element (5) is adjusted after the start-up process of the internal combustion engine (2) to a value at which the switching element (5) enters a slip operation and the internal combustion engine (2) is operated without disturbance at the level of the idle speed when a target drive speed (n_3_target) of the electric machine (3) is less than the idle speed of the internal combustion engine (2). Method according to one of claims 1 to 19, characterized in that the power flow of the parallel hybrid drive train (1) between the electric machine (3) and the output of the vehicle in the area of ​​the transmission (4) is reduced or completely interrupted by switching a friction clutch of the transmission (4) into a slip mode or by fully opening a switching element of the transmission (4), so that the output torque applied to the output during the start-up process of the internal combustion engine (2) is zero when a standstill of the vehicle is requested by a higher-level driving strategy or by a driver-side actuation of a brake pedal or a parking brake and / or a start of the internal combustion engine (2) is required due to a low charge level of an electrical storage device associated with the electric machine (3).

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