Method for operating a motor vehicle powertrain

Abstract

The invention relates to a method for operating the drivetrain of a motor vehicle, wherein a sailing mode is described in which, during travel of the motor vehicle, an internal combustion engine in the drivetrain is disconnected and switched off via an intermediate starting element from at least one drive wheel. To end the sailing mode, the internal combustion engine is started as a tow start, during which the starting element transmits torque between the internal combustion engine and the at least one drive wheel, and the internal combustion engine is subsequently towed by the at least one drive wheel.During the tow start procedure, any slippage of at least one drive wheel occurring during the towing of the internal combustion engine is determined, whereby a blocking notice is generated depending on the determined slippage occurring, with the presence of which further tow starts are prevented during a current driving cycle of the motor vehicle to terminate further sailing operations.

Description

[0001] The invention relates to a method for operating the drivetrain of a motor vehicle, wherein a coasting mode is implemented by disconnecting and switching off an internal combustion engine in the drivetrain from at least one drive wheel via an intermediate starting element during travel of the motor vehicle. To terminate the coasting mode, the internal combustion engine is started as a tow start, during which the starting element transmits torque between the internal combustion engine and the at least one drive wheel, and the internal combustion engine is subsequently towed by the at least one drive wheel. During the tow start, the slippage of the at least one drive wheel occurring while the internal combustion engine is being towed is determined. The invention further relates to a control unit, a computer program, and a data carrier.

[0002] In motor vehicle powertrains with internal combustion engines, it is common practice to provide a starting element between each engine and each drive wheel. This element allows the connection between the engine and the drive wheels to be established or disconnected depending on the driving situation. Such a starting element can be, for example, a starting clutch or a hydrodynamic torque converter. In some cases, a so-called coasting mode can also be implemented in a motor vehicle powertrain, allowing the vehicle to roll freely when the engine is not providing propulsion.The intermediate starting element then disconnects the internal combustion engine from the drive wheels, and the engine is subsequently switched off to save fuel during sailing. To end sailing, the engine is restarted, sometimes by towing the engine via the drive wheels as an alternative to using a starter motor. For this to work, the starting element must at least partially re-establish the connection between the engine and the drive wheels.

[0003] German patent application DE 10 2015 016 971 B4 discloses a method for operating a motor vehicle powertrain in which an internal combustion engine can be connected to the drive wheels of a drive axle via an intermediate starting element in the form of a clutch. The motor vehicle powertrain allows for coasting operation by disconnecting the internal combustion engine from the drive wheels of the drive axle by opening the clutch and then switching it off. To end coasting operation, the internal combustion engine can then be started in a first operating mode by an electric motor that acts as a starter for the internal combustion engine.On the other hand, a second operating mode allows for a tow start, in which the clutch, by at least partially engaging, permits torque transmission between the internal combustion engine and the drive wheels, resulting in the engine being towed by the drive wheels. To minimize the impact on the vehicle's driving performance during this tow start, the slip at the drive wheels is measured. If a certain threshold is exceeded, the electric motor is additionally engaged to start the internal combustion engine and drive the wheels, thereby reducing the slip.

[0004] Starting from the prior art described above, the object of the present invention is to increase the safety during the operation of a drive train of a motor vehicle that can be operated in a sailing mode.

[0005] This problem is solved from a process engineering perspective, starting from the preamble of claim 1 in conjunction with its characterizing features. The subsequent dependent claims each describe advantageous embodiments of the invention. A control unit with which the aforementioned method can be carried out is further the subject of claims 12 and 13. Claim 14 relates to a computer program product, and claim 15 to a data carrier.

[0006] According to the invention, a sailing mode is achieved in a method for operating the drivetrain of a motor vehicle by disconnecting and switching off an internal combustion engine in the drivetrain from at least one drive wheel via an intermediate starting element while the motor vehicle is in motion. To end the sailing mode, the internal combustion engine is started as a tow start, during which the starting element transmits torque between the internal combustion engine and the at least one drive wheel, and the internal combustion engine is subsequently towed by the at least one drive wheel. During the tow start, the slippage of the at least one drive wheel occurring while the internal combustion engine is being towed is measured.

[0007] At the beginning of the method according to the invention, the drivetrain of the motor vehicle is initially operated in a coasting mode, in which an internal combustion engine provided in the drivetrain is disconnected from at least one drive wheel and subsequently switched off. For the purposes of the invention, "disconnection" of the internal combustion engine from the at least one drive wheel means that after the disconnection, no further torque transmission is possible between the internal combustion engine and the at least one drive wheel, allowing the internal combustion engine and the at least one drive wheel to perform rotational movements independently of each other. During coasting, this disconnection is carried out to prevent the engine from being switched off, i.e., from being switched off, both while the motor vehicle is in motion and after the disconnection.to be able to bring the internal combustion engine to a standstill and to continue driving the motor vehicle by allowing at least one drive wheel to rotate freely without coupling to the internal combustion engine.

[0008] During sailing operation, the mechanical separation of the internal combustion engine from the at least one drive wheel is achieved via a starting element. This element is located in the drivetrain in the power flow between the internal combustion engine and the at least one drive wheel and, depending on its actuation state, either performs the mechanical separation or enables torque transmission between the internal combustion engine and the at least one drive wheel. This starting element is particularly preferably a friction clutch that, in a fully open state, separates the internal combustion engine from the at least one drive wheel, while, from an at least partially closed state, the friction clutch enables torque transmission between the internal combustion engine and the at least one drive wheel.

[0009] The at least one drive wheel in the vehicle's drivetrain can be driven by the internal combustion engine. For this purpose, the intermediate starting element establishes the drive connection between the internal combustion engine and the at least one drive wheel, thus enabling torque transmission from the internal combustion engine to the at least one drive wheel. Accordingly, the at least one drive wheel is assigned to the internal combustion engine in the drivetrain. Preferably, however, several drive wheels are assigned to the internal combustion engine, with a particular emphasis on paired assignment of drive wheels to the engine, whereby the paired drive wheels each form part of a drive axle of the vehicle.

[0010] In the inventive method, the coasting mode, i.e., the free rolling of the vehicle, is terminated by starting the internal combustion engine with a tow start. This tow start is achieved by pulling the internal combustion engine forward via the at least one drive wheel, for which purpose the intermediate starting element transmits torque between the internal combustion engine and the at least one drive wheel while the vehicle is in motion. If the starting element is designed as a friction clutch, this friction clutch is at least partially engaged. This torque transmission causes the at least one drive wheel to rotate the internal combustion engine, so that it can be started once the minimum necessary idle speed is exceeded.This tow start is particularly preferably carried out entirely without the aid of a starter motor associated with the internal combustion engine, so that starting the internal combustion engine during the tow start to end sailing operation takes place purely by towing. However, within the scope of the invention, starting the internal combustion engine during the tow start could also be carried out with the aid of a starter motor, for example, if a sufficiently high engine speed cannot be generated by the rotational movement of the at least one drive wheel.

[0011] During a tow start, the slippage that occurs at the at least one drive wheel when the internal combustion engine is towed is determined. In other words, the tow start process ascertains the slippage of the at least one drive wheel when the internal combustion engine is towed.

[0012] The invention now comprises the technical teaching that, depending on the determined slip, a blocking signal is generated, which, if present, prevents further tow starts to terminate further sailing operations during the current driving cycle of the motor vehicle. In other words, a blocking signal is issued depending on the slip generated during the tow start by the internal combustion engine pulling on the at least one drive wheel. If such a blocking signal is active, this means that during further sailing operations in the current driving cycle of the motor vehicle, the respective sailing operation cannot be terminated by a further tow start.

[0013] Operating a motor vehicle's powertrain in this way has the advantage of increasing the vehicle's safety when coasting ends. By generating a lockout signal during the current driving cycle, critical driving safety situations and high loads on powertrain components can be avoided when ending one or more further coasting phases. This is because determining the slip generated during a tow start allows the system to assess the impact of the engine's initial towing. If the tow start resulted in high slip, it can be assumed that a subsequent tow start during the current driving cycle will also lead to a similarly high slip.To avoid this, the inventive method prevents any further towing starts to terminate subsequent sailing operations when a blocking notice is present during the current driving cycle. Besides increasing the safety of the vehicle and preventing overloading, particularly of the intermediate starting element, this also avoids the effects on driving comfort when towing the internal combustion engine.

[0014] If a locking mechanism is present, further engine starts are prevented during the current driving cycle of the vehicle. A "driving cycle" refers to the period of time during which the vehicle's ignition is switched on. Specifically, when the ignition is switched off, any locking mechanism still in place at that time is deleted, so that when the next driving cycle is started by switching on the ignition, no locking mechanism is present.

[0015] In particular, the locking notice is generated if the measured slip exceeds a limit value. Thus, within the framework of the inventive method, the measured slip is compared with a predetermined limit value, which defines a permissible slip at the at least one drive wheel during a tow start. If the measured slip is above this limit value, the locking notice is generated.

[0016] According to one embodiment of the invention, a generated blocking notice is removed when at least one condition is met, thereby lifting the restriction on further tow starts. This allows the restriction on further tow starts to be lifted not only when the current driving cycle is terminated, but also when at least one condition is fulfilled. This at least one condition could be an increase in the vehicle's mass during the current driving cycle. Typically, an increase in the vehicle's mass results in lower slippage of at least one drive wheel during a subsequent tow attempt, thus potentially rendering the restriction on further tow starts no longer justified.When the motor vehicle is designed as a commercial vehicle, the vehicle mass can change, for example, through the temporary attachment of a semi-trailer or the temporary loading of the commercial vehicle, which would be detectable via an axle load sensor, a trailer status and / or a mass calculation.

[0017] According to one embodiment of the invention, during the tow start and before the torque transmission between the internal combustion engine and the at least one drive wheel, it is first checked whether at least one abort criterion for terminating the tow start is present. If the at least one abort criterion is present, the tow start is then aborted, whereas if the at least one abort criterion is not present, the tow start is continued. Advantageously, this allows for the consideration of further criteria that would preclude starting the internal combustion engine during the tow start.

[0018] In particular, exceeding a load limit due to stress, especially thermal stress, on the starting element is recognized as an abort criterion. If the starting element is already under high stress, performing the tow start could lead to an overload of the starting element and consequently to a safety-critical condition. Aborting the tow start due to exceeding a defined load limit, preferably a temperature limit of the starting element, reliably prevents this overload. Alternatively, but preferably additionally, falling below a weight limit is recognized as an abort criterion. This ensures that the tow start is also aborted if the vehicle is too light, for example, due to an insufficient load.It is particularly important to check, during the tow start and before the torque is transferred, whether either of the two aforementioned abort criteria is met.

[0019] According to one embodiment of the invention, the slip is determined by observing a deviation in the rotational speed of at least one drive wheel from the rotational speed of at least one non-driven wheel. This allows a reliable conclusion to be drawn about the slip occurring at the at least one drive wheel.

[0020] In a further embodiment of the invention, the blocking notice is also generated if an active intervention of at least one vehicle stability assistance system is detected during the tow start. This allows the system to identify whether a safety-critical condition would have occurred during the tow of the internal combustion engine, which was prevented by the active intervention of the at least one vehicle stability assistance system. The at least one vehicle stability assistance system can be an ABS or ESP system. Preferably, the active intervention of the at least one vehicle stability assistance system during the tow start is detected if the intervention time of the at least one vehicle stability assistance system exceeds a threshold value. Thus, only a significant intervention of the vehicle stability assistance system leads to the generation of a blocking notice.

[0021] Preferably, the tow launch is aborted by generating the blocking notice, thereby avoiding a safety-critical driving situation during the current sailing operation.

[0022] According to a further embodiment of the invention, starting the internal combustion engine during the current sailing cycle, if a locking signal is present and / or if the tow start has been aborted, is subsequently carried out as a starter start, in which the internal combustion engine is started via a starter motor and after the drive mechanism is disconnected from the at least one drive wheel. This ensures that, if a locking signal is present and / or if the current sailing operation has been aborted, the internal combustion engine is started via the starter motor, thus allowing the respective sailing operation to be terminated safely.

[0023] The invention further relates to a control unit configured to initiate a sailing mode in the drivetrain of a motor vehicle. This involves disengaging the internal combustion engine from at least one drive wheel via an intermediate starting element, followed by switching off the internal combustion engine. The control unit is further configured to initiate a tow-start to terminate the sailing mode. During this tow-start, the starting element transmits torque between the internal combustion engine and the at least one drive wheel, which then tows the engine.Furthermore, the control unit is configured to detect any slippage of at least one drive wheel occurring during the towing of the internal combustion engine during a tow start and, depending on the detected slippage, to generate a blocking signal. The presence of this signal during the current driving cycle prevents a further tow start to terminate further sailing operation. Particularly preferably, the control unit is also configured to implement one or more of the aforementioned variants of a method.

[0024] The method according to the invention can also be embodied as a computer program product which, when running on a processor, for example a processor of the aforementioned control unit, instructs the processor by means of software to carry out the associated process steps relating to the invention. In this context, a computer-readable medium on which a computer program product described above is stored in a retrievable manner is also part of the subject matter of the invention.

[0025] An advantageous embodiment of the invention, which is explained below, is illustrated in the drawings. It shows: Fig. 1. A schematic view of a motor vehicle; and Fig. 2 a flowchart of a method according to the invention for operating a drive train of the motor vehicle made of Fig. 1.

[0026] Out of Fig. Figure 1 shows a schematic view of a motor vehicle 1, which is in particular a commercial vehicle. The motor vehicle 1 comprises a drive train 2 in which an internal combustion engine 3 is provided. The internal combustion engine 3 is associated with a starter 4, which is preferably an electric starter in the form of an electric machine and by means of which the internal combustion engine 3 can be towed to start in the context of a starter start.

[0027] On the output side, the internal combustion engine 3 can also be connected via an intermediate starting element 5 to a transmission input of a downstream motor vehicle transmission 6, through which a drive torque provided by the internal combustion engine 3 can be transmitted, particularly with different gear ratios. The starting element 5 is specifically designed as a starting clutch. A differential 7 is connected downstream of the motor vehicle transmission 6 on the output side. This differential, acting as a transverse differential, distributes the drive torque transmitted via the motor vehicle transmission 6 to two drive wheels 8 and 9 of a drive axle 10 of the motor vehicle 1. In addition to the drive axle 10, the motor vehicle 1 also has a non-driven axle 11, which has two non-driven wheels 12 and 13.

[0028] In motor vehicle 1, a sailing mode can be implemented in a manner known in principle to those skilled in the art by disconnecting the internal combustion engine 3 from the motor vehicle transmission 6, and consequently from the two drive wheels 8 and 9, via the starting element 5 while the motor vehicle 1 is in motion. After this disconnection, the internal combustion engine 3 is then switched off to reduce fuel consumption during sailing. Due to the disconnection of the two drive wheels 8 and 9 from the internal combustion engine 3, the motor vehicle 1 can then roll freely during sailing.

[0029] With the motor vehicle 1 in sailing mode and during its ongoing driving cycle, the drive train 2 is then operated according to a method according to the invention, the flow diagram of which is shown in Fig.Figure 2 shows the process, which will now be described. The process is carried out by a control unit 14, which is a transmission control unit. The control unit 14 controls the starting element 5 and the vehicle transmission 6 and communicates via a data bus system with an engine control unit 15, which controls the internal combustion engine 3 and the starter 4.

[0030] The inventive method begins with the detection of the existing sailing operation in a first step S1 of the method. Following step S1, a second step S2 checks whether a request to terminate the current sailing operation exists. If this is not the case, the process returns to step S1 and the sailing operation continues, preferably transitioning back to step S2 after a defined time window.

[0031] If, however, a request to terminate the current sailing operation is detected in step S2, then in step S3 it is checked whether the sailing operation should be terminated by means of a tow start. In this process, the starting element 5, by at least partially closing, transmits torque between the internal combustion engine 3 and the vehicle transmission 6, and subsequently also to the drive wheels 8 and 9, so that the internal combustion engine 3 can be towed and started via the drive wheels 8 and 9. If the check in step S3 shows that a tow start is not requested, the process proceeds to step S4, in which the internal combustion engine 3 is started via the starter 4, with the starting element 5 still disconnecting it from the drive wheels 8 and 9.

[0032] If, however, a tow start is requested, step S3 is followed by a further step S5, in which it is checked whether a tow start is permitted or whether the vehicle mass of vehicle 1 has changed within the current driving cycle. If either of these two criteria is met, the system proceeds to step S6, while if a tow start is not permitted and the vehicle mass has not changed, the system proceeds to step S4 and accordingly starts the internal combustion engine 3 as a starter start.

[0033] In step S6, any existing block on a tow start is simply lifted, and the system then proceeds to step S7. In step S7, it is checked whether any abort criteria exist for terminating the tow start. This involves checking whether the current thermal load on the starting element 5 is below a defined load limit and whether the vehicle mass exceeds a predefined weight limit. If the current thermal load is below the load limit and the vehicle mass simultaneously exceeds the predefined weight limit, the system proceeds to step S8. Otherwise, the presence of an abort criterion is detected, and the tow start is aborted. This abort then triggers a return to step S4, and the starting of the internal combustion engine 3 is subsequently performed as a starter start.

[0034] Upon transition to step S8, the drag start of the internal combustion engine 3 is initiated by closing the starting element 5. During this process, the slip occurring at the drive wheels 8 and 9 is determined. This slip is calculated by observing the deviation of the rotational speeds of the drive wheels 8 and 9 from the rotational speeds of the non-driven wheels 12 and 13. If the slip occurring at the drive wheels 8 and 9 exceeds a limit value, the process proceeds to step S9. This transition to step S9 also occurs if the slip falls below the limit value and an active intervention by a vehicle stability assistance system of the vehicle 1 is detected for a defined period. This vehicle stability assistance system could, for example, be an ESP system.For a transition from step S8 to step S9, either the slip that occurs must exceed the limit value or the active intervention of the vehicle stability system must take place for the defined period of time.

[0035] In step S9, a blocking notice is entered, and the process then returns to step S5. When sailing operations are currently being terminated, returning to step S5 results in the blocking notice being recognized as a lack of permission for a tow launch, leading to a transition to step S4. Consequently, the transition from step S8 to step S9 also aborts the current tow launch, with the starting of internal combustion engine 3 and the resulting termination of sailing operations then taking place during the starter start.

[0036] If, however, in step S8 neither an exceedance of the limit value due to the occurring slip nor a sufficiently long intervention of the vehicle stability system is detected, the internal combustion engine 3 is towed by fully executing the tow start in step S10, which follows step S8. After successful towing, the method according to the invention is then terminated upon reaching step S11, to which the process also proceeds from step S4 after successful starter start.

[0037] If further sailing operations are carried out during the current driving cycle of the vehicle 1, the blocking notice, if any, set in step S9, leads to the prevention of further tow starts in step S5 when the method according to the invention is carried out. This is because a blocking notice set in step S5 leads to a transition to step S4. However, if an increase in the vehicle mass has occurred during the current driving cycle, for example by loading the vehicle 1, attaching a trailer, or coupling a semi-trailer, the process continues to step S6 even if a blocking notice is present, in which the blocking notice is then removed. If, however, the increase in vehicle mass is insufficient or the load on the starting element 5 is already too high, the check in step S7 nevertheless leads to the termination of a further tow start.

[0038] The safety of operating a propulsion system that can be operated in sailing mode can be increased by means of the method according to the invention. Reference sign 1 motor vehicle 2 Powertrain 3 Internal combustion engine 4 starter motors 5 Starting element 6 Automotive transmissions 7 Differential gears 8 drive wheel 9 drive wheel 10 drive axle 11 vehicle axle 12 wheel 13 wheel 14 Control unit 15 Engine control unit S1 to S11 individual steps QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2015 016 971 B4

[0003]

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