Method and control system for operating a drive train

A method for operating a drive train of a motor vehicle with a prime mover (1) including an internal combustion engine (2) and an electric machine (3), a separating clutch (6) connected between the internal combustion engine and the electric machine, and a transmission (5) connected between the prime mover (1) and a driven end (4) is provided. When at least one first operating condition is present, a previously decoupled internal combustion engine (2) is coupled such that the separating clutch (6) is actuated to engage. When at least one second operating condition is present, the coupling of the internal combustion engine is aborted, and an absolute value of a torque currently transmitted or currently transmittable by the separating clutch (6) is determined. The separating clutch (6) is disengaged at different rates depending on the absolute value of the torque currently transmitted or currently transmittable by the separating clutch (6).

CROSS-REFERENCE TO RELATED APPLICATION

The present application is related and has right of priority to German Patent Application No. 10 2019 207 659.9 filed on May 24, 2019, which is incorporated by reference in its entirety for all purposes.

FIELD OF THE INVENTION

The invention relates generally to a method for operating a drive train. Moreover, the invention relates generally to a control system for operating a drive train.

BACKGROUND

EP 1 826 088 A2 describes a method for operating a motor vehicle designed as a hybrid vehicle. The hybrid vehicle to be operated includes a prime mover encompassing an internal combustion engine and an electric machine. A separating clutch is connected between the internal combustion engine and the electric machine of the prime mover. A transmission is connected between the prime mover and a driven end. It is provided that, for the case in which a defined first operating condition has been met, the internal combustion engine is started. This takes place for the case in which the internal combustion engine is necessary in order to meet the driver demand. Moreover, it is provided that the start of the internal combustion engine is aborted if a second operating condition is present. This takes place for the case in which the brake is actuated again within a predetermined time period. In order to start the internal combustion engine, the separating clutch—which is connected between the internal combustion engine and the electric machine—is actuated, in order to couple the internal combustion engine.

For the case in which a coupling of an internal combustion engine is aborted, losses of comfort may have so far occurred. Namely, in the presence of an operating condition, on the basis of which the coupling of an internal combustion engine is aborted, if the separating clutch connected between the internal combustion engine and the electric machine is abruptly disengaged, this can result in an undesirably strong run-up of the internal combustion engine, whereby, on the one hand, noises and, on the other hand, vibrations in the drive train can be caused. In order to avoid this, it has previously only been possible to carry out the coupling process to the end despite the presence of an abort condition, and to couple the internal combustion engine again only after a defined time period has elapsed. This approach is also disadvantageous, however.

SUMMARY OF THE INVENTION

There is a need, for the case in which an abort of the coupling of the internal combustion engine is demanded, that this be carried out as quickly as possible with a high level of comfort. On the basis thereof, example aspects of the invention creates a new type of method for operating a drive train and a control system for operating a drive train.

For the case in which the at least one second defined operating condition is present, an absolute value of a torque currently transmitted or currently transmittable by the separating clutch is determined. The separating clutch is disengaged at different rates depending on the absolute value of the torque currently transmitted or currently transmittable by the separating clutch. With the aid of example aspects of the invention, it is possible, in the presence of an abort for the coupling of an internal combustion engine, to decouple the internal combustion engine with a high level of comfort within a short time.

According to one advantageous example refinement, for the case in which the absolute value of the torque currently transmitted or currently transmittable by the separating clutch is less than a limiting value, the separating clutch is disengaged, preferably in a stepwise manner, at a first speed. For the case in which the absolute value of the torque currently transmitted or currently transmittable by the separating clutch is greater than the limiting value, the separating clutch is disengaged at a second speed, which is less than the first speed, preferably continuously with a torque gradient along a ramp. In this way, it is particularly advantageously possible, in the presence of an abort for the coupling of an internal combustion engine, to decouple the internal combustion engine with a high level of comfort within a short time.

According to one advantageous example refinement, in order to disengage the separating clutch at the second speed, on the one hand, a torque gradient is determined, on the basis of which the separating clutch is disengaged in a torque-controlled manner; on the other hand, a specified rotational speed for the internal combustion engine is determined, on the basis of which the internal combustion engine is operated in a speed-controlled manner in overlap with the torque-controlled operation of the separating clutch. The internal combustion engine can be decoupled with a high level of comfort within a short time.

According to one advantageous example refinement, the torque gradient is determined depending on a torque, which must be decreased at the separating clutch, so that the electric machine can solely provide a driver-input torque, and/or depending on a current gradient of the transmission input torque. Preferably, the torque gradient is that much steeper, the higher the torque is, that must be decreased at the separating clutch, so that the electric machine can provide the driver-input torque. Moreover, the torque gradient is preferably that much steeper, the higher is the current gradient of the transmission input torque. This enables the advantageous determination of the torque gradient for the torque-controlled operation of the separating clutch.

According to one advantageous example refinement, the specified rotational speed for the internal combustion engine is determined depending on a current differential speed at the separating clutch and depending on the current gradient of the differential speed at the separating clutch. This enables the advantageous determination of the specified rotational speed for the speed-controlled operation of the internal combustion engine.

DETAILED DESCRIPTION

FIG. 1shows a diagram of a drive train of a motor vehicle designed as a hybrid vehicle.

The motor vehicle fromFIG. 1includes a prime mover1with an internal combustion engine2and an electric machine3. A transmission5is connected between the prime mover1and a driven end4. A separating clutch6is connected between the internal combustion engine2and the electric machine3. For the case in which the separating clutch6is disengaged, the internal combustion engine2is decoupled. When the separating clutch6is engaged, however, the internal combustion engine2is coupled. Moreover,FIG. 1shows, by way of example, a shift element7of the transmission5. The transmission5includes multiple such shift elements7, in order to make gears available.

Moreover,FIG. 1shows an engine control unit8, a transmission control unit9, and a hybrid control unit10. The engine control unit8is utilized for the closed-loop control and/or open-loop control of the operation of the internal combustion engine2. The transmission control unit9is utilized for the open-loop control and/or closed-loop control of the operation of the transmission5. The hybrid control unit10is utilized for the open-loop control and/or closed-loop control of the operation of the electric machine3and, inFIG. 1, of the separating clutch6. The hybrid control unit10can be an integral part of the transmission control unit9.

For the case in which driving takes place purely electrically via the electric machine3, the separating clutch6is disengaged and the internal combustion engine2is decoupled. In this case, the internal combustion engine2is preferably shut down. The internal combustion engine2can also run at idling speed in the decoupled condition, however. Depending on a defined first operating condition, a coupling of the internal combustion engine2can be demanded, in particular for the case in which the electric machine3alone cannot provide a driver-input torque. In this case, if the internal combustion engine2is shut down, the internal combustion engine2is started and the separating clutch6is engaged. However, if the internal combustion engine2is already running in this condition, it is only necessary for the separating clutch6to be engaged. Depending on at least one second defined operating condition, an abort for the coupling of the internal combustion engine2can be demanded, in particular, for example, via a driver-side actuation of a brake pedal and/or via a cancellation of a driver-input torque, for example, via a cancellation of an accelerator pedal actuation. In this case, the separating clutch6must be disengaged again.

Example aspects of the present invention now relates to such details, with the aid of which the coupling of the internal combustion engine2can be aborted in an advantageous manner, with a high level of comfort and within a short time.

According to example aspects of the invention, for the case in which the at least one second operating condition is present, which demands an abort for the coupling of the internal combustion engine2, a check is carried out to determine how high an absolute value is of a torque currently transmitted or transmittable by the separating clutch6. The separating clutch is disengaged at different rates depending on the absolute value of the torque currently transmitted or currently transmittable by the separating clutch.

For the case in which the absolute value of the torque currently transmitted or currently transmittable by the separating clutch6is less than a limiting value, the separating clutch6is disengaged at a first speed, preferably in a stepwise manner and completely.

However, for the case in which the absolute value of the torque currently transmitted or currently transmittable by the separating clutch6is greater than the limiting value, the separating clutch is not disengaged at the first speed and/or abruptly, but rather more slowly at a second speed, preferably, e.g., continuously along an, in particular, linear ramp having a defined torque gradient with respect to time.

This limiting value for the absolute value of the currently transmitted or currently transmittable torque of the separating clutch6can correspond to the drag torque of the separating clutch6increased by an offset.

For the case in which the separating clutch6is disengaged along a linear ramp, this torque gradient is constant during the disengagement.

In order to disengage the separating clutch6with the torque gradient, the separating clutch6is disengaged in a torque-controlled manner, in particular starting from the hybrid control unit10or the transmission control unit9.

The internal combustion engine2is operated in a speed-controlled manner in overlap, with respect to time, with the torque-controlled operation of the separating clutch6, wherein, for this purpose, a specified rotational speed for the internal combustion engine2is determined, with the aid of which the internal combustion engine2is actuated starting from the engine control unit8. The determination of the specified rotational speed for the internal combustion engine2can take place either in the hybrid control unit10or in the transmission control unit9, wherein the specified rotational speed is then made available to the engine control unit8.

The torque gradient for the torque-controlled operation of the separating clutch6is preferably determined depending on a torque, which must be decreased at the separating clutch6, so that the electric machine3can solely provide a driver-input torque. This torque is preferably determined with the following:
ΔMK0=(MFW−MGE)−(MEM-MIN−MEM-IST),

wherein

ΔMK0is the determined torque,

MGEis the current transmission input torque,

MEM-MINis the minimally possible torque of the electric machine, and

MEM-ISTis the current torque of the electric machine.

The aforementioned determination of the torque ΔMK0, which must be decreased at the separating clutch6, so that the electric machine3can make the driver-input torque available, is visualized inFIG. 2.FIG. 2shows, with respect to the time t, the torque profiles for the current driver-input torque MFW, for the current transmission input torque MGE, for the minimally possible torque MEM-MINof the electric machine3, and for the current torque MEM-ISTof the electric machine3.

The torque difference Δ1corresponds to the difference between the minimally possible torque MEM-MINof the electric machine3and the current torque MEM-ISTof the electric machine3. The torque difference Δ2corresponds to the difference between the driver-input torque MFWand the current transmission input torque MGE. The torque ΔMK0results from the difference Δ2−Δ1.

The torque gradient for the torque-controlled operation of the separating clutch6is preferably determined depending not only on the above-described torque ΔMK0, but rather preferably additionally depending on a current gradient of the transmission input torque MGE.

The torque gradient for the torque-controlled operation of the separating clutch6is that much steeper, the greater the torque ΔMK0is that must be decreased at the separating clutch6.

Moreover, the torque gradient for the torque-controlled operation of the separating clutch6is that much steeper, the greater is the current gradient of the transmission input torque MGE. Thus, e.g., the torque gradient for the torque-controlled operation of the separating clutch6may increase in correspondence with the current gradient of the transmission input torque MGE.

In overlap, with respect to time, with this torque-controlled operation, namely the torque-controlled disengagement of the separating clutch6depending on the torque gradient determined in the above-described way, the specified rotational speed for the internal combustion engine2is determined, on the basis of which the internal combustion engine2is operated in a speed-controlled manner in overlap with the torque-controlled operation of the separating clutch6.

The specified rotational speed for the internal combustion engine2is determined depending on a current differential speed at the separating clutch6and depending on the current gradient, with respect to time, of the differential speed at the separating clutch6.

The determination of the specified rotational speed for the internal combustion engine2is described in the following with reference to the diagram fromFIG. 3, whereinFIG. 3shows a graph, with respect to the time t, on the one hand, of a time profile of the rotational speed nEMof the electric machine3and, on the other hand, a rotational speed range Δn for the rotational speed of the separating clutch6spanning the rotational speed nEMof the electric machine3. This rotational speed range Δn is defined by a lower limiting value S1and an upper limiting value S2.

If the rotational speed at the separating clutch6lies within this rotational speed range Δn, the differential speed at the separating clutch6is less than a corresponding limiting value.

If the rotational speed at the separating clutch6lies outside this rotational speed range Δn, the differential speed at the separating clutch6is greater than the corresponding limiting value.

Moreover,FIG. 3shows two possible specified rotational speeds nVM-SOLL1and nVM-SOLL2for the internal combustion engine2, which each result from the current rotational speed nEMof the electric machine3plus an offset OF1or OF2, respectively. The arrows fromFIG. 3indicate, in each case, where the current rotational speed at the separating clutch6is located, wherein the slope of the particular arrow visualizes the magnitude of the current gradient of the differential speed at the separating clutch6.

For the case in which the current differential speed at the separating clutch6and the current gradient of the differential speed at the separating clutch6are each less than a corresponding limiting value, no specified rotational speed for the internal combustion engine2is determined, or the current rotational speed nEMof the electric machine3is determined as the specified rotational speed for the internal combustion engine2. In this case, the separating clutch6is considered to be engaged. This is the case, inFIG. 3, in the time intervals3and8. In these time intervals3and8, the rotational speed of the separating clutch6lies within the rotational speed range Δn and the gradient of the differential speed is low.

For the case in which the current differential speed at the separating clutch6and/or the current gradient of the differential speed at the separating clutch6are/is greater than the corresponding limiting value, a rotational speed is determined, as the specified rotational speed for the internal combustion engine2, which corresponds to the current rotational speed nEMof the electric machine3plus the offset. In this way, a synchronization of the separating clutch6is prevented. In the time intervals1,2,9,10fromFIG. 3, the specified rotational speed nVM-SOLL1is utilized as the specified rotational speed for the internal combustion engine2and, in the time intervals4,5,6,7fromFIG. 3, the specified rotational speed nVM-SOLL2is utilized as the specified rotational speed.

If the rotational speed of the separating clutch6lies outside the rotational speed range Δn, namely below the rotational speed range Δn, the current rotational speed nEMof the electric machine3, reduced by the offset OF1, is utilized as the specified rotational speed nVM-SOLL1.

However, if the rotational speed at the separating clutch6lies above the rotational speed range Δn, the rotational speed nEMof the electric machine3, increased by the offset OF2, is utilized as the specified rotational speed nVM-SOLL2, as the specified rotational speed for the internal combustion engine2.

If the rotational speed at the separating clutch6lies within the rotational speed range Δn, but the absolute value of the gradient of the differential speed at the separating clutch6is greater than the corresponding limiting value, then, depending on whether the gradient is positive or negative, either the rotational speed nEMof the electric machine3reduced by the offset OF1or the rotational speed nEMof the electric machine3increased by the offset OF2is utilized as the specified rotational speed, namely, in the case of a positive high gradient, the specified rotational speed nVM-SOLL2increased by the offset OF2and, in the case of a large negative gradient, the specified rotational speed nVM-SOLL1reduced by the offset OF1.

If the rotational speed of the internal combustion engine2is below the rotational speed nEMof the electric machine3, then, alternatively, the internal combustion engine2can also be stopped.

The offset values OF1and OF2can have the same absolute value but also deviate from each other such that the offset values OF1and OF2have different absolute values.

With the aid of example aspects of the invention, for the case in which, in the presence of at least one abort criterion for the coupling of the internal combustion engine2, the torque currently transmitted or currently transmittable by the separating clutch6is greater than the limiting value, a comfortable and also fast disengagement of the separating clutch6and, therefore, decoupling of the internal combustion engine2is possible and, in fact, without the risk of an undesirable noise development as well as without the risk of undesirable oscillations and/or vibrations in the drive train.

The invention also relates generally to a control system for operating the motor vehicle, which includes at least the transmission control unit9and the engine control unit8as well as the hybrid control unit10. The hybrid control unit10can be an integral part of the transmission control unit9. For the case in which at least one second defined operating condition for the abort of the coupling of the internal combustion engine2is present, the transmission control unit9and/or the hybrid control unit10check(s) the level of the torque currently transmitted or currently transmittable by the separating clutch6. Depending on the absolute value of the torque currently transmitted or currently transmittable by the separating clutch6, the transmission control unit9or the hybrid control unit10disengages the separating clutch6at different rates.

The control units8,9and10are therefore configured for implementing the method according to example aspects of the invention on the control side. For this purpose, the control units8,9and10of the control system include hardware-related and software-related means. The hardware-related means include data interfaces for exchanging data with assemblies contributing to the implementation of the method according to example aspects of the invention. Moreover, the hardware-related means include a processor for data processing and a memory for data storage. The software-related means include program components, which are utilized for implementing the method according to example aspects of the invention.

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