CROSS-REFERENCE TO RELATED APPLICATION

The present application is related and has right of priority to German Patent Application No. 10 2019 201 790.8 filed on Feb. 12, 2019, the entirety of which is incorporated by reference for all purposes.

FIELD OF THE INVENTION

The invention relates generally to a method for operating a drive train of a motor vehicle, and to a control unit suitable for operating a drive train of a motor vehicle.

BACKGROUND

Patent application DE 10 2004 002 061 A1 describes a method for the open-loop control and closed-loop control of a drive train of a hybrid vehicle. The drive train has an internal combustion engine and an electric machine, which are connectable to one another via a clutch unit. The internal combustion engine is startable by the electric machine. During this starting process, a shift element between the electric machine and an output shaft is adjusted such that a torque independent of the starting process is present at the output shaft.

Patent application DE 10 2010 061 826 A1 describes a method for accelerating this decoupling between the electric machine and the output shaft. Accordingly, in the presence of a decoupling demand, the electric machine is actuated such that it builds up a torque, for example, by outputting a specified rotational speed for the electric machine.

The problem addressed by the present invention is that of further developing this method.

SUMMARY OF THE INVENTION

The method is suitable for operating a motor vehicle drive train, which includes an internal combustion engine, an electric machine or motor, a separating clutch in the power flow between the internal combustion engine and the electric machine, and a transmission for providing different gears or gear speed ratios between an input shaft and an output shaft of the transmission. A rotor of the electric machine is connected to the input shaft directly or via a torque converter. A “direct connection” is understood to be a connection, which provides for a fixed speed ratio between the rotor and the input shaft. This fixed speed ratio can take place with a direct, rotationally fixed connection, or with an interconnected transmission having a constant ratio.

If the internal combustion engine is to be started, the separating clutch is transferred into an engaged condition or at least into a slip state starting from the disengaged condition, so that the electric machine can accelerate the internal combustion engine to a starting speed. During the starting process of the internal combustion engine, a torque-transmitting element between the electric machine and the output shaft is held in a slip state. The torque-transmitting element is, for example, a torque converter lockup clutch of a torque converter, a clutch between the electric machine and the input shaft of the transmission, or a shift element in the power flow of the transmission. As a result, vibrations arising during the starting process are decoupled from the output shaft, whereby the ride comfort of the motor vehicle is improved. In order to reliably reach and maintain the desired slip state of the torque-transmitting element during the starting process, the torque of the electric machine is increased.

According to the invention, in order to reliably reach and maintain the slip state of the torque-transmitting element between the electric machine and the output shaft, a decision is reached regarding which load direction the drive train will likely assume during the starting process. The two possible load directions are pull and push. The electric machine is operated either as a motor or as a generator, depending on the expected load direction, in order to reliably bring the torque-transmitting element into the slip state and to maintain the slip state.

Such an approach is advantageous, in particular, when the probability of a change in the load direction during the starting process of the internal combustion engine is high. This is the case, for example, when the present input torque demanded by the driver is close to zero. If the electric machine is operated as a motor in such a case, the input-side rotational speed of the torque-transmitting element increases in an appropriate manner. If a change in load direction from pull to push now takes place, the input-side rotational speed of the torque-transmitting element decreases again, briefly reaches a synchronous speed with respect to the output side of the torque-transmitting element, and, thereafter, decreases further. During the “crossing” of the synchronous speed, the torque-transmitting element changes from kinetic friction to static friction and, thereafter, back to kinetic friction. This transition is noticeable to occupants of the motor vehicle and has an adverse effect on comfort. This applies in the same way during an initial operation of the electric machine as a generator and a subsequent change of the load direction from push to pull.

Such a behavior is preventable, in many cases, by the approach according to the invention. By taking into account the load direction expected during the starting process and the appropriate operation of the electric motor as a motor or as a generator in order to reliably reach and maintain the slip state of the torque-transmitting element, the decoupling direction remains constant, provided the actual load direction during the starting process of the internal combustion engine corresponds to the expected load direction. A “crossing” of the synchronous speed is avoided in these cases.

Preferably, the electric machine is operated as a generator when the expected load direction is a pushing or coasting operation. The electric machine is therefore operated as a motor when the expected load direction is a pulling or traction operation.

In order to reliably bring the torque-transmitting element into the slip state, a pilot torque is preferably specified to or predetermined within a control unit of the electric machine. The sign of this pilot torque is therefore dependent on the expected load direction of the drive train during the starting process of the internal combustion engine.

According to one preferred embodiment, when there is a demand to start the internal combustion engine, the drive train is loaded in the expected load direction by the electric machine when the magnitude of a currently specified or desired input torque or the magnitude of a currently present or actual input torque at a given time is less than or equal to a limiting value. This is the case because, in particular, when the input torque is low, the probability of a change in load direction increases.

Preferably, the load direction expected during the starting process of the internal combustion engine is determined depending on the currently present input torque of the drive train. This is, for example, a computationally determined or calculated torque, which acts on the input shaft. The values necessary for such determination are present in a typical drive train control unit, such that a complex sensor unit is omitted. When the currently present input torque of the drive train is greater than or equal to zero, a pulling or traction operation is preferably expected as the load direction during the starting process. When the currently present input torque of the drive train is less than zero, a pushing or coasting operation is preferably expected as the load direction during the starting process.

Preferably, the load direction expected during the starting process of the internal combustion engine is determined depending on the currently present specified input torque. The specified input torque is determined, for example, based on the position of the accelerator pedal of the motor vehicle. Since the position of the accelerator pedal usually characterizes the drive demand of the driver, this dependence is particularly well suited for determining the expected load direction. Depending on the specific application, the position of the accelerator pedal is utilized either directly or in a filtered manner for determining the specified input torque. When the currently present specified input torque of the drive train is greater than or equal to zero, a pulling or traction operation is preferably expected as the load direction during the starting process. When the currently present specified input torque of the drive train is less than zero, a pushing or coasting operation is preferably expected as the load direction during the starting process.

Preferably, the expected load direction of the drive train during the starting process of the internal combustion engine is invariable for a predetermined time after being determined. As a result, a hysteretic behavior is achieved, so that a frequent change-over of the expected load direction is avoided.

Preferably, before the torque of the electric machine is increased, a contact force on the torque-transmitting element is reduced, in order to accelerate the establishment of a differential speed at the torque-transmitting element. Such an approach improves the comfort, since such increase of the torque is not noticeable, or is only negligibly noticeable, to the driver.

