This application claims the priority of German patent application no. 103 25 354.8, filed Jun. 5, 2003 (PCT International Application No. PCT/EP2004/004757, filed May 5, 2004), the disclosure of which is expressly incorporated by reference herein.
The invention relates to a method for operating the drive train of a motor vehicle.
German patent document DE 42 30 989 A1 discloses a method for operating a drive train with a prime mover in the form of an engine, a manual shift transmission, and a power divider which is not power-shiftable and is actuated by external force. A clutch actuated by foot force is arranged between the prime mover and the manual shift transmission. The power divider is arranged downstream of the manual shift transmission and has actuating members that can shift from road travel to cross-country travel with a high step-up ratio. The actuating members of the power divider are activated by a control device in the form of central control electronics.
In order to execute a shift, the vehicle driver must trigger a shift requirement by opening the clutch and actuate a switch in the form of a preselection switch in the interior of the motor vehicle. This shift requirement is then implemented by a control device.
U.S. Pat. No. 5,411,450 A also discloses a method for operating a drive train, a prime mover, an automatic transmission and a power divider that is not power-shiftable and is actuated by external force. Before the commencement of a shift of the power divider, a forward gear clutch of the automatic transmission is opened and the force flux between the prime mover and the power divider is thus interrupted. The shift in the power divider can subsequently be carried out. After completion of the shift, the forward gear clutch is closed again.
UK patent Document GB 2 304 835 A likewise describes a method for operating such a drive train. In the event of a shift, and consequently a change in the step-up ratio of the power divider, a suitable gear is selected automatically in the automatic transmission.
European patent document EP 0 947 739 A2 describes a method for operating a group transmission of a commercial vehicle. An automated main transmission is followed by a range group. In the event of a required shift of the range group, a check is made as to whether a rotational speed of the prime mover after the shift is lower than a minimum rotational speed. If so, the shift is not executed.
International patent document WO 00/021812 A describes a motor vehicle with an automated shift transmission and a device for preventing unwanted movement of the motor vehicle. In the event of an interrupted force flux between the prime mover and driven vehicle wheels, the device is activated to prevent rolling of the motor vehicle, specifically opposite to the desired direction of travel.
One object of the invention is therefore to provide a method for operating a motor vehicle, which makes it possible to operate the motor vehicle comfortably.
This and other objects and advantages are achieved by the method according to the invention, in which when a shift requirement is generated for the power divider, torque of the shifting members involved in the shift is reduced automatically by a control device before commencement of the shift. The shift requirement may be for a change of a step-up ratio of the power divider, or for a changeover between two-wheel and four-wheel drive. It may be triggered by the vehicle driver, for example by the actuation of a switch in the interior. Alternatively to this, the requirement may be generated by a control device as a function of operating variables of the motor vehicle and of environmental variables.
The reduction of the torque of the shifting members, for example gearwheels, sliding sleeve and/or synchronizing bodies, is necessary, since a shift (in particular a deselection of the shifted gear) is possible in non-power-shift transmissions only when no torque, or a very low torque is transferred by the shifting members which are in engagement.
The reduction can take place by means of the controlled setting of the output torque of the prime mover (for example, to zero), or by separating the connection between the prime mover and the power divider.
After the torque reduction has taken place, the shift is carried out by a suitable activation of externally driven actuating members; that is, without the assistance of the vehicle driver. The actuating members may be designed, for example, as electric motors or hydraulic or pneumatic piston/cylinder units. After the shift is completed, the control device again permits a torque at the shifting members. For this purpose, the output torque of the prime mover is set again according to an instruction from the vehicle driver, or the connection between the prime mover and power divider is restored. The instruction from the vehicle driver is derived from a degree of actuation of a power actuating member, for example the position of an accelerator pedal.
The transmission may be a manual shift transmission or an automated transmission.
The transmission is designed as an automatic transmission. In order to reduce the torque of the shifting members, the control device opens a clutch, interrupting a force flux between the prime mover and power divider which is produced by means of a positive or frictional connection. In particular, the control device opens a clutch in the automatic transmission (clutch also being understood as meaning a break in the automatic transmission). To open the clutch, the control device activates actuating members of the automatic transmission in a suitable form. Alternatively to a clutch in the automatic transmission, for example when an automated gearwheel change transmission is employed, a starting clutch arranged between the prime mover and the automatic transmission may also be opened. As a result of the opening of the clutch, the power divider is no longer connected to the prime mover, so that the drive train is separated and is consequently free of torque.
After the conclusion of the shift, the control device restores the force flux by closing the clutch.
The automatic transmission may be designed, for example, as an epicyclic transmission, continuously variable transmission, double clutch transmission or automated gearwheel change transmission. The motor vehicle may have a separate control device for simultaneously controlling the prime mover, the automatic transmission and the power divider.
Consequently, the torque of the shifting members can be reduced with a high degree of reliability, and it also becomes possible to shift the power divider reliably. Particularly in the case of distortions in the drive train (such as may occur, for example, on cross-country trips), the reduction in the torque of the shifting members by influencing the output torque of the prime mover is highly unreliable.
The motor vehicle has an activatable (for example, electrohydraulic) brake system by which a braking torque can be applied to the motor vehicle, independently of a position of a brake pedal, as a result of the activation of actuating members by a control device.
The control device of the power divider monitors the speed of the motor vehicle (and/or variables derived therefrom) during a shift of the power divider. Derived variables are, for example, a difference between a current speed and a speed of the commencement of the shift, acceleration of the motor vehicle or a direction of travel.
Based on the result of the monitoring, the control device activates the brake system at least indirectly, for example, by sending a requirement for a braking torque to the control device of the brake system, which implements the requirement. After completion of the shift, any requirement is canceled and therefore any braking torque which may be present is reduced.
The shift of the power divider may take up a few (for example, three), seconds during which time, the driver train is separated. (That is, the prime mover is not connected to the driven vehicle wheels, so that no torque from the prime mover can act on the vehicle wheels.) During this time, unwanted and uncontrolled movements of the motor vehicle may occur, for example when the motor vehicle is operated on a slope. By the speed of the motor vehicle and any activation of the brake system being monitored, active influence can be exerted on the movement of the motor vehicle in spite of the open drive train. Consequently, an especially reliable operation of the motor vehicle is ensured, and uncontrolled operation of the motor vehicle is prevented.
Upon the occurrence of a shift requirement, the control device calculates a rotational speed of the prime mover which occurs after the shift, based on the current speed of the motor vehicle and the step-up ratio in the drive train after the shift a rotational speed of the prime mover which occurs after the shift. This is relevant only when the step-up ratio of the power divider changes during the shift. A shift is carried out in the automatic transmission or the shift requirement is suppressed as a function of the calculated rotational speed.
The control device determines, in particular, a permitted range of the rotational speed of the prime mover after the shift. The range may be stored, for example, in the control device, or it may be determined as a function of operating variables of the motor vehicle, (for example, the speed, and/or environmental variables, such as the slope of the road. If the rotational speed can be brought into such range by simultaneously shifting the automatic transmission, the shift of the automatic transmission and of the power divider is carried out. Very high step-up ratio jumps (for example of 2.6) may lie between step-up ratios of power dividers. This may correspond approximately to a shift from 2nd to 5th gear, for example, in the case of a 7-gear epicyclic transmission. Consequently, in the event of a shift in the power divider and a step-up ratio of the automatic transmission which remains the same, the rotational speed of the prime mover changes sharply, and consequently rapidly exceeds the permitted range. This sharp change in rotational speed can be counteracted by a simultaneous change in the step-up ratio of the automatic transmission. To stay with the example mentioned, in the event of the shift of the power divider into a cross-country gear (that is, into a lower step-up ratio, the step-up ratio jump of 2.6), the rotational speed of the prime mover can be kept virtually constant by simultaneously shifting the automatic transmission from 2nd to 5th gear.
If, even as a result of a simultaneous shift of the automatic transmission, the rotational speed of the prime mover would lie outside the permitted range after the shift, the shift of the power divider is suppressed.
Consequently, a large proportion of required shifts of the power divider can also be executed. At the same time, however, the prime mover is prevented from entering an unpermitted operating state (in which, for example the rotational speed is too low or too high) after the shift.
When the method according to the invention is employed, no actuation of a clutch by the vehicle driver is necessary in order to execute a shift of the power divider. Should the driver wish to execute a shift, however, he or she needs only to trigger a shift requirement, and operation of the motor vehicle consequently becomes simpler and more comfortable for the vehicle driver.
In addition, by virtue of the method according to the invention, it is possible that the control device can decide whether a shift is appropriate, trigger such a shift, and carry it out without intervention by the vehicle driver.
In an embodiment of the invention, during the shift of the power divider, the control device automatically reduces the output torque of the prime mover, in which case an instruction from the vehicle driver via the power actuating member is ignored. After completion of the shift, the control device again permits an increase in the output torque, and the instruction from the vehicle driver is implemented again. The decrease and increase in the torque may take place, for example, along ramps.
Consequently, the vehicle driver can continue to actuate the power actuating member during a shift of the power divider, without the rotational speed of the prime mover rising undesirably and unnecessarily when the drive train is open. (During closing of the clutch, the risen rotational speed would in most instances have to be reduced again.) The operation of the motor vehicle consequently becomes simpler and more comfortable.
According to another feature of the invention, the control device activates the brake system when a false direction of travel that is, opposite to the desired direction of travel is detected. This may be determined, for example, from by comparing the current direction of travel with a position of the selector lever of the automatic transmission or with the direction of travel of the commencement of the shift. The current direction of travel may be determined by means of suitable rotational speed sensors on the vehicle wheels. The control device activates the brake system, in particular, to the standstill of the motor vehicle and subsequently holds the motor vehicle up to the conclusion of the shift. In this case, when the motor vehicle is subsequently started, rolling opposite to the desired direction of travel can likewise be prevented. The function of what is known as a hill holder can thereby be implemented.
A false direction of travel may occur, for example, during a shift of the power divider when the motor vehicle is driving up a steep slope at low speed, because the driver, train is separated, and no drive torque from the prime mover acts on the motor vehicle. Due to the slope downforce, the motor vehicle is decelerated and, in the most unfavorable case, is accelerated opposite to the original direction of travel. The motor vehicle could move toward motor vehicles which may possibly be following it. By activating the brake system, the unwanted movement can be prevented, and therefore safety-critical situations can be prevented. This makes it possible to operate the motor vehicle particularly reliably.
In an embodiment of the invention, the control device activates the brake system if a difference of the current speed from an initial speed at the commencement of the shift and/or a speed gradient, overshoot limit values. In particular, by means of a suitable braking torque, a constant differential speed or a constant speed gradient (that is, a constant acceleration), can be set.
This prevents the the vehicle speed from increasing too sharply during a shift of the power divider, such as might arise, for example, if a shift occurs when the motor vehicle is driving down a steep slope. That is, with the drive train separated, there is also no engine braking action on the motor vehicle, which may lead to a sudden sharp acceleration of the motor vehicle. Such acceleration may be very surprising to the vehicle driver, and may therefore lead to a safety-critical driving situation. By activating the brake system when one of the conditions mentioned is fulfilled, the safety-critical driving situations described cannot arise, thus resulting in a particularly reliable operation of the motor vehicle.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.