Abstract:
A method of operating a vehicle powertrain, includes: sensing a vehicle speed; selecting a plurality of control strategies; activating one of the plurality of control strategies, the control strategy including: (i) operating the vehicle in a stationary start-stop mode when the vehicle speed is below a first threshold; and (ii) operating the vehicle in a rolling stationary start-stop mode when the vehicle speed is above the first threshold but below a second threshold.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation and claims the benefit of German Patent Application No. DE 102011085395.2 titled “Method for Operating a Motor Vehicle and Drive System of a Motor Vehicle,” filed Oct. 28, 2011; and U.S. patent application Ser. No. 13/659,833 titled “Method for Operating a Vehicle Powertrain” filed Oct. 24, 2012, which are hereby incorporated in their entirety. 
     TECHNICAL FIELD 
     The present disclosure relates to vehicle powertrain control strategies. More particularly, the disclosure relates to a method for operating a motor vehicle having an internal combustion engine and to a drive system of the motor vehicle. 
     BACKGROUND 
     The drive motor of a motor vehicle serves primarily for generating a driving force which is transmitted to at least one driven wheel in order to move the motor vehicle. Modern drive motors include, for example, an internal combustion engine such as a spark-ignition engine or diesel engine. Since an internal combustion engine consumes fuel even when not transmitting a drive force, various methods have been proposed to reduce fuel consumption in driving situations were no driving force is required. 
     It is known, for example, from WO 2010/121861 A1 titled “Method for Operating a Vehicle Having a Freewheel Mode or a Rolling Mode”, to operate a vehicle as a function of the result of a plausibility check of an automatic speed control function or adaptive speed control function and/or other current vehicle operating data or vehicle state data in a driving mode in where frictional engagement in the drive train is interrupted and the drive motor is operated at an idling speed or switched off. However, this method does not optimize fuel efficiency and engine responsiveness in all driving situations. 
     Therefore it is desirable to provide a method for operating a motor vehicle that improves fuel efficiency and engine responsiveness in all driving situations. 
     SUMMARY 
     The present disclosure addresses one or more of the above-mentioned issues. Other features and/or advantages will become apparent from the description which follows. 
     One exemplary embodiment of the present disclosure relates to a method of operating a vehicle powertrain, the method includes: sensing a vehicle speed; selecting a plurality of control strategies; activating one of the plurality of control strategies, the control strategy including: (i) operating the vehicle in a stationary start-stop mode when the vehicle speed is below a first threshold; and (ii) operating the vehicle in a rolling stationary start-stop mode when the vehicle speed is above the first threshold but below a second threshold. 
     One exemplary embodiment of the present disclosure relates to a method for operating a motor vehicle having an internal combustion engine, the method includes: sensing a speed of the motor vehicle; sensing actuation of a speed control means; selecting a coasting mode when the vehicle is operating above a minimum speed of said coasting mode and the speed control means is not actuated; selecting a rolling mode when the vehicle is operating above a maximum speed of the coasting mode and the speed control means is not actuated; and selecting a creeping mode in which there is frictional engagement between the drive motor and at least one driven wheel of the motor vehicle, when the vehicle is operating above the maximum speed of a stationary mode and below a minimum speed of the rolling mode and when the speed control means is not actuated. 
     Another exemplary embodiment of the present disclosure relates to a drive system of a motor vehicle having a drive motor, including: a drive train for transmitting driving force to at least one driven wheel of the motor vehicle; and a control device for controlling the drive motor and an automatically actuable clutch, wherein the control device is designed to: (i) select a coasting mode above a minimum speed of said coasting mode when a speed control means is not actuated; (ii) select a rolling mode above a maximum speed of the coasting mode when the speed control means is not actuated; and (iii) select a creeping mode in which there is frictional engagement between the drive motor and at least one driven wheel of the motor vehicle, when the vehicle is operating above the maximum speed of a stationary mode and below a minimum speed of the rolling mode and when the speed control means is not actuated. 
     One advantage of the present disclosure is that it provides a method and drive system in which the fuel consumption is reduced without complex adaptation of the energy supply system being necessary and without significant impairment of the response behavior of the drive system. 
     The invention will be explained in greater detail below by way of example with reference to the figures, in which the same reference numbers are used in the figures for identical or essentially identical elements. The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the invention when taken in connection with the accompanying drawings. In the figures: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a collection of driving strategies corresponding to exemplary embodiments of the method according to the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the drawing, wherein like characters represent examples of the same or corresponding parts throughout the several views, there is shown a series of control strategies for a vehicle powertrain. The control strategies provide different fuel-savings and/or improve engine responsiveness in different driving conditions. Control strategies can be implemented by a control device, such as for example, an engine control unit (or ECU), powertrain control unit (PCU) or vehicle control unit (VCU). The various strategies include several different driving modes, examples of which are discussed hereinbelow, that are actuated according to the control strategy. The exemplary strategies below trigger driving modes according to vehicle speed and/or operation of cruise control (or a “coasting mode”). 
     Referring now to  FIG. 1 ,  FIG. 1  illustrates several different driving strategies (four of which are discussed in detail hereinbelow) according to which a motor vehicle can be operated and a drive system of a motor vehicle can be correspondingly actuated. In this embodiment, a current velocity of the motor vehicle is plotted on the y-axis. On the top the different strategies 1 to 8 are illustrated, denoted by the numbers above the corresponding column. 
     In the shown driving strategies of  FIG. 1 , a drive system is actuated in a stationary mode (SSS, “Static Start Stop”) when the vehicle is operating at a velocity below a speed, v 1 . The stationary mode is defined by an interrupted frictional engagement in the drive train. In the case of an automatic transmission with a torque converter, the frictional engagement can be closed via a torque converter. An internal combustion engine that serves as the drive motor is stopped in the stationary mode. A starter of the internal combustion engine can be disconnected from the engine by opening a starter clutch. The starter can also be engaged in order to expedite starting. If there is a request for driving force, for example, detected by actuation of the gas pedal, the internal combustion engine is started by the starter after the starter clutch has been closed, and the opened clutch in the drive train is closed and/or manual closing of the clutch is enabled. 
     According to strategy  1 , as shown in  FIG. 1 , the rolling mode (RSS, “Rolling Start Stop”) is selected starting from a velocity, v 1 , which corresponds to the maximum speed of the stationary mode and the minimum speed of the rolling mode when neither the brake pedal nor the gas pedal is actuated. In the rolling mode, the drive system is actuated by a control device in such a way that frictional engagement in the drive train is interrupted, for example, by opening a clutch and the drive motor is stopped. The starter can be connected to the drive motor in order to start the engine when necessary, for example when the gas pedal is actuated. 
     Starting from a velocity v 3 , as shown in  FIG. 1 , which represents the maximum speed of the rolling mode and the minimum speed of the coasting mode, a coasting mode (“SAIL”) is selected if neither the brake pedal nor the gas pedal is actuated. In the coasting mode, frictional engagement in the drive train is interrupted. In contrast to the rolling mode, the drive motor is in operation in the coasting mode, in particular at an idling speed. The starter is not connected to the drive motor. The coasting mode can be used up to a maximum speed v 5  of the motor vehicle. In other embodiments, v 5  can be lower. 
     A transition from one driving mode to another can take place by changing vehicle speed, for example, by slowing down and allowing the vehicle to come to a standstill or by increasing the speed when driving downhill. A driving mode can also be ended by actuating the gas pedal or the brake pedal. Likewise, it is possible to end a driving mode by deactivating an economy driving settings, by means of software or hardware switches, such as start/stop or on/off buttons. Other reasons for changing the driving mode can be, for example, passenger comfort and/or air conditioning, the state of charge for the battery, emission control of the engine, current altitude, ambient temperature, or engine temperature. 
     According to strategy  2 , as shown in  FIG. 1 , the maximum speed v 1  of the stationary mode corresponds to the minimum speed of a creeping mode (“CREEP”). The maximum speed v 2  of the creeping mode is the minimum speed of the rolling mode. As is apparent from  FIG. 1 , according to strategy  2 , the creeping mode is selected between the minimum speed and the maximum speed of the creeping mode v 1  and v 2  if neither the brake pedal nor the gas pedal is actuated. In the creeping mode, frictional engagement between the drive motor and the driven wheels is closed, the drive motor is operating at a corresponding rotational speed and the starter is disconnected from the drive motor. Above the maximum speed v 2  of the creeping mode the driving strategy  2  corresponds to strategy  1 . 
     According to strategy  3 , as shown in  FIG. 1 , the coasting mode is used up to a maximum speed v 4 . In the case of a velocity which is higher than v 4 , a passive deceleration mode (“DFSO Pass.” or “Deceleration Fuel Shut Off”) is selected provided that the driver does not actuate the brake pedal or the gas pedal. In the passive deceleration mode, the frictional engagement in the drive train is brought about by closing a corresponding clutch so that the drive motor runs at a rotational speed which corresponds to the current velocity of the vehicle and to a respectively engaged gear or a respective driving position, taking into account the slip of a torque converter if using an automatic transmission. The fuel supply to the internal combustion engine is, however, interrupted so that a deceleration effect of the engine occurs, and the effect is used according to driving strategy  3  to control coming to a standstill or driving downhill. The starter is not connected to the drive motor. The passive deceleration mode is used up to the maximum speed, v 5 , of the motor vehicle. In the case of a velocity below, v 4 , driving strategy  3  corresponds to strategy  1 . 
     As is shown by  FIG. 1 , strategy  4  corresponds to strategy  2  when there is a speed below v 3  and to strategy  3  when there is a speed above v 3 . In this respect, reference is made to the above explanations. 
     The speeds v 1  to v 5  do not have to have the same values in all the strategies. The speeds v 1  to v 5  can also depend on further parameters which characterize the respective driving situation. The strategies can each be configured in such a way that in the case of a velocity which is equal to a maximum speed or minimum speed, the driving mode which is assigned to the relatively low or relatively high velocity is selected. The speed at which another driving mode is selected can also be different in the case of a transition in the direction of a relatively high speed than in the case of a transition in the direction of a relatively low speed. 
     When the brake pedal is actuated, a driving mode (not illustrated in  FIG. 1 ), for example, an active deceleration mode which corresponds to the passive deceleration mode but includes an additional braking effect by the service brakes, can be selected. The selected driving mode can also depend on the current speed, further parameters of the vehicle or the driving situation. In addition, a driver can, in particular by actuating the gas pedal, bring about a change into the normal driving mode in which the motor vehicle is accelerated, for example, by a driving force which is generated in accordance with the position of the gas pedal, or one that is kept at a constant velocity. 
     Other driving strategies, for example, as shown in columns 5 through 8 can also be implemented. Those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention within the scope of the appended claims.