Abstract:
Disclosed is a control system and method for controlling starting of an engine in a hybrid vehicle. More specifically, a controller is implemented that confirms first and second brake and the first and second clutch are released so that the system is in a neutral condition, confirm that the engine is stopped, and control the first motor-generator and the second motor-generator to rotate the engine at a predetermined rotational speed to start the engine.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2011-0132870 filed in the Korean Intellectual Property Office on Dec. 12, 2011, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to a system and method for controlling a hybrid vehicle that controls a rotational speed of an engine to enable starting of the engine in a hybrid system that includes an engine, a first motor-generator, and a second motor-generator. 
     (b) Description of the Related Art 
     Generally, automatic transmissions use hydraulic pressure to shift gears in a multi step process to output the appropriate torque from a rotational torque of an engine/motor according driving conditions and driver demand. Some hybrid vehicles utilize two motor/generators (MG) and one engine that are connected through a planetary gear set and control the motor/generator to achieve a continuous variable shifting system or CVT. 
     The engine, the first and second motor/generators, and two planetary gear sets are used to continuously vary the output speed of a transmission according to driving conditions of the vehicle and driver demand. In particular, each speed of the first and second motor/generators are controlled, accordingly. 
     More specifically, a CVT can change steplessly through an infinite number of effective gear ratios between maximum and minimum values. This is much different than the traditional mechanical transmission that has a fixed number of gear ratios. A CVT allows the driving shaft to maintain a constant angular velocity over a range of output velocities. This can provide better fuel efficiency than other types of transmissions by enabling the engine to run at its most efficient revolutions per minute (RPM) for a range of vehicle speeds. CVT&#39;s also can maximize the performance of a vehicle by allowing the engine to turn at an RPM which produces peak power. Finally, a CVT does not strictly require the presence of a clutch, allowing for a clutch to be omitted from the overall system. By omitting the clutch, maintenance costs and manufacturing costs can be significantly reduced. 
     In these types of systems, the first motor/generator is often speed controlled according to the driving condition of the engine and the second motor/generator is torque controlled together with the engine to control the entire output torque. In a neutral mode, however, an engine is separated from a wheel shaft and all rotational elements (i.e., the wheels) are no longer forcible rotated. 
     The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in an effort to provide a control system and method of a hybrid vehicle having advantages of improving starting performance of an engine in a neutral mode when the engine is cold (−30° C.). 
     A control system and method of a hybrid vehicle that includes a first and a second planetary gear set, an engine, a first and second motor/generator, an output shaft, a first and second clutch and a first and second brake. More specifically, the first planetary gear set includes a first sun gear, a first planetary gear, a first ring gear, and a first carrier, and the second planetary gear set includes a second sun gear, a second planetary gear, a second ring gear, and a second carrier. An output shaft of the engine may be directly connected to the first carrier. The first motor-generator may be configured to rotate the first ring gear, and the second motor-generator may be directly connected to the second sun gear to rotate the second sun gear and the first sun gear through the second sun gear. 
     The first brake may be configured to apply friction to the first ring gear to reduce the rotational speed of the first ring gear, and the second brake may be configured to apply friction to the second ring gear to reduce the rotational speed of the second ring gear. The first clutch may selectively connect the first ring gear with the first carrier, and the second clutch may selectively connect the first carrier with the second ring gear. 
     In an exemplary embodiment of the present invention, a controller may be configured to confirm that the first and second brake and the first and second clutch are released so that the system is in a neutral condition, confirm that the engine has stopped operating, and operate/control the first motor-generator and the second motor-generator to rotate the engine at a predetermined rotational speed to start the engine. 
     In some exemplary embodiments of the present invention, the rotational speed of the second motor-generator may be controlled/calculated by a below formula 6. 
     
       
         
           
             
               
                 
                   
                     ω 
                     
                       MG 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                     Target 
                   
                   = 
                   
                     
                       
                         - 
                         
                           1 
                           
                             R 
                             1 
                           
                         
                       
                       ⁢ 
                       
                         ω 
                         
                           MG 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                       
                     
                     + 
                     
                       
                         
                           1 
                           + 
                           
                             R 
                             1 
                           
                         
                         
                           R 
                           1 
                         
                       
                       ⁢ 
                       
                         ω 
                         ENG 
                         Target 
                       
                     
                   
                 
               
               
                 
                   Formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   6 
                 
               
             
           
         
       
     
     A rotational speed of the first motor-generator may be controlled to ensure that the engine reaches a target speed, and the angular acceleration of the second ring gear may be controlled so that the torque that is transferred to the output shaft that is connected to the second ring gear reaches 0. 
     A target speed of the first motor-generator may be calculated by a below formula 2 for a target speed of the engine. 
     
       
         
           
             
               
                 
                   
                     ω 
                     
                       MG 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                   
                   = 
                   
                     
                       
                         - 
                         
                           1 
                           
                             R 
                             1 
                           
                         
                       
                       ⁢ 
                       
                         ω 
                         
                           MG 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                       
                     
                     + 
                     
                       
                         
                           1 
                           + 
                           
                             R 
                             1 
                           
                         
                         
                           R 
                           1 
                         
                       
                       ⁢ 
                       
                         ω 
                         ENG 
                         Target 
                       
                     
                   
                 
               
               
                 
                   Formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   2 
                 
               
             
           
         
       
     
     The angular acceleration of the second ring gear may be calculated by a below formula 3. 
     
       
         
           
             
               
                 
                   
                     
                       I 
                       
                         R 
                         ⁢ 
                         
                             
                         
                         ⁢ 
                         2 
                       
                     
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                         ω 
                         . 
                       
                       
                         R 
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                         2 
                       
                     
                   
                   = 
                   
                     
                       
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                           R 
                           2 
                         
                       
                       ⁢ 
                       
                         T 
                         
                           MG 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           2 
                         
                       
                     
                     + 
                     
                       
                         
                           R 
                           2 
                         
                         
                           R 
                           1 
                         
                       
                       ⁢ 
                       
                         T 
                         
                           MG 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           1 
                         
                       
                     
                   
                 
               
               
                 
                   Formula 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   3 
                 
               
             
           
         
       
     
     As described above, in the control system and method of a hybrid vehicle according to the exemplary embodiment of the present invention, an engine is effectively started in a neutral mode of a FHS4 (flexible hybrid system) by applying the above control to the starting process. Further, in the illustrative embodiment of the present invention, when the temperature of an engine is less than −30° C., the engine in the above described system is able to stably reach a target speed to secure startability of the engine even in colder temperatures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a gear shifting system of a hybrid vehicle according to an exemplary embodiment of the present invention. 
         FIG. 2  is a graph illustrating a gear shifting system of a hybrid vehicle as a lever type according to an exemplary embodiment of the present invention. 
         FIG. 3  is a graph illustrating a rotational speed of constituent elements in an engine starting process of a gear shifting system of a hybrid vehicle according to an exemplary embodiment of the present invention. 
         FIG. 4  shows formulas for controlling a gear shifting system of a hybrid vehicle according to an exemplary embodiment of the present invention. 
         FIG. 5  is a flowchart for controlling a first motor-generator for controlling a gear shifting system of a hybrid vehicle according to an exemplary embodiment of the present invention. 
         FIG. 6  is a flowchart for controlling a second motor-generator for controlling a gear shifting system of a hybrid vehicle according to an exemplary embodiment of the present invention. 
         FIG. 7  shows formulas for controlling a first and second motor-generator for controlling a gear shifting system of a hybrid vehicle according to an exemplary embodiment of the present invention. 
     
    
    
     DESCRIPTION OF SYMBOLS 
     
         
         
           
               100 : engine 
             MG 1 : first motor-generator 
             MG 2 : second motor-generator 
             PG 1 : first planetary gear set 
             r 1 : first ring gear, 
             s 1 : first sun gear 
             p 1 : first planetary gear 
             c 1 : first carrier 
             PG 2 : second planetary gear set 
             r 2 : second ring gear, 
             s 2 : second sun gear 
             p 2 : second planetary gear 
             c 2 : second carrier 
             BK 1 : first brake 
             BK 2 : second brake 
             CL 1 : first clutch 
             CL 2 : second clutch 
           
         
       
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present invention will be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. 
     It is understood that the term “vehicle” or “vehicular” or other similar term as used herein is inclusive of motor vehicles in general such as passenger automobiles including sports utility vehicles (SUV), buses, trucks, various commercial vehicles, watercraft including a variety of boats and ships, aircraft, and the like, and includes hybrid vehicles, electric vehicles, plug-in hybrid electric vehicles, hydrogen-powered vehicles and other alternative fuel vehicles (e.g. fuels derived from resources other than petroleum). As referred to herein, a hybrid vehicle is a vehicle that has two or more sources of power, for example both gasoline-powered and electric-powered vehicles. 
     Furthermore, the control logic of the present invention may be embodied as non-transitory computer readable media on a computer readable medium containing executable program instructions executed by a processor, controller or the like. Examples of the computer readable mediums include, but are not limited to, ROM, RAM, compact disc (CD)-ROMs, magnetic tapes, floppy disks, flash drives, smart cards and optical data storage devices. The computer readable recording medium can also be distributed in network coupled computer systems so that the computer readable media is stored and executed in a distributed fashion, e.g., by a server or a network. Additionally, although the exemplary embodiment is described as using one control unit to perform the above process, it is understood that the above processes may also be performed by a plurality of control units, controllers, processors or the like. 
     An exemplary embodiment of the present invention will hereinafter be described in detail with reference to the accompanying drawings. 
       FIG. 1  is a schematic diagram of a gear shifting system of a hybrid vehicle according to an exemplary embodiment of the present invention. As shown, a hybrid vehicle includes an engine  100 , a first planetary gear set PG 1 , a second planetary gear set PG 2 , a first motor-generator MG 1 , a first brake BK 1 , a first clutch CL 1 , a second clutch CL 2 , a second brake BK 2 , and a second motor-generator MG 2 . 
     The first planetary gear set PG 1  includes a first sun gear s 1 , a first planetary gear p 1 , a first ring gear r 1 , and a first carrier c 1 , and the output shaft of the engine  100  is configured to rotate the first sun gear s 1 . The output shaft of the first motor-generator MG 1  is configured to rotate the first ring gear r 1 , and the first brake BK 1  selectively locks the output shaft of the first motor-generator MG 1  and the first ring gear r 1 . 
     The first clutch CL 1  selectively connects the first ring gear r 1  with the first carrier c 1 , and the second clutch CL 2  selectively connects the first carrier c 1  with the second ring gear r 2 . The second brake BK 2  is configured to selectively lock the second ring gear r 2 , and the second carrier c 2  is directly connected to the output shaft. The first sun gear s 1  is directly connected to the second sun gear s 2 , and the second motor-generator MG 2  is configured to rotate the second sun gear s 2 . 
     In a neutral condition of an exemplary embodiment of the present invention, the first and second clutch CL 1  and CL 2  and the first and second brake BK 1  and BK 2  are disengaged and the first motor-generator MG 1 , the second motor-generator MG 2 , and the engine  100  are in the proper state for the starting process to begin. 
     When the engine  100  is stopped, the first motor-generator MG 1  and the second motor-generator MG 2  are speed controlled or torque controlled to induce the engine to reach a target speed for the starting the engine. As a result, the engine  100  is cranked (started smoothly so that the driver does not perceive the cranking of the engine. 
       FIG. 2  is a graph illustrating a gear shifting system of a hybrid vehicle as a lever type illustration according to an exemplary embodiment of the present invention. Referring to  FIG. 2 , the first motor-generator MG 1  generates torque to rotate the engine, and the second motor-generator MG 2  is torque controlled so that the speed thereof reaches 0. Accordingly, the first motor-generator MG 1  is speed controlled to induce the engine  100  to reach a target speed, and the second motor-generator MG 2  is controlled to a speed of 0. 
       FIG. 3  is a graph illustrating a rotational speed of constituent elements in an engine starting process of a gear shifting system of a hybrid vehicle according to an exemplary embodiment of the present invention. Referring to  FIG. 3 , the rotational speed of the engine is varied proportional to the rotational speed of the first motor-generator MG 1 , and the rotational speed of the second motor-generator MG 2  converges to 0 as a result of the applied control. Notably, however, in the second motor generator, a predetermined speed is generated early on. However, as time goes on, the speed of the second motor generator converges to 0. 
       FIG. 4  illustrates formulas for controlling a gear shifting system of a hybrid vehicle according to an exemplary embodiment of the present invention. Referring to  FIG. 4 , a formula 2 is induced by a formula 1, and the rotational speed of the first motor-generator MG 1  and the second motor-generator MG 2  can be drawn in the formula 2 so as to induce the engine  100  to reach a target speed. 
     
       
         
           
             
               
                 
                   
                     
                       ( 
                       
                         
                           I 
                           ENG 
                         
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                         ω 
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                       ENG 
                     
                   
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                       ENG 
                     
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                             1 
                           
                         
                       
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                         ω 
                         
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                         ω 
                         ENG 
                         Target 
                       
                     
                   
                 
               
               
                 
                   Formula 
                   ⁢ 
                   
                       
                   
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                   2 
                 
               
             
           
         
       
     
     I ENG  is a rotational inertia value of the engine  100 , I c1  is a rotational inertia moment of the first clutch CL 1 , {dot over (ω)} ENG  is a rotation angle acceleration of the engine  100 , τ ENG  is an output torque of the engine  100  (T MG1 =τ MG1 −{dot over (ω)} MG1 I MG1 ), τ MG1  is a torque of the first motor-generator MG 1 , {dot over (ω)} MG1  is angle acceleration of the first motor-generator MG 1 , and I MG1  is a inertia moment of the first motor-generator MG 1 . R 1  is a value representing the number of teeth of the first ring gear r 1  divided by the number of teeth of the first sun gear s 1 , ω MG1  is a rotation angle speed of the first motor-generator MG 1 , ω MG2  is a rotation angle speed of the second motor-generator MG 2 , and ω ENG   Target  is a target speed of the engine  100 . 
     Referring to back to  FIG. 4 , a speed of the second ring gear r 2  is calculated in a below formula 3. 
     
       
         
           
             
               
                 
                   
                     
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                   3 
                 
               
             
           
         
       
     
     I R2  is a rotational inertia moment of the second ring gear r 2 , {dot over (ω)} R2  is a rotational angle acceleration of the second ring gear r 2 , R 2  is a value equal to the number of the teeth in the second ring gear r 2  divided by the number of the teeth of the second sun gear, T MG2 =τ MG2 −{dot over (ω)} MG2 I MG2 , τ MG2  is a torque of the second motor-generator MG 2 , {dot over (ω)} MG2  is an angle acceleration of the second motor-generator MG 2 , I MG2  is an inertia moment of the second motor-generator MG 2 , T MG1 =τ MG1 {dot over (ω)} MG1 I Mg1 , τ MG1  is a torque of the first motor-generator MG 1 , {dot over (ω)} MG1  is an angle acceleration of the first motor-generator MG 1 , and I MG1  is an inertia moment of the first motor-generator MG 1 . 
       FIG. 5  is a flowchart for controlling a first motor-generator for controlling a gear shifting system of a hybrid vehicle according to an exemplary embodiment of the present invention. Referring to  FIG. 5 , the speed (ω MG1 ) of the first motor-generator MG 1  and the target speed (ω MG1   target ) of the first motor-generator MG 1  are inputted and are proportional integral (PI) controlled to speed control (τ MG   specontrol ) the torque of the first motor-generator MG 1  to generate the output torque (τ MG1 ) of the first motor-generator MG 1 . 
       FIG. 6  is a flowchart for controlling a second motor-generator for controlling a gear shifting system of a hybrid vehicle according to an exemplary embodiment of the present invention. Referring to  FIG. 6 , a target speed (ω MG2   target ) and a present speed (ω MG2 ) of the second motor-generator MG 2  are inputted and PI controlled. A factor 
             (       1     R   ⁢           ⁢   1       ⁢     τ     mg   ⁢           ⁢   1         )         
of the first motor-generator MG 1  is feedforward added thereto, and the target speed and present speed are processed to calculate torque (τ mg2 ) of the second motor-generator MG 2 .
 
     In an exemplary embodiment of the present invention, the first motor-generator MG 1  is used to rotate the engine  100  and simultaneously the torque of the second motor-generator MG 2  is used to make the second motor-generator MG 2  to diverge to 0 RPMs. The output torque of the first motor-generator MG 1  is calculated through a feedback control according to the speed of the engine  100 . Further, the torque of the second motor-generator MG 2  is calculated by feedback and PI controlling the feedforward torque of the first motor-generator MG 1  and the rotational speed of the second ring gear r 2 . 
       FIG. 7  shows formulas for controlling a first and second motor-generator for controlling a gear shifting system of a hybrid vehicle according to an exemplary embodiment of the present invention. In a below formula 4, the torque of the first motor-generator MG 1  is feedback controlled for the speed control of the first motor-generator MG 1 , wherein the formula shows the outputted torque.
 
τ MG1   Spdcontrol =τ MG1   F1B   Formula 4
 
     τ MG   SpdControl  is an output torque for speed control of the first motor-generator MG 1 , and τ MG1   F1B  is a feedback output torque of the first motor-generator MG 1 . In the formula 4,
 
τ Mg1   F1B =max( f   PI   N,Crank (ω MG1   Target −ω MG1 ),0)  Formula 5
 
Here, τ MG1   F1B  is a feedback output torque of the first motor-generator MG 1 , ω MG   Target  is a target speed of the first motor-generator MG 1 , and ω MG1  is a speed of the first motor-generator MG 1 .
 
     A relationship between a target speed of the first motor-generator MG 1 , a speed of the second motor-generator MG 2 , and a target speed of the engine  100  is shown in a below formula 6. 
     
       
         
           
             
               
                 
                   
                     ω 
                     
                       MG 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       1 
                     
                     Target 
                   
                   = 
                   
                     
                       
                         - 
                         
                           1 
                           
                             R 
                             1 
                           
                         
                       
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                           ⁢ 
                           
                               
                           
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                   6 
                 
               
             
           
         
       
     
     Here, ω MG   Target  is a target speed of the first motor-generator MG 1 , ω MG2  is a speed of the second motor-generator MG 2 , and ω ENG   Target  is a target speed of the engine  100 . 
     As shown in a below formula 7, the torque of the second motor-generator MG 2  is feedback and feedforward controlled to output torque for speed control of the second motor-generator MG 2 .
 
τ MG2   SpdControl =τ MG2   F1F +τ MG2   F1B   Formula 7
 
     τ MG   SpdControl  is an output torque for speed control of the second motor-generator MG 2 , τ MG2   F1F  is a feedforward output torque of the second motor-generator MG 2 , and τ MG2   F1B  is a feedback output torque of the second motor-generator MG 2 . In a formula 7, 
     
       
         
           
             
               
                 
                   
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                   8 
                 
               
             
           
         
       
     
     In a formula 7,
 
τ MG2   F1B   =f   PI   N,Crank (ω MG2   Target −ω MG2 )  Formula9
 
     Further, a target speed of the second motor-generator MG 2  is explained as a below formula 10 in an exemplary embodiment of the present invention.
 
ω MG2   Target =0  Formula 10
 
     While this invention has been described in connection with what is presently considered to be practical exemplary embodiments, it is to be understood that the invention is not limited to the disclosed embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the appended claims.