Abstract:
A system and a method for generating a line pressure of a continuously variable transmission (CVT) in a hybrid vehicle in which the line pressure of the CVT is maintained to be constant even in an idle stop state of an engine. Steps may include detecting operation of a brake; determining whether an idle stop condition is satisfied; detecting a hydraulic pressure of a control valve for controlling the line pressure of the continuously variable transmission when the idle stop condition is satisfied; comparing the hydraulic pressure of the control valve with a predetermined value; generating a pressure difference in the chamber when the hydraulic pressure of the control valve is smaller than or equal to the predetermined value; and actuating a cylinder for generating the line pressure according to the pressure difference in the chamber.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority to and the benefit of Korean Patent Application No. 10-2006-0053192 filed in the Korean Intellectual Property Office on Jun. 13, 2006, 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 a method for generating line pressure for a continuously variable transmission (CVT) in a hybrid vehicle. More particularly, the present invention relates to a system and a method for generating line pressure for a CVT in a hybrid vehicle in which the line pressure of the CVT is maintained as constant even in an idle stop state of an engine. 
   (b) Description of the Related Art 
   A hybrid vehicle is a vehicle having both an engine for generating power by combusting a fuel and a motor for outputting power of a battery. A hybrid vehicle controls operation of the engine and the motor, and thus, a fuel consumption rate can be enhanced and a noise and an exhaust gas can be reduced. 
   In addition, a hybrid vehicle has a characteristic of performing power delivery when the engine is restarted in an idle stop state of the engine. 
   Two methods for generating the line pressure for the CVT in a hybrid vehicle are known. The first method generates the line pressure by receiving power from a power source of the engine when the engine is operated. The second method generates the line pressure by using an additional drive motor. 
   However, the line pressure of the CVT is generated after the engine is started according to such methods. Thus, exhaust performance and power performance may be deteriorated during a delay time for generating sufficient line pressure. 
   In addition, speed and torque differences between an input element and an output element may occur during the delay time for generating sufficient line pressure. Thus, an impact may occur when the engine is restarted in the idle stop state of the engine. 
   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 
   Embodiments of the present invention provide a system and a method for generating line pressure for a continuously variable transmission (CVT) in a hybrid vehicle having advantages of enhancing exhaust performance and power performance of the hybrid vehicle by maintaining the line pressure of the CVT at a predetermined value in an idle stop state of an engine. 
   Embodiments of the present invention also provide a system and a method for generating the line pressure of the CVT having further advantages of reducing a starting impact when the engine is restarted in the idle stop state of the engine. 
   An exemplary system for generating line pressure for a continuously variable transmission in a hybrid vehicle according to an embodiment of the present invention may include: a brake for generating a hydraulic pressure by operation of a brake pedal; an oil tank storing an oil; an oil pump for pumping the oil stored in the oil tank according to the hydraulic pressure generated by the brake; a hydraulic pressure supply line for supplying the hydraulic pressure from the oil pump to the transmission; a hydraulic pressure withdrawal line connected to the hydraulic pressure supply line, and having a control valve for controlling the hydraulic pressure thereof; a transmission control unit for controlling opening and closing of the control valve; and a line pressure generating unit connected to the hydraulic pressure supply line, and generating the line pressure in an idle stop state. 
   The line pressure generating unit may include: a chamber divided into first and second portions by a diaphragm; a valve connected to the first portion of the chamber; and a cylinder for generating the line pressure in accordance with a pressure difference between the first and second portions of the chamber. 
   The second portion of the chamber may be connected to the brake and a pressure thereof may be maintained to be the same as a negative pressure of an engine. 
   A vacuum tank may be connected between the second portion of the chamber and the brake, and a pressure of the vacuum tank may be maintained to be the same as the negative pressure of the engine. 
   In addition, an exemplary system for generating the line pressure according to an embodiment of the present invention may further include: an acceleration pedal sensor for detecting operation of an acceleration pedal; and a hybrid control unit connected to the acceleration pedal sensor and controlling opening and closing of the valve. 
   The hybrid control unit may be connected to the transmission control unit, and may open the valve when the hydraulic pressure of the control valve is smaller than or equal to a predetermined value in the idle stop state. 
   An exemplary method for generating line pressure for a continuously variable transmission (CVT) in a hybrid vehicle by using a system for generating the line pressure of the CVT in the hybrid vehicle that generates the line pressure according to a pressure difference in a chamber divided into first and second portions may include: detecting operation of a brake; determining whether an idle stop condition is satisfied; and generating the line pressure when the idle stop condition is satisfied. 
   The generation of the line pressure may include: detecting a hydraulic pressure of a control valve for controlling the line pressure of the continuously variable transmission; comparing the hydraulic pressure of the control valve with a predetermined value; generating the pressure difference in the chamber when the hydraulic pressure of the control valve is smaller than or equal to the predetermined value; and actuating a cylinder for generating the line pressure according to the pressure difference in the chamber. 
   The generation of the pressure difference in the chamber may include: maintaining a pressure of the second portion of the chamber to be the same as a negative pressure of an engine; and opening a valve in order for a first portion of the chamber to have the same pressure as the atmosphere. 
   An exemplary method for generating the line pressure may further include: determining whether the engine is restarted; closing the valve when the engine is restarted; and operating an oil pump for pumping an oil. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram of an exemplary system for generating a line pressure for a continuously variable transmission in a hybrid vehicle according to one embodiment of the present invention. 
       FIG. 2  is a flowchart of an exemplary method for generating a line pressure for a continuously variable transmission in a hybrid vehicle according to one embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Exemplary embodiments of the present invention will hereinafter be described in detail with reference to the accompanying drawings. 
   As shown in  FIG. 1 , an exemplary system for generating line pressure for a continuously variable transmission (CVT) in a hybrid vehicle according to an embodiment of the present invention includes a brake  10 , an oil tank  30 , an oil pump  35 , a transmission control unit  40 , a line pressure generating unit  65 , and a hybrid control unit  55 . 
   The oil pump  35  is connected to a transmission  50  via a hydraulic pressure supply line  90 . A hydraulic pressure withdrawal line  95  is connected to the hydraulic pressure supply line  90  and the oil tank  30 . The brake  10  generates a hydraulic pressure in accordance with operation of a brake pedal  5 . The brake  10  includes a booster  15  and a master cylinder  20  for amplifying the hydraulic pressure. The oil tank  30  stores oil. The oil tank  30  is connected to the oil pump  35  and supplies the oil to the transmission  50 . 
   The oil pump  35  pumps the oil stored in the oil tank  30  in accordance with the hydraulic pressure generated in the brake  10 . In addition, the oil pumped by the oil pump  35  is supplied to the transmission  50  via the hydraulic pressure supply line  90 . 
   If an engine stops or is an idle stop state, the oil pump  35  stops, accordingly, and the oil is withdrawn via the hydraulic pressure withdrawal line  95 . 
   In addition, a control valve  45  is mounted on the hydraulic pressure withdrawal line  95 . The control valve  45  controls hydraulic pressure of the hydraulic pressure supply line  90  and the hydraulic pressure withdrawal line  95 . 
   The transmission control unit  40  can be realized by one or more processors activated by a predetermined program. The processor and program, as well as memory and other associated hardware and software may be selected and programmed by persons of ordinary skill in the art based on the teachings of the present invention contained herein. The transmission control unit  40  controls operation of the transmission  50 . In addition, the transmission control unit  40  is connected to the control valve  45  and controls opening and closing of the control valve  45 . The transmission control unit  40  is connected to the hybrid control unit  55 , and transmits the hydraulic pressure information of the control valve  45  to the hybrid control unit  55 . 
   The line pressure generating unit  65  is connected to a second hydraulic pressure supply line  97  and generates the line pressure of the CVT in the idle stop state of the engine. The line pressure generating unit  65  includes a chamber  105 , a valve  70 , and a plunger  85 . 
   The chamber  105  is divided into first and second portions by a diaphragm  100 . The valve  70  is connected to the first portion  75  of the chamber  105 , and a vacuum tank  60  is connected to the second portion  80  of the chamber  105 . 
   The valve  70  is opened and closed in accordance with a control signal of the hybrid control unit  55 . In the case that the valve  70  is opened, a pressure of the first portion  75  of the chamber  105  is maintained to be the same as an atmospheric pressure. On the contrary, in the case that the valve  70  is closed, the pressure of the first portion  75  of the chamber  105  is the same as a negative pressure of the engine. 
   The vacuum tank  60  is connected to the brake  10 , and the pressure of the vacuum tank  60  is the same as the negative pressure of the engine. Therefore, a pressure of the second portion  80  of the chamber  105  connected to the vacuum tank  60  is the same as the negative pressure of the engine. 
   In the case that the valve  70  is opened, the plunger  85  coupled to the diaphragm  100  is operated by the pressure difference between the first and second portions  75  and  80  of the chamber  105 , and generates the line pressure of the CVT. However, in the case that the valve  70  is closed, the first and second portions  75  and  80  of the chamber  105  have the same pressure, and thus the plunger  85  is not operated. 
   The hybrid control unit  55  can be realized by one or more processors activated by a predetermined program, and the predetermined program can be programmed to perform each step of a method for generating the line pressure of the continuously variable transmission in a hybrid vehicle according to an embodiment of this invention. Memory and other associated hardware and software as may comprise the hybrid control unit  55  may be selected and programmed by persons of ordinary skill in the art based on the teachings of the present invention as contained herein. 
   The hybrid control unit  55  is connected to the valve  70  and controls opening and closing of the valve  70 . In addition, the hybrid control unit  55  is connected to an acceleration pedal sensor (APS)  25 , and detects operation of the acceleration pedal (not shown). The hybrid control unit  55  determines whether a restarting condition of the engine is satisfied. 
   In addition, the hybrid control unit  55  is connected to the transmission control unit  40 , and receives a pressure signal of the control valve  45 . In addition, the hybrid control unit  55  compares the hydraulic pressure of the control valve  45  with a predetermined value. 
   If the hydraulic pressure of the control valve  45  is smaller than or equal to the predetermined value, the hybrid control unit  55  opens the valve  70  and generates the pressure difference in the chamber  105 . Therefore, the plunger  85  is operated in accordance with the pressure difference in the chamber  105  and generates the line pressure of the CVT. 
   Hereinafter, referring to the accompanying drawings and particularly  FIG. 2 , an exemplary method for generating the line pressure of the CVT in a hybrid vehicle according to an embodiment of the present invention will be described in detail. 
   When the engine is operated at step S 100 , the hybrid control unit  55  detects the operation of the brake  10  at step S 110 . If the brake  10  is operated, the hybrid control unit  55  determines whether the idle stop condition is satisfied at step S 120 . The idle stop condition may be determined to be satisfied when the speed of the vehicle is less than a predetermined speed (generally about 5 km/h), the position of a gear is in a neutral state, and the brake and clutch have been manipulated. 
   If an idle stop condition is not satisfied, the transmission control unit  40  operates the oil pump  35  and controls the hydraulic pressure of the hydraulic pressure supply line  90  and the hydraulic pressure withdrawal line  95  at step S 140 . If the idle stop condition is satisfied, the hybrid control unit  55  generates the line pressure of the CVT at step S 150 . 
   At the step S 150 , the transmission control unit  40  detects the hydraulic pressure of the control valve  45  at step S 130 , and the hybrid control unit  55  compares the hydraulic pressure of the control valve  45  with a predetermined value at step S 160 . The predetermined value is a pressure that is needed when the engine starts or restarts, and can be understood by a person of an ordinary skill in the art based on the teachings contained herein. 
   If the hydraulic pressure of the control valve  45  is smaller than or equal to the predetermined value, the hybrid control unit  55  generates a pressure difference in the chamber  105  at step S 170 . 
   At the step S 170 , the pressure of the second portion  80  of the chamber  105  connected to the brake  10  is maintained to be the same as the negative pressure of the engine at step S 180 . At this time, the hybrid control unit  55  opens the valve  70 , and so the pressure of the first portion  75  of the chamber  105  is the same as the atmospheric pressure at step S 190 . 
   In this case, the first portion  75  of the chamber  105  has a different pressure from the second portion  80  of the chamber  105 . Therefore, the plunger  85  is operated in accordance with the pressure difference in the chamber  105 . Accordingly, the line pressure of the CVT is generated at step S 200 . After that, the hybrid control unit  55  determines whether the engine is restarted at step S 210 . 
   If the engine is restarted, the hybrid control unit  55  closes the valve  70  and makes the pressure of the first portion  75  of the chamber  105  be the same as the negative pressure of the engine at step S 220 . In this case, the hybrid control unit  55  stops the operation of the plunger  85 . After that, the transmission control unit  40  operates the oil pump  35 , and the hydraulic pressure is supplied to the transmission  50  at step S 140 . 
   As described above, the line pressure of the CVT is generated in the idle stop state of the engine according to the present invention. Therefore, exhaust performance and power performance are improved according to the present invention. In addition, the restarting impact is reduced according to the present invention. 
   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.