CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 10-2010-0061506 filed in the Korean Intellectual Property Office on Jun. 28, 2010, 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 starting control of a hybrid vehicle which secures robust drivability when the hybrid vehicle starts. 
     (b) Description of the Related Art 
     A manual transmission vehicle is provided with various types of clutches (dry type/wet type). The clutches are mounted between an engine and a transmission so as to selectively transmit an engine output torque to the transmission. 
     When the manual transmission vehicle is started, a driver handles a clutch pedal and an accelerator pedal and controls the engine output torque so as to be supplied as a transmission input torque. 
     A clutch slip control is performed in an initial starting of the vehicle. The clutch slip control permits the vehicle to be started, reduces shock, jerk, or clutch judder which can occur when starting so as to provide a smooth starting, and prevents stall according to an urgent engine load. 
     An automatic transmission is provided with a torque converter instead of the clutch. The torque converter can increase the torque transmitted though a fluid-coupling but has a low transmitting efficiency. Therefore, fuel efficiency may be deteriorated. 
     Recently, because of demands for enhancement of fuel efficiency and reinforcement of exhaust gas regulations, eco-friendly vehicles have been researched. A hybrid vehicle is one type of such eco-friendly vehicles and attracts public attention. 
     The hybrid vehicle uses an engine and a motor as power sources, and enhances energy efficiency and reduces exhaust gas by selectively using the engine and the motor as the power sources. 
     In order to minimize cost and torque loss, a clutch operated by fluid, instead of a torque converter, is mounted between the engine and the motor in the hybrid vehicle. 
     The clutch mounted in the hybrid vehicle connects or disconnects the engine and the motor according to a driving condition such that torque transmitted to the transmission is optimized. 
     However, if the clutch slip control is not performed stably when connecting the engine with the motor, stall, drop, or flare may occur by overload of the engine. Accordingly, starting shock may occur. 
     In addition, since the clutch slip is not constant, robust drivability cannot be secured in a repetitive starting mode, and an excessive clutch slip causes clutch judder. Therefore, drivability may be deteriorated. 
     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 system and a method for starting control of a hybrid vehicle having advantages of providing robust drivability by performing a clutch slip control suitably. 
     One aspect of the present invention is to provide a system for starting control of a hybrid vehicle which includes an engine and a motor being power sources, a clutch connecting or disconnecting power delivery, and a clutch control unit controlling an operation of the clutch. The system may include a hybrid control unit controlling a clutch slip according to a starting demand, calculating a clutch slip torque, and requesting correction of an engine output torque; and an engine control unit controlling an engine speed to follow a target engine speed through a control of air amount and feedback control of ignition timing according to the correction request of the engine output torque received from the hybrid control unit. 
     The engine control unit may compensate the engine output torque lost by the clutch slip according to the correction request of the engine output torque through the control of the air amount and the feedback control of the ignition timing. 
     A method for starting control of a hybrid vehicle according to another aspect of the present invention may include: deciding a target torque according to starting demand; analyzing an engaging state of a clutch; calculating a clutch slip torque in a case that the clutch slips; requesting correction of an engine output torque according to the clutch slip torque; and correcting the engine output torque through control of air amount and feedback control of ignition timing according to the correction request of the engine output torque. 
     The target torque according to the starting demand may be calculated in a case the clutch is completely engaged, and the engine output torque may be controlled to follow the target torque. 
     The target torque according to the starting demand may be corrected by a clutch slip torque in a case of the clutch slip, and an engine speed may be controlled to follow the target engine speed. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of a system for starting control of a hybrid vehicle according to an exemplary embodiment of the present invention. 
         FIG. 2  is a flowchart of a method for starting control of a hybrid vehicle according to an exemplary embodiment of the present invention. 
     
    
    
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 &lt;Description of symbols&gt; 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 10: ECU 
                 20: HCU 
               
               
                   
                 30: MCU 
                 60: CCU 
               
               
                   
                 70: motor 
                 80: engine 
               
               
                   
                   
               
             
          
         
       
     
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, referring to the drawings, exemplary embodiments of the present invention will be described in detail. 
     As those skilled in the art would realize, the described embodiments may be modified in various different ways, all without departing from the spirit or scope of the present invention. Description of components that are not necessary for explaining the present invention will be omitted, and the same constituent elements are denoted by the same reference numerals in this specification. 
       FIG. 1  is a schematic diagram of a system for starting control of a hybrid vehicle according to an exemplary embodiment of the present invention. 
     As shown in  FIG. 1 , an exemplary embodiment of the present invention includes an engine control unit (ECU)  10 , a hybrid control unit (HCU)  20 , a motor control unit (MCU)  30 , a battery  40 , a battery management system (BMS)  50 , a clutch control unit (CCU)  60 , a motor  70 , an engine  80 , an ISG  81 , a clutch  90 , and a transmission  100 . 
     The ECU  10  is connected to the HCU  20  through a network and cooperates with the HCU  20  for controlling an operation of the engine  80 . 
     The HCU  20  controls each controller according to a driving demand and a vehicle state through the network so as to control output torques of the engine  80  and the motor  70 , and controls the clutch  90  according to a driving condition and a state of the battery  40  so as to operate the hybrid vehicle at an electric mode (EV), a hybrid mode (HEV), and an engine mode. 
     The HCU  20  controls a clutch slip according to a target torque through the CCU  60  when a starting demand is detected, calculates a clutch slip torque, and requests correction of an engine output torque to the ECU  10 . 
     The MCU  30  controls an operation of the motor  70  according to the control of the HCU  30 , and stores electric power generated at the motor  70  in the battery  40  through a regenerative braking. 
     The battery  40  supplies the power to the motor  70  at the hybrid mode (HEV) and the electric mode (EV), and is recharged therough the regenerative braking. 
     The BMS  50  information about a voltage, a current, and a temperature of a battery  40 , controls a state of charge (SOC), a recharge current, and discharge current, and transmits the information corresponding thereto to the HCU  20  through the network. 
     The CCU  60  controls actuators mounted in the transmission  100  according to the control of the HCU  20  so as to control a shift to a target shift-speed, engages or disengages the clutch  90  by controlling hydraulic pressure supplied to the clutch  90 , and controls the clutch slip according to the starting demand. 
     Output torque of the motor  70  is controlled by the control of the MCU  30 . 
     The engine output torque is controlled by the control of the ECU  20 , and an intake air amount is controlled by an ETC (not-shown). 
     The ISG  81  idle stops or starts the engine  80  according to a driving condition of the vehicle. 
     The clutch  90  is disposed between the engine  80  and the motor  70  and connects or disconnects the engine  80  and the motor  70  according to the drive mode (e.g., the engine mode, the hybrid mode, and the electric mode). 
     The transmission  100  achieves the target shift-speed by the control of the CCU  60  connected to the HCU  20  through the network. 
     In addition, the ECU  10  controls the engine output torque through control of air amount and feedback control of ignition timing according to the correction request of the engine output torque received from the HCU  20  through the network so as to maintain a target engine speed. 
     If it is assumed that ideal and precise information of an engine load is transmitted from the HCU  20  to the ECU  10 , the target engine speed can be maintained only by the control of the air amount. 
     However, since an error always exists in the clutch slip torque calculated by the HCU  20 , the ECU  10  controls the engine speed by the feedback control of the ignition timing as well as the control of the air amount so as to eliminate the effect of the error. 
     For example, a fluid operating the clutch  90  has different hydraulic characteristics according to a temperature thereof, and the hydraulic pressure according to the same signal may be changed in a transient region. 
     That is, since the calculated clutch slip torque (engine load) can be different from an actual engine load, the feedback control of the ignition timing is performed so as to compensate the difference between the calculated engine load and the actual engine load. 
     The clutch slip is calculated by the HCU  20  as follows. In addition, an error of the clutch slip may exist according to an error of a friction coefficient and an error of the estimated hydraulic pressure supplied to the clutch.
 
Clutch slip=estimated hydraulic pressure supplied to clutch/(μ× A×r×n )
 
     Herein, μ represents the friction coefficient, A represents a contact area, r represents an effective radius, and n represents the number of clutch disks. 
     During the ECU  10  controls the engine speed to follow the target engine speed, the error of the clutch slip torque should be within a predetermined range for effectively performing the feedback control. 
     That is, since the clutch slip torque is calculated within a controllable error range, the hydraulic pressure supplied to the clutch  90  should be lower than a maximum allowable hydraulic pressure supplied to the clutch  90 . The maximum allowable hydraulic pressure supplied to the clutch  90  is calculated as follows.
 
Maximum allowable hydraulic pressure=available engine torque/(μ× A×r×n )
 
     Herein, μ represents the friction coefficient, A represents a contact area, r represents an effective radius, and n represents the number of clutch disks. 
     The available engine torque differs according to a coolant temperature and an oil temperature. Therefore, the available engine torque is acquired through a number of experiments. 
     A method for starting control of a hybrid vehicle according to an exemplary embodiment of the present invention will be described in detail referring to  FIG. 2 . 
     In a state that the hybrid vehicle according to the present exemplary embodiment is stopped or drives at the electric mode (EV), the HCU  20  detects the starting demand of the driver at a step S 101  and calculates a target torque according to the starting demand of the driver at a step S 102 . 
     In addition, the HCU  20  detects a current state of the clutch  90  by analyzing the information received from the CCU  60  at a step S 103 , and determines whether the clutch  90  is completely engaged at a step S 104 . 
     If the clutch  90  is completely engaged at the step S 104 , the HCU  20  transmits the target torque of full load (F/L) or partial load (P/L) calculated according to the starting demand of the driver to the ECU  10  connected thereto through the network at a step S 105 . 
     After that, the ECU  10  corrects the engine output torque according to the target torque of the full load (F/L) or the partial load (P/L) transmitted from the HCU  10  through the network by the control of the air amount and the feedback control of the ignition timing and controls the engine output torque to follow the target torque at a step S 106 . 
     If the clutch  90  is not completely engaged at the step S 104 , the HCU  20  decides that the current state of the clutch  90  is a clutch slip state at a step S 107  and calculates the clutch slip torque at a step S 108 . 
     After the clutch slip torque is calculated at the step S 108 , the HCU  20  requests correction of the engine output torque to the ECU  10  through the network so as to compensate a load loss of the engine  80  by the clutch slip at a step S 109 . 
     The HCU  20  transmits the target torque according to the starting demand as well as the correction request of the engine output torque to the ECU  10 . 
     Therefore, the ECU  10  controls the engine speed to follow the target engine speed according to the target torque and the clutch slip torque received from the HCU  20  by the control of the air amount and the feedback control of the ignition timing at a step S 110 . Since the engine speed is closely related to the engine output torque, the ECU  10  may control the engine output torque to follow the corrected engine output torque. 
     If it is assumed that ideal and precise information of an engine load is transmitted from the HCU  20  to the ECU  10 , the target engine speed can be maintained only by the control of the air amount. 
     However, since an error always exists in the clutch slip torque calculated by the HCU  20 , the ECU  10  controls the engine speed by the feedback control of the ignition timing as well as the control of the air amount. 
     The fluid operating the clutch  90  has different hydraulic characteristics according to a temperature thereof, and the hydraulic pressure according to the same signal may be changed in a transient region. 
     Since the calculated clutch slip torque (engine load) can be different from an actual engine load, the feedback control of the ignition timing is performed so as to compensate the difference between the calculated engine load and the actual engine load. 
     At this time, the error of the clutch slip may exist according to the error of the friction coefficient and the error of the estimated hydraulic pressure supplied to the clutch  90 . Therefore, the ECU  10  controls the engine speed to follow the target engine speed only when the error of the clutch slip torque should be within the predetermined range. It is noted that the predetermined range of error may vary based on a variety of factors, for example, according to the type of vehicle, transmission engine, etc. and is not particularly limited and could be determined by one of skill in the art. However, an exemplary predetermined range of error can be about 0-10%. 
     It is exemplarily described that the present invention is applied to the hybrid vehicle, but the present invention can be applied to any automatic transmission (CVT, DCT, AMT, and so on) provided with the clutch. Since efficiency of the clutch is higher than that of the torque converter, fuel consumption and cost may further be reduced if applying the present invention to the automatic transmission. 
     According to the present invention, stall or drop may not occur by calculating a clutch slip according to a starting demand of a driver and compensating an engine output torque according to the clutch slip. Therefore, an operation of the engine may be stabilized and drive reliability may be enhanced. 
     In addition, starting responsiveness and initial acceleration feel according to a driver&#39; will may be improved. 
     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.
The present invention relates to a system and a method for starting control of a hybrid vehicle which secures robust drivability when the hybrid vehicle starts. A method for starting control of a hybrid vehicle according to another aspect of the present invention may include: deciding a target torque according to starting demand; analyzing an engaging state of a clutch; calculating a clutch slip torque in a case that the clutch slips; requesting correction of an engine output torque according to the clutch slip torque; and correcting the engine output torque through control of air amount and feedback control of ignition timing according to the correction request of the engine output torque.