Patent Publication Number: US-2013233268-A1

Title: Engine starting apparatus

Description:
TECHNICAL FIELD 
     The present invention relates to an engine starting apparatus for staring an engine in a vehicle such as, for example, a hybrid vehicle, which is provided with the engine and a motor. 
     BACKGROUND ART 
     As this type of apparatus, an apparatus for motoring or cranking an engine with a motor generator connected to a crankshaft of the engine (internal combustion engine) through a damper is known (e.g. refer to patent documents 1 and 2). 
     For example, the patent document  1  discloses a technology for increasing the number of times to motor and start the engine in the condition that there is little fuel left to be supplied to the engine. For example, the patent document 2 discloses a technology for starting fuel injection to the engine and ignition on the basis of a torsion angle of the damper at the start of the engine. 
     On the other hand, if the damper is included in a power transmission system (power train) for transmitting the power of the engine, torque variation at the start of the engine likely causes the resonance of the damper, thereby worsening the vibration of the power transmission system. In order to suppress the worse vibration of the power transmission system due to the resonance of the damper, what is known is a technology for applying, from the motor to the engine, a vibration controlling torque for suppressing the resonance of the damper in addition to a torque for cranking, in other words, a torque for increasing the number of revolutions of the engine (hereinafter referred to as a “cranking base torque” as occasion demands) in cranking the engine. The vibration controlling torque is controlled to vary depending on, for example, the position of a piston of the engine. 
     PRIOR ART DOCUMENT 
     Patent Document 
     
         
         Patent document 1: Japanese Patent Application Laid Open No. 2008-285085 
         Patent document 2: Japanese Patent Application Laid Open No. 2010-96096 
       
    
     DISCLOSURE OF INVENTION 
     Subject to be Solved by the Invention 
     However, if a torque obtained by applying the vibration controlling torque to the cranking base torque is outputted from the motor, depending on a time-dependent change in the cranking base torque and the vibration controlling torque, the maximum value of the torque to be outputted from the motor likely becomes greater than that in a case where only the cranking base torque is outputted from the motor and the power consumption of the motor likely increases. Thus, the rated output of a battery for supplying electric power to the motor (i.e. the maximum value of the electric power that the battery can output) needs to be increased, and it is hard to miniaturize the battery, which is technically problematic. 
     In view of the aforementioned conventional problem, it is therefore an object of the present invention to provide an engine starting apparatus capable of suppressing the vibration of the power transmission system due to the resonance of the damper at the start of the engine and suppressing the power consumption of the motor. 
     Means for Solving the Subject 
     The above object of the present invention can be achieved by an engine starting apparatus installed in a vehicle provided with: an engine; a motor capable of cranking the engine; a power transmission system, including a damper, for transmitting power of the engine to drive wheels; and a battery capable of supplying electric power to the motor, the engine starting apparatus provided with: a target torque setting device for setting a sum of a cranking base torque for cranking the engine and a vibration controlling torque for suppressing vibration of the power transmission system due to resonance of the damper, as a target torque to be outputted by the motor in cranking the engine; and a motor controlling device for controlling the motor to output the set target torque, the target torque setting device having a base torque controlling device for setting the cranking base torque to a first torque value if the number of revolutions of the engine is less than or equal to predetermined number of revolutions of the engine and for controlling the cranking base torque such that the cranking base torque starts to be reduced at a time point at which a piston of the engine is at a top dead center or in a compression stroke and such that the cranking base torque is a second torque value which is less than the first torque value at a time point at which the piton is in an expansion stroke if the number of revolutions of the engine is greater than the predetermined number of revolutions of the engine. 
     According to the engine starting apparatus of the present invention, when the engine is started, the motor is controlled by the motor controlling device to output the target torque to the engine from the motor and the engine is cranked. 
     The target torque is set by the target torque setting device. The target torque setting device sets the sum of the cranking base torque and the vibration controlling torque as the target torque. The cranking base torque is a torque to be outputted by the motor in order to crank the engine, in other words, in order to increase the number of revolutions of the engine. The cranking base torque is controlled by the base torque controlling device. Here, the “number of revolutions of the engine” of the present invention means the number of revolutions per unit time of the crankshaft of the engine, and it corresponds to a rotational speed of the crankshaft, or a moving speed of the piston of the engine. The vibration controlling torque is a torque to be outputted by the motor in order to suppress the vibration of the power transmission system due to the resonance of the damper. Typically, the vibration controlling torque is controlled to vary depending on the position of the piston of the engine. The vibration controlling torque is controlled such that the direction of the torque varies between a case where the piston of the engine is in the compression stroke (in other words, in a period in which the piston transfers from a bottom dead point to a top dead point) and a case where the piston of the engine is in the expansion stroke. More specifically, the vibration controlling torque is controlled to reduce a torque outputted by the motor if the piston is in the compression stroke, and the vibration controlling torque is controlled to increase the torque outputted by the motor if the piston is in the expansion stroke. By applying the vibration controlling torque as described above to the engine, it is possible to suppress the vibration of the power transmission system due to the resonance of the damper. 
     In the present invention, in particular, the base torque controlling device (i) sets the cranking base torque to the first torque value if the number of revolutions of the engine is less than or equal to the predetermined number of revolutions of the engine and (ii) controls the cranking base torque such that the cranking base torque starts to be reduced at the time point at which the piston of the engine is at the top dead center or in the compression stroke and such that the cranking base torque is the second torque value which is less than the first torque value at the time point at which the piton is in the expansion stroke if the number of revolutions of the engine is greater than the predetermined number of revolutions of the engine. In other words, the cranking base torque is set to the first torque until the number of revolutions of the engine increases the predetermined number of revolutions of the engine, and the cranking base torque is controlled to start to be reduced at the time point at which the piston is at the top dead center or in the compression stroke after the number of revolutions of the engine reaches the predetermined number of revolutions of the engine (typically, at a time point at which the piston is at the top dead point or in the compression stroke for the first time after the number of revolutions of the engine reaches the predetermined number of revolutions) and to be the second torque value which is less than the first torque value at the time point at which the piton is in the expansion stroke following the compression stroke. 
     Thus, for example, it is possible to reduce the power consumption of the motor in the expansion stroke after the number of revolutions of the engine reaches the predetermined number of revolutions of the engine, in comparison with a case where the cranking base torque is set to the first torque value even in the expansion stroke after the number of revolutions of the engine reaches the predetermined number of revolutions of the engine. Therefore, it is possible to reduce the rated output of the battery (the maximum value of the electric power that the battery can output) for supplying the electric power to the motor, thereby miniaturizing the battery. Incidentally, as described above, the vibration controlling torque is typically controlled to reduce the torque outputted by the motor if the piston is in the compression stroke and to increase the torque outputted by the motor if the piston is in the expansion stroke. Thus, if the cranking base torque is set to the same torque value between the compression stroke and the expansion stroke, the target torque is maximal in the expansion stroke. 
     Here, in the expansion stroke, the rotation of the crankshaft is accelerated by the expansion of the air compressed in a cylinder in the compression stroke. Thus, in the expansion stroke, the number of revolutions of the engine tends to easily increase than in the compression stroke. Thus, in the present invention, the cranking base torque in the expansion stroke is reduced less than the cranking base torque in the compression stroke by the rotation of the crankshaft being accelerated by the expansion of the air compressed in the cylinder in the compression stroke,. This makes it possible to avoid a wasteful increase in the power consumption of the motor in the expansion stroke while increasing the number of revolutions of the engine. 
     As explained above, according to the engine starting apparatus of the present invention, it is possible to suppress the vibration of the power transmission system due to the resonance of the damper at the start of the engine and to suppress the power consumption of the motor. 
     In one aspect of the engine starting apparatus of the present invention, the base torque controlling device controls the cranking base torque such that the cranking base torque is greater than the first torque value in at least one portion of a period in which the piston in the compression stroke. 
     According to this aspect, it is possible to reduce or prevent that an increase in the number of revolutions of the engine is suppressed by the compressed air in the cylinder of the engine, in the period in which the piston is in the compression stroke. This makes it possible to suppress an increased difference in the rate of increase in the number of revolutions of the engine between the period in which the piston is in the compression stroke and a period in which the piston is in the subsequent expansion stroke. Thus, the vibration of the power transmission system can be also suppressed. 
     The operation and other advantages of the present invention will become more apparent from embodiments explained below. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic configuration diagram conceptually showing the configuration of a hybrid vehicle in a first embodiment. 
         FIG. 2  is a conceptual view for explaining an outline of a method of setting a MG 1  command torque in the first embodiment. 
         FIG. 3  is a flowchart showing a flow of controlling a cranking base torque in the first embodiment. 
         FIG. 4  is a graph showing one example of a time-dependent change in the cranking base torque and the number of revolutions of an engine in the first embodiment. 
         FIG. 5  is a conceptual view for explaining an outline of a method of setting a MG 1  command torque in a comparative example. 
         FIG. 6  is a flowchart showing a flow of controlling a cranking base torque in a second embodiment. 
         FIG. 7  is a graph showing one example of a time-dependent change in the cranking base torque and the number of revolutions of the engine in the second embodiment. 
         FIG. 8  is a flowchart showing a flow of controlling a cranking base torque in a third embodiment. 
         FIG. 9  is a graph showing one example of a time-dependent change in the cranking base torque and the number of revolutions of the engine in the third embodiment. 
         FIG. 10  is a graph showing one example of a time-dependent change in the cranking base torque and the number of revolutions of the engine in the comparative example. 
     
    
    
     MODE FOR CARRYING OUT THE INVENTION 
     Hereinafter, embodiments of the present invention will be explained with reference to the drawings. 
     First Embodiment 
     An engine starting apparatus in a first embodiment will be explained with reference to  FIG. 1  to  FIG. 4 . 
     Firstly, the entire configuration of a hybrid vehicle to which the engine starting apparatus in the embodiment is applied will be explained with reference to  FIG. 1 . 
       FIG. 1  is a schematic configuration diagram conceptually showing the configuration of the hybrid vehicle in the embodiment. 
     In  FIG. 1 , a hybrid vehicle  10  in the embodiment is provided with an electronic control unit (ECU)  100 , an engine  200 , a motor generator MG 1 , a motor generator MG 2 , a power dividing mechanism  300 , a power control unit (PCU)  400 , a battery  500 , a transmission mechanism  600 , a differential gear  610 , a transmission shaft  620 , a damper  700 , a crank position sensor  800 , and drive wheels FR and FL. 
     The ECU  100  is provided with a central processing unit (CPU), a read only memory (ROM), and a random access memory (RAM) or the like, and it is an electronic control unit capable of controlling all the operations of the hybrid vehicle  10 . The ECU  100  is configured to perform various controls in the hybrid vehicle  10 , in accordance with a control program stored in the ROM or the like. The ECU  100  functions as one example of the “engine starting apparatus” of the present invention. Specifically, the ECU  100  functions as one example of each of the “target torque setting device” and the “motor controlling device” of the present invention. 
     The engine  200  is a reciprocating engine and is configured to function as the power source of the hybrid vehicle  10 . The engine  200  has such a configuration that a plurality of cylinders are disposed in a cylinder block. Moreover, the engine  200  is configured such that a force generated when an air-fuel mixture including fuel is compressed in a compression stroke in each cylinder and the compressed air-fuel mixture is ignited spontaneously or by an ignition operation of a spark plug or the like is converted to a rotary motion of a crankshaft  210  through a piston and a connecting rod. The rotation of the crankshaft  210  is transmitted to the drive wheels FR and FL through the power dividing mechanism  300  and the transmission mechanism  600 , whereby the hybrid vehicle  10  can be driven. Incidentally, the “engine” of the present invention includes, for example, a two-cycle or four-cycle reciprocating engine or the like and conceptually includes an engine (internal combustion engine) configured to have at least one cylinder and to extract a force generated when the air-fuel mixture including various fuels such as gasoline, light oil or alcohol is burned in a combustion chamber within the cylinder, as a driving force through a physical or mechanical transmitting device such as a piston, a connecting rod, and a crankshaft, as occasion demands. As long as the concept is satisfied, the configuration of the “engine” of the present invention is not limited to that of the engine  200  but may have various aspects. 
     The engine  200  is provided with a crank position sensor  810 . The crank position sensor  810  can detect a crank angle CA, which is a rotation angle of the crankshaft  210 , and the number of revolutions of the engine Ne, which is the number of revolutions per unit time. The crank position sensor  810  is electrically connected to the ECU  100 , and the crank angle CA and the number of revolutions of the engine Ne detected are recognized by the ECU  100  with a certain or uncertain period. 
     The motor generator MG 1  is a motor generator and has a power running function for converting electrical energy into kinetic energy and a regeneration function for converting kinetic energy into electrical energy. The motor generator MG 1  is configured to function as a generator for charging the battery  500  or a generator for supplying electric power to the motor generator MG 2 , and as a motor for cranking the engine  200 . Incidentally, the motor generator MG 1  is one example of “motor” of the present invention. 
     The motor generator MG 2 , as in the motor generator MG 1 , has the power running function for converting electrical energy into kinetic energy and the regeneration function for converting kinetic energy into electrical energy. The motor generator MG 2  is configured to function mainly as a motor for assisting (aiding) the output of the engine  200 . The motor generator MG 2  can transmit power to the drive wheels FR and FL through the power dividing mechanism  300 , the transmission mechanism  600 , the differential gear  610 , and the transmission shaft  620 . 
     Incidentally, the motor generators MG 1  and MG 2  may be configured, for example, as synchronous motor generators. Each of the motor generators MG 1  and MG 2  is provided with a rotor having a plurality of permanent magnets on an outer circumferential surface and a stator in which a three-phase coil for forming a rotating magnetic field is wound; however, of course, it may have another configuration. 
     The power dividing mechanism  300  is provided with a carrier  310 , a first planetary gear mechanism  320 , a ring gear  330 , a propeller shaft  340 , a ring gear  350 , and a second planetary gear mechanism  360 . 
     The first planetary gear mechanism  320  has a sun gear  321  corotationally coupled with the rotating shaft of the motor generator MG 1  and a planetary gear  322  coupled with the carrier  310 . The crankshaft  210  is coupled with the planetary gear  322  of the first planetary gear mechanism  320  through the damper  700  and the carrier  310 . The planetary gear  322  is coupled with the ring gear  330  located on the outer circumference of the first planetary gear mechanism  320 . 
     Thus, the rotation of the engine  200  (i.e. the rotation of the crankshaft  210 ) is transmitted to the sun gear  321  and the ring gear  330  through the carrier  310  and the planetary gear  322 , and an output torque of the engine  200  is divided into two systems. 
     The propeller shaft  340 , which is the rotating shaft of the ring gear  330 , is coupled with the transmission mechanism  600 , through which the output torque from the engine  200  is transmitted to the drive wheels FL and FR 
     The propeller shaft  340  is coupled with the ring gear  350  at an end opposite to an end where the propeller shaft  340  is coupled with the ring gear  330  which is coupled with a planetary gear  362  of the second planetary gear mechanism  360 . 
     A sun gear  361  of the second planetary gear mechanism  360  is coupled with the rotating shaft of the motor generator MG 2  and transmits the rotation of the motor generator MG 2  to the propeller shaft  340 . 
     The PCU  400  includes an inverter capable of converting direct-current (DC) power extracted from the battery  500  into alternating-current (AC) power and supplying it to the motor generators MG 1  and MG 2  and capable of converting AC power generated by the motor generators MG 1  and MG 2  into DC power and supplying it to the battery  500 . The PCU  400  is a control unit capable of individually controlling the input/output of the electric power between the battery  500  and each motor generator. The PCU  400  is electrically connected to the ECU  100 , and the operations of the PCU  400  are controlled by the ECU  100 . 
     The battery  500  is a chargeable storage battery which functions as an electric power source associated with the electric power for the power running of the motor generators MG 1  and MG 2 . 
     The transmission mechanism  600  is coupled with the power dividing mechanism  300 , and it is a mechanism for transmitting the torque outputted from the engine  200  and the motor generator MG 2  to the drive wheels FL and FR through the differential gear  610  and the transmission shaft  620 . 
     The damper  700  is, for example, a torsional damper. The damper  700  is disposed between the crankshaft  210  and the power dividing mechanism  300 , and has a function of attenuating torque vibration between them. 
     The drive wheels FL and FR transmit to a road surface the torque transmitted through the transmission mechanism  600 . In  FIG. 1 , one wheel on the left side and one wheel on the right side are shown. The hybrid vehicle  10  is actually provided with four wheels in total, which are front, rear, left, and right wheels, including the drive wheels FL and FR. 
     Next, the start of the engine  200  in the hybrid vehicle  10  will be explained with reference to  FIG. 2 . 
     In the hybrid vehicle  10  configured as described above with reference to  FIG. 1 , at the start of the engine  200 , the engine  200  is cranked by the motor generator MG 1  under the control of the ECU  100 . Specifically, the ECU  100  sets a MG 1  command torque which is a target torque to be outputted by the motor generator MG 1  in cranking the engine  200  and controls the motor generator MG 1  to output the MG 1  command torque. 
       FIG. 2  is a conceptual view for explaining an outline of a method of setting the MG 1  command torque in the embodiment. Incidentally,  FIG. 2  shows a graph showing one example of a time-dependent change in a cranking base torque, a graph showing one example of a time-dependent change in a vibration controlling torque, and a graph showing one example of a time-dependent change in the MG 1  command torque. 
     As shown in  FIG. 2 , the ECU  100  sets the sum of the cranking base torque and the vibration controlling torque, as the MG 1  command torque. 
     The cranking base torque is a torque to be outputted by the motor generator MG 1  in order to crank the engine  200 , in other words, in order to increase the number of revolutions Ne of the engine  200 . The cranking base torque is controlled basically to be set to a first torque value BT 1  at the beginning of the cranking and to a second torque value BT 2  after the number of revolutions of the engine Ne becomes greater than the predetermined number of revolutions of the engine Ne 1 . Incidentally, the control of the cranking base torque will be explained later in detail. 
     The vibration controlling torque is a torque to be outputted by the motor generator MG 1  in order to suppress the vibration of a power train (i.e. a power transmission system including the damper  700 , the power dividing mechanism  300 , the transmission mechanism  600 , and the like for transmitting the power of the engine  200  to the drive wheels FL and FR) due to the resonance of the damper  700 . The vibration controlling torque is controlled to vary depending on the position of a piston of the engine  200 . The vibration controlling torque is controlled such that the direction of the torque varies between a case where the piston of the engine  200  is in a compression stroke and a case where the piston of the engine  200  is in an expansion stroke. More specifically, as shown in  FIG. 2 , the vibration controlling torque is controlled to reduce a torque outputted by the motor generator MG 1  if the piston of the engine  200  is in the compression stroke, and the vibration controlling torque is controlled to increase the torque outputted by the motor generator MG 1  if the piston of the engine  200  is in the expansion stroke. In other words, the vibration controlling torque is set to a negative torque value if the piston of the engine  200  is in the compression stroke, and the vibration controlling torque is set to a positive torque value if the piston of the engine  200  is in the expansion stroke. Incidentally, in  FIG. 2 , a torque value in a direction of rotating the motor generator MG 1  to crank the engine  200  is set to be positive, and a torque value in a direction of rotating it opposite to the above direction is set to be negative. By applying the vibration controlling torque as described above to the engine  200 , it is possible to suppress the vibration of the power train due to the resonance of the damper  700 . 
     Next, the control of the cranking base torque in the embodiment will be explained in detail with reference to  FIG. 3  and  FIG. 4 . 
       FIG. 3  is a flowchart showing a flow of controlling the cranking base torque in the embodiment.  FIG. 4  is a graph showing one example of the time-dependent change in the cranking base torque and the number of revolutions of the engine in the embodiment. Incidentally, the graph which shows one example of the time-dependent change in the cranking base torque also shows one example of a time-dependent change in a cylinder inner pressure P, which is a pressure within the cylinder of the engine  200 . 
     In  FIG. 3  and  FIG. 4 , if the cranking of the engine  200  is started, the number of revolutions of the engine  200  is obtained by the ECU  100  (step S 100 ). In other words, the ECU  100  obtains from the crank position sensor  810  the number of revolutions of the engine detected by the crank position sensor  810 . Incidentally, as describe above with reference to  FIG. 2 , the cranking base torque is set to the first torque value BT 1  at the beginning of the craning. By applying the torque from the motor generator MG 1 , the number of revolutions of the engine  200  is increased. 
     Then, it is judged by the ECU  100  whether or not the number of revolutions of the engine Ne is greater than the predetermined number of revolutions of the engine Ne 1  (step S 20 ). Incidentally, in  FIG. 4 , a time point at which the number of revolutions of the engine Ne reaches the predetermined number of revolutions of the engine Ne 1  is shown as a time point Tne 1 . 
     If it is judged that the number of revolutions of the engine Ne is greater than the predetermined number of revolutions of the engine Ne 1  (the step S 20 : Yes), the crank angle CA is obtained by the ECU  100  (step S 30 ). In other words, the ECU  100  obtains from the crank position sensor  810  the crank angle CA detected by the crank position sensor  810 . 
     Then, it is judged by the ECU  100  whether or not the piston of the engine  200  is at a top dead center (TDC) (step S 40 ). The ECU  100  judges whether or not the piston of the engine  200  is at the TDC on the basis of the obtained crank angle CA. Incidentally, in  FIG. 4 , a time point at which the piston of the engine  200  reaches the TDC for the first time after the time point Tne 1  at which the number of revolutions of the engine Ne reaches the predetermined number of revolutions of the engine Ne 1  is shown as a Ttdc 1 . 
     If it is judged that the piston of the engine  200  is at the TDC (the step S 40 : Yes), a falling flag of the cranking base torque is set to be ON by the ECU  100  (step S 60 ). Here, the falling flag of the cranking base torque is a flag indicating whether or not the cranking base torque is reduced from the current torque value. If the falling flag of the cranking base torque is ON, the ECU  100  reduces the cranking base torque from the current torque value, and if the falling flag of the cranking base torque is OFF, the ECU  100  maintains the cranking base torque at the current torque value. In other words, if judging that the piston of the engine  200  is at the TDC, the ECU  100  sets the falling flag of the cranking base torque to be ON and reduces the cranking base torque from the first torque value BT 1 . More specifically, as shown in  FIG. 4 , the ECU  100  controls the cranking base torque to start to be reduced from the first torque value BT 1  at the time point Ttdc 1  at which the piston of the engine  200  reaches the TDC for the first time after the number of revolutions of the engine Ne becomes greater than the predetermined number of revolutions of the engine Ne 1  (i.e. after the time point Tne 1 ) and to be the second torque value BT 2  during the expansion stroke after the TDC. 
     Thus, for example, it is possible to reduce the power consumption of the motor generator MG 1  in the expansion stroke after the number of revolutions of the engine Ne reaches the predetermined number of revolutions of the engine Ne 1 , in comparison with a case where the cranking base torque is set to the first torque value BT 1  even in the expansion stroke after the number of revolutions of the engine Ne reaches the predetermined number of revolutions of the engine Ne 1 . Therefore, it is possible to reduce the rated output of the battery  500  (the maximum value of the electric power that the battery  500  can output) for supplying the electric power to the motor generator MG 1 , thereby miniaturizing the battery  500 . The miniaturization of the battery  500  makes it possible to reduce the vehicle weight of the hybrid vehicle  10 , to improve a fuel consumption rate, and to reduce cost. 
     If it is judged that the piston of the engine  200  is not at the TDC (the step S 40 : No), it is judged by the ECU  100  whether or not the number of revolutions of the engine Ne is greater than the predetermined number of revolutions of the engine Net (step S 50 ). 
     If it is judged that the number of revolutions of the engine Ne is greater than the predetermined number of revolutions of the engine Ne 2  (the step S 50 : Yes), the falling flag of the cranking base torque is set to be ON by the ECU  100  (the step S 60 ). 
     If it is judged that the number of revolutions of the engine Ne is not greater than the predetermined number of revolutions of the engine Ne 2  (i.e. that the number of revolutions of the engine Ne is less than or equal to the predetermined number of revolutions of the engine Ne 2 ) (the step S 50 : No), the falling flag of the cranking base torque is set to be OFF by the ECU  100  (step S 70 ). In other words, the ECU  100  maintains the cranking base torque at the first torque value BT 1 . 
     On the other hand, if it is judged that the number of revolutions of the engine Ne is not greater than the predetermined number of revolutions of the engine Ne 1  (the step S 20 : No), the falling flag of the cranking base torque is set to be OFF by the ECU  100  (the step S 70 ). 
     Next, with reference to  FIG. 5 , an explanation will be given on a method of setting a MG 1  command torque by an engine starting apparatus in a comparative example and on an effect by the control of the cranking base torque in the embodiment. 
       FIG. 5  is a conceptual view for explaining an outline of the method of setting the MG 1  command torque in the comparative example. Incidentally,  FIG. 5  shows a graph showing one example of a time-dependent change in the cranking base torque in the comparative example, a graph showing one example of a time-dependent change in the vibration controlling torque in the comparative example, and a graph showing one example of a time-dependent change in the MG 1  command torque in the comparative example. 
     As shown in  FIG. 5 , the engine starting apparatus in the comparative example is configured to be different from the engine starting apparatus in the embodiment in that the cranking base torque is set to the first torque value BT 1  even in the expansion stroke after the number of revolutions of the engine Ne reaches the number of revolutions of the engine Ne 1  (i.e. after the time point Tne 1 ). The engine starting apparatus in the comparative example is configured to be substantially the same as the engine starting apparatus in the embodiment in other points. 
     According to such a comparative example, the MG 1  command torque is maximal in the expansion stroke after the time point Tne 1  at which the number of revolutions of the engine Ne reaches the number of revolutions of the engine Ne 1  (refer to a portion surrounded by a dashed line circle C 1  in  FIG. 5 ). Thus, the power to be outputted by the motor generator MG 1 , in other words, the power consumption of the motor generator MG 1  (i.e. MG 1  power consumption) is also maximal in the expansion stroke after the time point Tne 1  (refer to a portion surrounded by a dashed line circle C 2  in  FIG. 5 ). 
     Here, in the expansion stroke, the rotation of the crankshaft is accelerated by the expansion of the air compressed in the cylinder in the compression stroke. Thus, like this comparative example, if the cranking base torque is maintained at the first torque value BT 1  as in the compression stroke even in the expansion stroke, the rotation of the crankshaft is wastefully accelerated. In other words, according to the comparative example, the MG 1  power consumption is increased due to the wasteful acceleration. As a result, it is hard to reduce the rated output of the battery. 
     However, according to the embodiment, as described above, the cranking base torque is controlled to start to be reduced from the first torque value BT 1  at the time point Ttdc 1  at which the piston of the engine  200  reaches the TDC for the first time after the number of revolutions of the engine Ne becomes greater than the predetermined number of revolutions of the engine Ne 1  (i.e. after the time point Tne 1 ) and to be the second torque value BT 2  during the expansion stroke after the TDC. Thus, it is possible to reduce the power consumption of the motor generator MG 1  during the expansion stroke and to reduce the rated output of the battery  500 . 
     As explained above, according to the engine starting apparatus in the embodiment, it is possible to suppress the vibration of the power train due to the resonance of the damper  700  at the start of the engine  200  and to suppress the power consumption of the motor generator MG 1 . 
     Second Embodiment 
     An engine starting apparatus in a second embodiment will be explained with reference to  FIG. 6  and  FIG. 7 . 
       FIG. 6  is a flowchart showing a flow of controlling the cranking base torque in the second embodiment.  FIG. 7  is a graph showing one example of a time-dependent change in the cranking base torque and the number of revolutions of the engine in the second embodiment. Incidentally, in  FIG. 6 , the same steps as those in the control of the cranking base torque in the first embodiment shown in  FIG. 4  will carry the same step numbers, and the explanation thereof will be omitted as occasion demands. 
     In  FIG. 7 , the engine starting apparatus in the second embodiment is configured to be different from the engine starting apparatus in the first embodiment described above in the point of controlling the cranking base torque to start to be reduced from the first torque value BT 1  at a time point Tcs 1  at which the piston of the engine  200  reaches the compression stroke for the first time after the number of revolutions of the engine Ne becomes greater than the predetermined number of revolutions of the engine Ne 1  (i.e. after the time point Tne 1 ) and to be the second torque value BT 2  during the expansion stroke after the compression stroke. The engine starting apparatus in the second embodiment is configured to be substantially the same as the engine starting apparatus in the first embodiment described above in other points. 
     In  FIG. 6 , if it is judged that the number of revolutions of the engine Ne is greater than the predetermined number of revolutions of the engine Ne 1  (the step S 20 : Yes), the crank angle CA is obtained by the ECU  100  (the step S 30 ). Then, it is judged by the ECU  100  whether or not the piston of the engine  200  is in the compression stroke (step S 42 ). The ECU  100  judges whether or not the piston of the engine  200  is in the compression stroke on the basis of the obtained crank angle CA. 
     If it is judged that the piston of the engine  200  is in the compression stroke (the step S 42 : Yes), the falling flag of the cranking base torque is set to be ON by the ECU  100  (the step S 60 ). 
     If it is judged that the piston of the engine  200  is not in the compression stroke (the step S 42 : No), it is judged by the ECU  100  whether or not the number of revolutions of the engine Ne is greater than the predetermined number of revolutions of the engine Ne 2  (the step S 50 ). 
     In other words, in the embodiment, as shown in  FIG. 7 , the ECU  100  controls the cranking base torque to start to be reduced from the first torque value BT 1  at the time point Tcs 1  at which the piston of the engine  200  reaches the compression stroke for the first time after the number of revolutions of the engine Ne becomes greater than the predetermined number of revolutions of the engine Ne 1  (i.e. after the time point Tne 1 ) and to be the second torque value BT 2  during the expansion stroke after the compression stroke. 
     Thus, according to the embodiment, as in the first embodiment described above, for example, it is possible to reduce the power consumption of the motor generator MG 1  in the expansion stroke after the number of revolutions of the engine Ne reaches the predetermined number of revolutions of the engine Ne 1 , in comparison with the case where the cranking base torque is set to the first torque value BT 1  even in the expansion stroke after the number of revolutions of the engine Ne reaches the predetermined number of revolutions of the engine Ne 1 . 
     Third Embodiment 
     An engine starting apparatus in a third embodiment will be explained with reference to  FIG. 8  and  FIG. 9 . 
       FIG. 8  is a flowchart showing a flow of controlling the cranking base torque in the third embodiment.  FIG. 9  is a graph showing one example of a time-dependent change in the cranking base torque and the number of revolutions of the engine in the third embodiment. Incidentally, in  FIG. 8 , the same steps as those in the control of the cranking base torque in the first embodiment shown in  FIG. 4  will carry the same step numbers, and the explanation thereof will be omitted as occasion demands. 
     In  FIG. 9 , the engine starting apparatus in the third embodiment is configured to be different from the engine starting apparatus in the first embodiment described above in the point of controlling the cranking base torque to be greater than the first torque value BT 1  in at least one portion of a period in which the piston of the engine  200  is in the compression stroke for the first time after the number of revolutions of the engine Ne becomes greater than the predetermined number of revolutions of the engine Ne 1  (i.e. after the time point Tne 1 ). The engine starting apparatus in the third embodiment is configured to be substantially the same as the engine starting apparatus in the first embodiment described above in other points. 
     In  FIG. 8 , if it is judged that the number of revolutions of the engine Ne is greater than the predetermined number of revolutions of the engine Ne 1  (the step S 20 : Yes), the crank angle CA is obtained by the ECU  100  (the step S 30 ). Then, it is judged by the ECU  100  whether or not the piston of the engine  200  is in the compression stroke (step S 32 ). The ECU  100  judges whether or not the piston of the engine  200  is in the compression stroke on the basis of the obtained crank angle CA. 
     If it is judged that the piston of the engine  200  is in the compression stroke (the step S 32 : Yes), a base torque addition ΔBT according to the crank angle CA is calculated by the ECU  100  (step S 34 ). In other words, if judging that the piston of the engine  200  is in the compression stroke, the ECU  100  calculates the base torque addition ΔBT according to the crank angle CA and adds the calculated base torque addition ΔBT to the cranking base torque. Namely, as shown in  FIG. 9 , the ECU  100  controls the cranking base torque to be a third torque value BT 3  which is greater than the first torque value BT 1  in at least one portion of the period in which the piston of the engine  200  is in the compression stroke for the first time after the number of revolutions of the engine Ne becomes greater than the predetermined number of revolutions of the engine Ne 1  (i.e. after the time point Tne 1 ). Incidentally, the third torque value BT 3  is a value obtained by adding the base torque addition ΔBT to the first torque value BT 1 . 
     Thus, according to the embodiment, it is possible to reduce or prevent that an increase in the number of revolutions of the engine is suppressed by the compressed air in the cylinder of the engine  200 , in the period in which the piston of the engine  200  is in the compression stroke. This makes it possible to suppress an increased difference in the rate of increase in the number of revolutions of the engine between the period in which the piston of the engine  200  is in the compression stroke and a period in which the piston of the engine is in the subsequent expansion stroke. Thus, the vibration of the power train for transmitting the power of the engine  200  can be also suppressed. 
       FIG. 10  is a graph showing one example of a time-dependent change in the cranking base torque and the number of revolutions of the engine in the comparative example described above with reference to  FIG. 5 . 
     In  FIG. 10 , the engine starting apparatus in the comparative example sets the cranking base torque to the first torque value BT 1  even in the expansion stroke after the number of revolutions of the engine Ne reaches the number of revolutions of the engine Ne 1  (i.e. after the time point Tne 1 ), as described above with reference to  FIG. 5 . 
     Here, in the expansion stroke, the rotation of the crankshaft is accelerated by the expansion of the air compressed in the cylinder in the compression stroke. Thus, like this comparative example, if the cranking base torque is maintained at the first torque value BT 1  as in the compression stroke even in the expansion stroke, that increases a difference in the rate of increase in the number of revolutions of the engine between the period in which the piston of the engine  200  is in the compression stroke and the period in which the piston of the engine is in the subsequent expansion stroke (in other words, between before and after the time point Ttdc 1 ) (refer to a portion surrounded by a dashed line circle C 3  in  FIG. 10 ). This may increase the vibration of the power train for transmitting the power of the engine  200 . 
     However, according to the embodiment, as described above, the cranking base torque is controlled to be the third torque value BT 3  which is greater than the first torque value BT 1  in the period in which the piston of the engine  200  is in the compression stroke for the first time after the number of revolutions of the engine Ne becomes greater than the predetermined number of revolutions of the engine Ne 1  (i.e. after the time point Tne 1 ). Thus, it is possible to suppress an increased difference in the rate of increase of in the number of revolutions of the engine between before and after the time point Ttdc 1 . 
     The present invention is not limited to the aforementioned embodiments, but various changes may be made, if desired, without departing from the essence or spirit of the invention which can be read from the claims and the entire specification. An engine starting apparatus, which involves such changes, is also intended to be within the technical scope of the present invention. 
     DESCRIPTION OF REFERENCE CODES 
     
         
           10  hybrid vehicle 
           100  ECU 
           200  engine 
           210  crankshaft 
           300  power dividing mechanism 
           400  PCU 
           500  battery 
           600  transmission mechanism 
           610  differential gear 
           620  transmission shaft 
           700  damper 
           810  crank position sensor 
         FL, FR drive wheel 
         MG 1 , MG 2  motor generator