Abstract:
In an engine starting apparatus, together with a one-way clutch, a pinion is pushed toward a ring gear of an engine mounted in a vehicle. The one-way clutch has an idling torque smaller than a torque of the ring gear that tries to turn the pinion when the pinion is pushed to the ring gear. By a control device, a pinion pushing device is enabled to operate when i) the revolution speed of the ring gear is larger than a revolution speed of the pinion and ii) a relative revolution speed between the revolution speed of the ring gear and the revolution speed of the pinion is a desired value.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims the benefit of priority from earlier Japanese Patent Application No. 2009-106554 filed Apr. 24, 2009, the description of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Technical Field of the Invention 
     The present invention relates to an engine starting apparatus which is able to engage a pinion of a starter with a freewheeling ring gear in the course of an engine stop process to restart the engine. 
     2. Related Art 
     Providing vehicles with an idle stop system is an important approach to reducing CO 2  as one of the countermeasures against global warming. The idle stop system is a system, for example, that stops fuel injection to an engine to automatically stop the engine when the vehicle is stopped at an intersection due to a stop signal or in pause due to traffic jam or the like. 
     Conventional idle stop systems have been configured to automatically stop an engine after the vehicle has been fully stopped. In order to further improve the effect of reducing CO 2 , it is effective to elongate an engine stop period. Elongating the engine stop period may be specifically achieved by a system that stops an engine before the vehicle speed runs out (i.e. during the deceleration preceding the vehicle stop), converting from the conventional systems that stop the engine after the vehicle has been fully stopped. It is expected that such a system that elongates the engine stop period may significantly improve the effect of reducing CO 2 , compared to the conventionally used idle stop systems. 
     However, this system raises an issue incurred in a potential restart of an engine after the engine has entered an engine stop process. Specifically, in conventional starters, the pinion of the starter cannot be engaged with the ring gear of the engine unless the engine is fully stopped. This means that, in the case where an engine is restarted using a conventional starter, the engine cannot be restarted from the point when the engine has entered the engine stop process up to the point when the engine is completely stopped. There may be a situation, for example, that the traffic light at an intersection is red and the vehicle is decelerated accordingly followed by the output of a stop command to allow the engine to enter the engine stop process, and that, then, the traffic light quickly turns green. In such a situation, conventional starters cannot immediately restart the engine, which may cause trouble to the following vehicle and impose a psychological burden on the user. Accordingly, in order to use the idle stop function while the vehicle is decelerating, it is essential to enable restart of the engine when the engine is in the engine stop process. 
     In order to realize restart during the engine stop process, the pinion of the starter is required to be in engagement with a ring gear in rotation. A technique as a method of realizing such a restart is disclosed in WO2007/101770. Specifically, this patent document discloses a method of restarting an engine using a starting device that includes a first RPM detecting means that detects the number of revolutions of a ring gear, a second RPM detecting means that detects the number of revolutions of the pinion of a starter or the number of revolutions of a motor, and a motor revolution control driver that controls the number of revolutions of the motor. In this starting device, the number of revolutions of the pinion is controlled by the motor revolution control driver based on the number of revolutions detected by the first and second RPM detecting means, for synchronization with the number of revolutions of the ring gear. As a result, the pinion is engaged with the ring gear. 
     The method disclosed in WO2007/101770 (the method of synchronizing the number of revolutions of a pinion with that of a ring gear to establish engagement between the gears) is an ideal method in the case where gears distanced from each other are brought into engagement with each other. However, this method has a large problem of requiring a motor revolution control driver that controls the number of revolutions of a motor. Generally, an MOS transistor as a control element is used as a motor revolution control driver to perform voltage control (e.g., pulse width control, so-called PWM control). However, starter motors have a low voltage (usually 12 V) in spite of having a large output. Therefore, this necessitates the use of an MOS transistor having a large current capacity exceeding 500 A and thus greatly raises the cost as a result. 
     In addition, achieving synchronization between the numbers of revolutions of a pinion and a ring gear may require feedback control of the numbers of revolutions. As a result, a long time will be taken for the synchronization. Therefore, in many cases, there is a concern that synchronization is unlikely to be completed during the very short time in which the engine speed is decreasing. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in light of the problems set forth above and has as its object to provide an on-vehicle engine starting apparatus which is able to engage a starter&#39;s pinion with an engine&#39;s ring gear, which is in the state of decreasing revolutions, during the short time of an engine stop process to thereby restart the engine. 
     In order to achieve the object, an engine starting apparatus is provided which comprises an electric motor which receives current to generate a rotational force, an output shaft that has an outer periphery surface and rotates by the rotational force, a one-way clutch that is helical-spline-fitted to the outer periphery surface of the output shaft, a pinion that receives the rotational force via the one-way clutch, a pinion pushing device that pushes, together with the one-way clutch, the pinion toward a ring gear of an engine, the one-way clutch having an idling torque smaller than a torque of the ring gear that tries to turn the pinion when the pinion is pushed to the ring gear, and a current switching device that turns on/off the current supplied to the motor. The apparatus further comprises a revolution speed detecting device that detects a revolution speed of the ring gear, and a control device. The control device enables the pinion pushing device to operate when the revolution speed of the ring gear detected by the revolution speed detecting device is larger than a revolution speed of the pinion acquired from a revolution speed of the motor and a relative revolution speed between the revolution speed of the ring gear and the revolution speed of the pinion is a desired value. This control device is able to control the operations of the pinion pushing device and current switching device independently from each other. 
     In the case where engine restart is requested while the number of revolutions of the ring gear is decreasing in an engine stop process, the engine starting apparatus of the present invention actuates the pinion pushing device when the ring gear and the pinion rotate at predetermined relative numbers of revolutions (the number of revolutions of the ring gear&gt;the number of revolutions of the pinion) to thereby allow the pinion to be pushed to the ring gear side integrally with the one-way clutch. 
     The actuation of the pinion pushing device brings the end face of the pinion into contact with the end face of the ring gear. When the pinion is pressed against the ring gear being applied with a predetermined load, the number of revolutions of the pinion instantaneously synchronizes with that of the ring gear with the idling of the one-way clutch. This is because the rotational torque of the one-way clutch in an idling state is set smaller than the rotational torque with which the ring gear attempts to rotate the pinion. 
     From the instance of the synchronization as well, the revolutions of the ring gear still continue decreasing. In this case, however, the pinion will not decrease revolutions synchronized with the revolutions of the ring gear because the one-way clutch is on the connecting side (torque transmitting side). Accordingly, the ring gear will separate from the pinion in the direction opposite to the direction of revolutions, whereby engagement is established between the pinion and the ring gear. 
     It should be appreciated that the engine speed does not have to be directly detected, but a crank angle sensor or the like may be used. 
     It is preferred that, in the foregoing configuration, the output shaft provides an axial direction which is along a longitudinal direction of the output shaft, the ring gear has a first periphery surface on which a plurality of teeth are formed, the teeth of the ring gear having a first axial end face facing the pinion and being directed in the axial direction, the pinion has a second periphery surface on which a plurality of teeth are formed, the teeth of the pinion having a second axial end face facing the ring gear and being directed in the axial direction, and recesses are formed on at least one of the first axial end face and the second axial end face and formed in a direction crossing a rotational direction of the ring gear and the pinion. 
     With this configuration, the pinion is pushed with the actuation of the pinion pushing device. Then, when the end face of the pinion comes into contact with the end face of the ring gear, the recess formed in the pinion end face, for example, will be caught by the teeth of the ring gear. In this way, the revolutions of the pinion can instantaneously follow (synchronize with) those of the ring gear, thereby promptly establishing engagement. 
     It is also preferred that frictional coefficient increasing means is formed on at least one of the first axial end face and the second axial end face to increase a frictional force thereon. 
     With this configuration, the pinion is pushed with the actuation of the pinion pushing device. Then, when the end face of the pinion comes into contact with the end face of the ring gear, frictional force between the both end faces will be increased by the frictional coefficient increasing means. In this way, the revolutions of the pinion can instantaneously follow (synchronize with) those of the ring gear, thereby promptly establishing engagement. 
     Preferably, the recesses are chamfered portions formed at least one of the ring gear and the pinion, the chamfered portions being at least one of i) chamfered portions crossing both the first periphery surface and the first axial end face and ii) chamfered portions crossing both the second periphery surface and the second axial end face. 
     With this configuration, it is highly probable that the teeth of the pinion and the teeth of the ring gear are caught with each other after in the axial direction after the pinion has come into contact with the ring gear. Thus, reliability in the synchronization of the revolutions between the pinion and the ring gear can be enhanced. In a vehicle having an idle stop function, it is required to consider the case where the engine may be started without using the idle stop function, i.e. started in a conventional manner, for a certain number of times. In this regard, formation of the chamfered portions can ensure the engagement performances based on both of the startup using the idle stop function and the startup in the conventional manner. 
     Still preferably, the frictional coefficient increasing means is composed of a plurality of grooves. It is preferred that each of the grooves has a depth which is smaller than a module of the pinion and the ring gear. For example, the depth is smaller than 1/n of the module (n is a positive integer of 9 or less). The module is a size (i.e., height) of each tooth of each of the pinion and ring gear. 
     With this configuration, the frictional coefficient increasing means can be easily formed using a means, such as a knurling tool, which can facilitate processing. 
     It is also preferred that the motor is a brush type of DC motor having an armature, a rectifier arranged at the armature, a brush made in contact with a surface of the rectifier, and a spring pushing the brush to the surface of the rectifier, wherein the armature has a torque larger than the idling torque of the one-way clutch. 
     With the actuation of the pinion pushing device, the pinion is pressed by the ring gear and thus the revolutions of the pinion will follow and synchronize with the revolutions of the ring gear. After the synchronization as well, the ring gear still continues decreasing the number of revolutions. Thus, the torque of the ring gear works on the pinion such that the revolutions of the pinion are decreased. In this regard, since the one-way clutch structured integrally with the pinion is on the connecting side (torque transmitting side), the torque that works on the pinion such that the revolutions of the pinion are decreased will be transmitted to the motor side. 
     Meanwhile, in the motor of the present invention, a braking force works on the revolutions of the armature when the brush is pressed against the surface of the rectifier by the brush spring. Accordingly, the armature is unlikely to be rotated from the ring gear side. As a result, the pinion will not decrease its revolution speed synchronizing with the decreasing revolutions of the ring gear. This will permit easy deviation between the teeth of the pinion and the teeth of the ring gear. Thus, the time required for achieving engagement between the pinion and the ring gear can be shortened. 
     Preferably, the engine starting apparatus further comprises a reduction device which reduces a rotational speed of the motor and transmits the reduced rotational speed of the motor to the output shaft. 
     The torque of the ring gear, which works on the pinion such that the revolutions of the pinion are decreased, may be transmitted to the motor side. In such a case, an arrangement of the reduction gear between the motor and the output shaft may allow the armature to be more unlikely to be rotated from the ring gear side. Thus, it is ensured that the teeth of the pinion and the teeth of the ring gear are easily deviated (separated), whereby the time taken for completing engagement between the pinion and the ring gear is further shortened. 
     Preferably, the control device includes a delay device that allows the current switching device to start to operate when a predetermined period of time has passed since the start of a pushing operation of the pinion. 
     According to the present invention, the pinion can be fully engaged with the ring gear and then, in this fully engaged state, current is passed to the motor to start the engine. Thus, the pinion and the ring gear can be prevented from being damaged due to potential incomplete engagement therebetween when the revolutions of the ring gear are decreasing in the engine stop process. As a result, the life of each of the gears can be improved in a vehicle having an idle stop function, in which the starter is actuated for a number of times. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a general view, with partly cut, illustrating a starter incorporated in an engine starting apparatus according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view illustrating a pinion-pushing solenoid and a motor electrification switch of the starter; 
         FIG. 3  is an electric circuit diagram illustrating the engine starting apparatus of the starter; 
         FIGS. 4A to 4D  are explanatory views illustrating an operation in a first situation, in which a pinion engages with a ring gear which is decreasing revolutions in an engine stop process; 
         FIGS. 5A to 5D  are explanatory views illustrating an operation in a second situation, in which a pinion engages with a ring gear which is decreasing revolutions in an engine stop process; 
         FIG. 6  is a graph illustrating engine speed in an engine stop process with time being indicated on the horizontal axis; 
         FIG. 7  is a diagram illustrating the ring gear and the pinion as viewed from the axial direction; 
         FIG. 8  is a diagram illustrating an example of a frictional coefficient increasing means formed in a pinion end face; and 
         FIG. 9  is a schematic diagram illustrating the configuration of a motor with a brush. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the accompanying drawings, hereinafter will be described embodiments of an engine starting apparatus according to the present invention. 
     Referring to  FIGS. 1 to 9 , an embodiment of the engine starting apparatus will now be described. 
     The engine starting apparatus is used for an idle stop system that automatically controls stop and restart of an on-vehicle engine. The engine starting apparatus includes a starter  1  (shown in  FIG. 1 ), an ECU (electronic control unit)  2  (shown in  FIG. 3 ), and an RPM detector  4  (shown in  FIG. 3 ). The starter  1  starts an engine (i.e., internal combustion engine) mounted on a vehicle. The ECU  2  controls the operation of the starter  1 . The RPM detector  4  detects a signal indicative of the number of revolutions of a ring gear  3  attached to a crank shaft of the engine and outputs the detected signal to the ECU  2 . 
     As shown in  FIG. 1 , the starter  1  includes an electric motor  5 , an output shaft  6 , a pinion movable body (described later), a shift lever  7 , a pinion-pushing solenoid  8 , a battery  9 , and a motor electrification switch  10 . In the present embodiment, directions can be defined such that longitudinal directions of the output shaft  6  are axial directions AX, radially extending directions from the output shaft  6  along a plane perpendicular to the axial directions are radial directions RA, and directions circulating around the axial directions along the plane perpendicular to the axial directions are circumferential directions CR. 
     The motor  5  generates torque in response to current supply thereto. The output shaft  6  rotates being transmitted with the torque generated by the motor  5 . The pinion movable body is provided such that it is axially movable on the outer periphery of the output shaft  6 . The pinion-pushing solenoid  8  has a function of pushing the pinion movable body in the direction opposite to the motor (leftward in  FIG. 1 ) via the shift lever  7 . The motor electrification switch  10  opens/closes a motor contact which is provided at a motor circuit to pass current from the battery  9  (see  FIG. 3 ) to the motor  5 . 
     The motor  5  is an electric dc motor with a brush, including a field magnet  11 , armature  14  and a brush  16 . The field magnet  11  is configured by a plurality of permanent magnets. The armature  14  includes an armature shaft  12  with its one end being provided with a rectifier  13 . The brush  16  is arranged being in contact with an outer peripheral surface of the rectifier  13  (hereinafter referred to as a “rectifier surface”) and pressed against the rectifier surface by a brush spring  15  (see  FIG. 9 ). The field magnet  11  of the motor  5 , which is made up of the permanent magnets, may be replaced by a field electromagnet made up of a field coil. 
     The output shaft  6  is arranged being aligned with the armature shaft  12  via a reduction gear  17 . Thus, the revolutions of the motor  5  are transmitted being reduced by the reduction gear  17 . 
     The reduction gear  17  is a known planetary reduction gear, for example, in which a planetary carrier  17   b  that picks up the orbital motion of a planetary gear  17   a  is provided being integrated with the output shaft  6 . 
     The pinion movable body is configured by a clutch  18  and a pinion  19 . 
     The clutch  18  includes a spline sleeve  18   a , an outer  18   b , an inner  18   c , a roller  18   d  and a roller spring (not shown). The spline sleeve  18   a  is helical-spline-fitted to the outer periphery of the output shaft  6 . The outer  18   b  is provided being integrated with the spline sleeve  18   a . The inner  18   c  is relatively rotatably arranged at the inner periphery of the outer  18   b . The roller  18   d  is located between the outer  18   b  and the inner  18   c  to connect/disconnect torque therebetween. The roller spring has a role of biasing the roller  18   d . The clutch  18  is provided as a one-way clutch that unidirectionally transmits torque from the outer  18   b  to the inner  18   c  via the roller  18   d.    
     The pinion  19  is integrated with the inner  18   c  of the clutch  18  and relatively rotatably supported by the outer periphery of the output shaft  6  via bearings  20 . 
     The pinion-pushing solenoid  8  and the motor electrification switch  10  have a solenoid coil  21  and a switch coil  22 , respectively, each of which forms an electromagnet when current is passed. A fixed core  23  is arranged between the solenoid coil  21  and the switch coil  22  so as to be commonly used by these coils. The outer periphery of the pinion-pushing solenoid  8  is covered with a solenoid yoke  24 , while the outer periphery of the motor electrification switch  10  is covered with a switch yoke  25 . The solenoid yoke  24  and the switch yoke  25  are integrally and continuously formed in the axial directions AX to provide a single overall yoke. In other words, as shown in  FIG. 1 , the solenoid  8  and the switch  10  are integrally configured in series in the axial directions AX, disposed being parallel to the motor  5 , and fixed to a starter housing  26 . 
       FIG. 2  is a cross-sectional view illustrating the pinion-pushing solenoid  8  and the motor electrification switch  10  of the starter  1 . As shown in  FIG. 2 , the overall yoke has a bottomed cylindrical shape with one axial end (first end E 1 ) (left side in  FIG. 2 ) being provided with an annular bottom and the other axial end (second end E 2 ) being opened. The outer diameter of the overall yoke is made even from the first end E 1  to the second end E 2 . However, the inner diameter of the switch yoke  25  is ensured to be larger than that of the solenoid yoke  24 . Accordingly, the thickness of the switch yoke  25  is smaller than that of the solenoid yoke  24 . In other words, the inner peripheral surface of the overall yoke has a step between the solenoid yoke  24  and the switch yoke  25 . 
     The fixed core  23  is inserted from an open end that is the second end E 2  of the overall yoke (open end of the switch yoke  25 ) into the inside of the switch yoke  25 . The inserted fixed core  23  has a radially outer end face on the first end E 1  side. This radially outer end face is brought into contact with the step provided at the inner peripheral surface of the overall yoke, between the solenoid yoke  24  and the switch yoke  25 , to determine the axial position of the fixed core  23 . 
     Referring to  FIGS. 2 and 3 , hereinafter is described the configurations of the pinion-pushing solenoid  8  and the motor electrification switch  10 , except for the overall yoke (the solenoid yoke  24  and the switch yoke  25 ) and the fixed core  23 . 
     The pinion-pushing solenoid  8  includes the solenoid coil  21 , a plunger  27  and a joint  28 . The solenoid coil  21  is arranged along the inner periphery of the solenoid yoke  24  that forms a part of the overall yoke on the first end E 1  side. The plunger  27  is disposed being opposed to one radially inner attractive surface S 1  of the fixed core  23  and is permitted to be axially movable along the inner periphery of the solenoid coil  21 . The joint  28  transmits the movement of the plunger  27  to the shift lever  7 . 
       FIG. 3  is an electric circuit diagram illustrating the engine starting apparatus of the starter  1 . As shown in  FIG. 3 , the solenoid coil  21  has an end connected to a connector terminal  29  and the other end grounded being fixed to a surface of the fixed core  23 , for example, by welding or the like. An electrical wiring connected to a starter relay  30  is connected to the connector terminal  29 . 
     The starter relay  30  is subjected to on/off control of the ECU  2 . When the starter relay  30  is controlled and turned on, current is passed from the battery  9  to the solenoid coil  21  via the starter relay  30 . 
     When the fixed core  23  is magnetized with the supply of current to the solenoid coil  21 , the plunger  27  is attracted to the attractive surface S 1  of the fixed core  23  against the reaction force of a return spring  31  disposed between the fixed core  23  and the plunger  27 . Then, when the current supply to the solenoid coil  21  is stopped, the plunger  27  is pushed back by the reaction force of the return spring  31  in the direction opposite to the fixed core  23  (leftward in  FIG. 2 ). The plunger  27  has substantially a cylindrical shape with a cylindrical hole being formed at its radially central portion. The cylindrical hole is open at one axial end of the plunger  27  and bottomed at the other end thereof. 
     The joint  28  having a shape of a rod is inserted into the cylindrical hole of the plunger  27  together with a drive spring (not shown). Thus, the joint  28  has an end portion projected from the cylindrical hole of the plunger  27 . This end portion of the joint  28  is formed with an engagement groove  28   a  with which one end portion of the shift lever  7  engages. The other end portion of the joint  28  is provided with a flange portion. The flange portion has an outer diameter that enables the flange portion to be slidably movable along the inner periphery of the cylindrical hole. The flange portion, being loaded by the drive spring, is being pressed against the bottom face of the cylindrical hole. 
     With the movement of the plunger  27 , an end face  19   a  (see  FIG. 1 ) of the pinion  19  pushed in the direction opposite to the motor comes into contact with an end face  3   a  (see  FIG. 1 ) of the ring gear  3 . Then, the drive spring is permitted to bow while the plunger  27  is permitted to move and attracted to the attractive surface S 1  of the fixed core  23 . Thus, the drive spring accumulates reaction force that allows the pinion  19  to engage the ring gear  3 . 
     The motor electrification switch  10  includes the switch coil  22 , a movable core  32 , a contact cover  33 , two terminal bolts  34  and  35 , a pair of fixed contacts  36 , and a movable contact  37 . The switch coil  22  is arranged along the inner periphery of the switch yoke  25  forming a part of the overall yoke on the second end E 2  side. The movable core  32  faces the other radially inner attractive surface  52  of the fixed core  23  and is permitted to be movable in the axial directions AX of the switch coil  22 . The contact cover  33 , which is made of resin, is assembled, blocking the open end, i.e. the second end E 2 , of the overall yoke (the open end of the switch yoke  25 ). The two terminal bolts  34  and  35  are fixed to the contact cover  33 . The pair of fixed contacts  36  are fixed to the two terminal bolts  34  and  35 . The movable contact  37  electrically connects/disconnects between the pair of fixed contacts  36 . 
     As shown in  FIG. 3 , the switch coil  22  has one end connected to an external terminal  38 , and the other end grounded being fixed, for example, to a surface of the fixed core  23  by welding or the like. The external terminal  38  is provided being projected out of an axial end face of the contact cover  33 , for connection to an electrical wiring connected to the ECU  2 . 
     The switch coil  22  has a radially outer peripheral side on which an axial magnetic path member  39  is arranged to form a part of a magnetic path. Also the switch coil  22  has an axial side opposite to the fixed core, on which a radial magnetic path member  40  is arranged to form a part of the magnetic path. 
     The axial magnetic path member  39  has a cylindrical shape and is inserted into the switch yoke  25  along the inner periphery thereof with substantially no gap being provided therebetween. The axial magnetic path member  39  has an axial end face on the first end E 1  side, which axial end face is brought into contact with a radially outer end face of the fixed core  23  to determine the axial position of the member  39 . 
     The radial magnetic path member  40  is arranged perpendicular to the axis of the switch coil  22 . The radial magnetic path member  40  has a radially outer end face on the first end E 1  side, which surface is brought into contact with an axial end face of the axial magnetic path member  39  to constrain the position of the member  40  with respect to the switch coil  22 . The radial magnetic path member  40  has a round opening at its radial central portion so that the movable core  32  can move therethrough in the axial directions AX. 
     The fixed core  23  is magnetized upon supply of current to the switch coil  22 . Then, the movable core  32  is attracted to the attractive surface S 2  of the fixed core  23  against the reaction force of the return spring  41  disposed between the fixed core  23  and the movable core  32 . When the current supply to the switch coil  22  is stopped, the movable core  32  is pushed back in the direction opposite to the fixed core  23  (rightward in  FIG. 2 ) by the reaction force of the return spring  41 . 
     The contact cover  33  has a cylindrical leg portion  33   a . The leg portion  33   a  is inserted into the switch yoke  25  along the inner periphery thereof, the switch yoke  25  forming a part of the overall yoke on the second end E 2  side. The contact cover  33  is arranged, with the end face of the leg portion  33   a  being in contact with a surface of the radial magnetic path member  40 , and caulked and fixed to the open end, i.e. the second end E 2 , of the overall yoke. 
     The terminal bolt  34 , one of the two terminal bolts, is a B terminal bolt  34  to which a battery cable  42  (see  FIG. 3 ) is connected. The terminal bolt  35 , the other of the two terminal bolts, is an M terminal bolt  35  to which a motor lead  43  (see  FIGS. 1 and 3 ) is connected. The pair of fixed contacts  36 , which are provided separately from (or may be provided integrally with) the two terminal bolts  34  and  35 , are electrically in contact with the two terminal bolts  34  and  35  inside the contact cover  33  and mechanically fixed to the contact cover  33 . 
     The movable contact  37  is arranged so that the distance from the movable contact  37  to the movable core is larger than the distance from the pair of fixed contacts  36  to the movable core (rightward in  FIG. 2 ). The movable contact  37  is in reception of the load of a contact-pressure spring  45  and pressed against an end face of a resin rod  44  fixed to the movable core  32 . It should be appreciated that the initial load of the return spring  41  is set larger than that of the contact-pressure spring  45 . Therefore, when the switch coil  22  is de-energized, the movable contact  37  is seated on an inner seat  33   b  of the contact cover  33 , with the contact-pressure spring  45  being contracted. 
     The motor contact mentioned above is formed of the pair of fixed contacts  36  and the movable contact  37 . Being biased by the contact-pressure spring  45 , the movable contact  37  comes into contact with the pair of fixed contacts  36  with a predetermined pressing force. Resultantly, current is passed across the pair of fixed contacts  36  via the movable contact  37  to thereby dose the motor contact. When the movable contact  37  is drawn apart from the pair of fixed contacts  36 , the current across the pair of fixed contacts  36  is shut down to thereby open the motor contact. 
     a) Referring to  FIGS. 4A to 4D  and  FIGS. 6 to 9 , an operation is described taking as an example a first situation in which engine restart is requested while the number of revolutions of the ring gear  3  is decreasing in an engine stop process. 
       FIG. 4A  illustrates a process in which the pinion  19  moves forward to the ring gear  3  which is decreasing the number of revolutions.  FIG. 4B  illustrates a state where the end face  19   a  of the pinion  19  is in contact with the end face  3   a  of the ring gear  3 .  FIG. 4C  illustrates a process in which the positions of the pinion  19  and the ring gear  3  are relatively deviated in the direction of revolutions.  FIG. 4D  illustrates a state where the pinion  19  is brought into engagement with the ring gear  3  in a decelerating state. 
       FIG. 6  is a graph illustrating engine speed Neg in the engine stop process with time being indicated on the horizontal axis. In  FIG. 6 , “X” indicates a point of generation of an engine stop signal, “Cm” indicates a point when an engine restart request is given by the driver&#39;s free will, “Sp” indicates an actuation start point of the pinion-pushing solenoid  8 , “δN” indicates relative numbers of revolutions of the ring gear  3  and the pinion  19 , and “Mp” indicates an actuation start point of the motor electrification switch  10 . 
     After generation of an engine stop signal at the point X of  FIG. 6 , an engine restart request may be given by the driver at the point Cm. Then, the ECU  2  permits the RPM detector  4  to input the number of revolutions of the ring gear  3  at the time the request has been given. If the number of revolutions of the ring gear  3  is lower than a predetermined number of revolutions, the starter relay  30  is controlled and turned on at the point (point Sp of  FIG. 6 ) when the relative numbers of revolutions of the ring gear  3  and the pinion  19  have reached δN. At this point, the number of revolutions of the motor  5  is “0” because the motor electrification switch  10  has not been actuated (no current is passed to the switch coil  22 ). Accordingly, the relative numbers of revolutions will be expressed as: δN=the number of revolutions of the ring gear  3 . 
     When the starter relay  30  is closed, current is supplied from the battery  9  to the solenoid coil  21  of the pinion-pushing solenoid  8 . Then, the plunger  27  is moved, being attracted to the magnetized fixed core  23 . With the movement of the plunger  27 , the pinion movable body (the clutch  18  and the pinion  19 ) is pushed in the direction opposite to the motor via the shift lever  7 . Then, as shown in  FIGS. 4A and 4B , the pinion  19  moves forward to the ring gear  3  which is decreasing the number of revolutions. As a result, the end face  19   a  of the pinion  19  is pressed against the end face  3   a  of the ring gear  3  applied with a predetermined load F 1 . In this case, a rotational torque T 1  with which the ring gear  3  attempts to rotate the pinion  19  can be expressed by the following Formula (1):
 
 T 1= F 1× rp×μ 1  (1)
 
where μ1 is a frictional coefficient between the end face  19   a  of the pinion  19  and the end face  3   a  of the ring gear, rp is a pitch circle radius of the pinion  19  (see  FIG. 7 ).
 
     In this case, a rotational torque T 2  of the clutch  18  in an idling state may be set smaller than the rotational torque T 1  (T 1 &gt;T 2 ). Thus, the revolutions of the pinion  19  catch up and synchronize with the revolutions of the ring gear  3 . In this regard, at least either the end face  19   a  of the pinion  19  or the end face  3   a  of the ring gear  3  may be formed with a frictional coefficient increasing means, so that the frictional coefficient may be increased at each of the teeth of either the pinion  19  or the ring gear  3 . 
     For example, as shown in  FIG. 8 , which is an illustration of the end face  19   a  of the pinion  19 , a plurality of grooves  19   b  may be formed in the end face  19   a . In this case, each of the grooves  19   b  may have a depth which is smaller than a module of the pinion and the ring gear. Preferably, the depth is smaller than 1/n of the module (n is a positive integer of 9 or less). The module is defined as a size (i.e., height) of each tooth of each of the pinion and ring gear. Thus, the frictional force between the end face  19   a  of the pinion  19  and the end face  3   a  of the ring gear  3  will be increased when both of the end faces are brought into contact with each other. Accordingly, the revolutions of the pinion  19  can instantaneously synchronize with the revolutions of the ring gear  3 . 
     From the point of synchronization as well, the ring gear  3  still continues decreasing revolutions. However, since the clutch  18  is now on the connecting side (torque transmitting side), the rotational torque which is received by the pinion  19  from the ring gear  3  will be a torque T 3  that rotates the armature  14  of the motor  5 .  FIG. 9  is a schematic diagram illustrating the configuration of the motor  5  with a brush. As shown in  FIG. 9 , in the case where the brush  16  is pressed against the outer periphery of the rectifier  13  having a radius rc with a frictional coefficient μc, the rotational torque T 3  that rotates the armature  14  can be expressed by the following formula (2):
 
 T 3= F 2× rc×μc   (2)
 
     In this case, the rotational torque T 3  for rotating the armature  14  may be set larger than the rotational torque T 2  of the clutch  18  in an idling state (T 3 &gt;T 2 ). Thus, the frictional force caused between the end faces of the pinion  19  and the ring gear  3  will be smaller than the rotational torque T 3  that rotates the armature  14 . Therefore, the pinion  19  will not decrease the number of revolutions keeping synchronization with the revolutions of the ring gear  3 . Instead, as shown in  FIG. 4C , the ring gear  3  will be deviated with respect to the pinion  19  in the direction opposite to the direction of revolutions (rightward in  FIG. 4C ). As a result, as shown in  FIG. 4D , each of the teeth of the pinion  19  is pushed between the teeth of the ring gear  3  to thereby achieve engagement between the pinion  19  and the ring gear  3 . 
     After completion of the engagement between the pinion  19  and the ring gear  3  and then after expiration of a predetermined time (point Mp of  FIG. 6 ), the ECU  2  outputs a turn-on signal to the motor electrification switch  10 . 
     When current is passed through the switch coil  22  of the switch  10 , the movable core  32  is attracted to the fixed core  23  to allow the movable contact  37  to come into contact with the pair of fixed contacts  36 . Then, being biased by the contact-pressure spring  45 , the motor contact is closed. As a result, current is supplied from the battery  9  to the motor  5  to generate torque in the armature  14 . The torque is then transmitted to the output shaft  6  via the reduction gear  17 . Further, the torque of the output shaft  6  is transmitted to the pinion  19  via the clutch  18 . Since the pinion  19  has already been in engagement with the ring gear  3 , the revolutions of the pinion  19 , as they are, are transmitted to the ring gear  3 . In this way, as plotted with the broken line in the graph of  FIG. 6 , the engine speed Neg increases, whereby the engine is restarted. 
     b) Referring to  FIGS. 5A to 5D , an operation is described taking as an example a second situation in which engine restart is requested while the number of revolutions of the ring gear  3  is decreasing in an engine stop process. 
     In the second situation, when the pinion movable body (the clutch  18  and the pinion  19 ) is pushed to the ring gear side with the actuation of the pinion-pushing solenoid  8 , a chamfered portion  19   c  formed in each of the teeth of the pinion  19  is caught by a chamfered portion  3   b  formed in each of the teeth of the ring gear  3 . The chamfered portion  19   c  of the pinion  19  and the chamfered portion  3   b  of the ring gear are also examples of the recesses recited in claim  2  of the present invention. As shown in  FIG. 5B , the chamfered portion  19   c  is formed at a corner of each tooth of the pinion  19 , and the chamfered portion  3   b  is formed at a corner of each tooth of the ring gear  3 . These chamfered portions (the recesses of the present invention) may be formed in either one of the pinion  19  and the ring gear  3 . 
     As shown in  FIG. 5B , in the second situation, when each chamfered portion  19   c  of the pinion  19  is caught by each chamfered portion  3   b  of the ring gear  3 , the revolutions of the pinion  19  instantaneously synchronize with the revolutions of the ring gear  3 . In this regard, similar to the first situation, the rotational torque T 2  of the clutch  18  in an idling state is set smaller than the rotational torque T 1  that rotates the pinion  19  from the ring gear  3  side, while the rotational torque T 3  that rotates the armature  14  is set larger than the rotational torque T 2  of the clutch  18  in an idling state. 
     Even from the instant when the revolutions of the pinion  19  synchronize with the revolutions of the ring gear  3 , the number of revolutions of the ring gear  3  still continues decreasing. Accordingly, as shown in  FIG. 5C , the ring gear  3  will be deviated with respect to the pinion  19  in the direction opposite to the direction of revolutions (rightward in  FIG. 5C ). As a result, as shown in  FIG. 5D , each of the teeth of the pinion  19  is pushed between the teeth of the ring gear  3  to thereby achieve engagement between the pinion  19  and the ring gear  3 . After completion of the engagement between the pinion  19  and the ring gear  3  and then after expiration of a predetermined time (point Mp of  FIG. 6 ), the ECU  2  outputs a turn-on signal to the motor electrification switch  10 . Resultantly, the torque of the motor  5  is transmitted from the pinion  19  to the ring gear  3 , whereby the engine is restarted. 
     In the engine starting apparatus of the present invention, the pinion-pushing solenoid  8  is actuated to permit the end face  19   a  of the pinion  19  to be in contact with the end face  3   a  of the ring gear  3 . With this contact, the end face  19   a  of the pinion  19  is pressed against the end face  3   a  of the ring gear  3  with the predetermined load F 1 . Meanwhile, the rotational torque T 2  of the clutch  18  in an idling state is set smaller than the rotational torque T 1  with which the ring gear  3  in a decelerating state attempts to rotate the pinion  19 . Therefore, the revolutions of the pinion  19  can instantaneously synchronize with the revolutions of the ring gear  3 . As a result, engagement can be promptly established between the ring gear  3  and the pinion  19 . 
     According to the configuration and scheme described above, the expensive motor revolution control driver disclosed in WO2007/101770 will not be needed. Accordingly, the engine starting apparatus can be provided at low cost. 
     In the conventional art disclosed in WO2007/101770, the number of revolutions has to be fed back in permitting the number of revolutions of the pinion  19  to synchronize with that of the ring gear. However, with the engine starting apparatus of the present invention, the revolutions of the pinion  19  can be instantaneously synchronized with the revolutions of the ring gear  3 . Thus, the number of revolutions does not have to be fed back. In addition, when engine restart is requested while the number of revolutions of the ring gear is decreasing, the pinion  19  can be reliably brought into engagement with the ring gear to restart the engine in a short time. 
     The engine starting apparatus of the present invention is different from the conventional engine starting apparatuses using starters (i.e. the apparatuses in which the end face  19   a  of the pinion  19  comes into contact with the end face  3   a  of the ring gear  3  being applied with a predetermined load, and then engagement is forcibly established by the torque of the motor  5 ). Specifically, the engine starting apparatus of the present invention utilizes the inert revolutions (i.e., revolutions due to inertia) of the ring gear  3  in the engine stop process, for the engagement of the pinion  19  with the ring gear  3 . Therefore, the load imposed between the teeth of the pinion  19  and the teeth of the ring gear  3  is mitigated, exerting an effect of significantly reducing wearing between the ring gear  3  and the pinion  19 . Thus, the engine starting apparatus of the present invention can be appropriately used for an idle stop system in which the number of actuations of the starter  1  is significantly increased. 
     In the conventional engine starting apparatuses using starters, the pinion  19  has been brought into engagement with the ring gear that remains stationary, utilizing the torque of the motor  5 . Therefore, if the engagement is unsuccessful once, the relative numbers of revolutions of the pinion  19  and the ring gear  3  will be increased with time, no longer enabling engagement. In this regard, with the engine starting apparatus of the present invention, the revolutions of the pinion  19  are synchronized with those of the ring gear  3  during the process in which the number of revolutions of the ring gear  3  is decreasing, and then engagement is established. Thus, the relative numbers of revolutions of the pinion  19  and the ring gear  3  will be approximated with time, whereby engagement can be easily achieved. Accordingly, compared to the conventional engine starting apparatuses using starters, the engine starting apparatus of the present invention can significantly and highly reliably reduce the probability of failure of engagement between the pinion  19  and the ring gear  3 . 
     (Modifications) 
     In the embodiment described above, the starter relay  30  has been turned on to actuate the pinion-pushing solenoid  8  (at this point, current has not yet been supplied to the switch coil  22  of the motor electrification switch  10 ) under the conditions where: the number of revolutions of the ring gear  3  at the point when engine restart is requested is lower than a predetermined number of revolutions; and the relative numbers of revolutions of the ring gear  3  and the pinion  19  have reached δN (the number of revolutions of the ring gear  3 =δN). However, when the number of revolutions of the ring gear  3  at the point when engine restart is requested is higher than the predetermined number of revolutions, the switch  10  may be actuated prior to the actuation of the solenoid  8 , followed by actuating the solenoid  8  at the point when the relative numbers of revolutions of the ring gear  3  and the pinion  10  have reached δN. In this case, it is not required to wait for the number of revolutions of the ring gear  3  to become equal to or lower than the predetermined number of revolutions. Accordingly, engine restart can be carried out in a short time. 
     In this modification, the relative numbers of revolutions of the ring gear  3  and the pinion  19  can be determined based on the number of revolutions of the ring gear  3  detected by the RPM detector  4 , and a predetermined logic set according to an estimated ascending curve of the number of revolutions of the motor (rising curve of the motor  5 ).