Abstract:
In a starter for starting an on-vehicle engine, a solenoid is provided to push a pinion gear. The solenoid has an electromagnetic coil composed of a single coil and electrically separated from a motor circuit, a fixed core, and a plunger. Supply of excitation current to the electromagnetic coil allows the fixed core to be magnetized to attract the plunger. Hence, a movement of the plunger results in a push of the movable member toward the ring gear. A switch is provided in the circuit and has a contact, a movable core, and a switch coil functioning as an electromagnet attracting the movable core in response to supply of current to the switch coil. A movement of the movable core results in on/off switching operations of the switch. The switch is allowed to operate independently of the solenoid when both the switch and solenoid are controlled.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims the benefit of priority from earlier Japanese Patent Application Nos. 2009-102214 filed Apr. 20, 2009, 2009-281589 filed Dec. 11, 2009, 2010-9832 filed Jan. 20, 2010, and 2010-092197 flied Apr. 13, 2010, the description of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1.Technical Field of the Invention 
     2.Related Art 
     In vehicles with engines (i.e., internal combustion engines), a starter is usually used to start the engines. Though a variety of types of starters are known, one type of such starters is provided with a magnet field type of electric motor having an armature and a field coil, a solenoid, and a switch. The solenoid is used to push, using a shift lever, a pinion gear toward a ring gear attached to an on-vehicle engine. The switch turns on/off a main contact arranged in an electric circuit for driving the motor (known as a motor circuit), in which the motor circuit drives the motor by supplying current from a battery to the motor. This kind of starter is disclosed by Japanese Utility Model No 56-42437. 
     In this configuration, the solenoid and the switch can be operated independently of each other. For example, only the solenoid is driven first to make the pinion gear to engage with the ring gear, and then the switch is operated to close the main contact so that the current is supplied to the motor. By this sequential operation technique, the motor can be driven to start the engine after completion of engagement between the pinion gear and the ring gear. 
     In the foregoing starter, the solenoid to push the pinion gear has an electromagnetic coil composed of two coils. These two coils are an attraction coil to generate a magnetic force necessary for attracting the plunger and a retention coil to generate a magnetic force necessary for retaining the attracted plunger. It is usually required that both one end of the attraction coil and one end of the retention coil be electrically connected to a connector or other electric terminal members. Further, the other end of the attraction coil is electrically connected to fixed contacts of the main contact, so that when the main contact in the motor circuit is closed by the electric switch, the attraction coil is short-circuited via the main contact, that is, no current passes through the attraction coil. 
     Furthermore, in the starter disclosed above, the electromagnetic coil of the solenoid and the filed coil of the motor are electrically connected by a wiring member with each other. This electrical connection intends to allow current to flow to the field coil via the electromagnetic coil without closing the main contact whenever the pinion gear is brought into contact with the ring gear axially pushed by the solenoid. In other words, the current flows through the field coil via the electromagnetic coil. This current flow makes the armature of the motor rotates slightly, thus making the pinion gear rotates slightly in response to transmission of the slight rotation of the motor armature to the pinion gear, thus allowing the pinion gear and the ring gear meshes on each other. 
     However, in the structure disclosed by the foregoing starter, the electric circuitry is complicated, resulting in a larger number of parts necessary for the electric circuit. In addition, various working steps are required for manufacturing the starter. Such working steps include a step in which one end of the attraction coil and one end of the retention coil are electrically connected to, for example, a connector, a step in which the other end of the attraction coil is electrically connected to the fixed contacts of the main contact, and a step in which the electromagnetic coil of the solenoid to push the pinion gear and the field coil of the motor are mutually electrically connected by a conductive wire. These many working steps result in an increase in the manufacturing costs of the starter. 
     Additionally, the foregoing disclosed starter has a difficulty that permanent magnets cannot be used as the magnetic field system of the motor. That is, this starter is obliged to employ a field coil as its magnetic field system. The disclosed technique by the foregoing publication cannot be applied to permanent magnet field type of motors which use permanent magnets in their magnetic field system. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in consideration of the foregoing circumstances, and it is an object of the present invention to provide a starter which has the solenoid to push the pinion gear and the switch to switch open/close the main contact of the motor circuit, wherein the solenoid and the switch can be controlled independently of each other, manufacturing steps can be reduced in number by simplifying the electric circuitry, and permanent magnets and a field coil can be adopted selectively by the magnetic field system of a motor. 
     In order to achieve the object, the present invention provides, as one aspect thereof, a starter for a vehicle having an engine with a ring gear, comprising: an electric motor that generates a torque in response to reception of electric power supplied from a battery via an electric circuit electrically connecting the battery and the motor, the circuit relaying the power; an output shaft that rotates in response to reception of the torque from the motor, the output shaft having a longitudinal direction defined as an axial direction; a movable member having a pinion gear that transmits the torque to the ring gear and being movable on the output shaft together with the pinion gear in the axial direction; a solenoid having an electromagnetic coil composed of a single coil and electrically separated from the circuit, a fixed core, and a plunger, supply of excitation current to the electromagnetic coil allowing the fixed core to be magnetized to attract the plunger so that a movement of the plunger results in a push of the movable member toward the ring gear in the axial direction; and a switch which is provided in the circuit and which has a contact, a movable core, and a switch coil functioning as an electromagnet attracting the movable core in response to supply of current to the switch coil, a movement of the movable core resulting in on/off switching operations of the switch, the switch being allowed to operate independently of the solenoid when both the switch and solenoid are controlled. 
     As described, the solenoid pushing the pinion gear has a single electromagnetic coil electrically separated from the circuit for the motor. Hence, the electric circuitry can be simplified compared to the conventional. In addition, the foregoing working steps which have been necessary for manufacturing electromagnetic coils with two coils (consisting of an attraction coil and a retention coil) become unnecessary. 
     The starter according to the present invention can adopt any of a permanent magnet and a field coil as its motor field system. Even when adopting the field coil, it is not required to introduce a step of eclectically connecting the field coil and the electromagnetic coil of the pinion-pushing solenoid. Hence, a simplified electric circuitry leads to a reduction in the number of electric parts. The number of manufacturing steps can be reduced, which results in starter manufacturing with saved costs. 
     As a second aspect, the present invention provides an apparatus for starting an engine mounted in a vehicle, comprising: a starter; an excitation circuit through which the excitation current flows from an on-vehicle battery to the electromagnetic coil; a starter relay that connects the battery and the excitation circuit; a diode having a cathode and an anode, the cathode being electrically connected to a positive potential side point of the electromagnetic coil and the anode being electrically connected to the ground; and a controller that controls excitation and non-excitation operations of the electromagnetic coil via the starter relay. 
     In this engine starting apparatus, in response to a drive signal form the controller, the starter relay is closed (turned on), an excitation current flows from the battery to the electromagnetic coil of the pinion-pushing solenoid via the starter relay. When the controller then commands the current to stop, the starter relay is opened (turned off), thereby cutting off the excitation current. This will cause a counter electromotive force (i.e., a surge voltage) across the electromagnetic coil due to its inductance. 
     However, the diode is connected in parallel to the electromagnetic coil with its cathode connected to the positive potential side of the electromagnetic coil and its anode connected to the ground. Hence, the counter electromotive force can be absorbed well by the diode, whereby no current flows though the starter relay on account of the counter electromotive force. No arc discharge occurs between the contacts of the starter relay, reducing wearing of the contacts, leading to a longer duration of life of the starter. 
     Preferably, the apparatus is mounted in an idle stop apparatus which is capable of automatically controlling a stop and a restart of the engine, wherein the Idle stop apparatus restarts the engine during a period of time a time instant at which the engine starts to stop to a time instant at which the engine stops completely, the engine rotating during the period of time due to inertia of the engine rotation. 
     In this preferred example, since the operations of both the solenoid pushing the pinion gear and the switch for current supply to the motor can be controlled independently of each other, it is possible to restart the engine during its rotation due to its inertia after an engine stop is instructed by an idle stop apparatus. In this situation, the switch can be activated before the activation of the solenoid, so that the motor starts rotating prior to a movement of the pinion gear to the ring gear of the engine. This means that the pinion gear meshes with the ring gear in a state where a relative difference between the rotation numbers of the ring gear rotating due to inertia and that of the pinion gear is reduced. Hence, the mesh between both the gears becomes reliable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a cross-sectional view illustrating a starter according to a first embodiment of the present invention; 
         FIG. 2  is a cross-sectional view illustrating a solenoid unit (a pinion-pushing solenoid and a motor electrification switch) according to the first embodiment; 
         FIG. 3  is an electrical circuit diagram illustrating an apparatus for starting an engine according to the first embodiment; 
         FIG. 4  is an electrical circuit diagram illustrating an apparatus for starting an engine according to a second embodiment of the present invention; 
         FIG. 5  is a cross-sectional view illustrating a solenoid unit according to a third embodiment of the present invention; 
         FIG. 6  is a cross-sectional view illustrating an electromagnetic switch used for a starter according to conventional art; 
         FIG. 7  is a graph illustrating spring characteristics and attraction force characteristics of an electromagnetic switch used for a starter according to conventional art and a pinion-pushing solenoid of the present invention; and 
         FIG. 8  is a cross-sectional view illustrating a solenoid unit according to a fourth embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     With reference to the accompanying drawings, hereinafter will be described some embodiments of the present invention. 
     (First Embodiment) 
     Referring to  FIGS. 1 to 3 , an apparatus for starting an on-vehicle engine according to a first embodiment of the present invention will be described. 
     The apparatus for starting the engine of the first embodiment includes a starter  1  that starts an on-vehicle engine EG.  FIG. 1  is a cross-sectional view illustrating the starter  1 . In the first embodiment, the apparatus for starting the engine EG is loaded in a vehicle having an idle stop system. For example, the idle stop system is able to automatically stop the engine EG when the vehicle is in pause at an intersection by a stop signal or in pause due to traffic jam or the like. 
     As shown in  FIG. 1 , the starter  1  includes a motor  2 , an output shaft  3 , a shift lever  4 , a pinion movable body (described later), pinion-pushing solenoid  5 , a battery (see  FIG. 3 ) and a motor electrification switch  7 . In the present embodiment, the output shaft has a longitudinal direction, so that directions along the longitudinal direction can be defined as an axial direction AX, directions radiating from the axial direction AX along a plane perpendicular to the axial direction AX can be defined as a radial direction RA, and directions around the axial direction AX can be defined as a circumferential direction CR. 
     The motor  2  generates torque. The output shaft  3  is rotated being transmitted with the torque of the motor  2 . The pinion movable body is provided so as to be axially movable (leftward and rightward in  FIG. 1 ) along the output shaft  3 . The pinion-pushing solenoid  5  has a function of pushing the pinion movable body in a direction opposite to the motor (leftward in  FIG. 1 ) via the shift lever  4 . The motor electrification switch  7  opens/closes a main contact (described later) provided at a motor circuit which supplies current from the battery  6  to the motor  2 . 
       FIG. 3  is an electrical circuit diagram illustrating the apparatus for starting the engine EG according to the first embodiment. For example, as shown in  FIG. 3 , the motor  2  is a commutator motor that includes a field magnet  8 , a rectifier  9 , an armature  10  and a brush  11 . The field magnet  8  is configured by a plurality of permanent magnets. The armature  10  has an armature shaft with its one being provided with the rectifier  9 . The brush  11  is provided on the outer periphery of the rectifier  9 . The field magnet  8  of the motor  2 , which is made up of the permanent magnets, may be replaced by a field electromagnet made up of a field coil. 
     The output shaft  3  is disposed being aligned with the armature shaft via a reduction gear (not shown). The torque of the motor  2  is transmitted being reduced by the reduction gear. 
     The reduction gear is a known planetary reduction gear, for example, in which a planetary carrier that picks up the orbital motion of a planetary gear is provided being integrated with the output shaft  3 . 
     The pinion movable body is configured by a clutch  12  and a pinion gear  13 , which will be described later. 
     The clutch  12  includes a spline sleeve  12   a  (see  FIG. 1 ), an outer, an inner, a roller and a roller spring. The spline sleeve  12   a  is helical-spline-fitted to the outer periphery of the output shaft  3 . The outer is provided being integrated with the spline sleeve  12   a . The inner is relatively rotatably arranged at the inner periphery of the outer. The roller is located between the outer and the inner to connect/disconnect torque therebetween. The roller spring has a role of pressing the roller. The clutch  12  is provided as a one-way clutch that unidirectionally transmits torque from the outer to the inner via the roller. 
     The pinion gear  13  is integrated with the inner of the clutch  12  and relatively rotatably supported by the outer periphery of the output shaft  3  via bearings (not shown). 
     The pinion-pushing solenoid  5  and the motor electrification switch  7  have a solenoid coil (i.e., an electromagnetic coil)  14  and a switch coil  15 , respectively, each of which forms an electromagnet when current is passed. A fixed core  16  is arranged between the solenoid coil  14  and the switch coil  15  so as to be commonly used by these coils. Meanwhile, a solenoid case  17  and a switch case  18  are continuously formed in the axial direction AX. Specifically, the solenoid case  17  and the switch case  18  are integrally formed to provide a single overall case. In other words, as shown in  FIG. 1 , the pinion-pushing solenoid  5  and the motor electrification switch  7  are arranged in series in the axial direction AX to integrally configure a solenoid unit and are fixed to a starter housing  19  so as to be parallel to the motor  2 . The solenoid case  17  also serves as a magnetic yoke of the pinion-pushing solenoid  5 , while the switch case  18  also serves as a magnetic yoke of the motor electrification switch  7 . 
       FIG. 2  is a cross-sectional view illustrating the solenoid unit (the pinion-pushing solenoid  5  and the motor electrification switch  7 ). As shown in  FIG. 2 , the overall case 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 case is made even from the first end E 1  to the second end E 2 . However, the solenoid case  17  forming a part of the overall case at the first end E 1  side is ensured to be thicker than the switch case  18  forming a part of the overall case at the second end E 2  side. In other words, the inner peripheral surface of the overall case has a step between the solenoid case  17  and the switch case  18 . 
     The fixed core  16  is configured being divided into an annular core plate  16   a  and a core portion  16   b  caulked along the inner periphery of the core plate  16   a  for fixation. The core plate  16   a  has an outer circumferential surface on the coil side (first end E 1  side) in the thickness-wise direction, which surface is brought into contact with the step provided at the inner periphery of the overall case, to thereby constrain the position of the fixed core  16  on the coil side. 
     Referring to  FIGS. 2 and 3 , hereinafter are described the configurations of the pinion-pushing solenoid  5  and the motor electrification switch  7 , excepting the overall case (the solenoid case  17  and the switch case  18 ) and the fixed core  16 . 
     a) The pinion-pushing solenoid  5  includes the solenoid coil  14 , a plunger  20  and a joint  21 . The solenoid coil  14  is arranged along the inner periphery of the solenoid case  17  that forms a part of the overall case on the first end E 1  side. The plunger  20  is made of iron and disposed being opposed to the care portion  16   b  of the fixed core  16  and is permitted to be axially movable along the inner periphery of the solenoid coil  14 . The joint  21  transmits the movement of the plunger  20  to the shift lever  4 . 
     The solenoid coil  14  is made up of a single coil and has an end which is connected to an external connector terminal  22  (see  FIG. 3 ) and the other end which, for example, is connected and fixed to a surface of the core plate  16   a  by welding or the like, for grounding. 
     The external connector terminal  22  is connected to an electrical wiring  44  so that excitation current can be passed from the battery  6  to the solenoid coil  14  via a starter relay  23  (see  FIG. 3 ). 
     The solenoid coil  14  has an inner periphery at which a cylindrical sleeve  24  is disposed to slidably hold the outer periphery of the plunger  20 . 
     When the fixed core  16  is magnetized with the supply of current to the solenoid coil  14 , the plunger  20  is attracted to one end face of the core portion  16   b  against the reaction force of a return spring  25  disposed between the core portion  16   b  and the plunger  20 . Then, when the current supply to the solenoid coil  14  is stopped, the plunger  20  is pushed back by the reaction force of the return spring  25  in the direction opposite to the core portion  16   b  (leftward in  FIG. 2 ). 
     The plunger  20  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  20  and bottomed at the other end thereof. 
     The joint  21  having a shape of a rod is inserted into the cylindrical hole of the plunger  20  together with a drive spring  26 . Thus, the joint  21  has an end portion projected from the cylindrical hole of the plunger  20 . This end portion of the joint  21  is formed with an engagement groove  21   a  with which one end portion of the shift lever  4  engages. The other end portion of the joint  21  is provided with a flange portion  21   b . The flange portion  21   b  has an outer diameter that enables the flange portion  21   b  to be slidably movable along the inner periphery of the cylindrical hole. The flange portion  21   b , being loaded by the drive spring  26 , is being pressed against the bottom face of the cylindrical hole. 
     With the movement of the plunger  20 , an end face of the pinion gear  13  pushed in the direction opposite to the motor via the shift lever  4  comes into contact with an end face of a ring gear  27  (see  FIG. 1 ) which is attached to a crank shaft of the engine EG. Then, the drive spring  26  is contracted while the plunger  20  is permitted to move and attracted to the one end surface of the core portion  16   b . Thus, the drive spring  26  accumulates reaction force that allows the pinion gear  13  to engage the ring gear  27 . 
     b) The motor electrification switch  7  includes the switch coil  15 , a movable core  28 , a contact cover  29 , two terminal bolts  30  and  31 , a pair of fixed contacts  32 , and a movable contact  33 . The switch coil  15  is arranged along the inner periphery of the switch case  18  forming a part of the overall case on the second end E 2  side. The movable core  28  is opposed to the core portion  16   b  of the fixed core  16  and is permitted to be movable in the axial direction AX. The contact cover  29 , which is made of resin, is assembled, blocking the open end, i.e. the second end E 2 , of the overall case (the open end of the switch case  18 ). The two terminal bolts  30  and  31  are fixed to the contact cover  29 . The pair of fixed contacts  32  are fixed to the two terminal bolts  30  and  31 . The movable contact  33  electrically connects/disconnects so between the pair of fixed contacts  32 . 
     The switch coil  15  is made up of a single coil and has one end which is connected to an external connector terminal  34  (see  FIG. 3 ), and the other end which, for example, is connected and fixed to a surface of the core plate  16   a  by welding or the like, for grounding. 
     The external connector terminal  34  is connected to an electrical wiring  45  so that excitation current can be passed from the battery  6  to the switch coil  15  via a motor relay  35  (see  FIG. 3 ). The external connector terminals  22  and  34  are each formed, for example, of a metal plate terminal. Ends of the respective plate terminals are provided being externally projected in the axial direction AX from the contact cover  29 . 
     The switch coil  15  has a radially outer peripheral side on which an axial magnetic path member  36  is arranged to form a part of a magnetic path. Also, the switch coil  15  has an axial side opposite to the fixed core, on which a radial magnetic path member  37  is arranged to form a part of the magnetic path. 
     The axial magnetic path member  36  has a cylindrical shape and is inserted into the switch case  18  along the inner periphery thereof with substantially no gap being provided therebetween. An end face of the axial magnetic path member  36  on the first end E 1  side is brought into contact with the outer peripheral surface of the core plate  16   a  to determine the axial position of the member  36 . 
     The radial magnetic path member  37  is arranged perpendicular to the axial direction AX. The radial magnetic path member  37  has a radially outer end surface on the first end E 1  side, which surface is brought into contact with an axial end face of the axial magnetic path member  36  to constrain the position of the member  37  with respect to the switch coil  15 . The radial magnetic path member  37  has a round opening at its radially central portion so that the movable core  28  can move therethrough in the axial direction AX. 
     The fixed core  16  is magnetized upon supply of current to the switch coil  15 . Then, the movable core  28  is attracted to the other end face of the core portion  16  against the reaction force of the return spring  38  disposed between the core portion  16   b  and the movable core  28 . When the current supply to the switch coil  15  is stopped, the movable core  28  is pushed back in the direction opposite to the core portion (rightward in  FIG. 2 ) by the reaction force of the return spring  38 . 
     The contact cover  29  has a cylindrical trunk portion  29   a . The trunk portion  29   a  is inserted into the switch case  18  along the inner periphery thereof, the switch case  18  forming a part of the overall case on the second end E 2  side. The contact cover  29  is arranged, with the axial end face of the trunk portion  29   a  being in contact with a surface of the radial magnetic path member  37 , and caulked and fixed to the open end, i.e. the second end E 2 , of the overall case. 
     The terminal bolt  30 , one of the two terminal bolts, is connected to a battery cable  39  (see  FIG. 3 ). The terminal bolt  31 , the other of the two terminal bolts, is connected to a motor lead  40  (see  FIGS. 1 and 3 ). This motor lead  40  serves as an electric circuit connecting the battery  6  and the motor  2  (that is, serves as a motor circuit). 
     The pair of fixed contacts  32 , which are provided separately from (or may be provided integrally with) the two terminal bolts  30  and  31 , are electrically fixed to the two terminal bolts  30  and  31  inside the contact cover  29 . 
     The movable contact  33  is arranged so that the distance from the movable contact  33  to the movable core is larger than the distance from the pair of fixed contacts  32  to the movable core (rightward in  FIG. 2 ). The movable contact  33  is in reception of the load of a contact spring  42  and pressed against an end face of a resin rod  41  fixed to the movable core  28 . It should be appreciated that the initial load of the return spring  38  is set larger than that of the contact spring  42 . Therefore, when the switch coil  15  is de-energized, the movable contact  33  is seated on an inner seat  29   b  (see  FIG. 2 ) of the contact cover  29 , with the contact spring  42  being contracted. 
     The main contact is formed of the pair of fixed contacts  32  and the movable contact  33 . Being biased by the contact spring  42 , the movable contact  33  comes into contact with the pair of fixed contacts  32  with a good pressing force. Resultantly, current is passed across the pair of fixed contacts  32  to thereby close (turn on) the main contact. When the movable contact  33  is drawn apart from the pair of fixed contacts  32 , the current across the pair of fixed contacts  32  is shut down to thereby open (turn off) the main contact. 
     The operation of the starter  1  will be described. 
     The operation of the starter  1  is controlled by an ECU (electronic control unit)  43  through the starter relay  23  and the motor relay  35 . 
     a) The case where the engine EG is normally started (i.e. the case where the user turns on an ignition switch (not shown) to start the engine EG in the state where the engine EG is fully stopped) 
     When an engine start signal issued by a turn-on operation of the ignition switch is inputted, the ECU  43  outputs a drive signal (turn-on signal) to the starter relay  23 . Then, the starter relay  23  is turned on so that current is passed from the battery  6  to the solenoid coil  14  of the pinion-pushing solenoid  5 , for magnetization of the core portion  16   b . Then, the plunger  20  is permitted to move being attracted to the magnetized core portion  16   b . With the movement of the plunger  20 , the pinion movable body (the clutch  12  and the pinion gear  13 ) is pushed in the direction opposite to the motor via the shift lever  4 . Then, an end face of the pinion gear  13  comes into contact with an end face of the ring gear  27  and stops. 
     After expiration of a predetermined period (e.g., 30 to 40 ms) from the issuance of the engine start signal, the ECU  43  outputs a drive signal (turn-on signal) to the motor relay  35  to turn on the motor relay  35 . Thus, current is passed from the battery  6  to the switch coil  15  of the motor electrification switch  7  to allow the movable core  28  to be attracted to the core portion  16   b . Then, the movable contact  33  is brought into contact with the pair of fixed contacts  32  and biased by the contact spring  42  to thereby close the main contact. As a result, current is supplied to the motor  2  to generate torque in the armature  10 . The torque is then transmitted to the output shaft  3  via the reduction gear. The torque of the output shaft  3  is further transmitted to the pinion gear  13  via the clutch  12 . When the pinion gear  13  rotates up to a position that enables engagement with the ring gear  27 , the pinion gear  13  is permitted to engage the ring gear  27  by the reaction force accumulated in the drive spring  26 . Thus, the torque is transmitted from the pinion gear  13  to the ring gear  27 , whereby the engine EG is started. 
     b) The case where engine restart is requested in an engine stop in process performed by an idle stop system, and where the engine EG is restarted during inert revolutions prior to the full stop of the engine EG. 
     When conditions for automatically stopping the engine EG (e.g. the vehicle speed being zero, the brake pedal being stepped on, and the like) from an idling state are met, the ECU  43  outputs an engine stop signal to stop fuel injection and supply of intake air. As a result, the engine EG enters an engine stop process, whereby the ring gear  27  starts decreasing revolutions. When engine restart is requested while the ring gear  27  is decreasing revolutions (prior to the full stop of the engine revolutions), the ECU  43  outputs a drive signal (turn-on signal) to the motor relay  35 . Upon output of the drive signal, the motor relay  35  is turned so that current is passed from the battery  6  to the switch coil  15 . As a result, the main contact is closed to pass current to the motor  2 , thereby generating torque in the armature  10 . 
     Then, the ECU  43  outputs a drive signal (turn-on signal) to the starter relay  23 . When the starter relay  23  is turned on, current is passed from the battery  6  to the solenoid coil  14  to operate the pinion-pushing solenoid  5 . With the operation of the pinion-pushing solenoid  5 , the pinion movable body is pushed in the direction opposite to the motor via the shift lever  4 . Resultantly, the end face of the pinion gear  13  is brought into contact with the end face of the ring gear  27 . Then, at the point when both of the gears  13  and  27  have rotated to the positions enabling engagement, the engagement between these gears is achieved. Thus, the torque of the motor  2  is transmitted from the pinion gear  13  to the ring gear  27 , whereby the engine EG is restarted. 
     In the starter  1  of the present embodiment, the solenoid coil  14  of the pinion-pushing solenoid  5  is formed of a single coil, and the solenoid coil  14  is electrically separated from the motor circuit (i.e. the solenoid coil  14  is not connected to the motor circuit). Therefore, the circuit configuration can be simplified. In other words, some processes (e.g., a process of connecting one end of an attraction coil and one end of a holding coil to connectors or the like, and a process of electrically connecting the other end of the attraction coil to the fixed contacts  32  disposed on the motor side and configure the main contact) can be eliminated. These processes would have otherwise been required if the solenoid coil  14  is configured by two coils; an attraction coil and a holding coil. 
     In the starter  1  of the present embodiment, the field magnet  8  of the motor  2  is not required to be limited to a field electromagnet. Thus, either of a permanent magnet and a field coil may be usable. Use of a field coil will not necessitate establishing, connection between the solenoid coil  14  of the pinion-pushing solenoid  5  and field coil via an electrical wiring. 
     In this way, the circuit configuration of the starter  1  can be simplified to thereby reduce the number of parts and the number of manufacturing processes. As a result, the starter  1  can be provided at low coast. 
     Further, the starter  1  of the present embodiment enables independent operation of the pinion-pushing solenoid  5  and the motor electrification switch  7 . Therefore, when engine restart is requested during the engine stop process performed by an idle stop system, the engine EG can be restarted during the inert revolutions prior to the full stop. In this case, as described in the above item (b) explaining operation, the motor electrification switch  7  is operated prior to the operation of the pinion-pushing solenoid  5 . Specifically, current supply to the switch coil  15  prior to the solenoid coil  14  will permit the motor  2  to rotate prior to the movement of the pinion movable body toward the ring gear  27 . Therefore, engagement between the pinion gear  13  and the ring gear  27  can be achieved in the state where the relative numbers of revolutions of these gears in inert revolutions have been decreased. Thus, the engine EG startability can be enhanced, while the starting noise can be reduced. 
     Furthermore, the pinion-pushing solenoid  5  and the motor electrification switch  7  are arranged in series in the axial direction AX. Hence, compared to a structure in which the solenoid and switch are arranged in the circumferential direction CR, an area occupied when viewed in the axial direction AX. In other words, an occupied size in the radial direction RA of the motor  2  is kept smaller. Hence, the solenoid unit according to the present embodiment can be arranged in a mounting space which is almost the same as a space required to mount a conventional type of starter electromagnetic switch with one plunger for both pushing a pinion gear and opening/closing the main contact. 
     Further, compared to a configuration in which the pinion-pushing solenoid  5  and the motor electrification switch  7  are independent of each other in respect of their arrangement and structures, the solenoid unit of the present embodiment is still advantageous in that the number of parts and manufacturing costs can be reduced. Unifying the cases of the solenoid  5  and switch  7  improves resistance to vibration applied. 
     The switch coil  15  is a single coil, so that, compared to the two-coil type of switch coil, a winding step can be shortened in time and the circuitry can be simplified. For the two-coil type of switch coil, two terminal lines for grounding are necessary, while the one-coil type of switch coil needs only one ground-side terminal line. Hence, a step for processing the ground terminal line can be facilitated. 
     (Second Embodiment) 
     Referring now to  FIG. 4 , hereinafter is described an apparatus for starting an on-vehicle engine according to a second embodiment of the present invention. 
     In the second and the subsequent embodiments as well as in the modifications provided below, the components identical with or similar to those in the first embodiment are given the same reference numerals for the sake of omitting explanation. 
     The second embodiment is associated with prolonging lives of the contacts used in the starter relay  23  and the motor relay  53  described in the first embodiment. 
     Since the configurations of the starter  1  and the solenoid unit (the pinion-pushing solenoid  5  and the motor electrification switch  7 ) are the same as those in the first embodiment, the explanation Is omitted. 
     The solenoid coil  14  of the first embodiment has not been formed of two coils, an attraction coil and a holding coil. Instead, the solenoid coil  14  of the first embodiment has been formed of a single coil in which one end is connected to the starter relay  23  and the other end is grounded. Therefore, when the starter relay  23  is turned off and the solenoid coil  14  is de-energized, a counter electromotive force (i.e., a surge voltage) is generated by the inductance of the solenoid coil  14 . With the generation of the counter electromotive force, current is passed through the starter relay  23 . As a result, arc discharge occurs across the contacts of the starter relay  23 . Hence, the second embodiment is directed to avoiding such arc discharges, while still gaining the various advantages described in the first embodiment. 
     With reference to  FIG. 4 , characteristics of a circuit configuration of the second embodiment, which differ from those in the first embodiment, are specifically described below.  FIG. 4  is an electrical circuit diagram illustrating the apparatus for starting an engine according to the second embodiment. 
     In the pinion-pushing solenoid  5 , a diode  46  is in parallel connected to the solenoid coil  14 . Likewise, in the motor electrification switch  7 , a diode  47  is in parallel connected to the switch coil  15 . In other words, in the pinion-pushing solenoid  5 , the cathode of the diode  46  is connected to the positive-potential side point, that is, the terminal  22 , of the solenoid coil  14  and the anode is connected to the grounding side. Likewise, in the motor electrification switch  7 , the cathode of the diode  47  is connected to the positive-potential side point, that is, the terminal  34 , of the switch coil  15  and the anode is connected to the grounding side. 
     With the above configuration, when the starter relay  23  is turned off to de-energize the solenoid coil  14 , the counter electromotive force generated in the solenoid coil  14  can be absorbed by the diode  46 . Specifically, the solenoid coil  14  is permitted to short-circuit by the diode  46  so that the counter electromotive force generated in the solenoid coil  14  can be absorbed by the diode  46 . Thus, since no current passes through the starter relay  23 , arc discharge will not occur across the contacts of the starter relay  23 . As a result, wearing of the contacts of the starter relay  23  can be suppressed, whereby the lives of the contacts can be suppressed from being shortened. 
     In the same way, when the motor relay  35  is turned off to de-energize the switch coil  15 , the counter electromotive force generated in the switch coil  15  can be absorbed by the diode  47 . Thus, since no current passes through the motor relay  35 , arc discharge will not occur across the contacts of the motor relay  35 . As a result, wearing of the contacts of the motor relay  35  can be suppressed, whereby the lives of the contacts can be suppressed from being shortened. 
     The two diodes  46  and  47  can be accommodated in a casing of the solenoid unit, which casing is formed of the overall case (the solenoid case  17  and the switch case  18 ) and the contact cover  29 . In this case, not being exposed to the outside, the diodes  46  and  47  can be prevented from being deteriorated. In addition, since the diodes  46  and  47  can be connected within the casing of the solenoid unit, connector terminals are not required to be newly provided. 
     In this way, in the second embodiment, the lives of the contacts used in the starter relay  23  and the motor relay  35  can be prolonged. The prolongation of the lives of the contacts is particularly effective in a vehicle installing an idle stop system. 
     Specifically, the number of restarts of the engine EG is drastically increased (e.g., by a factor of about ten) in a vehicle installing an idle stop system, compared to a vehicle not installing an idle stop system. Therefore, preventing wearing of contacts of the starter relay  23  and the motor relay  35  for the prolongation of the lives of the contacts is of extreme importance in the circumstances where use of idle stop systems is prevailing, and may also lead to enhancing reliability of the idle stop systems. 
     (Third Embodiment) 
     Referring to  FIGS. 5 to 7 , an apparatus for starting an on-vehicle engine according to a third embodiment of the present invention is described. 
     The third embodiment is different from the first and second embodiments in that a tapered projection  20   a  is provided at the plunger  20  of the pinion-pushing solenoid  5 . 
       FIG. 5  is a cross-sectional view illustrating a solenoid unit of the third embodiment. As shown in  FIG. 5 , the plunger  20  of the pinion-pushing solenoid  5  is provided with the projection  20   a  having a tapered shaped. Specifically, the plunger  20  has an end face, in a radially inner side of which the tapered projection  20   a  is provided being projected to and axially opposed to the core portion  16   b . Meanwhile, the core portion  16   b  has an axial end face in which a tapered recess  16   c  is formed so that the projection  20   a  of the plunger  20  can be fitted thereto when the plunger  20  has been attracted to the core portion  16   b.    
     Building up the plunger  20  by providing the tapered projection  20   a  at the end face may allow lots of magnetic flux to pass through the projection  20   a . Therefore, compared to the electromagnetic switches of the conventional starters, the starter of the present embodiment can improve saturation of the flux density to thereby increase the attraction force.  FIG. 6  is a cross-sectional view illustrating an electromagnetic switch used for a conventional starter. The “electromagnetic switches of the conventional starters” herein refers to an electromagnetic switch, as shown in  FIG. 7 , in which a single movement of the plunger  20  carries out both pushing a pinion movable body and opening/closing a main contact, or refers to an electromagnetic switch not provided with the tapered projection  20   a  at the end face of the plunger  20 , which end face is opposed to the core portion  16   b  (i.e. the plunger  20  with a flat end face). 
       FIG. 7  is a graph illustrating spring characteristics and attraction force characteristics of an electromagnetic switch used for a conventional starter and the pinion-pushing solenoid of the present invention. 
     The electromagnetic switch of a conventional starter has a contact spring  41  (see  FIG. 6 ) that pushes contacts, as well as the return spring  25  and the drive spring  26 . Therefore, as indicated by the broken line (b) in  FIG. 7 , a required value of the attraction force becomes large at the time of achieving contact (at the time when the movable contact  33  has contacted the fixed contacts  32 ). As a plunger gap (the value indicated on the horizontal axis in  FIG. 7 ) becomes smaller, the inclination of the attraction force characteristics becomes drastically large. 
     On the other hand, the pinion-pushing solenoid  5  of the present invention only has a function of pushing the pinion movable body toward the ring gear  27 , while the function of opening/closing the main contact is performed by the motor electrification switch  7 . Therefore, the required value of attraction force can be made small when the plunger gap has a size corresponding to the size at the time of achieving contact. In this regard, as indicated by the solid line (a) in  FIG. 7 , the attraction force can be increased in the present invention by providing the tapered projection  20   a  at the radially inner side of the end face of the plunger  20 . The increase in the attraction force will lead to a decrease in the number of turns of the electromagnetic coil  14 , thereby making it possible to make the electromagnetic coil  14  more compact in it size. 
     In addition, the inclination of the attraction force characteristics can be made small, whereby the attraction force characteristics may be permitted to turn to the characteristics more suitable for the spring characteristics. 
     As described above, the pinion-pushing solenoid  5  of the present embodiment is provided with the projection  20   a  at the radially inner side of the end face of the plunger  20 . Therefore, the return spring  25  can be arranged radially outside of the plunger  20  and the core portion  16   b , Specifically, as shown in  FIG. 5 , one end of the return spring  25  is held by a spring-holding recess  20   b  formed in a radially outer portion of the plunger  20 . The other end of the return spring  25  is held by a spring-holding recess  16   d  formed in a radially outer portion of the core portion  16   b . Thus, the return spring  25  is arranged close to the inner periphery of the sleeve  24 . 
     In this case, a lubricant, such as grease, may be applied to the inner peripheral surface of the sleeve  24 , so that the plunger  20  can smoothly move along the inner periphery of the sleeve  24 . In this regard, with the arrangement of the return spring  25  close to the inner periphery of the sleeve  24  as mentioned above, the lubricant dropped from the inner peripheral surface of the sleeve  24  can be temporarily collected between wire portions of the return spring  25 . Then, when the plunger  20  has been attracted to the core portion  16   b  with the contraction of the return spring  25 , the lubricant is pushed out from between the wire portions of the return spring  25  and returns to the inner periphery of the sleeve  24 . Thus, lubricating properties can be maintained between the sleeve  24  and the plunger  20 . 
     (Fourth Embodiment) 
     Referring to  FIG. 8 , an apparatus for starting an on-vehicle engine according to a fourth embodiment of the present invention is described. 
       FIG. 8  is a cross-sectional view illustrating a solenoid unit of the fourth embodiment. In the fourth embodiment, the core portion  16   b  is provided with a projection  16   e . The projection  16   e  has a tapered shape and is formed so as to be axially opposed to the plunger  20  of the pinion-pushing solenoid  5 . 
     More specifically, as shown in  FIG. 8 , the core portion  16   b  has an end face, in a radially inner side of which the tapered projection  16   e  is provided being projected to and axially opposed to the plunger  20 . Meanwhile, the plunger  20  has an axial end face in which a tapered recess  20   c  is formed so that the projection  16   e  of the core portion  16   b  can be fitted thereto when the plunger  20  has been attracted to the core portion  16   b.    
     Building up the core portion  16   b  by providing the tapered projection  16   e  at the end face may allow lots of magnetic flux to pass through the projection  16   e . Therefore, similar to the second embodiment and compared to the electromagnetic switch of the conventional starter shown in  FIG. 6 , the starter of the present embodiment can improve saturation of the flux density to thereby increase the attraction force. 
     Similarly to that described in the second embodiment, the increase in the attraction force will lead to a decrease in the number of turns of the electromagnetic coil  14 , thereby making it possible to make the electromagnetic coil  14  more compact in it size. 
     (Modifications) 
     In the first embodiment, the pinion-pushing solenoid  5  and the motor electrification switch  7  have been arranged in series in the axial direction AX to integrally configure a solenoid unit. Alternatively, however, the solenoid  5  and the switch  7  may be separately configured. 
     The diodes  46  and  47  of the second embodiment are not necessarily accommodated in the casing of the solenoid unit, but may be arranged outside the casing. The same applies to the case where the solenoid  5  and the switch  7  are separately configured. For example, the diode  46  may be arranged outside the casing of the solenoid  5 , with the cathode being connected to the external connector terminal  22  and the anode being connected to the grounding side (e.g. to the solenoid case  17 ). Similarly, the diode  47  may be arranged outside the casing of the switch  7 , with the cathode being connected to the external connector terminal  34  and the anode being connected to the grounding side (e.g. to the switch case  18 ). 
     For the sake of completeness, it should be mentioned that the various embodiments and modifications explained so far are not definitive lists of possible embodiments of the present invention. The expert will appreciate that it is possible to combine the various construction details or to supplement or modify them by measures known from the prior art without departing from the basic inventive principle.