Abstract:
An improved DC electrical starting motor and method for starting internal combustion engines that reduces starter motor noise in the period after the engine starts by effecting breaking of the starter motor shaft at that time. Preferably the braking is accomplished by regenerative braking.

Description:
BACKGROUND OF INVENTION 
     This invention relates to a starter motor for an engine for a vehicle such as a motorcycle or the like and to a method for starting such engines and reducing starter noise at the time of the starting operation. 
     In an engine for a vehicle such as a motorcycle, a starter motor is used at the time of engine starting to rotate an engine shaft under battery power. The starter motor is generally a DC motor driven through a relay by operation of a manual switch such as main switch or a starter switch of the vehicle. In this case, the switch is first turned ON to rotate the starter motor, for cranking. When the engine is started after the starter motor load drops to zero, resulting in a maximum rotational speed. At this moment, the switch is turned OFF to stop the power supply to the starter motor. As a result, the starter motor output shaft is disconnected from the engine shaft by a one way clutch type of device and it rotates idly by its inertia and stops after gradually reducing its rotational speed. 
     At the time of the engine starting described above, during the time while the starter motor is stopping its rotation after the power supply to the starter motor is OFF it generates an abnormal and unpleasant noise. The cause of this phenomenon may be understood by reference to FIG.  1 . In this figure the horizontal axis represents time and the vertical axis represents both starter motor rotational speed and noise level. 
     The power source of the starter motor is turned ON at a time t 0  and begins to rotate for cranking. When the engine is started, the starter motor load drops to zero and the rotational speed increases to a maximum. At a time t 1  when this state is reached, the starter switch is turned OFF by hand. 
     As a result, the starter motor rotates idly as a result of its inertia, decreasing its rotational speed gradually and stopping eventually at a time t 2 . Between the times t 1  and t 2 , an abnormally high noise is generated. The noise at this point is an abnormal and unpleasant one and unusually is louder than the engine noise or even that of the starter motor during the actual starting operation. 
     This abnormal noise is caused by the starter motor yoke of its stator resonating when the natural frequency of the yoke coincides with the number of times of cogging reaction at a specific motor speed. This resonance frequency corresponds to a frequency determined by the least common multiple of the number of slots of an armature and the number of magnetic poles of magnets, or the cogging number/rotation, and the cogging reaction produced at a specific motor speed. In an actual measurement shown in FIG. 1, the cogging number is the least common multiple of 28 for a motor with fourteen slots and four-pole magnets. An abnormal noise is generated at the time of the rotational speed of 5100 rpm. In this case, the resonance frequency is expressed as follows: 
     
       
         (28×5100/60)×2=4760 Hz.  
       
     
     More specifically, an armature connected to the output shaft of the starter motor is formed of a plurality of radially disposed cores. Electrical coils are wound on these cores and face a plurality of magnets on the inside surface of the starter motor yoke. The armature is rotated through successive attractions of magnetic forces of the magnets. 
     When the cores of the armature pass across the magnets and its polarities are changed, the armature changes its rotational torque, generating cogging with a perturbed movement. Therefore, the larger the magnetic forces are, the greater cogging is generated, resulting in an abnormal noise due to the reaction. 
     Normally the material of the permanent magnets is a ferrite-based magnetic material. However, neodymium-based magnets made from a magnetic material containing Nd of a rare metal element or its compound known as high-energy magnets are preferred because they permit a higher output starter motor for a given size. If such neodymium-based magnets are used, since the magnetic forces are great, the problem of an abnormal noise due to the cogging reaction is amplified. 
     It is therefore a principle object of this invention to provide an improved starter motor arrangement and method of starting an internal combustion engine that reduces noise during the starting operation. 
     SUMMARY OF INVENTION 
     A first feature of this invention is adapted to be embodied in a starter arrangement for an internal combustion engine. The starter arrangement comprises a DC electrical motor having an output shaft in starting arrangement with a shaft of the engine for starting the engine. A battery is provided for selectively energizing terminals of the DC electrical motor for driving the engine shaft to start the engine. A braking arrangement brakes the rotation of the starter motor output shaft when the engine starts. 
     In a preferred embodiment of this first feature, the braking arrangement comprises a switching arrangement that connects the terminals of the DC electrical motor to the battery to charge the battery upon the deenergization of the terminals for stopping the driving of the DC electrical motor by regeneratively braking the rotation of said DC electrical motor. 
     Another feature of the invention is adapted to be embodied in a method for starting an engine with a DC electrical motor and reducing starter motor noise. A DC electrical motor has an output shaft in starting arrangement with a shaft of the engine for starting the engine. A battery is also provided. The method comprises the steps of selectively energizing terminals of the DC electrical motor for driving the engine shaft to start the engine upon operator demand and the rotation of the starter motor output shaft is braked when the engine starts. 
     In a preferred embodiment of this other feature the starter motor output shaft is braked by connecting the terminals of the DC electrical motor to the battery to charge the battery upon starting of the engine to regeneratively brake the rotation of the DC electrical motor. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a graphical view showing the noise and speed of a prior art type of starter motor during engine starting operation. 
     FIG. 2 is a cross sectional view of a starter motor constructed and operated in accordance with the invention. 
     FIG. 3 is a cross sectional view taken along the line  3 — 3  in FIG.  2 . 
     FIG. 4 is a cross sectional view taken along the line  4 — 4  in FIG.  2 . 
     FIG. 5 is a circuit diagram of the starter motor. 
    
    
     DETAILED DESCRIPTION 
     Referring now in detail to the drawings and initially primarily to FIGS. 2-4, a starter motor for an internal combustion engine (not shown) is indicated generally by the reference numeral  11 . The starter motor  11  is comprised of a stator  12  formed of a cylindrical yoke  13  and four permanent magnets  14 , of arc-shaped cross section, bonded on the inside surface of the yoke. The permanent magnets  14  are preferably formed from a neodymium (Nd)-based magnetic material that is magnetized after being bonded to the yoke  13 . 
     An armature or rotor, indicated generally at  15  rotatably mounted inside the stator  12  in a manner to be described shortly. The armature  15  is comprised of a core  16  facing the magnets  14  and fixed to a starter motor output shaft  17 . A commutator  18  is fixed adjacent to the core  16  on one end of the starter motor output shaft  17 . 
     The core  16 , as shown in FIGS. 2 and 3, is formed of a plurality of radially disposed core teeth  19 . In the illustrated embodiment there are  14  core teeth  19 . Electrical coils (not shown) are wound around the core pieces  19 . The commutator  18  is formed of a plurality of contact pieces  21  corresponding in number to the core pieces  19  and that are electrically connected to the coil ends, as is well known in the art. Two sets of two brushes  22  and  23  (FIG. 4) held by respective brush holders  24  and  25  are juxtaposed to the commutator  18  at its outside circumference. The brushes  22  and  23  are pressed against the contact pieces  21  of the commutator  18  by coil springs  26 . 
     Fitted to the opposite ends sides of the cylindrical yoke  13  are a front cover  27  (FIG. 2) covering the left side of the yoke  13  as seen in the figure and a rear cover  28  covering the right side of the yoke. on the figure, collectively forming, with the yoke  13 , a motor case indicated generally by the reference numeral  29 . The starter motor output shaft  17  is journalled for rotation on the front cover  27  and the rear cover  28 , respectively by bearings  31 . 
     On the rear cover  28  (FIGS. 2 and 4) is provided a positive terminal  32  for power supply from the positive electrode of a battery (described later by reference to FIG. 5 mounted on the vehicle. The positive terminal  32  is suitably connected to the brushes  22  on the positive electrode side. The brushes  23  on the negative electrode side (ground side) are connected to the end closure  28  by grounding fasteners  33 . The motor case  29  is grounded to the associated engine by a mounting bracket  34  that fixes the starter motor  11  to the engine thus acting as a negative terminal. 
     On the front cover  27  is mounted an oil seal  35  (FIG. 2) for preventing ingress of oil into the motor case  29  from the associated engine, and an O-ring  36  for sealing the mounting portion to the engine. On the starter motor output shaft  17  at the engine side end is provided a pinion gear  37  meshing with an flywheel gear (not shown) to rotate the engine shaft for starting. Some form of one way device such as a one way clutch is provided in this connection to permit the engine shaft from driving the starter motor once the engine has started to run under its own power, as is well known in this art. 
     Inside the rear cover  28  covering the commutator  18  at the end of the starter motor output shaft  17  is fixed a disk-like brush carrier 38 . The brush holders  24  and  25  are affixed to the brush carrier 38  at four positions spaced radially at right angles to hold the opposing two positive electrode brushes  22  and opposing two negative electrode (grounding) brushes  23 . As has been noted, the brushes  22 ,  23  are biased radially inwardly toward the commutator  18  by the coil springs  26 . The positive electrode brushes  22  are connected to the positive terminal  32 , and the negative electrode brushes  23  to the negative (grounding) terminal  34 . 
     FIG. 5 is a diagram of a circuit for driving the starter motor  11 . The starter motor  11  is connected to the aforementioned battery  39  through a relay  41 . Power supply from the battery  39  to the motor is switched ON/OFF through operation of a main or starter switch  42 . 
     The relay  41  is comprised of a solenoid winding  43 . The winding  43  encircles an armature that carries a contact plate  44 . The relay  41  further comprises first and second contacts  45 ,  46  with which the contact plate  44  comes in contact. 
     When the switch  42  is closed, the magnetic force of the solenoid  43  causes the contact plate  44  to move toward the right as shown in FIG. 5, closing the first contact  45 . Thus power supply from the battery  39  is ON for energization of the starter motor  11 . As a result, the starter motor  11  is rotated to rotate the engine. 
     After the engine is started by this cranking, the switch  42  is opened. Then, the solenoid  43  is disconnected from the battery  39 , and the contact plate  44  is returned to the left on the figure by a spring (not shown) and comes in contact with the second contact  46  to close it (the state shown in FIG.  5 ). 
     As a result of the second contact  46  being closed, the positive and the negative electrodes of the starter motor  11  are connected. Thus, power supply from the battery  39  to the starter motor  11  is OFF, energization of the starter motor  11  is stopped, and the motor begins to rotate idly by inertia. At this moment, the positive and the negative electrode of the motor are connected, so that the starter motor  11  acts as a generator, producing regenerative electromotive force. Whereby, the function of regenerative braking is effected and the starter motor  11  is stopped quickly. The starter noise caused by the prior art as described by reference to FIG. 1 is thus substantially reduced if not totally eliminated. 
     Thus it should be readily apparent that the described apparatus and starting method achieves the goals set out above in a low cost and highly effective manner. Of course those skilled in the art will understand that the embodiment described is only a preferred embodiment of the invention and various changes and modifications may be made without departing from the spirit and scope of the invention, as defined by the appended claims.