Starter motor assemblies are used to start vehicle engines, such as engines in heavy duty vehicles. The conventional starter motor assembly broadly includes an electric motor, a solenoid, and a drive mechanism.
The starter motor is placed in operation when a user closes an ignition switch on the vehicle and energizes the solenoid. Energization of the solenoid moves a solenoid shaft (also referred to herein as the “plunger”) in an axial direction. Movement of the solenoid plunger closes electrical contacts, thereby delivering full power to the electric motor. Movement of the solenoid plunger also moves a pinion of the drive mechanism into engagement with the engine flywheel gear. The electric motor delivers torque to the pinion. The pinion, in turn, causes the flywheel to rotate, thereby cranking the vehicle engine.
Once the vehicle engine starts, the operator of the vehicle opens the ignition switch, de-energizing the solenoid assembly. As a result of this deenergization, the magnetic field that caused the plunger to move decreases and is overcome by a return spring, causing the plunger to return to its original position. As the plunger moves to its original position, the pinion is pulled away from the ring gear, and the vehicle engine operates free of the starter motor.
Many starter motors include features that facilitate engagement of the pinion with the vehicle ring gear. One example of such a feature is known as a “soft-start” arrangement. Soft-start arrangements generally allow some limited power to be provided to the electric motor before the pinion engages the ring gear. As a result, the electric motor and pinion provide a “soft start” torque which helps the pinion clear any abutment with the ring gear, thus encouraging the pinion teeth to fully mesh with the ring gear teeth.
Soft-start arrangements typically utilize two coils, i.e., a pull-in coil and a hold-in coil. Both the pull-in coil and the hold-in coil are initially energized when the ignition switch is turned on, allowing current to flow through both coils. The electric field created by energization of the two coils encourages the plunger of the solenoid assembly to move in the axial direction, thus moving the pinion toward engagement with the ring gear of the engine flywheel. The pinion is driven by the electric motor of the soft-start arrangement such that the electric motor provides rotational torque to the pinion.
The electric motor of the soft-start arrangement is in series with the pull-in coil. Thus, the resistance of the pull-in coil limits current flowing through the electric motor during the process of pinion engagement with the ring gear. Because only limited current flows through the electric motor, the torque provided by the electric motor and the associated pinion are also limited (relative to the normal cranking torque) during the process of pinion engagement with the ring gear. As the pinion moves toward engagement with the ring gear, it freely rotates. However, once the pinion is abutted with the ring gear, the rotational speed of the pinion is limited as frictional drag between the pinion and ring gear prevents rapid acceleration of the pinion. Thus, the pinion rotates into full mesh with the ring gear at a relatively slow rotational speed (relative to the normal cranking speed). This relatively slow rotational speed of the pinion allows the pinion to more easily mesh with the ring gear.
When the plunger is moved to the point where the plunger contact disc engages the electrical contacts, the pull-in coil is effectively short circuited, and full power is delivered to the electric motor. The hold-in coil then holds the plunger in place in order to maintain engagement of the pinion with the ring gear during engine cranking.
Starter motors with soft-start arrangements are generally very effective in starting vehicle engines. However, some minor issues with soft-start arrangements occasionally exist with certain situations. One situation where an issue may exist is a heavy-duty application when two starter motors with soft-start arrangements are used to crank a single engine. In this situation, the two starter motors are connected electrically in parallel across a 24V battery pack on the vehicle. This arrangement of two starter motors works quite well for actual starting of the engine. However, the two starter motors operate independent of each other, and do not always provide full cranking power at the same point in time. This time difference may be 0.25 seconds or larger. Because of this, a noise may be encountered as the first starter motor is fully engaged with the ring gear and cranking the engine while the second starter motor is still trying to engage the ring gear. Accordingly, it would be desirable to provide a dual starter motor arrangement that provides for reduced noise over existing dual starter motor arrangements. It would also be desirable if such a dual starter motor arrangement could be implemented with only limited additional costs than existing dual starter motor arrangements.