Both Otto cycle and Diesel cycle internal-combustion engines require the pistons to be moving before the ignition phase of the cycle. This means that the engine must be set in motion by an external force before it can power itself. Originally, a hand crank was used to start engines, but it was inconvenient and rather hard work to crank the engine up to speed. It was also highly dangerous. Even though cranks had an overrun mechanism to prevent it, when the engine started, a crank could begin to spin along with the crankshaft. The operator had to pull away immediately, or else risk a broken wrist, or worse. Moreover, as engines evolved, they became larger and compression ratios increased, making hand cranking an increasingly difficult matter.
The modern starter motor is a series-wound direct current electric motor with a solenoid switch (similar to a relay) mounted on it. When low-current power from the starting battery is applied to the solenoid (the thin, grey wire in the image above), usually through a key-operated switch, it pushes out a small pinion gear on the starter motor's shaft and meshes it with the ring gear on the flywheel of the engine. The solenoid also closes high-current contacts (powered through the thick red cable in the image) for the starter motor and it starts to run. Once the engine starts, the key-operated switch is opened, a spring in the solenoid assembly pulls the pinion gear away from the ring gear, and the starter motor stops. Modern starter motors have a "bendix" — a gear and integral freewheel, or overrunning clutch, that enables the flywheel to automatically disengage the pinion gear from the flywheel when the engine starts.
Chrysler and Ford both contributed to the starter market, with two types that were very different to those used on vehicles today.
Chrysler manufactured a gear reduction starter employing a small gear to drive a larger gear attached to the starter's pinion gear shaft. This allowed lower current to be drawn from the battery to run the starter, and still had the initial torque needed to turn the flywheel approximately at 200 rpm. This starter is also smaller and integrates the starter solenoid in the starter case, instead of having it mounted externally. Since this design weighs less, it has also been adapted to some light aircraft engines, where minimizing weight is very important.
Ford's version was slightly more complicated. The engineers at Ford Motor Company used a "positive engagement" style starter. This type of starter eliminated the solenoid, replacing it with a moveable armature and a separate starter relay. An armature is a part made of ferromagnetic metal that is magnetized by a coil of copper ribbon wound around it, creating an electromagnet. The Ford starter operated as follows:
1. The operator closed the key-operated starting switch.
2. A small electric current flowed through the starter relay coil, closing the contacts and sending a large current to the starter motor assembly.
3. The armature moved a pinion gear to engage the flywheel ring gear, and simultaneously closed a pair of heavy-duty contacts supplying current to the starter motor winding.
4. The starter motor cranked the engine until it started. An overrunning clutch in the pinion gear uncoupled the gear from the ring gear.
5. The operator released the key-operated starting switch, cutting power to the starter motor assembly.
6. A spring retracted the armature, and with it, the pinion gear.
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