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What is a Starter Motor?
An automobile self-starter (commonly "starter motor" or simply "starter") is an electric motor that initiates rotational motion in an internal combustion engine before it can power itself.
Electric starter motor
The modern starter motor is either a permanent-magnet or a series- or series-parallel wound direct current electric motor with a solenoid switch (similar to a relay) mounted on it. When current from the starting battery is applied to the solenoid, usually through a key-operated switch, it pushes out the drive pinion on the starter driveshaft and meshes the pinion with the ring gear on the flywheel of the engine. Before the advent of key-driven starters, most electric starters were actuated by foot-pressing a pedestal located on the floor, generally above the accelerator pedal.
The solenoid also closes high-current contacts for the starter motor, which begins to turn. 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. The starter's pinion is clutched to its driveshaft through an overrunning sprag clutch which permits the pinion to transmit drive in only one direction. In this manner, drive is transmitted through the pinion to the flywheel ring gear, but if the pinion remains engaged (as for example because the operator fails to release the key as soon as the engine starts), the pinion will spin independently of its driveshaft. This prevents the engine driving the starter, for such back drive would cause the starter to spin so fast as to fly apart. However, this sprag clutch arrangement would preclude the use of the starter as a generator if employed in hybrid scheme mentioned above; unless modifications are made.
This overrunning-clutch pinion arrangement was phased into use beginning in the early 1960s; before that time, a Bendix drive was used. The Bendix system places the starter drive pinion on a helically-cut driveshaft. When the starter motor begins turning, the inertia of the drive pinion assembly causes it to ride forward on the helix and thus engage with the ring gear. When the engine starts, back drive from the ring gear causes the drive pinion to exceed the rotative speed of the starter, at which point the drive pinion is forced back down the helical shaft and thus out of mesh with the ring gear.
An intermediate development between the Bendix drive developed in the 1930s and the overrunning-clutch designs introduced in the 1960s was the Bendix Folo-Thru drive. The standard Bendix drive would disengage from the ring gear as soon as the engine fired, even if it did not continue to run. The Folo-Thru drive contains a latching mechanism and a set of flyweights in the body of the drive unit. When the starter motor begins turning and the drive unit is forced forward on the helical shaft by inertia, it is latched into the engaged position. Only once the drive unit is spun at a speed higher than that attained by the starter motor itself (i.e., it is back driven by the running engine) will the flyweights pull radially outward, releasing the latch and permitting the overdriven drive unit to be spun out of engagement. In this manner, unwanted starter disengagement is avoided before a successful engine start.
Chrysler Corporation contributed materially to the modern development of the starter motor. In 1962, Chrysler introduced a starter incorporating a gear train between the motor and the driveshaft. Rolls Royce had introduced a conceptually similar starter in 1946, but Chrysler's was the first volume-production unit. The motor shaft has integrally-cut gear teeth forming a drive gear which mesh with a larger adjacent driven gear to provide a gear reduction ratio of 3.75:1. This permits the use of a higher-speed, lower-current, lighter and more compact motor assembly while increasing cranking torque. Variants of this starter design were used on most vehicles produced by Chrysler Corporation from 1962 through 1987. The Chrysler starter made a unique, readily identifiable sound when cranking the engine.
This starter formed the design basis for the offset gear reduction starters now employed by about half the vehicles on the road, and the conceptual basis for virtually all of them. Many Japanese automakers phased in gear reduction starters in the 1970s and 1980's. Light aircraft engines also made extensive use of this kind of starter, because its light weight offered an advantage.
Those starters not employing offset gear trains like the Chrysler unit generally employ planetary epicyclic gear trains instead. Direct-drive starters are almost entirely obsolete owing to their larger size, heavier weight and higher current requirements. Ford also issued a nonstandard starter, a direct-drive "movable pole shoe" design that provided cost reduction rather than electrical or mechanical benefits. This type of starter eliminated the solenoid, replacing it with a movable pole shoe and a separate starter relay. 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. One of the pole shoes, hinged at the front, linked to the starter drive, and spring-loaded away from its normal operating position, swung into position. This 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 pole shoe, and with it, the pinion gear.
This starter was used on Ford vehicles from 1973 through 1990, when a gear-reduction unit conceptually similar to the Chrysler unit replaced it.
Some gas turbine engines and Diesel engines, particularly on trucks, use a pneumatic self-starter. The system consists of a geared turbine, an air compressor and a pressure tank. Compressed air released from the tank is used to spin the turbine, and through a set of reduction gears, engages the ring gear on the flywheel, much like an electric starter. The engine, once running, powers the compressor to recharge the tank.
Another method, for large diesel engines, uses additional valves in cylinder heads. Compressed air is let in the cylinders so that its pressure pushes pistons down when appropriate; at the upward piston movement, air is discharged through normal exhaust valves.
Since large trucks typically use air brakes, the system does double duty, supplying compressed air to the brake system. Pneumatic starters have the advantages of delivering high torque, mechanical simplicity and reliability. They eliminate the need for oversized, heavy storage batteries in prime mover electrical systems.
Auxiliary starter engine
A large, high power Diesel engine, such as those used in off-road heavy equipment, may have a small gasoline-powered engine attached to the side as a starter.
These were also sometimes called pony engines. On some applications, they shared the same cooling system and oil supply. As the pony engine warmed up, it circulated warm coolant and warm oil in the diesel engine. In addition to making it easier to crank, it improved the service life.
Another way to provide for shutting off a car's engine when it is stopped, then immediately restarting it when it's time to go, is by employing a static-start engine. Such an engine requires no starter motor, but employs sensors to determine the exact position of each piston, then precisely timing the injection and ignition of fuel to turn over the engine.
While this concept is elegant in theory, and quite doable with sensors and precision crankshaft braking to position the piston(s) at the right position(s), obviously this will mean the piston(s) will be required to hold compressed air-fuel mixture for a considerable length of time at position past the TDC (Top Dead Centre) for the engine to be turned over with the power stroke(s) when the time comes. However, pistons and piston rings are not perfect seals in current engine production / design practices, due to costs / manufacturing / metal expansion tolerances considerations. The normal gas blow-by which is normally negligible, due to the very short time interval of pistons travel at speed, will leak away past the rings to the crankcase if the pistons are required to hold the compressed mixture for a long time.
Unless extremely small clearances in the fit of the pistons and rings with the cylinders are designed into the engine to hold the mixture indefinitely (as in some situations, one can be away for days or weeks before driving the car), eventually all the mixture will leak away in time. Other considerations include maintenance services on the engine, such as changing spark plugs and malfunctions in the pistons positioning system; these will release all the fuel mixture inside the cylinders and render the engine non-startable, unless there is the starter to provide the necessary assistance. For Diesel engines, this concept will require even higher precisions than the extremely high tolerances the Spark Ignition Engines already require; due to their extremely high compression ratios. These ultra-tight tolerances will be detrimental to the free movement of the pistons - if not impossible - once the engine reaches operating temperature, due to metal expansion. Thus, with the current automotive technologies and engine designs, we will probably not see the starter disappear anytime soon.