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An automobile starter motor

A starter motor (also starting motor or starter) is an electric motor for rotating an internal-combustion engine so as to initiate the engine's operation under its own power.


A 1920s era self-starter

Typical starter installed underneath and toward the rear of an automobile engine

Both Otto cycle and Diesel cycle internal-combustion engines require an external source of power to start, unlike steam engines, as well as electric, hydraulic and air motors. Internal combustion engines use torque from the power strokes of other cylinders, or stored energy in the flywheel (single-cylinder engines) to compress incoming air (and gasoline vapor, for gasoline engines) before it is ignited to develop power.

Originally, a hand-crank was used to start engines, but it was inconvenient, difficult, and dangerous to crank-start an engine. Even though cranks had an overrun mechanism, when the engine started, the crank could begin to spin along with the crankshaft and potentially strike the person cranking the engine. Additionally, care had to be taken to retard the spark in order to prevent backfiring; with an advanced spark setting, the engine could kick back (run in reverse), pulling the crank with it, because the overrun safety mechanism works in one direction only.

Although users were advised to cup their fingers under the crank and pull up, it felt natural for operators to grasp the handle with the fingers on one side, the thumb on the other. Even a simple backfire could result in a broken thumb; it was possible to end up with a broken wrist, or worse. Moreover, increasingly larger engines with higher compression ratios made hand cranking a more physically demanding endeavour.

While the need was fairly obvious—as early as 1899, Clyde J. Coleman applied for U.S. Patent 745,157 for an electric automobile self-starter—inventing one that worked successfully in most conditions did not occur until 1911 when Charles F. Kettering of Dayton Engineering Laboratories Company (DELCO) invented and filed for U.S. Patent 1,150,523 for the first useful electric starter. (Kettering had replaced the hand crank on NCR's cash registers with an electric motor five years earlier.) One aspect of the invention lay in the realization that a relatively small motor, driven with higher voltage and current than would be feasible for continuous operation, could deliver enough power to crank the engine for starting. At the voltage and current levels required, such a motor would burn out in a few minutes of continuous operation, but not during the few seconds needed to start the engine. The starters were first installed by Cadillac on production models in 1912. These starters also worked as generators once the engine was running, a concept that is now being revived in hybrid vehicles. The Model T relied on hand cranks until 1919; by 1920 most manufacturers included self-starters, thus ensuring that anyone, regardless of strength or physical handicap, could easily start a car with an internal combustion engine.

Before Chrysler's 1949 innovation of the key-operated combination ignition-starter switch,[1] the starter was often operated by the driver pressing a button mounted on the floor or dashboard. Some vehicles had a pedal in the floor that manually engaged the starter drive pinion with the flywheel ring gear, then completed the electrical circuit to the starter motor once the pedal reached the end of its travel. Ferguson tractors from the 1940s had an extra position on the gear lever that engaged the starter switch, ensuring safety by preventing the tractors from being started in gear.[2]

Electric starter

  1. Main Housing (yoke)
  2. Overrunning clutch, and Pinion gear assembly
  3. Armature
  4. Field coils with Brushes attached
  5. Brush-carrier
  6. Solenoid

The modern starter motor is either a permanent-magnet or a series-parallel wound direct current electric motor with a starter solenoid (similar to a relay) mounted on it. When current from the starting battery is applied to the solenoid, usually through a key-operated switch, the solenoid engages a lever that pushes out the drive pinion on the starter driveshaft and meshes the pinion with the starter ring gear on the flywheel of the engine.

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, or if there is a short and the solenoid remains engaged), the pinion will spin independently of its driveshaft. This prevents the engine driving the starter, for such backdrive 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 were made. Also, a standard starter motor is only designed for intermittent use which would preclude its use as a generator; the electrical components are designed only to operate for typically under 30 seconds before overheating (by too-slow dissipation of heat from ohmic losses), to save weight and cost. This is the same reason why most automobile owner's manuals instruct the operator to pause for at least ten seconds after each ten or fifteen seconds of cranking the engine, when trying to start an engine that does not start immediately.

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, backdrive 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 backdriven 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.

Gear reduction

Chrysler Corporation contributed materially to the modern development of the starter motor. In 1962, Chrysler introduced a starter incorporating a geartrain between the motor and the driveshaft. Rolls Royce had introduced a conceptually similar starter in 1946,[citation needed] but Chrysler's was the first volume-production unit. The motor shaft has integrally cut gear teeth forming a pinion which meshes 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.[3] Variants of this starter design were used on most rear- and four-wheel-drive vehicles produced by Chrysler Corporation from 1962 through 1987. It makes a unique, distinct sound when cranking the engine, which led to it being nicknamed the "Highland Park Hummingbird"—a reference to Chrysler's headquarters in Highland Park, Michigan.[4]

The Chrysler gear-reduction starter formed the conceptual basis for the gear-reduction starters that now predominate in vehicles on the road. Many Japanese automakers phased in gear reduction starters in the 1970s and 1980s.[citation needed] Light aircraft engines also made extensive use of this kind of starter, because its light weight offered an advantage.

Those starters not employing offset geartrains like the Chrysler unit generally employ planetary epicyclic geartrains instead. Direct-drive starters are almost entirely obsolete owing to their larger size, heavier weight and higher current requirements.[citation needed]

Movable pole shoe

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. This starter operates as follows: The driver turns the key, activating the starter switch. A small electric current flows through the switch-type starter solenoid, closing the contacts and sending large battery current to the starter motor. One of the pole shoes, hinged at the front, linked to the starter drive, and spring-loaded away from its normal operating position, is swung into position by the magnetic field created by electricity flowing through its field coil. This moves the starter drive forward to engage the flywheel ring gear, and simultaneously xclosesx OPENS a pair of contacts supplying current to the rest of the starter motor winding. Once the engine starts and the driver releases the starter switch, a spring retracts the pole shoe, which pulls the starter drive out of engagement with the ring 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.

Pneumatic starter

Main article: Air start system

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.

Aircraft with large gas turbine engines are typically started using a large volume of low-pressure compressed air, supplied from a very small engine referred to as an auxiliary power unit, located elsewhere in the aircraft. After starting the main engines, the APU often continues to operate, supplying additional power to operate aircraft equipment. Alternately, aircraft engines can be rapidly started using a mobile ground-based pneumatic starting engine, referred to as a start cart or air start cart.

On larger diesel generators found in large static installations and especially on ships, a pneumatic starting gear is used. The air motor is normally powered by compressed air at pressures of 10–30 bar. The air motor is made up of a center drum about the size of a soup can with four or more slots cut into it to allow for the vanes to be placed radially on the drum to form chambers around the drum. The drum is offset inside a round casing so that the inlet air for starting is admitted at the area where the drum and vanes form a small chamber compared to the others. The compressed air can only expand by rotating the drum which allows the small chamber to become larger and puts another one of the cambers in the air inlet. The air motor spins much too fast to be used directly on the flywheel of the engine, instead a large gearing reduction such as a planetary gear is used to lower the output speed. A Bendix gear is used to engage the flywheel.

On large diesel generators and almost all diesel engines used as the prime mover of ships will use compressed air acting directly on the cylinder head. This is not ideal for smaller diesels as it provides too much cooling on starting. Also the cylinder head needs to have enough space to support an extra valve for the air start system. The air start system operates very similar to a distributor in a car. There is an air distributor that is geared to the camshaft of the diesel engine, on the top of the air distributor is a single lobe similar to what is found on a camshaft. Arranged radially around this lobe are roller tip followers for every cylinder. When the lobe of the air distributor hits one of the followers it will send an air signal that acts upon the back of the air start valve located in the cylinder head causing it to open. The actual compressed air is provided from a large reservoir that feeds into a header located along the engine. As soon as the air start valve is opened the compressed air is admitted and the engine will begin turning. It can be used on 2-cycle and 4-cycle engines and on reversing engines. On large 2-stroke engines less than one revolution of the crankshaft is needed for starting.

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.

Hydraulic starter

Some diesel engines from 6 to 16 cylinders are started by means of a hydraulic motor. Hydraulic starters and the associated systems provide a sparkless, reliable method of engine starting at a wide temperature range. Typically hydraulic starters are found in applications such as remote generators, lifeboat propulsion engines, offshore fire pumping engines, and hydraulic fracturing rigs. The system used to support the hydraulic starter includes valves, pumps, filters, a reservoir, and piston accumulators. The operator can manually recharge the hydraulic system; this cannot readily be done with air or electric starting systems, so hydraulic starting systems are favoured in applications wherein emergency starting is a requirement.

Hydraulic Starter

Hydraulic Starter

Donkey or pony engine

Used on larger engines a small petrol engine was used to turn the main engine to start it. Caterpillar used this system on its early diesels and still used it after suitable electric starters were in common usage as it saved having large batteries & electrics on the tractors which often worked in remote sites. it was used on machine from the Caterpillar D2 right up to the Caterpillar D9 until the 1970s. On some models the exhaust gases from the donkey engine warm the inlet to the main engine.

Other methods

Before the advent of the starter motor, engines were started by various methods including wind-up springs, gun powder cylinders, and human-powered techniques such as a removable crank handle which engaged the front of the crankshaft, pulling on an airplane propeller, or pulling a cord that was wound around an open-face pulley (as still commonly used on very small engines for lawn mowers and chain saws).

The behaviour of an engine during starting is not always predictable. The engine can kick back, causing sudden reverse rotation. Many manual starters included a one-directional slip or release provision so that once engine rotation began, the starter would disengage from the engine. In the event of a kickback, the reverse rotation of the engine could suddenly engage the starter, causing the crank to unexpectedly and violently jerk, possibly injuring the operator. For cord-wound starters, a kickback could pull the operator towards the engine or machine, or swing the starter cord and handle at high speed around the starter pulley.

Self starting

Some modern gasoline engines with twelve or more cylinders always have at least one piston at the beginning of its power stroke and are able to start by injecting fuel into that cylinder and igniting it.



See also

  • Coffman starter
  • Hucks starter
  • Hybrid Synergy Drive
  • Vincent Hugo Bendix


  1. "Chrysler Family Debut", Popular Mechanics April 1949, p.122
  2. Ralph W. Sanders. Vintage farm tractors: the ultimate tribute to classic tractors, 98. 
  3. Chrysler Corporation (November, 1961), The 1962 Starting Motor and Alternator, Chrysler Corporation. 
  4. LaChance, David (2007-06), "Memorable Mirada", Hemmings Classic Car, Retrieved on . ""…the twitterings of the Highland Park hummingbird, Mopar's famous gear-reduction starter…"". 

External links

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