The modern starter motor is either a permanent magnet or a series/series-parallel wound direct current electric motor. A solenoid switch is mounted on it. When current from the battery is applied to the solenoid through a key-operated switch, it pushes out the drive pinion on the starter drive shaft and meshes the pinion with the ring gear on the flywheel of the engine.
Gear-reduction starters are almost entirely obsolete owing to their larger size, heavier weight and higher current requirements. A direct-drive, movable pole shoe design are mainly used for 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.
Some gas turbine engines and Diesel engines (from trucks and earth moving equipment) use pneumatic starters. These starters consist of a geared turbine, an air compressor and a pressure tank. Compressed air released from the tank is used to spin the turbine. Apart from it, a set of reduction gears engages the ring gear on the flywheel. The engine, once running, powers the compressor to recharge the tank.