Starter Motors

Internal combustion engines are generally started by rotating the flywheel until the engine fires and continues to run on its own power. The flywheel is usually rotated by a starter motor fed with current from the battery. Petrol engines have to be started at a speed of 50-60 r.p.m.; diesel engines require about 100 r.p.m.

The shaft of the starter is provided with a pinion (a small gear wheel) which, on commencement of the starting operation, is shifted forward until it engages with the toothed rim on the flywheel. The driving motor of the starter then rotates the flywheel. When the engine has started up, the starter pinion is disengaged from the flywheel and retracted. The various types of starter which are in present-day use differ mainly in the manner in which engagement and disengagement is effected.

Pinion shift starter: When the starter button (inside the car) is actuated, the connection between the battery and the solenoid switch is established.

A powerful magnetic field is set up in the magnetic coil of this switch, so that the armature of the switch is pulled forward such a distance that the tilting bridge closes the contact I, with the result that the auxiliary coil and the holding coil are energised. This causes the armature to be drawn, slowly rotating, into the magnetic field of the auxiliary coil and to engage with the toothed rim of the flywheel.

As a result of this movement the release lever is freed and closes the contact II on the tilting bridge. The main coil is now energised, the armature rotates, and the engine is turned. When the starter button is released, the current ceases to flow through the armature, and the latter is pulled back, and thus disengaged from the flywheel, by the return spring.

Inertia gear drive starter (Bendix type starter): In this type of starter the pinion can be shifted along the armature shaft on a quick screw thread. When the starter button is actuated, the armature of the solenoid switch is attracted and the pinion is pushed forward, while it rotates, by the engaging lever. In the second stage the solenoid switch closes, the current ceases to flow through the draw-in coil, the armature of the starter begins to rotate and screws the pinion completely forward until it engages with the gear rim on the flywheel.

The solenoid switch is held in position by the holding coil. The starter motor can now turn the engine. When the latter fires, the engagement lever is pulled back to the home position by the return spring; when the engine begins to rotate faster than the starter, the pinion is disconnected from the armature shaft by a roller-operated freewheel device.

With all types of starter it is possible to perform the first stage of operation by means of a pedal, as exemplified by a gear-shift starter. The design is simpler than in the case of the Bendix type starter because here the pinion is not rotated while it engages with the gear rim.