thumb|Distributor (side view) with input shaft (bottom) and vacuum advance unit (top right)
thumb|Ignition system diagram with distributor at the top right
A distributor is an electric and mechanical device used in the ignition system of older spark-ignition engines. The distributor's main function is to route electricity from the ignition coil to each spark plug at the correct time.
Design
A distributor consists of a rotating arm ('rotor') that is attached to the top of a rotating 'distributor shaft'. The rotor constantly receives high-voltage electricity from an ignition coil via brushes at the centre of the rotor. As the rotor spins, its tip passes close to (but does not touch) the output contacts for each cylinder. As the electrified tip passes each output contact, the high-voltage electricity is able to 'jump' across the small gap. This burst of electricity then travels to the spark plug (via high tension leads), where it ignites the air-fuel mixture in the combustion chamber.
On most overhead valve engines, the distributor shaft is driven by a gear on the camshaft, often shared with the oil pump; on most overhead camshaft engines, the distributor shaft is attached directly to a camshaft.
Older distributor designs used a cam on the distributor shaft that operates the contact breaker (also called points). Opening the points causes a high induction voltage in the ignition coil.
By the 1980s and 1990s, distributors had been largely replaced by electronic ignition systems.
See also
- List of auto parts