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. This design was superseded by an electronically controlled ignition coil with a sensor (usually
Hall effect or optical) to control the timing of the ignition coil charging.
Ignition advance In older distributors, adjusting the
ignition timing is usually achieved through both
mechanical advance and
vacuum advance.
Mechanical advance adjusts the timing based on the engine speed (rpm), using a set of hinged weights attached to the distributor shaft. These weights cause the breaker points mounting plate to slightly rotate, thereby advancing the ignition timing.
Vacuum advance typically uses
manifold vacuum to adjust the ignition timing, for example to improve fuel economy and driveability when minimal power is required from the engine. Most distributors used on
electronic fuel injection engines use electronics to adjust the ignition timing, instead of vacuum and centrifugal systems. This allows the ignition timing to be optimised based on factors other than engine speed and manifold vacuum. ==Direct ignition== Since the early 2000s, many cars have used a 'coil-on-plug'
direct ignition system, whereby a small ignition coil is located directly above the spark plug for each cylinder. This design means that high-voltage electricity is only present in the small distance between each coil and the spark plug. See
Saab Direct Ignition. == Gallery ==