Adistributor is an electric and mechanical device used in theignition system of olderspark ignition engines. The distributor's main function is to route electricity from theignition coil to eachspark plug at the correct time.
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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 anignition coil viabrushes at the centre of the rotor. As the rotor spins, its tip passes close to (but does not touch) the output contacts for eachcylinder. As the electrified tip passes each output contact, the high-voltage electricity is able to 'jump' across the small gap.[1] This burst of electricity then travels to thespark plug (viahigh tension leads), where it ignites the air-fuel mixture in the combustion chamber.
On mostoverhead valve engines, the distributor shaft is driven by a gear on thecamshaft, often shared with theoil pump; on mostoverhead camshaft engines, the distributor shaft is attached directly to a camshaft.
Older distributor designs used acam on the distributor shaft that operates thecontact breaker (also calledpoints). Opening the points causes a highinduction voltage in the ignition coil.[1] This design was superseded by an electronically controlled ignition coil with a sensor (usuallyHall effect or optical) to control the timing of the ignition coil charging.
In older distributors, adjusting theignition timing is usually achieved through bothmechanical advance andvacuum 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 usesmanifold 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 onelectronic 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.
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. SeeSaab Direct Ignition.
The first mass-produced electric ignition was theDelco ignition system, which was introduced in the 1910Cadillac Model 30. In 1921,Arthur Atwater Kent Sr invented the competingUnisparker ignition system.[2]
By the 1980s and 1990s, distributors had been largely replaced byelectronic ignition systems.
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