Direct-ignition engine

A distributor is an enclosed rotating shaft used in spark-ignition internal combustion engines that have mechanically-timed ignition. The distributor's main function is to route secondary, or high voltage, current from the ignition coil to the spark plugs in the correct firing order, and for the correct amount of time. Except in magneto systems, the distributor also houses a mechanical or inductive breaker switch to open and close the ignition coil's primary circuit.

The first reliable battery operated ignition was developed by Dayton Engineering Laboratories Co. (Delco) and introduced in the 1910 Cadillac. This ignition was developed by Charles Kettering and was considered a wonder in its day. Atwater Kent invented his Unisparker ignition system about this time in competition with the Delco system. By the end of the 20th century mechanical ignitions were disappearing from automotive applications in favor of inductive or capacitive electronic ignitions fully controlled engine control units (ECU), rather than directly timed to the engine's crankshaft speed.

A distributor consists of a rotating arm or inside the distributor cap, on top of the distributor shaft, but insulated from it and the body of the vehicle (ground). The distributor shaft is driven by a gear on the camshaft on most overhead valve engines, and attached directly to a camshaft on most overhead cam engines. (The distributor shaft may also drive the oil pump.) The metal part of the rotor contacts the high voltage cable from the ignition coil via a spring-loaded carbon brush on the underside of the distributor cap. The metal part of the rotor arm passes close to (but does not touch) the output contacts which connect via high tension leads to the spark plug of each cylinder. As the rotor spins within the distributor, electric current is able to jump the small gaps created between the rotor arm and the contacts due to the high voltage created by the ignition coil.

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Wikipedia