There are some exceptions however, as some engines (many Alfa Romeo cars, some 1980s Nissans) have two spark plugs per cylinder, so there are two leads coming out of the distributor per cylinder. The distributor cap has one post for each cylinder, and in points ignition systems there is a central post for the current from the ignition coil coming into the distributor. The distributor cap is the cover that protects the distributor's internal parts and holds the contacts between internal rotor and the spark plug wires. Additionally, eliminating vacuum and centrifugal advance results in a simpler and more reliable distributor. This allows more accurate control of ignition timing, as well as the ability to alter timing based on factors other than engine speed and manifold vacuum (such as engine temperature). On such distributors, the timing advance is controlled electronically by the engine computer. Most distributors used on electronically fuel injected engines lack vacuum and centrifugal advance units. The remaining secondary (high voltage) circuit stayed essentially the same, using an ignition coil and a rotary distributor. This also eliminates any problem with breaker follower or cam wear, and by eliminating a side load it extends distributor shaft bearing life. As this is a non-contacting device and the ignition coil is controlled by solid state electronics, a great amount of maintenance in point adjustment and replacement was eliminated. The capacitor is connected parallel to the breaker points, to suppress sparking to prevent excessive wear of the points.Īround the 1970s the primary breaker points were largely replaced with a Hall effect sensor or optical sensor. Usually there is also a capacitor attached to the distributor. In addition, the distributor has a vacuum advance unit that advances the timing even further as a function of the vacuum in the inlet manifold. The distributor also houses the centrifugal advance unit: a set of hinged weights attached to the distributor shaft, that cause the breaker points mounting plate to slightly rotate and advance the spark timing with higher engine revolutions per minute (rpm). Opening the points causes a high induction voltage in the system's ignition coil. The distributor shaft has a cam that operates the contact breaker (also called points). 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. 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. (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 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. By the end of the 20th century mechanical ignitions were disappearing from automotive applications in favor of inductive or capacitive electronic ignitions fully controlled by engine control units (ECU), rather than directly timed to the engine's crankshaft speed.Ī distributor consists of a rotating arm or rotor inside the distributor cap, on top of the distributor shaft, but insulated from it and the body of the vehicle ( ground). Atwater Kent invented his Unisparker ignition system about this time in competition with the Delco system. This ignition was developed by Charles Kettering and was considered a wonder in its day. (Delco) and introduced in the 1910 Cadillac Model 30. The first reliable battery operated ignition was the Delco ignition system developed by Dayton Engineering Laboratories Co. Except in magneto systems and many modern computer controlled engines that use crank angle/position sensors, the distributor also houses a mechanical or inductive breaker switch to open and close the ignition coil's primary circuit. 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. Upper right is Distributor.Ī distributor is an enclosed rotating shaft used in spark-ignition internal combustion engines that have mechanically timed ignition.
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