The distributor in the ignition system of an internal combustion engine is a device which routes the high voltage in the correct firing order to the spark plugs.
It consists of a rotating arm or rotor inside the distributor cap, on top of the distributor shaft. The rotor contacts the central high voltage cable from the coil via a spring loaded carbon brush. The rotor arm passes close to (but does not touch) the output contacts which connect via high tension cables to the spark plug of each cylinder. Within the distributor, the high voltage energy is able to jump the small gap from the rotor arm to the contact.
The distributor shaft has a cam that operates the contact breaker. Opening the points causes a high induction voltage in the ignition system's coil.
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 rpm. 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. Usually there is also a capacitor attached to the distributor. The capacitor is connected parallel to the breaker points, to suppress sparking and prevent wear of the points.
Around the 1970s the primary breaker points were largely replaced with Hall effect sensors. As this is a non-contacting device and the primary circuit is controlled by solid-state electronics, a great amount of maintenance in point adjustment and replacement was eliminated. This also eliminates any problem with breaker follower or cam wear, and by eliminating a side load extends distributor shaft bearing life. The remaining secondary (high voltage) circuit was as described above, using a single coil and a rotary distributor.
Modern engine designs are tending to do away with the distributor and coil, instead performing the distribution function in the primary circuit electronically and applying the primary pulse to individual coils on top of each spark plug (Direct Ignition or coil-on-plug). This avoids the need to switch very high voltages, which is very often a source of trouble, especially in damp conditions.
Function
The purpose of the distributor is to distribute the high tension voltage surges to the individual spark plugs in the correct sequence, and at the correct instant in time, in the engine cycle.
Since each high voltage surge is triggered by the opening of the primary circuit, it is convenient to locate a single set of contacts in the distributor housing, and operate them by lobes formed on a cam which is driven by the distributor shaft.
The number of cam lobes corresponds to the number of engine cylinders. Ignition pulses occur as each lobe opens the contacts to break the circuit, and then allows them to close, re-establishing the primary circuit in readiness for the next opening.
An insulated rotor arm with a brass electrode is keyed to the shaft directly above the cam, and rotates within a moulded insulated distributor cap held by clips on the distributor housing. The cap has the same number of connecting outlets for the ignition leads as there are engine cylinders, and a central terminal locates the high tension lead from the ignition coil. An internal spring loaded carbon brush conducts each ignition surge from the central terminal to the centre of the rotor electrode as it turns.
By arranging the surge to occur when the rotor electrode is opposite a fixed electrode inside the cap the high voltage bridges the small gap between them, driving current through the ignition lead for that cylinder and bridging the gap at the spark plug.
By positioning the rotor arm opposite the electrode for Number One cylinder when it is at the end of it's compression stroke, then connecting successive leads in the engine firing order, each surge occurs in the correct sequence.
Source: CDX Global & Wikipedia - en.wikipedia.org
