A distributor is an electric and mechanical device used in the

ignition system Ignition systems are used by heat engines to initiate combustion by igniting the fuel-air mixture. In a spark ignition versions of the internal combustion engine (such as petrol engines), the ignition system creates a spark to ignite the fuel-ai ...

of older

spark-ignition engine A spark-ignition engine (SI engine) is an internal combustion engine, generally a petrol engine, where the combustion process of the air-fuel mixture is ignited by a spark from a spark plug. This is in contrast to compression-ignition engines, ty ...

s. The distributor's main function is to route electricity from the ignition coil to each spark plug at the correct time.

Design

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 Cam or CAM may refer to: Science and technology * Cam (mechanism), a mechanical linkage which translates motion * Camshaft, a shaft with a cam * Camera or webcam, a device that records images or video In computing * Computer-aided manufacturin ...

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 This design was superseded by an electronically controlled ignition coil with a sensor (usually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 The Hall effect is the production of a voltage, potential difference (the Hall voltage) across an electrical conductor that is wikt:transverse, transverse to an electric current in the conductor and to an applied magnetic field wikt:perpendicul ...

or optical) to control the timing of the ignition coil charging.

Ignition advance

In older distributors, adjusting the

ignition timing In a spark ignition internal combustion engine, ignition timing is the timing, relative to the current piston position and crankshaft angle, of the release of a spark in the combustion chamber near the end of the compression stroke. The need ...

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

History

The first mass-produced electric ignition was the Delco ignition system, which was introduced in the 1910 Cadillac Model 30. In 1921, Arthur Atwater Kent Sr invented the competing ''Unisparker'' ignition system. By the 1980s and 1990s, distributors had been largely replaced by electronic ignition systems.

References