A distributor is an electric and mechanical device used in the

ignition system An ignition system generates a spark or heats an electrode to a high temperature to ignite a fuel-air mixture in spark ignition internal combustion engines, oil-fired and gas-fired boilers, rocket engines, etc. The widest application for spark i ...

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, ...

s. The distributor's main function is to route electricity from the

ignition coil An ignition coil (also called a spark coil) is an induction coil in an automobile's ignition system that transforms the battery's voltage to the thousands of volts needed to create an electric spark in the spark plugs to ignite the fuel. So ...

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

via

brushes A brush is a common tool with bristles, wire or other filaments. It generally consists of a handle or block to which filaments are affixed in either a parallel or perpendicular orientation, depending on the way the brush is to be gripped durin ...

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 A cylinder (from ) has traditionally been a three-dimensional solid, one of the most basic of curvilinear geometric shapes. In elementary geometry, it is considered a prism with a circle as its base. A cylinder may also be defined as an infin ...

. 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 High tension leads or high tension cables or spark plug wires or spark plug cables, colloquially referred to as HT leads, are the wires that connect a distributor, ignition coil, or magneto to each of the spark plugs in some types of internal comb ...

), where it ignites the air-fuel mixture in the combustion chamber. On most

overhead valve engine An overhead valve (OHV) engine, sometimes called a ''pushrod engine'', is a piston engine whose valves are located in the cylinder head above the combustion chamber. This contrasts with earlier flathead engines, where the valves were located b ...

s, the distributor shaft is driven by a gear on the camshaft, often shared with the oil pump; on most

overhead camshaft engine An overhead camshaft (OHC) engine is a piston engine where the camshaft is located in the cylinder head above the combustion chamber. This contrasts with earlier overhead valve engines (OHV), where the camshaft is located below the combustion ...

s, the distributor shaft is attached directly to a camshaft. Older distributor designs used a

cam Calmodulin (CaM) (an abbreviation for calcium-modulated protein) is a multifunctional intermediate calcium-binding messenger protein expressed in all eukaryotic cells. It is an intracellular target of the secondary messenger Ca2+, and the bin ...

on the distributor shaft that operates the

contact breaker A contact breaker (or "points") is a type of electrical switch, found in the ignition systems of spark-ignition internal combustion engines. The switch is automatically operated by a cam driven by the engine. The timing of operation of the switch ...

(also called ''points''). Opening the points causes a high

induction Induction, Inducible or Inductive may refer to: Biology and medicine * Labor induction (birth/pregnancy) * Induction chemotherapy, in medicine * Induced stem cells, stem cells derived from somatic, reproductive, pluripotent or other cell t ...

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 difference (the Hall voltage) across an electrical conductor that is transverse to an electric current in the conductor and to an applied magnetic field perpendicular to the current. It was dis ...

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 f ...

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 Manifold vacuum, or engine vacuum in an internal combustion engine is the difference in air pressure between the engine's intake manifold and Earth's atmosphere. Manifold vacuum is an effect of a piston's movement on the induction stroke and the ...

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 Manifold injection is a mixture formation system for internal combustion engines with external mixture formation. It is commonly used in engines with spark ignition that use petrol as fuel, such as the Otto engine, and the Wankel engine. In a mani ...

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 Saab Direct Ignition is a capacitor discharge ignition developed by Saab Automobile, then known as Saab-Scania, and '' Mecel AB'' during the 1980s. It was first shown in 1985 and put into series production in the Saab 9000 in 1988. One of the fi ...

Gallery

History

The first mass-produced electric ignition was the

Delco ignition system The Delco ignition system, also known as the Kettering ignition system, points and condenser ignition or breaker point ignition, is a type of inductive discharge ignition system invented by Charles F. Kettering. It was first sold commercially on ...

, which was introduced in the 1910

Cadillac Model 30 The Cadillac Model Thirty is an automobile that was introduced in December 1909 by the Cadillac Division of General Motors, and sold through 1911. It was the company's only model for those years and was based on the 1907 Model G. The 1912 Model ...

. In the 1920s, Arthur Atwater Kent Sr invented the competing ''Unisparker'' ignition system. By the end of the 20th century, distributors had been largely replaced by

electronic ignition An ignition system generates a spark or heats an electrode to a high temperature to ignite a fuel-air mixture in spark ignition internal combustion engines, oil-fired and gas-fired boilers, rocket engines, etc. The widest application for spark i ...

systems.

References

{{Authority control Electric power distribution Engine components Ignition systems