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Typical distributor with distributor cap.
Also visible are mounting/drive shaft (bottom), vacuum advance unit (right) and capacitor (centre). Spinterogeno.JPG
Typical distributor with distributor cap.
Also visible are mounting/drive shaft (bottom), vacuum advance unit (right) and capacitor (centre).
Car ignition system. Upper right is Distributor. Car ignition system.svg
Car ignition system. Upper right is Distributor.

A distributor is an enclosed rotating switch used in spark-ignition internal combustion engines that have mechanically timed ignition. The distributor's main function is to route 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 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 first reliable battery operated ignition was the Delco ignition system developed by Dayton Engineering Laboratories Co. (Delco) and introduced in the 1910 Cadillac Model 30. 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. [1] 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.


A 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). 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. [2]

The distributor shaft has a cam that operates the contact breaker (also called points). Opening the points causes a high induction voltage in the system's ignition coil. [2]

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). 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 to prevent excessive wear of the points.

Around the 1970s[ citation needed ] the primary breaker points were largely replaced with a Hall effect sensor or optical sensor. 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. This also eliminates any problem with breaker follower or cam wear, and by eliminating a side load it extends distributor shaft bearing life. The remaining secondary (high voltage) circuit stayed essentially the same, using an ignition coil and a rotary distributor.

Most distributors used on electronically fuel injected engines lack vacuum and centrifugal advance units. On such distributors, the timing advance is controlled electronically by the engine computer. 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). Additionally, eliminating vacuum and centrifugal advance results in a simpler and more reliable distributor.

Distributor cap

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.

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. 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. Another implementation is the wasted spark system, where a single contact serves two leads, but in that case each lead connects one cylinder. In General Motors high energy ignition (HEI) systems there is no central post and the ignition coil sits on top of the distributor. Some Toyota and Honda engines also have their coil within the distributor cap. On the inside of the cap there is a terminal that corresponds to each post, and the plug terminals are arranged around the circumference of the cap according to the firing order in order to send the secondary voltage to the proper spark plug at the right time.

The rotor is attached to the top of the distributor shaft which is driven by the engine's camshaft and thus synchronized to it. Synchronization to the camshaft is required as the rotor must turn at exactly half the speed of the main crankshaft in the 4-stroke cycle. Often, the rotor and distributor are attached directly to the end of one of the (or the only) camshaft, at the opposite end to the timing drive belt. This rotor is pressed against a carbon brush on the center terminal of the distributor cap which connects to the ignition coil. The rotor is constructed such that the center tab is electrically connected to its outer edge so the current coming in to the center post travels through the carbon point to the outer edge of the rotor. As the camshaft rotates, the rotor spins and its outer edge passes each of the internal plug terminals to fire each spark plug in sequence.

Engines that use a mechanical distributor may fail if they run into deep puddles because any water that gets onto the distributor can short out the electric current that should go through the spark plugs, rerouting it directly to the body of the vehicle. This in turn causes the engine to stop as the fuel is not ignited in the cylinders. [3] This problem can be fixed by removing the distributor's cap and drying the cap, cam, rotor and the contacts by wiping with tissue paper or a clean rag, by blowing hot air on them, or using a moisture displacement spray e.g. WD-40 or similar. Oil, dirt or other contaminants can cause similar problems, so the distributor should be kept clean inside and outside to ensure reliable operation. [4] Some engines include a rubber o-ring or gasket between the distributor base and cap to help prevent this problem. The gasket is made of a material like Viton or butyl for a tight seal in extreme temperatures and chemical environments. [5] This gasket should not be discarded when replacing the cap. Most distributor caps have the position of the number 1 cylinder's terminal molded into the plastic. By referencing a firing order diagram and knowing the direction the rotor turns, (which can be seen by cranking the engine with the cap off) the spark plug wires can be correctly routed. Most distributor caps are designed so that they cannot be installed in the wrong position. Some older engine designs allow the cap to be installed in the wrong position by 180 degrees, however. The number 1 cylinder position on the cap should be noted before a cap is replaced.

The distributor cap is a prime example of a component that eventually succumbs to heat and vibration. It is a relatively easy and inexpensive part to replace if its bakelite housing does not break or crack first. Carbon deposit accumulation or erosion of its metal terminals may also cause distributor-cap failure.

As it is generally easy to remove and carry off, the distributor cap can be taken off as a means of theft prevention. Although not practical for everyday use, because it is essential for the starting and running of the engine, its removal thwarts any attempt at hot-wiring the vehicle.

Direct and distributorless ignition

Modern engine designs have abandoned the high-voltage distributor and coil, instead performing the distribution function in the primary circuit electronically and applying the primary (low-voltage) pulse to individual coils for each spark plug, or one coil for each pair of companion cylinders in an engine (two coils for a four-cylinder, three coils for a six-cylinder, four coils for an eight-cylinder, and so on).

In traditional remote distributorless systems, the coils are mounted together in a transformer oil filled coil pack, or separate coils for each cylinder, which are secured in a specified place in the engine compartment with wires to the spark plugs, similar to a distributor setup. General Motors, Ford, Chrysler, Hyundai, Subaru, Volkswagen and Toyota are among the automobile manufacturers known to have used coil packs. Coil packs by Delco for use with General Motors engines allow removal of the individual coils in case one should fail, but in most other remote distributorless coil pack setups, if a coil were to fail, replacement of the whole pack would be required to fix the problem.

More recent layouts utilize a coil located very near to each spark plug known as coil-near-plugs, or directly on top of each spark plug known as direct ignition (DI) or coil-on-plug (COP). This design avoids the need to transmit very high voltages, which is often a source of trouble, especially in damp conditions.

Both direct and remote distributorless systems also allow finer levels of ignition control by the engine computer, which helps to increase power output, decrease fuel consumption and emissions, and implement features such as cylinder deactivation. Spark plug wires, which need routine replacement due to degradation, are also eliminated when the individual coils are mounted directly on top of each plug, since the high voltages and fields exist only over a very short distance from the coil to the plug.

Wasted spark

The distributor can be eliminated on four-stroke engines by using the wasted spark principle. An ignition pulse is delivered to two cylinders at the same time,chosen so that one cylinder is in an exhaust stroke while the other is about to begin the power stroke. The spark in the cylinder on the exhaust phase is wasted. Each end of the ignition coil winding is connected to a spark plug and they fire in pairs.

A single-cylinder engine has only one spark plug and so needs no distributor. Ignition systems on such engines may produce a wasted spark during the exhaust stroke.

See also

Related Research Articles

Ignition magneto Electricity-producing device

An ignition magneto, or high-tension magneto, is a magneto that provides current for the ignition system of a spark-ignition engine, such as a petrol engine. It produces pulses of high voltage for the spark plugs. The older term tension means voltage.

Engine tuning

Engine tuning is the adjustment or modification of the internal combustion engine or Engine Control Unit (ECU) to yield optimal performance and increase the engine's power output, economy, or durability. These goals may be mutually exclusive; an engine may be de-tuned with respect to output power in exchange for better economy or longer engine life due to lessened stress on engine components.

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 ignition internal combustion engines is in petrol (gasoline) road vehicles such as cars and motorcycles.

Contact breaker

A contact breaker 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 is set so that a spark is produced at the right time to ignite the compressed air/fuel mixture in the cylinder of the engine. A mechanism may be provided to slightly adjust timing to allow for varying load on the engine. Since these contacts operate frequently, they are subject to wear, causing erratic ignition of the engine. More recent engines use electronic means to trigger the spark, which eliminated contact wear and allows computer control of ignition timing.

A wasted spark system is a type of ignition system used in some four-stroke cycle internal combustion engines. In a wasted spark system, the spark plugs fire in pairs, with one plug in a cylinder on its compression stroke and the other plug in a cylinder on its exhaust stroke. The extra spark during the exhaust stroke has no effect and is thus "wasted". This design halves the number of components necessary in a typical ignition system, while the extra spark, against much reduced dielectric resistance, barely impacts the lifespan of modern ignition components. In a typical engine, it requires only about 2–3 kV to fire the cylinder on its exhaust stroke. The remaining coil energy is available to fire the spark plug in the cylinder on its compression stroke.

The Hyundai Beta engines are 1.6 L to 2.0 L I4 built in Ulsan, South Korea.

High energy ignition, also known as H.E.I., is an electronic ignition system designed by the Delco-Remy Division of General Motors. It was used on all GM vehicles, at least in the North American market, from 1975 through the mid-1980s. There were many design variations over the years, and provisions for computer controls were added for some applications starting in the late 1970s. A predecessor system called "Unitized Ignition" was optional on 1972 and 1973 Pontiacs.

Alfa Romeo Twin Spark engine Motor vehicle engine

Alfa Romeo Twin Spark (TS) technology was used for the first time in the Alfa Romeo Grand Prix car in 1914. In the early 1960s it was used in their race cars to enable it to achieve a higher power output from its engines. And in the early and middle 1980s, Alfa Romeo incorporated this technology into their road cars to enhance their performance and to comply with stricter emission controls.

Ignition coil Automobile fuel ignition system component

An ignition 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. Some coils have an internal resistor, while others rely on a resistor wire or an external resistor to limit the current flowing into the coil from the car's 12-volt supply. The wire that goes from the ignition coil to the distributor and the high voltage wires that go from the distributor to each of the spark plugs are called spark plug wires or high tension leads. Originally, every ignition coil system required mechanical contact breaker points and a capacitor (condenser). More recent electronic ignition systems use a power transistor to provide pulses to the ignition coil. A modern passenger automobile may use one ignition coil for each engine cylinder, eliminating fault-prone spark plug cables and a distributor to route the high voltage pulses.

Capacitor discharge ignition

Capacitor discharge ignition (CDI) or thyristor ignition is a type of automotive electronic ignition system which is widely used in outboard motors, motorcycles, lawn mowers, chainsaws, small engines, turbine-powered aircraft, and some cars. It was originally developed to overcome the long charging times associated with high inductance coils used in inductive discharge ignition (IDI) systems, making the ignition system more suitable for high engine speeds. The capacitive-discharge ignition uses capacitor discharge current to the coil to fire the spark plugs.

Ignition timing

In a spark ignition internal combustion engine, Ignition timing refers to 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 following outline is provided as an overview of and topical guide to automobiles:

Low tension coil

A low tension coil is an electrical device used to create a spark across the points of an ignitor on early 1900s gasoline engines, generally flywheel engines, hit and miss engines, and other engines of that era. In modern electronic terms, a low tension coil is simply a large inductor, an electrical device that stores energy for brief periods. The term "low tension" was the terminology of the day used to differentiate it from the term "high tension", and generally meant "low voltage" (tension) as opposed to "high voltage" (tension). High tension coils produce high voltages, generally meant to produce a spark across a spark plug.

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 combustion engine. "High tension lead" or "cable" is also used for any electrical cable carrying a high voltage in any context. Tension in this instance is a synonym for voltage. High tension leads, like many engine components, wear out over time. Each lead contains only one wire, as the current does not return through the same lead, but through the earthed/grounded engine which is connected to the opposite battery terminal. High tension may also be referred to as HT.

Inductive discharge ignition systems were developed in the 19th century as a means to ignite the air–fuel mixture in the combustion chamber of internal combustion engines. The first versions were low tension coils, then low-tension and in turn high-tension magnetos, which were offered as a more effective alternative to the older-design hot-tube ignitors that had been utilized earlier on hot tube engines. With the advent of small stationary engines; and with the development of the automobile, engine-driven tractors, and engine-driven trucks; first the magneto and later the distributor-type systems were utilized as part of an efficient and reliable engine ignition system on commercially available motorized equipment. These systems were in widespread use on all cars and trucks through the 1960s. Manufacturers such as Ford, General Motors, Chrysler, Citroen, Mercedes, John Deere, International Harvester, and many others incorporated them into their products. The inductive discharge system is still extensively used today.

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 the 1912 Cadillac and was manufactured by Delco. Over time, this system was used extensively by all automobile and truck manufacturers on spark ignition, i.e., gasoline engines. Today the system is still widely used in coil-on-plug, coil-near-plug and in coil packs in distributorless ignitions. An alternative ignition system used in automobiles has been capacitor discharge ignition, which is primarily found now as aftermarket upgrade systems. Electronic ignition was a common term for Kettering inductive ignition with the points replaced with an electronic switch such as a transistor.

Magneto Electricity-producing machine

A magneto is an electrical generator that uses permanent magnets to produce periodic pulses of alternating current. Unlike a dynamo, a magneto does not contain a commutator to produce direct current. It is categorized as a form of alternator, although it is usually considered distinct from most other alternators, which use field coils rather than permanent magnets.

Austin 25/30 Car model

The Austin 25-30 is a motor car. It was the first automobile produced by newly established British car manufacturer Austin.

Trembler coil

A trembler coil, buzz coil or vibrator coil is a type of high-voltage ignition coil used in the ignition system of early automobiles, most notably the Benz Patent-Motorwagen and the Ford Model T. Its distinguishing feature is a vibrating magnetically-activated contact called a trembler or interrupter, which breaks the primary current, generating multiple sparks during each cylinder's power stroke. Trembler coils were first used on the 1886 Benz automobile, and were used on the Model T until 1927.


  1. "Cadillac History | Kanter Car Tales". kanter-car-tales.com. Retrieved 2016-02-12.
  2. 1 2 "How the ignition system works". How a Car Works. Retrieved 2016-02-12.
  3. "Misfire from driving through "wave of water" likely result of wet distributor cap — a problem that's not costly to fix". Post and Courier. Retrieved 2016-02-12.
  4. mitmaks (2014-03-23), Cleaning distributor cap , retrieved 2016-02-12
  5. "Rubber Gaskets and Soft Gaskets - Mercer Gasket & Shim". Mercer Gasket & Shim. Retrieved 2016-02-12.