Spark plug wires (also called high tension leads) are electrical cables used by older

internal combustion engine An internal combustion engine (ICE or IC engine) is a heat engine in which the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit. In an internal comb ...

s to transmit high-voltage electricity from the

distributor A distributor is an electric and mechanical device used in the ignition system of older spark-ignition engines. The distributor's main function is to route electricity from the ignition coil to each spark plug at the correct time. Design ...

to the

spark plugs A spark plug (sometimes, in British English, a sparking plug, and, colloquially, a plug) is a device for delivering electric current from an ignition system to the combustion chamber of a spark-ignition engine to ignite the compressed fuel/air ...

. ''Tension'' in this instance is a synonym for

voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

. High tension may also be referred to as HT. Many modern engines use

coil-on-plug ignition An ignition coil is used in the ignition system of a spark-ignition engine to transform the battery voltage to the much higher voltages required to operate the spark plug(s). The spark plugs then use this burst of high-voltage electricity to ig ...

, a design that does not use spark plug wires.

Design

Spark plug wires have an outer insulation several times thicker than the conductor, made of a very flexible and heat-resistant material such as

silicone In Organosilicon chemistry, organosilicon and polymer chemistry, a silicone or polysiloxane is a polymer composed of repeating units of siloxane (, where R = Organyl group, organic group). They are typically colorless oils or elastomer, rubber ...

EPDM rubber EPDM rubber (ethylene propylene diene monomer rubber) is a type of synthetic rubber that is used in many applications. EPDM is an M-Class rubber under ASTM standard D-1418; the ''M'' class comprises elastomers with a saturated polyethylene chai ...

. The thick insulation prevents

arcing An electric arc (or arc discharge) is an electrical breakdown of a gas that produces a prolonged electrical discharge. The current through a normally nonconductive medium such as air produces a plasma, which may produce visible light. An ar ...

from the cable to an earthed engine component. A rubber "boot" covers each terminal. Dielectric grease can be used to improve insulation; a small amount can be applied in the inside of the rubber boot at each end of each wire to help seal out moisture. Printing on spark plug wires may include a brand name, insulation thickness (in millimeters), insulation material type, cylinder number, and conductor type (suppressor or solid wire). The wire from each spark plug is just long enough to reach the distributor, without excess. 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 (negative terminal on modern engines). Each end of a spark plug wire has a metal terminal that clips onto the spark plug and distributor, coil, or magneto. There are dedicated spark plug wire pliers, tools designed for removing the terminal from a spark plug without damaging it. To reduce radio frequency interference (RFI) produced by the spark being radiated by the wires, which may cause malfunction of sensitive electronic systems in modern vehicles or interfere with the car radio, various means in the spark plug and associated lead have been used over time to reduce the nuisance: # Copper conductors (no suppression) # Resistor in spark plug with copper conductor # Compressed carbon powder as conductor in the lead to act as a resistor # Stainless steel wire wound as a coil in the lead with a resistance of about 1300 ohms/meter since 1980s. This acts as an inductor and a resistor

Coil wire

Older ignition systems with a separate

ignition coil An ignition coil is used in the ignition system of a spark-ignition engine to transform the battery voltage to the much higher voltages required to operate the spark plug(s). The spark plugs then use this burst of high-voltage electricity to ig ...

use a ''coil wire'' between the ignition coil and the

. A coil wire is of the same construction as a spark plug wire, but generally shorter and with different terminals. Some distributors have an ignition coil built inside them, eliminating the need for a separate coil wire, such as the

High Energy Ignition 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. The syste ...

(HEI) system used by General Motors in the 1970s and 1980s. Some engines instead used multiple ignition coils (one for each pair of cylinders) built into a coil pack, eliminating the need for a distributor and coil wire.

Alternatives

Many modern car engines use

, which eliminates the need for spark plug wires.

Diesel engine The diesel engine, named after the German engineer Rudolf Diesel, is an internal combustion engine in which Combustion, ignition of diesel fuel is caused by the elevated temperature of the air in the cylinder due to Mechanics, mechanical Compr ...

s do not use spark plugs, and therefore do not use spark plug wires either.

References

{{Automotive engine , collapsed Auto parts Ignition systems Electric power distribution Engine components

Design

Coil wire

Alternatives

See also

References