What’re spark plug wires?

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Spark plug wires carry high voltage to spark plugs in internal combustion engines. Traditional systems use wires to connect ignition coil to distributor and spark plugs, while distributorless systems connect coil packs directly to spark plugs. Mishandling wires in high voltage systems can cause serious injury. Wires can be damaged by heat and short to ground, causing misfires.

Spark plug wires are a component found in many internal combustion engines. They usually consist of a conductive core surrounded by several insulating layers, since they are designed to carry very high voltages. In traditional ignition systems, spark plug wires connect the ignition coil to the distributor and the distributor to the spark plugs. With distributorless ignition systems (DIS), the spark plug wires will typically connect the coil packs directly to the spark plugs. Certain systems, such as coil on plug, place the coil packs directly on the spark plugs and do not use spark plug wires.

Many internal combustion engines work by igniting a mixture of fuel and air with the help of a high voltage spark. This spark is typically provided to the cylinder by a spark plug, although it is generated by an ignition coil or similar device. The design of the coils can vary, but they generally work by running voltage across a primary coil, then turning it off. When voltage to the primary is removed, the magnetic field generated by the coil can collapse, generating significantly higher voltage in a secondary winding. If the ignition system has a distributor, this high voltage can be sent through a special spark plug wire, often known as the coil wire.

In a system with a distributor, the high voltage spark can be transferred through a rotor to the correct spark plug wire. The camshaft spins a rotor inside the distributor, allowing the spark to be properly timed and sent to the right cylinder during its compression stroke. Typically, one spark plug wire is used per cylinder, although some applications use two spark plugs for each, and therefore twice as many spark plug wires.

Distributorless systems work similarly as far as the spark plug wires are concerned. The method of generating the spark at the correct time is different, although each cylinder will usually have an associated coil pack and a spark plug wire connecting the two. Distributorless ignition systems can operate at substantially higher voltages than traditional systems, so extra care must be taken when dealing with spark plug wires in those applications. Serious injury or even death can result from mishandling a DIS spark plug wire.

Spark plug wires are often routed near or even on top of very hot components such as exhaust manifolds. The back exterior of a spark plug wire is typically a heat resistant material such as silicone rubber, although heat damage is still quite common. If the protective insulating layers of a spark plug wire are damaged, the wire can short to ground and cause a misfire. The gap in a spark plug can create enormous electrical resistance, so it is possible for spark plug wires to short even if there is no visible damage.

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