Ignition wires (ignition cables)

Safety

The location in which ignition wires are installed exposes them to high levels of stress and strain.

At first glance, the external appearance of ignition wires and ignition cable harnesses belies the complexity within. They must rise to the toughest of technical challenges inside the engine compartment, where temperatures induced by catalytic converters and more powerful and more compact engines can reach up to 200 degrees Celsius and electronic ignition systems are producing high voltages of up to 40,000 V. Brass and stainless steel contacts oxidise as they age. The electrical resistance of the cable rises, along with the risk of the ignition coils failing.

PVC Hypalon ignition wires, which just a few years ago were more than able to rise to this challenge, have long since fell by the wayside. Today's cars have even seen the use of class F silicone, which can withstand temperatures of up to 220 degrees Celsius and exhibits a dielectric strength of up to 40,000 V. They protect against energy loss and provide optimum protection against vibrations, water, acids, oil and petrol. With corresponding connectors they are able to meet the most stringent of requirements where interference suppression and electromagnetic compatibility (EMC) are concerned.

Environmental protection

Because plasticisers escape from the plastics from which sheaths are made, the sheaths become brittle over time. High temperatures as well as contact with oil or fuel vapours can accelerate this process or even dissolve the plastic completely. Damage to the insulation results in ignition voltage being conducted to earth. The consequences are misfiring and the engine running erratically, with unburned fuel getting into the catalytic converter.

Every time a spark plug misfires, valuable petrol is injected but not combusted. As a result, there is a significant increase in environmental pollution due to the increased in consumption per kilometre alone. Furthermore, the unburned fuel in the catalytic converter can ignite explosively, causing damage that will prevent the catalytic converter from rendering hazardous substances like carbon monoxide, nitrogen oxide and hydrocarbons harmless and requiring it to be replaced.

Depreciation

As ignition wires are exposed to high levels of stress and strain, the wires must be checked regularly and replaced at the first sign of ageing. Faults affecting the ignition system and the catalytic converter usually only come to light during the statutory exhaust gas test. This regular test is an important precautionary measure which can save you hard cash. After all, identifying and rectifying damage at an early stage does not cost a lot. Purchasing ignition cables complete with connectors will cost much less than having to replace a catalytic converter on account of consequential damage.

Function

Ignition wires must conduct the necessary voltage (U) to the spark plug with minimum possible losses. Depending on how the vehicle is designed, this is achieved using:

• A mechanical spark distributor and distributor cap
• A fully electronic ignition module
• A fully electronic semi-direct ignition or double spark ignition coil

Since the ignition voltage (U) of up to 36,000 volts is in the high-voltage range, the ignition cables have to be protected accordingly against overvoltage. The ignition voltage must never permeate the insulation and flow to ground, since this could cause misfiring.

Although the fundamental aim is low-loss conduction, resistors are used in all ignition wire systems. Looking into the electrical technology, it is clear that this is not necessarily a contradiction. All electrically-operated devices create electromagnetic fields of greater or lesser strength. In most cases they are only negligible, but under some circumstances they are undesired (e.g. interference with radio reception). The ignition system requires optimal low-pass interference attenuation in order to ensure interference-free operation of radios, communications equipment and control units for engines or gearboxes. The assumption that resistors reduce ignition energy and thus engine performance has been proved to be mistaken.

The resistors which are used are dimensioned so that they are hardly noticeable. The ignition cable systems offered by brand manufacturers combine the best interference suppression with optimum ignition performance. The unit of measurement for resistance (R) is the ohm. For ignition cables this value lies in the range of a few thousand ohm or "kiloohm". The purpose of this resistance, as already described, is to reduce electromagnetic radiation. This is achieved by limiting the current (I) through the ignition wire and simultaneously ensuring that the spark plug also receives the necessary voltage (U). The mathematical formula for this is U = R * I.