Ignition Coil Quality
Demand for replacement ignition coils was traditionally satisfied by units sourced from OE suppliers. As the market has matured an aftermarket offering has emerged giving customers a greater choice, but buyers should beware of cheap products that do not offer matching quality. Intermotor coils are manufactured and tested to OE standards to ensure that they meet the high standards and performance criteria of today’s vehicles.
A brief overview
Ignition coils consist of two windings of wire arranged around a soft iron core. The primary winding is fed with 12 volts DC and the secondary winding, which has significantly more turns of wire, is the output end of the coil. The greater the difference in the number of turns between the two coils, the greater the voltage output.
When an electrical current flows through the primary coil it creates a magnetic field around the wire. As soon as the current is turned off, the magnetic field collapses, inducing a very high voltage current flow in the secondary winding. This current flow is distributed to the spark plug.
When dealing with such high voltages care must be taken to ensure that there is no opportunity for the current to deviate from the designed route. This is known as ‘arcing’ and causes heat damage to the coil and associated components.
The primary resistance of replacement coils should always match the OE unit. Any deviation from the set parameters causes the whole electrical circuit to perform differently, leading to possible ECU burnout or even an electrical fire.
With up to 40,000 volts passing through it, the wire selected for the secondary winding must be premium quality and incorporate a varnish or shellac insulation. While it is very difficult to eliminate faults or breaks in this coating, a premium quality wire may have a maximum of 3 breaks per 10 metres of wire, while a lower quality wire will have many more. Keeping these break points to a minimum is vital because it is at the break point that the high voltage has the potential to arc.
Low grade steel is not very efficient at transferring electrical current, so coils which use poor quality steel produce a low voltage and low spark strength.
Epoxy or potting compound
Modern coils employ an epoxy resin or “potting” to insulate the windings. The process of “potting” or filling a coil with resin must take place in a vacuum to ensure that the coil is correctly filled and that there are no voids or bubbles which could lead to arcing. This filling process cannot be carried out successfully in the normal atmosphere.
The resin needs to have a high dielectric strength (resistance to passing of a current) and also needs to have a degree of flexibility as a coil in its normal environment can be subject to temperatures as high as 120°C and as low as -20°C. Rapid changes between these temperatures can cause a non-flexible epoxy to crack.
Plastic bodies and bobbins
The plastics used to form the coil casing and bobbins around which the windings are formed must have a high dielectric strength. Poor quality plastics with low dielectric strength or low quality plastics may crack under the changes in temperature. Intermotor’s ignition coil programme consists of over 360 references giving very high vehicle parc coverage.