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Twin-spark Coil



Assignment

In contrast to the single-spark coil the twin-spark coil has two primary- and two secondary windings. Thereby, it generates one ignition spark in two spark plugs at the same time. This type of ignition coil can only be used in engines with an even number of cylinders.

Function

The primary winding is the same as in all other coil-types. Only the secondary coil has a spark plug connected to each end. Thus e-g., in a six-cylinder in-line engine the spark plugs of the cylinders 1 and 6, 2 and 5, 3 and 4 ignite together, in a four-cylinder in-line engine it's the spark plugs 1 and 4, 2 and 3. The spark in one plug jumps from the center electrode to the earth and in the other one from the earth to the center electrode. Thereby, between compression- and work stroke in the highly compressed cylinder a strong main-spark is produced, in the cylinder which is in overlap and without compression a weak additional spark is produced. In the picture, on the top left, three twin-spark coils are integrated into one unit.

Switch-on voltage

It poses a problem for other coil ignitions. What is meant here, is the (relative high) voltage, which is generated when switching on the primary current in the secondary coil again. As far as the polarity is concerned, it is laid out contrary to the tension of the primary coil, in fact, it hinders it's build-up. This tension could of course, at the completely wrong time, quite unnecessarily, jump over to the spark plug. In single-spark coils this is prevented by diodes using the opposite polarity. In the distributor this was not a problem because, not only on the spark plug but between the rotor-finger and the distributor-cap a small amount of sparking also occured. This additional spark-gap is also found in the twin-coil, in the form of the serially switched second spark plug. Therefore, in most cases, a diode is not necessary here. 08/08

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