IG­NI­TION COILS

Motorcycle News (UK) - - This Week -

Put sim­ply, the ig­ni­tion coil con­verts the low volt­age of the mo­tor­cy­cle bat­tery (ap­prox­i­mately 12v) into the huge volt­age re­quired to jump the gap at the spark plug (around 20,000v). The coil is made up of a pri­mary wind­ing, which is a made from heavy gauge wire, and a sec­ondary wind­ing of thou­sands of turns of thin­ner wire. Cur­rent flow­ing through the pri­mary wind­ing cre­ates a mag­netic field, which os­cil­lates when the ig­ni­tion con­tact breaker opens, gen­er­at­ing the larger volt­age in the sec­ondary wind­ing. The huge amount of stored en­ergy in the coil wants to find its way to earth and it does so by trav­el­ling down the bike’s HT lead and through the spark plug.

Many bikes have twinout­put coils to save space. Th­ese op­er­ate on a wasted spark sys­tem which means that both spark plugs at­tached to the coil fire at the same time but one of them sparks with­out fuel, so there is no det­o­na­tion. All this hap­pens at unimag­in­able speed – the coils on a sports­bike near the red­line are send­ing 233 elec­tri­cal pulses per sec­ond, or one ev­ery 14 mil­lisec­onds.

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