Dual coil ignition

[tommd]

TL;DR Summary

Dual coil ignition

I have come across a dual coil ignition system as used by Jaguar on their V12 in the 1980's. It uses two ignition coils with their primary windings wired in parallel. The primary coil has its secondary winding wired to the distributor and then to the spark plugs as is standard practice. However, the auxiliary coil has it secondary winding output sealed off. The purpose of the system was to provide a stronger spark to the plugs, always a difficult task with the very short dwell time of a V12 engine. The attached diagram is about the only information I have been able to find on it. The red was added by me to show the secondary windings. I can see that the buildup of energy in the aux coil primary winding can discharge into the primary coil to increase total spark energy. But I do not quite follow the rest. The description says the coil acts as a capacitor. How? I do read that an open secondary on a transformer can have a capacitive element at higher frequencies. Is that happening in the aux coil secondary winding? If not, what is the purpose of the aux coil secondary? It is open circuit, so is it being used at all? Is only the aux primary being used?
Any insight would be appreciated.
Tom

 

[tech99]

It is a puzzling circuit because the capacitance of the idling secondary can only affect the primary windings. But my hunch is that it is nothing to do with capacitance. When the second coil is connected in parallel, the inductance across the electronic driver is halved. The time constant of an L-R circuit is given by L/R, so doing this will halve the time constant and allow the driver to charge the inductor in half the time.