Ignition coil powered by AC. Why?

[Mat1603]

Hello.
I have a question about ignition system in old motorbike. The high voltage ignition coil is powered by alternating current. It's working properly but i can't understand why. I was thinking there should be a DC like in cars.

 

[barry]

Your ignition coil is a transformer; transformers only work with AC (or, actually in the case of automotive coils, pulsed DC).

 

[Mat1603]

Exactly should be powered by pulsed DC, but in this case the HV coil is connected directly to the coil which produce AC, and the second HV coil terminal is connected to the breaker which disconnecting the HV coil from ground when spark should appear.

The HV coil is constantly charged and discharged. What will happen when breaker disconnect the coil when it be discharged? No spark? In this case spark strenght will be random too.

 

[barry]

What "coil which produce AC" are you talking about? Do you mean the magneto? The way that type of ignition works is that the switch (what you call "breaker") opens at the time when the magnetic field is maximum, not randomly.

 

[Mat1603]

Yes, magneto. I just checked the magnet and it has 8 pairs of poles, so the current in HV coil is changing 16 times per one crankshaft rotation what is 22.5 degree. Well, if I change the ignition advance about 10 degree that can have a huge impact at spark strength, or there will be no spark at all.

 

[barry]

Right. That's why the switch opening has to be synchronized with the magneto; it's not random.

 

[dick_freebird]

I've had old simple dirt bikes that were either AC or rectified
AC with no battery. It's just penny pinching (or whatever
the Japanese equivalent of a penny is). If you've got a
kick starter and no lights, and it runs, then why be fancy?