Ignition Timing Question

This may be a dumb question, but I can't find the answer I am looking for. The question is this: How does the ignition system know which time to fire when the piston is at (or near) TDC? I assume that the timing of the magnetic pulse from the keyed flywheel magnet tells the ignition system when the pistion is at (or a few degrees before) TDC, but how does the ignition system know when the piston is at the top of the compression stroke (when a spark is needed), and when the piston is at TDC after just finishing the exhaust stroke? The crank shaft and flywheel are in the exact same position at both times.

Another way of asking this question is: What times the ignition system to the timing of the valves? I suppose one solution is to say that the ignition system fires twice per cycle - once when needed, and a second time at the top of the exhaust stroke. That doesn't seem like it would be the right answer.

Thanks

I suppose one solution is to say that the ignition system fires twice per cycle - once when needed, and a second time at the top of the exhaust stroke. That doesn't seem like it would be the right answer.

Oh, but it is. The ignition is triggered from the flywheel near TDC on every revolution.

Thanks for the reply.

Firing twice per cycle means that the ignition system has to supply twice the current as needed, which I would think leads to an unnecessary oversizing of the stator. I guess any weight savings by having a smaller stator are more than offset by the weight, cost and complexity of having a sensor on one of the cams to detect valve position?

It is.... what it is....

Just a precise high speed hall sensor.

Edited by KTM_Pat

The current is only twice as much on a total wattage per minute basis. Otherwise, the current requirement per event is no different, and doesn't require any extra "oversizing" of the stator. If anything, the flywheels in these are ridiculously small, if you ask me. The gas cap is almost bigger.

StkFlywl.jpg

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