Spark at TDC

I have a question regarding how the engine knows to send a spark on every other TDC stroke, when the stator/magneto is reading off the crank/flywheel? In my thinking, some sort of sensor would need to be reading the cam(s) to know which TDC stroke to fire on, but this is obviously not the case on my 08 YZF 450. Anyone else thought about this?

it fires every other stroke. (ie compression and exhaust)

Correct, it fires every other stroke, but how does it know not to send a signal to spark on every other stroke?

Mose

It creates a spark at the correct time before every TDC. That way it doesn't have to figure anything out. The one that occurs at TDC between the exhaust and intake stroke does nothing because the incoming fuel/air mix is nowhere near dense enough to be ignited at that point.

Thanks for clearing this up. I was thinking that some type of detination would take place if it was firing between the exhaust and intake stroke.

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