Question about spark

[facepants]

How does the engine know when to send out spark?

I'm trying to understand how the timing works and I understand the valve timing, but I cant figure out how it knows when to send out the spark.  I thought it had to do with the position of the marks on the pulse generator coil, but now I'm not so sure.

For example, take the right piston.  During the four stroke cycle (two full rotations of the r.t. mark on the pulse generator coil), the piston is at TDC twice.  Does it spark each time it's at TDC and the r.t. mark is aligned with the left mark (twice for the right cylinder during the four stroke cycle)? Or does it somehow know which part of the four stroke cycle it should spark and only spark once during the four stroke cycle?

[dgyver]

It sparks twice per cycle. compression and exhaust.

[ducati_nolan]

Yeah most bikes have a "wasted spark" ignition system where the plug fires every time the cylinder is at the top of it's stroke, compression and exhaust. I think it's for emmisions. I had an old CB200 where the points were triggered by the cam, which made more sense