When exactly do the spark plugs fire in the 912ULS?

Yes, it's the "wasted-spark" system.  The sparks fire on two cylinders are the same time, but only one of them is igniting a fuel mixture (the other is just finishing pushing out exhaust).  It's much simpler that way.

WasteFUL?  The sparks don't cost anything, and I believe the "exhaust" spark actually jumps the gap at a much lower voltage. Not much extra wear on those $3 NGK plugs...

There is a little (Actually, a LOT) of Artistic license in those statements.

It is Not 3 sparks per 4 cycles.  Just Two!

The plugs fire Near TDC on the Compression stroke while simultaneously, the other plug fires Near (Not AT) TDC on the Exhaust stroke.  The same BTDC in Both cases!

In fact the Plugs fire in time with the Crankshaft and NOT the Cam shaft.

The engine does not "Know" which plug is on the Compression Stroke or the Exhaust Stroke.

It fires both Front(1+2) or Rear(3+4) plugs knowing that one of them will need the spark, but not which one that one is.

Both plugs in the same cylinder do not fire "Simultaneously" either.

The "A" Module fires at 26° BTDC and the "B" Module fires 4° later at 22° BTDC.

The compression sequence is 1-4-2-3.

The Firing sequence is (1+2), (3+4), (1+2), (3+4). A spark occures somewhere (Actually in Two Places) every 180°

Most owner do not care or need this much explanation, so the simplified version suffices.

This is the way nearly every auto on the road today fires the plugs.

It eliminates the need for Points, Rotors, Distributors, timing adjustments, etc.

The plug firing in the exhaust gases is relatively cold and does not experience excess wear from the extra sparking.

Most vehicles on the road today go their entire life without ever changing plugs at all.

Autos use this system even though they are almost always fuel injected? I believed the ECU whether for a car or airplane could time the spark for each cylinder individually.

Edit: nvm. I forgot that you needed one coil for each circuit and wasted-spark would save on coils. Wikipedia says some systems called direct-ignition use one coil per plug but others will have one plug for a pair a la the wasted-spark system you mentioned.

Tyler Hathaway wrote:

Yes, it's the "wasted-spark" system.  The sparks fire on two cylinders are the same time, but only one of them is igniting a fuel mixture (the other is just finishing pushing out exhaust).  It's much simpler that way.

WasteFUL?  The sparks don't cost anything, and I believe the "exhaust" spark actually jumps the gap at a much lower voltage. Not much extra wear on those $3 NGK plugs...

I think technically the sparks cost a little by causing resistance at the "magneto" but it's probably so little that it doesn't matter with 100 horses moving it.

And didn't Rotax switch away from NGK because they didn't want to be associated with aviation anymore?

Yes Rotax have their own brand now.  The old ones still work just fine, though – a whole set will cost you about the price of a single new one.  

Either way it's pretty silly to worry about any running cost associated with the wasted spark.