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

Before we just blow this off ... and keeping in mind that I have waaay too much time on my hands ... and that I am a self proclaimed Geek Extraordinaire ... Plus, Enquiring minds want to know!  

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This is just another back of the envelope calculation, but it should put the issue to rest.

A typical ignition system draws about 1 amp from the power bus.

1 amp at 12 volts is 12 watts.  1 hp is 745 watts, so a typical ignition steals about 12/745 = 0.016 hp (1/60th). 

But we have Two, so 1/30th of a hp for the pair.

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Looking at it from the other direction...

A typical spark is about 100mJ (mili-Joules) (0.100 J)

At 5000 rpm 10,000 sparks are produced per minute. 167 per second.  Time 0.1J  per spark = 16.7J per ignition.

16.7J / 745 J per Hp = .022hp (1/45 hp).

And again we have two, so lets be generous and call it 1/25th hp.

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1/25th or 1/30th of a hp ??? . . . pretty good agreement for a first approximation. !!!

So what could we do with another 1/30th hp???

That would be enough power to increase the rate of climb of a 1000lb aircraft by just over a 1/4 inch per minute.  That is Inch NOT Foot per minute.

And as P.J. said, " ... It's ... so little that it doesn't matter ..."

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I think this should settle the big worry over the cost of Wasted Sparks and everyone should sleep well tonight.  

Bill Hertzel wrote:

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.

Both A and B fire simultaneously at 26° BTDC on the 912. Only on the 914 is the B module firing 4° later (74-00-00 page 6 and 7 in the heavy maintenance manual).

Simple answer. It fires every stroke. 

Roger Lee wrote:

Simple answer. It fires every stroke. 

And it is way more common than you might imagine.
It happens in any engine that does Not have a distributor.
This would include most lawn mowers.
You can see the wasted spark happen in This Video.
Skip to 6:10 in the video if you have ADHD.

Roger Lee wrote:

Simple answer. It fires every stroke. 

I know. Bill explained it very well. I just needed to nitpick that module A and B fire simultaneously.