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Discussion: Wasted spark on leading & trailing plugs, rotary 
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I was just having a chat to wheeler_express and he told me that a few guys run wasted spark coils hooked between the leading and trailing plugs on their rotaries.

I explained that wasted spark usually works because one plug is sitting in highly ionised exhaust gas which is effectively a short circuit and thus effectively a ground (very low resistance).

My thoughts are that if you fire the same wasted coil into the same cylinder at the same time, the Voltage requirement could be doubled, or at least raised due to having two high impedance gaps to jump.

Also, your spark energy will be split across both plugs and neither will get full heat.

My advice was to use two single post coils per rotor, each with an independent ignitor, and each of those with an independent ignitor driver. Then you've only got the CPU to blame/fail, the rest is duplicated completely. Application is planes, btw.

I'm interested to hear others thoughts on this. Shoot holes through my theories, go right ahead!

Fred.

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Sun May 05, 2013 10:04 pm
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QFP80 - Contributor

Joined: Tue Aug 19, 2008 4:05 pm
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Yes you are effectively doubling the voltage requirements and maybe doubling your chance of misfire since both plugs need to ionise to form a current path inside the pressurised chamber.
I was also under the impression that a delayed spark on the trailing plugs was desired in some cases, but not necessarily always. Tying both plugs to the same coil eliminates any chance of trying delayed spark on the trailing plugs.


Sun May 05, 2013 11:10 pm
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You can pretty much just fire the primaries (no secondaries at all) and suffer no significant loss of power. A fraction of delay on the secondaries is beneficial over simultaneous firing.

The different rate of approaching rotor would likely mean both are not equal and one would fire first, however I've had a wine, so I'm not going to try to guess which that might be.

Fred.

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FreeEMS.org - the open source engine management system
FreeEMS dev diary and its comments thread and my turbo truck!
n00bs, do NOT PM or email tech questions! Use the forum!
The ever growing list of FreeEMS success stories!


Sun May 05, 2013 11:15 pm
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