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  #11  
Old 11-24-2020, 11:49 AM
airguy's Avatar
airguy airguy is offline
 
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Location: Garden City, Tx
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Quote:
Originally Posted by Ralph Inkster View Post
The active EI may switch back & forth as you adjust the throttle & the engine really is running on which ever EI has the most advance at that moment.
Would be interesting to see the variances.
Well, sorta... and that brings up a good point that I'd like to hear some about, from some folks that know more about this than I do.

There are two plugs lighting the mixture, and the flame front meets in the middle - kinda. If one plug fires say, one degree, of rotation earlier than the other, then they both still light some mixture but the meeting point of the flame front is changed and the arrival of peak pressure is delayed (or advanced) somewhat. Does anybody know with any degree of relative certainty how much deviation between firing points is available before we are essentially firing on just one plug? How many degrees of rotation on the engine will occur for the flame front to be generated and travel to the other plug, rendering it essentially inert for its later spark?
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Last edited by airguy : 11-24-2020 at 11:54 AM.
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  #12  
Old 11-24-2020, 12:03 PM
lr172 lr172 is offline
 
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Quote:
Originally Posted by airguy View Post
Well, sorta... and that brings up a good point that I'd like to hear some about, from some folks that know more about this than I do.

There are two plugs lighting the mixture, and the flame front meets in the middle - kinda. If one plug fires say, one degree, of rotation earlier than the other, then they both still light some mixture but the meeting point of the flame front is changed and the arrival of peak pressure is delayed (or advanced) somewhat. Does anybody know with any degree of relative certainty how much deviation between firing points is available before we are essentially firing on just one plug? How many degrees of rotation on the engine will occur for the flame front to be generated and travel to the other plug, rendering it essentially inert for its later spark?
Pretty simple to test. Changes in total or effective timing correlate with changes in EGT (related mostly to where peak pressure occurs). Several threads discussing why this is. Timing advance is a composite of timing from both ignitions. If you drop one ignition (reducing total timing) and the EGT stays constant, then the ignition you turned off was providing no meaningful contribution to the combustion process in a way that impacts where peak pressure occurs. While it may still be starting a unique combustion process, generally speaking, all that really matters is where peak pressure occurs.

I believe that in most single EI installations, if you drop the mag, EGT will rise, providing confirmation that it is in fact contributing to where peak pressure occurs and indirectly engine performance.
Larry
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Last edited by lr172 : 11-24-2020 at 12:30 PM.
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  #13  
Old 11-24-2020, 12:34 PM
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rv6ejguy rv6ejguy is offline
 
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Once the flame front is established, at rich mixtures, around .002 seconds for the flame fronts to meet on a 360 Lyc, at lean mixtures around .003 seconds (round figures). Call that around 28 deg of crank rotation at 2400 rpm (rich) if I've done the calcs correctly.

There is more to it than that however. Crank angle vs rate of pressure rise is altered and thus torque delivered is altered.

Without a dyno, hard to say how much advance it would take on one plug to make around the same power although you could fiddle with this in flight with one ignition shut down and keep advancing until you saw the same TAS as before with both firing.

Would be a fascinating experiment.
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Last edited by rv6ejguy : 11-24-2020 at 12:42 PM.
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  #14  
Old 11-24-2020, 08:32 PM
Captain Avgas Captain Avgas is offline
 
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Originally Posted by rmarshall234 View Post
^^^

This is why I believe it is so important to have a timing display in the cockpit and I think the folks over at SDS are the only one's providing it.
Well, I agree that having a timing display is extremely important but as Ralph Inkster correctly pointed out in Post #11 the statement that you can only have a timing display with SDS EI is not true.

You can instal optional timing displays for both PMags and Lightspeed. I have a display for the readout on my Lightspeed Plasma 11 and to be honest I would not entertain the idea of having EI without a timing display. I want to see that when I push in the throttle for full power take-off my timing retards to exactly 25 degrees BTDC....and that while I'm idling it is around 38-40 degrees BTDC. No way known I'm going to have the EI choosing the timing for me and not knowing what it is...absolutely no way.
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  #15  
Old 11-25-2020, 07:13 AM
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Walt Walt is offline
 
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Quote:
Originally Posted by KayS View Post
Hi All, i'm at phase 1 with my 7 (IO-375/Hartzell CS) that is equipped with two electronic ignitions. Lately i started to test LOP-operation and the entire detonation topic came up, so here's one of the questions i have:

if you reduce RPM (with the old magnetos) the pressure peaks inside the cylinders increase and move closer to TDC which results in an decreased safety margin in regard to detonation. i hope i got that right.

but if your lycoming is equipped with EI, lowering the RPM should result in a ignition timing closer to TDC and therefore the combustion event is moved to the right, which compensates the effect of reduced RPM...?

to sum it up: as long as you don't choose agressive timing, EI in general reduces the likelihood of detonation?

Thanks
Kay
Not sure if you're running the high compression 370 (9.6) but if you are, I would be very careful with advanced timing.
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