What's new
Van's Air Force

Don't miss anything! Register now for full access to the definitive RV support community.

do electronic ignitions increase detonation margin?

KayS

Well Known Member
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
 
I would say it depends on your EI.
How is it programmed ?
Allso the mixture is a part of the risk for detonation.
Lean mixture and high manifold pressure and low rpm will definetly take you
closer to the risk for detonation.
Rich mixture reduces the risk for detonation.
Superior airpart has a Operation Manual.
Page 86 will give you an idea of the detonation margins.
https://www.superiorairparts.com/files/1014/0311/7842/SVIOM01.pdf
I would not say that EI in general reduces the risk for detonation.
 
Depends on how the map is set up. Some ignition maps top out the RPM advance relatively early - like 2000 RPM. The rest of the tuning is handled by load (MP). In this case, a slight reduction in RPM will have little change on timing.

But load has a major influence on detonation and if your EI is adjustable, you can very easily reduce timing in high load conditions to increase your detonation safety margin.
 
I would say it depends on your EI.
How is it programmed ?
Allso the mixture is a part of the risk for detonation.
Lean mixture and high manifold pressure and low rpm will definetly take you
closer to the risk for detonation.
Rich mixture reduces the risk for detonation.
Superior airpart has a Operation Manual.
Page 86 will give you an idea of the detonation margins.
https://www.superiorairparts.com/files/1014/0311/7842/SVIOM01.pdf
I would not say that EI in general reduces the risk for detonation.

Under many conditions, LOP reduces cylinder pressures and lowers CHTs, both of which reduce the likelihood of detonation.
 
to sum it up: as long as you don't choose agressive timing, EI in general reduces the likelihood of detonation?

In general? No.

BTW, let's stop treating electronic ignition and variable timing as the same thing. Spark generation and spark timing are separate subjects. Several of the available EI's can be run with fixed timing, and given a weekend, I can probably deliver a Slick with variable timing. Points ignition incorporated variable timing a century ago.
 
Come on Dan, everybody knows that you need sensors, microprocessors, and programming to do spark advance! Everybody! :D

In the spirit of Bob "I could do the same thing with two op-amps" Pease:
 

Attachments

  • Muscle-Car-Vacuum-Advance-Distributor-56a1569f5f9b58b7d0be695b.jpg
    Muscle-Car-Vacuum-Advance-Distributor-56a1569f5f9b58b7d0be695b.jpg
    223.7 KB · Views: 153
I would say it depends on your EI.
How is it programmed ?
Allso the mixture is a part of the risk for detonation.
Lean mixture and high manifold pressure and low rpm will definetly take you
closer to the risk for detonation.
Rich mixture reduces the risk for detonation.
Superior airpart has a Operation Manual.
Page 86 will give you an idea of the detonation margins.
https://www.superiorairparts.com/files/1014/0311/7842/SVIOM01.pdf
I would not say that EI in general reduces the risk for detonation.


^^^

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.

High cylinder head temps are the concern and will begin to go through the roof with detonation. It's important to witness in real time, the relationship between timing, mixture and cylinder head temps. You can do that with a cockpit display, I wouldn't have it any other way.
 
i installed one PMAG (with jumper) and one Lightspeed Plasma 2. Don't ask me how i came to that setup 10 years ago.

Reason i am asking is i wanted to know if i have to be extra precautious with two EI's in regard of detonation or is it maybe a little bit more "relaxed".

I understand herewith that it doesn't make a difference and i will be extra careful when testing LOP and stuff with lower RPM.
 
Reason i am asking is i wanted to know if i have to be extra precautious with two EI's in regard of detonation...

You have to take extra precautions with advanced spark timing, as compared to fixed timing. How the spark was generated (EI, points, or flint and steel) makes no practical difference.
 
i installed one PMAG (with jumper) and one Lightspeed Plasma 2. Don't ask me how i came to that setup 10 years ago.

Reason i am asking is i wanted to know if i have to be extra precautious with two EI's in regard of detonation or is it maybe a little bit more "relaxed".

I understand herewith that it doesn't make a difference and i will be extra careful when testing LOP and stuff with lower RPM.

As you know, you are dealing with two different advance curves. You could see them working by monitoring their optional panel displays, if installed (Simpson timing display for Lightspeed, & EI Commander for PMag), you could determine which ign system the engine is running on at any specific RPM & MAP setting. 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.
 
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?
 
Last edited:
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
 
Last edited:
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.
 
Last edited:
^^^

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.
 
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.
 
Back
Top