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  #211  
Old 02-01-2021, 12:16 PM
nomocom's Avatar
nomocom nomocom is offline
 
Join Date: Jun 2009
Location: Caldwell ID
Posts: 264
Default Low voltage pre-ignition?

Quote:
Originally Posted by pecanflyboy View Post

#3 If you have a weak battery, poorly grounded engine or starter, or corrosion in the starter relay or cable terminals, you can get a backfire during the start. It has nothing to do with the accuracy of the SIM timing during start, and will not be corrected by timing several degrees after TDC. There fix is usually to check the health of your electrical system (weak battery in your case).

It has to do with the system voltage dropping below the required voltage (8.5VDC) for the SIM to operate. I believe the SIM charges the coils as the engine approaches TDC on compression. If the system voltage drops below the operating voltage of the SIM, it will shut down and it is possible for the field to collapse and a spark to be released before TDC. You will see this as the starter struggles to get the blade over compression, and the engine kicks back. Check the voltage at the starter teminal when the starter is engaged to check the health of your electrical system.
Earlier in this thread, starting timing is covered nicely IMO, so I'm not going to rehash that. However, the low voltage- lets take that one head on. I hope this is wrong. If you right, that is a major design failure. A good design doesn't risk grenading the system when a failure mode that has a 100% chance of occurring occurs. Cold happens, weak batteries happen, masters get left on, etc. Please tell me a spark doesn't get triggered by an event (low voltage) unrelated to piston position.

** another thought. If for some reason power dips when you are 50 degrees BTDC, and you are WOT, full power. (you win the bad luck lottery)

Update- I've heard back from Bill. He insists a low voltage will not cause a firing event. But also says that if the prop stalls (before TDC) the pre-charged coil pack can't hold it forever. It could fire substantially before TDC. In this edge case I prefer the impulse coupling. Preignition event at WOT? Clearly, one can shut off the SIM (ground the P-Lead), WOT, full power and the code manages to shut down each cylinder appropriately. It has been done many times. However, what is the timing behavior if voltage is removed, WOT, full power? Is it a lottery or a safe shutdown?
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1990 RV-3 (now apart, upgrades in the works)
1959 C172 O-360

Last edited by nomocom : 02-01-2021 at 03:31 PM. Reason: additional thought/ correction
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  #212  
Old 02-02-2021, 07:45 AM
pecanflyboy pecanflyboy is offline
 
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Location: Granbury, Texas
Posts: 124
Default

Any electronic system operated or installed outside its design parameters, has the potential to operate improperly. Not a design fault.
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  #213  
Old 02-07-2021, 12:17 PM
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Default Design

Hang with me a moment, it is on topic. Good friends of ours purchased a car, based somewhat on our good experiences with Subaru Outbacks. They got the new model at the time, a 2018, fully loaded- every option. They park in a locked and attached garage. Several times, they found the car with a drained battery. It could happen in a couple days. Turns out, the modern key fob and the car would have a battery depleting conversation. And yes, after a few of these, the dealer points out somewhere, in the 400 page manual, one is instructed to not leave the keys in the car or near the car. In fact, take them out, and even better, drop them in a faraday cage (coffee can) so these battery depleting conversations don't occur.
I tell this story to make a point about good design. Yes, it was mentioned in the manual, and perhaps it behaved as expected according to the engineering specifications, yet for those of us who park in a garage, camp next to the car, the only benefit this behavior has is it makes a great case study in stupid design. Those designers get an F. In the same fashion, if an aircraft electronic ignition system doesn't safely handle low or no voltage situations, another F, whether or not it is behaving as expected.
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1990 RV-3 (now apart, upgrades in the works)
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  #214  
Old 02-07-2021, 03:32 PM
pecanflyboy pecanflyboy is offline
 
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As the poster mentioned in his update, Surefly Tech Supported confirmed the hypothetical would not happen after engine start. The poster who mentioned a backfire on start stated it was due to a weak battery.

Bottom line is that if you are going to have an electronic ignition, then you need a dependable electrical system. The ignition coil is magnetized by current flow. When you remove the current flow, this magnetic field collapses and a surge of current is able to jump the gap in the spark plug. A magneto does this with points that open the magnetic circuit, allowing the field to collapse, creating a spark. An electronic ignition does this with a solid state circuit board. If you remove power from that circuit board when the coil is charged, you may have enough energy to get a spark. This is common with all electronic ignitions.

A weak battery, bad starter, corroded electrical connections etc. will cause the starter to hang as the piston compresses the cylinder. The voltage will drop dramatically. If the voltage drops below the operating limit of the any EIS, it may spark. But, we are talking about just a few degrees BTDC, extremely slow rotation of the engine, hence the back spin of the engine.

I suspect the reason that SF told you itís a non-issue after start is because the system is designed to only charge the coil while the engine is in a safe zone for a spark? I would guess the FAA testing would have addressed this scenario.

No system is without compromise. Fortunately, you have a choice. A magneto that has a 500 inspection requirement due to multiple failures. Or an EIS that appears to have solid reliability in the field.
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  #215  
Old 02-07-2021, 03:57 PM
BlndRvtr BlndRvtr is offline
 
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Location: NE Where
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What is the high time SIM out in the field?

George
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  #216  
Old 02-08-2021, 02:40 PM
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nomocom nomocom is offline
 
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Default hours?

Quote:
Originally Posted by BlndRvtr View Post
What is the high time SIM out in the field?

George
Not me, 53 tach hours. I'd think some schools are likely clients and those folks might have some high hours. It would be nice to hear from them.
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1990 RV-3 (now apart, upgrades in the works)
1959 C172 O-360
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  #217  
Old 02-20-2021, 02:20 PM
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MiserBird MiserBird is offline
 
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Location: Cornelia, GA
Posts: 64
Default SureFly stumble cure

Quote:
Originally Posted by pecanflyboy;

We have found that turning the ignition switch slowly between positions masks this issue on most aircraft. (edit: This worked for me and most I've talked to with this issue)
Thanks so much for the tip! I was having to reduce the power on mag check to minimize the SureFly boot-up stumble, but slowly turning my Bendix key switch when switching back to the SureFly completely cured the problem.
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KAJR - Cornelia, GA

1975 Cardinal RG N1593H since Mar 1993
1980 RV-3 N3VR Bought flying Sept 2017

Dues current 2021

Last edited by Mike S : 02-20-2021 at 03:12 PM.
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  #218  
Old 02-24-2021, 05:08 PM
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nomocom nomocom is offline
 
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Default where?

Quote:
Originally Posted by MiserBird View Post
Thanks so much for the tip! I was having to reduce the power on mag check to minimize the SureFly boot-up stumble, but slowly turning my Bendix key switch when switching back to the SureFly completely cured the problem.
Is your Surefly installed left or right? And your switch position sequence is off, right, left, both?

thanks
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1990 RV-3 (now apart, upgrades in the works)
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  #219  
Old 02-24-2021, 05:26 PM
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MiserBird MiserBird is offline
 
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Location: Cornelia, GA
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SureFly is on the left, PMag on the right, you're correct on the switch sequence.
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KAJR - Cornelia, GA

1975 Cardinal RG N1593H since Mar 1993
1980 RV-3 N3VR Bought flying Sept 2017

Dues current 2021
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