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Ignition systems
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Monkeh:

--- Quote from: bdunham7 on October 01, 2021, 05:05:18 am ---
--- Quote from: David Hess on October 01, 2021, 04:58:06 am ---Do all piston aircraft still have dual magnetos with two spark plugs per cylinder for redundancy?

As I recall, the test is to change the magneto selector from both to magneto 1 and then magneto 2 and the engine RPM is only suppose to drop by like 100 RPM or something.  But I remember reading about US aircraft on Pacific island bases during World War 2 where they said maintenance was such a problem that aircraft often would not even run without both magnetos selected.

--- End quote ---

All conventional (old style) piston aero engines are like this, yes, but there are examples of newer systems out there flying around.  But they aren't just for redundancy, you lose some power and the engine runs hotter with only one spark plug going.  Some old rotary engines were spark plug fouling monsters, so having dual ignitions helps there too.

--- End quote ---

Even relatively recently, some otherwise normal inline car engines have had dual spark for a cleaner burn, and let me tell you, those plugs go on for a while.
Circlotron:
I have made ignition systems for a living for the last 27 years.
One of the most overlooked issues with a capacitor discharge ignition is it's woefully low efficiency, at least when using conventional coils. The short answer is - a 1uF capacitor charged to 450VDC has 100mJ of energy. Discharge it into a coil primary, and once the spark in a running engine has begun it takes about 1500V to maintain the spark. Under these conditions a modern E-core coil will send about 35mJ to the spark gap. An older points type canister coil delivers about 25mJ. if you ignore plug lead resistance the majority of the losses are in the coil secondary. with a 1uF 450V CDI the coil secondary current will be close to 400mA peak. If the coil secondary resistance is say 6000 ohms then there will be 1500V across the spark gap and 2400V lost in the coil secondary resistance. So right way we can see the efficiency would be only 38.5%. One particular inductive ignition I make delivers a measured 150mJ to the spark gap. This is a huge jump compared to a CDI.

I posted a thread about this to a high end car forum about this a while back.

https://www.speed-talk.com/forum/viewtopic.php?t=42274
Circlotron:
Best thing you can do of course is have a CDI initiate the spark and before it goes out have an inductive ignition jump in and maintain the spark.

https://www.speed-talk.com/forum/viewtopic.php?f=1&t=45340
SteveyG:
Modern coils are just HV generators, and will produce a high voltage the entire time the trigger signal is applied

Coil packs vary in design, but for the most part work the same way, however some will use a wasted spark system whereas others will only fire on the correct cylinder

Ignition coil + points are pulsed by the ignition timing system.
paulca:
Interesting, I thought CDI was more common, maybe because I ride motorcycles.  Which use CDI a lot.


--- Quote from: SteveyG on October 01, 2021, 08:52:59 am ---Coil packs vary in design, but for the most part work the same way, however some will use a wasted spark system whereas others will only fire on the correct cylinder

--- End quote ---

Was watching a video about Harley Davidsons.  They really are a pile of junk.  The engineering is many places is, well, really, really poor.  It's not even like they evolved and adapted with the times/tech, they don't/didn't.  And.... are now dieing.

They used a single crank pin for 2 pistons, not because that was a good choice, but because they couldn't be bothered to have two crank pins, balance the engine and have a sane firing interval.  They use a 45* firing interval as a result.  So BANG, BANG................ BANG BANG.  Then because they were cheap skates they decided that per-cylinger ignition was too much effort so they just fire both plugs.  This results in the BANG, BANG, POP.....  The POP is caused by the engine being so poorly made there is enough combustable exhaust gas in the exhaust strike to ignite!

With an unbalanced crank, the bike shakes violently when idleing.  So they mounted the engine on rubber.... with a cheap and nasty steel frame that flexed.  Resulting in many deaths caused by death wobble oscilations.  They added a counter balance (to fix the unbalanced pistons) such that the vibration of the unbalanced mass was forward/back, not up/down, hence the shaking.  The only saving grace is that when moving, some of that insane vibration is muted by the road motion.

I think the video making summed it up by saying, Harley's, you either get them or you don't.  I don't.
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