Modern Engines & Ignition Systems

Discussion of aircraft electrical system design, construction, and problems.

Modern Engines & Ignition Systems

Postby daleandee » Mon Jan 22, 2018 10:34 pm

OK ... I am aware that this topic might open up a can of worms and that is NOT my intent at all. I would like to begin a logical discussion about the merits of dual ignition vs single ignition or a system somewhere between the two. It's correct to say that dual ignition systems are safer but are they really? I had an experience on my Aerovee many years ago where I fouled both plugs on one cylinder. Needless to say the fault was with me but to say that two ignitions guarantee that cylinders will always fire is not accurate.

Some builders are using single ignition set ups on their VW conversions with some success. I have read (and tend to believe) that the initial reason for dual ignitions was that mags weren't that reliable back in the day, the cylinders were quite large requiring dual sparks to properly ignite the mixture, and that because of low voltage coils small plug gaps were used that were prone to being fouled by the lead in the fuel.

Todays modern electronic ignition systems are extremely reliable, ignitions coils can produce an amazingly hotter spark allowing for larger plug gaps, and many are now using unleaded Mogas so plugs fouling is greatly reduced.

The Corvair uses a semi-dual ignition system in that there are two trigger sources (electronic/points, dual points, or dual electronic) which drive two separate high voltage coils that are then sent through an automatic coil selector unit. Admittedly it is single ignition after the distributor cap with one wire & plug per cylinder but these systems have proven to be very robust. I have never fouled a plug in my Corvair flight engine. I now use iridium plugs but used standard spark plugs for a number of years prior to that.

This system is electrically dependent but so is every engine that uses electronic ignition, electronic fuel injection, or requires a fuel pump that runs full time. A good alternator and battery are a redundant system for power as long as the pilot has a warning whenever the alternator goes off-line or battery/system voltage drops.

The reason this comes to mind is seeing the threads concerning the second plug holes in the VW heads and the concerns that come with that along with some complaining of the noise created by the B&S magnetrons.

Would it be possible to place four hall effect sensors instead of two in the rear flywheel area and time them so that you would have dual electronic ignition sources and feed those into four dual wire high output coils? This would eliminate the magnetrons and add two more electronic ignition coils. Of course this would still require dual plugs. The other option would be to drive a dual ignition system from a low profile distributor cap/rotor system similar to the Corvair set up?

There may be many ways to approach this so I'd be interested to know if anyone has considered this and what approach seems reasonable from another view point. BTW ... my '74 Beetle with single ignition and a weak spark points system has yet to foul a spark plug.

Any discussion? Have I lost my mind?

Dale Williams
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Re: Modern Engines & Ignition Systems

Postby kmacht » Mon Jan 22, 2018 11:00 pm

Having lost the trigger magnet on my aerovee when it came unglued and departed the engine around 10 hours into flight testing I was happy to have the mags working as a backup system. With the aerovee you really need both systems. The motor doesn't turn fast enough to fire the mags when starting and once running the mags are needed to keep the engine running if either of the hall effect sensors, the coils, or the trigger magnet fail.

I think the key is to have redundancy where the most likely failure areas are locates. In the corvair it is the points and coils which both have redundant systems. The plugs, distributor body, and plug wires should have a low failure rate and done need duplicates.

I think it was just simpler in the aerovee to put in a second set of plugs than to try and the the mags and electronic systems together.

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Re: Modern Engines & Ignition Systems

Postby daleandee » Mon Jan 22, 2018 11:20 pm

kmacht wrote:Having lost the trigger magnet on my aerovee when it came unglued and departed the engine around 10 hours into flight testing I was happy to have the mags working as a backup system.


Hi Keith,

It goes without saying that if there is a concern with the magnets coming loose and failing the system then that must be corrected. Perhaps a different/better system could be found ...

Dale
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Re: Modern Engines & Ignition Systems

Postby mike.smith » Tue Jan 23, 2018 12:35 am

daleandee wrote:It goes without saying that if there is a concern with the magnets coming loose and failing the system then that must be corrected. Perhaps a different/better system could be found ...


To play devil's advocate, let's look at a typical automobile. If a fuel filter can get clogged failing the system, then that must be corrected. If an onboard computer fails failing the system, then that must be corrected. If a tire gets punctured and goes flat failing the system then that must be corrected. Hmmm... None of these have been corrected. The point is simply that everything has a failure mode. Nothing is perfect. There are tradeoffs for everything. Just because something can fail does not automatically make it a liability. For anything mechanical, having a backup is the wisest choice of all.
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Re: Modern Engines & Ignition Systems

Postby lutorm » Tue Jan 23, 2018 3:58 am

If one wants a dual ignition, the Aerovee magnetrons seem quite good as the stupid-simple, self-contained, system.

It would be quite easy to replace the fixed-time secondary ignition with an aftermarket electronic ignition system. The only hardware needed would be to replace the trigger magnet with a toothed trigger wheel since you can keep the existing coils. You could have ignition timing that varies with RPM and manifold pressure which should have potential for lower cruise fuel consumption since you can advance the ignition timing. All the while the magnetrons could keep firing at their fixed 28 degrees or whatever it is and be there for backup and better full-throttle ignition.

On the other hand, a Sonex isn't really a traveling machine, so I'm not sure it's worth the complexity. On a Long-Ez or something it would be a no-brainer to me.
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Re: Modern Engines & Ignition Systems

Postby kmacht » Tue Jan 23, 2018 7:43 am

mike.smith wrote:Nothing is perfect. There are tradeoffs for everything. Just because something can fail does not automatically make it a liability. For anything mechanical, having a backup is the wisest choice of all.


To clear something up, I wasn't implying that the trigger magnet coming loose is any sort of wide spread issue. As far as I know it was only my engine that it has ever happened to. Having the magnetron as a secondary system is what kept the engine running and I didn't even know it happened until I went to try and start the plane again the next day.

In the engineeringv world we have what is called a fmea for every design. FMEA stands for failure mode effects analysis. You look at every piece of a part or system and list out every possible failure mode for it. For each failure mode you then list out the possible consequences when the failure happens, how likely the failure is to happen and what mitigations are in place if it does happen. Each of those categories are given a score of 1 to 10 with 10 being the worst and 1 being the best. Those scores are then multiplied against each other for each failure mode and the aggregate scores are ranked from highest to lowest. It gives you a way to quantitatively assess risk for each part and focus your efforts where it matters.

Using the trigger magnet as an example, it would have a mid to low FMEA score.

How likely: It isn't very likely to happen as I'm the only one I know that has had an issue to date.

Consequence: If it does happen the engine will still run because of the magnetron. It won't restart if the engine is shut off which isn't likely to be done in flight unless maybe you are in a Xenos.

Mitigation: The only down side is that there isn't anything in place to prevent if from happening. The magnet is press-fit/glued in place. There isn't a secondary retention feature such a set screw to keep it there if the fit is in the loose side.

If you really wanted to do this to analyze how necessary each component in a dual ignition system is you could. It would help you sort out what parts need backups and which ones don't. It would have to be done for each type if engine though. The failures on an aerovee system could be different than those components on a corvair or a Lycoming. There is risk everywhere, the key is to be able to assess and quantify it.

Keith
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Re: Modern Engines & Ignition Systems

Postby Sonerai13 » Tue Jan 23, 2018 9:53 am

kmacht wrote:The motor doesn't turn fast enough to fire the mags when starting...


Just wanted to clear this up. This statement is not true. Early AeroVee engines had only the magnetron ignition and started just fine. You just need a good starter and a good battery. Yes, they start better with the secondary ignition, but to say that they won't start on the magnetrons alone is inaccurate.
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Re: Modern Engines & Ignition Systems

Postby kmacht » Tue Jan 23, 2018 10:17 am

I would say that my experience contradicts that. When I lost the trigger magnet the engine would not start. The engine turned over good and fast, battery was fully charged but it would not fire. Maybe the earlier aerovees had a different starter?

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Re: Modern Engines & Ignition Systems

Postby Sonerai13 » Tue Jan 23, 2018 12:04 pm

kmacht wrote:I would say that my experience contradicts that. When I lost the trigger magnet the engine would not start. The engine turned over good and fast, battery was fully charged but it would not fire. Maybe the earlier aerovees had a different starter?


Well Keith, your sample size is one. Yes, maybe your engine won't start on the magnetrons alone. But that does not mean that ALL AeroVee engines won't start on magnetrons alone. Yes, the early AeroVee engines did use a different starter, but I've started them with the current SkyTec starter on magnetrons alone, so it can be done.
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Re: Modern Engines & Ignition Systems

Postby RodgerC » Tue Jan 23, 2018 4:40 pm

I too lost a magnet, however I suspect that it was due to the trigger shaft assembly on which it is mounted coming loose and that increased the gyro forces acting on it. The service bulletin was issued days after I reported it to Sonex that required longer mounting screws.

The Skytec starter wouldn’t start it and thus my pride and joy was stranded at a remote airfield for a month until the new components arrived in Oz from the USA. (You USA flyers have nothing to complain about when it comes to delivery time or cost)

Interestingly the replacement assembly had the magnet retained by peening its housing with a centre punch. Not a bad idea, but a stepped pocket may have been more professional.

So the positive take home message for all Aerovee punters is to include in your preflight a check that the trigger shaft assembly is not loose. And yes, I remove my cowls after EVERY flight in prep for the next preflight.
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