Spark redundancy.

[Geraldc]

How do people do redundant spark using two feeds to one spark plug?
I am thinking of using computer spark alongside magneto fixed advance as a dual system into one plug.
Most auto conversion failures are ignition related.

 

[103]

How do people do redundant spark using two feeds to one spark plug?
I am thinking of using computer spark alongside magneto fixed advance as a dual system into one plug.
Most auto conversion failures are ignition related.

Difficult to do with a Magneto unless you can feed an externally sourced spark into the distributor end. Easy to do if you have a automotive distributor. The Corvair typically uses a MSD 8210 coil combiner to run two spark sources to a single plug. Very reliable and flight proven but as with anything there are weak links. Particular attention to detail wire selection and ring lug termination is paramount.

REF http://www.n56ml.com/electrical/

 

[pictsidhe]

You could do it with high voltage diodes. The testing needed to make something more reliable than a properly maintained modern automotive single spark system is likely beyond most homebuilders.

 

[Geraldc]

The Corvair typically uses a MSD 8210 coil combiner

You could do it with high voltage diodes

Thanks to both posters.The MSD unit looks like it works like a diode.
I have a distributorless system so maybe need 4 units.

 

[WonderousMountain]

This probably doesn't help,
but my plan is to excite-time
from each of two camshaft stators.

 

[Dan Thomas]

Magnetos generate an alternating current to the coil, so one plug fires positive, the next negative, the third positive, and so on. Diodes won't work there.

 

[slociviccoupe]

Seeiously when is the last time a coil on plug or coil near plug system failed? And when it did the whole engine didnt quit, you just loose one cylinder. Only system needing a backup would be a distributed system using one coil. But heres the thing, you still only have one distributor. And only one plug per hole. You are safer with a proven set of coils like the ign-1a (ls based coil near plug) .
And on the topic of mags on airplanes or dual mags or dual plugs. Dual plugs are needed to light the fuel mixture in such a large bore. Same reason early hemi's had dusl plugs. Its not for redundancy. Loose a mag or foul a plug you still loose power not all completely but do loose some power.

 

[Toobuilder]

Yep. Taking a highly reliable and proven single ignition system and cobbling a bunch of parts onto it is almost certainly going to reduce system reliability.

Not sure where you got the statistic that auto conversions suffer from a rash of ignition failures, but "car" ignition failure is extremely rare these days. I'm wondering if the failed "auto conversion" ignitions were stock, or "improved"?

 

[pictsidhe]

If it were allowed here, I'd put a lot of money on auto conversion ignition failure being due to the conversion. Failure of OEM parts has been exceedingly rare since the 90s. I have had several 30yo cars with original coil, pickup and electronics. The HT stuff does still wear, but shouldn't fail if you do any regular maintenance. I've seen more failures of aftermarket 'better' stuff than OEM. Guess what I fit to anything now?

 

[Protech Racing]

The crank triggers and cam position sensors fail first . Maybe at street 100kmiles .
I would consider a redundant , switchable crank trigger.
The coil failures are usually associated with a huge spark plug gap that forces the spark to look for a short cut to ground, inside of the coil.

 

[rv6ejguy]

Use Hall Effect crank sensors. We've never had an electrical failure on one in millions of hours collectively. Magnetic reluctor types are a different story...

 

[Dan Thomas]

Electronic ignition relies 100% on the airplane's electrical system. That's where the troubles usually lie. One wants to make that system as foolproof and reliable as possible.

 

[Dan Thomas]

Seeiously when is the last time a coil on plug or coil near plug system failed? And when it did the whole engine didnt quit, you just loose one cylinder.

Losing one cylinder in a four-banger is almost like losing two cylinders. That dead cylinder, instead of contributing, is robbing power from the rest as it takes in air, compresses it and exhausts it. We had a 150 eat a valve; one cylinder dead and the airplane could not maintain altitude. And that was with no compression happening, since the valve head was gone.

In an airplane, where the engine is working at very high power levels all the time (unlike a car), every cylinder needs to be working at max efficiency.

 

[WonderousMountain]

Should have mentioned I'll be using twin plugs.
So you have actual redundancy, not quasi duality.

 

[Riggerrob]

I was under the impression that most airplane engines have two spark plugs per cylinder because one was not enough to ignite all the gasoline. Dual magnetos for redundancy was a secondary benefit.
OTOH modern automobile engines are specifically designed to ignite all the fuel with a single spark plug. Modern cars run hundreds of thousands of kilometres with minimal maintenance to ignition systems.
Why any one would want to mess with a ridiculously reliable system is a mystery to me.

Adding a second ignition system is about blindly adhering to tradition without understanding the background history and technology of the tradition. ... like too many military traditions.

 

[Dan Thomas]

Two magnetos were necessary for safety because magnetos are notoriously unreliable. They're a lot better than they were but they still fail. The manufacturers of magnetos stipulate 500-hour inspections (Slick) or 400-hour (Bendix) to catch the stuff that's wearing or corroding or whatever, but since FAR 43 is so soft on a lot of stuff it seldom gets done and owners run their magnetos until they fail. To me, that's just plain stupid. An owner will carefully preflight his airplane, making sure the oil and fuel are up and there's no bird nest under the cowl and that the runup goes well, but they'll run the mags, vacuum pumps and alternators until they quit. And they usually quit at some inconvenient or dangerous place or time. A magneto can do more than quit, too: it can wear the plastic gears in its distributor so that they slip and the thing starts sending sparks to the wrong cylinders at bad times such as the intake stroke, making the engine run so rough it just about quits. The mag switch is there to shut that bad mag off, but I once watched an RV trying to make it to the airport with the engine barking and coughing from a bad mag. I presume he made it, since there was no news about any accident.

Poor training, there. How many of you were ever taught to try cycling the mag switch if the engine started running real rough and carb heat didn't fix it?

New engines are now certified under revised FAR 33. In the Appendix regarding continued airworthiness we find this:

The Instructions for Continued Airworthiness must contain the following manuals or sections, as appropriate, and information:
<snip>
(6) Scheduling information for each part of the engine that provides the recommended periods at which it should be cleaned, inspected, adjusted, tested, and lubricated, and the degree of inspection the applicable wear tolerances, and work recommended at these periods. However, the applicant may refer to an accessory, instrument, or equipment manufacturer as the source of this information if the applicant shows that the item has an exceptionally high degree of complexity requiring specialized maintenance techniques, test equipment, or expertise. The recommended overhaul periods and necessary cross references to the Airworthiness Limitations section of the manual must also be included. In addition, the applicant must include an inspection program that includes the frequency and extent of the inspections necessary to provide for the continued airworthiness of the engine.

See that reference to Airworthiness Limitations? That makes the inspections mandatory by law. No more running stuff to failure on new engine designs.

 

[rv7charlie]

While it's true that in 'big bore' a/c engines having dual plugs helps achieve complete combustion, I'm not so sure that was the original reason for dual ignition systems.

An O-200 has the same bore diameter as a GM 327 cu in V-8 and it's only slightly larger than the GM straight sixes that were developed in the same time frame as the O200. And it turns at half the rpm (or less). The auto engines from that day ran fine on one ignition; just not bullet-proof reliably.

I don't recall ever having a carb or fuel injection experience a hard fail on me, but I've had numerous mag failures (bullet-proof reliability wasn't a requirement on the tractors they were sourced from).

Having said that, automotive engine controllers are much more reliable on a MTBF basis. Unfortunately, the bucket full of external sensors, ignitors, injectors, etc color the reliability thing a bit.

Even more important is the fact that using that highly refined automotive controller in an airplane can be, and often is, quite dangerous. We don't have access to the source code, so we don't know where the 'snakes' are hiding when we alter the operating modes, delete sensors, etc on controllers that have code to run an entire car, including everything from the transmission to alternator charge rate to door locks. We're left with using some form of 'aftermarket' controller, that will never have the hundreds of thousands (likely millions) of hours of testing & debugging that the automotive controller has gone through. Ross at SDS is fond of saying (quite reasonably) that no alt engine mfgr has proven his package reliable until it's seen at least 500 hrs in the field. I know they extensively test their controllers, and they've begun to accumulate a lot of hours in customer planes. Does he run one controller, or two?

Charlie

 

[Protech Racing]

I use Microsquirt controllers on my race cars that allow it. When I get done tuning on the dyno, I remove the inputs from the sensors. Ready to race the only inputs are crank position and map value. 2 inputs.. No air temp no TPS, nothing. I run them without feed back, open loop only on race day . They have a self tune option also .
This reduces the failure points .
The little model engines that use EFI have only 4 inputs also. Temp, rpm, air density, TPS, AFAIK.

 

[pfarber]

Seeiously when is the last time a coil on plug or coil near plug system failed?

Have you driven a ford lately? Early 2000 F150 were famous for shooting plugs right out the head (bad engineering, not enough threads for the plug to hold). They also had COPs which were prone to breaking.

I get your point, but you really have to start with a good motor as a foundation, THEN research the issues. I wouldn't add redundant spark to the plugs 'just because' . You still have multiple points of failures... bad plug, fouling, bad wire etc. that a second spark will no fix.

Even the FAA doesn't REQUIRE dual plugs/ignition. The certification specs simply say 'either a reliable system, or redundant systems'. Yes, that's the FAA's logic. But given that magneto's are garbage that should have died in the 80's, its still that vague in the certification standards.

 

[Chris Matheny]

I guess I'll weigh in. As an automotive shop owner, I still see ignition failures of coils, plugs and connections. I still see vvt cam phaser failures, I see direct and port injector failures. None are immune. Some better than others. This is why I am adding redundant spark, all sensors, and also ECU's to my conversion. I am also ridding my engine of VVT as its not even close to necessary with the loads a propeller puts on the engine. If you can make something less likely to fall from the sky without reinventing the wheel I say go for it.