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Magnetos - question/s.


flying dog
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Ok, I'm not the brightest bulb in the box/sharpest tool in the shed/ what ever.

 

I was reading about Mags and CDI ignition systems in one of the threads.

 

Got me curious.

 

Some people were talking about starting engines with one mag - specifically saying right or left - because of the timing advance.

 

Then when the engine is running, you turn on both.

 

I'm still confused why you start an engine with ONE magneto on rather than both. But that's another question.

 

Then the thread was talking about "mixtures" of CDI and Mag ignition systems.

 

Why would you do that?

 

Obviously vibrations are not good and points will be susceptable to "damage" from the vibrations.

 

Where as Mags are imune to that problem - so I understand.

 

Why can't you put hall effect switching on CDI?

 

Better shut up now, I think I have painted a big enough target on myself.

 

Let's see what happens now.

 

 

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FD the vibrations affect the Coils mostly particularly magnetos that have moving coils as distinct from moving magnets. The points are pretty substantial and normal vibrations wouldn't affect their action to any practical extent..

 

You start on the "impulse " for two reasons. It is retarded at the time it uses the impulse function which gives a hot spark at almost zero revs. That's why you don't lean on a prop, or be in it's arc.

 

Once you get above a slow idle it goes to advance which is where the other magneto is close to.(which is turned ON when the engine starts). If you start with "BOTH" the non impulse magneto may fire and kick back against the starter or (if being swung by hand) the hand of the person starting it. The Impulse mechanism incorporates a heavy clock type spring and a trip mechanism which hits a stop when a pivoting part of the trip mechanism projects out to contact the stop which arrests it's movement for a short time and then it clicks past rapidly under the action of the heavy spring and gives the hotter but retarded spark.. Once the engine is running centrifugal force makes this mechanism inoperative as it swings to a new position and makes no contact with the stop. That's enough for a while.. Nev

 

 

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Nev, I'm going to have to sit down and digest that.

 

Thanks.

 

But I am still not seeing a picture forming.

 

You say:

 

You start on the "impulse" for two reasons.

 

What is THE "impulse"?

 

Then you go on:

 

Once you get above a slow idle it goes to advance which is where the other magnet is close to.

 

It?

 

"where the other magneto is close to"?

 

When the engine is running, EITHER mag can be turned off and the engine keeps running.

 

If one was for starting and the other for running, then if you turn off the "wrong" one, the engine would stall/stop.

 

 

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Both magneto's are timed fairly close to the same timing when NOT affected by the impulse action. Some engines have two magnetos set up for impulse. It's in the drive to the magneto right at where it couples.(fits to the accessories case). You really only need one, to get it started. Once the engine is running turning off one mag or the other has much the same effect because the impulse mechanism is not operating and they are working in the advance position. Remember how you check each mag before flight, but you operate with BOTH turned on always unless one of them is faulty and you detect it IN the AIR. and with some faults it runs better with the faulty mag OFF. OFF is earthed. IF the switch doesn't earth the magneto is LIVE. It Fails to ON as far as the switch and it's wires is concerned...

 

.On larger (radials) they ( Impulse mechanisms) are not used . They have other methods of lighting the fires at low engine speeds. to get you going. Nev.

 

 

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The impulse coupling is a gadget between the magneto and the engine, containing a powerful spring and a pawl. When the engine is turned slowly, the pawl engages and stops the magneto, about 30 degrees before top dead centre on the firing stroke, so the spring winds up. As the engine passes top dead centre, the pawl is forced out of engagement, so the spring gives the magneto a sharp enough "kick" that it fires. If you are leaning on the propeller when this happens, you're likely to lose at least an arm. As the engine starts to spin, the pawl is held out of engagement by centrifugal force, so the magneto then behaves normally.

 

Jabiru engines do not use magnetos; they have a form of breakerless ignition; however it needs the starter to spin the engine fairly fast.

 

 

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Then the thread was talking about "mixtures" of CDI and Mag ignition systems.

Why would you do that?

Some people have a fear of full electrical failure which would shut down your electronic ignition so like to have a mag that is self energising and doesn't rely on any other system as backup.

 

 

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There has been a practice of replacing one of the magnetos on a Lycoming or Continental, with a CDI system, which is claimed to improve the cruise fuel economy by advancing the ignition. There are two things against this - firstly, it alters the status of the engine from being type-certificated to being experimental; this may have (will have, in Australia) the effect of prohibiting flight in controlled airspace, as I understand the current rules.

 

Secondly, it alters the engine's vibratory inputs into the propeller. If you have a certificated metal or composite propeller, or a wood propeller that is ground-adjustable, it must be vibration-cleared for the engine; you can find which propellers are cleared for which engines, by looking up the propeller Type Certificate Data Sheet. However, that clearance is only valid if the engine fully conforms to its certificated condition. There have been a number of propeller blade failures, including one fatal one involving an almost new Hartzell on an RV-6 in Australia, from this cause. If you're running a fixed-pitch wood prop, this is unlikely to shake a blade off, but it may make it more likely for the propeller hub face to char, which loosens the propeller, so both blades come off.

 

 

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Ok, thanks for the replies.

 

Allow me to re-cap what I have read:

 

There is this "impulse" thing which kicks the mag/prop/engine at/near TDC when starting to give the spark.

 

When the engine fires up, that is disconnected and the mag functions norally.

 

WRT: CDI.

 

Having one changes the timing of when it (the CDI) fires wrt the prop/engine and TDC.

 

How can that be good? The mag will still fire at it's time and so throw out the system - wouldn't it?

 

How would having one CDI send vibrations to the prop? The only way I can see that is with "bad" timing wrt the mag.

 

Therefore it needs adjusting to remove this difference.

 

Jabs and mags. Funny, I thought they do have mags. I fly 160, 170 and 230 (and a while ago LSA) and from what I was told it is a mag.

 

I do know the LSA does, because I once flew one and it's alternator was kaput (dunno why I actually flew it) and when I landed and needed to re-start it, the battery was FLAT!

 

Maybe they said mag to "simplify" their explination. Dunno.

 

 

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The two-magneto dual ignition system on Lycomings and Continentals (and all large radials, except the Guiberson) has a fixed advance, normally around 25 degrees BTDC (tho that varies a bit from one model to another). The impulse coupling provides 25 degrees retard, so it make it possible to hand-swing the propeller. Or for a fairly small starter motor to start an engine that has around two litres capacity per cylinder. The fixed advance means the two magnetos fire simultaneously, which is necessary to get full power from a large cylinder - and also to increase the detonation margin at lean mixture. Detonation is caused by the pressure rise as the flame-front travels across the combustion chamber; the further the flame-front has to travel, the greater the tendency to detonation. In a big-bore cylinder, the combustion chamber shape has been developed to take advantage of the shorter flame-front travel provided by dual ignition. If you lose one magneto, it's advisable to keep the mixture full rich - tho that is not commonly taught, except that if the engine runs rough, or the EGT rises noticeably, most pilots go to full rich automatically.

 

The concept of having one of the magnetos replaced by a CDI system seems to be, if I understand it correctly, that at reduced-power cruise, the fuel economy might benefit from a bit more ignition advance. However that loses the advantage of the shorter flame-front travel when both plugs fire simultaneously; and as lean mixture settings such as one uses anyway for cruise economy take the engine closer to detonation, this longer flame travel is liable to result in the combustion in the part of the cylinder furthest from the plug fired by the CDI system, getting much closer to detonation than with the correct dual ignition setup. Even if detonation is not sufficient to show up as increased EGT, this can result in a sharp pressure rise late in the combustion process, which puts a sharp edge on the torque pulse reaching the propeller.

 

The second plug firing late, has no effect if the flame front from the first plug has already passed it.

 

NO Jabiru engine has a magneto, in the sense normally understood. It does have distributors, rather than being a "wasted-spark" system. The jab ignition system is self-powering, so it does NOT rely on the battery; it's all out in the open, so you can see how the ignition is excited by magnets on the ring-gear disc. However, the contact-breaker points are replaced by an electronic gizmo that detects the rate of rise of the current in the primary winding. Because the rise-point occurs a bit earlier at higher RPM, this provides sufficient ignition advance. It's a clever system, but it won't produce sufficient power to produce a spark unless the engine is turning fairly fast on the starter. I'm not familiar with how the slow-start boost setup works.

 

 

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Dafydd,

 

If you lose one magneto, it's advisable to keep the mixture full rich - tho that is not commonly taught, except that if the engine runs rough, or the EGT rises noticeably, most pilots go to full rich automatically.

 

Hang on.....

 

Here is how I thought things worked:

 

The fuel is pumped in, compressed and the spark makes the BANG.

 

If there the air/fuel mixture is good, the temps are good.

 

If the fuel is too low, the cyl.... pistons get hot, the valves, etc and you get damage.

 

That is because there is too much oxegene that wants to keep things happening. Yeah, ok, badly explained, but I hope that is good enough for this.

 

However: If there is TOO MUCH fuel, it isn't burnt and so things run cooler. Also there is more "smoke" put out because of the un-bunred fuel.

 

So if - as you said - a mag is lost, all the fuel wouldn't be burned and so the EGT would go down. Right?

 

 

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If you enrich the mixture when you lose one magneto, you are not necessarily improving the engine running on one magneto. Too rich a mixture with two mags is still too rich with one. If there is excess fuel for one mag to burn, where is the fuel going? I reckon it goes out the exhaust, still burning and that would result in increased EGT, but lowered CHT.

 

The reason most pilots go full rich when one mag goes down is that the trained response to engine problems is to go full rich and then start analysing what could be the problem.

 

 

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I am not going to try to write a textbook on this subject, in this thread. The combustion process is affected by a lot of factors, many of which are incompletely understood. But the fuel-air mixture burns, it does NOT explode (at least, if the engine is to stay together more that a few seconds). Losing a mag. does NOT necessarily mean all the fuel isn't burned; it means it isn't burned at the right time.

 

If you can find a copy, look up "The High-Speed Internal-Combustion engine" by Ricardo; that's the classic textbook on this subject. There's a vast amount of later information on this, on the internet; but it's difficult to put it all together unless you have read Ricardo's explanation first. However, the characteristic frequency of detonation is related to the cylinder bore diameter, which tends to confirm that it's a result of the rate of propagation of the flame front. (I managed to confirm that during detonation testing of the Jabiru 2200; you'll find the formula in the literature if you look for it.)

 

I agree that going to full rich is not necessarily the best way to fix an engine running problem - but it's normally a least-worst initial response.

 

 

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THAT might seem intuitive but I would use rich mixture to nurse the engine. It will darken the oil and waste bit of fuel. When you have a rich mixture the hydrogen is always consumed first and the carbon laden bit last and what is not burned is expelled as soot. This will give a "torching exhaust" with a dull red flame. I don't think it. is that hot. Nev

 

 

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Yeah, ok. Appologies.

 

Got got CHT and EGT mixed up - maybe.

 

Dafyyd, I am not expecting you to go into the full "blow by blow" of what is going on.

 

I am only discussing it. I am not having a go at you.

 

It is nice to be able to talk about things and that is why I asked. I don't know and would like to ask and learn.

 

Granted I therefore should research as well. But asking people and talking about it is a good way.

 

Nowadays it seems to be "STFW" (as opposed to RTFM).

 

And as much as there is good info out there, there is just as much bad. So as I said: I am only discussing it with the intention of learning a bit more than I know.

 

 

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Yeah, ok. Appologies.Got got CHT and EGT mixed up - maybe.

 

Dafyyd, I am not expecting you to go into the full "blow by blow" of what is going on.

 

I am only discussing it. I am not having a go at you.

 

It is nice to be able to talk about things and that is why I asked. I don't know and would like to ask and learn.

 

Granted I therefore should research as well. But asking people and talking about it is a good way.

 

Nowadays it seems to be "STFW" (as opposed to RTFM).

 

And as much as there is good info out there, there is just as much bad. So as I said: I am only discussing it with the intention of learning a bit more than I know.

No, that's OK; but it's not my major field of expertise. I've told you what I am sure of. I am in an outlying area and have only limited broadband capacity, so I cannot indulge in much in the way of a discussion. Nor do I really have the time. I try to alert people to potential hazards; and propeller blade failure is one of them when you start playing about with the engine's ignition system. You'll find a reference to this on the U.K. PFA site.

 

 

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Thanks. Shall do.

Newcastle TAFE have the 4th edition in their library. The bit Dafydd didn't mention was that enriching the mixture up to ~125% of chemically correct increases the resistance to detonation a lot - see also post 18 - and the power a little (I have both of his copies of Ricardo, heh heh...)

 

 

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