Magnetos

Let'sgoflying!

Let'sgoflying!

Touchdown! Greaser!
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Feb 23, 2005
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Dave Taylor
Sometimes I get reading stuff and I want to get deeper into it, but more questions arise and past training or online things are only partial, or no help.

I am thinking about the 'alignment of components' and timing in magnetos. Tom will surely be interested. Maybe Lance?
Offer any corrections:

The rotating magnet has the cam affixed to one end. The lobes of the cam are aligned to the magnet on the shaft, so that when the magnet passes the coil, it can make the best spark possible by opening the contactor points at this time. (if the magnet is well past the coil when the points open; weak or no spark results)
During assembly, the points are adjusted using the timing light to have them open at the best time possible...when the magnet is producing peak flux in the coil. The e-gap is thus set.

The magnet passes the coil each time a piston approaches tdc and the distributor gear rotates much more slowly; 1/4 that speed in a 4 banger, 1/6 that speed in a 6 cylinder, the IO720(8 cylinder) I suppose would be 1/8th that speed.....so that the 'finger' will pass each distributor post every time the magnet passes the coil.

We clock the distributor gear to the rotating magnet so that the points open when the metal finger (a TCM term) is passing a post on the distributor tower. The nylon distributor gear is marked appropriately ie I have seen CCW/CW/L/R (also LB and RB)
Can anyone explain:
-about LB and RB ie boost coil mags?
-why the alignment is a few degrees off for the 'B' type mags?
-why the CW and CCW (or L and R) marks are not at the same place?

Then we install the mag so that it is firing #1 cylinder (when properly set using the timing marks on the distributor gear, the 'finger' is set at the #1 post).
Then we fine tune this setting to eg, 25btdc when we time the mag to the engine.
 
Couple of things: The magnet has two poles and generates current in the primary of the coil twice per revolution. For a four-cylinder engine, the distributor gear turns at half the rate of the magnet (which turns at engine speed). For a six, the distributor turns at 1/3.

The points open at about 7 degrees past the neutral point of the magnet with the coil frame (neutral is 90 degrees past full-register of the magnet with the frame, where maximum magnetic flux is passing from the magnet thru the frame). The coil's primary is an inductor, which means that once electron flow is induced in it by the moving magnet, the flow then generates its own field as it begins to collapse, and that field causes continued flow. The points are set to open when the magnetic flux differential is at a maximum. The "E-gap" is that angle between neutral and points opening. This setup leads to more confusion than anything else about magnetos, since it's not intuitive. The average dude thinks that the biggest zap comes if the magnet is aligned with the coil frame, which is not true. It's more than 90 degrees later.

Many magneto parts are designed to fit multiple applications. Some engines rotate their mags in the same direction as the crankshaft, while others rotate them opposite. The distributor gear has CW or CCW or maybe just R and L markings on it so you can set the timing finger to the correct position. The magneto shaft will often be a different part number for L or R rotations, since the cam has to be in a somewhat different location to take advantage of E-gap.

Magneto rotation is as viewed from the gear end. Engine rotation is viewed from behind the engine, in the cockpit, facing forward. Nothing like consistency, huh?

Condensers, or capacitors, are often explained as simply there "to prevent arcing at, and burning of, the points." Just try running a mag (or any contact-point ignition) without the condenser. It won't work. The condenser is a diversion for the electron flow as the points begin to open, which stops the arcing alright, but that condenser fills quickly and flow soon stops. By that time the points are too far open to allow arcing to start, and the primary's field collapses and we get a good spark. If the condensser wasn't there, arcing would occur at the points. Not only would they burn, but that arcing represents continued electron flow and gradual reduction of the magnetic field through the secondary winding, which won't produce the spark. We need that flow to stop very quickly so the field collapses suddenly to produce the spark. And if you read the Slick service manual, they'll also point out that the condenser aids in starting the next primary flow cycle before the points close again and short it; the magnet's rotation generates an AC flow in the primary, and the condenser helps to "bounce" the flow in that opposite direction for the next spark. That phenomenon would apply only to magnetos, not battery-points ignition, by the way.

Dan
 
Let'sgoflying! said:
I did!

The FAA manual...I have always found those more theoretical and less practical. Still useful however.
Click to expand...

Just remember the Slick has a pin to place in a hole to tell you that the mag is at the firing point for #1, get the pin in the hole, place the engine at firing position and install the mag. then time with a light.

With Bendix simply look thru the top inspection hole and turn the mag until you see the red tooth on the distributor gear, insert the mag holding tool, then place the engine at firing position and install the mag. then time with a light.

Don't forget to compensate for the impulse coupling.
 
Let'sgoflying! said:
I did!

The FAA manual...I have always found those more theoretical and less practical. Still useful however.
Click to expand...

Like any textbook, it can only cover the theory. There are far too many variables in the practical world to address. Every magneto manufacturer uses the same basic theory, but the mechanical layout is considerably different between makes of mags and the way the engine manufacturers mount them. The typical Lyc or Continental drives them via gears inside the accessory case, for instance, and mag-to-engine timing is achieved by rotating the mag a little to get the points opening at the specified spark advance. My old Auster's Gipsy Major VII had external mag drives with toothed rubber couplings having differing numbers of teeth on each side that meshed with two toothed wheels; backing the mags off and rotating the rubbers one tooth would shift the timing a tiny bit. Not really convenient, but at least there were no hassles with leaking mag shaft seals letting engine oil into the mag.

Dan
 
I'd love to know what thoughts were going on in the heads of guys who invented these. Were they modified from another application and applied to internal combustion engines?

"OK, we need a device that will supply a hot spark, at exactly the right time, on a continuous reliable basis, inside a high pressure chamber - using its own internal electrical supply." Easy for us to say 'No Problem' from the vantage-point of 2011.
 
Those guys were genii in my mind, they spent interminable hours with paper napkins and pencils, figuring out how to spin wheels and gears, levers and cams to make those old mechanical devices work. I would have loved it, I started pulling apart my parent's alarm clock at age 5 and tape recorders, etc soon after. Every bicycle and the one motorcycle I had got torn apart so I could investigate how they made them!
 
Let'sgoflying! said:
Those guys were genii in my mind, they spent interminable hours with paper napkins and pencils, figuring out how to spin wheels and gears, levers and cams to make those old mechanical devices work. I would have loved it, I started pulling apart my parent's alarm clock at age 5 and tape recorders, etc soon after. Every bicycle and the one motorcycle I had got torn apart so I could investigate how they made them!
Click to expand...

the number that returned to service is the key to success.

or

how many ever worked again.



:)
 
Figure this one out.......

my Warner is a 7 cylinder single row radial, it has Bendix DF7 mags on it, they have 4 point opening cam lobes. yet it fires each cylinder at 28 BTDC.

The timing can be changed 8 degrees by rotation of the mag, and by advancing or retarding the mag drive gear 1 tooth. for each 3 degrees required.
 
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