Magneto Question

[plongson]

Man, this is bugging me...So the mags are out of the Cherokee with a Lycoming 0320 for service and repair. Before sending them off, I popped off the cover to look at the points...what is puzzling me is the rotor has TWO lobes to break the points. I'm thinking back to the old days in autos when they had points and condenser to service and the lobes equaled the number of cylinders...ie. 4 cyl./4 lobes, 6 cyl/6 lobes, 8 cyl/8 lobes on the distributor cam.

What am I missing here on these aircraft magnetos?? School me!

 

[plongson]

I think I got it figured out...the rotor in the mag is geared so it's turning half the speed of the engine thus allowing each cycle of the points to be directed to 4 spark plugs....JFYI

 

[Dan Thomas]

The rotor is turning at crankshaft speed. There are two firings per crank revolution. The rotor's plastic gear drives the distributor gear at half the rotor's speed, so the distributor turns once per two crank revolutions, four firings.

The rotor in the mag for a six-cylinder engine is geared to the engine so it turns 1.5 times the crank speed, and the distributor rotor is geared so it turns 1/3 the rotor speed. So it takes three turns of the rotor, two crank revs, to make the distributor turn once. The rotor's cam for the points still has only two lobes.

 

[Dan Thomas]

Now we need to discuss the stuff that creates the spark. That gets real interesting and there are things not readily apparent to most folks.

 

[NordicDave]

Good Mike Busch EAA webinar on how magnetos work, I found helpful:

 

[plongson]

Hate to admit it, but it kept me awake for a while contemplating how the mag timing worked out. I kind'a had it in my head but your explanation brought it to life...THANKS!!