[ARC5] Magnetoes

[Roy Morgan]

On May 13, 2014, at 3:20 PM, Robert Eleazer <releazer at earthlink.net> wrote:

> If the ignition switch is set to "Both" that means that neither mag is grounded and that both spark plugs in each cylinder are firing.
> 
> When the ignition switch is set to "Off" that means both mags are grounded.

I plead faint memory (I posted errors in my message about aircraft magnetos, and had forgotten the fact that engines continue to run even with total electrical failure).

This link at Wikipedia:
http://en.wikipedia.org/wiki/Aircraft_engine_controls
has the following about “Ignition Switches”:

• Ignition Switch - Activates the magnetos by opening the grounding or 'p-lead' circuit; with the p-lead ungrounded the magneto is free to send its high-voltage output to the spark plugs. In most aircraft the ignition switch also applies power to the starter motor during engine start. In piston aircraft engines, the battery does not generate the spark for combustion. This is accomplished using devices called magnetos. Magnetos are connected to the engine by gearing. When the crankshaft turns, it turns the magnetos which mechanically generate voltage for spark. In the event of an electrical failure, the engine will continue to run. The Ignition Switch has the following positions:
	• Off - Both magneto p-leads are connected to electrical ground. This disables both magnetos, no spark is produced.
	• Right - The left magneto p-lead is grounded, and the right is open. This disables the left magneto and enables the right magneto only.
	• Left - The right magneto p-lead is grounded, and the left is open. This disables the right magneto and enables the left magneto only.
	• Both - This is the normal operating configuration, both p-leads are open, enabling both magnetos.
	• Start - The pinion gear on the starter motor is engaged with the flywheel and the starter motor runs to turn the engine over. In most cases, only the left magneto is active (the right p-lead is grounded) due to timing differences between the magnetos at low RPMs.
(The reference is: "MS94-8A Service Bulletin”
http://www.tcmlink.com/serviceBulletins/pdf/MS94-8A.pdf  )

> Interesting thing about magnetos - they require special switches for grounding. … I guess that a switch rated at 125V/5 amps cannot handle the thousands of volts produced by a magneto - it shorts across it in short order.

I think that magnetos and the controls on lawn mowers, and perhaps race cars, are controlled a bit differently.  I remember the little metal tab next to the spark plug on lawn mowers, and on a more recent one, the damnable safety bar at the handle bar that somehow got released JUST as I finally and at great effort got the thing to run.

Gordon White, who apparently has a race car, reports:

> Actually, a mag switch shorts the low-voltage side of the mag. I have such a switch on my race car, in fact two, one a toggle on the dash and one a button on the brake handle (Yes, no foot brake!!) They work fine.


Roy Morgan
RoyMorgan at alum.mit.edu
K1LKY Since 1958