Powerplant Electrical - Magnetos

how fast does a distributor rotor spin in relation to crankshaft speed? does this vary with # of cylinders?

1/2 crankshaft speed, does not vary (since it is a 4-stroke motor, the cylinders only fire on every-other crankshaft rotation. therefore, the distributor only needs to fire each cylinder once for each crank rotation.

what is the typical range for coming-in speeds on magnetos?

100 to 200 rpm

what does the acronym "FADEC" mean?

Full Authority Digital Engine Control

when does maximum flux occur in the rotational cycle of the rotor in a magneto?

Max flux occurs when the magnet poles are aligned with the pole shoes.

what is the formula for determining the relationship between magneto (magnet rotor) speed and engine crankshaft speed?

Number of cylinders / (2 x # of poles in rotor magnet)

briefly describe a booster magneto.

a booster magneto was a hand-cranked, cockpit mounted magneto that used a "trailing finger" to retard the timing during engine startup. these were used up through the end of WWII.

how are magnetos disabled during a magneto check?

by grounding them. selecting the right magneto to test removes the ground from that magneto, but not the left. selecting the left magneto is the opposite. selecting "both" removes the grounds from both magnetos.

what effect does retarded (late) timing have on engine operation?

engine will idle ok, at high rpm engine will not produce its rated power, may produce high EGT readings

how often does the direction of the flux produced by the rotor reverse?

every 180 degrees of rotation.

in most cases, where are the retard breaker points for a shower of sparks ignition system located?

in the left magneto. during starting these points retard the ignition timing, and the right magneto is grounded so it will not interfere.

what effect does advanced (early) timing have on engine operation?

poor low-end performance, better high-end performance, may cause detonation

how is electrical current induced in the primary windings?

the magnetic field produced by the rotor as it spins travels through the iron core inside the primary coils. this induces current flow in the primary coils, which alternates direction with the direction changes of the flux. (every 180 degrees)

what causes the delay in flux collapse of the coil field as it spins to the neutral position?

the magnetic flux created in the primary coil resists the collapse of the rotor magnetic field, causing the delay in collapse. it will delay the collapse of the field until the rotor passes the neutral position, at which point the opening of the breaker points causes the field to rapidly collapse.

what induces the high voltage current used for the spark sent to the spark plugs?

the spinning rotor creates a magnetic field that travels through the primary winding core, which induces current flow in the primary windings. this current flow creates its own magnetic field that extends across the secondary windings. when this field collapses, a high voltage current is induced in the secondary windings that is sent to the spark plugs.

describe correct timing for normal engine operation

typically 20 to 30 degrees before TDC, spark ignites fuel/air mixture before piston reached TDC so that burn will be complete as piston reaches top of the stroke for greatest power production

what rotor position has the least amount of flux? what is it called?

when the magnet poles are aligned 90 degrees form the pole shoes, called "neutral" position

what happens in the shower of sparks system once the engine is started?

when the start switch is disengaged, the ground to the right magneto is removed so it will begin to function. also, battery power is removed from the starting vibrator, and the retard points are removed from the circuit. at this point the magneto begins to function normally.

how does a "shower of sparks" induction vibrator system work during engine startup?

when the starter switch is engaged, the right magneto is grounded to disable it. battery power is fed to the starting vibrator, which is a coil that opens the contacts supplying power to it, causing its magnetic field to collapse and allow the contacts to close again. this causes the cycle to repeat very rapidly as long as power is supplied, and creates a pulsating DC current that is then sent to the un-grounded left magneto. this current runs in a parallel circuit, one branch to the retard points, and the other branch to the primary coil. when both the retard points AND the normal operation points are open, pulsating DC current flows through the primary coil to ground, creating the high-voltage current in the secondary coil for firing the spark plugs. if either set of points is closed, the current will travel through the closed points to ground and the primary coil will not induce a high voltage current in the secondary coil.