Pistons 1 - Week 3

what does CAR 533 state

each engine ignition system must have a dual ignition system with at least two spark plugs for each cylinder and two separate electrical circuits with separate sources of electrical energy or equivalent in-flight reliability

what do nearly all aircraft piston engines use for an ignition system

magnetos

a magneto is a

permanent ac generator that uses electromagnetic and induced current principles to develop high voltage energy to fire a set of spark plugs

less dense air at altitude leading to the high voltage spark getting misrouted is known as

flashover

a side effect of flashover that leads to carbon tracks that form a path to ground

carbon tracking

carbon tracking ________ the possibility of flashover

increases

the magnetic circuit consists of

rotating permanent magnet, pole shoe extensions, pole shoes, coil core

the greatest density of flux lines through the coil core occurs when

a pair of magnetic poles are fully aligned with the pole shoes

rotating magnets are made from

alnico (alloy of aluminum, iron, nickel, cobalt) or permalloy

the pole shoes, extensions, and core are constructed of

several laminated layers of high grade soft iron

maximum lines of flux through the core when the magnetic field is the strongest

full register

no lines of flux through the core

null

primary current attempts to hold the flux in the core at a high value until

the rotating magnet reaches the neutral position

point at which the greatest magnetic field stress exists (degrees beyond the the neutral position)

efficiency gap (e-gap)

primary circuit consists of

insulated magneto coil, a set of breaker points, capacitor

primary winding in a magneto coil consists of

180-200 turns of 18 gauge copper wire

when are the breaker points opened

e-gap position

used to reduce arcing in the primary circuit

capacitor

circuit that produces the high voltage energy required to cause a spark

secondary circuit

components that make up the secondary circuit

secondary winding in the coil, distributor

amount of current induced into the secondary winding related to two factors

rate at which magnetic field around primary field collapses, ratio of windings between primary and secondary windings

the secondary winding is capable of producing a

20000 volt pulse

the distributor rotor must complete one revolution for

every two revolutions of the crankshaft to fire all the cylinders of the engine

a magneto can fire all the spark plugs once for each

720 degrees of crankshaft rotation

backup path to ground connected to the secondary winding

safety gap

all low tension magnetos utilize these to boost the strength of the spark

transformers

P-lead stands for

primary lead

any wiring connected to the primary of the coil is called

p-lead

when the ignition switch is in off position

contacts are closed and both L and R circuits are completed to ground

ignition switch set to R

right magneto operates left grounded

ignition switch set to L

left magneto operates right grounded

ignition switch set to both

L and R contacts open and both p-lead circuits open

magneto checks are done

whenever running the engine to ensure ignition switch is operating normally

a check when the engine is running is known as a

live magneto check

to check a magneto what happens to the switch

briefly position the switch to off position then quickly back to both while observing the tachometer

what is the expected outcome of a magneto check

when switch in off position engine should slow and almost stop until switch is returned to both position

if an engine does not respond during a magneto check the problem may lie

in the p-lead or ignition switch

if engine rpms drop to zero during a check the problem is

a short circuit between the magneto and the ignition switch

ignition timing in aircraft is

not variable

firing order represents

the sequence in which the ignition event occurs in different cylinders of an engine

firing order on single row radial engines

the odd cylinders fire in succession first followed by the even numbered cylinders

firing order for 14 cylinder twin row radial engines

add 9/subtract 5 rule

firing order for 18 cylinder twin row radial engines

add 11/subtract 7 rule

firing orders on non radial engines are determined

by the manufacturer

the ignition harness

transfers the high voltage produced by the magneto to the spark plugs

ignition harness construction

stranded or single coil conductor encased in one or two layers of silicone insulation with a braided metal shield on top to reduce electromagnetic interference

transmits the short pulse of high voltage from magneto into the combustion chamber

spark plug

main components of a spark plug

metal shell, insulator, electrode assembly

high number means the plug is

hot (less heat transfer)

low number means the plug is

cold (more heat transfer)

threaded on both ends one end into cylinder head other end into ignition harness

metal shell

used for spark plug removal/installation

special 6 point socket

made of a ceramic material that is fragile

insulator

two varieties of electrodes

mass electrode (nickel), fine wire (platinum/iridium)

critical for spark plug operation

gap between the electrodes

spark plugs must be serviced every

100hrs

spark plugs must be installed with

new gaskets