Small Engines/ Ag. 221
IGNITION THEORY
Objectives
 Identify basic terms and definitions
associated with magneto ignition systems
 List and identify parts related to a magneto
ignition system
 Describe the principles of magneto ignition
systems
 Identify the function of individual parts
related to ignition systems
Terms and Definitions
 Amperage
 A unit for measuring the rate of current flow
through wires.
 Similar to the flow of water in pipes
 Voltage
 A unit of electrical force or pressure
 Comparable to water pressure
 Circuit
 The complete path of electric current
 Magneto
 An alternator with
permanent magnets used
to generate current for
the ignition of an internal
combustion engines
 Ohm
 Measure of the resistance to current flow through
a wire or other current carrying material
 Primary circuit
 Current generated from flywheel magnet and
armature, carried through the breaker points and
condenser (points closed)
 Secondary circuit
 current generated from the primary circuit,
carried through the coil and spark plug wire
(secondary wire) to the spark plug (points open)
Ignition Parts
 armature
 Breaker points (older engines)
 crankshaft
 coil
 Condenser (older engines)
 Electronic ignition module (conversion)
 flywheel
 Flywheel key
 Point plunger (older, points and ignition
systems)
 Spark plug
Magneto Ignition
Ohm’s Law
 Ohm’s law – a pressure of one volt (E) will
force a current (I) of one amp through a
resistance of one ohm (R).
 E=I * R
 Amperage (I) and resistance (R) are directly
proportional to voltage (E).
 If amperage or resistance increases then
voltage will also increase
 An ignition system of an engine requires
10,000 to 12,000 volts to create a sufficient
arc across the spark plug
 To obtain that high of voltage a coil wire is
used to increase resistance which directly
increases voltage
How Electricity is Produced
 If a wire is passed through a magnetic field
and electrical current will flow on the wire.
 A larger wire or a larger number of wires that
are passed through a magnetic field will
increase the amount of the current
 If a current flows on a wire a magnetic field is
produced around the outside of the wire
 Resistance (R) can be used to reduce the
current (I) and increase the voltage (E).
 In a small engine
 Magnet is located on the outer edge of the
flywheel
 Coil of wires is in a stationary position outside of
the flywheel
 The magnet passes by the coil with each
revolution of the flywheel, creating electricity on
every revolution
Magneto Ignition
 Consists of two circuits
 Primary circuit
 Secondary circuit
Spark Plug
Breake
r Points
Condenser
Kill
Switch
Primary Circuit
 Primary coil winding;
larger gauge wire
 Points
 Condensor
 Or an electronic
ignition module
Secondary Circuit
 Secondary coil
winding; smaller
gauge wire
 Spark plug
Spark Plug
Breaker
Points
Condenser
Kill