Download Small Engine Ignition Theory - Georgia Ag-Ed Portal

SMALL ENGINE
TECHNOLOGY
IGNITION SYSTEMS
IGNITION SYSTEMS
THEORY
Faraday's Law and Ignition
How do you obtain 40,000 volts across a
sparkplug in an small engine when you
have only 12 volts DC to start with? The
essential task of firing the sparkplugs to
ignite a gasolene-air mixture is carried out
by a process which employs Faraday's
law.
Magnetism
1. Molecules are the smallest particles of matter which are recognizable.
2. In most materials, the magnetic poles of adjoining molecules are arranged in a random pattern, so
there is no magnetic force
Magnetism
3. A magnetized substance has all molecules in alignment, north to south.
4. Individual molecules combine magnetic forces to produce a strong overall magnetic force.
Magnetism
5.
The fact that there is a close relationship between electricity and magnetism serves as the basis
for making a workable magneto.
6. If a conductor, such as copper wire, is moved so that it cuts magnetic lines of force, an electron flow
is induced in the conductor.
7. A conductor that is not moving and not cutting magnetic lines of force will not induce electrical
current. Electricity will flow when the magnetic lines of force are being cut by the wire.
8. A coil of wire with current flowing through it will produce a magnetic field around itself and around
each turn of wire in the coil.
Faraday's Law
Any change in the magnetic environment of a coil of wire will cause a voltage
(emf) to be "induced" in the coil. No matter how the change is produced, the
voltage will be generated. The change could be produced by changing the
magnetic field strength, moving a magnet toward or away from the coil,
moving the coil into or out of the magnetic field, rotating the coil relative to
the magnet, etc.
Faraday's Law
 The primary winding of the ignition coil is wound with a small number of
turns and has a small resistance. Applying the battery to this coil causes a
sizable DC current to flow. The secondary coil has a much larger number of
turns and therefore acts as a step-up transformer. But instead of operating
on AC voltages, this coil is designed to produce a large voltage spike when
the current in the primary coil is interrupted. Since the induced secondary
voltage is proportional to the rate of change of the magnetic field through
it, opening a switch quickly in the primary circuit to drop the current to
zero will generate a large voltage in the secondary coil according to
Faraday's Law. The large voltage causes a spark across the gap of the
sparkplug to ignite the fuel mixture. For many years, this interruption of
the primary current was accomplished by mechanically opening a contact
called the "points" in a synchronized sequence to send high voltage pulses
to the sparkplugs. One of the drawbacks of this process was that the
interruption of current in the primary coil generated an inductive backvoltage in that coil which tended to cause sparking across the points. The
system was improved by placing a sizable capacitor across the contacts so
that the voltage surge tended to charge the capacitor rather than cause
destructive sparking across the contacts. Using the old name for capacitors,
this particular capacitor was called the "condenser".
Types of ignition systems
1. Magneto Power (A spark-ignition system which receives its source of power from a
magnet rotating near an armature).
a. With breaker-points
-"Mechanical Breaker Ignition" (MBI) system is a flywheel magneto inductive
system. It employs mechanical breaker contacts for the timing and triggering
of the system.
b. Solid-State (Breakerless)
(1) "Transistor Controlled Ignition" (TCI) system is an inductive system.
Semiconductors (transistors, diodes, etc.) are used for switching purposes
instead of mechanical breaker contacts.
(2) "Capacitor Discharge Ignition" (CDI) system has no moving parts and
stores its primary energy in a capacitor and uses semiconductors for the
timing and triggering of the system.
Components of ignition system
1.
Magneto - Self-contained units that produce electrical current for ignition without any outside primary source of electricity.
a.
Basic parts of the magneto are:
(1) Permanent magnets.
(2) High tension coil with laminated iron core and primary secondary windings.
(3) Breaker points and breaker cam.
(4) Condenser.
(5) High tension spark plug wire.
(6) Spark plug
Ignition coil
1.
2.
Used in a magneto system and operates like a transformer.
The coil contains two separate winding of wire insulated from each other and wound around a common laminated iron core.
3.
4.
a.
Primary winding - Winding of heavy gage wire with few turns.
b. Secondary winding - Winding of light gage wire with many turns.
When electric current is passed through the primary winding, a magnetic field is created around the iron core.
When the current is stopped, the magnetic field collapses rapidly, cutting through the secondary windings. This rapid cutting
of the field by the wire in the coil induces high voltage in the secondary circuit.
The high secondary voltage, in turn causes a spark to jump the spark plug gap and ignite the air-fuel mixture.
5.
COILS (Armature)
Transformer

A transformer makes use of Faraday's law and the ferromagnetic properties of an
iron core to efficiently raise or lower AC voltages. It of course cannot increase power
so that if the voltage is raised, the current is proportionally lowered and vice versa.
Advantages of a solid state ignition system
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
Elimination of ignition system maintenance.
No breaker points to burn, pit or replace.
Increased spark plug life.
Easy starting, even with fouled plugs.
A flooded engine will start easily.
Higher spark output and faster voltage rise.
Spark advance is electronic and automatic. It never needs adjusting.
Electronic unit is hermetically sealed and unaffected by dust, dirt, oil, or moisture.
System delivers uniform performance throughout component life and under adverse
operating conditions.
(j) Improves idling and provides smoother power under load.
Magneto cycle
a.
As the flywheel turns, the magnets pass the legs of the laminated core of the coil. When the north pole of the magnet is over
the center leg of the coil, current passes across the bottom of the lamination and up the side leg to the south pole.
b.
As the flywheel continues to turn the north pole of the magnet comes over the side leg and the south pole is over the center
leg of the lamination. Now the lines of force move from the north pole down through the side leg and up through the center
leg and the coil to the south pole. At this point, the lines of force have reversed direction.
c.
Field reversal takes place in the center leg of the lamination and coil. The reversal induces low voltage current in the
primary circuit through the breaker points. Current flowing in the primary winding of the coil creates a primary magnetic
field of its own, which reinforces and helps maintain the direction of the lines of force in the center leg of the lamination.
It does this until the magnets' pole move into a position where they can force the existing lines of force to change direction
in the center leg of the lamination just before this happens the breaker points are opened by the cam.
d.
Opening of the points breaks the primary circuit and the primary magnetic field collapses through the turns of the
secondary winding. The condenser makes the breaking of the primary current as instantaneous as possible by absorbing
the surge of primary current to prevent arcing between the breaker points.
e.
As the magnetic field collapses through the secondary winding of coil, high voltage current is induced in the secondary
winding. At exactly the same time, the charge stored in the condenser surges back into the primary winding and reverses
the direction of current in the primary windings. This change in direction sets up a reversal in direction of the magnetic
field cutting through the secondary and helps increase the voltage in the secondary circuit. The potential of the high voltage
causes secondary current to arc across the spark plug gap.
Faraday's Law and Ignition
The transistor switches are contained in a solid-state Ignition
Control Module. Modern coil designs produce voltage pulses up in
the neighborhood of 40,000 volts from the interruption of the 12 volt
power supplied by the battery.
Servicing Nonbattery Ignition
Systems
 A magneto applies magnetism to supply electricity in ignitions where there is no
battery. The magneto is turned by the crankshaft, which rotates when the manual
recoil starter is pulled. Here's how to service a non-battery ignition system:
Step 1: Service magneto.
Step 2: Service ignition controller.
Step 3: Service spark plug(s).
Step 4: Service flywheel.
If you need to service a magneto, use the following directions:
Step 1: Remove the magneto cap and clean surfaces with a small, clean paintbrush.
Wipe any excess oil away.
Step 2: Service the ignition controller (mechanical-breaker, capacitor-discharge, or
transistor-controlled) as described at the bottom of this page.
Servicing Mechanical-Breaker
Ignitions

For many years, mechanical-breaker ignition systems were the most popular of all ignition
systems. The high voltage electricity from the coil is turned on and off using contact points and a
condenser. The spark must be correctly timed to reach the spark plug at the exact moment when
the piston is at the top of its travel and the fuel-air mixture is fully compressed. Here's how to
service a mechanical-breaker ignition:
Step 1: Remove the cover from the stator plate to expose the breaker points and condenser.
Step 2: Manually turn the crankshaft until the high point of the cam lobe opens the contact
points. Inspect the points for uneven wear or damage. If necessary, replace the breaker points
and condenser.
Step 3: Slightly loosen the points setscrew and place the appropriate thickness gauge between
the two contacts. (Check your owner's or service manual for the correct gap.) Move the points
setscrew until the thickness gauge is touching both contacts but can be withdrawn without
moving them.
Step 4: Tighten the points setscrew.
Step 5: Check the points gap with the thickness gauge again. Tightening the setscrew may have
changed the gap.
Step 6: Clean the points with lint-free paper to remove any oil left by the thickness gauge.
Note: Some mechanical-breaker ignitions can be set using a dwell meter. If you have a dwell
meter, refer to the unit's operating instructions and the ignition specifications to determine what
dwell angle setting is correct and how to set it.
CHECKING THE FLYWHEEL
MAGNET STRENGTH
Place a 10" #3 1/4" straight blade
screwdriver against the flywheel magnet
and release the screwdriver. If the
magnets are good they should hold the
screwdriver out in a straight position
FLYWHEEL KEY
Inspect key for partial shearing.
I
f sheared, replace.
Check flywheel and crankshaft keyways
for damage. If damaged, replace with
new parts. Install Flywheel, Nut
and/or Clutch
Servicing Flywheels

The flywheel on a small gas engine is a simple part that requires little
service. The most important part of servicing a flywheel is to inspect it for
damage periodically. Here's how to service a flywheel:
Step 1: Remove the lead from the spark plug (to ensure that the engine
doesn't start), then rotate the flywheel by hand and inspect it for wobble
and obvious damage. Check edges and cooling fins, looking for cracks and
missing pieces that can make the flywheel -- and the engine -- rotate out
of balance.
Step 2: To inspect the inside of the flywheel, use a flywheel puller or a
knock-off tool to remove the flywheel from the end of the crankshaft.
Step 3: Inspect the magnets on the inside of the flywheel, if so equipped.
Wipe all surfaces clean, removing rust, oil, and debris.
CHECK ARMATURE AIR GAP
 With the use of a flat feeler gauge measure the air gap
between the armature and the flywheel magnets. Do
this at both of the stator poles. The armature air gap
should be between .010-.014.
Install governor blade and armature.
The mounting holes in the armature la
minations are slotted.
Push armature up as far as possible
and tighten one mounting screw to hold
armature in place.