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Technical Training
Magneto Ignition Operation
© STIHL Incorporated
Magneto 2011
“Any sufficiently advanced technology is indistinguishable from magic.”
--Arthur C. Clarke, author of 2001: A Space Odyssey
This lesson will look at how a magneto works, and de-mystify the “magic”
involved in creating the spark.
All STIHL engines and most small engines use a magneto ignition system to
create the spark for combustion to take place.
This type of ignition is capable of creating up to 15,000 volts of electricity at
the spark plug without needing any outside supply of electricity, such as a
battery.
A magneto ignition is self contained, small, lightweight, and ideal for hand
held power equipment.
© STIHL Inc., Virginia Beach, VA 2013 US/STR
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Magneto Design
„
There are basically two magneto systems that you are likely to see used on small
engines:
„ Breaker points with condenser
„ CDI, or capacitor discharge ignition
Points & Condenser under
Starter
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One-piece Solid State
CDI Ignition Module
Ignition Coil by
Flywheel
The breaker points type of magneto is essentially obsolete, and will usually
be found only on older equipment.
STIHL began using solid state magneto ignition components as far back as
the 041 chain saw in 1967. By the 1980’s most of the Power Equipment
Industry had started using solid state ignition systems for most applications.
Breaker points, as in the picture above from a trimmer, begin to wear and
deteriorate as soon as the engine starts to run.
They require maintenance and service on a regular basis.
The points act as a switch to cause the ignition coil to send the spark through
the high tension lead to the spark plug.
Now we use solid state electronic components to replace the switch action
that was performed by the breaker points.
Many manufacturers, including STIHL, made kits available to replace the
points and condenser on their ignition systems with solid state modules. Now
most are integral within the ignition module.
© STIHL Inc., Virginia Beach, VA 2013 US/STR
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Magneto Components
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Laminated Steel
Armature
Primary Winding
Secondary Winding
Spark Plug Gap
Switch: Points and
Condenser or Solid State
Magnet
Steel Pole Shoes
This is a simplified graphic illustration to show the components in a typical
ignition system. The armature is made up of thin laminations of steel stacked
together with the primary winding wrapped around one leg of the armature and
connected to the switch. The switch could be a set of points and condenser or a
solid state transistor type switch. Wrapped on top of the primary winding is the
secondary winding that is connected to the spark plug through the high tension
lead.
The flywheel has a magnet embedded inside that is touching two pole shoes
that are exposed on the outer edge of the flywheel.
As the flywheel spins by the armature current is induced in the windings and
12,000 to 15,000 volts can be generated to fire the spark plug.
CDI modules like the one in the above picture are a very compact and sealed
one piece design.
Early solid state ignitions were bulky and often of a multi piece design.
There is a solid state circuit board epoxied inside the module.
The circuit board and windings are sealed in epoxy to protect them
the elements and from vibration.
© STIHL Inc., Virginia Beach, VA 2013 US/STR
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Magnetism and Electricity
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„
„
A permanent magnet is embedded in the flywheel
There are two steel pole shoes on either side of the magnet
The magnet may be a man made ceramic type or a smaller rare earth type
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There is usually only one magnet in the flywheel. The flywheel will also have
steel embedded in the opposite side so that it will be in balance when it
spins. In most cases the flywheel is also responsible for providing cooling air
and may be part of the starter system as well.
This is what a typical flywheel from a STIHL product looks like on the inside.
The magnet may be a man-made ceramic material, or a rare earth material,
and will have very strong magnetic properties.
What you see on the circumference of the flywheel, where the nut is being
held by magnetism, are the steel pole shoes. This gives the flywheel a north
and south magnetic pole as it rotates, which is necessary for the ignition
module to work properly.
The polarity does matter, and some models will have flywheels with different
polarity than others.
© STIHL Inc., Virginia Beach, VA 2013 US/STR
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Magnetic Fields
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„
Every magnet has a north pole and a south pole,
just like the earth, which is actually a really big
magnet
„
Magnets have a magnetic field surrounding them
The earth’s magnetic field is what makes a compass work and what causes
the Northern Lights at the North Pole, also called the Aurora Borealis.
Scientists also believe that the earth's magnetic field protects us from
harmful radiation from the sun.
All magnets are surrounded with a magnetic field. Iron filings can be used to
illustrate this.
The invisible energy reaching out from one pole to the other are called lines
of flux.
The lines of flux are three dimensional, not flat, as the magnetic field viewer
shows.
© STIHL Inc., Virginia Beach, VA 2013 US/STR
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Magnetic Lines of Flux
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„
„
Notice how the iron filings illustrate the magnetic lines of flux reaching out of the
pole shoes on the flywheel
As the flywheel goes by the armature the lines of flux reach out and travel up and
through the armature
These lines of flux will interact with any conductors they come near.
A conductor is anything that will transmit electricity.
Examples of conductors would be:
ƒ Most metals - both ferrous and non-ferrous, are conductors
(ferrous metals are magnetic)
ƒ Copper is a non-ferrous conductor.
Examples of insulators would be:
ƒ Non-metallic materials - plastic, glass, rubber
© STIHL Inc., Virginia Beach, VA 2013 US/STR
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Armature Construction
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„
„
„
Most magnetos are similar in
construction
A steel or soft iron core, usually “U”
shaped, is made of thin stampings,
laminated together, called the
armature
They are slightly insulated from
each other by a thin coating of
oxide
The magnetic lines of flux travel
through the armature as the
flywheel spins by
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If the core was solid it would have strong eddy currents that would cause
heat and make the coil less efficient than it is by using laminations.
The eddy currents are reduced by using laminations that are slightly
insulated from one another.
The laminated core transfers the magnetic lines of flux through the windings
without being magnetized itself.
As the flywheel passes the legs of the armature, the magnetic flux lines
travel up through the metal armature, as illustrated by the picture at the
bottom of the slide. Because the primary and secondary windings are
wrapped around one leg of the armature, the lines of flux cut through these
windings, inducing a current, just like in the galvanometer experiment. As the
magnet moves by the legs of the laminated armature, the polarity of the
magnetic field changes direction, so alternating current is generated in the
primary winding.
© STIHL Inc., Virginia Beach, VA 2013 US/STR
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Magneto Construction
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„
„
„
The primary winding is wrapped around the
armature first
The secondary winding is wrapped around the
primary and is held slightly away by a plastic
separator to keep the wires in place until the
epoxy is poured into the housing
Each winding is one continuous length of wire
Secondary Winding
Primary Winding
The primary winding is the low voltage side of the coil or magneto. It
usually consists of around 20,000 turns of coated copper wire wound tightly
around the armature. One end is grounded and the other is connected to the solid
state controller, or the breaker points in the “old days”.
The secondary windings consist of as many as 200 turns of a very fine wire
wrapped around the primary, and is the high voltage side. One end is grounded
and the other is connected to the high tension wire and the spark plug. As the
magnetism passes through the armature the lines of flux cut through the primary
winding and current builds up in the winding, which in turn creates another
magnetic field that is now cutting through the secondary winding.
When the energy in the primary is released, the concentrated magnetic field in the
windings collapses, and this process creates high voltage, anywhere from 10,000
to 15,000 volts or more, depending on the design. The high voltage is able to
jump the gap of the spark plug, under up to 150 P.S.I., to ground, to complete the
circuit and ignite the air fuel mix in the combustion chamber.
© STIHL Inc., Virginia Beach, VA 2013 US/STR
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Magic or Science?
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By using a laminated steel core for the armature, and wrapping the primary
and a secondary winding around the armature, the magnetic lines of flux can
be directed and amplified to ultimately give us the high voltage needed to fire
the spark plug.
This is happening on every revolution of the flywheel, so at 12,000
RPM, the ignition module is firing the spark plug 200 times a second.
Maybe it is magic after all!
© STIHL Inc., Virginia Beach, VA 2013 US/STR
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Ignition Timing and Spark Advance
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„
„
„
Ignition timing refers to the point at which the plug fires in relation to where the
piston is in the cylinder as it comes up on the compression stroke
It usually fires a little before TDC
It is measured as degrees of crankshaft rotation before TDC
In the combustion chamber, the best situation is for the plug to fire and the
compressed air-fuel mix to burn evenly across the top of the piston. It all
happens very fast, but it is a controlled burn, as opposed to an uncontrolled
explosion.
For this to happen, the plug needs to fire a little before TDC on the
compression stroke. This is known as spark advance.
If the plug fires too soon then the timing is too advanced, and the burning
gases push back against the piston, creating a loss of power and extreme
pressures on engine parts. If the timing is set with too much advance the
engine will be very difficult to start, and may kick back and pull the starter
handle out of the operator’s hand.
If the plug fires too late then the timing is retarded, and combustion is
incomplete and again there is a loss of power, as well as increased exhaust
emissions.
The best time for the plug to fire is not the same at all times. It should be
retarded, or close to TDC when trying to start, and advance away from TDC
as RPM increases.
© STIHL Inc., Virginia Beach, VA 2013 US/STR
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Effects of Ignition Timing on Engine Performance
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Ignition w/o timing advance
Best tuning of ignition timing
35
Ignition Timing [deg BTDC]
30
Best tuning for power,
torque, temperature,
emissions
(depends on cylinder
design and cooling)
Better acceleration,
lower fuel consumption
25
20
Speed limiter to
avoid excessive rpm
15
“Smoother” idle speed
10
5
Starting
500
1000
Idle Speed
2000
3000
Load Area
5000
8000
10000
12000
Over Speed
0
100
Smooth starting,
cold acceleration behavior
15000
Revolution [rpm]
The red line in this slide shows the best tuning of ignition timing for easy
starting, a smooth idle, and best performance at all engine speeds.
Notice that at starting there is approximately 7 to 17 degrees of advance,
and that it increases RPM goes up, with the speed limiter kicking in around
12,000 RPM in this example.
The blue line shows that without any means of spark advance, the timing
would remain static at around 28 degrees of advance. This was normal for
engines with the points and condenser type of ignition, which typically did not
have any means of spark advance. Early CDI ignition systems did not have
any way to advance the timing either.
An engine with the timing set at 28 degrees of advance would require more
effort to start that an engine with the timing set at 8 degrees of advance.
© STIHL Inc., Virginia Beach, VA 2013 US/STR
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Digital Timing Control
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The controller board inside this ignition module is digital, with a
microprocessor and a quartz clock built in, and it gets the power it needs to
operate from the electricity created from the magnet in the flywheel.
When a small engine is being cranked by hand, to prevent a starter handle
snap-back or backfire, the plug should not fire at all until a certain speed is
reached.
As the previous slide shows, for the best performance, fuel efficiency, and
the least amount of emissions, the timing should actually change, advancing
as engine RPM increases. Very few engines equipped with a conventional
breaker point magneto had these capabilities.
The circuit board is designed to have the ignition timing point set near TDC
for easy and safe starting. It measures the time between rotations of the
flywheel with the built in clock, and using this precise measurement,
calculates engine speed and adjusts the ignition timing for more advance as
RPM increases. If RPM exceeds a preset limit, it will drop out a few firing
pulses, or retard the timing, to prevent over-revving. This design also
provides outstanding smoothness under all operating conditions, especially
at idle.
Remember that not all CDI modules will have a microprocessor controller to
provide variable timing, but most new engine designs produced in the last
ten years or so do.
© STIHL Inc., Virginia Beach, VA 2013 US/STR
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Capacitor Discharge Ignition
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diode
primary
winding
spark
plug
magnet
capacitor
secondary
winding
transistor
With a breaker points ignition system the spark plug fires at the same timing
point regardless of the RPM, unless there is a mechanical advance system
of some kind.
To be able to have a timing curve and fire the plug at different timing points
as RPM increases, the charge generated in the primary winding is stored in
the capacitor. The digital microprocessor can then activate the transistor,
similar to the switch action of a set of breaker points, to fire the plug at the
correct timing point regardless of how far past the module the flywheel has
rotated. At the same time, the digital microprocessor can simply not close
the transistor to engage the rev-limiter, or take other actions depending on
the command software installed. This system provides flexibility for the
engineer to incorporate features for easy starting, best performance and
economy, and engine protection from over speeding.
Of course there is more to this system than shown here, but this is a good
basic explanation to get a sense of how modern microprocessor ignition
systems function.
© STIHL Inc., Virginia Beach, VA 2013 US/STR
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Primary Purpose of a Spark Plug:
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„To
ignite the fuel and air mixture
„To
remove heat from the combustion chamber
Center electrode with
copper core
Groundelectrode
Gasket
Glass seal resistance
Seals
Insulator
Ribs
Terminal Nut
When the ignition module sends the voltage to the spark plug the spark is
created across the spark plug gap. The ignition must be able to create enough
voltage to jump the spark plug gap while under the pressure created in the
combustion chamber on the compression stroke. It takes more energy to jump
the gap under compression than it does to jump the same gap at atmospheric
pressure. The spark ignites the vaporized fuel molecules that are in the path of
the spark between the center electrode and the ground electrode, setting off a
chain reaction that burns through the rest of the air-fuel mix.
The temperature of the spark plug’s firing end must be low enough to prevent
pre-ignition but hot enough to prevent fouling out from the combustion gases
being burned. This is referred to as the heat range.
The spark plug works as a heat exchanger to remove excessive thermal heat
from the combustion chamber. Because the spark plug is firmly tightened into
the metal cylinder, heat flows readily through the connection and can be
dissipated through the cooling fins at the top of the cylinder.
The spark plug does not create heat, it just starts the burning process, but it
does remove heat from the combustion chamber to prepare for the next
combustion event.
A resistor type spark plug helps reduce the electrical noise from sparking and
protects any nearby electronic circuits from interference.
© STIHL Inc., Virginia Beach, VA 2013 US/STR
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Heat Range
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„The
heat range of a
spark plug
determines it’s ability
to dissipate heat
More
insulator is
exposed to
the
combustion
chamber
Less
insulator is
exposed to
the
combustion
chamber
5
hot
6
7
cold
The rate of heat transfer is determined by the porcelain insulator nose length
and how much surface area is exposed to the heat generated during the
combustion process.
It is very important to have the correct heat range plug in any engine. If the plug
is too cold, starting and performance will suffer.
If it is too hot, the piston can actually get a hole burned through the top.
Spark plugs also vary by thread size and length, and how they seal against the
head, so it is extremely important to have the correct plug for any engine
application.
Always verify the spark electrode gap is correct before installing a new or
cleaned plug. All STIHL engines require a 0.020” gap.
© STIHL Inc., Virginia Beach, VA 2013 US/STR
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STIHL Ignition Module Testing Procedure
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This test tree can be used to discover any faults that might be preventing the
module from indicating spark. The solid state modules used on STIHL and
by most manufacturers today have microchip controllers in them, and there
is basically no feasible way to test a module with a meter and compare it with
specifications. A spark tester that loads the module through the spark plug,
such as the ZAT 4, will indicate if spark is present, however, even then, the
module may not start the engine. This can be frustrating, but the only
efficient way to be sure about a module is to verify all the systems on the
engine, and use the ignition test tree as a checklist for the ignition
components.
Modules can have intermittent faults, heat soak loss of spark that reverses
after the module cools down, or even show spark but not start the engine.
The final verification of a faulty module may be replacing it with a known
good module for comparison.
© STIHL Inc., Virginia Beach, VA 2013 US/STR
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Summary
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„ The
science and principles of operation of how magnetos create the spark
necessary for combustion has been examined
„ How
an ignition module and a flywheel are constructed and use the science
to create a spark, at the right time, and automatically adjust the ignition timing
for starting and running has been discussed
„ An
understanding that goes beyond “magic” and will be of benefit in
understanding how STIHL ignition modules operate should now be in place
Do you have any questions?
© STIHL Inc., Virginia Beach, VA 2013 US/STR
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