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© STIHL Inc. Bronze Magneto US/STR June 2010 1

Bronze Level TrainingMagneto Ignition Operation

“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.


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Ignition Coil by Flywheel

Points & Condenser under Starter

One-piece Solid State CDI Ignition Module

Magneto DesignThere are basically two magneto systems that you are likely to see used on small engines:

Breaker points with condenser CDI, or capacitor discharge ignition

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 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.


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Magneto Components

Primary Winding Secondary Winding

Spark Plug Gap

Switch: Points and Condenser or Solid State

Laminated Steel Armature

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 from the elements and from vibration.


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Magnetism and Electricity

A permanent magnet is embedded in the flywheelThere are two steel pole shoes on either side of the magnetThe magnet may be a man made ceramic type or a smaller rare earth type

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.


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Magnetic Fields

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

. The lines of flux are three dimensional, not flat, as the magnetic field viewer shows.


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Magnetic Lines of Flux

Notice how the iron filings illustrate the magnetic lines of flux reaching out of the pole shoes on the flywheelAs 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:

- both ferrous and non-ferrous, are conductors

(ferrous metals are magnetic)is a non-ferrous conductor.

Examples of insulators would be:Non-metallic materials - plastic, glass, rubber


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Flux Interaction With a Conductor

When a magnet is moved near a conductor, an electrical current is produced in the conductor

This is known as electromagnetic induction

The amount of current produced is effected by the strength of the magnet and the speed it is moving. In this demonstration, the two metal plugs are the same weight and size, but one is magnetized and the other is not.What will happen when the un-magnetized metal plug is dropped through the copper tube?

What will happen when the magnetized plug is dropped through the copper tube?

The magnetic field is interacting with the copper conductor and the electric current being generated is pushing back against the magnet, slowing it down. This interaction is known as eddy currents. Remember that copper is not magnetic.


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Electromagnetic Induction

Based on experiments done in the early 1800’s, Michael Faraday is credited with the discovery that if a copper coil of wire was exposed to a moving magnetic field, an electric current is induced in the wireThe plastic tube has one long piece of 24 gage coated copper wire wrapped tightly around it

The galvanometer is a very sensitive volt meter. What happens when the magnet is passed through the tube from end to end?

indicating electricity is being generatedAs the magnet slides through the tube, the lines of flux interact with the turns of wire, which are insulated from each other, and induce electricity in the turns, or windings, of copper wire, which is indicated by the meter. If the wire was wrapped around the tube over and over, back and forth in layers on top of each other, more current would be generated. The engineer decides on how many turns of wire, the size of the wire, and the type and strength of the magnet used, based on what the desired voltage and current should be for the application.


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Armature ConstructionMost magnetos are similar in constructionA steel or soft iron core, usually “U”shaped, is made of thin stampings, laminated together, called the armatureThey 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

The copper tube experiment demonstrated “eddy currents”. 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.


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Magneto Construction

The primary winding is wrapped around the armature firstThe 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 housingEach winding is one continuous length of wire

Primary Winding

Secondary Winding

The primary winding is the low voltage side of the coil or magneto. Itusually consists of around 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 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.


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Magic or Science?

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 12,000RPM, the ignition module is firing the spark plug times a second. Maybe it is magic after all!


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Ignition Timing and Spark Advance

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 strokeIt usually fires a little before TDCIt 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.


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Effects of Ignition Timing on Engine Performance

100 500 1000 2000 3000 5000 8000 10000 12000 150000

5

10

15

20

25

30

35

Revolution [rpm]

Igni

tion

Tim

ing

[deg

BTD

C]

“Smoother” idle speed

Better acceleration,lower fuel consumption

Smooth starting,cold acceleration behavior

Starting Idle Speed Load Area

Over Speed

Best tuning for power,torque, temperature,

emissions(depends on cylinderdesign and cooling)

Speed limiter toavoid excessive rpm

Ignition w/o timing advance Best tuning of ignition timing

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.


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Digital Timing Control

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.


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Capacitor Discharge Ignition

diode

capacitor

primary winding

secondary winding

spark plug

transistor

magnet

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.


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Ground-electrode

Insulator

Center electrode with copper core

GasketSeals Ribs

Terminal NutGlass seal resistance

To ignite the fuel and air mixture

To remove heat from the combustion chamber

Primary Purpose of a Spark Plug:

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.


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Heat Range

5 hot

6 7 cold

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

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.


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STIHL Ignition Module Testing Procedure

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.


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Summary

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?

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