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Magneto (electrical)
Encyclopedia
A magneto is an electrical generator
that uses permanent magnet
s to produce alternating current.
Magnetos adapted to produce pulses of high voltage
are used in the ignition system
s of some gasoline-powered internal combustion engine
s to provide power to the spark plug
s. The magneto is now confined mainly to engines where there is no available electrical supply, for example in lawnmowers and chainsaw
s. It is also widely used in aviation
piston engines even though an electrical supply is usually available. This is because a magneto ignition system is more reliable than a battery-coil
system. People discussing magnetos and coils
used in early internal-combustion engines generally used the term "tension" instead of the more modern term "voltage."
, particularly in the combustion chamber of a high-compression engine, requires a great voltage (or higher tension) than can be achieved by a simple magneto. The high-tension magneto combines an alternating current magneto generator and a transformer
. A high current at low voltage is generated by the magneto, then transformed to a high voltage (even though this is now a far smaller current) by the transformer.
The first person to develop the idea of a high-tension magneto was Andre Boudeville, but his design omitted a condenser (capacitor
); Frederick Richard Simms
in partnership with Robert Bosch
were the first to develop a practical high-tension magneto.
Magneto ignition was introduced on the 1899 Daimler
Phönix. This was followed by Benz, Mors
, Turcat-Mery
, and Nesseldorf, and soon was used on most cars up until about 1918 in both low voltage (voltage for secondary coils to fire the spark plugs) and high voltage magnetos (to fire the spark plug directly, similar to coil
ignitions, introduced by Bosch in 1903).
. In the inductor magneto, the magnet
is rotated and the coil remains stationary.
On each revolution, a cam
opens the contact breaker
one or more times, interrupting the current
, which causes the electromagnetic field
in the primary coil to collapse. As the field collapses there is a voltage induced (as described by Faraday's Law
) across the primary coil. As the points open, point spacing is such that the voltage across the primary coil would arc across the points. A capacitor
is placed across the points which absorbs the energy stored in the primary coil. The capacitor and the coil together form a resonant circuit which allows the energy to oscillate from the capacitor to the coil and back again. Due to the inevitable losses in the system, this oscillation decays fairly rapidly.
A second coil, with many more turns than the primary, is wound on the same iron core to form an electrical transformer
. The ratio of turns in the secondary winding to the number of turns in the primary winding, is called the turns ratio. Voltage across the primary coil results in a proportional voltage being induced across the secondary winding of the coil. The turns ratio between the primary and secondary coil is selected so that the voltage across the secondary reaches a very high value, enough to arc across the gap of the spark plug.
In a modern installation, the magneto only has a single low tension winding which is connected to an external ignition coil
which not only has a low tension winding, but also a secondary winding of many thousands of turns to deliver the high voltage required for the spark plug(s). Such a system is known as an "energy transfer" ignition system. Initially this was done because it was easier to provide good insulation for the secondary winding of an external coil than it was in a coil buried in the construction of the magneto (early magnetos had the coil assembly externally to the rotating parts to make them easier to insulate—at the expense of efficiency). In more modern times, insulation materials have improved to the point where constructing self contained magnetos is relatively easy, but energy transfer systems are still used where the ultimate in reliability is required such as in aviation engines.
or other source of energy, the magneto is a compact and reliable self-contained ignition system, which is why it remains in use in many general aviation
applications.
Since the beginning of World War I
in 1914, magneto-equipped aircraft engines have typically been dual-plugged
, whereby each cylinder
has two spark plug
s, with each plug having a separate magneto system. Dual plugs provide both redundancy should a magneto fail, and better engine performance (through enhanced combustion). Twin sparks provide two flame fronts within the cylinder, these two flame fronts decreasing the time needed for the fuel charge to burn, thereby burning more of the fuel at a lower temperature and pressure . As the pressure within a cylinder increases, the temperature rises
; and if there is only a single plug, the unburnt fuel away from the original flame front can self-ignite, producing a separate unsynchronized flame front. This leads to a rapid rise in cylinder pressure, producing engine "knock"
. Higher octane fuel delays the time required for auto-ignition at a given temperature and pressure, reducing knock; so by burning the fuel charge faster, two flame fronts can decrease an engine's octane requirement. As the size of the combustion chamber determines the time to burn the fuel charge, dual ignition was especially important for the large-bore aircraft engine
s around World War II
.
, and battery circuit. This was done to improve engine efficiency without sacrificing reliability. Magnetos were once considered a more reliable ignition source, but have the disadvantage of having fixed timing. This means that the timing must be a compromise setting, which is neither the best for low RPM nor the best for high RPM. On the other hand, battery ignition systems have almost always had a timing advance system which can set the timing to the best setting for the speed the engine is turning, improving power output and fuel efficiency. As the reliability of battery ignition systems improved, the magneto fell out of favor for general automotive use, but may still be found in sport or racing engines.
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...
that uses permanent magnet
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
s to produce alternating current.
Magnetos adapted to produce pulses of high voltage
High voltage
The term high voltage characterizes electrical circuits in which the voltage used is the cause of particular safety concerns and insulation requirements...
are used in the ignition system
Ignition system
An ignition system is a system for igniting a fuel-air mixture. Ignition systems are well known in the field of internal combustion engines such as those used in petrol engines used to power the majority of motor vehicles, but they are also used in many other applications such as in oil-fired and...
s of some gasoline-powered internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the engine...
s to provide power to the spark plug
Spark plug
A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.Spark plugs have an insulated central electrode which is connected by...
s. The magneto is now confined mainly to engines where there is no available electrical supply, for example in lawnmowers and chainsaw
Chainsaw
A chainsaw is a portable mechanical saw, powered by electricity, compressed air, hydraulic power, or most commonly a two-stroke engine...
s. It is also widely used in aviation
Aviation
Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...
piston engines even though an electrical supply is usually available. This is because a magneto ignition system is more reliable than a battery-coil
Ignition coil
An ignition coil is an induction coil in an automobile's ignition system which transforms the battery's 12 volts to the thousands of volts needed to create an electric spark in the spark plugs to ignite the fuel...
system. People discussing magnetos and coils
Low tension coil
A low tension coil is an electrical device used to create a spark across the points of an ignitor on early 1900s gasoline engines, generally flywheel engines, hit and miss engines, and other engines of that era. In modern electronic terms, a low tension coil is simply a large inductor, an...
used in early internal-combustion engines generally used the term "tension" instead of the more modern term "voltage."
History
Firing the gap of a spark plugSpark plug
A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.Spark plugs have an insulated central electrode which is connected by...
, particularly in the combustion chamber of a high-compression engine, requires a great voltage (or higher tension) than can be achieved by a simple magneto. The high-tension magneto combines an alternating current magneto generator and a transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
. A high current at low voltage is generated by the magneto, then transformed to a high voltage (even though this is now a far smaller current) by the transformer.
The first person to develop the idea of a high-tension magneto was Andre Boudeville, but his design omitted a condenser (capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
); Frederick Richard Simms
Frederick Richard Simms
Frederick Richard Simms was a British mechanical engineer, businessman, prolific inventor and motor industry pioneer. Simms coined the words "petrol" and "motorcar"...
in partnership with Robert Bosch
Robert Bosch
Robert Bosch was a German industrialist, engineer and inventor, founder of Robert Bosch GmbH.-Biography:...
were the first to develop a practical high-tension magneto.
Magneto ignition was introduced on the 1899 Daimler
Daimler Motoren Gesellschaft
Daimler-Motoren-Gesellschaft was a German engine and later automobile manufacturer, in operation from 1890 until 1926. Founded by Gottlieb Daimler and Wilhelm Maybach, it was based first in Cannstatt...
Phönix. This was followed by Benz, Mors
Mors (automobile)
The Mors automobile factory was an early French car manufacturer. It was one of the first to take part in automobile racing, beginning in 1897, due to the belief of the company founder, Émile Mors, in racing's technical and promotional benefits...
, Turcat-Mery
Turcat-Méry
-External links:*...
, and Nesseldorf, and soon was used on most cars up until about 1918 in both low voltage (voltage for secondary coils to fire the spark plugs) and high voltage magnetos (to fire the spark plug directly, similar to coil
Induction coil
An induction coil or "spark coil" is a type of disruptive discharge coil. It is a type of electrical transformer used to produce high-voltage pulses from a low-voltage direct current supply...
ignitions, introduced by Bosch in 1903).
Operation
In the type known as a shuttle magneto, the engine rotates a coil of wire between the poles of a magnetMagnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
. In the inductor magneto, the magnet
Magnet
A magnet is a material or object that produces a magnetic field. This magnetic field is invisible but is responsible for the most notable property of a magnet: a force that pulls on other ferromagnetic materials, such as iron, and attracts or repels other magnets.A permanent magnet is an object...
is rotated and the coil remains stationary.
On each revolution, a cam
Cam
A cam is a rotating or sliding piece in a mechanical linkage used especially in transforming rotary motion into linear motion or vice-versa. It is often a part of a rotating wheel or shaft that strikes a lever at one or more points on its circular path...
opens the contact breaker
Contact breaker
A contact breaker is a type of electrical switch, and the term typically refers to the switching device found in the distributor of the ignition systems of spark-ignition internal combustion engines.-Purpose:...
one or more times, interrupting the current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
, which causes the electromagnetic field
Electromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...
in the primary coil to collapse. As the field collapses there is a voltage induced (as described by Faraday's Law
Faraday's law of induction
Faraday's law of induction dates from the 1830s, and is a basic law of electromagnetism relating to the operating principles of transformers, inductors, and many types of electrical motors and generators...
) across the primary coil. As the points open, point spacing is such that the voltage across the primary coil would arc across the points. A capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
is placed across the points which absorbs the energy stored in the primary coil. The capacitor and the coil together form a resonant circuit which allows the energy to oscillate from the capacitor to the coil and back again. Due to the inevitable losses in the system, this oscillation decays fairly rapidly.
A second coil, with many more turns than the primary, is wound on the same iron core to form an electrical transformer
Transformer
A transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...
. The ratio of turns in the secondary winding to the number of turns in the primary winding, is called the turns ratio. Voltage across the primary coil results in a proportional voltage being induced across the secondary winding of the coil. The turns ratio between the primary and secondary coil is selected so that the voltage across the secondary reaches a very high value, enough to arc across the gap of the spark plug.
In a modern installation, the magneto only has a single low tension winding which is connected to an external ignition coil
Ignition coil
An ignition coil is an induction coil in an automobile's ignition system which transforms the battery's 12 volts to the thousands of volts needed to create an electric spark in the spark plugs to ignite the fuel...
which not only has a low tension winding, but also a secondary winding of many thousands of turns to deliver the high voltage required for the spark plug(s). Such a system is known as an "energy transfer" ignition system. Initially this was done because it was easier to provide good insulation for the secondary winding of an external coil than it was in a coil buried in the construction of the magneto (early magnetos had the coil assembly externally to the rotating parts to make them easier to insulate—at the expense of efficiency). In more modern times, insulation materials have improved to the point where constructing self contained magnetos is relatively easy, but energy transfer systems are still used where the ultimate in reliability is required such as in aviation engines.
Aviation
Because it requires no batteryBattery (electricity)
An electrical battery is one or more electrochemical cells that convert stored chemical energy into electrical energy. Since the invention of the first battery in 1800 by Alessandro Volta and especially since the technically improved Daniell cell in 1836, batteries have become a common power...
or other source of energy, the magneto is a compact and reliable self-contained ignition system, which is why it remains in use in many general aviation
General aviation
General aviation is one of the two categories of civil aviation. It refers to all flights other than military and scheduled airline and regular cargo flights, both private and commercial. General aviation flights range from gliders and powered parachutes to large, non-scheduled cargo jet flights...
applications.
Since the beginning of World War I
World War I
World War I , which was predominantly called the World War or the Great War from its occurrence until 1939, and the First World War or World War I thereafter, was a major war centred in Europe that began on 28 July 1914 and lasted until 11 November 1918...
in 1914, magneto-equipped aircraft engines have typically been dual-plugged
Dual ignition
Dual Ignition is a system for spark-ignition engines, whereby each cylinder has every critical ignition component duplicated, including spark plugs, magnetos and ignition circuits, rather than the more usual single spark plug or circuit...
, whereby each cylinder
Cylinder (engine)
A cylinder is the central working part of a reciprocating engine or pump, the space in which a piston travels. Multiple cylinders are commonly arranged side by side in a bank, or engine block, which is typically cast from aluminum or cast iron before receiving precision machine work...
has two spark plug
Spark plug
A spark plug is an electrical device that fits into the cylinder head of some internal combustion engines and ignites compressed fuels such as aerosol, gasoline, ethanol, and liquefied petroleum gas by means of an electric spark.Spark plugs have an insulated central electrode which is connected by...
s, with each plug having a separate magneto system. Dual plugs provide both redundancy should a magneto fail, and better engine performance (through enhanced combustion). Twin sparks provide two flame fronts within the cylinder, these two flame fronts decreasing the time needed for the fuel charge to burn, thereby burning more of the fuel at a lower temperature and pressure . As the pressure within a cylinder increases, the temperature rises
Charles's law
Charles' law is an experimental gas law which describes how gases tend to expand when heated. It was first published by French natural philosopher Joseph Louis Gay-Lussac in 1802, although he credited the discovery to unpublished work from the 1780s by Jacques Charles...
; and if there is only a single plug, the unburnt fuel away from the original flame front can self-ignite, producing a separate unsynchronized flame front. This leads to a rapid rise in cylinder pressure, producing engine "knock"
Engine knocking
Knocking in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.The...
. Higher octane fuel delays the time required for auto-ignition at a given temperature and pressure, reducing knock; so by burning the fuel charge faster, two flame fronts can decrease an engine's octane requirement. As the size of the combustion chamber determines the time to burn the fuel charge, dual ignition was especially important for the large-bore aircraft engine
Aircraft engine
An aircraft engine is the component of the propulsion system for an aircraft that generates mechanical power. Aircraft engines are almost always either lightweight piston engines or gas turbines...
s around World War II
World War II
World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...
.
Impulse coupling
Because the magneto has low voltage output at low speed, starting an engine is more difficult. Therefore some magnetos have an impulse coupling, a springlike mechanical linkage between the engine and magneto drive shaft which "winds up" and "lets go" at the proper moment for spinning the magneto shaft. The impulse coupling uses a spring, a hub cam with flyweights, and a shell. The hub of the magneto rotates while the drive shaft is held stationary, and the spring tension builds up. When the magneto is supposed to fire, the flyweights are released by the action of the body contacting the trigger ramp. This allows the spring to unwind giving the rotating magnet a rapid rotation and letting the magneto spin at such a speed to produce a spark.Automobile
Some aviation engines as well as some early luxury cars have had dual-plugged systems with one set of plugs fired by a magneto, and the other set wired to a coil, dynamoDynamo
- Engineering :* Dynamo, a magnetic device originally used as an electric generator* Dynamo theory, a theory relating to magnetic fields of celestial bodies* Solar dynamo, the physical process that generates the Sun's magnetic field- Software :...
, and battery circuit. This was done to improve engine efficiency without sacrificing reliability. Magnetos were once considered a more reliable ignition source, but have the disadvantage of having fixed timing. This means that the timing must be a compromise setting, which is neither the best for low RPM nor the best for high RPM. On the other hand, battery ignition systems have almost always had a timing advance system which can set the timing to the best setting for the speed the engine is turning, improving power output and fuel efficiency. As the reliability of battery ignition systems improved, the magneto fell out of favor for general automotive use, but may still be found in sport or racing engines.
See also
- Ignition systemIgnition systemAn ignition system is a system for igniting a fuel-air mixture. Ignition systems are well known in the field of internal combustion engines such as those used in petrol engines used to power the majority of motor vehicles, but they are also used in many other applications such as in oil-fired and...
- Faraday's law of inductionFaraday's law of inductionFaraday's law of induction dates from the 1830s, and is a basic law of electromagnetism relating to the operating principles of transformers, inductors, and many types of electrical motors and generators...
- Induction coilInduction coilAn induction coil or "spark coil" is a type of disruptive discharge coil. It is a type of electrical transformer used to produce high-voltage pulses from a low-voltage direct current supply...
- TransformerTransformerA transformer is a device that transfers electrical energy from one circuit to another through inductively coupled conductors—the transformer's coils. A varying current in the first or primary winding creates a varying magnetic flux in the transformer's core and thus a varying magnetic field...