A magneto ignition needs no battery: a magnet on the flywheel sweeps past a coil and induces a current in a few turns of thick wire, and when a cam opens the contact points that current stops suddenly, the magnetic field collapses, and a second winding of many fine turns delivers a pulse of thousands of volts to the spark plug. A magneto is the simplest self-contained ignition a settlement can run, and a telephone-ringing magneto is not a substitute.
Part of Internal Combustion
How a magneto generates and times a spark, what its parts are, and how it is set up, maintained and kept safe.
A Collapsing Field Makes The Spark
The principle is the one in Electromagnetic Induction: a changing magnetic field induces a voltage in a coil. Inductor Theory gives the second half: disconnecting a coil from its current throws a spark, because the inductor resists any change of current.
A magneto uses both. A permanent magnet turns with the engine (often set into the flywheel) and passes a coil on a laminated iron core. The coil has two windings: a primary of few turns of heavier wire, and a secondary of many turns of fine wire wound over it. While the points are closed the primary carries the induced current. The cam opens the points at the chosen moment; the primary current stops, the field collapses fast, and the voltage induced in the secondary is the primary’s sudden change multiplied by the turns ratio. A condenser (capacitor, see Capacitors) across the points absorbs the primary’s spark so the points do not burn. The secondary lead goes to the plug.
This is different from a battery-and-coil system, which uses the same coil with the current taken from a battery and a distributor to send the pulse to each cylinder. The magneto does without the battery.
Not The Telephone Magneto
The corpus has an article on a magneto in the telephone domain, Magneto Generator. It is a hand-cranked generator that produces 60 to 90 volts AC at 15 to 25 Hz to ring a bell. That is a different machine with a different job: it has no points, no condenser and no high-voltage winding, and 60 to 90 volts cannot jump a plug gap in compressed gas. It shares the name and the principle of induction, nothing else.
What You Need
- A salvaged magneto or flywheel magneto from a small engine, which is the practical choice; making one needs a strong permanent magnet (Permanent Magnets), a laminated core, accurately wound primary and secondary windings with good insulation, and a condenser.
- Contact points, a cam and a condenser in good order.
- A feeler gauge for the points gap.
- A spark plug and a way to see the spark away from fuel.
Method: Check And Set A Magneto
Step 1: Look at the points. They should be clean, flat and not burned or pitted. Dress with a fine file or stone and wipe.
Step 2: Set the points gap with the cam on its high point. The maker’s figure sets the gap, which also sets how long the current builds and how late the points open. A wrong gap shifts the spark timing.
Step 3: Check the condenser. A failed condenser lets the points arc and burn, and the spark is weak or absent.
Step 4: Turn the engine and look for a spark on a plug held against bare metal of the engine, in a place free of fuel and gas.
Step 5: Time the spark to the engine’s marks; see Ignition Timing.
Worked Example: Voltage Ratio And Spark Rate
Turns ratio. Assume a primary of 200 turns and a secondary of 20,000 turns, a ratio of 20,000 ÷ 200 = 100. If the primary produced a pulse of 200 V when the points opened (an assumed figure for the arithmetic), the open-circuit secondary pulse would be 100 × 200 = 20,000 V. A real magneto delivers less, because of leakage and the load, and the voltage needed to jump the plug gap depends on the gap and the compression; no figure is sourced here, but it is thousands of volts.
Spark rate. A single-cylinder four-stroke at 3,000 rpm needs one spark per two revolutions, so 3,000 ÷ 2 = 1,500 sparks a minute. A simple “wasted spark” magneto fires every revolution, so 3,000 a minute, with half of them landing on the exhaust stroke and doing nothing.
Common Mistakes
- Using the wrong magneto. A telephone ringer will not fire a plug.
- A burned points surface or a dead condenser. The spark fades as the engine warms.
- Letting oil or damp get on the points. The contact becomes intermittent.
- Storing magnets near heat or knocking them. Heat and shock weaken magnets (recalled, flagged).
Safety
The spark lead carries thousands of volts and a spark can light fuel
The current is small but the shock is sharp and can make you jerk into a flywheel or belt. The corpus explains in Lethal Current Levels that current, not voltage, is what kills, and that 10 to 20 mA is the let-go threshold. Do not hold the plug lead while cranking. Test for spark away from open fuel, spilt fuel and gas, because the spark is an ignition source for petrol, ethanol vapour and producer gas alike.
What’s Next
- Learn next: Spark Plug — where the pulse goes
- Combine with: Ignition Timing — when it should arrive