Ignition magneto

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An ignition magneto, also called a high‑tension magneto, is an old ignition system used in spark‑ignition engines. It uses a magneto and a transformer to make high‑voltage pulses for the spark plugs. The name “high‑tension” just means high voltage.

A simple magneto uses permanent magnets to make electricity, but it normally produces only low voltage. To fire a spark plug, the system includes a transformer that boosts the voltage, though this lowers the output current.

When the spark points begin to open, a capacitor across the points absorbs energy and slows the voltage rise, helping the points open fully. A second coil with many more turns, wound on the same iron core, forms the transformer. The turns ratio (secondary turns divided by primary turns) sets how high the spark‑plug voltage will be.

The primary voltage rises to hundreds of volts, and the secondary voltage reaches tens of thousands of volts, enough to jump the spark plug gap. Magnetos produce less voltage at low engine speeds, making starting harder. To help, many magnetos use an impulse coupling—parts that wind up a spring while the engine cranks and then release to spin the magneto fast at the right moment to produce a spark.

History shows the first practical high‑tension magneto was developed in the late 1890s by Frederick Simms with Robert Bosch and colleagues. The Bosch magneto ignition was used in Daimler engines on the Zeppelin in 1900. The first car to use magneto ignition was the 1901 Mercedes 35 hp, followed by other early cars. Magnetos were widely used in cars, with both low‑voltage and high‑voltage versions.

They were mostly replaced by ignition coils fed from a battery, because a battery can supply a strong spark even at low speeds and makes starting easier. Still, ignition magnetos (often in pairs) were used in aircraft piston engines well into the 21st century.

In aviation, magnetos have advantages over many electronic ignition systems: they don’t rely on an outside power source, so if the aircraft’s electrical system fails, the magnetos can keep delivering sparks as long as the engine runs.