A magneto is a self-contained electrical generator built into an engine that produces the high-voltage spark needed to ignite fuel. Unlike a standard ignition system that draws power from a battery, a magneto generates its own electricity using spinning magnets and coils of wire. You’ll find magnetos in aircraft piston engines, lawnmowers, chainsaws, older motorcycles, and other small engines where simplicity and independence from a battery are valuable.
How a Magneto Creates a Spark
The core principle is electromagnetic induction: when a magnet moves past a coil of wire, it creates a voltage in that coil. A magneto harnesses this effect in a compact package to produce pulses of electricity timed to each firing stroke of the engine.
Here’s the sequence. A set of strong permanent magnets spins near a U-shaped iron core (called an armature) wrapped with two coils of wire, one layered over the other. The inner coil, called the primary coil, has roughly 200 turns of thick wire. The outer coil, the secondary coil, has around 20,000 turns of very thin wire. As the magnets sweep past the armature, they build up a magnetic field that induces a small current in the primary coil.
At the exact moment the magnetic field peaks, a switch opens and abruptly cuts off current flow through the primary coil. That sudden interruption causes a voltage spike of about 200 volts in the primary coil. Because the secondary coil has roughly 100 times more turns of wire, it amplifies that spike to approximately 20,000 volts. That jolt travels to the spark plug, jumps across the plug’s gap, and ignites the air-fuel mixture in the cylinder.
In older magnetos, the switch that breaks the primary circuit is a mechanical set of contact points (also called breaker points) paired with a small capacitor. Most modern magnetos replace those mechanical points with a solid-state electronic module, often labeled “electronic ignition,” which does the same job with no moving parts to wear out.
Where the Magnets Actually Sit
The physical layout varies depending on the engine. In small engines like lawnmowers and chainsaws, the permanent magnets are embedded directly in the engine’s flywheel. Every time the flywheel spins past the armature and coils (which are mounted to the engine block), the magneto fires. There’s no separate magneto housing to speak of; the flywheel itself is part of the system.
In aircraft engines and some older automotive engines, the magneto is a self-contained unit bolted to the engine. It has its own internal rotating magnet driven by the engine’s gear train, plus the coils, switching mechanism, and a distributor cap that routes the high-voltage pulse to the correct cylinder at the right time. Everything the magneto needs to function is inside the housing, which is why it’s often described as “self-contained.”
Why Magnetos Don’t Need a Battery
This is the magneto’s defining advantage. Because the spinning magnets generate electricity mechanically, the system works as long as the engine is turning. A dead battery won’t stop a magneto-equipped engine from running. That independence makes magnetos ideal for equipment used far from repair shops (chainsaws, outboard motors, portable generators) and for aircraft, where losing electrical power shouldn’t mean losing the engine.
Small gasoline engines rely on this trait heavily. A pull-start lawnmower has no battery at all. When you yank the cord, the flywheel spins, the magnets pass the coil, and the spark plug fires. The entire ignition system is just magnets, wire, and a switch.
Dual Magnetos in Aircraft Engines
Aviation takes the magneto’s simplicity one step further with redundancy. Federal regulations require certified aircraft piston engines to have two completely independent ignition systems, each with its own magneto and its own set of spark plugs. Every cylinder gets two spark plugs, one fired by the left magneto and one by the right.
The reason is straightforward: ignition failures in piston engines are relatively common. If one magneto quits in flight (say the breaker points fail or the coil shorts out), every cylinder keeps firing on the remaining spark plug. All exhaust temperatures rise slightly, and the pilot notices a modest power loss, but the engine keeps running. Even a single failed spark plug barely registers; the other plug in that cylinder picks up the load. The pilot flies to the destination and gets the bad magneto repaired on the ground.
This redundancy is why most piston aircraft have a magneto switch with four positions: OFF, LEFT, RIGHT, and BOTH. Pilots check each magneto individually during the pre-takeoff run-up. A small RPM drop when switching from BOTH to a single magneto is normal, since you’re running on half the spark plugs. A large drop or rough running signals a problem.
Fixed Timing Is a Trade-Off
One limitation of a traditional magneto is that it fires at a fixed point in the engine’s rotation, typically 20 to 25 degrees before the piston reaches the top of its stroke. That timing doesn’t change regardless of engine speed or power setting. It’s a compromise: the timing is optimized for one operating condition (usually full power at sea level) and slightly less efficient everywhere else.
Modern electronic ignition systems can vary the timing automatically, advancing it at higher altitudes or lower power settings to extract more efficiency from the fuel. Some electronic systems advance timing to 35 degrees at cruise altitude. This is one reason newer aircraft and automotive engines have largely moved to electronic ignition, though magnetos remain standard in certified aircraft engines due to their proven reliability and the slow pace of aviation certification.
Common Failure Points
Magnetos are mechanically simple, but they do wear out. The most frequent trouble spots are the internal contact points (in older designs), which erode over thousands of open-close cycles. The capacitor that protects those points can also degrade, leading to excessive arcing and accelerated point wear. Distributor caps crack, allowing voltage to leak to the wrong place. Coil insulation breaks down over time, especially in high-heat environments, eventually causing misfires or a complete loss of spark.
Loose connections and poor grounding can also cause intermittent problems that are harder to diagnose. Environmental exposure, vibration, and simple age all contribute. In aircraft, magnetos are inspected on a regular maintenance schedule precisely because these failures are predictable and gradual rather than sudden.
A Brief History
The magneto dates to the earliest days of internal combustion engines. Robert Bosch delivered his first magneto ignition device on October 8, 1887, to a mechanical engineering company near Stuttgart. Those early units generated sparks for stationary engines. Bosch improved the design by replacing bar magnets with U-shaped ones, making the devices lighter, more reliable, and more powerful. By 1891, his workshop was producing over 100 units a year, and magneto ignition accounted for more than half of its revenue. The real breakthrough came when Bosch adapted the magneto for use in automobiles, turning a small workshop into one of the world’s largest engineering companies.
Magnetos dominated automotive ignition until battery-and-coil systems took over in the 1920s and 1930s. They persisted far longer in aviation and small engines, where their battery-free operation remained a decisive advantage. Today, millions of small engines and thousands of aircraft still rely on the same basic principle Bosch refined over 130 years ago.