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What component of an aircraft generates the electric power to start the aircraft's engine?


For a Rotax 912 engine, I think the magneto generates the electricity, but I'm not sure; I also think about the ignition coil or electronic ignition.

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  • $\begingroup$ Magneto generates the electric power for spark ignition, not starting. $\endgroup$ Apr 9 at 17:41
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For most small planes, it is the battery, which turns the electric starter motor. A generator driven by the engine or by a small propeller in the slipstream then charges up the battery while the plane is flying.

Some history:

a few older and small planes had a pull cord in the cockpit with which the pilot could pull-start the engine, like a lawn mower. For engines with no starter at all, the pilot or their assistant would grab the propeller and swing it around by muscle power to start the engine (be careful!).

Some very old planes had starters operated by a crank that you stuck into a hole in the engine cowling, and turned by arm power. This worked for small engines but was too hard to do for bigger engines. In that case, the crank turned a small flywheel up to great speed and then the engine was geared into the flywheel through a reduction gear drive, and the energy stored in the rotating flywheel turned the engine to start it. This was used in the P-26 "Peashooter" plane.

In later years, a small electric motor spun the flywheel up to speed instead of a hand crank. The Antonov An-2 still uses this type of starter, called an inertia starter, as does the Dromader crop duster with the radial engine.

Another method of starting an airplane engine was to explode a blank shotgun shell in a closed chamber and send the hot gas through a tube to a special cylinder geared onto the crankshaft, which thereby turned the crankshaft to start the engine. This is called a Coffman starter and it plays a key role in the old movie "The Flight Of The Phoenix". In some versions of this principle, the gases from the shotgun shell were sent right into one of the cylinders of the engine itself, forcing the piston down and turning the crankshaft.

This idea saw use in more modern times, where a jet engine could be started by burning a charge of solid rocket motor fuel called a cartridge inside a sealed chamber and using the hot gas to spin up the exhaust turbine. F-4 Phantom jet fighters could be started with cartridges like this in just a few minutes even in Alaskan winters- without the use of a start cart.

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    $\begingroup$ "Another method of starting an airplane engine was to explode a blank shotgun shell in a closed chamber and send the hot gas through a tube to one of the cylinders in the engine, forcing its piston violently down and thereby turning the crankshaft." Wow!!!! $\endgroup$ Apr 9 at 17:26
  • $\begingroup$ @RafaelVega, yes, it's called a coffman starter and it plays a key role in the movie "The Flight Of The Phoenix". $\endgroup$ Apr 9 at 18:08
  • $\begingroup$ The battery doesn't GENERATE the electric power, it merely stores generated power for later use. In many light planes, electricity is generated by an alternator, just as in a car. In fact, the alternator on my Piper Cherokee was a '60s Chrysler part. $\endgroup$
    – jamesqf
    Apr 10 at 4:21
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The magneto generates the spark for the ignition system. While the engine is running it drives an alternator which produces electricity to recharge the batteries and provides power to the aircrafts systems. When the engine is off, the aircrafts power is derived solely from the battery. The battery is used to power the starter motor which gets the engine going.

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  • $\begingroup$ Critically there are two sources; the battery to turn the engine over and the magneto to generate a spark. Although other ignition systems are commonplace in automotive engines these days, magnetos are still used in aircraft engines because they are very simple and therefore hopefully reliable. $\endgroup$
    – Frog
    Apr 10 at 4:52
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As your question goes, "power to start the engine" may be understood as the power needed to rotate the engine until it is able to rotate itself by its own power. It may or may not be electrical in nature.

It may be the pilot's muscle power in very small engines (just like in smaller motorcycles), a starter battery for bigger engines (just like in cars) or power from a land-bound infrastructure (for big jets).

If you think about the electricity in the spark plugs of a piston engine, there is a catch - electricity for sparks is needed not only to start the engine, but for running the engine as well.

"Magneto" is just a small synchronous generator mounted directly on the engine crankshaft. It serves two purposes - providing power and timing for the electric spark. It is used in small engines (not only in aircraft) that can be started without electricity present (think motorcycle or a lawnmower). The magneto circuit is completely separate from the other possible electric equipment.

Bigger engines that are started by a battery power usually don't have a magneto. Their electricity needs are served by the starter battery (when starting) or by an attached generator (when running) - just like in most cars.

The ignition coil doesn't produce electricity. It just transforms it to the voltage needed for the sparks.

The electronic ignition is just a method to control the ignition coil. It doesn't produce electricity by itself.

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A larger turbine-powered aircraft such as a Grumman G-1 Gulfstream uses an auxiliary power unit to start each of two 1800 hp Rolls Royce turbine engines. The APU, as it is called, is a self-contained unit in the tail portion of the aircraft. Specifically, the APU is started by battery, usually from a ground assisted cart. The cart is essentially a truck with a self-latching-plug electrical cable connected to a male-to-female receptacle on the aircraft. Such cart services are provided by most executive airports in the United States. Quite typically, while the aircraft is in ground operation (pre-flight for instance) the APU powers the aircraft. Use of the APU is discontinued when all aircraft systems are flight ready.

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