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Watching older planes startup I noticed they crank for a little bit before starting. Why do they also shoot out flames when starting? Mostly in Mustangs and Spitfires.

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    $\begingroup$ "shoot out flames", "Spitfire" Hmm... $\endgroup$
    – Arthur
    Aug 22, 2022 at 10:29

2 Answers 2

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On many aircraft engines, there is a significant distance between the carburetor and the intake valves, so it takes several turns of the crank to get the fuel-air mix all the way out to the cylinders through the induction pipes.

In addition, you want all the cylinders in the engine to be charged like this before you turn on the mags. This requires at the very least that the crankshaft be turned through two complete rotations before mags on both. You monitor this by counting the number of propeller blades that pass by the windscreen as you crank. Four blades on the prop x 2 revolutions = 8 blade passages, then mags on.

Lastly, note that two complete turns after the mix has made it to the cylinders will put some air/fuel mix into some of the exhaust stacks by the time you turn the mags on, and when the engine catches the hot exhaust will set that on fire inside the stack- and flames will then come shooting out.

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    $\begingroup$ Just to mention that WW II inline-V engines, used in mustangs and spitfires, had a gearbox, giving crankshaft and propeller different rotation speeds. The four-stroke operation also takes - worst case - one and a halve turn of the crankshaft until a specific cylinder gets its fuel-air charge. $\endgroup$
    – Bobby J
    Aug 21, 2022 at 9:01
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Aircraft engines are started by priming instead of using a choke. The priming system injects fuel right at the intake ports. On a fuel injected engine, the fuel injectors themselves are used for priming.

On a carbureted engine, or an engine with a pressure carburetor like the Merlin (equivalent to throttle body injection in a car), there will be dedicated priming injectors at the intake ports that are only used for priming, and they can be pressurized either by an electric pump or a manual syringe type hand pump. The Merlin has a priming system operated by an electric pump with a priming toggle switch.

The priming process sprays a quantity of fuel which coats the intake passage in the vicinity of the primer, just upstream of the intake port and takes some time to evaporate. The evaporating fuel is added to the fuel being injected, or being metered at the carburetor, to provide a mixture that is much richer even than the engine running normally with the mixture setting at full rich, enough to keep a cold engine engine running for 30 seconds or so. In really cold weather, prestart priming may not be enough and you may need to operate the primer after starting to keep it running without dying right away.

A choke on a carbureted car does the same thing by restricting the intake and skewing the carburetor toward extra rich, for starting and for driving operability while the engine is cold, because you want to drive it right away after starting.

Aircraft engines don't use chokes because of the risks of having a potential obstruction device in the intake, and most of the time they only need enough enrichment to get going initially, because unlike a car they aren't going to be called to do any real work until warmed up.

On bigger engines with lots of cylinders, you want to have all cylinders charged with the priming fuel for the smoothest start, BEFORE you introduce ignition. So you will crank it for two crank rotations before switching on mags, counting 6 blades or 8 blades depending on the prop. Depending on the engine, you may run the primer while cranking, or before cranking.

When you do this, the cylinders that got charged first, because they were on the intake stroke when you engaged the starter, end up completing a full 4 stroke cycle, passing some of the priming raw fuel charge out the exhaust valve, so you have a bit of raw fuel in the exhaust port and stack. On the Merlin, with an exhaust pipe of maybe 12 inches long, this gives the little light show during start. You'll notice it only happens on certain exhaust stacks, the ones that got charged first. If you over prime the engine, by holding the priming switch too long, you tend to get a bigger flames-and-light show. You're just holding a switch and counting off one-thousand-two-thousand, or whatever it is, in your head, so it's not hard to overdo it.

On a warm engine, there's no need to prime and you normally won't see any flames, or just a hint of flames, during start.

On large radials, the exhaust system ducting is much longer and priming fuel is usually consumed before it exits the exhaust pipe, and you see less flame or none at all, during start. Unless of course the pilot over primes and you get the nice light show as well.

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    $\begingroup$ And there I thought they just did it because it looks and sounds really cool. $\endgroup$ Aug 23, 2022 at 13:36
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    $\begingroup$ I’m a “too much information” kind of guy, in that I really appreciate answers that provide a deeper intuition and not just “the answer.” Thanks for this. $\endgroup$
    – Max R
    Sep 2, 2022 at 19:34

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