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Spark plug and wire on the Argus As 014 pulse jet engine.

I've read conflicting accounts of the spark plug's use.

In one account, the spark plug is used only during the engine's startup: Acetylene gas from a ground source is fed into the engine's combustion chamber. The spark plug, also powered by a ground source, sparks to ignite the acetylene, which heats the combustion chamber to a "blood red", hot enough to ignite the air-gasoline mixture during engine mainstage. Mainstage combustion maintains the heat necessary to ignite subsequent fuel.

Other accounts have the spark plug in use as a part of the normal operation of the engine.