A hot-tube ignitor was an early device that fit onto the cylinder head of an internal-combustion engine, used to ignite the compressed fuel/air mixture by means of a flame heating part of the tube red-hot.
The hot tube usually enters the combustion chamber at the valve block, to the side of the main cylinder bore in a side-valve, and after an ignition event remains full of a small residue of spent gasses as the piston pushes the majority of such out of the exhaust valve.
On the induction stroke, fresh charge cannot ignite arbitrarily as the hot part of the tube is shielded by these spent gasses.
Most later styles used a fixed burner and varied tube lengths to change ignition timing.
The tubes used were typically 6 to 12 inches long, which tended to make them impractical for use on anything but large engines (e.g., stationary motors in factories).