Definition of hot tube in English:

hot tube

noun

Engineering
historical
  • (In some early internal-combustion engines) a metal or porcelain tube, closed at the end, which projected from one end of the cylinder and was heated externally by a flame, so that it ignited the mixture forced into it during the compression stroke; usually attributive in "hot-tube ignition".

Origin

Late 19th century; earliest use found in U.S. Patents.