Propylene (C₃H₆) is a colorless, flammable, liquified gas with a faintly sweet odor. It has high heat release in its primary and secondary flames. The heat release in the primary flame cone is similar to acetylene. The BTU capacity of the outer flame is superior to that of acetylene. Propylene combines the qualities of an acetylene flame with the secondary heating capacity of propane. The fuel gas burns hotter than propane; however, the cutting speed should be calculated on a case-by-case-basis before choosing this as the most economical choice as your fuel gas.