is the term encompassing a broad range of
distillate and residual fuels identified by
ASTM grades 1 through 6. Grade No. 1, a kerosene-type
fuel, is a light distillate fuel that has the lowest boiling range. No. 2
fuel oil, popularly called heating oil, has a higher
boiling range and is commonly used in home
heating. It is comparable in boiling range to
diesel fuel. Grades 4, 5, and 6 are called heavy fuel oils (HFO),
or residual fuel oils; they are composed largely of heavy pipe still
bottoms. Because of their high
viscosity, No. 5 and No. 6 fuel oils require
preheating to facilitate pumping and burning. No. 6 fuel oil is also called
Bunker C fuel oil.
Also see distillation.