D6 is a type of residual fuel,
mainly used in power plants and larger ships. The fuel is
required to be preheated before it can be used. It is not
possible to use it in smaller engines or vessels/vehicles where
it is not possible to pre-heat it. D6 is its name in the USA. In
other parts of the world it has other names. Number 6 fuel oil is
a high-viscosity residual oil requiring preheating to 220 – 260
°F (104 – 127 °C).
Residual means the material
remaining after the more valuable cuts of crude oil have boiled
off. The residue may contain various undesirable impurities
including 2 percent water and one-half percent mineral soil. D6
fuel is also known as residual fuel oil (RFO), by the Navy
specification of Bunker C, or by the Pacific Specification of
PS-400 D6 fuel is used mostly for generators.