Heavy fuel oil (HFO), or “residual fuel oil” is based on the high viscosity, tar-like mass which remains after crude oil has been cracked to produce lighter hydrocarbon products such as methane, hydrogen, petrol (gasoline), distillate diesel fuels and heating oils or feedstocks for lubricants.
As a residual product, HFO is a relatively inexpensive fuel – typically its costs around 30% less than distillate fuels. It thus became the standard fuel for large marine diesel engines during the oil crises of the 1970s and 1980s. Its use required extensive adaptation of the injection system and other components of low and medium speed engines - which are still the only reciprocating engines capable of running on HFO.
In the meantime, large two-stroke and four stroke diesel engines attain long times between overhaul and ever lower exhaust gas emissions operating on HFO.