Page 164 - Introduction to Marine Engineering

Chapter 8

Fuel oils, lubricating oils and

their treatment

Crude oil is, at the present time, the source of most fuel oils for marine
use. Synthetic fuels are being developed but will probably be too
expensive for ship propulsion. Solid fuel, such as coal, is returning in a
small way for certain specialised trade runs. The various refined
products of crude oil seem likely to remain as the major forms of marine
fuel.
The refining process for crude oil separates by heating and distillation
the various fractions of the oil. Paraffin fuel would be used in gas
turbine plants, gas oil in high- and medium-speed diesel engines and
crude oils in slow-speed and some medium-speed diesels. Paraffin and
gas oil are known as 'distillates', which are free flowing, easily stored and
can be used without further treatment. Residual fuels, however, are very
viscous or thick at normal temperatures, and require heating before use.
Additional treatment to remove harmful chemicals or sulphur may be
required for all or some of the refined products, depending upon their
application. Finally blending or mixing of the various oils is done to
provide a range of commercial fuels for different duties.

Fuel oils

Fuel oils have various properties which determine their performance
and are quoted in specifications. The specific gravity or relative density is
the weight of a given volume of fuel compared to the weight of the same
volume of water expressed as a ratio, and measured at a fixed
temperature. Viscosity is a resistance to flow. A highly viscous fuel will
therefore require heating in order to make it flow. Measurement of
viscosity is by Redwood, Saybolt or Engler instrument flow times for a
given volume of fuel.
The ignition quality of a fuel is measured by the time delay between
injection and combustion, which should be short for good controlled
burning. Ignition quality is indicated as cetane number, diesel index and
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