Page 165 - Introduction to Marine Engineering

Fuel oils, lubricating oils and their treatment 15!

calculated cetane index; the higher the value the better the ignition
quality of the fuel.
The flash point is a figure obtained and used mainly to indicate the
maximum safe storage temperature. The test determines the tempera-
ture at which the fuel will give off sufficient vapours to ignite when a
flame is applied. Two values are possible: an open flash point for
atmospheric heating, and a closed flash point when the fuel is covered
while heating.
Low-temperature properties are measured in terms of pour point and
cloud point. The pour point is slightly above the temperature at which the
fuel just flows under its own weight. It is the lowest temperature at which
the fuel can be easily handled. At the cloud point waxes will form in the
fuel. Below the cloud point temperature, pipe or filter blocking may
occur.
The carbon residue forming property of a fuel is usually measured by
the Conradson method. Controlled burning of a fuel sample gives a
measure of the residual carbon and other remains.
Sulphur content is of importance since it is considered a cause of engine
wear. A maximum limit, expressed as a percentage by weight, is usually
included in specifications.
The calorific value of a fuel is the heat energy released during
combustion. Two values are used, the more common being the Higher
Calorific Value, which is the heat energy resulting from combustion.
The Lower Calorific Value is a measure of the heat energy available and
does not include the heat energy contained in steam produced during
combustion but passing away as exhaust. The measurement is obtained
from a bomb calorimeter test where a small fuel quantity is burnt under
controlled conditions.
The various fuel properties have different effects on performance of
the engine and the storage and handling requirements of the system.
Blending and the use of various additives will also influence both the
engine and the system.
Viscosity will affect jerk-type injector pumps and injector operation
since the liquid fuel is the operating medium. The pump mechanism is
lubricated by the fuel which, if it is of low viscosity, will cause wear.
Cloud point and pour point values are important when considering
the lowest system operating temperatures. Wax deposited in filters and
fuel lines will cause blockages and may restrict fuel flow to the engine.
The cetane number or diesel index will determine injection timing
and also influences the combustion noise and production of black
smoke. The temperature in a fuel system should be progressively
increased in order to deliver fuel at the correct viscosity to the injectors
or burners. System cleanliness is also very important to reduce wear on
the many finely machined parts in the fuel injection equipment. Regular