Miscellaneous

Fuel

Properties of distillate and residual fuels

Density

This is the mass per unit volume and is generally given as kg/m3 at 15OC and is specifically given as that found in a vacuum. The density reduces with increasing temperature and also reduces in air although the latter by only about 0.1%. These characteristics are important to allow proper description for commercial transactions and also for preparation ( say purifiers) and in use ( main engines). Density may also be given as specific gravity, this is a ratio of mass of the fuel to the same volume of water at the same temperature.

Viscosity

This is a measure of a fuels resistance to flow or more specifically resistance to shear of adjacent molecules. It is one of the most important characteristics of a fuel. It governs the method of handling, storage, pumping and combustion.

The coefficient of viscosity is referred to as the absolute velocity and has many accepted units of value, the SI units are MPa/s and is defined as the force required to shear a set area of liquid at contant velocity.

Kinematic viscosity is this is the ratio of dynamic viscosity of a fluid over its density. The units are accepted as stokes or centistokes although the SI unit is m2/s or mm2/s. The viscosity of a fuel generally varies considerably with temperature and this is particularly the case with resif=dual fuels.

Flash Point

This is the temperature of a fuel at which sufficient vapour is given off that momentary ignition occurs if an external flame is introduced. This temperature si quoted as Open or Closed. The closed temperature is about 15'C lower than the open

Pour Point

This is the temperature at which and below wax crystals begin to form. This is important for pumping

Specific energy or Calorific value

This is the total energy chemically available by a unit fuel. Heavier fuels tend to have lower values.

Ignition quality

Is a property related to distillated fuel and is that quality of combustibility during combustion process in a diesel engine, which causes ignition delay. It is a relative value on a scale of 0 to 100, known as cetane number. Paraffin as non-combustible substance is taken for zero and Cetane (C16H34) a highly combustible substance is taken as 100.


Low Sulphur Fuels

Sulphur contained in the fuel forms metallic sulphides that coat the internal surfaces of the fuel injection equipment including the fuel pumps and the fuel injectors. These sulphides have low shear resistance and act as EP additives similar to that found in lubrication oils. Extremely low sulphur fuels in use on the automotive transport industry have led to the use of lubricity additives. In the marine environment the reduction in sulphur content has been less dramatic.

Marpol Annex VI(regulation 14) and the creation of Sulphur Emission Control Area means it wil be a requirement to use only fuels with a certain maximum sulphur content. In the addition to the increased cost of these low sulphur fuels it is necessary to factor in the possibility of increased wear and tear on the engine components.

Low sulphur fuels are normally low viscosity oils such as gas oil. Carefull planning has to be done both at the design level ( to ensure sufficient storage capacity) and at the operational and maintenance levels due to the known difficulties in changing over from a heated fuel to a non heated or one with reduced heating capacity.