Fuel properties of Aircraft:
EASE OF FLOW
The ease of flow of a fuel is mainly a question of viscosity, but impurities such as ice, dust, wax, etc., may cause blockages in filters and in the fuel system generally.
Most liquid petroleum fuels dissolve small quantities of water and if the temperature of the fuel is reduced enough, water or ice crystals are deposited from the fuel. Adequate filtration is therefore necessary in the fuel system. The filters may have to be heated, or a fuel de-icing system fitted, to prevent ice crystals blocking the filters. Solids may also be deposited from the fuel itself due to the solidification of waxes or other high molecular weight hydrocarbons. Distillates heavier than kerosene, such as gas oil, generally have a pour point temperature too high for use in aircraft operating in low temperatures. If these fuels were to be used, some form of heating in the aircraft's tanks and fuel system would be necessary. Such heating would obviously be an unreasonable complication.
EASE OF STARTING
The speed and ease of starting of gas turbines depends on the ease of ignition of an atomised spray of fuel. This ease of ignition depends on the quality of the fuel in two ways:
a) The volatility of the fuel at starting temperatures.
b) The degree of atomisation, which depends on the viscosity of the fuel as well as the design of the atomiser.
The viscosity of fuel is important because of its effect on the pattern of the liquid spray from the burner orifice and because it has an important effect on the starting process. Since the engine should be capable of starting readily under all conditions of service, the atomised spray of fuel must be readily ignitable at low temperatures. Ease of starting also depends on volatility, but in practice the viscosity is found to be the more critical requirement. In general, the lower the viscosity and the higher the volatility, the easier it is to achieve efficient atomisation.
The exact proportion of air to fuel required for complete combustion is called the theoretical mixture and is expressed by weight. There are only small differences in ignition limits for hydrocarbons, the rich limit in fuels of the kerosene range being 5:1 air/fuel ratio by weight and the weak limit about 25:1 by weight.
Flammable air/fuel ratios each have a characteristic rate of travel for the flame which depends on the temperature, pressure and the shape of the combustion chamber. Flame speeds of hydrocarbon fuels are very low and range from 0.3 - 0.6 m/sec. These low values necessitate the provision of a region of low air velocity within the flame tube, in which a stable flame and continuous burning are ensured.
Flame temperature does not appear to be directly influenced by the type of fuel, except in a secondary manner as a result of carbon formation, or of poor atomisation resulting from a localised over-rich mixture. The maximum flame temperature for hydrocarbon fuels is roughly 2,000?C. This temperature occurs at a mixture strength slightly richer than the theoretical, owing to dissociation of the molecular products of combustion, which occurs at the theoretical mixture. Dissociation occurs above about 1,400?C and reduces the energy available for temperature rise.
The problem of the flame becoming extinguished in flight is not perfectly understood, but it appears that the type of fuel is of relatively minor importance. However, wide cut gasoline's are more resistant to extinction than kerosene and engines are easier to relight using wide cut fuel. This is due to the higher vapour pressure of these fuels.
The calorific value is a measure of the heat potential of a fuel. It is of great importance in the choice of fuel, because the primary purpose of the combustion system is to provide the maximum amount of heat with the minimum expenditure of fuel. The calorific value of liquid fuels is usually expressed in megajoules (MJ) per litre. When considering calorific value, it should be noted that there are two values which can be quoted for every fuel, the gross value and the net value. The gross value includes the latent heat of vaporisation and the net value excludes it. The net value is the quantity generally used. The calorific value of petroleum fuels is related to their specific gravity. With increasing specific gravity (heavier fuels) there is an increase in calorific value per litre but a reduction in calorific value per kilogram. Thus, for a given volume of fuel, kerosene gives an increased aircraft range when compared with gasoline, but weighs more. If the limiting factor is the volume of the fuel tank capacity, a high calorific value by volume is the more important.
The tendency of a turbine fuel to corrode the aircraft's fuel system depends on two factors:-
b) Other corrosive substances, notably sulphur compounds.
The water which causes corrosion is dissolved water. It leads to corrosion of the fuel system, which is particularly important with regard to the sticking of sliding parts, especially those with small clearances and only small or occasional movement.
Corrosion can also be caused by secondary effects, such as biological corrosion caused by plant spores, which are not killed off by the cracking process. Kerosene and diesel suffer from this form of contamination.
EFFECTS OF BY-PRODUCTS OF COMBUSTION
Carbon deposition in the combustion system indicates imperfect combustion and may lead to:-
a) A lowering of the surface temperature on which it is deposited, resulting in buckled flame tubes because of the thermal stresses set up by the temperature differences.
b) Damage to turbine blades caused by lumps of carbon breaking off and striking them.
c) Disruption of airflow through the turbine, creating turbulence, back-pressure and possible choking of the turbine, resulting in loss of efficiency.
It appears that carbon deposition depends on the design of the combustion chamber and the aromatic content of the fuel. (Aromatics are a series of hydrocarbons based on the benzene ring). The higher the aromatic content, the greater the carbon deposits.
Sulphur will affect the turbine. Every effort is made to keep the sulphur content as low as possible in aviation turbine fuels. Unfortunately, removal of the sulphur involves increased refining costs and decreased supplies and so some sulphur is therefore permitted.