CHAPTER No . 1
INTRODUCTION
The gas turbine works on the principle as the piston engine; air is taken in, compressed, heated and expanded. Combustion in the gas turbine is however continuous. There is no need for valves or sparking plugs in the gas turbine, the whole mechanism is simpler and lighter in weight. Moreover, since power is generated continuously, much power can be produced by a relatively small engine. The fuel is burnt in a combustion chamber. There are no fluctuating pressure, and the combustion chamber can be of relatively light construction.
Aviation fuels are used in the aircraft engine. Basically aviation fuels are of two types depending upon the type of engine. They are Aviation gasoline & Jet fuels.Aviation gasoline is used in the reciprocating piston engine. But today these engines are not longer used for military purposes except for training, transport, air craft and helicopters, because the very high power required for modern military air craft are beyond the range of piston engine, which become too large, heavy and complex above 3500 hp. Though the piston engines are still used in civil aircraft, but are rapidly superseded by " Jet engines" which permit greater speeds and larger planes. Fuel consumption is much higher but fuel quality is less critical. Super constellations (a plane) took 10 hours for the flight from London to New York with forty passengers, using 25,000 litres of aviation gasoline. Modern Jet planes do the trip in 6 hours with ninety passengers, but consume 90,000 litres of kerosene.
1.1 THE AVIATION PISTON ENGINE:There is a little difference in principle between the automotive and the aviation piston engines although there are significant differences in detail. In aviation piston engine the fuel is stored in tanks, situated usually in the wings, and is pumped from there to a carburettor where it is vaporized and automized by the induced air. The power generated depends basically on the amount of the charge, and is thus limited by the size of the cylinder. The amount of the charge depends also on the density of the air. With a reduced quantity of air the power would fall off and flying at altitude will be impossible. The engine is therefore `supercharged' or boosted.1.2 THE JET ENGINE OR THE AVIATION GAS TURBINE ENGINE:Supercharging has an effect similar to increasing the compression ratio, since more charge is delivered and compressed in to the volume of the combustion space. Normally the aero-engines have compression ratios about 6:1 to 8.7:1 as compared with 7:1 to 10:1 for automotive engines. Aero-engine may air cooled or liquid-cooled according to the arrangement of cylinders. The former were used in past generations of fighters but the latter predominate in civil air craft.
Water is not suitable as a coolant because aero-engines run hotter than motorcar engines, and weight limitations preclude large coolers; a mixture of glycol and water is normally used.
The number of cylinders in an air-craft engine can range from four to twenty-eight and may be arrange in line or radically.
In operation air is sucked into the front of the unit and compressed in the compressor before delivery to the combustion chamber where fuel is injected, atomized and burnt. The expansion of the air due to the heat of combustion operates the turbine driving the compressor, and in the process the gases lose part of their energy. They are then ejected at high speed from the jet at the rear, and their momentum (mxv) propels the aircraft. The combustion temperature is very high, therefore the gases are cooled by the addition of excess air before passing to the turbine (with some loss in efficiency).
The weight of air drawn in by the compression decreases with altitude as the density of air decreases. Thus the power required to maintain the air craft at a given speed is reduced as altitude increases, and it is more economical to operate Jet aircraft at high altitude, in general they fly at heights of upto 40,000 feet.
1.3 AVIATION GASOLINE:
Aviation gasoline, in general, similar to motor gasoline except that they have a slightly narrow boiling range, about 40 to 170oC, and a lower vapour-pressure, 7psi maximum. Until recently the antiknock quality of these fuels was higher than that of motor fuels, but today the octane number of the best quality motor spirits are in the aviation gasoline range. Gasoline of this type are made up of specially straight run and synthetic components. Such as Iso-octane, obtained from straight run gasoline by super fractionation, alkylates and aromatic. The later are obtained from reformats by distillation or solvent extraction. Iso-octane is a special component produce in the refining process of specialized equipment. Small amounts of Iso pentane and aeromatic (ring) compounds are also used. The isopentane allows the correct volatility to be achieved in the final fuel blend. Aeromatics are used to improve mixture ratings. However, these aeromatics must be limited to achieve other specification. Grade 80 Aviation gasoline (Av gas) may also contain straight run gasoline but their components lower octane ratings makes it un-suitable for higher octane blends. Approved additives include alkyl, lead and antiknock additives. Other additives are also used to then control lead deposit formation. Color-dyes are required in most grader for safety identification. Another common and required additive includes oxidation inhibitors to improve storage stability and inhibit gum formation. These anti oxidant additives also help to prevent lead compounds precipitation (separation). Other additives such as corrosion inhibitors, fuel system icing inhibitors and static dissipaters additives may also be included by agreement with the user, by the military or by some foreign specifications. All other additives are forbidden.
An aviation fuel must conform quiet rigid specifications, most of
which have been laid by the armed services. In U.K. there are five normal
grades of aviation gasoline ranging an antiknock value from 73 - 115. In
USA there are four grades ranging from 80 - 115. These must be stable and
clean products with out appreciable gum or gum forming compounds.
1.3.1 CHARACTERISTICS:
1 CARBURETTOR ICING:2 VAPOUR LOCK:
It is common to aero and motor engine. It is avoided in aero engines by the use of fuel injectors or injectors carburettors and by heating the intake air when necessary.
The atmospheric temperature falls continuously as altitude increases and it is essential that the fuel should not freeze solid. A maximum freezing point is therefore specified for aviation gasolines i.e. - 76oF.
The vapour locking tendencies of gasoline increases with altitude and vapour lock can occur when an air craft, after standing on the ground in a warm atmosphere climbs quickly to high altitudes. The vapour pressure of aviation gasoline must therefore be lower than that of motor gasoline. Aviation gasoline must contain fewer higher boiling constituents than does motor gasoline, and consequently must have a lower final boiling point.3 FREEZING POINT:
Another potential hazard of low temperature operation is the freezing of entrained water to form ice particles which could clog filters and reduces fuel flow. It is therefore a specification requirement that aviation gasoline should be free from water.
4. ANTIKNOCK VALUE:
Detonation is common to both aero and automotive engines. It does little actual harm in the automize engine, but can vary serious in the aero engines with its large cylinders and high specific out put, which leads to the burning of the piston. It is essential therefore that aviation gasoline should have good antiknock properties. The CFR motor and research methods used for rating are of limited use for aviation gasoline. Two other methods are used known respectively as the F3 or 1C method and the F4 or 3C methods. The F4 method rates a fuel higher than does the F3 method and in practice, it is usual to characterize a fuel by both figures, e.g. the 115 /145 grade has an F3 performance of 115 and F4 of 145.
1.4 JET FUELS:
Basically the jet engine is not particularly fuel conscious, and can operate on any clean burning fuel. Kerosene is a suitable fuel and is widely used, but since there is only a limited amount of kerosene in any crude. A distillate of wider distillation range is also used. Wide range distillate is produced by including some of the gasoline fraction, and this naturally introduces a some what greater fire hazard which has lead to some controversy in the use of this type of fuel for civil air craft.
1.4.1 CHARACTERISTICS:
Fuels for jet engine must be free from acidic materials and corrosive sulphur compounds to prevent corrosion of metal equipment and there is a rigid specification to ensure this. Although combustion of the inevitably produces sulphur gases little or no effect on the combustion chamber or turbine blades is experienced provided the sulphur compounds of the fuel is with in 0.4% by weight generally specified by service authorities.LOW TEMPERATURE CHARACTERISTICS:
Water can exist in a fuel either in suspension or in solution. It is essential that jet fuel be entirely free from suspended water because of the low temperature to which it is exposed. Suspended water present in the jet fuel can generally be removed by setting, although this may take a long time. Water in solution is more difficult matter, it cannot be removed by any practical means. Ice particles may form below 0oC and although there is only about 0.005 to 0.01 % by weight of dissolve water in the fuel. The large quantity of fuel used in a long high altitude flight may produce sufficient ice to block the fine filters use for the removal of extraneous solids. Dissolve water has given less trouble in practice than had been expected because exposure of the fuel to low temperature and pressure during flight reduces the amount of dissolve water.
At extremely low temperature a fuel may have a high viscosity. Although a boosted pump is use to deliver fuel to the engine, the oil feed to this pump from the oil tank is by gravity and it is essential that it should not be impeded by too high viscosity fuel with final boiling points below 300oC generally have viscosities sufficiently low for them to flow to the pump its temperatures down to their freezing point. The limiting temperature of use would be determine by solidification of the fuel in the tanks, and this would not occur until well below the freezing point, firstly, because the freezing point is the temperature at which solid being to separate under arbitrary test conditions and not the temperature at which the fuel solidifies and secondly because of mechanical agitation inherent in the system. In practice therefore the specification of a maximum freezing point corresponding to the lowest service temperature. Likely to be experienced provides an ample margin of safety. A maximum freezing point of -40oC or -60oC, according to the type of the service, ensures adequate flow under all practical conditions including starting at very low temperatures.
SUPER SONIC CONDITIONS:
Super sonic speed are designated as "mach numbers" equal to the ratio of the speed of the air craft to the speed of the sound, i.e. a mach number of 1 equals the speed of sound, a mach number of 2 equals twice the speed of sound. At a mach number of 2.2 the skin temperature of the fuel tank may attain 120oC, at a mach number of 3 it may be 260oC. The fuel must therefore be stable at high temperatures, present normal grades are satisfactory up to mach number 2.2 but better stability is likely to be required at higher speeds.
CORROSION:
1.5 JET FUEL GRADES:
Jet fuel comes in a variety of grade designator. Here are the
common civilian and military grades in use today.
JET A (JP-1): It is a narrow cut kerosene product. This is the standard commercial and a general aviation grade available. It usually contains no additives but may be additised with anti-icing chemicals.
JET A-1 (JP - 2 ): It is identical to Jet-A with the exception of freeze point. It is the fuel of choice for long haul flights where the fuel temperature may fall to near the freeze point often contain a static dissipater additives.
JET B (J P - 3 ): It is a wide cut kerosene with lighter gasoline type naphtha components. It contains static dissipater and has a very low flash point.
J P - 4 : JP - 4 is a blended component of gasoline and light naphtha. The blending ratio is from 55 - 60% JP - 1 and 40 - 45% gasoline. Some time the percentage may be vary.
JP - 4 is a military designated for a fuel like Jet - B but contains a full additive package including corrosion inhibition, anti-icing and static dissipater. It was the primary fuel of United State Air Force for decades but is being phased out in favour of JP - 8.
J P - 5: It is another military fuel. It has higher flash point (140 F min) and was designated for use by the US Navy on board air craft carriers. It contains anti-ice and corrosion inhibitors.
J P - 7: