Downloaded from https://www.everyspec.com FM 1-506 Additives in JP fuels include oxidation and corrosion inhibitors, metal deactivators, and icing inhibitors. Icing inhibitors also function as biocides to kill microbes in aircraft fuel systems. 1500 o F (815 o C); therefore, the gases at entrance to the turbine vanes cannot exceed about 1600 o F (898 o C) for more than brief periods. Fuel-Air Ratio Should mixing of JP fuels become necessary, there is no need to drain the aircraft fuel system before adding the new fuel. Due to the different specific gravities of these fuels, mixing them will affect the turbine engine’s performance.
Be sure to consult appropriate technical manuals for additional information and procedures. The fuel-air ratio is so low in gas turbine engines that if fuel and air were uniformly mixed, the mixture would not ignite. Complete combustion of both fuel and air is obtained with isooctane at a fuel-air ratio of 0.066. This is the theoretical value which would produce complete combustion if given sufficient time to reach equilibrium. In practice, complete combustion of either fuel or air requires an excess of one or the other. Thus, if all of the air is to be burned excess fuel must be present and a fuel-air ratio of about 0.080 is required.
Control of power output is largely determined by means of fuel-air ratio. Increasing fuel-air ratio increases the quantity of air and the temperature at which it is discharged at the exhaust jet pipe. When changing to a fuel with a different specific gravity, externally adjusted fuel controls and fuel flow dividers on some engines may require retrimming or readjustment for optimum performance. COMBUSTION Mixing fuel with air and burning it would seem to be a very simple process, but this apparent simplicity is deceptive. Problems encountered are with distribution knot.kj ignition timing, and so forth.
In an internal com- bustion engine, the combustion process is the rather rapid reaction between fuel and oxygen. This process liberates the potential energy contained in the fuel supplied to the engine. In a gas turbine engine atmospheric air is taken in and compressed; fuel is then burned in the compressed air, which then expands through a turbine that drives a compressor. Combustion efficiency in a well-designed combus- tion chamber using the most favorable fuel may be as high as 98 percent at sea level.
On the other hand, it may fall to as low as 40 percent at extremely high altitudes with a badly designed combustion chamber and unsuitable fuel As combustion efficiency is reduced, a point is reached when the turbine does not develop enough power to drive the compressor. Increasing the fuel supply in order to maintain or increase engine speed may not result in increased engine RPM, and the unburned fuel may ex- tinguish the flame. This is known as rich-mixture blowout. Rich-Mixture Blowout The combustion problem would not be so great if weight and space were not so important in aircraft gas turbine engines. Without such limitation the air supply for the compressors could be divided.
A portion suitable to the desired power output could be burned at ap- proximately the chemically correct fuel-air ratio in a low-velocity combustion chamber. Design of the com- bustion chamber is such that less than a third of the total volume of air entering the chamber is permitted to mix with the fuel. The excess air bypasses the fuel nozzles and is used to cool the hot surfaces and to mix with and cool the burned gases before they enter the turbines. Lean-Mixture Die-Out In contrast to rich-mixture blowout, lean-mixture die-out occurs when the mixture is too lean to bum under conditions of efficient combustion.
When combustion efficiency is very low, the mixture may be ignited from an external source (such as an igniter plug). It will then either be extinguished when the spark ceases or may bum so slowly that the flame is carried out through the turbine. Lean-mixture die-out can also occur when the fuel supply is reduced in order to decrease engine speed. Gas turbine engines produce work in proportion to the amount of heat released internally. Most of this heat is obtained by burning fuel although some heat originates when air is compressed in the compressor.
Low fuel-air ratio is required to keep the temperature of the gases delivered to the turbine down to a value which the turbine wheel can tolerate. With present materials of construc- tion, the highly stressed blades or buckets of the turbine wheel cannot stand a temperature of more than about Temperature Limitations Rich mixture blowout and lean mixture die-out have been virtually eliminated through the refinement of fuel delivery systems. LUBRICATION Lubrication is a very important part of power plant operation. An engine allowed to operate without 1-4