The jet engines are essentially a machine designed for the purpose of producing high velocity gasses at the jet nozzle . The engine is started by rotating the compressor with the starter ,
the outside air enter to the engine . The compressor works on this incoming air and delivery it to the combustion or burner section with as much as 12 times or more pressure the air had at the front . At the burner or
combustion section , the ignition is igniting the mixture of fuel and air in the combustion chamber with one or more igniters which somewhat likes automobile spark plugs. When the engine has started and its compressor is rotating
at sufficient speed , the starter and igniters are turn off. The engine will then run without further assistance as long as fuel and air in the proper proportions continue to enter the combustion chamber. Only 25% of the air is taking part in
the actual combustion process . The rest of the air is mixed with the products of combustion for cooling before the gases enter the turbine wheel . The turbine extracts a major portion of energy in the gas stream and uses this energy
to turn the compressor and accessories . The engine's thrust comes from taking a large mass of air in at the front and expelling it at a much higher speed than it had when it entered the compressor . THRUST , THEN , IS
EQUAL TO MASS FLOW RATE TIMES CHANGE IN VELOCITY .
The more air that an engine can compress and use , the greater is the power or thrust that it can produce . Roughly 75% of the power generated inside
a jet engine is used to drive the compressor . Only what is left over is available to produce the thrust needed to propel the airplane .
JET ENGINE EQUATION
Since Fuel flow adds some mass to the air flowing through the engine , this must be added to the basic of thrust equation . Some formular do not consider the fuel flow effect when
computing thrust because the weight of air leakage is approximately equal to the weight of fuel added . The following formular is applied when a nozzle of engine is " choked " , the pressure is such that the gases are treveling
through it at the speed of sound and can not be further accelerated . Any increase in internal engine pressure will pass out through the nozzle still in the form of pressure . Even this pressure energy cannot turn into velocity energy
but it is not lost .
FACTORS AFFECTING THRUST
The Jet engine is much more sensitive to operating variables . Those are:
1.) Engine rpm. 2.) Size of nozzle area. 3.) Weight of fuel flow. 4.) Amount of air bled from the compressor. 5.) Turbine inlet temperature. 6.) Speed of aircraft (ram pressure rise).
7.) Temperature of the air. 8.) Pressure of air 9.) Amount of humidity. Note ; item 8,9 are the density of air .
ENGINE STATION DESIGNATIONS
Station designations are assigned to the varius sections of gas turbine engines to enable specific locations within the engine to be easily and accurately identified.
The station numbers coincide with position from front to rear of the engine and are used as subscripts when designating different temperatures and pressures at the front , rear , or inside of the engine.
For engine configurations other than the picture below should be made to manuals published by the engine manufacturer.
N = Speed ( rpm or percent ) N1 = Low Compressor Speed N2 = High Compressor Speed N3 = Free Turbine Speed P = Pressure T = Temperature t = Total
EGT = Exhaust Gas Temperature EPR = Engine Pressure Ratio ( Engine Thrust in term of EPR ). Pt7 / Pt2
Ex.: Pt 2 = Total Pressure at Station 2 ( low pressure compressor inlet ) Pt 7 = Total Pressure at Station 7 ( turbine discharge total pressure )