Turbojets, Turbofans, Turboprops
The turbojet uses a series of fan-like compressor blades to bring air into the engine and compress it. An entire section of the turbojet engine performs this function, which can be compared to the compression stroke of the reciprocating engine. In this section, there is a series of rotor and stator blades. Rotor blades perform somewhat like propellers in that they gather and push air backward into the engine. The fixed stator blades serve to straighten the flow of this air as it passes from one set of rotor blades to the next).
As the air continues to be forced further into the engine, it travels from the low-compression set of rotors and stators to the high-compression set. This last set puts what we might say is the final squeeze on the air.
The combustion chamber receives the high-pressure air, mixes fuel with it, and burns the mixture. The hot, very high-velocity gases produced strike the blades of the turbine and cause it to spin rapidly. The turbine is mounted on a shaft which is connected to the compressor. Thus, the spinning is what causes the compressor sections to function. After passing the turbine blades, the hot, highly accelerated gases go into the engine's exhaust section.
The exhaust section of the jet engine is designed to give additional acceleration to the gases and thereby increase thrust. The exhaust section also straightens the flow of the gases as they come from the turbine. Essentially the exhaust section is a cone mounted in the exhaust duct, more often referred to as the tailpipe. The shape of the tailpipe varies, depending on the design operating temperatures and the speed-performance range of the engine.
With all the heat produced in the turbojet engine, how it is kept from overheating? Like most aircraft reciprocating engines, the jet is also air-cooled. Of all the air coming into the compressor section, only about 25 percent is used to produce thrustthe remaining 75 percent passes around the combustion chamber and turbine area to serve as a coolant.
The turbofan engine has gained popularity for a variety of reasons. One or more rows of compressor blades extend beyond the normal compressor blades, with a result that four times as much air is pulled into a turbofan engine as in a simple turbojet. However, most of this excess air is ducted through bypasses around the power section and out the rear with the exhaust gases. Also, a fan burner permits the burning of additional fuel in the fan airstream. With the burner off, this engine can operate economically and efficiently at low altitudes and low speeds. With the burner on, the thrust is doubled by the burning fuel, and it can operate on high speeds and high altitudes fairly efficiently. The turbofan has greater thrust for takeoff, climbing, and cruising on the same amount of fuel than the conventional turbojet engine.
With better all-around performance at a lower rate of fuel consumption and less noise from its operation, it is understandable that most new military and civilian jet-powered airplanes are fitted with turbofan engines.
Jet Engine Thrust
The force produced by a jet engine is expressed in terms of pounds of thrust. This is a measure of mass or weight of air moved by an engine times acceleration of the air as it goes through the engine. Technically, if the aircraft were to stand still and the pressure at the exit plane of the jet engine was the same as the atmospheric pressure, the formula for the jet engine thrust would be:
Imagine an aircraft standing still, capable of handling 215 pounds of air per second. Assume the velocity of the exhaust gases to be 1,500 feet per second. The thrust would then be:
If the pressure at the exit plane is not the same as the atmospheric pressure and the aircraft were not standing still, the formula would be something different.
It is not practical to try to compare jet engine output in terms of horsepower (apples and oranges). As a rule of thumb, however, you might remember that at 375mph one pound of thrust equals one horsepower; at 750mph one pound of thrust equals two horsepower.
Strictly speaking, the turboprop engine it is not a jet, but an effort to combine the best features of turbojet and propeller aircraft. The first is more efficient at high speeds and high altitudes; the latter is more efficient at speeds under 400 mph and below 30,000'. The turboprop uses a gas turbine to turn a propeller. Its turbine uses almost all the engine's energy to turn its compressor and propeller, and it depends on the propeller for thrust, rather than on the high-velocity gases going out of the exhaust.
The gas turbine can turn a propeller with twice the power of a reciprocating engine. Reduction gears slow the propeller below the turbine's rpm, and this must be done because of the limitations of propellers. No propeller is capable of withstanding the forces generated when it is turned at the same rate as that of the gas turbine. Even so, the turboprop engine receives fairly extensive use in military and civilian aviation circles.
In summary, aircraft turbine engines may be classified as turbojet, turbofan, or turboprop. As a group, turbine engines have many advantages over reciprocating engines, the most obvious being the capability of higher-altitude and higher-speed performance. Vibration stress is relieved as a result of rotating rather than reciprocating parts. Control is simpler because one lever controls both speed and power. With the large airflow, cooling is less complicated. Spark plugs are used only for starting, and the continuous ignition system of reciprocating engines is not needed. A carburetor and mixture control are not needed.
Major disadvantages have been high fuel consumption and poor performance at low power setting, low speeds, and low altitudes, but turboprop and turbofan developments have greatly improved aircraft turbine engines in these areas.