Jet engines
Jet engine is a jet engine propelled by a jet of hot exhaust gases formed from heated and expanded air that is drawn into the engine via a compressor, typically a centrifugal or axial type. They are typically gas turbine engines. The opposite of airbreathing jet engines are non-airbreathing jet engines, such as rocket engines, for example, which are propelled by a jet of hot gases created by the chemical reaction of two or more compounds internally. While the majority of the mass flow of an airbreathing jet engine is provided by air taken from outside of the engine and heated internally, using energy stored in the form of fuel, a rocket engine's fuel provides both the energy and the mass flow to create thrust.
All practical airbreathing jet engines are internal combustion engines that directly heat the air by burning fuel, with the resultant hot gases used for propulsion via a propulsive nozzle, although other techniques for heating the air have been experimented with (such as nuclear jet engines). Most modern jet engine designs are turbofans, which have largely replaced turbojets. These modern engines use a gas turbine engine core with high overall pressure ratio (about 40:1 in 1995) and high turbine entry temperature (about 1800 K in 1995), and provide a great deal of their thrust with a turbine-power fan stage, rather than with pure exhaust thrust as in a turbojet. These features combine to give a high efficiency, relative to a turbojet. A few jet engines use simple ram effect (ramjet) or pulse combustion (pulsejet) to give compression.Dr. Hans von Ohain and Sir Frank Whittle are both recognized as being the co-inventors of the jet engine. Each worked separately and knew nothing of the other's work. Hans von Ohain is considered the designer of the first operational turbojet engine. Frank Whittle was the first to register a patent for the turbojet engine in 1930. Hans von Ohain was granted a patent for his turbojet engine in 1936. However, Hans von Ohain's jet was the first to fly in 1939. Frank Whittle's jet first flew in in 1941.
Sir Frank Whittle was an English aviation engineer and pilot, the son of a mechanic, Frank Whittle joined the Royal Air Force or RAF as an apprentice. He joined an RAF fighter squadron in 1928 and became a test pilot in 1931. The young RAF officer was only 22 when he first thought to use a gas turbine engine to power an airplane. While often regarded as the father of modern jet propulsion systems, the young Frank Whittle tried without success to obtain official support for study and development of his ideas. He had to persist his research on his own initiative and received his first patent on turbojet propulsion in January 1930.
With private financial support, he began construction of his first engine in 1935. This engine, which had a single-stage centrifugal compressor coupled to a single-stage turbine, was successfully bench tested in April 1937; it was only a laboratory test rig, never intended for use in an aircraft, but it did demonstrate the feasibility of the turbojet concept. The modern turbojet engine used in many British and American aircraft is based on the prototype that Frank Whittle invented.
The firm of Power Jets Ltd., with which Whittle was associated, received a contract for a Whittle engine, known as the W1, on July 7, 1939. This engine was intended to power a small experimental aircraft. In February 1940, the Gloster Aircraft Company was chosen to develop the aircraft to be powered by the W1 engine - the Pioneer. The historic first flight of the Pioneer took place on May 15, 1941, with Flight Lieutenant P. E. G. Sayer as pilot.
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| Fig: How Jet Engine Works |
The basic idea of the turbojet engine is simple. Air taken in from an opening in the front of the engine is compressed to 3 to 12 times its original pressure in compressor. Fuel is added to the air and burned in a combustion chamber to raise the temperature of the fluid mixture to about 1,100°F to 1,300° F. The resulting hot air is passed through a turbine, which drives the compressor. If the turbine and compressor are efficient, the pressure at the turbine discharge will be nearly twice the atmospheric pressure, and this excess pressure is sent to the nozzle to produce a high-velocity stream of gas which produces a thrust.
Substantial increases in thrust can be obtained by employing an afterburner. It is a second combustion chamber positioned after the turbine and before the nozzle. The afterburner increases the temperature of the gas ahead of the nozzle. The result of this increase in temperature is an increase of about 40 percent in thrust at takeoff and a much larger percentage at high speeds once the plane is in the air.
The turbojet engine is a reaction engine. In a reaction engine, expanding gases push hard against the front of the engine. The turbojet sucks in air and compresses or squeezes it. The gases flow through the turbine and make it spin. These gases bounce back and shoot our of the rear of the exhaust, pushing the plane forward.
Jet engines move the airplane forward with a great force that is produced by a tremendous thrust and causes the plane to fly very fast.
All jet engines, which are also called gas turbines, work on the same principle. The engine sucks air in at the front with a fan. A compressor raises the pressure of the air. The compressor is made up of fans with many blades and attached to a shaft. The blades compress the air. The compressed air is then sprayed with fuel and an electric spark lights the mixture. The burning gases expand and blast out through the nozzle, at the back of the engine. As the jets of gas shoot backward, the engine and the aircraft are thrust forward.
The image above shows how the air flows through the engine. The air goes through the core of the engine as well as around the core. This causes some of the air to be very hot and some to be cooler. The cooler air then mixes with the hot air at the engine exit area.
A jet engine operates on the application of Sir Isaac Newton's third law of physics: for every action there is an equal and opposite reaction. This is called thrust. This law is demonstrated in simple terms by releasing an inflated balloon and watching the escaping air propel the balloon in the opposite direction. In the basic turbojet engine, air enters the front intake and is compressed, then forced into combustion chambers where fuel is sprayed into it and the mixture is ignited. Gases which form expand rapidly and are exhausted through the rear of the combustion chambers. These gases exert equal force in all directions, providing forward thrust as they escape to the rear. As the gases leave the engine, they pass through a fan-like set of blades (turbine) which rotates the turbine shaft. This shaft, in turn, rotates the compressor, thereby bringing in a fresh supply of air through the intake. Engine thrust may be increased by the addition of an afterburner section in which extra fuel is sprayed into the exhausting gases which burn to give the added thrust.
At approximately 400 mph, one pound of thrust equals one horsepower, but at higher speeds this ratio increases and a pound of thrust is greater than one horsepower. At speeds of less than 400 mph, this ratio decreases.In a turboprop engine, the exhaust gases are also used to rotate a propeller attached to the turbine shaft for increased fuel economy at lower altitudes. A turbofan engine incorporates a fan to produce additional thrust, supplementing that created by the basic turbojet engine, for greater efficiency at high altitudes. The advantages of jet engines over piston engines include lighter weight with greater power, simpler construction and maintenance with fewer moving parts, and efficient operation with cheaper fuel.