Engines used in the earlier parts of 20th century had deficiencies in spite of their advantages. The advantages used in those times were Turboprop engines. These engines could produce only 10% of their thrust from the exhaust jet. They could not attain high speed.
Researches were conducted further, which led to the developments of Turbofan engines. Turbofan engines combined the hot air jet with bypassed air from a fan. This created a quieter engine with greater boost at low speeds, making it a popular choice for commercial airplanes. And due to generation of more thrust for nearly the same amount of fuel, it is highly fuel-efficient
An actual jet engine does not operate quite as simply as a balloon, although the basic principle is the same. More important than pressure imbalance is the acceleration due to high velocities of the jet leaving the engine. This is achieved by forces in the engine that enable the gas to flow backward forming the jet. Newton's second law shows that these forces are proportional to the rate at which the momentum of the gas is increased. For a jet engine, this is related to the rate of mass flow multiplied by the rearward-leaving jet velocity. Newton's third law, which states that every force must have an equal and opposite reaction, shows that the rearward force is balanced by a forward reaction, known as thrust. This thrusting action is similar to the recoil of a gun, which increases as both the mass of the projectile and its muzzle velocity are increased. High-thrust engines, therefore, require both large rates of mass flow and high jet-exit velocities, which can only be achieved by increasing internal engine pressures and by increasing the volume of the gas by means of combustion.
Jet-propulsion devices are used primarily in high-speed, high-altitude aircraft, in missiles, and in spacecraft. The source of power is a high-energy fuel that is burned at intense pressures to produce the large gas volume needed for high jet-exit velocities. The oxidizer required for the combustion may be the oxygen in the air that is drawn into the engine and compressed, or the oxidizer may be carried in the vehicle, so that the engine is independent of a surrounding atmosphere. Engines that depend on the atmosphere for oxygen include turbojets, turbofans, turboprops, ramjets, and pulse jets. Non atmospheric engines are usually called rocket engines.
Jet power as a form of propulsion has been known for hundreds of years, although its use for propelling vehicles that carry loads is comparatively recent. The earliest known reaction engine was an experimental, steam-operated device developed about the first century B.C. by the Greek mathematician and scientist Hero of Alexandria. Known as the Aeolipile, Hero's device did no practical work, although it demonstrated that a jet of steam escaping to the rear drives its generator forward. The aeolipile consisted of a spherical chamber into which steam was fed through hollow supports. The steam was allowed to escape from two bent tubes on opposite sides of the sphere, and the reaction to the force of the escaping steam caused the sphere to rotate.
The development (1629) of the steam turbine is credited to the Italian engineer Giovanni Branca, who directed a steam jet against a turbine wheel, which in turn powered a stamp mill. The first recorded patent for a gas turbine was obtained in 1791 by the British inventor John Barber.
In 1910, seven years after the first flights by the American inventors Orville and Wilbur Wright, the French scientist Henri Marie Coanda designed and built a jet-propelled biplane, which took off and flew under its own power with Coanda as pilot. Coanda used an engine that he termed a reaction motor, but, discouraged by the lack of public acceptance of his aircraft, he abandoned his experiments.
Parts of a Turbofan Engine
The different parts of a Turbofan engine are as shown in Fig 10 below:-
Fan - The fan is the first component in a turbofan. The fan pulls air into the engine. The large spinning fan sucks in large quantities of air. It then, speeds the air up and splits it into two parts. One part continues through the "core" or center of the engine, where it is acted upon by the other engine components. The second part "bypasses" the core of the engine, instead traveling through a duct that surrounds the core to the back of the engine where it produces much of the force that propels the airplane forward.
Compressor - The compressor is the first component in the engine core. The compressor squeezes the air that enters it into smaller areas, resulting in an increase in the air pressure. This results in an increase in the energy potential of the air. The squashed air is forced into the combustion chamber.
Combustor - In the combustor the air is mixed with fuel and then ignited. This process results in high temperature, high energy airflow. The fuel burns with the oxygen in the compressed air, producing hot expanding gases.
Turbine - The high energy airflow coming out of the combustor goes into the turbine, causing the turbine blades to rotate. This rotation extracts some energy from the high-energy flow that is used to drive the fan and the compressor. The gases produced in the combustion chamber move through the turbine and spin its blades. The task of a turbine is to convert gas energy into mechanical work to drive the compressor.
Nozzle - The nozzle is the exhaust duct of the engine. The energy depleted airflow that passed the turbine, in addition to the colder air that bypassed the engine core, produces a force when exiting the nozzle that acts to propel the engine, and therefore the airplane, forward. The combination of the hot air and the cold air are expelled and produce an exhaust which causes a forward thrust. The nozzle may be preceded by a mixer, which combines the high temperature air coming from the engine core with the lower temperature air that was bypassed in the fan. This results in a quieter engine than if the mixer was not present.
Afterburner - In addition to the basic components of a gas turbine engine, one other process is occasionally employed to increase the thrust of a given engine. Afterburning (or reheat) is a method of augmenting the basic thrust of an engine to improve the aircraft takeoff, climb and (for military aircraft) combat performance. Afterburning consists of the introduction and burning of raw fuel between the engine turbine and the jet pipe propelling nozzle, utilizing the unburned oxygen in the exhaust gas to support combustion. The increase in the temperature of the exhaust gas increases the velocity of the jet leaving the propelling nozzle and therefore increases the engine thrust. This increased thrust could be obtained by the use of a larger engine, but this would increase the weigh and overall fuel consumption. In other words Afterburner is a device for increasing the thrust (forward-directed force) of a gas-turbine (jet) airplane engine. Additional fuel is sprayed into the hot exhaust duct between the turbojet (engine) and the tailpipe. The fuel ignites, providing a burst of speed. Afterburning is used for a short increase of power during takeoff, or during combat in military aircraft
As an example for the turbofan engine consider the Rolls-Royce Tay turbofan engine.This Rolls-Royce Tay turbofan engine pushes nearly three times as much air through the bypass ducts as it pushes through the central core of the engine, where the air is compressed, mixed with fuel, and ignited. Turbofan engines like the Rolls-Royce Tay are not as powerful as turbojets, but they are quieter and more efficient.
The turbofan engine is an improvement on the basic turbojet. Part of the incoming air is only partially compressed and then bypassed in an outer shell beyond the turbine. This air is then mixed with the hot turbine-exhaust gases before they reach the nozzle. A bypass engine has greater thrust for takeoff and climb, and increased efficiency; the bypass cools the engine and reduces noise level.
In some fan engines the bypass air is not remixed in the engine but exhausted directly. In this type of bypass engine, only about one-sixth of the incoming air goes through the whole engine; the remaining five-sixths is compressed only in the first compressor or fan stage and then exhausted. Different rotational speeds are required for the high- and low-pressure portions of the engine. This difference is achieved by having two separate turbine-compressor combinations running on two concentric shafts or twin spools. Two high-pressure turbine stages drive the 11 high-pressure compressor stages mounted on the outer shaft, and 4 turbine stages provide power for the fan and 4 low-pressure compressor stages on the inner shaft. To move an airplane through the air, thrust is generated by some kind of propulsion system. Most modern airliners use turbofan engines because of their high thrust and good fuel efficiency.
An example of an engine of this type is the JT9D-3 jet engine, which weighs about 3850 kg (about 8470 lb) and can develop a takeoff thrust of about 20,000 kg (about 44,000 lb). This is more than double the thrust available for the largest commercial planes before the Boeing 747.