The jet engine is also known as a reaction engine. A reaction engine is any such engine that generates thrust by expelling a reaction mass, which then produces force in the opposite direction in which the given weight is expelled. It is due to Newton’s Third law of motion, which states that every action produces an equal and opposite reaction. As such, rocket engines, water jets, etc. can be classified as reaction engines, but the term “Jet Engine” in this article describes an ‘air-breathing’ reaction engine.
In this article, we focus mainly on the gas turbines that intake a volume of air, compress it, add more energy to it (generally through the combustion of a fuel mixture) and then eject it through a nozzle that produces thrust. It is henceforth, needless to say, that the aircraft we use today are called jet airplanes, not because of any intrinsic quality that makes it so, but because of its use of jet engines for propulsion.
Jet engines were one of the late technologies to enter the second World War. Since then, it has seen tremendous improvements in its performance and complexity, and as such, any composition that makes no mention of its incredible history, as a technology, is utterly incomplete.
History of Jet Engines
A rudimentary form of jet power dates back to the Aeolipile, which was a device, no more than a curiosity that directed steam to exit two nozzles, which caused the machine to spin rapidly. The King of Mysore, India, Hyder Ali, was the first ruler to use jet propulsion against the British East India Company.
It was, however, during the Second World War that a basic form of the modern gas turbine saw serious developments. It is then that engineers began to realize that the speed an aircraft could fly at was approaching its limits due to propeller efficiency, which decreases as the tips of the propeller blades approach the sound barrier. Hence, if aircraft performance were to proceed beyond such a wall, then an entirely new technique of propulsion needs to be invented. It was the motivation behind the development of the gas turbine engine.
In 1928, RAF College Cranwell cadet Frank Whittle submitted his idea for a turbojet to his superiors and, consequently filed his first patent on 16th January 1930. In 1935, Hans Von Ohain began work on a similar design. His designs saw a few iterations, such as the HeS 1, which used hydrogen, supplied under external pressure as its fuel, unlike the gasoline-powered engine, under development by Frank Whittle. The subsequent HeS 3, however, utilized gasoline for power and produced 5kN of thrust. This engine, incorporated into the Heinkel-developed He 178 airframe, became the world’s first jet-powered aircraft to take to the skies.
The Junkers Motoren 004 jet engine was used in the Messerschmitt Me 262, which became the first jet-powered airplane to be put into active service. The Me 262, however, entered the war too late due to delays in the 004 engine’s availability and, henceforth, did not make enough contributions in improving the german air positions before the war came to a close.
Frank Whittle set up a company for manufacturing turbojet engines, named Power Jets Ltd. The Power Jets-made turbojet engines powered the Gloster Meteor, which entered service with the RAF in July 1944. By the 1950s, jet engines were almost universal in combat aircraft, and by the 1960s, jet engines became commonplace in civilian airplanes as well, while the old piston engine was left behind to power low-cost niche roles, such as cargo flights.
Today, with the advent of the high-bypass Turbofan Jet engines, fuel efficiency has been brought down to the level of piston engines (which are still more fuel-efficient than turbojet engines at low speeds and lower altitudes) while maintaining similar thrust as that of a turbojet at high speeds and high altitudes.
Basic working of an Air- Breathing Jet Engine
The primary method in which a jet engine functions broadly consists of 4 parts – Intake, Compression, Combustion, Expansion, and Exhaust. In the intake stage, the air is sucked in from the front of the engine into the compressor. The compressor, consisting of multiple steps (single-stage consists of a rotor and a stator), compresses a continuously flowing stream of air by incrementally squeezing it into tighter and tighter spaces with every stage. This is done to increase the potential energy in every air molecule resulting in more power output from a physically smaller engine.
The compressed air is then directed to the combustion chamber where fuel is mixed into the air and consequently ignited, which further causes the air to expand. This hot air, which is exhausted from the combustor, with incredible kinetic energy, is then directed through another set of turbine stages, which start spinning as the kinetic energy of the moving air imparts rotational motion to the turbine assembly. The turbine, in turn, spins the central shaft, which rotates the intake fan and the compressor assembly, and the cycle continues.
Types of Jet Engines
In this article, we focus majorly on four types of jet engines, which have different purposes, performance, and efficiency curves, and as such, generally have a well-defined role they are made for. These include:
1. Turbojet Engines
These types of jet engines were the first kind to be developed and probably the simplest ones to understand and implement. The working of a turbojet is the same as described in the section above. They were in extensive use following the Second World War but consequently fell out of favor after the invention of the turbofan. They had a very high fuel consumption at low speeds and low altitudes without the power output to justify the same but had its advantages in the simplicity of its design and manufacture. A top-rated aircraft that used a turbojet engine is the McDonnell Douglas F4 Phantom.
2. Turboprop Engines
Turboprops can be loosely thought of as an engine where the radial piston assembly used in old WWII planes has been replaced by a turbojet. The energy that is imparted by the expanding hot air at the end of the combustion stage is used up, as much as possible for spinning the central shaft, which is, in turn, is connected to a propeller through a gearbox and spins the same, as demonstrated in the picture above. The propeller generates thrust by forcing the air in front of it, backwards at a much higher speed than ambient. They are extremely fuel-efficient but very slow as compared to modern speeds of flying and do not fly at high altitudes. They are hence used in short-haul flights or in transport aircraft. Examples include The Airbus A400M.
3. Turboshaft Engines
The turboshafts are very similar to the turboprops. So much so, that a single unit is often sold in both configurations at the same time. The energy from the exhaust gases at the end of the combustion stage is ideally, completely used, and converted into output shaft power. The major difference lies in the fact that the propeller is generally attached to the main body of the engine in a Turboshaft, which is not the case in a Turboprop and hence such an engine is structurally designed to take the added load. They are majorly used in applications that require a continuous high-power output in a reliable and lightweight package, e.g. in Helicopters, Tanks, Hovercrafts , and stationary equipment.
4. Turbofan Engines
The Turbofans are, by far, the most popular type of engine in use today. The engine is fed air through an intake fan, placed in front of the core engine. The mass flow of the air is divided into two parts, and one part flows around the core engine and the other part through it. The ratio of the mass flow of air around the core engine to the atmosphere through it is called the Bypass Ratio. The core engine is very much like a standard Turbojet engine.
The net thrust produced is hence the sum of the thrust produced by the cold air accelerated by the intake fan, flowing around the core engine and the thrust produced by the hot air exiting the same. In this ingenious design, the cold air actually ends up creating most of the total thrust. Thus, in terms of performance, this kind of engine can be as fuel-efficient as a turboprop (high-bypass ratio) at speeds and altitudes that can rival a turbojet.
Other than these four basic types, there are many more types of jet engines and gas turbine engines alone. An excellent example can be a Ramjet, where the compression of the air is done by the speed of the air moving into the engine apart, which is then combusted and exhausted to produce thrust. Needless to say, these require very high rates even to start functioning but are very fuel-efficient at very high speeds and can sustain speeds up-to Mach 5, or 5 times the speed of sound, all the while requiring no moving parts to function.
A great many incredible achievements in the field of jet propulsion have already been made, and today, we stand on the shoulder of giants and look further beyond the horizon. It is, however, our staunch belief that air travel and transport can only get greener, faster, and more efficient with the times ahead of us.