SafekipediaJet engine
Adapted from Wikipedia · Discoverer experience
A jet engine is a type of reaction engine that produces movement by shooting out a fast stream of heated air. This creates a force called thrust, which pushes the aircraft forward through jet propulsion. Though the term can include other types of engines like rocket engines or water jet engines, most of the time when we talk about jet engines we mean air-breathing engines such as turbojet or turbofan engines.
These engines work by drawing in air, compressing it, mixing it with fuel, and then burning it to create a powerful stream of exhaust. This process, known as the Brayton thermodynamic cycle, allows jet engines to be very efficient for flying long distances. Modern jet aircraft use high-bypass turbofan engines, which are better for flying at slower speeds while still being fast and saving fuel compared to older engine types.
Over the years, jet engines have become much stronger and more reliable. In the 1950s, a typical jet engine produced about 5,000 pounds of thrust, but by the 1990s, some engines could produce more than 115,000 pounds of thrust. This improvement, along with better fuel efficiency, made it possible for airplanes to fly across oceans without stopping for fuel, changing the way people travel around the world.
History
Main article: History of the jet engine
See also: Timeline of jet power
Jet engines have a long history, with early ideas dating back to ancient times. The basic principle was shown by a simple device called the aeolipile, made by Hero of Alexandria in 1st-century Egypt, which used steam power to spin a sphere. Later, people used rocket-like devices for flight, like Lagâri Hasan Çelebi in 1633.
The real development of jet engines began in the early 1900s. Before World War II, engineers knew that regular airplane engines had limits. They needed something new to go faster. The key was the gas turbine, which uses the engine's own power to keep running.
During World War II, two men, Frank Whittle in Britain and Hans von Ohain in Germany, built the first real jet engines. In 1939, von Ohain's engine powered the world's first jet plane, the He 178. Soon after, in 1944, both the German Messerschmitt Me 262 and the British Gloster Meteor became the first jet fighters in combat. After the war, jet engines changed aviation forever, leading to faster, higher-flying planes for both military and civilian use.
Uses
Jet engines are used to power many different machines. They help jet aircraft fly and can also power cruise missiles and unmanned aerial vehicles. In the form of rocket engines, they are important for model rocketry, spaceflight, and military missiles.
Jet engines have also been used in high-speed cars, like drag racers. One famous car, ThrustSSC, set the land speed record using a jet engine. They are also used in factories and on ships, helping to generate electricity and power pumps. These engines can be changed to work in many ways, and some parts are also used in engines that power helicopters and propeller planes.
Types of jet engine
Main article: Airbreathing jet engine
Jet engines make aircraft move forward by pushing out a fast stream of heated air. The most common type used today is the turbofan engine, which works well at speeds just below the speed of sound.
A turbojet engine compresses air, mixes it with fuel, burns it, and then lets the hot air shoot out to create thrust. Turbofans are similar but have a fan that pushes some air around the main engine, making them more efficient for planes like airliners. Other types include propfans, which combine features of turboprops and turbofans, and ramjets and scramjets, which rely on the speed of the aircraft to compress the air before burning fuel.
| Type | Description | Advantages | Disadvantages |
|---|---|---|---|
| Motorjet | Works like a turbojet but a piston engine drives the compressor instead of a turbine. | Higher exhaust velocity than a propeller, offering better thrust at high speed | Heavy, inefficient and underpowered. Example: Caproni Campini N.1. |
| Pulsejet | Air is compressed and combusted intermittently instead of continuously. Some designs use valves. | Very simple design, used for the V-1 flying bomb and more recently on model aircraft | Noisy, inefficient (low compression ratio), works poorly on a large scale, valves on valved designs wear out quickly |
| Pulse detonation engine | Similar to a pulsejet, but combustion occurs as a detonation instead of a deflagration, may or may not need valves | Maximum theoretical engine efficiency | Extremely noisy, parts subject to extreme mechanical fatigue, hard to start detonation, not practical for current use |
Other types of jet propulsion
Main article: Rocket engine
Rockets are a type of engine that produces thrust like jet engines, but they do not need air from the atmosphere. Instead, they carry their own oxygen and other materials needed for the reaction. This allows rockets to work anywhere, including in space where there is no air. Rockets are used for launching satellites, exploring space, and even landing on the Moon in 1969. They are especially useful for very high speeds or when very strong pushes are needed.
Main article: Pump-jet
A water jet, or pump-jet, is a system used in boats that pushes a jet of water to move the boat forward. This can be done with a special kind of propeller inside a tube or with a spinning compressor and a nozzle. The pump-jet needs a separate engine, like a Diesel engine or a gas turbine, to power it.
| Type | Description | Advantages | Disadvantages |
|---|---|---|---|
| Rocket | Carries all propellants and oxidants on board, emits jet for propulsion | Very few moving parts. Mach 0 to Mach 25+; efficient at very high speed (> Mach 5.0 or so). Thrust/weight ratio over 100. No complex air inlet. High compression ratio. Very high-speed (hypersonic) exhaust. Good cost/thrust ratio. Fairly easy to test. Works in a vacuum; indeed, works best outside the atmosphere, which is kinder on vehicle structure at high speed. Fairly small surface area to keep cool, and no turbine in hot exhaust stream. Very high-temperature combustion and high expansion-ratio nozzle gives very high efficiency, at very high speeds. | Needs lots of propellant. Very low specific impulse – typically 100–450 seconds. Extreme thermal stresses of combustion chamber can make reuse harder. Typically requires carrying oxidizer on-board which increases risks. Extraordinarily noisy. |
| Type | Description | Advantages | Disadvantages |
|---|---|---|---|
| Turborocket | A turbojet where an additional oxidizer such as oxygen is added to the airstream to increase maximum altitude | Very close to existing designs, operates in very high altitude, wide range of altitude and airspeed | Airspeed limited to same range as turbojet engine, carrying oxidizer like LOX can be dangerous. Much heavier than simple rockets. |
| Air-augmented rocket | Essentially a ramjet where intake air is compressed and burnt with the exhaust from a rocket | Mach 0 to Mach 4.5+ (can also run exoatmospheric), good efficiency at Mach 2 to 4 | Similar efficiency to rockets at low speed or exoatmospheric, inlet difficulties, a relatively undeveloped and unexplored type, cooling difficulties, very noisy, thrust/weight ratio is similar to ramjets. |
| Precooled jets / LACE | Intake air is chilled to very low temperatures at inlet in a heat exchanger before passing through a ramjet and/or turbojet and/or rocket engine. | Easily tested on ground. Very high thrust/weight ratios are possible (~14) together with good fuel efficiency over a wide range of airspeeds, Mach 0–5.5+; this combination of efficiencies may permit launching to orbit, single stage, or very rapid, very long distance intercontinental travel. | Exists only at the lab prototyping stage. Examples include RB545, Reaction Engines SABRE, ATREX. Requires liquid hydrogen fuel which has very low density and requires heavily insulated tankage. |
| Type | Description | Advantages | Disadvantages |
|---|---|---|---|
| Water jet | For propelling water rockets and jetboats; squirts water out the back through a nozzle | In boats, can run in shallow water, high acceleration, no risk of engine overload (unlike propellers), less noise and vibration, highly maneuverable at all boat speeds, high speed efficiency, less vulnerable to damage from debris, very reliable, more load flexibility, less harmful to wildlife | Can be less efficient than a propeller at low speed, more expensive, higher weight in boat due to entrained water, will not perform well if boat is heavier than the jet is sized for |
General physical principles
Jet engines are reaction engines that produce thrust by sending out a fast stream of gas backward. This action pushes the engine—and anything attached to it, like an airplane—forward.
Jet engines can use fuel stored in tanks or they can take in air from the outside, speed it up, and push it out to create thrust. One important part of a jet engine is the propelling nozzle, which speeds up the exhaust gases to create a powerful jet.
These engines work best when the exhaust pressure matches the surrounding air pressure, but this is hard to achieve all the time, especially at high speeds. The efficiency of a jet engine depends on many factors, including how well the air and gases flow through the engine and how much fuel it uses to make thrust. Engineers work to make engines more efficient by smoothing out airflow and reducing energy losses.
| ṁ e | = the mass rate of hot combustion exhaust flow from the core engine |
| ṁo | = the mass rate of total air flow entering the turbofan = ṁc + ṁf |
| ṁc | = the mass rate of intake air that flows to the core engine |
| ṁf | = the mass rate of intake air that bypasses the core engine |
| vf | = the velocity of the air flow bypassed around the core engine |
| vhe | = the velocity of the hot exhaust gas from the core engine |
| vo | = the velocity of the total air intake = the true airspeed of the aircraft |
| BPR | = Bypass Ratio |
Operation
In a jet engine, different parts spin very fast, and special gauges help monitor how fast they are going. For example, some engines have gauges labeled N1 to watch the front part, and N2 to watch another part. These gauges show the speed as a percentage instead of actual rotations per minute, making it easier to understand.
Images
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