SOME TYPES OF JET ENGINES

 JET ENGINES

                Jet engines date back to the invention of the aeolipile before the first century AD. This device directed steam power through two nozzles to cause a sphere to spin rapidly on its axis. So far as is known, it did not supply mechanical power and the potential practical applications of this invention did not receive recognition. Instead, it was seen as a curiosity.

                Jet propulsion only took off, literally and figuratively, with the invention of the gunpowder-powered rocket by the Chinese in the 13th century as a type of fireworks, and gradually progressed to propel formidable weaponry. However, although very powerful, at reasonable flight speeds rockets are very inefficient and so jet propulsion technology stalled for hundreds of years.

                A jet engine is a reaction engine discharging a fast moving jet that generates thrust by jet propulsion in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, and pulse jets. In general, jet engines are combustion engines but non-combusting forms also exist. The term jet engine loosely refers to an internal combustion air breathing jet engine. These typically consist of an engine with a rotary (rotating) air compressor powered by a turbine ("Brayton cycle"), with the leftover power providing thrust via a propelling nozzle. Jet aircraft use these types of engines for long-distance travel. Early jet aircraft used turbojet engines which were relatively inefficient for subsonic flight. Modern subsonic jet aircraft usually use high-bypass turbofan engines. These engines offer high speed and greater fuel efficiency than piston and propeller aero engines over long distances.

APPLICATIONS

i. Jet engines power aircraft, cruise missiles and unmanned aerial vehicles.

ii. In the form of rocket engines they power  model rocketry, spaceflight, and military missiles.

iii. Jet engines have propelled high speed cars, particularly drag racers, with the all-time record held by a rocket car.

iv. Jet engine designs are frequently modified for non-aircraft applications, as industrial gas turbines or marine power plants.

v. These are used in electrical power generation, for powering water, natural gas, or oil pumps, and providing propulsion for ships and locomotives. Industrial gas turbines can create up to 50,000 shaft horsepower.

vi. Jet engines are also sometimes developed into, or share certain components such as engine cores, with turbo shaft and turboprop engines, which are forms of gas turbine engines that are typically used to power helicopters and some propeller-driven aircraft.

VARIOUS TYPES OF JET ENGINES

Water jet

For propelling boats; squirts water out the back through a nozzle.

Motor jet

Most primitive air breathing jet engine. Essentially a supercharged piston engine with a jet exhaust. Higher exhaust velocity than a propeller, offering better thrust at high speed.

Turbojet

A tube with a compressor and turbine sharing a common shaft with a burner in between and a propelling nozzle for the exhaust. Uses a high exhaust gas velocity to produce thrust. Has a much higher core flow than bypass type engines. Simplicity of design, efficient at supersonic speeds.

Turbofan

A turbofan is a type of jet engine, similar to a turbojet. It essentially consists of a large ducted fan with a smaller diameter turbojet engine mounted behind it that provides propulsion and also powers the fan. Part of the airstream from the ducted fan passes through the turbojet, providing oxygen to burn fuel to power the turbojet. But part, usually most, of the flow bypasses the turbojet, and is accelerated by turbine blades acting like a propeller. The combination of these two processes produces thrust more efficiently than other jet designs. Turbofans have a net exhaust speed that is much lower than a turbojet. This makes them much more efficient at subsonic speeds than turbojets, and somewhat more efficient at supersonic speeds up to roughly Mach 1.6.

All of the jet engines used in currently manufactured commercial jet aircraft are turbofans. They are used commercially mainly because they are highly efficient and relatively quiet in operation. Turbofans are also used in many military jet aircraft.

Low-bypass Turbofan

One- or two-stage fan added in front bypasses a proportion of the air through a bypass chamber surrounding the core. Compared with its turbojet ancestor, this allows for more efficient operation with somewhat less noise. This is the engine of high-speed military aircraft, some smaller private jets, and older civilian airliners such as the Boeing 707, the McDonnell Douglas DC-8, and their derivatives.

High-bypass Turbofan

First stage compressor drastically enlarged to provide bypass airflow around engine core, and it provides significant amounts of thrust. Compared to the low-bypass turbofan and no-bypass turbojet, the high-bypass turbofan works on the principle of moving a great deal of air somewhat faster, rather than a small amount extremely fast. Most common form of jet engine in civilian use today- used in airliners like the Boeing 747, most 737s, and all Airbus aircraft.

Scramjet

Similar to a ramjet without a diffuser; airflow through the entire engine remains supersonic. Few mechanical parts, can operate at very high Mach numbers (Mach 8 to 15) with good efficiencies.

Ramjet

Intake air is compressed entirely by speed of oncoming air and divergent shape, and then it goes through a burner section where it is heated and then passes through a propelling nozzle. Very few moving parts, Mach 0.8 to Mach 5+, efficient at high speed (> Mach 2.0 or so), lightest of all air-breathing jets (thrust / weight ratio up to 30 at optimum speed), cooling much easier than turbojets as no turbine blades to cool.

Pulsejet

Air is compressed and combusted intermittently instead of continuously. Some designs use valves. Very simple design, commonly used on model aircraft.

Pulse detonation engine

Similar to a pulsejet, but combustion occurs as a detonation instead of a deflagration, may or may not need valves.

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 10.0 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 exo-atmospheric which is kinder on vehicle structure at high speed, fairly small surface area to keep cool, and no turbine in hot exhaust stream

Air-augmented rocket

Essentially a ramjet where intake air is compressed and burnt with the exhaust from a rocket.

Turbo-rocket

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.

Pre-cooled jets

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.