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An airbreathing jet engine (or ''ducted 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),〔"Gas Turbine Technology Evolution: A Designer's Perspective" Bernard L.Koff Journal of Propulsion and Power Vol20 No4 July–August 2004 Fig.34/41〕 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. The original air-breathing gas turbine jet engine was the turbojet, which compresses and heats air and then exhausts it as a high speed, high temperature jet to create thrust. While these engines are capable of giving high thrust levels, they are most efficient at very high speeds (over Mach 1), due to the low-mass-flow, high speed nature of the jet exhaust. Modern turbofans are a development of the turbojet; they are basically a turbojet that includes a new section called the ''fan stage''. Rather than using all of its exhaust gases to provide direct thrust like a turbojet, the turbofan engine extracts some of the power from the exhaust gases inside the engine and uses it to power the fan stage. The fan stage accelerates a large volume of air through a duct, bypassing the ''engine core'' (the actual gas turbine component of the engine), and expelling it at the rear as a jet, creating thrust. A proportion of the air that comes through the fan stage enters the engine core rather than being ducted to the rear, and is thus compressed and heated; some of the energy is extracted to power the compressors and fans, while the remainder is exhausted at the rear. This high-speed, hot-gas exhaust blends with the low speed, cool-air exhaust from the fan stage, and both contribute to the overall thrust of the engine. Depending on what proportion of cool air is bypassed around the engine core, a turbofan can be called ''low-bypass'', ''high-bypass'', or ''very-high-bypass'' engines. ''Low bypass'' engines were the first turbofan engines produced, and provide the majority of their thrust from the hot core exhaust gases, while the fan stage only supplements this. These engines are still commonly seen on military fighter aircraft, since they provide more efficient thrust at supersonic speeds and have a narrower frontal area, minimizing aerodynamic drag. Their comparatively high noise levels and subsonic fuel consumption are deemed acceptable in such an application, whereas although the first generation of turbofan airliners used low-bypass engines, their high noise levels and fuel consumption mean they have fallen out of favor for large aircraft. ''High bypass'' engines have a much larger fan stage, and provide most of their thrust from the ducted air of the fan; the engine core provides power to the fan stage, and only a proportion of the overall thrust comes from the engine core exhaust stream. A high-bypass turbofan functions very similarly to a turboprop engine, except it uses a many-bladed ''fan'' rather than a multi-blade propeller, and relies on a duct to properly vector the airflow to create thrust. Over the last several decades, there has been a move towards ''very high bypass'' engines, which use fans far larger than the engine core itself, which is typically a modern, high efficiency two or three-spool design. This high efficiency and power is what allows such large fans to be viable, and the increased thrust available (up to 100,000lbs per engine in engines such as the General Electric GENx), have allowed a move to large twin engine aircraft, such as the Boeing 777, as well as allowing twin engine aircraft to operate on long overwater routes, previously the domain of 3-engine or 4-engine aircraft. Jet engines were designed to power aircraft, but have been used to power jet cars and jet boats for speed record attempts, and even for commercial uses such as by railroads for clearing snow and ice from switches in railyards (mounted in special rail cars), and by race tracks for drying off track surfaces after rain (mounted in special trucks with the jet exhaust blowing onto the track surface). ==Types of airbreathing jet engines== Airbreathing jet engines are nearly always internal combustion engines that obtain propulsion from the combustion of fuel inside the engine. Oxygen present in the atmosphere is used to oxidise a fuel source, typically a hydrocarbon-based jet fuel. The burning mixture expands greatly in volume, driving heated air through a propelling nozzle. Gas turbine powered engines: *turbojet *turbofan Ram powered jet engine: *ramjet *scramjet Pulsed combustion jet engine: *pulse detonation engine *pulse jet engine *motorjet 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Airbreathing jet engine」の詳細全文を読む スポンサード リンク
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