An airbreathing jet engine (or ducted jet engine) is a jet engine in which the exhaust gas which supplies jet propulsion is atmospheric air, which is taken in, compressed, heated, and expanded back to atmospheric pressure through a propelling nozzle. Compression may be provided by a gas turbine, as in the original turbojet and newer turbofan, or arise solely from the ram pressure of the vehicle's velocity, as with the ramjet and pulsejet.
All practical airbreathing jet engines heat the air by burning fuel. Alternatively a heat exchanger may be used, as in a nuclear-powered jet engine. Most modern jet engines are turbofans, which are more fuel efficient than turbojets because the thrust supplied by the gas turbine is augmented by bypass air passing through a ducted fan.
Background The original air-breathing gas turbine jet engine was the turbojet. It was a concept brought to life by two engineers, Frank Whittle in England UK and Hans von Ohain in Germany. The turbojet 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 turbojets that include a new section called the fan stage. Rather than using all their exhaust gases to provide direct thrust like a turbojet, turbofan engines extract some of the power from the exhaust gases inside the engine and use 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, because they have a smaller frontal area which creates less ram drag at supersonic speeds leaving more of the thrust produced by the engine to propel the aircraft. 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.
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 75,000 lbs per engine in engines such as the Rolls-Royce Trent XWB or General Electric GENx), have allowed a move to large twin engine aircraft, such as the Airbus A350 or 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 jet engines: turbojet turbofan
Ram powered jet engine: ramjet scramjet
Pulsed combustion jet engine: pulse detonation engine pulse jet engine motorjet
Turbojet engine
Two engineers, Frank Whittle in the UK and Hans von Ohain in Germany, developed the turbojet concept independently into practical engines during the late 1930s.
Turbojets consist of an inlet, a compressor, a combustor, a turbine (that drives the compressor) and a propelling nozzle. The compressed air is heated in the combustor and passes through the turbine, then expands in the nozzle to produce a high speed propelling jet
Turbojets have a low propulsive efficiency below about Mach 2 and produce a lot of jet noise, both a result of the very high velocity of the exhaust. Modern jet propelled aircraft are powered by turbofans. These engines, with their lower exhaust velocities, produce less jet noise and use less fuel. Turbojets are still used to power medium range cruise missiles due to their high exhaust speed, low frontal area, which reduces drag, and relative simplicity, which reduces cost.
Turbofan engine
Most modern jet engines are turbofans. The low pressure compressor (LPC), usually known as a fan, compresses air into a bypass duct whilst its inner portion supercharges the core compressor. The fan is often an integral part of a multi-stage core LPC. The bypass airflow either passes to a separate 'cold nozzle' or mixes with low pressure turbine exhaust gases, before expanding through a 'mixed flow nozzle'.
In the 1960s there was little difference between civil and military jet engines, apart from the use of afterburning in some (supersonic) applications. Today, turbofans are used for airliners because they have an exhaust speed that is better matched to the subsonic flight speed of the airliner. At airliner flight speeds, the exhaust speed from a turbojet engine is excessively high and wastes energy. The lower exhaust speed from a turbofan gives better fuel consumption. The increased airflow from the fan gives higher thrust at low speeds. The lower exhaust speed also gives much lower jet noise.
The comparatively large frontal fan has several effects. Compared to a turbojet of identical thrust, a turbofan has a much larger air mass flow rate and the flow through the bypass duct generates a significant fraction of the thrust. The additional duct air has not been ignited, which gives it a slow speed, but no extra fuel is needed to provide this thrust. Instead, the energy is taken from the central core, which gives it also a reduced exhaust speed. The average velocity of the mixed exhaust air is thus reduced (low specific thrust) which is less wasteful of energy but reduces the top speed. Overall, a turbofan can be much more fuel efficient and quieter, and it turns out that the fan also allows greater net thrust to be available at slow speeds.
Thus civil turbofans today have a low exhaust speed (low specific thrust – net thrust divided by airflow) to keep jet noise to a minimum and to improve fuel efficiency. Consequently, the bypass ratio (bypass flow divided by core flow) is relatively high (ratios from 4:1 up to 8:1 are common), with the Rolls-Royce Trent XWB approaching 10:1. Only a single fan stage is required, because a low specific thrust implies a low fan pressure ratio.
Turbofans in civilian aircraft usually have a pronounced large front area to accommodate a very large fan, as their design involves a much larger mass of air bypassing the core so they can benefit from these effects, while in military aircraft, where noise and efficiency are less important compared to performance and drag, a smaller amount of air typically bypasses the core. Turbofans designed for subsonic civilian aircraft also usually have a just a single front fan, because their additional thrust is generated by a large additional mass of air which is only moderately compressed, rather than a smaller amount of air which is greatly compressed.
Military turbofans, however, have a relatively high specific thrust, to maximize the thrust for a given frontal area, jet noise being of less concern in military uses relative to civil uses. Multistage fans are normally needed to reach the relatively high fan pressure ratio needed for high specific thrust. Although high turbine inlet temperatures are often employed, the bypass ratio tends to be low, usually significantly less than 2.0.
Turboprop and turboshaft
Turboprop engines are jet engine derivatives, still gas turbines, that extract work from the hot-exhaust jet to turn a rotating shaft, which is then used to produce thrust by some other means. While not strictly jet engines in that they rely on an auxiliary mechanism to produce thrust, turboprops are very similar to other turbine-based jet engines, and are often described as such.
In turboprop engines, a portion of the engine's thrust is produced by spinning a propeller, rather than relying solely on high-speed jet exhaust. Producing thrust both ways, turboprops are occasionally referred to as a type of hybrid jet engine. They differ from turbofans in that a traditional propeller, rather than a ducted fan, provides the majority of thrust. Most turboprops use gear-reduction between the turbine and the propeller. (Geared turbofans also feature gear reduction, but they are less common.) The hot-jet exhaust is an important minority of thrust, and maximum thrust is obtained by matching the two thrust contributions. Turboprops generally have better performance than turbojets or turbofans at low speeds where propeller efficiency is high, but become increasingly noisy and inefficient at high speeds.
Turboshaft engines are very similar to turboprops, differing in that nearly all energy in the exhaust is extracted to spin the rotating shaft, which is used to power machinery rather than a propeller, they therefore generate little to no jet thrust and are often used to power helicopters.Mattingly, p. 12
Propfan
A propfan engine (also called a "propjet", "unducted fan", "open rotor", or "ultra-high bypass") is a jet engine that uses its gas generator to power an exposed fan, similar to turboprop engines. Like turboprop engines, propfans generate most of their thrust from the propeller and not the exhaust jet. The primary difference between turboprop and propfan design is that the propeller blades on a propfan are highly swept to allow them to operate at speeds around Mach 0.8, which is competitive with modern commercial turbofans. These engines have the fuel efficiency advantages of turboprops with the performance capability of commercial turbofans.Sweetman, Bill (2005). The Short, Happy Life of the Prop-fan . Air & Space/Smithsonian. 1 September 2005. While significant research and testing (including flight testing) has been conducted on propfans, none have entered production.
Major components
Major components of a turbojet including references to turbofans, turboprops and turboshafts:
Cold Section