Airbreathing jet engine
[2] Compression may be provided by a gas turbine, as in the original turbojet and newer turbofan,[3][4] or arise solely from the ram pressure of the vehicle's velocity, as with the ramjet and pulsejet.
[6] 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.
[3] It was a concept brought to life by two engineers, Frank Whittle in England UK and Hans von Ohain in Germany.
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.
[1] The burning mixture expands greatly in volume, driving heated air through 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[3] Turbojets have a low propulsive efficiency below about Mach 2[citation needed] and produce a lot of jet noise, both a result of the very high velocity of the exhaust.
The additional duct air has not been ignited, which gives it a slow speed, but no extra fuel is needed to provide this thrust.
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.
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.
Although high turbine inlet temperatures are often employed, the bypass ratio tends to be low, usually significantly less than 2.0.
Producing thrust both ways, turboprops are occasionally referred to as a type of hybrid jet engine.
It is named after George Brayton (1830–1892), the American engineer who developed it, although it was originally proposed and patented by Englishman John Barber in 1791.
Initially as the aircraft gains speed down the runway, there will be little increase in nozzle pressure and temperature, because the ram rise in the intake is very small.
Although the penalty is zero at static conditions, it rapidly increases with flight speed, causing the net thrust to be eroded.
Above Mach 1.0, with a subsonic inlet design, shock losses tend to decrease net thrust, however a suitably designed supersonic inlet can give a lower reduction in intake pressure recovery, allowing net thrust to continue to climb in the supersonic regime.
In 1988 an Ethiopian Airlines Boeing 737 ingested pigeons into both engines during take-off and then crashed in an attempt to return to the Bahir Dar airport; of the 104 people aboard, 35 died and 21 were injured.
[16] Jet engines have to be designed to withstand the ingestion of birds of a specified weight and number, and to not lose more than a specified amount of thrust.
The weight and numbers of birds that can be ingested without hazarding the safe flight of the aircraft are related to the engine intake area.
The plane ditched in the Hudson River after taking off from LaGuardia International Airport in New York City.
The outcome of an ingestion event and whether it causes an accident, be it on a small fast plane, such as military jet fighters, or a large transport, depends on the number and weight of birds and where they strike the fan blade span or the nose cone.
If a jet plane is flying through air contaminated with volcanic ash, there is risk that ingested ash will cause erosion damage to the compressor blades, blockage of fuel nozzle air holes and blockage of the turbine cooling passages.
It was the case of British Airways Flight 9 which flew through volcanic dust at 37,000 ft. All 4 engines flamed out and re-light attempts were successful at about 13,000 ft.[18] One class of failure that has caused accidents is the uncontained failure, where rotating parts of the engine break off and exit through the case.
Prior to the United 232 crash, the probability of a simultaneous failure of all three hydraulic systems was considered as high as a billion-to-one.
believe that jet engines are also a source of global dimming due to the water vapour in the exhaust causing cloud formations.
At low altitudes this is not thought to be especially harmful, but for supersonic aircraft that fly in the stratosphere some destruction of ozone may occur.
They consist of three sections; an inlet to compress incoming air, a combustor to inject and combust fuel, and a nozzle to expel the hot gases and produce thrust.
[29] The bleed flow, 20% at Mach 3, was returned to the engine via 6 external tubes to cool the afterburner liner and primary nozzle as well as to provide extra air for combustion.
Finally, pure hydrogen is not found in nature, and must be manufactured either via steam reforming or expensive electrolysis.
The idea is also being researched by the EU for a concept to achieve non-stop antipodal supersonic passenger travel at Mach 5 (Reaction Engines A2).
This user interface decision has been made as a human factors consideration, since pilots are more likely to notice a problem with a two- or 3-digit percentage (where 100% implies a nominal value) than with a 5-digit RPM.
