2024 Author: Howard Calhoun | [email protected]. Last modified: 2023-12-17 10:16
Surging is an extremely unpleasant phenomenon that can lead to complete destruction of the engine and death of people. He entered our lives along with the era of jet aircraft, which began in the 50-60s of the last century. Now this phenomenon no longer poses an absolutely fatal danger. Thanks to the accumulated experience and technical solutions, aircraft engine surge can be prevented in many cases, but this violation continues to occur today.
Engines of passenger liners
Contrary to popular belief, modern passenger liners are only 20-30 percent jet aircraft. That is how much thrust the reactive component of a modern turboprop engine gives. The remaining 70-80 percent is obtained from the thrust of the good old propeller rotated by the turbine. True, this propeller is completely different from the classic aircraft propellers of previous generations of aircraft. To understand the phenomenon of aircraft engine surge, it is necessary to get at least a minimal understanding of the structure of a modern aircraft engine. The engine diagram is shown in the picture.
The principle of operation of an aircraft engine
A modern engine, usually hanging under the wing in a beautiful engine nacelle fairing, is structurally a two-circuit system. The outer circuit is a fan, which actually operates on the principle of a classic aircraft propeller and is a combination of many blades, which are perfectly visible from the front of every modern aircraft. The second task of this fan is to take in air for the internal circuit of the engine. The latter is much more complicated and represents the following chain of units interacting with each other: the already mentioned fan pumps air into the compressor, which compresses it to the required values, then the heated and compressed air enters the combustion chamber, where it mixes with aviation kerosene and ignites.
Outgoing gases (combustion products) turn the blades of the turbine that rotates the fan (they are on the same shaft), after which the rest of the exhaust gases are ejected through the nozzle and add thrust, acting on the jet principle. If you remove the outer contour, you get a purely jet engine, and so it is a turboprop. Well, or turbofan - as you like more.
Of course, this description of the operation of the engine is extremely simplified. An aircraft engine is the most complex technical system. While there are many countries in the world that can design and manufacture aircraft, there are far fewer countries that have the technology to design and manufacture turboprops.
Surge phenomenon
The word pompage itself comes from French and means "pumping", "pumping". Its essence is in violation of the normal flow of air in the engine. Its longitudinal oscillations occur along the entire air path, causing an imbalance in the ratio of air and fuel in the combustion chamber. If the imbalance is in favor of fuel, the over-enriched mixture explodes; if the imbalance is in favor of air, combustion stops.
Outwardly, the surge of the aircraft engine looks like a chain of explosions, accompanied by strong vibration and the ejection of flame from the nozzle. The sequence of events in this process can be imagined as follows: lack of air, an excess of fuel in the combustion chamber, an explosion in the combustion chamber, a sharp acceleration of the turbine, a sharp jump in fan speed, an excess of air, cessation of combustion, a decrease in speed, again lack of air. The cycle is complete.
If all these events are repeated repeatedly, vibration and local explosive phenomena can lead to very sad consequences, including a jet plane crash.
Reasons
The reasons for the surge of the aircraft engine can be different. The most common is damage to the turbine blades due to wear or a foreign object entering the air intake. For example, a bird or a stone from the runway. It can also be caused by crew errors, overfueling at start-up, engine running at extreme limits, and atmospheric phenomena such as whirlwinds or excessivelow pressure in hot weather.
How to prevent and eliminate surge
Modern engines have sensors installed throughout the airways. Based on the readings of the sensors, the built-in automation immediately makes changes to the fuel supply mode and compressor parameters. In the engine itself, instead of one, two or even three shafts are used to ensure its stable operation in the event of surge phenomena, disconnecting the direct connection between the fan and the turbine.
Turbine surge, if it occurs in level flight, is eliminated by simply temporarily turning off the engine or reducing its speed, after which it is possible to carry out the so-called "cold purge", that is, to blow out excess fuel from the combustion chamber with an oncoming air flow.
The most dangerous thing is the surge on takeoff, when the decision point is passed. However, modern airliners are able to continue taking off even with one failed engine.
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