modern jet engine

The Modern Jet Engine

Tim Takeoff
04.01.2019
2 pictures
6 minutes

A commercial jet engine is a wonder of technology. Over time, engines in the aviation industry have become quieter, more efficient and, of course, more powerful. But how does this kind of engine generally work? Has it actually got an “ignition key”?

The second question is quicker to explain than the first one: of course, there is no “ignition key”. In principle, starting an engine is highly complex. In the past, it had to be started by a flight engineer and many parameters need to be monitored so that malfunctions can be identified immediately.

The FADEC

Strictly speaking, a so-called “full authority digital engine control” unit, FADEC for short, controls most processes in a modern aircraft. It replaces the control functions and responds fully automatically to almost any situation. Modern-day cockpits often have just a switch to start the engine and control the supply of aviation fuel, while all other parameters are entered or monitored indirectly via the on-board computer. The previously calculated take-off thrust and performance settings for the cruising flight are entered here. Whether you want to save time or fuel – the computer and FADEC take care of it!

But how does this kind of engine work?

An engine is made up of different sections. If you start at the front, you see the large blades – the fan – with the naked eye. This generates around 90 percent of the overall thrust in modern aircraft. This ratio is referred to as “bypass ratio”, which we have already reported on in this blog article. If you look a little deeper you discover that the engine is divided into two sections. You can “look right through” from front to back on the outer “casing” because the fan pushes the air on an unimpeded path into the “open air”. You can even see the first stages of the compression of the compressor in the inner section. What’s more difficult to determine from the outside is that this compressor has many more stages. One stage consists of a rotating impeller, the rotor and a stationary wheel, the stator.

Just hot air?

The compressor continuously compresses the intake air (like a turbocharger in a car) until it has reached a high base temperature. High pressure generates heat according to the principles of physics. This hot air now reaches the combustion chamber and the aviation fuel comes into play here in addition to the air. It is injected into the combustion chamber through several “fuel nozzles” where it can self-ignite and continue to run if the temperatures are sufficiently high. The explosion and expansion of the gases now taking place is discharged in a controlled manner through multiple blades in different stages of the “turbine”. These powered blades in turn drive the fan right at the front, and the compressor, through an internal shaft. The fan forms the first compressor stage, so to speak.null

But what comes first, air or fuel? The start-up procedure.

How is this kind of engine started? Of course, fuel is burned in the final stage but it has to reach a certain speed before the combustion reaction can take place. Compressed air, so-called “bleed air”, is used to start the engine rotating from a standstill. This can be supplied externally via an “air-start system” but as power still needs to be supplied to the electricity and air conditioning system in an aircraft while it is on the ground, an auxiliary turbine is used, the “auxiliary power unit” – APU. This is permanently installed in the rear fuselage and can be operated completely independently.

“Cleared for engine start!”

As soon as the pilots are cleared for engine start, the FADEC releases compressed air from the APU to one of the engines. This is sufficient to get the compressor to a minimum speed of approximately 20 percent and fuel is fed into the turbine once the speed has been reached. As the temperature isn’t quite sufficient to initiate self-ignition (the autoignition temperature of aviation fuel is around 220 degrees Celsius), spark plugs are arranged around the combustion chamber. They generate a spark that ignites the air-fuel mixture and the turbine now drives the fan and compressor through a shaft, as described above. The exhaust gas temperatures begin to rise and the engine will now keep “running” as long as there is a supply of aviation fuel. The igniters are switched off by the FADEC once the exhaust gas reaches a certain temperature because they are no longer needed.

“Bleed air”

All other engines are “fired up” using the one engine that is already running and air is “drawn off” the compressor. This intake air, also known as “bleed air”, then additionally powers the air conditioning system in the aircraft. The pilots switch off the APU once the engines are started and it is no longer needed. By contrast, stopping the engine is easier: the fuel supply is simply switched off and the engine then slowly returns to its idle state. If you listen carefully the next time you take a flight, you can clearly hear the point at which an engine is “fired up”. There is suddenly a loud humming noise after the engine purrs into action and slowly starts up. It can also be heard clearly in this video:

Thick fuel lines …

Currently the largest engine in the world, the General Electric GE90, mounted on the Boeing 777, generates 52 tons of thrust on each side. The aviation fuel lines to the engine are so powerful that approximately 5 litres of fuel can pass through the pipe every second during take-off at full throttle. The fan diameter is over 3 metres so the whole fuselage of a Boeing 737 would fit through it.

… and expensive materials

The blades are made of special materials so as to maximise their efficiency and the fan itself consists of a composite structure made of carbon fibre and titanium. The compressor and turbine are subjected to high temperatures so ceramic structures are primarily used here. Each individual blade, whether in the fan or in the rotor, is ventilated from the inside, ensuring that the distance to the outer casing can always be adjusted very precisely despite the extreme forces at work. A wonder of technology! This of course has its price as a single compressor blade in the fan of the GE90 engine costs approximately 80,000 euros.

Downsizing is bang on trend

As engines are becoming more reliable, the popular four-engine aircraft are slowly disappearing from the market, such as the Boeing 747 or the Airbus A340. Modern two-engine aircraft offer similar reliability and are much more economical. Specific examples of these include the Boeing 787 Dreamliner, the new Boeing 777X and the Airbus A350 or A330 Neo and they will replace all the older models on the long-haul routes in the coming years. Only the Airbus A380 holds its own, particularly on routes with extremely high capacities.

Aircraft with gears?

The focus is on modern technology even in the short and medium-haul segment. The Airbus A320 Neo and the Boeing 737 Max already have the latest geared turbofan technology where the shaft used to connect the fan to the compressor has a gear transmission just like in a car. This allows the fan to rotate slower than the compressor and still pushes through a tremendous volume of air so you can literally fly in a “higher gear” fully automatically. Practical, don’t you think?

You can find out more about how the modern jet engine works in this video (in English):

by Tim Takeoff

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