Thousands of passenger planes take to the skies every day. Many of them are airliners from manufacturers such as Boeing, Airbus, Bombardier, and Embraer. A turbofan, which is a special kind of engine, powers most of these aircraft. This turbine propulsion system uses combustion to turn a ducted fan to generate thrust. This is a complex piece of machinery, and even though this technology advances every year, the basic principles remain the same.
There are four stages to the turbofan’s functionality: Intake, compression, combustion, exhaust. Let us look at these in greater detail. First, a ducted fan ingests large volumes of air (1). Most of this air stream bypasses around the central core and the rest enters the low-pressure compressor. Rotating blades spin against stationary blades and compress the incoming air (2). The second area is a high-pressure compressor where the highest elevation of pressure and temperature occurs (3). The air here becomes about 1000 degrees hotter than the ingested air and is now at the desired pressure. A diffuser slows the air down directly after compression (4). The air then enters the combustion chamber to be mixed with fuel and ignited (5). This exhaust moves into the high-pressure turbine and rotates it (6). Next, the air passes through the larger low-pressure turbine (7). The air is then accelerated one last time as it exists the exhaust nozzle and accounts for some of the thrust (8).
There are two major component sections of the turbofan, the low spool, and the high spool. The low spool is the ducted fan, low pressure compressor, and the low-pressure turbine connected via a single shaft. The high spool is the high-pressure compressor and the high-pressure turbine connected via a single shaft that first around the low spool shaft. When the ignited air moves the high-pressure turbine, it in turn moves the high-pressure compressor. At the same time, as the exhaust gasses rotate the low-pressure turbine, this drives the low-pressure compressor and the main ducted intake fan.
When the air is initially ingested by the ducted fan, most of the air is bypassed away from the central core. Because of the high fan speeds and the narrowing area around the core, this bypass air generates majority of the thrust. It also provides cooling and plays an integral part in the fuel efficiency of the turbofan engine.
Starting a turbofan is more complicated than a traditional propeller. Not only does the main fan and compressors must spin at a certain RPM to ingest and compress air, but an even higher RPM must be achieved to be able to ignite and sustain a burn inside the combustion chamber for continuous self-sustaining operation. This means that something has to start the rotation of these components. An electric motor is usually not capable of this.
Modern airliners usually have Auxiliary Power Units that provide electricity, bleed air for cabin conditioning, and startup air for the turbofans. Once enough rotation is created via the APU, a single engine can be started. The air from the first operational turbofan can be piped as startup air for the second engine. This is why airliners start one engine at a time. A huffer cart is an option if the APU is not working and can provide the same startup air flow.
Modern turbofan engines are technological marvels and propel hundreds and thousands of aircraft all around the world on a daily basis. Despite the fact that very much engineering and design efforts go into the making of these turbofans, the principles of thermodynamics and physics remain the same.
A turbofan for a modern airliner is a complex and intricate piece of equipment. The basic principles of operation, however, have always remained the same. Ingest air with a large fan, bypass most of it, compress the leftovers, ignite fuel and air mixture, decompress and increase pressure, and exhaust as hot gas and large fan rotational power.
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