How a turbofan engine works
The turbofan engine is a type inherited from the turbojet engine, with increased performance, less noise and better fuel economy.
This kind of engine has a big fan in front of it and the air coming into the engine flows in two paths, the main path and the bypass path.
The bigger the bypass ratio, having a bigger fan, the less noise, lower fuel consumption, lower emissions and high performance is achieved. But the high bypass turbofan can´t get supersonic speeds that is why military aircraft use low bypass turbofans or turbojet engines.
• Low bypass ratio turbofan:
This type of engine has one to three small fans in front of it to produce enough thrust, the diameter of the engine is small, they generate moderate to high level of sound and are commonly found in these aircraft:
Pratt & Whitney JT8D used in DC-9, MD-80, Boeing 707, 727, 737-200 with bypass ratio of 0.96:1
Rolls Roice RB183 Tay used in Fokker 100 and Gulfstream IV, with a bypass ratio of 3.1:1
• High bypass ratio turbofan:
This are new generation engines having a big fan in front of it driven by the last turbine stage shaft. Having a big fan reduce the noise and improves fuel consumption, making it the best choice of the airlines today for subsonic flights.
They are commonly found in these aircraft:
CFM56-5 Series used in Airbus A318, A319, A320, A321, A340 with a bypass ratio of 5.4:1 to 6.6:1
CFM56-7 Series used in Boeing 737-600/-700/-800/-900 with a bypass ratio of 5.1:1 to 5.3:1
GEnx-1B70 used in Boeing 787-8/-9/-10 with a bypass ratio of 9.6:1
GEnx-2B67 used in Boeing 747-8 with a bypass ratio of 8.6:1
Rolls Roice Trent XWB used in Airbus A350 with a bypass ratio of 9.3:1
Pratt & Whitney PW1127G used in Airbus A320neo with a bypass ratio of 12:1
Pratt & Whitney PW1521G used in Airbus Bombardier CSeries with a bypass ratio of 12:1
Schematic of a turbofan engine
The fan is in front of the engine. It is where the propulsion begins. The air flow impacting the fan diverts in two paths : primary and secondary or bypass air. The primary air flow goes through the compressor stage to the combustion chambers.
The bypass air is pushed by the fan being the main thrust of the engine.
The primary air flow goes through the compressor stages turning in the same way the fan does. Some designs include low compressor stage and high compressor stage in the same core.
The function of the compressor is to raise the pressure and temperature of the air before entering the combustion chamber. The compression ratio commonly used in new turbofan engines is about 30.
Pratt & Whitney PW 2037 High Pressure Compressor rotor module costing aprox. 250,000.00 USD
• Combustion chamber:
The high pressure and high temperature air reaching 600 ºC coming from the compressor stage enters the combustion chambers.
Each combustion chamber has one fuel injector and in an alternate pattern there is a glow plug too, for starting the engine
When the fuel burns mixed with the compressed air it reaches 1000 ºC.
The high temperature burned mixture getting out of the combustion chamber enters the turbine stage, pushing the blades and making a rotational movement of the multiple turbine stages.
In the low bypass type turbofans there is a single shaft joining the turbine, compressor and fan stages. In most advanced high bypass ratio type turbofans there is a shaft for low pressure turbine and compressor stage, high pressure turbine and high pressure compressor stage and fan, turning at different speed and in opposite direction to reduce vibration.
the hot air getting out of the turbine stage passes through the exhaust, squeezing the air flow and accelerating it, for increased thrust.
The bigger the bypass ratio, the lower the exhaust is putting thrust to the total thrust of the engine, being the bypass air pushed by the fan the main thrust component.
Air flow animation
This is a very good animation showing the components of a turbofan engine, how they spin and the air flow.
The Trent 500 is used in Airbus A340-500 & A340-600
In this video you can see the HP compressor joined with the HP turbine by a shaft, and the fan, LP compressor and LP turbine in another shaft turning in the opposite direction.
The Pratt & Whitney JT8D is used in the Boeing 727, 737-100, DC-9 and MD-80´s
The JT8D is a low bypass ratio turbofan
The Spinner in a CFM56-3 turbofan engine is the black cone with a white stripe mounted in the center of the front fan and you can see in the upper left there is a sensor for temperature measurement for the computers (FADEC) to adjust the air fuel ratio.
The APU delivers the compressed air to drive the pneumatic starter motor, making the internal components spin and begin the combustion cycle, as illustrated in this CFM56 engine.
The CFM LEAP family represents the engines of choice for the next-generation single-aisle aircraft. The LEAP-1A is an option on the Airbus A320neo; the LEAP-1B is the exclusive powerplant for the Boeing 737 MAX; and the LEAP-1C is the sole Western powerplant for the COMAC C919. These engines had garnered more than 8,000 operates from more than 50 customers across the globe..
The LEAP-X1C by CFM International will be used in the C919 in China
Rolls Royce Trent 892 used in the Boeing 777
Reverse Thrust Systems
Almost all turbofan engines used in commercial aircraft has Reverse Thrust Systems installed for helping brake the aircraft in the landing procedure.
The system works directing forward, rather than backwards, the exhaust gases by a system of deflectors, and there are different kind of systems.
Clam-shell, Bucket and Cold-Air are the common Reverse Thrust Systems used in modern turbofan engines.
Bucket type doors used in Boeing 737-200
Bucket type doors used in Fokker70
Cold air system used in Boeing 747
Turbofan Pratt & Whitney PW4090 used in the Boeing 777 costs US$10 to US$12 million.
Pratt & Whitney PurePower 1000G is a geared turbofan engine. In a geared turbofan, a (planetary) reduction gearbox between the fan and the LP shaft allows the latter to run at a higher rotational speed thus enabling fewer stages to be used in both the LP turbine and the LP compressor, increasing efficiency and reducing weight.
The PW1000G architecture comprises one fan upstream, followed by a reduction gearbox that allows the low pressure compressors to run faster than the fan, two low pressure compressor stages, 8 high pressure compressor stages, 2 high pressure turbines (to keep the high pressure compressors running) and 3 low pressure turbines that keep the low pressure compressors and the fan running. A listing of the number of bladed disks is referred to as the “stage count”, and in the case of the PW1000G family of engines is:
1-G-3-8-2-3, with the “G” referring to the gear system that links the fan(1 bladed disk) with the low pressure compressor (3 bladed disks).
Gearbox for reduce the rotation of the fan.
Behind the big fan is the engine core
Assembling a turbofan engine
Regular engine washing has been found to reduce fuel consumption by up to 1.0% a year. This means thousands of dollars for small operators and millions of dollars for major airlines. Engine washing would also increase EGT margin which in turn would make the engine and gaspath more efficient in terms of producing more power for the same or lower amount of fuel consumption.