A turboprop engine and a turbofan engine both have a compressor, a combustor, and two turbines.
In the turboprop engine,
1. Air enters the engine and passes into the compressor.
2. The compressor increases the density of the air, which passes into the combustor.
3. In the combustor, fuel mixes with the air, and the fuel-air mixture is ignited.
4. The combustion gases expand rapidly to the rear, rotating both the turbines.
5. The first turbine is connected to, and rotates, the compressor rotor. As a result, once combustion occurs, the engine continues to run until it is shut down or it runs out of fuel.
6. The second turbine is connected to a shaft that passes through the center of the engine to a gearbox.
7. The rotational energy of the turbine shaft passes through the gears within the gearbox to rotate the propeller, which powers the aircraft. The gears reduce the speed of rotation of the turbine shaft to a level that can be used by the propeller.
In the turbofan engine,
1. A fan is added at the front of the engine. The fan draws in air, a small portion of which passes into the compressor. The greater portion of the air, however, bypasses the engine by flowing through a large duct that surrounds the engine.
2. The compressor increases the density of the air, which passes into the combustor.
3. In the combustor, fuel mixes with the air, and the fuel-air mixture is ignited.
4. The combustion gases pass rapidly to the rear, rotating both the turbines.
5. The first turbine is connected to, and rotates, the compressor rotor. As a result, once combustion occurs, the engine continues to run until it is shut down or it runs out of fuel.
6. The second turbine is connected to a shaft that passes through the center of the engine to the drive the fan to continue to draw in air.
The thrust produced by a turbofan engine is the product of the mass of air/combustion gases displaced times the rate of acceleration of the air/combustion gases. The rapid expansion of the combustion gases to the rear also accelerates the displacement of the "bypass" air. To produce thrust.
If the bypass ratio of a turbofan engine is 9 to 1, 90% of the thrust is attributable to the bypass air. However, the vital acceleration is provided by the expansion of the combustion gases.
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