According to relevant sources, a turboprop engine is a turbine that drives a propeller via a reduction gear as shown in the above diagram (NASA 4). As such, the exhaust gases drive the power shaft, which in turn drives the reduction gear assembly through a shaft as shown (Husain 98; Sickle 205). The reduction gearing is essential because optimum propeller performances in these engine designs are reached at slower speeds than the engines’ operating revolution per minute. At slow airspeeds, these engine designs are fuel efficient and operate effectively (FOPPGS 1).
Turbofan engines were developed to incorporate some of the best features that exist in the already mentioned engines (FOPPGS 1). For example, these engines have been designed to generate additional thrust by directing a secondary airflow around the combustion chamber (NASA 4). Additionally, the bypass helps in cooling the engine, as well as helps in reducing exhaust noises. “In a turbofan engine, therefore, the bypass ratio refers to the ratio of the mass airflow passing through the fan divided by the mass airflow passing through the engine core” (FOPPGS 2).
However, part of the inlet airflow is not directed toward the compressor, combustor, and turbine, but is rather bypassed through a duct, which ends in a nozzle. Since air leaves the nozzle at a speed that is higher than the intake velocity, thrust is produced by momentum exchange with the airframe. During the intake phase, the pressure, temperature and volume of the gases remain constant.
The compressor is an arrangement of blades on a rotating disk, whose main function is to force air to flow into the engine as it reduces its volume and increases its pressure. Torque is required to change the momentum of the working fluid, forcing it to follow the curved surface of the blades. The work required to drive the compressor comes from the engine itself, by means of a shaft connecting the compressor and the turbine.