The propulsion system widely used by transport planes and small subsonic aircrafts running in low speed are usually turboprop engines. Turboprop engines are composed of an intake fan, compressor, combustor, turbine and nozzle. One distinct factor that makes this engine different from other jet engines is that it uses most of its generated energy to run the turbine that would provide the needed energy to run the propeller. The propeller and the engine both help in the aircraft's thrust but it is the propeller that provides most of the said action.
Jet engines in general work in such a way that it applies Newton's third law of motion which states that for every action, there is an equal and opposite reaction. The air that is compressed by the compressors would be sprayed with fuel and ignited in the combustion chamber. The force created would then be reciprocated by the force exerted by the air as it escapes through the rear portions of the engine, thus, it would induce forward thrust as it escapes. A better example of this same phenomenon is when a balloon's air is released though its inflating hole wherein the escaping air would force the balloon to loft to the opposite direction.
The turboprop engine's propulsion system is mainly composed of two parts - the engine core and the propeller. The difference of turboprop's engine, as stated earlier, is that most of the energy produced during gas combustion is used to run the turbine than to utilize it to produce thrust as what most turbojet engines do. Since the engine provides lesser thrust due to more energy is consumed to run the drive shaft which is connected to an additional turbine, the next main component, which is the propeller, compensate for this as it provides most of the aircraft's thrust.
THE STAGES OF HOW TURBOPROP ENGINES WORK IN EACH ENGINE PART
FAN - Entry Point of Air
This is the first entry point of a large amount of air. The blades of this fan, which is mainly titanium, increases the velocity of air and splits it up into two. The first part of the air is forced to the main engine parts for further processing. The second part, on the other hand, goes through the duct surrounding the engine core then to the rear part of the engine where it would meet with the air that entered the engine core which would help in providing thrust to the engine.
COMPRESSOR - Area of Air Compression
The part of the air from the fan that enters the engine core would first enter the compressor where air pressure is increased. The compressors in jet engines are made of rotating fans with many blades or rotors that are connected to the shaft and these blades are responsible in compressing the air.
Turboprop engine compressors are axial-flow compressors. These types of compressors are usually composed of many components of rotating aerofoils and air diminishes as it passes through these components to maintain the needed axial Mach number. The high polytropic efficiency that reaches at about 90% of these axial-flow compressors help in increasing the energy potential of air that will pass through the next parts of the engine.
COMBUSTOR - Area of Gas and Fuel Ignition
The compressed air will be sprayed with fuel through more or less 20 nozzles in the combustor. The combination of air and fuel will be ignited resulting to high temperature and high energy of the flowing air. The ignited air, which has oxygen, would cause expansion of these hot gases. The heat in the combustor that reaches about 27000C can be tolerated by ceramic materials that are used in making the inner lining of combustors.
TURBINE - Area of Energy Distribution
The high temperature air produced by the combustor would further undergo gas expansion and would help in rotating the blades of the turbine. In turboprop engines, a propeller is connected to the turbine shaft that is propelled by the energy from the high energy airflow that is useful in saving fuel in low altitude flights. Some of the energy generated by the turbine is used to spin the blades of the compressor that is connected to it and also to rotate the fan in front of the engine.
NOZZLE - Exhaust of Air
Most jet engines would depend their thrust from the exhaust duct. However in turboprop engines, only some of the exhaust is further expanded in the nozzle since a part of the energy created during combustion had been used to run the blades of the turbine. The thrust produced as air is exhausted to the atmospheric pressure is due to the force created by combination of the portion of air that passed through the engine core and the air that bypassed the input fan. The forceful exhaustion is responsible for the forward movement of the aircraft during flight. Some turboprop nozzles are preceded by a mixer that reduces the noise produced by the engine by combining the high temperature gas from the engine core and the low temperature air that passes through the duct that surrounds the engine core.
Modern turboprop engines have propellers that have smaller diameter but they are composed of more blades to cope up with greater flight speed. However, turboprop engines are still designed for aircrafts that are of low speed like small commuter planes because they become less efficient with increased speed. The low jet velocity of propellers of turboprop engines as well as their high cost makes them useful only in low speed planes and those aircrafts that require short runways such as STOL (short-takeoff and landing) aircrafts.