Turbofan vs Turboprop
To overcome disadvantages in the performance of the turbojet engines at subsonic speeds, such as efficiency and noise, advanced variants were built based on the turbojet engines. Turbofans were developed as early as 1940s, but not used due to less efficiency until 1960s, when Rolls-Royce RB.80 Conway became the first production turbofan engine.
Turboprop engines are another variant built on the turbojet engine, and use the turbine to produce shaft work to drive a propeller. They are a hybrid of early reciprocating engine propulsion and newer gas turbine powered propulsion. Also, turboprop engines can be seen as a turboshaft engine with propeller connected to the shaft through a reduction gear mechanism.
More about Turbofan Engine
Turbofan engine is an advanced version of the turbojet engine, where the shaft work is used to drive a fan to take in large amounts of air, compress, and direct through the exhaust, to generate thrust. Part of the air intake is used to drive the jet engine in the core, while the other portion is directed separately through a series of compressors and directed through the nozzle without undergoing combustion. Because of this ingenious mechanism the turbofan engines are less noisy and deliver more thrust.
High Bypass Engine
The bypass ratio of air is defined as the ratio between the mass flow rates of air drawn through a fan disk that bypasses the engine core without undergoing combustion, to the mass flow rate passing through the engine core that is involved in combustion, to produce mechanical energy to drive the fan and produce thrust.
In a high bypass design, most of the thrust is developed from the bypass flow, and in the low bypass, it is from the flow through the engine core. High bypass engines are usually used for commercial applications for their less noise and fuel efficiency, and low bypass engines are used where higher power to weight ratios are required, such as military fighter aircraft.
More about Turboprop Engine
Turboprop engine is an advanced version of the turbojet engine, where the shaft work is used to drive a propeller through a reduction gear mechanism attached to the turbine shaft. In this form of jet engines, majority thrust is generated by the propeller reaction and the exhaust generates a negligible amount of usable energy; hence mostly not used for thrust.
The propellers in turboprop engines are usually a constant speed (variable pitch) type, similar to propellers used in larger reciprocating aircraft engines. While most modern turbojet and turbofan engines use axial-flow compressors, turboprop engines usually contain at least one stage of centrifugal compression.
Propellers lose efficiency as aircraft speed increases, but very efficient at flight speeds below 725 km/h. Hence turboprops are normally not used on high-speed aircraft and are used to power small subsonic aircraft. Some exceptions exist, such as Airbus A400M and Lockheed Martin C130, which are large military freighters, and turboprops are used for high-performance short-takeoff and landing requirements of these aircrafts.
What is the difference between Turbofan and Turboprop Engine?
• In turbofan engines, a gas turbine engine is used to drive a fan to generate the thrust while, in turboprops, it is used to drive a propeller.
• In turbofan engine, thrust generated is a combination of bypass flow and gas turbine exhaust, while turboprops generate thrust almost completely by the propellers.
• Turbofans perform with good efficiency at both supersonic and transonic flight, but a turboprop can be only used in subsonic flight.
Diagram Source:
http://en.wikipedia.org/wiki/File:Turboprop_operation-en.svg
http://en.wikipedia.org/wiki/File:Turbofan_operation.svg