Turbine engines are classified according to whether the compressor is centrifugal flow, axial flow or a combination of both centrifugal and axial. The type of engine is further classified by the path the air takes through the engine and how power is produced. There are four different types of turbine engines – turbojet, turboprop, turbofan and turboshaft.
A turbojet engine was first developed in Germany and England prior to World War II and is the simplest of all jet engines. The four sections of a turbojet engine are the compressor, combustion chamber, turbine section and exhaust. The compressor passes air at a high rate of speed to the combustion chamber which contains the fuel inlet and igniter. Expanding air drives the turbine and accelerated exhaust gases provide thrust. These engines are limited on range and endurance and today are mostly used in military aviation. They are known for being slow to respond to throttle applications at slow compressor speeds.
Between 1939 and 1942, a Hungarian designer, Gyorgy Jendrassik designed the first turboprop engine. However, the design was not implemented into an actual aircraft until Rolls Royce converted a Derwint II into the RB50 Trent which flew on September 20, 1945 as the first turboprop jet engine. A turboprop engine drives a propeller through a reduction gear, allowing optimum propeller performance to be achieved at much slower speeds than the operating RPM. With their ability to perform well at slow airspeeds and fuel efficiency, turboprop engines are often used in small, commuter aircraft and agricultural applications due to their greater reliability offsetting their higher initial cost. One of the most reliable turboprop engines is the Pratt & Whitney PT6A.
Turbofan jet engines were designed to merge the best features of the turbojet and turboprop. By diverting a secondary airflow around the combustion chamber, additional thrust was created. Two separate streams of air pass through a turbofan engine. One passes through the engine core while the second bypasses the core. The Gloster E28/39 which flew for the first time on May 15, 1941 was one of the first times a turbofan engine was used for military or commercial aircraft.
The fourth type of jet engine is known as the turboshaft. Most of the energy produced by the expanding gases drives a shaft connected to a turbine through a single stage of reduction gearing rather than producing jet thrust. Turboshaft engines are predominantly used by helicopters. The first turboshaft engine was built by the French firm, Turbomeca in 1949.
Turbine engines have several advantages over reciprocating engines, including less vibration, increased aircraft performance and reliability. In addition, each type of turbine engine has its own advantages and disadvantages. For more information about the types of turbine engines, visit www.covingtonaircraft.com.