Gas turbine engines are the power plants for all flying aircraft/helicopters (also known as aero-engines or aero-derivative gas turbines) and the source of power in various industrial applications such as automobiles, tanks, marine vessels, and electric power production. Gas turbines are classified as follows:
TURBOJET ENGINES
In the 1940s, turbojet engines were the first type of jet engine utilized to propel aircraft. This aircraft engine revolutionized air transportation. It significantly decreased air travel costs and increased safety on board. Additionally, the turbojet enabled higher speeds, including supersonic ones. Compared to piston-driven engines, it featured a far higher ratio of thrust to weight, which immediately translated to longer ranges, greater payloads, and less expensive maintenance. Military fighters and fast business jets use turbojet engines:
Turbojet engines may be classified as a single spool or double spool.
Turbojets are also categorized as either afterburning or non-afterburning. An afterburner is used to boost thrust. Afterburners are only used by fighter aircraft when essential. The Concorde was the only civil transport aircraft equipped with an afterburner.
Turbojet engines may be also classified as nuclear or nonnuclear engines. However, all the nuclear engine projects were abandoned in favor of safety.
THE TURBOFAN ENGINES
The turbofan engine was designed as a compromise between the turboprop and turbojet engines. It includes a large internal propeller and two streams of air flowing through the engine. The primary stream travels through all the components like a turbojet engine, while the secondary air passes through the fan and is either ducted outside through a second nozzle identified as the cold nozzle or mixes with the hot gases leaving the turbine(s) and both are expelled from a single nozzle.
Turbofan engines have better performance and fuel economy than turbojet at low power settings, low speeds, and low altitudes.
TURBOPROP ENGINES
Turboprop engines combine the best features of turbojet and piston engines. The former is more efficient at high speeds and high altitudes. Consequently, commuter aircraft and military transports tend to feature turboprop engines. A turboprop engine differs from a turbojet engine in that the design is optimized to produce rotating shaft power to drive a propeller,
instead of thrust from the exhaust gas. The turboprop uses a gas turbine to turn a large propeller.
The shaft that connects the propeller to the turbine is also linked to a gearbox that controls the propeller’s speed. The propeller is most efficient and quiet when the tips are spinning at just under supersonic speed.
Turboprop engines may be further classified into two groups, depending on the turbine driving the propeller.
In the first group, the propeller is driven by the same gas turbine driving the compressor.
In the second group, an additional turbine (normally denoted as a free-power turbine) turns the propeller.
PROPFAN ENGINES
Propfan engines are sometimes identified in former Soviet Union countries as turbopropfan (like the D-27 engine) or unducted fans.
There are two categories:
- Single rotation
- Counterrotating
The single rotation has a single forward unducted fan that combines the advantages of the propeller of a turboprop and the fan of a turbofan engine
The second type has two propellers, one behind the other that rotates in opposite directions (CR). Counterrotating designs offer the best performance. It has some similarities with the aft fan type, but it is composed here of two counterrotating fans.
ADVANCED DUCTED FAN
These designs are essentially turbofans with large, swept fan blades that have pitch control and reduction gearing similar to propfans, but the fans are enclosed in ducts like the turbofan.
There are two basic designs: one with geared, variable pitch, single-propeller fan and the other with counter-rotating blades with pitch control. The need for such engines has been spurred by increasing airplane traffic, which raises noise, environmental, and fuel consumption issues.
ADVANCED DUCTED FAN
These designs are essentially turbofans with large, swept fan blades that have pitch control and reduction gearing similar to propfans, but the fans are enclosed in ducts like the turbofan.
There are two basic designs: one with geared, variable pitch, single-propeller fan and the other with counter-rotating blades with pitch control. The need for such engines has been spurred by increasing airplane traffic, which raises noise, environmental, and fuel consumption issues.
