Types of Gas Turbine Engines

Posted on August 6, 2019 John Jones

In a typical reciprocating combustion engine, as seen in automobiles and propeller-driven aircraft, the functions of intake, compression, combustion, and exhaust all take place in the same combustion chamber. Therefore, each must have exclusive occupancy of the chamber during its part in the combustion cycle. Gas turbines, however, have separate sections for each function, and all functions are performed simultaneously without interruption.

These sections typically consist of:

An air inlet, where the air enters the engine
A compressor section
A combustion section
A turbine section
An exhaust section
An accessory section
Systems used in starting, lubrication, fuel supply, and auxiliary purposes, such as anti-icing, cooling, and pressurization

There are four gas turbine engines used to power jet aircraft: turbofans, turboprops, turboshaft, and turbojet. While turbojets were the first type of turbine engine to be developed, they are noisy and have high fuel consumption at the speeds most airliners fly at, so turbojets are fairly limited in use

Most turbine-driven aircraft use turbofan engines. A turbofan has a large fan or set of fans at the front of the engine that produces about eighty percent of the thrust from the engine, with less noise and fuel consumption. Turbofan engines have more than one shaft in the engine, with most having two. These two-shafted engines use two spools (the compressor, shaft, and turbines), divided between a high-pressure spool and low-pressure spool.

Turbofans are either low bypass or high bypass. The amount of air bypassed around the core determines the bypass ratio: if, for example, a turbofan has 100 pounds per second of air flowing through the fan, and 20 pounds per second flowing through the core, the engine has a 5:1 bypass ratio. Some low-bypass turbofan engines are used in speeds above .8 Mach, and use afterburners to increase thrust. By adding more fuel nozzles and a flame holder in the exhaust system, extra fuel can be sprayed and burned which gives large increases to thrust for a short time.

Turboprop engines are gas turbine engines that turn propellers through a speed reduction gear box. This type of engine is most efficient at 300 to 400 mph, and can use shorter runways than other aircraft. Eighty to eighty five percent of the energy that the engine produces is used to drive the propeller, while the rest exist the exhaust as thrust.

Lastly, turboshaft engines transfer horsepower to a shaft that turns a helicopter transmission or serves as an auxiliary power unit (APU). APUS are used to provide electrical power and bleed air on the ground, and as a backup generator in flight. Turboshaft engines can come in a variety of configurations and horsepower range.