Gas Turbine Engine Types

 

BASIC GAS TURBINE Types of Gas Turbine Engines

TYPES OF GAS TURBINE ENGINES

1

M.Omar Yazdani

PIA Training Centre

July 2009

 

BASIC GAS TURBINE Types of Gas Turbine Engines

TURBOJET ENGINE If an aircraft gas turbine engine uses only the thrust developed within the engine to produce its propulsive force, it is a turbojet engine. Because they have no added feature such as a fan, propeller or free turbine, turbojets are sometimes referred to as straight jets. Two kinds: -> Centrifugal flow compressor type -> Axial Axia flow compressor typeand in some engines, both a Either type can have one lor more compressors centrifugal compressor and an axial flow compressor are incorporated. Because the efficiency of a turbojet is sustained sust ained at high altitude and airspeed, engine of this type are well suited for f or high flying, high-speed aircraft that operate over a sufficient range to make the climb to their best operating altitude worthwhile. But, high thrust at low airspeed is not a turbojet characteristics. To be at their best, turbojets need the ram air pressure at their inlet that comes only with an appreciable forward f orward speed. Turboj Turbojet et powered aircraft therefore need long runways for takeoff. 2

M.Omar Yazdani

PIA Training Centre

July 2009

 

BASIC GAS TURBINE Types of Gas Turbine Engines

 

3

M.Omar Yazdani

PIA Training Centre

July 2009

 

BASIC GAS TURBINE Types of Gas Turbine Engines

Schematic diagram showing the operation of a centrifugal flow turbojet engine. The compressor is driven via the turbine stage and throws the air outwards, requiring it to be redirected parallel to the axis of thrust.

4

M.Omar Yazdani

PIA Training Centre

July 2009

4

 

BASIC GAS TURBINE Types of Gas Turbine Engines

Schematic diagram showing the operation of an axial flow turbojet engine. Here, the compressor is again driven by the turbine, but the air flow remains parallel to the axis of thrust.

M.Omar Yazdani

PIA Training Centre

July 2009

5

 

BASIC GAS TURBINE Types of Gas Turbine Engines

 An afterburner  afterburner or or "reheat "reheat jetpipe jetpipe"" is a device added to the rear of the jet engine. It provides a means of spraying fuel directly into the hot exhaust, where it ignites and boosts available thrust significantly; a drawback is its very high fuel consumption rate. Afterburners are used mostly on military aircraft, but the two supersonic civilian transports, Concorde and the TU144, also utilized afterburners afterburner (or (or reheat reheat)) is an additional component added to some jet An afterburner  engines, primarily those on military supersonic aircraft. Its purpose is to provide a temporary increase in thrust, both for supersonic flight and for takeoff (as the high wing loading typical of supersonic aircraft designs means that take-off speed is very high). On military aircraft the extra thrust is also useful for combat situations. This is achieved by injecting additional fuel into the jet pipe downstream (i.e. after ) of the turbine. The advantage of afterburning is significantly increased thrust; the disadvantage is its very high fuel consumption and inefficiency, inefficiency, though this is often regarded as acceptable for the short periods during which it is usually used.

M.Omar Yazdani

PIA Training Centre

July 2009

6

 

BASIC GAS TURBINE Types of Gas Turbine Engines

Turboprop engines are a type of aircraft power plant that use a gas turbine to drive a propeller. propeller. The gas turbine is designed specifically for this application, with almost all of its output being used to drive the propeller.. The engine's exhaust propeller gases contain little energy compared to a jet engine and play a minor role in the propulsion of the aircraft. The propeller is coupled to the turbine through a reduction gear that converts the high RPM, low torque output to low RPM, high torque. The propeller itself is normally a constant speed (variable pitch) type similar to that used with larger reciprocating aircraft engines.

M.Omar Yazdani

PIA Training Centre

July 2009

7

 

BASIC GAS TURBINE Types of Gas Turbine Engines

Turboprops very efficien efficient t atpropeller modest flight flig ht exhaust) speeds (below (belo w 450 mph) because theare jet velocity of the (and is relatively low. Due to the high price of turboprop engines, they are mostly used where highperformance short-takeoff short-takeoff and landing (STOL) capability and efficiency at modest flight speeds are required. In a civilian aviation context, the most common application of turboprop engines istoinreciprocating small commuter aircraft, where their greater reliability as compared engines offsets their higher initial cost. Turboprop engines are gas-turbine engines that deliver almost all of their power to a shaft to drive a propeller propeller.. Turboprops Turboprops remain popular on very small or slow aircraft, as Hercules small commuter transports, such as thesuch C-130 and P-3airliners, Orion. and military

M.Omar Yazdani

PIA Training Centre

July 2009

8

 

BASIC GAS TURBINE Types of Gas Turbine Engines

Turboprops havereason a fairlyisstrict spot at lose speeds below about mph (700 km/h). The that sweet all propellers efficiency at high450 speed, occurs just below supersonic due to an effect known as wave drag that occurs just speeds.. This powerful form of drag has a sudden onset, and led to the speeds concept of a sound barrier when it was first encountered in the 1940s. In the casethat of athe propeller, propeller thisprop effect can happen timeofthe prop even is spun fast enough tips of,the travel near theany speed sound, if the aircraft is sitting still.

M.Omar Yazdani

PIA Training Centre

July 2009

9

 

BASIC GAS TURBINE Types of Gas Turbine Engines

 A turbofan is a type of aircraft gas turbine engine that provides propulsion using a combination of a ducted fan and an jet exhaust nozzle. Part of the airstream from the ducted fan passes through the core, providing oxygen to burn fuel to create power. power. However, However, the rest of the air flow bypasses the engine core and mixes with the faster stream from the core. The rather slower bypass airflow produces thrust more efficiently than the high-speed air from the core, and this reduces the specific fuel consumption.

M.Omar Yazdani

PIA Training Centre

July 2009

10

 

BASIC GAS TURBINE Types of Gas Turbine Engines

Turbofans have a net exhaust speed that is much lower than a turbojet. This makes them much more efficient at subsonic speeds than turbojets, and somewhat more efficient at supersonic speeds up to roughly Mach1.6, but have also been found to be efficient when used with continuous afterburner at Mach 3 and above. However, the lower speed also reduces thrust at high speeds.  All of the jet engines used in currently manufactured commercial jet aircraft are turbofans. They are used commercially mainly because they are highly efficient and relatively quiet in operation. Turbofans Turbofans are also used in many military jet aircraft. The difference between a turbofan and a propeller, besides direct thrust, is that the intake duct of the former slows the air before it arrives at the fan face. As both propeller and fan blades must operate at subsonic inlet velocities to be efficient, ducted fans allow efficient operation at higher vehicle speeds.

M.Omar Yazdani

PIA Training Centre

July 2009

11

 

BASIC GAS TURBINE Types of Gas Turbine Engines

If the turboprop is better at moderate flight speeds and the turbojet is better at very high speeds, it might be imagined that at some speed range in the middle a mixture of the two is best. Such an engine is the turbofan (originally termed bypass turbojet by the inventors at Rolls Royce). Another name sometimes used is ducted fan. fan.

M.Omar Yazdani

PIA Training Centre

July 2009

12

 

BASIC GAS TURBINE Types of Gas Turbine Engines

Schematic diagram illustrating a 2-spool, low-bypass turbofan engine with a mixed exhaust, showing the low-pressure (green) and high-pressure (purple) spools. The fan (and booster stages) are driven by the low-pressure turbine, whereas the highpressure compressor is powered by the high-pressure turbine

M.Omar Yazdani

PIA Training Centre

July 2009

13

 

BASIC GAS TURBINE Types of Gas Turbine Engines

 An un undu duct cted ed fa fan n or propfan is a modified turbofan engine, with the fan placed outside of the engine nacelle on the same axis as the compressor blades. Propfans are also known as ultra-high bypass (UHB) engines and, most recently, recently, open rotor jet engines. The design is intended to offer the speed and performance of a turbofan, with the fuel economy of a turboprop. The propfan concept was intended to deliver 35% better fuel efficiency than contemporary turbofans, and in this they succeeded. In static and air tests on a modified DC-9, propfans reached a 30% improvement. This efficiency comes at a price, as one of the major problems with the propfan is noise, particularly in an era where aircraft are required to comply with increasingly strict Stage III and Stage IV noise requirements.

M.Omar Yazdani

PIA Training Centre

July 2009

14

 

Basic GaGAS Gas s Turbine Turbine BASIC TURBINE Types of Gas Turbine Engines

 A turboshaft engine is a form of gas turbine which is optimized to produce shaft power, power, rather than jet thrust. In principle, a turboshaft engine is similar to a turbojet, except the former features additional turbine expansion to extract heat energy from the exhaust and convert it into output shaft power. Turboshaft Turbo shaft engines are commonly used in applications which require a sustained high power output, high reliability, reliability, small size and light weight. These include helicopters, auxiliary power units, boatsand ships, tanks, hovercraft, and stationary equipment.

M.Omar Yazdani

PIA Training Centre

July 2009

 

15

 

BASIC GAS TURBINE Types of Gas Turbine Engines

In most designs the gas generator and power section are mechanically separate so that they may each rotate at different speeds appropriate for the conditions. This is referred to as a free power turbine. turbine. A free power turbine can be an extremely useful design feature for vehicles, as it allows the design to forego the weight and cost of complex multi-ratio transmissions and clutches. The general layout of a turboshaft is similar to that of a turboprop. The main difference is that a turboprop is structurally designed to support the loads created by a rotating propeller, as the propeller is not attached to anything but the engine itself. In contrast, turboshaft engines usually drive a transmission which is not structurally attached to the engine. The transmission is attached to the vehicle structure and supports the loads created instead of the engine. However,, in practice many of the same engines are built in both turboprop However and turboshaft versions, with only minor differences.