The Adaptable Gas Turbine
Whether creating electricity or moving planes, this engine continues to inspire innovation
In an aircraft gas turbine, all of the turbine power is used to drive the compressor (which may also have an associated fan or propeller). The gas flow leaving the turbine is then accelerated to the atmosphere through an exhaust nozzle to provide thrust or propulsion power. Gas turbine or jet engine thrust power is equal to the momentum increase in the mass flow from engine inlet to exit, multiplied by the flight velocity. The actual thrust force produced in the engine (and pulling the plane forward) is the summation of all the axial components of pressure forces on the internal surfaces of the engine exposed to the gas path flow.
A jet engine can be small enough to be handheld and produce a few pounds of thrust (1 pound of thrust is equivalent to 4.45 newtons of force) to be used on model airplanes or military drones. (The retired Swiss pilot Yves Rossy, nicknamed “Jetman,” attached four such small jet engines—each producing 50 pounds of thrust or about 223 newtons—to a back-mounted wing and flew across the English Channel in 2008 and over the Grand Canyon in 2011.) On modern commercial jet aircraft, gas turbines are typically in the range of 30,000 pounds of thrust (or 136,000 newtons), with the largest currently at about 100,000 pounds of thrust (445,000 newtons) on Boeing’s long-range 777 airplanes.
The jet engine shown in the figure at right is a turbofan engine, with a larger-diameter compressor-mounted fan. Thrust is generated by air passing through the fan alone (called bypass air) and through the gas generator itself. The combination of mechanisms greatly increases the fuel efficiency of the engine. With a large frontal area to pull in a higher mass of air (with the trade-off that the configuration does engender higher aerodynamic drag forces at cruising flight velocities), the turbofan engine generates peak thrust at takeoff speeds. It is therefore most suitable for commercial aircraft, which need most of their lift to get off the ground, not to maneuver once in the air. In contrast, a turbojet does not have a fan and generates all of its thrust from air that passes through the gas generator. Turbojets have smaller frontal areas (and thus lower drag at high flight velocities) and generate peak thrusts at high speeds, making them most suitable for fighter aircraft that travel at much higher velocities than commercial craft.
In nonaviation gas turbines, only part of the turbine power is used to drive the compressor. The remainder is used as output shaft power to turn an energy conversion device, such as an electrical generator, or to compress natural gas in a pipeline so it can be transported. Shaft power land-based gas turbines can get very large (with an output as high as 375 megawatts, enough to power about 300,000 homes). The unit shown in the figure at right is called an industrial or frame machine. It is constructed for ruggedness and long life, so weight is not a major factor as it is with a jet engine. Typically frame machines are designed conservatively but have made use of technical advances in jet engine development when it has made sense to do so.
Lighter-weight gas turbines derived from jet engines and used for nonaviation applications are called aeroderivative gas turbines. Aeroderivatives are used to drive natural gas pipeline compressors, power ships and produce electric power. They are used particularly to provide peaking and intermediate power for electric utilities, because they can start up quickly. Peaking power supplements a utility’s normal output during high-demand periods, such as summer air conditioning in major cities.