This article reviews the development of geared turbofan (GTF) engines. GTF engines have a hub-mounted epicyclic gearbox that drives the front-mounted fan at lower rotational speeds than the engine turbine section that powers the fan. The turbine driving the fan is most efficient at high-rotational speeds. The fan operates most efficiently and creates less noise at lower rpm. The operating gear reduction ratio also permits increasing the engine’s bypass ratio with larger fans. Gear trains are one of the oldest known machines, and none is more closely identified by the general public with the profession of mechanical engineering. Pratt & Whitney is in production of their first generation of GTF engines in the 18,000–30,000 lbt range, which power twin engine single-aisle, narrow body 70–200 passenger aircraft. The GTF combines existing jet engine technology with the well-established mechanical engineering technology of gears.
Gears Steer New Engine Designs
The coterie of geared turbofan jet engine companies is growing. Rolls-Royce is now developing a geared turbofan (GTF) for its future engines in the 25,000-110,000 pound-thrust (lbt) range, slated for production in the next decade . This major OEM will join Pratt & Whitney and Honeywell, who both have been designing, developing and producing GTF engines for some years.
GTF engines have a hub-mounted epicyclic gearbox that drives the frontmounted fan at lower rotational speeds than the engine turbine section that powers the fan. The turbine driving the fan is most efficient at high rotational speeds. The fan operates most efficiently and creates less noise at lower rpm. By lowering fan blade tip speeds by means of gearing, engineers can more easily satisfy fan blade and disk stress limits and avoid the onset of power-robbing supersonic fan blade flows.
The operating gear reduction ratio also permits increasing the engine's bypass ratio with larger fans. Bypass ratios - the mass of fan air bypassed around the engine for every unit mass of air through the engine - can be increased, which improves the propulsion efficiency of the turbofan engine.
The net result is a great reduction in fan generated noise and as much as a double digit reduction in engine fuel consumption. Both of these attributes are causing airlines to demand from airframe companies, new commercial aircraft that mount the GTF engines.
Gear trains are one of the oldest known machines and none is more closely identified by the general public, with the profession of mechanical engineering. Gears use the principle of the lever to alter the speed and torque carried by shafts, and can be traced back as far as 3000 BC in use in China.
One of the most famous of ancient gear assemblies is the Antikythera Mechanism , recovered in 1900 from a shipwreck off the coast of Greece. Possibly constructed in Rhodes in 150-100 BC, the mechanism is an astronomical analog calculator (or orrery) that was probably used as one of the first analog computers to show celestial positions of the sun and moon, the time of solar eclipses and the dates of Olympic and Pan-Hellenic games. The Antikythera Mechanism has some 30 intermeshing gears, which include an epicyclic gear train.
So here we are, two thousand years later using the same type of gear train to improve the performance of modern gas turbines. The name epicycle goes back to Greek astronomy, where planets were believed to move in circular orbits, with the earth as center - a geocentric system. Such orbits could not explain why at times, planets moved backward, relative to the earth-bound observer. Ptolemy (150 AD) explained such retrograde motion by superposing small circles - epicycles - on the original assumed circular orbit.
Currently, a geared fan epicyclic gearbox consists of a center sun gear, mounted on the driving turbine shaft. The sun gear meshes with normatively, five equallysized surrounding pinion gears, which also mesh with an encompassing annular ring gear. A circular carrier houses the five pinion gear shafts to support and position them.
If the carrier is fixed to the engine casing, the ring gear drives the fan. The pinion gears, now fixed as they transmit motion from sun to ring gear, are now called star gears. If the ring gear is fixed the carrier rotates to drive the fan. The pinion gears now rotate about the sun gear, and are called planet gears. A planetary gearbox can have higher gear ratios than a star gearbox.
Current Production GTFs
Honeywell first started developing geared fans almost 50 years ago . In 1968, then as the Garrett Air Research Phoenix Division, they developed their 3500 lbt TFE731 business jet engine from an existing auxiliary power unit (APU). Given the high rotational speed of the APU low pressure turbine (about 20,000 rpm), to avoid excessive fan tip speeds, Garrett engineers developed a epicyclic gearbox (about 8.5 inches in diameter and with a 1.8:1 gear ratio), which allowed the TFE731 to have a 2.5:1 bypass ratio (high for 1972, when it was certified). Still in production, it has been one of the most successful small gas turbine aircraft engines, with over 13,000 units produced.
Pratt & Whitney is in production of their first generation of GTF engines in the 18,000 - 30,000 lbt range, which power twin engine single-aisle, narrow body 70 - 200 passenger aircraft . As an example, their PW1100-JM is currently powering the Airbus A320neo, with airlines reporting up to 20% in fuel savings. The epicyclic gearbox (about 20 inches in diameter) has journal bearings for its star gears rather than roller element bearings, with transmitted power as high as 30,000 hp. The gear ratio is 3:1, yielding a bypass ratio of 12:1. Even small inefficiencies in its double helical gear teeth and bearings could generate enough heat to “cook” gearbox lubricating oil. Testing has shown that the P&W GTF gearboxes must be at least 99.3% efficient to avoid that problem.
One of my colleagues, Kazem Kazerounian (currently our Dean of Engineering at UConn) who is a gear systems researcher and an early consultant for P&W on gears, has some observations on possible future work on GTF gearboxes:
The challenges of light-weight, high-powered epicyclic gear systems include large deflections and vibration induced in the relatively thin ring gears (as the planets/ stars pass), and the possibilities of large displacement of the center of the sun gear.
New developments include using Herringbone bevel gears (bevel gears of opposite directions to cancel axial thrust) and using spiral bevel gears instead of straight bevels. Additional advantages in smoothness and load carrying capacity might be obtained by phasing the two bevel gears that constitute the Herringbones, so that teeth on both sides do not enter the mesh simultaneously.
There is significant room for optimization if designers consider nonstandard, or even non-involute gearing. This is uncharted territory in gear design, that might decide the future leaders in GTF design and manufacturing.
New technologies evolve based on the chaotic and constant recombining of existing technologies . The GTF combines existing jet engine technology with the wellestablished mechanical engineering technology of gears.