Not so simple machines: the geared turbofan may be a revolutionary jet engine, but it's just the latest device to employ precision gearing.
And yet, if you flip to the second page of the Times, you can find that traditional, old-fashioned machinery is still appreciated. Hardly a day goes by when small square ads for luxury timepieces don't appear in those pages, often without listed prices (on the theory that if you have to ask, you really can't afford one). People aren't buying such watches because they need to tell time with improved accuracy; cheap quartz watches, relying on atomic scale phenomena, are accurate to about a second a day. Instead, the attraction is a combination of high-quality materials and the emotional impact of precision-engineered parts--mostly gears--working together in an intricate network to produce satisfactory, results using purely mechanical means.
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 are also integral to a new engine that has the potential to change commercial aviation. Pratt & Whitney's geared turbofan jet engine will have significantly better fuel economy and much quieter operation. This new geared turbofan has a hub-mounted gearing system that drives the front-mounted engine fan at lower speeds, permitting as much as 16 percent lower fuel consumption and noise some 15 to 20 dB lower than the latest regulations. It is scheduled for airline operation by 2016 on new aircraft such as the Airbus 320neo, the new Mitsubishi regional jet, Bombardier C-Series, and the Russian Irkut MS-21.
P&W has been developing this new turbofan engine since the 1980s, and its engineers are lauding the GTF as "disruptive technology." It is disruptive in the sense that it interrupts the normal course of aircraft jet engine development, providing a favorable step change in performance.
"Technologies evolve," writes economist and engineer W. Brian Arthur in his book The Nature of Technology, "based on the chaotic and constant recombining of existing technologies." The P&W GTF combines existing jet engine technology with the well-established mechanical engineering technology of gears. And it's not the first time that gears--gears from Connecticut, in fact--have been involved in a disruptive technological break.
Noted gear expert Darle Dudley traced the art and technology of gears as far back as 3000 BC, to Chinese travelers crossing the Gobi Desert using two-wheeled chariots. One type of chariot had an advanced guidance system: a wheel-mounted differential gear mechanism comprising wooden pin teeth would operate an outstretched arm so that it always pointed south.
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 used probably as one of the first analog computers to predict celestial positions of the sun and moon, the time of solar eclipses, and the dates of Olympic and Pan-Hellenic games. Although heavily corroded by its 2,000 years in the sea, it has been determined to have had at least 30 metal gears, with triangular teeth.
In more recent centuries, a decent history of the development and use of gears could be written without losing one's focus on Connecticut, a U.S. state known for industrial manufacturing. Eli Whitney, who graduated from Connecticut's Yale College in 1765, invented the cotton gin the following year. The gin (a name probably derived from "engine") was a breakthrough machine that successfully separated cottonseed from raw cotton fibers; it used gears in many of its models to transmit power to the separation mechanism. Historians point out that the cotton gin made growing cotton very profitable and set the plantation-economy South on a collision course with the industrial North. Gears, it can be said, set in motion the events leading to the American Civil War.
Another Yale student, Josiah Willard Gibbs, was awarded the first engineering Ph.D. degree in the U.S. for his 1863 thesis, "On the Form of the Teeth of Wheels in Spur Gearings." As Gibbs relates in the thesis, his work dealt with "... the necessary relations between the surfaces of the teeth of wheels, in order that they satisfy the various conditions of practical utility. ..." Gibbs went on to be a renowned American engineer, scientist, thermodynamicist, mathematician, and professor at Yale.
During the 1800s and into the 1900s, Connecticut had a thriving clock and industry, with names like Terry, Seth Thomas, Ingraham, Sessions, Gilbert, and Waterbury Clock (which became Timex.) Clockwork gear manufacture included the early use of cast brass and wood, and later, brass stamping with an emphasis on interchangeable parts, a technology that had been pioneered earlier by Eli Whitney with firearm components.
Connecticut companies also led in the manufacture of machine tools such as lathes, milling and boring machines, and screw thread machines, all of which required close tolerance gear trains. As an engineering student, I worked at one such company, the Hallden Machine Company in Thomaston. They made large flying shear machines, used to cut strip steel into sheets, as the strip steel came off a steel plant rolling mill at many feet per second. The flying shear moved at the speed of the strip steel, as it cut it into sheets. Precision elliptical gearing was used to drive and control the flying shear to provide for operation over a range of strip speeds and sheet lengths as required.
In spite of all this rich history, however, it's fair to say that no set of Connecticut gears has had to work at such a high level of precision as those in the geared turbofan.
The first commercial jetliners were powered by turbojets--jet engines in which all thrust was provided by gases that went through the engine from inlet to exhaust nozzle, exiting in a single high velocity jet. The resulting momentum increase provides the thrust force necessary for flight, but kinetic energy is "wasted" in the exiting jet.
A more efficient design is the turbofan jet engine, so-named for a ducted fan mounted in the front. Air drawn into the fan is divided, with some flowing out of the fan into the jet engine itself and the remainder bypassing the engine. The lower velocity bypassed air and the higher velocity engine air combine downstream to produce thrust with a larger mass flow at an average velocity lower than the high velocity jet flow.
With a large frontal area, the commercial aircraft turbofan is designed to produce peak thrust at takeoff, with most of thrust produced by air drawn in by the fan and bypassing the jet engine core itself. Bypass ratios--the mass of fan air bypassed for every unit mass of air through the engine--currently can be as high as 8.4:1, as in General Electric's 100,000 pound thrust GE90 engine that is used on Boeing 777s. Frank Whittle, one of the inventors of the jet engine, first proposed the addition of a fan to a jet engine in a 1930 patent. He called it a thrust augmenter, because its addition does increase thrust and reduce fuel consumption (that is, it makes for lower thrust specific fuel consumption).
For subsonic flight, the propulsive efficiency, [[eta].sub.p] of a turbofan is also higher than that of a turbojet. This efficiency is defined as the useful propulsive power (the product of thrust and flight velocity, [V.sub.o]) divided by jet power (rate of change of the kinetic energy of gases through the engine). This simplifies to
[[eta].sub.p]= 2 / [V.sub.e] / [V.sub.o] + 1
where [V.sub.e] is a suitable average of the lower velocity bypass air and the higher velocity jet exhaust. That equation shows that a turbojet engine with a high value of [V.sub.e] / [V.sub.o] has a low propulsive efficiency, while a turbofan engine with low values has a corresponding high propulsion efficiency. Since [V.sub.e] is largely determined by bypass flow in a high-bypass engine, the fan pressure ratio (which could by 1.6 for a bypass ratio of 8:1) is the key parameter. Lowering it to decrease [V.sub.e] and increase [[eta].sub.p] means increasing the bypass ratio.
While driving the fan with the jet engine compressor and turbine spool is a necessity, the two components have diverging needs. On the one hand, the compressor and turbine within the engine are most efficient at high rotational speeds. But the fan operates most efficiently and creates less noise at lower rpm, and by lowering fan tip speeds, engineers can satisfy stress and supersonic flow limitations.
The solution Pratt & Whitney engineers came up with involves separate gear-connected shafts for the fan and the compressor and turbine spool, so that each component can run closer to its optimal operating speed. The geared turbofan uses a fan hub-mounted epicyclic gear train. (For reference, a hand cranked pencil sharpener uses epicyclic gearing.) A "sun" gear is mounted to the shaft running through the compressor and turbine. That sun gear meshes with five surrounding intermediate gears. A carrier, stationary with respect to the engine casing, supports the five intermediate gears which mesh with an outer ring gear attached to and driving the fan.
Since the carrier is fixed, causing the five intermediate gears to be fixed in position, they are called "star" gears, meshing with the sun gear. (If the ring gear was fixed and the carrier was coupled to the fan, the intermediate gears would be termed "planetary.") All five star gears have small-diameter, high-load carrying journal bearings, and have double helical teeth chosen for load sharing capacity and smoother meshing action.
Gearboxes add weight, to be sure, but that's compensated for by the elimination of components in the engine core. A conventional engine in the same thrust range would normally have 21 to 23 stages of bidding; the GTF engine, with its core operating at non-compromised fan conditions, has only 17 stages. Also, the core engine parts that have been eliminated involve high cost materials, such as superalloys that can withstand high temperatures, while the gearbox materials are less exotic, providing a potential cost benefit.
Even so, it is critical that the GTF gearbox be very efficient in transmitting power from the compressor/turbine spool to the fan. The transmitted power is as high as 18,000 hp for the Mitsubishi engine and 30,000 hp for the Airbus 320neo engine. (By convention, the gearbox power is listed in horsepower-which was James Watt's original power unit, used to convince mine owners to buy steam engines to replace horses used for mine pumping.) Even small inefficiencies such as gear tooth mismatch and bearing misalignment would generate enough heat to "cook" gearbox lubricating oil. Testing has shown that GTF gearboxes must be at least 99.3 percent efficient to avoid that problem.
P&W engineers found that some 80 percent of the heat load in the gearbox actually comes from churning and foaming of the oil. Much of their design effort has gone into getting lubrication oil quickly into and out of the unit, to prevent heat load buildup.
A key facility for developing the GTF gearbox has been a specially designed four-square gear test rig at P&W's Middletown plant. The test rig is variable speed, with two GTF gear boxes working against each other and a torque load applicator. Two 600 hp electric motors provide power to operate a test and to overcome any losses in the two test GTF gearboxes. With this test rig, P&W engineers can simulate as many as 15,000 takeoffs in about 20 days of continuous running. The orientation of the GTF test gearboxes can be adjusted with respect to gravity to simulate different flight conditions.
After an extensive program using this four-square rig and a long history of gearbox experience associated with their very popular turboprop gas turbines at Pratt & Whitney Canada, P&W engineers are convinced their new GTF engines will have a bright future.
Currently, P&W's first generation geared turbofan engines use a gear reduction of 3:1, with a resulting bypass ratio of 12:1. The resultant fuel consumption is as much as 16 percent lower than with current conventional fan engines. Noise reduction is substantial, with the characteristic turbofan whine being replaced by a lower frequency "whoosh."
If the 3:1 gear ratio is successful, they plan to move on to higher thrust models - and to higher gear ratios. At a 4:1 ratio, for instance, the bypass ratio can be increased to 15:1. That would lead to potentially greater fuel savings, and possibly create a greater disruption in the jet engine market.
Jet engines are some of the largest and most expensive industrial products our society produces. That the most advanced new model relies on humble gearworks, much the way ancient chariots or luxury watches do, is pretty satisfying.
PATHWAY TO EFFICIENCY
Most of the air drawn in by the fan (far left) bypasses the engine completely. But around 9 percent is compressed by two stages of compressor blades.
The compressed air is mixed with jet fuel and combusted. The exhaust is expanded through two sets of turbine blades (far right) which drive the engine.
The gearbox, immediately behind the fan, Drives the fan at a slower, ore efficient speed.
LEE S. LANGSTON is an ASME Fellow and professor emeritus of the mechanical engineering department at the University of Connecticut in Storrs.
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|Author:||Langston, Lee S.|
|Date:||Jan 1, 2013|
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