Machine Design

Gears Put A New Spin On Turbofan Performance

A new jet engine with a gear reduction between the fan and compressor optimizes engine-component speeds and boosts the turbofan’s efficiency.

How do you break into a market dominated by a pair of companies manufacturing proven products? And in industries like commercial aircraft where safety is paramount, trying new designs is a risky option. Those are the challenges facing Pratt & Whitney, East Hartford, Conn., which wants to supply more engines for 120 to 180-passenger airliners, a market segment it once ruled with its JT8D engines.

This market will make up over 65% of all commercial aircraft sold over the next two years and currently two companies have a stranglehold on it. CFM International, Cincinnati, is the solid leader, having sold over 8,500 of its CFM56 engines. International Aero Engines AG, East Hartford, Conn., with about 1,200 of its V2500 engines in service, is a distant second. Although Pratt & Whitney owns 30% of IAE, it now wants to independently break into this lucrative market. It plans on doing this with the PW8000 geared turbofan.

The engine will be the first turbofan to use gears for enhanced performance. P&W engineers have added a gearbox to the conventional turbofan so engine components spin at more efficient speeds. Although some critics consider gears unreliable for the performance required by airlines, the move will succeed if geared turbofans set new standards for jet engines.

It’s easy to see why gears are beneficial in turbofans. Ideally the engine’s fan should turn more slowly than the compressors and turbines. Until recently this couldn’t happen because the turbines and compressors share the same shaft. As a result turbofan designers make a tradeoff in which the turbine and compressors turn below their optimum speed while the fan turns faster than its ideal speed. Although a gear reducer may seem an obvious solution, this addition has been avoided primarily because adding gears increases the risk of costly maintenance and failures. But after years of testing geared designs, engineers have devised an engine that is not only reliable but offers efficiency improvements that pay off in ways even the designers didn’t expect.

The PW8000’s 17-in. gearbox mounts on the shaft just behind the fan, letting engineers optimize the speed of each component. The lowpressure turbine, running at about 9,000 rpm, provides input to the gearbox. After a 3:1 reduction, the fan runs at about 3,000 rpm. This lets the low-pressure turbine and compressor do more work while the fan runs quieter than if all components ran at the same speed. Although this setup improves efficiency, the gearbox is also the subject of skepticism.

Reliability is often a concern with gears because of the constant friction and stress exerted on gear teeth. To address this concern, P&W engineers tried to maximize smooth operation and minimize stress. The PW8000 gearbox uses “self-centering” technology that is said to eliminate misalignment. P&W has also tweaked the gearbox bearings and gears themselves. The result is a gearbox with efficiencies that exceed 99% according to P&W. This produces a heat load less than half what engineers expected. To put this into perspective, the 32,000-hp gearbox is barely larger than a car’s transmission. And if an automobile transmission were this efficient, it would weigh around two lb.

A downside of adding gearboxes to jet engines, however, is the added weight that goes along with them. But because the gearbox lets the engine run more efficiently, engineers could save weight in other areas such as the turbines and compressors. For instance the PW8000 will have 35% fewer stages (13 instead of 20) than conventional turbofans and 52% fewer compressor and turbine airfoils. This also means fewer parts to repair or replace and P&W expects to lower operating costs by 10% and maintenance costs by almost 30%. All this translates into annual savings estimated at $600,000 for 120 to 180-passenger planes.

And the design is proven, says the company. The gear system has undergone over 1,000 hr of component testing and more than 1,000 hr of full-scale tests. A 25% larger 40,000-shaft-hp gearbox has run in a test engine for over 100 hr and P&W has invested over $350 million in testing over the past 10 years. It now says the new engine could enter commercial service as early as 2002.

Two types of compressors, centrifugal and axial flow, are used on jet engines. Axial-flow compressors pressurize air by alternately passing it through rotors and stators. Centrifugal-flow compressors collect air near the center of an impeller and increase its velocity by spinning it to the impeller’s outer edge where a diffuser slows the air to increase its pressure. The PW8000 uses axial-flow compressors, which have high compression ratios and, therefore, high efficiency. Multistage compressors have several sets of rotors and stators to increase air compression even further. Air enters the compressor through airfoil- shaped blades called inlet guide vanes before passing through guide vanes on the rotors and stators. These vanes move air through the compressor in directions ideal for compression.

Efficiency of axial-flow compressors can be boosted by increasing the number of compression stages. Efficiency can also be increased by using separate compressors and turbines. In twinspool turbofans air from the fan enters a lowpressure compressor before flowing into a highpressure compressor on its way to the combustor. The two compressors run on separate concentric shafts with turbines at their other ends.

The PW8000’s low-pressure compressor is driven by a low-pressure turbine, which also drives the gearbox and, ultimately, the fan. A separate high-pressure turbine drives a high-pressure compressor that gets air to its peak pressure before combustion. The high-pressure compressor on the PW8000 has a 12:1 pressure ratio, and the overall pressure ratio is about 40:1.

Turbines on jet engines also have airfoil-shaped blades to directionally guide exhaust gas. Unlike compressors, however, the blades guide the gas to extract the most kinetic energy from it. The extracted energy is carried by the turbine shaft to drive the compressors and, on the PW8000, the gearbox that drives the fan.

The PW8000 has fewer stages because the engine has an impressive 11:1 bypass ratio (11 parts of air go around the engine core for every one part going through it). Bypass air gives the geared turbofan 90% of its propulsive power.

The engine’s 76- in. fan is about 11 in. larger in diameter than fans on similar, conventional engines, letting it turn more slowly. When designing the slower fans, engineers used computational fluid dynamics to get the blade tips to move air efficiently. So now the fan performs the same amount of work as faster fans which helps reduce fuel consumption by 9%. This contributes to reduced operating costs and increases the aircraft’s range.

Lowering fan speed also reduces noise, a long-time goal of federal restrictions on aircraft and airports. The PW8000 should operate 30 dB below these restrictions, qualifying it for use at noise-restricted airports. Federal emissions standards, another hurdle for engine designers, won’t keep the PW8000 grounded either. The combustor uses an advanced burner that produces emissions at least 40% below current regulations, according to P&W.

Jet Engine Basics
The basic turbojet engine consists of a compressor, a combustor, and a turbine. Air enters the engine at atmospheric pressure and passes through the compressor. Compressed air then travels through the combustor, where it mixes with fuel and is burnt. Hot exhaust gas rushes through the turbine on its way out the nozzle. Hot gas leaving the nozzle creates thrust and pushes the engine and aircraft forward. A common shaft has the compressor on one end and the turbine on the other. Hot gas spinning the turbine also spins the compressor.

Increased airflow produces more thrust, so adding a fan at the front of the engine boosts both airflow and thrust. But turbofans don’t push all the air they move through the combustor. Most of the air flows around the engine core, cools the engine, and creates thrust. The ratio of the amount of air going around the core to the amount of air going through the core is called the bypass ratio. Engines with higher bypass ratios operate more efficiently than those with lower bypass ratios. Turbofans characteristically have high bypass ratios compared to other jet engines, such as turbojets. Because of this, they have become the most common type of jet engine in service today.

Twin-spool turbofans have one spool with a high-pressure turbine driving a high-pressure compressor on one shaft. The other spool uses a separate concentric shaft with a low-pressure turbine driving a low-pressure compressor and fan. Two spools let components run at more efficient speeds. The low-pressure compressor, for instance, spins slower than the high-pressure compressor to more closely match incoming-air speed. Adding a gearbox to a turbofan further optimizes component speeds by allowing lower fan speeds and higher low-pressure turbine and compressor speeds.

© 2010 Penton Media, Inc.

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