05 January 2016
Building the next generation of aircraft engines
Everyone remembers the Wright brothers, who made the first powered, heavier-than-air flights by human beings on a beach in North Carolina in 1903. Few, by contrast, remember Charlie Taylor, a mechanic at the brothers' bicycle business in Dayton, Ohio. Yet it was Taylor who, by building an internal-combustion engine out of aluminium castings rather than iron ones, created a device both light enough and powerful enough to lift Orville and Wilbur into the sky.
Engine design has always been crucial to aviation. To start with, more powerful versions of the piston-driven motor pioneered by Taylor ruled the roost. Then, a radical, new approach emerged as the designs of Frank Whittle, a British engineer, ushered in the jet age. The jet has since evolved into the turbofan, whose gaping intakes have—as seasoned air travellers will have noticed—grown larger and larger over the years, to accommodate ever bigger and better fans. And now, as 2015 turns into 2016, another new design is being rolled out. This is the geared turbofan, which is available as an option on the A320neo, the latest product of Airbus, Europe's biggest aerospace group.
Geared turbofans, as their name suggests, include a gearbox as part of the mechanism. Those on the A320neo are the brainchildren of engineers at Pratt & Whitney, a division of United Technologies, an American conglomerate. Designing and building geared turbofans, which Pratt & Whitney brands "PurePower", is a gamble. The firm has spent two decades and more than $10 billion developing them. Connecting an engine's inlet fan to the compressor and turbine in its core through a gearbox should give better fuel economy and make the thing quieter—both desirable outcomes. But the bigger the engine the bigger the forces on the gearbox and the more likely it is that something will go wrong. So, though gearboxes are found in turboprops (jet engines that turn a propeller) and in a few executive jets, no one had until now managed to scale one up to cope with the 30,000 horsepower delivered by the core of an airliner's engine.
Pratt & Whitney has persevered because it thinks the conventional, ungeared turbofan is reaching its limits, and that only by adding a gearbox can airlines achieve the performance and economy which will be required of them in the future. Airlines, though, are notoriously conservative, and are wary of new, complicated kit like gearboxes, which are yet one more thing that can go wrong. So Pratt & Whitney has had its work cut out to persuade them.
Meshing it together
A jet engine works according to Newton's third law of motion: to every action there is an equal and opposite reaction. The reaction is forward movement. The action which provokes that is the ejection from the back of the jet of fast-moving gas. This gas generates thrust in proportion to its mass and to the speed with which it is being ejected. In the early, ear-splitting jet engines designed by Whittle and his contemporaries, the thrust came from air that entered the engine's core at the front (see diagram) where it was squeezed by a compressor, mixed with fuel and ignited to produce hot gases that rushed out of the rear. Though the mass of this exhaust gas was small, its velocity was high, so the resulting thrust kept an aircraft fitted with such an engine aloft. The compressor, meanwhile, was turned by a turbine propelled by the exhaust gases.
A turbofan works in a broadly similar way, but with a fan also turned by the turbine to push some of the air around, rather than through, the core. Though this core-bypassing air is not moving as fast as the exhaust gases, there is a lot of it—so it, too, produces a great deal of thrust. The upshot is a system that is more efficient and quieter than earlier jet engines.
The proportion of air going around the core compared with that going through it is known as the bypass ratio. Some of the latest turbofans have bypass ratios as high as 9:1. It is to achieve this that fans (and therefore inlets) have increased in size. But as fan blades get longer, their tips travel faster—and now those tips are going at close to the speed of sound. Accelerating them any further would cause shock waves, and these might result in dangerous vibrations.
A gearbox gets around this by letting the fan turn more slowly than the compressor and the turbine. This means the fan can be made bigger (and can thus accelerate a greater volume of air) without slowing everything else down to its rev rate. This arrangement permits all parts to be engineered for optimal performance. As a result, PurePower has a bypass ratio of 12:1.
Doing all of this does, though, require an utterly reliable gear box. Pratt & Whitney uses advanced nickel-based alloys for the components of the box itself. The fan blades are made from a lightweight alloy of aluminium and lithium. And the turbine is composed of titanium aluminide, a substance developed in collaboration with MTU, a German firm, that has twice the strength of the conventional cast alloys used to make turbines.
The upshot is that a pair of PurePower engines slung under an A320neo's wings promise to reduce fuel consumption by 15% compared with a standard A320. This could save an airline more than $1.5m a year per aircraft in fuel costs. Geared turbofans also give the plane a longer range and are markedly less noisy.
There have, inevitably, been teething problems. Industry reports suggest that the geared turbofan needs a slightly longer period to cool down than was expected, to avoid uneven wear when it is restarted. This might sound trivial, but at a busy airport it could cost a plane its take-off slot. For that reason Qatar Airways, which had been expected to be the first to take delivery of the A320neo, is believed to have postponed receipt. The honour of being first now looks like going to Lufthansa, a German carrier. Pratt & Whitney says the geared turbofan meets or exceeds all its performance requirements. During routine flight testing, ways to improve the engine were identified, but the company adds that any modifications will be minor.
Looking for efficiency
Whether geared turbofans will sweep all before them remains to be seen—and depends, at least in part, on the response of Pratt & Whitney's two big rivals in the jet-engine business, General Electric (another American firm) and Rolls-Royce (a British one). These companies are also working on more efficient aircraft engines. Both, though, think improvements can still be squeezed from the conventional turbofan design without resorting to a gearbox. General Electric, in partnership with Snecma, a French firm, is offering a rival engine, called the CFM Leap, for the A320neo.
This will be available later in 2016 and is claimed by the partners to provide fuel savings similar to those of a geared arrangement. The Leap is a conventional turbofan, but it is made using some unconventional techniques. These include new composite materials and also additive manufacturing (popularly known as 3D printing). Rolls-Royce, too, aims to get greater efficiency from its turbofan designs, though it does also have a gearbox-development programme, with a view to making a geared turbofan that might enter service on large passenger aircraft in around a decade's time.
As to PurePower itself, so far the opinion of airlines is divided. Airbus has taken orders for more than 4,400 A320neos. About a third of these will sport PurePower, a third Leap, and in the cases of the remaining third, the customer has yet to make up his mind.
Pratt & Whitney, though, does not plan to be tied only to Airbus. It is also offering versions of PurePower to firms trying to break the duopoly enjoyed on short-to-medium-range aircraft by that firm and Boeing. Bombardier of Canada is one such. Its competitor to the A320 is called the CSeries.
The Mitsubishi Regional Jet, from Japan, is another plane which Pratt & Whitney hopes might use PurePower. And there are also the MC-21, a 180-seat airliner from Irkut, a Russian aerospace company better known for its Sukhoi fighter jets, and the E-Jet from Embraer, of Brazil. Whether Pratt & Whitney's PurePower play will pay off remains to be seen, but as Charlie Taylor knew over 100 years ago, gearing up for success does mean taking risks.