THAT jet engines have evolved over the past few decades will be apparent to any seasoned air traveller. Early jet engines had narrow inlets and were very noisy, but as the diameter of the fans at the front increased, the engines became quieter. Compared with a rowdy 1960s jet, a modern turbofan is some 80% quieter and burns as little as half as much fuel—thus producing fewer greenhouse gases. But with regulation of aviation emissions likely in the coming years in order to combat climate change, jet engines must become cleaner and more frugal still.

When a jet engine is running, a fan at the front draws in air, which is then compressed and burned with fuel to produce hot, rapidly expanding gases which roar out of the back. On the way out, these gases drive a turbine which is connected to the fan via a shaft, thus drawing in more air and keeping the process going. But not all the air passing through an engine needs to go through the turbine. Over the years the fans have grown larger and the fraction of the air that passes through the turbine has fallen. The latest “high bypass” turbofans, such as the Rolls-Royce Trent, push around nine times more air around the core of the engine than through it. Such high-bypass engines rely on a bigger but slower-moving volume of air to provide thrust. Their turbines can be smaller, since not all the air is passing through them, and this in turn means they are quieter and use less fuel.

But the aviation industry has set itself a tough target: a 50% reduction in fuel consumption by 2020, to cut CO 2 emissions in half. Building more aerodynamic aircraft and operating them more efficiently (by reducing air-traffic-control delays, for instance) could produce about two-thirds of those savings. But the rest will have to come from better engines. Research in areas such as new materials and improved blade design will provide incremental improvements to high-bypass turbofans. But it is unclear whether these will be enough to achieve the required fuel savings. A completely new type of engine may be needed.

The solution devised by Pratt & Whitney (P&W), a division of United Technologies, is a “geared turbofan” engine called the PurePower PW1000G. Unlike a conventional turbofan, it uses a gearbox rather than a shaft between the fan and the turbine. Turbines run most efficiently at high speeds, and fans at low speeds, so turbofan engines have to compromise between the two, because the engine's design requires them to turn at the same speed. A gearbox, however, allows the turbine to operate at a high speed while driving the fan at a lower speed. In February P&W said that in tests, this design had proved capable of “double digit” improvements in fuel efficiency and emissions, and a 50% reduction in noise.

Some airlines, however, are wary of gearboxes. They worry that replacing a simple shaft with a complex gearbox will increase maintenance costs and make it more likely that something will go wrong. P&W disagrees. Bolted under the wing of an Airbus A340 as part of its test programme, the PW1000G endured more than 75 hours of operation, including many extreme manoeuvres, and proved its durability, says Bob Saia, who is in charge of engine development at P&W. Moreover, he adds, the gearbox is made from the same kind of steel that is already used in the small gearboxes that take power from turbofans for aircraft systems.

P&W will now use the data gathered from its test flights to finalise the engine's design. The PW1000G is due to enter service in 2013 powering two new short-haul aircraft being built by Mitsubishi and Bombardier. It could also be scaled up for use on bigger aircraft. Some in the industry think geared turbofans could eventually reduce fuel consumption by 20-25%.

There is another way to build a greener jet engine: by bypassing the turbine to an even greater extent, with an open rotor. This is a bit like going back to propellers. A number of designs use two rings of stubby, counter-rotating blades made from composite materials. Unlike the blades on old-fashioned propeller engines, these blades spin around at the back of the engine. Rolls-Royce and General Electric (GE) are studying this approach, though they are also keeping their options open by working on improvements to conventional turbofans, too. GE carried out test flights with an open-rotor engine in the 1980s, and reckoned it would use 30% less fuel than similar-sized engines of the time. But the engine was noisy, and there were concerns about what would happen if one of the blades broke off and tore into the aircraft's fuselage.

All this is difficult for aircraft-makers, who are used to being able to hang competing engines off the same wing. Airlines can then choose which airframe to buy, and which engines, and there is competition in both fields. But the geared turbofan and the open rotor are so different from each other that they will need different airframes. For safety reasons, open-rotor engines might have to be mounted at the back, for example. At the moment Boeing is concentrating on its new 787, a medium-sized, wide-bodied aircraft, and its rival Airbus is delivering the first examples of its new A380 super-jumbo. But before long the two companies will turn their attention to designing replacements for the smaller 737 and A320, the most numerous aircraft in the sky. At that point, they will have to decide which, if either, of these new engines they want to adopt.