More Efficient Jet Engine Gets in Gear

An aircraft engine that uses a novel gearing system could decrease pollution and increase the energy efficiency of air travel.

U.S. engine maker Pratt and Whitney, which developed the engine has signed a deal that should see the engine enter service in 2016 on the next-generation Airbus A320. The design promises to lower noise pollution and nitrous oxide emissions significantly and reduce fuel consumption by up to 15 percent. The fuel saving should mean that each aircraft will emit 3,600 fewer tons of carbon dioxide annually, says Tom Enders, CEO and president of Airbus.

The new gear system decouples the engine’s turbine, which turns as air flows through it, from the large air intake fan at its front, making it possible to run both at optimum speeds, says Graham Webb, chief engineer at Pratt and Whitney’s Next Generation Product Family Group in East Hartford, Connecticut.

The concept of a geared turbofan is not new, says Magdy Attia, an aerospace engineer at Embry-Riddle Aeronautical University in Daytona Beach, Florida. The first designs for this type of engine gearing date back to the 1980s. “But the industry is very resistant to change,” he says. “Gear boxes are heavy and notorious for being very problematic.”

If the problems can be overcome, the rewards in terms of efficiency gains should be significant, says Attia. He says other engine makers are likely to follow suit, developing their own geared systems.

The new engine developed by Pratt and Whitney is a turbofan. Like other jet engines, it burns fuel as compressed air is passed through; the resulting air-gas mix expands to generate thrust. But in a turbofan engine, a fan at the front forces slower-moving air around the engine and mixes it with the faster hot gases to create additional thrust.

Indeed, most of the thrust generated by a turbofan engine comes from this bypassed air. Thus, a smaller jet engine is required, so less fuel is burned. The space around the engine also helps reduce noise.

The size of the chamber around the engine relative to the engine itself is called the bypass ratio. “The trend in modern engines is to go with higher and higher bypass ratios, because they are more efficient,” says Attia. Currently, the average ratio is around 8:1. The gearing system allows the ratio in the new engine to be 12:1.

The inlet fans at the front of turbofan engines are driven by turbines that are rotated by the combusted gas-air jet. The new engine lets the inlet fan rotate at a third of the speed of the turbine, improving the propulsive efficiency. “This allows both [the turbine and the inlet fan] to run at their optimal speeds,” says Webb.

The design also allows other elements of the engine to be simplified. “We were able to remove several low-pressure turbine stages and two low-pressure compressor stages,” Webb says. “This adds up to more than 1,500 fewer airfoils in the engine, which significantly reduces maintenance costs for airlines.”

Pratt and Whitney has been testing a geared turbofan demonstrator since 2008 and has conducted more than 75 hours of test flights. Over the next two years, 15 engines will enter testing, with engine certification expected by late 2012.