Whirlybirds go green

tauringhuang.

Jul 23rd 2008
From Economist.com

Diesel engines could make helicopters—and other aircraft—more efficient and cleaner

THE ability to take off and land vertically, and to hover, makes helicopters useful machines. Unfortunately, because they lack fixed wings to help provide lift, they are also expensive to operate. Helicopters need powerful engines to drive their rotors, and that means they use a lot of fuel. So could they benefit from one of the developments being used to make car engines more efficient, namely the spread of diesel engines? EADS, Europe’s biggest aerospace company, seems to think they could. One of its subsidiaries, Eurocopter, has begun work on a diesel-powered helicopter. And EADS is not alone in bringing diesels to the air. Makers of fixed-wing aircraft, too, are studying them. Not for flying across the Atlantic, of course. They will never compete with jets. But where smaller planes are concerned, diesels are starting to look like a serious alternative to standard piston engines.

The first diesel-powered aircraft, a modified Stinson, took to the air in 1928. But, despite the fact that diesels were tried out in airships and some early Junkers bombers, they never really caught on. Their chief drawback was their weight. Both of a diesel’s big advantages on the ground, its efficiency and its torque (pulling power), are the result of its high compression of the mixture of fuel and air in its cylinders. That high compression, however, generates powerful forces, and only a big, heavy engine was strong enough to contain them. Hence, diesels were used mostly in heavy machines, such as locomotives and lorries.

Now the weight penalty is starting to diminish. New casting and manufacturing methods allow lighter, stronger components to be used. Computerised fuel-injection has improved diesels’ performance, and thanks to their inherent strength, they can be turbocharged to boost power even further. As a consequence, many of the least thirsty cars now on the road have diesel engines, and these same advances are being put to use in a new generation of diesel engines for use in small fixed-wing aircraft.

Diamond Aircraft, an Austrian company, is making a diesel-powered light aircraft which it says uses just 17 litres of fuel an hour instead of the 35 litres that a similarly sized plane with a traditional piston engine would consume. General Atomics, a Californian company, has performed a similar trick. It has begun supplying diesel-powered versions of its Predator unmanned aerial vehicle, known as Sky Warriors, to American forces in Iraq and Afghanistan. The Sky Warrior is intended for long-range reconnaissance missions, rather than as a weapons platform.

Eurocopter’s contribution is to propose fitting an advanced, turbo-charged diesel engine into one of its EC120 helicopters. Normally this aircraft, like many commercial helicopters, is powered by a jet-turbine engine. (Only the smallest helicopters, like Robinson R22s, still use piston engines.) Guillaume Faury, Eurocopter’s engineering chief, hopes to have a prototype diesel-powered EC120 flying by 2012. If the adapted helicopter works well enough, the company might build a twin-engined version too.

Eurocopter calculates that although an unloaded diesel EC120 would still be heavier than a jet-turbine version, it would use less fuel per kilogram lifted. This would not bring much benefit on short flights, but on longer ones (say, two or three hours) the weight of the extra fuel needed for the jet turbine would compensate for the inherent heaviness of the diesel engine, meaning that the diesel would bring a significant saving of fuel. That would make diesel helicopters ideal for missions such as search-and-rescue, and for border patrols. For the whirlybirds at least, this would be less thirsty work.