If engineering design is all about trade-offs, the compromises are especially pronounced in propulsion for small military UAVs. The little piston engines for such aircraft traditionally run on gasoline, an unwelcome fuel in military service. They also waste much of that fuel in their search for high power/weight ratios, and for the same reason they are highly tuned, contributing to noise and vibration, unhelpful for their typical missions of surveillance and reconnaissance.

A new, unidentified U.S. small UAV is avoiding those compromises by employing an engine technology already in widespread civilian use on the ground, from snowmobiles to rickshaws. While specifications are undisclosed, something like a 40% increase in range or endurance has probably been achieved.

The customer and function of the aircraft for the latest powerplant are also undisclosed, but the engine supplier, Orbital Australia Ltd., says its client operates continuous worldwide intelligence, surveillance and reconnaissance operations for land and maritime missions. Orbital's technology will allow the engine to burn JP-5 and JP-8, “thus satisfying a U.S. Defense Department initiative to eliminate gasoline fuels for safety and logistic reasons”—a fairly clear hint at the military background of the customer.

“The program target is to develop a small engine package that will be durable, fuel-efficient and lightweight, all key factors to provide the desired range and payload capability,” says Orbital. “A further advantage of the Orbital technology is extended range compared to the standard engine typically used in this class of small UAV.”

Other key design objectives are low noise and vibration—presumably for stealth and stable camera-mounting. Spark-ignition also helps hold down vibration, since the internal forces of the engine are not as great as in a diesel.

The development contract is worth AU$4.5 million ($4 million). Orbital will do the work this year and next, making prototype and initial low-volume production engines in Perth, although it plans to set up assembly, testing and support facilities in the U.S.

Traditionally, designers of piston engines for such aircraft use gasoline because it can be readily burned in a spark-ignition engine; the massive construction of engines for compression-ignition does not suit aviation. But military operators dislike that highly flammable fuel; many, including the U.S. armed services, are phasing it out.

Also, a two-stroke cycle, with every cylinder firing on every revolution, is needed to maximize power-to-weight ratios—but, when combined with a carburetor or conventional port fuel-injection, a traditional two-stroke engine exhausts some fuel before burning.

The Orbital engine chosen by the unnamed U.S. customer uses heavy fuel, such as the kerosene-based JP-5 and JP-8 commonly stocked for other military equipment, and ignites it with a spark. The two-stroke cycle receives the fuel by direct injection, timed to avoid the opening period of the exhaust ports. And the fuel-injection system achieves fine atomization to promote rapid vaporization of the lower-volatility fuel to achieve the same or better combustion as gasoline.

The key feature of Orbital's technology is acceleration of the fuel with air during injection. The fuel enters the cylinder more finely atomized than it would otherwise and so evaporates more quickly and completely, forming something close to an ideally even mixture with the air. The result is not only good burning, as thus injected, heavy fuels can be ignited with a spark. Compared with equivalent gasoline-carburetor or port fuel-injected engines, the range advantage comes mainly from avoiding loss of unburned fuel.

Orbital has been working for 25 years with the technology, which it first developed for an unsuccessful internal combustion engine that featured an unusual orbital motion, hence the name of the company. The technology has been applied to recreational vehicles, such as snowmobiles, for a high power/weight ratio, and to rickshaws in India.

For aviation, Orbital intends to offer engines rated at 2–120 kw; outside of that range it believes other technologies become competitive.

Orbital Engineering Director Geoff Cathcart says there is not one key obstacle to making the technology work, and the many interrelated challenges have notably included software for managing the engine under widely varying operating conditions. For example, kerosene has a low effective octane number, so it all too readily pre-combusts at high temperatures—which, inconveniently, are encountered mainly at sea level, where the greatest power is needed. At the low temperatures of high altitudes, on the other hand, kerosene is not eager to burn at all.

Orbital has chosen larger cylinder sizes for a given power and therefore slower running. Weight rises, but not proportionately, and there is a gain in propulsive efficiency (and, presumably, a reduction in noise) from turning the propeller more slowly.

Vibration is also attacked by attention to engine balancing, prolonging engine and airframe life, as well as improving sensor mountings and thus performance.

The size, output and fuel consumption of the new engine has not been disclosed. The name cannot be revealed, either, because Orbital's people, like piston-engine designers the world over, have designated their product by its cylinder capacity. Internally, the engine family is called Redback, after a small and deadly Australian spider.

Orbital has begun at the bottom of the aero-engine power range that it intends to address. Textron unit AAI Corp. contracted last year for a “tiny two-stroke engine” from Orbital that produces 4 hp. (3 kw) and is supposed to meet “aggressive fuel economy, weight and noise-reduction targets.” Scant details of that design should give clues to this year's more secret engine. The AAI engine has a single cylinder, is made of light materials and will fit into a shoe box, for powering an aircraft for the U.S. Navy and Special Operations Command. The technology extends range “by up to 40 percent, or allows AAI to increase the payload,” Orbital said last year.

“Engineering development to reduce its noise emissions included refinements to the engine block, exhaust system, speed of revolution and propeller design,” the company says.