Aviation gasoline has been the fuel of choice for lighter aircraft for decades, but with leaded avgas becoming hard to find in some regions and environmentally unacceptable in others, diesel-cycle engines burning jet fuel are gaining ground.

While aviation diesels have low specific fuel consumption (sfc), the problem with them has been their power and weight. They have not been powerful enough for use in larger, higher-performance general aviation (GA) aircraft. And power-to-weight ratio has been too low for them to be usable in light helicopters.

This is changing, with ground tests now underway of a new aviation diesel replacing the turboshaft engine in an Airbus Helicopters EC120 (see photo). Flight tests of the HIPE AE440 high-compression engine (HCE) are to begin by mid-year under Europe’s Clean Sky research program. The turbocharged V8 has been developed by aviation-diesel producer Austro Engine and race-car engine maker TEOS Powertrain Engineering.

Clean Sky is a €1.6 billion ($1.69 billion) public-private partnership under which European Union research funding is matched by industry and other participants. Led by Airbus Helicopters, the HCE program is one of the major technology demonstrations under the Green Rotorcraft project within Clean Sky, which began in 2007 and runs to 2017.

Based in Austria, Austro already is planning production of the 330-kw (440 hp) AE440 for the fixed-wing market, to meet demand for a jet-fuel engine with higher power and lower fuel consumption than offered by its in-production 125-kw AE300 diesel. The light helicopter market may take longer to penetrate, but a competitive diesel-cycle engine could yet breathe new life into the sector.

Clean Sky’s goal is to demonstrate reductions of 30% in sfc, 40% in carbon-dioxide and 50% in nitrogen-oxide emissions in the EC120’s turbine engine. A 30% reduction in direct operating cost and a doubling of range with the same fuel are also targets. The AE440 has a weight-to-power ratio of 0.8 kg/kw (1.3 lb./hp) and a fuel consumption of 235 g/kwh (0.386 lb./hp-hr.) at takeoff power.

The keys to the AE440’s performance are its aluminum construction and fuel injection, both of which come from the automotive industry. Austro’s four-cylinder AE300 is based on a Mercedes diesel and is a cast steel engine. The AE440 is an all-new engine made of lightweight, machined aluminum. In production, this will be changed to less costly but still lighter cast aluminum, as used in race cars.

Common-rail direct fuel injection operating at a 1,800 bar (26,000 psi) provides precise injection of less fuel at higher pressure for improved atomization and combustion, and reduced fuel consumption. Two high-pressure fuel rails feed individual injector valves digitally controlled by the engine management system.

To enable it to directly replace the faster-turning Turbomeca Arrius turboshaft in the EC120 testbed, the AE440 has a multiplier gearbox so it can drive the rotors via the existing transmission. For fixed-wing applications, the engine will drive the propeller via a reduction gearbox.

With €9.3 million ($9.9 million) in funding under Clean Sky, the research program has not been without its challenges. Austro and TEOS have built five engines and completed 300 hr. of tests so far. Component failures have caused delays, which is not unusual in engine development. This and the recent decision to add 100 hr. of flight testing have stretched the program to 47 from 39 months.

But the AE440 is Austro’s first clean-sheet engine, and the company acknowledges that without Clean Sky it would not have taken on development of a powerplant that could be the breakthrough needed to bring fuel-efficient, low-emission aviation diesels to a wider market.