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In the framework of a European research program, Airbus Helicopters in 2016 completed a flight-test program with a diesel engine on an H120 single. Test results have been analyzed and the program was a technical success, Airbus claims. However, an application still has to be found, as no engine manufacturer has picked up the work yet. This may explain why Airbus has been quiet about the program.
The goal, as part of the EU’s Clean Sky public-private “technology initiative,” was to demonstrate a diesel engine is a viable alternative to a turboshaft on a light single. The expected benefit was a reduced fuel burn, thanks to the greater efficiency. Challenges, meanwhile, were the torque oscillation of a piston engine, its weight and cooling.
With a diesel engine, a light single would have a greater endurance. The tested H120 could have flown eight hours, Christian Mercier, Airbus Helicopters’ chief research engineer for new engines, told Vertical. It would thus be highly suitable for operations such as power line surveillance, he said.
An additional advantage is better hot-and-high performance.
The company refers to a “high-compression engine,” rather than “diesel,” perhaps to keep away from the negative publicity that has hit diesel engines in the automotive sector in recent years.
After a two-month evaluation on an iron bird followed by one month of ground tests with the H120, five flight hours were enough to check every specification was met, according to Mercier. The trials of the 330-kilowatt (440-shaft-horsepower) powerplant were thus “a complete success,” Mercier said.
At takeoff power, the fuel consumption (using kerosene) was cut by 30 percent over that of a turboshaft. At cruise power (around 130 shp), it was “more than halved.” The average reduction, calculated over a mission, was 42 percent.
The solution to torque oscillation was the dampening system Airbus and its Clean Sky partners designed. The diesel engine was thus made compatible with a helicopter gearbox.
Another challenge was cooling. A piston engine’s thermodynamics call for a more complex system. The “power pack” on the H120 included a large fan and added non-negligible weight to an already heavy engine. Nevertheless, the engine and accompanying cooling system are much cheaper than a comparable turboshaft, according to Mercier.
At 0.8 kg/kW (including the cooling system), the weight index density is twice lower than that of a turboshaft but much better than piston engines in conventional cars. That number was achieved thanks to the input of a company specializing in car racing, Teos Powertrain Engineering. A power density of 0.6 kg/kW is within reach, Mercier said.
Teos and Austro Engine (part of the Diamond Aircraft group) co-designed the V8 engine. However, Airbus retains its intellectual property, Mercier stated. No established engine-maker seems to have expressed interest yet.
In addition to helicopters, the engine is deemed suitable for fixed-wing aircraft in general aviation, as well as auxiliary power units for commercial aircraft.