Turbomeca plans to reduce the specific fuel consumption (SFC) of its turboshaft engines by 37 percent by 2030, and at the Helitech 2011 show discussed the strategies it is implementing to accomplish that change. While the company plans to make changes to the engine machinery, it expects much of the reduction to come from engine-airframe integration and new practices that make more efficient use of the engine.
In the last 20 years, the engine manufacturer has made relatively little progress in SFC reduction (15 percent), said senior v-p for product and market strategy Charles Claveau. The limited SFC reduction is the result of a scale effect, he explained. “When you want to cut the SFC, you increase the pressure ratio. This means you head for a smaller last stage in the compressor. At some point, you simply cannot manufacture the tiny blades you would like to,” he said.
Still, there is some hope to improve the turboshaft’s efficiency. “We can gain 15 percent in cruise SFC; one improvement would be to add inlet guide vanes upstream the centrifugal compressor,” Claveau said. However, such a change makes the engine more complex.
Engine-aircraft integration will also help reduce specific fuel consumption by as much as 5 percent, Claveau asserted. Today’s engines are deliberately oversized for safety reasons, such as to provide redundancy on a twin. However, an oversized engine burns much more fuel in cruise. Claveau suggested designing the engine without the 30-second one-engine-inoperative emergency power requirement and instead designing the system so the battery’s energy could be “injected” into the turboshaft, boosting it during that 30 seconds.
In other words, this is a first step into a hybrid cycle. So what’s the bottom line? “If you decrease the size of the engine by 7 percent, you cut the SFC by 5 percent,” Claveau said.
Another reason twins consume so much fuel is that they are in full twin-engine mode all the time, even in cruise. Why not simply stop one of the engines when less power is needed? Turbomeca tested the possibility on a Bell 206 with Rolls-Royce engines, Claveau said. He added he understands, though, that a pilot might be reluctant to deliberately shut down an engine in flight. A tradeoff could be to have one engine running at a special idle speed, yielding a 12-percent reduction in SFC, Claveau predicted.
In a more distant future (after 2030), engines could be fitted with heat exchangers. The principle is to use the thermal power downstream the turbine to heat the air just upstream the combustor. This is extremely efficient, Claveau said, potentially reducing SFC by 20 percent. This idea is not a new one, but progress has been slow because of the exchanger’s huge weight and size.
Another exotic idea would be to replace part of the hot section with a pulse-detonation engine.
The Bordes, France-based engine maker has assigned itself other 2020 objectives in addition to the SFC pursuit. Among them: cut the frequency of in-flight shutdowns by a factor of 10, to one per million flight hours; extend the mean time between failures to 1,000 hours for the entire engine (including accessories); and offer time between overhaul of 6,000 hours at entry into service.