Nov 28 2018
While hybrid-electric cars are turning out to be relatively common, analogous technology used in airplanes presents an entirely different set of challenges. Now, aerospace engineers at the University of Illinois are tackling some of these challenges in an effort to develop a more sustainable substitute to fossil fuels to power airplanes.
This figure shows the (a) parallel and (b) series drivetrain models. (Image credit: Department of Aerospace Engineering, University of Illinois)
Jet fuel and aviation gasoline are easy to store on an airplane. They are compact and lightweight when compared to the amount of energy they provide. Unfortunately, the actual combustion process is very inefficient. We’re harnessing only a small fraction of that energy but we currently don’t have electrical storage systems that can compete with that.
Phillip Ansell, Assistant Professor, Department of Aerospace Engineering, College of Engineering, University of Illinois.
According to Ansell, while it may seem logical to add more numbers of batteries to fly farther, it works against the objective of making an airplane as lightweight as possible. “That’s one of the big barriers we run into when designing battery-powered electrified aircraft. The current technology has very significant range disadvantages. But strong fuel-burn advantages.”
Together with current doctoral student Gabrielle Wroblewski and former aerospace undergraduate student, Tyler Dean, Ansell used a sequence of simulations to design the flight performance of hybrid-electric aircraft.
We started with an existing twin-engine aircraft and looked at how we might create a hybrid-electric drivetrain for it using existing off-the-shelf hardware. We wanted to know how well it would perform. If I used a certain set of drivetrain components, I want to know how far the aircraft could fly, how much fuel does it burn, how fast can if climb—all of the overall flight performance changes.
Phillip Ansell, Assistant Professor, Department of Aerospace Engineering, College of Engineering, University of Illinois.
Eventually, the team developed a flight-performance simulator to precisely represent the real flight performance of a Tecnam P2006T aircraft on a common mission to include take-off, climb, cruise, descent, and landing, together with adequate reserves to fulfill FAA regulations. Within the simulation, transition segments were added during climb and descent operations where all other flight control variables, including the propeller rotation rate, flap deployment, and throttle setting, were set to imitate the input from a standard pilot, or alternatively, prescribed according to the aircraft flight manual.
Once the simulator was configured to gather baseline performance data, the simulation was integrated with a parallel hybrid drivetrain. The team evaluated the fuel economy and the sensitivity range against the level of electrification, electric motor power density, and battery-specific energy density. A series hybrid-electric drivetrain was used to inspect the same sensitivities.
According to Ansell, on the whole, significant improvements can be realized in fuel efficiency of a specified aircraft configuration through a hybrid-electric drivetrain, although these gains are strongly dependent on the combined differences in the needed mission range and the degree of drivetrain electrification. Both of these factors affect not only the weight allocation of fuel systems and battery but also the weight scaling imposed by electrical motor components and internal combustion engine. Generally, the highest fuel efficiency can be achieved if a hybrid architecture is utilized with as much electrification in the drivetrain as is allowed within a specified range requirement.
Improvements in fuel efficiency were demonstrated to be especially useful for short-range missions, which can be considered as a good sign because range constraints act as one of the major bottlenecks with regards to the feasibility of hybrid aircraft. Through this analysis, developments in hybrid component technologies also made it possible to forecast the variations in the aircraft’s range capabilities.
For example, the propulsion system today could be configured to have 25 percent of its propulsive power come from an electric motor. However, it would only be able to fly about 80 nautical miles. Fast forward to projections for lighter battery technologies for roughly the year 2030 and the same aircraft could fly two and a half to three times as far. The range increase is nonlinear, so the largest improvements can be seen for the most immediate improvements with battery specific energy density, with gradually diminishing returns for that same proportional increase in specific energy. One interesting and unexpected result we observed, however, came about when comparing the parallel and series hybrid architectures. Since the parallel architecture mechanically couples the shaft power of the engine and motor together, only one electrical machine is needed. For the series architecture, a generator is also needed to convert the engine power to electrical power, along with a larger motor than the parallel hybrid configuration to drive the propulsor. Unexpectedly, this aspect made the parallel architecture more beneficial for improved range and fuel burn almost across the board due to its lighter weight. However, we did observe that if significant improvements are made in maturing electrical motor components in the very long term, we may actually someday see better efficiency out of series-hybrid architectures, as they permit a greater flexibility in the placement and distribution of propulsors.
Phillip Ansell, Assistant Professor, Department of Aerospace Engineering, College of Engineering, University of Illinois.
The researchers opted to design the Tecnam P2006T aircraft through a range of performance variables featured in published articles by the aircraft manufacturer. That specific aircraft was partly selected by the researchers because NASA has been developing its X-57 aircraft, which has advanced propellers for high lift.
“This study was being conducted for NASA, and use of this aircraft also allowed our results to be better applicable to the X-57 concept vehicle,” stated Ansell. “Using our data, they will be able to have at least a ballpark idea about how the hybrid system will perform without the other distributed propulsion modifications.”
Ansell added that even now, not much is known about propulsion electrification in relation to how a vehicle should be constructed, engineered, flown. “Our study helps inform those discussions. We looked only at battery storage systems though there are many more that can be implemented, each with their own advantages and disadvantages. This study allowed us to look at what types of advancements need to be made in motor technology, in battery technology, etc.”
The study, “Mission Analysis and Component-Level Sensitivity Study of Hybrid-Electric General Aviation Propulsion Systems,” was performed by Tyler Dean, Gabrielle Wroblewski, and Phillip Ansell, and has been reported in the Journal of Aircraft.
The research was funded by NASA Neil A. Armstrong Flight Research Center under Small Business Technology Transfer in association with Rolling Hills Research Corporation.