Phillip Ansell, assistant professor in the Department of Aerospace Engineering in the College of Engineering at the University of Illinois explains the benefits and downsides of fossil fuels. “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.”
Ansell and colleagues are striving to use simulations to increase the range of hybrid aircraft. Batteries provide their own set of problems. He explains that adding more batteries to go farther may have a certain logic, for example, but increases weight – a big detriment.
“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.”
eVTOL News recently noted that over 125 electric or hybrid eVTOL aircraft are under development or now flying. Because a large proportion of these craft rely on lifting the total weight of the vehicle and its occupants with only downward-pushing rotors or fans, the lift has to equal the weight, obviating the use of only batteries for energy storage.
Hybrids rule for this application, and several groups are researching and developing improved hybrid technology to meet the need. Since Uber is leading the way on a great many fronts, and asking for vehicles that might weigh 5,000 pounds at their all-up weight, hybrids are currently the only way to haul the load any appreciable distance.
Ansell, former aerospace undergraduate student Tyler Dean, and current doctoral student Gabrielle Wroblewski, set a baseline performance simulation of the twin-engine testbed in its original configuration, and then performed a series of simulations to model the performance of hybrid-electric variants of that aircraft.
Modeling some of their work on that of the X-57 team at NASA’s Armstrong Flight Research Center at Edwards Air Force Base in the Mojave Desert of California, the team chose a Tecnam P2006T as their base aircraft. Interestingly, the Illinois team is investigating both parallel hybrid and serial hybrid propulsion on their model – Parallel hybrid systems combine the output from the internal combustion engine with that of the electric motor. Serial hybrids power their motors from a central engine/generator that charges the batteries for a number of electric motors.
Ansell explains the process. “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.”
The University reports, “After configuring the simulator to collect baseline performance data, a parallel hybrid drivetrain was integrated into the simulation. The researchers compared the sensitivity of range and fuel economy to the level of electrification, battery specific energy density, and electric motor power density. The same sensitivities were studied with a series hybrid-electric drivetrain.”
Ansell looks for “substantial improvements in fuel efficiency in a given aircraft configuration” for a given configuration, with variations for the amount of electrification and the required mission range. Short-range missions are best for fuel efficiency, evidenced by the 25-percents propulsive power coming from an electric motor in the current version – but only for 80 miles range. Ansell and his team anticipate that by 2030, battery advances will increase that range “two and a half to three times.”
Ansell adds one fascinating comparison. “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.”
The study, “Mission Analysis and Component-Level Sensitivity Study of Hybrid-Electric General Aviation Propulsion Systems,” was conducted by Tyler Dean, Gabrielle Wroblewski, and Phillip Ansell. It appears in the Journal of Aircraft.
This project was supported by NASA Neil A. Armstrong Flight Research Center under Small Business Technology Transfer in collaboration with Rolling Hills Research Corporation.