Fundamental Studies of Variable-Voltage Hybrid-Electric Powertrains

A variable-voltage hybrid-electric powertrain is constructed and tested to acquire data and understand the fundamental characteristics of the system. The powertrain is examined component by component, with over 500 test points, from the engine alone to the engine–generator, to the engine–generator with four distributed propulsors, in an instrumented test bed. The principal conclusion is that generator voltage is a key parameter that needs careful control relative to rotor speed to minimize engine-specific fuel consumption. For any operating state—defined by rotor torque and revolutions per minute (RPM)—the generator voltage should be minimized. In general, the system is influenced more by the engine–generator than electric motors. Hence greater rotor torque and lower rotor RPM is desired in general, implying the need for collective control on the rotor. The overall understanding gained from this work is that the effectiveness of a hybrid-electric powertrain for vertical take-off and landing is closely coupled with controls and aeromechanics. Reliable design and simulation will require integration with these disciplines—a powerplant designed in isolation will be suboptimal. The data reported in this paper can provide a basis to build and validate models that can be used for this purpose.