The aviation industry is currently focused on research and development of propulsion systems that produce less emissions, are more efficient, and can provide better range/endurance. Hybrid-electric systems have shown a promising potential in reducing emissions. Current battery technologies do not have the energy density required to meet most application requirements. While combustion technology has improved in efficiency over the decades;​ hybrid technology is required to take the next big step. In addition to the environmental benefits parallel hybrid technology provides other benefits such as operating mode variety, redundancy, and higher endurance.

In this thesis research, the primary goal is to model, evaluate and validate a mathematical model developed for a parallel hybrid-electric propulsion system. This model will be used in Model Based Design (MBD) to predict system performance, improve component selection, and optimize operation. For this thesis the modelling and design was completed for small-scale unmanned aerial vehicles (UAVs). A test bench was redesigned to handle the power produced by the combustion engine and electric motor (EM). For the experimental configuration a 50cc Corvid-50 combustion engine was used which produces 2.8kW at 7000RPM. This engine was combined with a SKP 6485 electric motor capable of 4.12kW continuously with a maximum speed of 8364RPM. Both power units are coupled together with a Mayr 500.301.0 type 4 electromagnetic clutch rated for 40Nm at 7000RPM. The propulsion system is connected to an electric dynamometer with programmable load capable of simulating any power profile. Telemetry for the system is collected through National Instruments hardware, electronic speed controller, and engine control unit.

During operation the test bench is able to operate in five different modes. Combustion and electric-only operation are capable by disabling the clutch for electric-only and running the electric motor at zero current for combustion. When running the system in a hybrid configuration there are three command modes:​ dual speed, throttle and speed, speed and current. Each of these command modes dictate which power unit governs speed while the other has direct torque control. Each of these hybrid modes provides the opportunity for regeneration and boost modes when requested. The test bench generates the experimental data required to model the propulsion system with accuracy. Using datasheets, unloaded runs, governing equations, and controller values component level models were created to complete initial simulations. The model and test iterative process repeated until the system level model responded well. To further develop the simulation model a virtual flight mission was created.

The test bench and Simulink model ran the virtual flight mission in combustion, electric, and hybrid modes. These different runs provided the data required to assess the model’s accuracy and demonstrate the difference between each propulsion technology. For these tests the simulation was able to predict speed and torque within a range of 1-12\% for steady-state operation between flight segments. The starting torque of the electric motor to initiate combustion was modelled to represent cold starts where the torque range was between 2-3Nm. Over the length of the fifteen-minute flight mission run the simulation predicted battery charge and fuel consumption within 5\%.

Energy density of each propulsion type was analyzed for the components used on the test bench. This showed that combustion power has the highest available energy density at 1.17MJ/kg;​ electric power is substantially lower at 0.35MJ/kg. The initial energy density of the hybrid system is 0.71MJ/kg but can be further optimized. By optimizing the energy masses for the hybrid-electric system an energy density equal to the combustion engine was accomplished with a 0.77kg mass reduction. This optimization process can be taken further by improving the command sequence of the system to incorporate regeneration, clutch disengagement, and throttle curve modification to reduce fuel flow.

The results of this research project created a simulation model and test bench capable of high-power flight tests. Both the model and test bench will continue to develop;​ further increasing the ability to design, optimize, and test parallel hybrid systems. This will provide the experience and knowledge to design, build, and integrate a power unit ready for flight testing.