Hybrid Engines

Electrification can arguably help ameliorate the overall performance as the biggest advantage of  Hybrid Electric Propulsion Systems (HEPS) is the fact that the engine operates close to its design point to produce the required thrust and maintain all the secondary systems running, well below their maximum power setting. Hence, and apart from extending the life of the engine, the rotational speed of the shaft decreases, and overall pressure gradients decrease, containing noise emissions. SFC, in conjunction with greenhouse gas emissions, is also dramatically reduced since the engine can be optimized to operate inside tighter margins, thus securing the highest thermal efficiency of the propulsive unit.

There are two principal philosophies under examination at LFMT that integrate the hybrid electric technology inside the propulsion system. The first philosophy is the Parallel Scheme and the second one is the Turboelectric-Motor Serial Scheme. At the Parallel Scheme the engine works cooperatively with the batteries and the electrical systems to cover the power requirements of the platform. At the Turboelectric-Motor Serial Scheme the engine works as a power generator unit coupled with an Electric Motor (EM), charging the batteries that power the propulsors/electric fans (E-Fans). In general, an airbreathing engine has a high-power density, lacks vibrations and, as far as power generation is concerned, it is proven to be an ideal solution.

Hybrid and electrified propulsion systems, as a propulsion solution, can be incorporated in UAVs and play a key role in UAV design procedures by integrating the propulsion system with the airframe geometry. Such integration can be achieved by novel Boundary Layer Injestion (BLI) engine configurations that operates constantly at the wake of the UAV. The main advantage of such layouts and propulsion system configurations, is the utilization of low energy air (dirty air) for the propulsion mechanism, thus leading to the necessary energy top-up of the airflow at the back-end of the airframe.

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