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Hybrid-electric propulsion has emerged as a promising technology to mitigate the adverse environmental impact of civil aviation. Boosting conventional gas turbines with electric power improves mission performance and operability. In this work the impact of electrification on pollutant emissions and direct operating cost of geared turbofan configurations is evaluated for an 150-passenger aircraft. A baseline two-and-a-half-shaft geared turbofan, representative of year 2035 entry-into-service technology, is employed. Parallel hybridization is implemented through coupling a battery-powered electric motor to the engine low-speed shaft. A multi-disciplinary design space exploration framework is employed comprising modelling methods for multi-point engine design, aircraft sizing, performance and pollutant emissions, mission and economic analysis. A probabilistic approach is developed considering uncertainties in the evaluation of direct operating cost. Sensitivities to electrical power system technology levels, as well as fuel price and emissions taxation are quantified at different time-frames. The benefits of lean direct injection are explored along a realistic mission scenario, demonstrating 32% NOx savings compared to traditional rich-burn, quick-mix, lean-burn technologies. The impact of electrification on the enhancement of lean direct injection benefits is investigated. For hybrid-electric powerplants, the take-off-to-cruise turbine entry temperature ratio is 2.5% lower than the baseline, extending the corresponding NOx reductions to the level of 46%. This work sheds light on the environmental and economic potential and limitations of a hybrid-electric propulsion concept towards a greener and sustainable civil aviation. INTRODUCTION Mitigation of the adverse environmental impact of civil aviation constitutes a major objective for manufacturers, operators and regulatory and advisory bodies (Darecki et al., 2011; Scherer, 2019). According to the 20-Year Air Passenger Forecast of the International Air Transport Association (IATA), the foreseen 3.5% compound annual growth rate will lead to a doubling in passenger numbers compared to today’s levels (IATA, 2019). Responding to this demand, a series of revolutionary propulsion system architectures has been proposed to address the challenging future requirements (Epstein, 2014; Pornet et al., 2015; Bowman et al., 2018; Sahoo et al. 2020). Electrified propulsion has emerged as a promising concept for enhanced fuel efficiency, reduced gaseous emissions and improved operability. The potential impact of hybrid electric propulsion in regional aviation was assessed for entryinto-service (EIS) technology of 2035 (Pornet et al., 2014). Block fuel reduction of up to 20% was achieved for a hybrid electric twin-engine narrow-body aircraft, with a single electric propulsor operated in cruise. However, this figure did not account for the life-cycle energy production requirements. Similarly, weight penalties eliminated performance improvements of turbo-electric aircraft featuring a boundary layer ingesting propulsor at the fuselage aft-end (Giannakakis et al., 2019). A more conservative, boosted concept was proposed for an 150-pax aircraft with a low-speedshaft-coupled electric motor to provide power during take-off (Zhao et al., 2019). Despite operability improvements, there was no significant fuel saving with 2035 technology levels assumed for all electrical components. NASA reported 6-14% reductions in direct operating cost (DOC) for an 150-pax high-span hybrid electric aircraft relative to a non-electrified reference (Bradley et al., 2015). Nevertheless, these benefits were outperformed when costs associated with increased aircraft weight and complexity were included. The importance of charging the batteries with electricity from renewables to achieve concurrent reductions in CO2 emissions and DOC was highlighted for regional hybrid electric aircraft (Hoelzen et al., 2018). However, shifting to renewable energy ground-based plants includes

Enabling the Potential of Hybrid Electric Propulsion Through Lean-Burn-Combustion Turbofans