Hybrid Power Units (HPUs) for EVTOL Urban Air Mobility (UAM) Vehicles: A Conceptual Analysis and Future Research Directions

Abstract

This article explores hybrid power systems (HPSs) as a promising solution to enhance payload (PL) capacity and flight endurance (FE), promoting the adoption of electric vertical takeoff and landing (eVTOL) technology for Urban Air Mobility (UAM). First, a review of relevant aviation technologies, including fully electric and hybrid eVTOL vehicles, is conducted. Second, an algorithm is introduced, systematically utilizing key performance metrics such as maximum takeoff weight (MTOW), power-to-MTOW, fuel weight (FW), FE, and total energy generation (TE). A new metric, usable weight (UW), is proposed to quantify the increase in FE or PL capacity. Third, five distinct hybrid power unit (HPU) case studies are analyzed based on the proposed algorithm, incorporating a supercar piston engine (PE), a turboshaft engine (TSE), a Wankel engine, a fixed-wing adapted engine, and a new proposed engine. Each HPU is virtually run with a predefined flight profile. This analysis estimates FE, TE, and total FW, highlighting that the proposed PE represents a trade-off between power, weight, and fuel efficiency. Fourth, a comparative framework evaluates the TE output of five HPUs and a fuel cell (FC)-battery hybrid system from the literature, each with a maximum power output of 450 kW, for a 2000 kg MTOW eVTOL and a powertrain weight limit of 560 kg (including fuel). Fifth, seven commercial aircraft PEs were assessed for their suitability as prime movers in eVTOL UAM applications. The analysis identified only one candidate engine, with a power-to-weight ratio (PWR) of 1.4 kW/kg, capable of delivering FE of ≥1 hour. Finally, virtual aviation engines with power outputs up to 450 kW (dual 225 kW configuration) and PWR spanning 1-3 kW/kg were investigated. Results demonstrate that engines with PWR ≤1.1 kW/kg fail to satisfy the minimum power density requirements for sustained eVTOL FE (≥1 hour). By establishing a systematic framework for HPU prime mover selection, this study advances the design of next-generation eVTOLs for UAM with medium-to-long endurance capabilities, while simultaneously providing actionable insights to connect academic research with industrial development. © 2025 IEEE.