The Impact of Shaft Power Extraction on Small Turbofan Engines: A Thermodynamic and Exergy-Based Analysis for No-Bleed Architectures

In “no-bleed” engine architectures, bleed air is replaced by shaft power extraction to run the subsystems, avoiding the inefficiencies of traditional bleed systems. This approach is increasingly used in small turbofan engines, prompting analysis of its impact on engine performance and exergy efficiency. A small high-bypass turbofan engine was modeled in software under two control strategies: constant thrust (CT) and constant speed (CS), with shaft power extraction up to 18 kW. Exergy analysis evaluated efficiency losses and sustainability metrics (exergy efficiency, environmental effect factor, and exergetic sustainability index). Simulations indicate that an 18 kW shaft power extraction increases SFC by 13.6% (CT) and 42.1% (CS). Exergy efficiency rises from 47.3% to 50.7% (CT) and 54.2% (CS). However, these power draws also increase irreversibility and the environmental effect factor (EEF) grows from 0.678 to 0.732 (CT) and 0.744 (CS), while the exergetic sustainability index (ESI) drops from 1.48 to 1.34, signaling reduced sustainability at high extraction. Maintaining constant thrust during extraction incurs smaller fuel consumption and exergy efficiency penalties than constant speed control. The findings highlight the need for adaptive control strategies (e.g., limiting extraction levels or using variable-geometry components) to mitigate losses and enhance sustainability in no-bleed engine designs.