Turbine Power Distribution and Energy Pathways in Free-Turbine Turboshaft Engines: A Comparative Thermodynamic Study

This paper presents a thermodynamic analysis of free-turbine turboshaft engines, focusing on the quantitative distribution of turbine power and related energy parameters between the gas generator turbine and the free power turbine. The study is based on an analytical calculation model combining catalog specifications and validated experimental data, applied to a series of turboshaft engines from different manufacturers with similar free-turbine architectures and power classes ranging from approximately 960 kW to 2100 kW. The comparative analysis is conducted at take-off conditions for the engine series, while a detailed regime-dependent investigation from idle to take-off is performed for the TV2-117A reference engine. The results indicate that, at take-off, the gas generator turbine typically absorbs between 55% and 66% of the total turbine power to drive the compressor, whereas the free power turbine delivers the remaining 34% to 45% as usable shaft output. For all analyzed engines, the total actual specific enthalpy drop of the expansion process exceeds 98% of the available thermal potential, demonstrating efficient turbine energy utilization. Total turbine temperature drops are found to range between approximately 335 K and 565 K, depending on engine power class and cycle characteristics. In the case of the TV2-117A engine, the gas generator turbine power share decreases from about 75% at idle to roughly 65% at take-off, confirming a clear regime-dependent redistribution of expansion work. Thermal efficiency values at take-off vary between approximately 23% and 31% across the analyzed engine series. Unlike previous studies primarily focused on single-engine modeling or control strategies, this work introduces a unified and experimentally validated multi-engine thermodynamic framework that quantifies internal turbine power distribution patterns and provides transferable design-oriented benchmarks for free-turbine turboshaft engines.