Recognizing the attention currently devoted to the environmental impact of aviation, this two-part publication introduces two new aircraft propulsion concepts for the timeframe beyond 2030. Part one focuses on the exhaust heat generated steam injection engine concept featuring in-flight condensation and recovery of the used water. In the second part, the free-piston composite cycle engine concept is presented. Based on consistent thermodynamic descriptions, preliminary designs and initial performance studies, the concepts potentials are analyzed and subsequently discussed. A third but separate publication, building upon those two concepts, presents the procedure for demonstrating their feasibility with numerical simulation and test bench experiments up to a technology readiness level of four.

The development of both concepts is an effort to drastically reduce climate-impacting emissions from aviation by 2050. The conventional gas turbine engine used in today's civil aviation has undergone enormous developments since its invention. At present, it seems possible that the Joule-Brayton cycle-based gas turbine will meet the 2030 emission reduction targets through continuous but increasingly expensive technological improvements. However, the reduction of emissions beyond this point requires novel concepts to overcome the approaching physical limits of the known gas turbine cycle.

A composite cycle engine is an integrated assembly of at least two heat engine cycles featuring independent compression, heat source and expansion operating on the same working fluid. The free-piston composite cycle engine concept is composed of a gas turbine topped with a free-piston system. The latter is a self-powered gas generator in which the internal combustion process drives an integrated air compressor. Here, several free-piston motors replace the high-pressure core of the gas turbine. Through the piston engines’ conceptual ability to work at much higher temperatures and pressures, the overall system efficiency can significantly be increased.

The concept of an aero composite cycle engine has been presented most recently. Published studies already identified significant cycle efficiency improvement potentials compared to conventional gas turbine engines due to the higher temperature and pressure levels of the internal combustion engine. However, the weight penalty of the investigated crankshaft-connected heat engine prevented further improvements.

The proposed free-piston composite cycle engine provides a conceptual design which enables lower weight and size, as no mechanical transmission and lubrication system is required. The absence of a crankshaft and connecting rods eliminates reactive forces and reduces mechanical losses and, thus, allows for higher mean piston velocities. Facilitated through air lubrication, higher cylinder wall temperatures are possible. In particular, the reduction of wall heat losses allows to keep the waste heat of the internal combustion within the composite core engine. Furthermore, the possibility of sequential-burning in a second combustion chamber offers the potential to not only reduce CO~2~ emission but also to keep the production of NO~x~ on a minimum level.


Aero Engine Concepts Beyond 2030: Part 2 \u2014  The Free-Piston Composite Cycle Engine

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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