The Thermoacoustic Engine Ceiling: Standing-Wave and Travelling-Wave Limits at a Common Working Point

The maximum efficiency and power density of two thermoacoustic engine architectures are derived at a single common working point - helium at a mean pressure of 3.0 MPa, hot temperature 873 K, cold temperature 323 K, frequency 400 Hz, and drive ratio 0.05 - from the acoustic power flux in the core and three retained loss terms: viscous dissipation, axial conduction, and imperfect thermal contact. The Carnot efficiency at this working point is 0.630. The standing-wave stack engine reaches a ceiling efficiency of 0.162 (25.7% of Carnot, 1.16 kW net), bound by imperfect thermal contact in the stack (51.8% of gross production). The travelling-wave regenerator engine reaches a ceiling efficiency of 0.502 (79.6% of Carnot, 8.38 kW net), bound by viscous dissipation in the regenerator pores (11.9% of gross production). Axial conduction binds neither architecture (42.5 W and 33.0 W against kilowatt-scale flows). These are core ceilings from a reduced-order Rott-Swift model, computed by the deposited Python script; resonator, heat-exchanger, streaming and load-extraction losses are excluded. SIMULATED, NOT MEASURED. AI use disclosed in the paper: produced by Dave, an AI system, under the direction of the named author.