Study of piston trajectory parameters on the performance of a kinetic Stirling engine

The Stirling engine is a type of external heat engine that is capable of extracting thermal energy from differential temperature sources and converting the energy into boundary work. Since the Stirling cycle is realized through four polytropic processes, the shape of the pressure-volume trajectory of the working medium fundamentally governs the output performance of the engine. Traditional alpha-, beta-, gamma-type engines have fixed piston trajectories, bounded by mechanical linkages, usually following approximately sinusoidal curves. Free-piston variants are capable of tailored piston trajectories if active displacer regulation is added, but still constrained by the spring-mass-damper dynamics. Currently, either kinetic or free-piston engine results in polytropic processes of the gas. During these processes, the boundary work may hinder heat transfer, potentially reducing the engine thermodynamic performance. To guide syntheses of more efficient engines, the key trajectory parameters need to be studied. This paper starts from the thermal-pressure-structural modeling of the engine, evaluates the 4 key processes during an ideal Stirling cycle, and proposes the trajectory parameters group, which finely tunes the gas curve as well as pistons' trajectories, allowing for a detailed look into the relationship among the work medium the interactions between the pistons and the engine performance. Through establishment of a prototype engine, the models are validated through an inhouse designed and assembled test bed. Results show that manipulation of the gas volume curve is key to achieve isochoric heat addition or rejection, and the pistons trajectories determine the throttling loss performance. By applying isochoric heat transfer and reduced throttling condition, the net output work of the engine can be improved by nearly 100% against the sinusoidal baseline.