Design and Analysis of a Resonating Free Liquid-Piston Engine Compressor

This paper presents the design and simulation of a resonating free liquid-piston compressor (FLPC) equipped with a separated combustion chamber. The FLPC is a proposed device that utilizes combustion of a hydrocarbon fuel to compress air into a high-pressure supply tank, thus potentially serving as a portable power supply candidate for untethered pneumatic systems of human-scale power. The energetic merits of the FLPC concept have been outlined and demonstrated in previous work, and this new design aims at meeting its intended power density, all while maintaining an adequate energy density in a compact and simple device. In this new design, the free “piston” consists of a slug of water (or another incompressible fluid) trapped between two high-stiffness elastomeric diaphragms, thus providing perfect blow-by sealing and near zero friction, while adequately presenting the dynamic elements needed for smooth, continuous operation at desirable resonant frequencies. The device is essentially a tuned resonator whereby the inertia of the liquid piston and the elasticity of the diaphragms are selected to achieve a given resonant frequency. The passive dynamics of the engine are exploited to achieve efficiency through over-expansion, and to enable a return stroke with the small investment made in elastic energy with each power stroke. Additionally, the implementation of a separated combustion chamber – along with built-in actuated, high-flow intake and exhaust valves – ensures the feasibility of such desirable frequencies by decoupling the injection dynamics from the free-piston dynamics. The design and implementation of the device is shown, and simulated results are discussed.