Reducing Piston Mass in a Free Piston Engine Compressor by Exploiting the Inertance of a Liquid Piston

A dynamic model of a free liquid piston that exploits piston geometry to produce a high inertance was developed for use in a free piston engine compressor. It is shown that for the size scale targeted, advantageous piston dynamics can be achieved with a reduced piston mass compared to a rigid piston design. It is also shown that the viscous losses associated with the liquid piston are negligible for the application discussed. The slow dynamics achieved by the liquid piston also allow for reduced valve sizes for the compressor, creating a more energy-dense device on a systems level. Other advantages gained by this design compared to prior work are discussed, including the elimination of a separated combustion chamber, smaller (integrated) pump check valve, and the capability of more balanced operation for a singlepiston compressor. A dynamic model of the proposed high inertance liquid piston is presented and simulation results are discussed. INTRODUCTION Energetic limitations have long plagued the development of untethered human-scale robotic systems. Typically, systems are actuated by DC servo motors powered by NIMH or Li-ion batteries. Given the low energy density of state-of-the-art rechargeable batteries, operational times of these systems in the 100W range are restrictive [1]. A second and related concern for small-scale untethered applications is the relatively low power density of electromagnetic actuators. One approach to address problems of low energy density batteries and low power density actuators is to avoid the electromechanical domain and utilize pneumatic actuation. Power supplies for pneumatic systems also need to be addressed, since portable tanks that could carry enough air for a useful operation time would be size and weight prohibitive. Traditional air compressors are also too heavy to be used effectively as on-board air supplies for the scale of interest. Goldfarb, et al [2] have shown the viability of using catalytic decomposition of hydrogen peroxide to produce hot gas to directly drive pneumatic actuators. Riofrio, et al [3] designed a free piston compressor specifically for a lightweight untethered air supply for actuation of traditional pneumatic cylinders and valves, using hydrocarbon fuels as an energy source. The piston, acting as an inertial load, converts the thermal energy on the combustion side of the engine into kinetic energy, which in turn compresses air into a reservoir to be used for a pneumatic actuation system. A second device by Riofrio et al [4], a free liquid-piston compressor (FLPC), was designed using a liquid trapped between elastomeric diaphragms as a piston. The liquid piston eliminated the blow-by and friction losses of standard piston configurations [4]. This device incorporated a combustion chamber that was separated from an expansion chamber. Once the high pressure combustion gasses were vented into the expansion chamber, PV work was converted to inertial kinetic energy of the piston. The separated combustion chamber kept air/fuel injection pressure high prior to ignition for efficient combustion, and allowed for air/fuel injection that was decoupled from power and return strokes of the engine cycle. The separated combustion chamber and the high pressure injection of both air and fuel allowed for an engine devoid of intake and compression strokes. This work continues investigation of a free liquid piston compressor power source, focusing on exploiting the geometry of the liquid piston to create a high inertance, which advantageously slows the dynamics of the system without the penalty of adding more mass. Modeling and simulation of the high inertance free liquid piston is conducted, and implications on the performance of a free-piston engine compressor utilizing this liquid piston concept are discussed. LIQUID PISTON INERTANCE Consider a fluid filled pipe approximated with three regions of effective lengths 1 L , 2 L , and 3 L , with distinct cross sectional areas and liquid masses as shown in Fig. 1. This configuration represents the liquid chamber between two moving seals, such as solid pistons or elastomeric diaphragms. An external force acting on either of the moving seals will cause fluid flow through the chamber. FIGURE 1. THREE REGIONS OF A GENERIC HIGH INERTANCE LIQUID PISTON CONTAINED BY DIAPHRAGMS OR SLIDING PISTONS ON BOTH ENDS The power flowing through the fluid filled pipe of Fig. 1, in response to the left and right boundaries moving, can be represented as the time derivative of the kinetic energies in each of the flow regions: