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. 1.0 INTRODUCTION The work presented in this paper is intended to address the current energetic limitations in untethered robotic systems of human-scale power output (in the neighborhood of 100 W, as defined in [1]). The existing body of work in such systems is mostly in the electromechanical domain, where the actuation is carried out by DC servo motors, and the source of electrical energy is typically NIMH batteries. From a design and controls perspective, these electromechanical systems provide convenient working platforms due to the relative ease of servo control. However, from an energetic perspective, they are fundamentally constrained by the low energy density of the batteries (180 kJ/kg), in terms of their active duration between charges. A state of the art example is the Honda P3 humanoid robot, whose operation time ranges between 15 and 25 minutes before its 30-kg battery pack needs to be replaced. A relatively new approach to developing such robotic systems is being undertaken in the pneumatic domain, where motion is typically generated via linear pneumatic actuators. On-board air supply has shown to be a non-trivial issue, since standard air compressors are too heavy for the intended target scale, as are portable tanks with enough compressed air to supply the actuators for a useful duration of time. Goldfarb, et al [2] have shown the viability of utilizing hot gas released by the catalytic decomposition of hydrogen peroxide to drive pneumatic actuators, whereby the on-board supply of hot gas is carried out by a small tank of hydrogen peroxide in line with a small catalyst pack. This work presents an alternative approach for developing an on-board cool air supply, via a free liquid-piston compressor (FLPC). Put simply, the FLPC serves the function of converting chemically stored energy of a hydrocarbon fuel into pneumatic potential energy of compressed air. More specifically, it extracts the energy by producing combustion of a stoichiometric mixture of propane and air, and the combustion-driven free liquidpiston acts as an air pump to produce the compressed air. The main objective of this idea is to exploit the high massspecific energy density of hydrocarbon fuels (46,350 kJ/kg) and the high mass-specific power density of linear pneumatic actuators (approximately 5 times greater than with gear head motors [3]), in order to provide at least an order of magnitude greater combined energy and power density (power supply and actuation) than state of the art electrical power supply and actuation systems. A more indepth energetic merit analysis of such a free piston compressor device is provided in [4]. The idea of using a free piston combustion-based device as a pump has been around since the original free-piston patent by Pescara in 1928 [5]. Junkers developed a free piston compressor that became widely used by German submarines through World War II [6]. The automotive industry conducted a large amount of research in the 1950s. Ford Motor Company considered the use of a free piston device as a gasifier in 1954 [7]. General Motors presented the “Hyprex” engine in 1957 [8]. Such endeavours were aimed at an automotive scale engine and were largely unsuccessful. In more recent times, the free piston engine concept has been considered for small-scale power generation. Aichlmayr, et. al. [9,10] have considered the use of a free piston device as an electrical power source on the 10 W scale meant to compete with batteries. Beachley and Fronczak [11], among others, have considered the design of a free-piston hydraulic pump. McGee, et. al. have considered the use of a monopropellant-based catalytic reaction as an alternative to combustion, as applied to a free piston hydraulic pump [12]. A previous free piston compressor device has been designed and experimentally shown in [13]. It was built with standard pneumatic equipment, and meant as proof of concept introduced in [14]. Figures 1 and 2 show a schematic and experimental setup of this device. The operational logistics are as follows: (1) two magnets hold the piston to the left while high-pressure fuel and air are injected into the combustion side, (2) sparked combustion occurs and the force on the piston exerted by the combustion pressure overcomes the magnetic holding, (3) the piston loads up with kinetic energy as it travels to the right and the combustion gases expand down to atmospheric pressure, (4) still in mid-stroke, the combustion gases reach atmospheric pressure (over-expansion) and go slightly below, causing a breathe-in check valve to allow fresh air to quickly enter the chamber and cool down the combustion products, all while (5) the air in the rod-side of the piston is pumped into the high pressure air reservoir, and finally (6) the piston reaches the end of its stroke and the entire process takes place again from right to left. High pressure air reservoir Propane or other self pumping fuel Fuel Valve Fuel Valve Air Valve Air Valve Exhaust Valve Exhaust Valve Pneumatic power ports Breathe-in check valve Breathe-in check valve Spark Spark Outlet check valves Inlet check valves Magnets Figure 1: Schematic of previous free piston compressor. Fuel Valve Figure 2: Experimental setup of previous free piston compressor It should be noted that due to the over-expansion and breathe-in in the combustion chamber, the free piston compressor is self-cooling and has a quiet exhaust. Additionally, the use of high-pressure injection of air and fuel allows for the device to operate without a starter or separate starting cycle. These features, more thoroughly discussed in [4] are conceptually fundamental to the free piston compressor and constitute a starting point for emerging research. While this previous device successfully demonstrated the energetic merit potential of a free piston compressor device, it fell short of achieving an adequate power density for its intended application, mostly due to the limitations of utilizing standard pneumatic cylinders and valves. As outlined in [4], these limitations include: high physical dead volume in the pump, high surface area-to-volume ratio in the combustion chamber, low combustion pressure, low frequency of operation, and finally losses attributed to seal friction, blowby and metal-to-metal collisions. The FLPC presented in this paper addresses all these issues by escaping the constraints of standard pneumatic equipment and specifically matching the desired dynamic behavior of the system with custom-built equipment. The following section introduces the FLPC design and outlines its features and principle of operation, and presents a properly scaled, custom-fabricated FLPC; section 3 presents a full simulation of the device, along with its yielded results and a discussion; and finally, section 4 offers some conclusions. 2.0 DESIGN The main feature introduced in the FLPC design is the use of a liquid slug as the piston, trapped in between two elastomeric diaphragms. This approach aims at obtaining perfect blow-by sealing and essentially zero friction, as well as ridding the system from the losses and noise associated with metal-to-metal collisions. Additionally, this allows for the walls of the pump to be shaped such that the liquid piston can fully match their contour and thus drastically reduce the amount of dead volume presented in the chamber. Figure 3 below shows a simplified schematic of Air Valve Mixture Valve