Experimental Operation and Characterization of a Free Piston Compressor

The ongoing design evolution of a free piston compressor (FPC) is presented in this paper. The FPC is a proposed device that utilizes combustion of a hydrocarbon fuel to compress air into a high-pressure supply tank. This device is designed to extract chemically stored energy from the fuel and convert it to potential energy of compressed air, while achieving high conversion efficiency relative to other smallscale portable power supply systems. The chemically stored energy of the hydrocarbon fuel is first converted into kinetic energy of the free piston by the end of the combustion phase. Subsequently, the moving piston acts as a pump and air compressor during a compression phase. The proposed technology is intended to provide a compact and efficient pneumatic power supply source appropriate for human-scale robots. The design and implementation of this version of the FPC is shown, and experimental results relating all phases (combustion, expansion and pumping) are discussed. The total efficiency of the system is experimentally measured and compared to its theoretical prediction. 1.0 INTRODUCTION The need for an effective portable power supply for untethered human-scale robots has increasingly become a matter of interest in robotics research. Current prototypes of humanoid robots, such as the Honda P3, Honda ASIMO and the Sony QRIO, show significant limitations in the duration of their power sources in between charges (the operation time of the Honda P3, for instance, is only 25 minutes). This becomes a strong motivation for the implementation of a more adequate source of power. The motivation details are discussed more thoroughly in [1]. This paper presents the design of a free piston compressor (FPC) as a power supply for pneumatically actuated systems. The FPC 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 piston acts as an air pump to produce the compressed air. The FPC, coupled with pneumatic actuators, is intended as an alternative to electrical batteries coupled with electrical motors. The main objective of this idea is to exploit the high mass specific energy density of hydrocarbon fuels and the high mass specific power density of linear pneumatic actuators, 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. Given their inherent penalization for carrying their own mass, the total energetic merit of an untethered power supply and actuation system is a combined measure of the source energy density of the energetic substance being carried, the efficiency of conversion to controlled mechanical work, the energy converter mass, and the power density of the actuators. With regard to a battery powered electric motor actuated system, the efficiency of conversion from stored electrochemical energy to shaft work after a gear head can be high (~ 50% to 80%) with very little converter mass (e.g. PWM amplifiers); however, the energy density of batteries is relatively low (about 180 kJ/kg for NiMH batteries), and the power density of electrical motors is not very high (on the order of 50 W/kg) rendering them heavy in relation to the mechanical work that they can output. With regard to the hydrocarbon-based pneumatic power supply and actuation approach presented here, the converter mass is high relative to a battery/motor system, and the total conversion efficiency is shown in [2] to be low in relative terms. However, the energy density of hydrocarbon fuels is in the neighborhood of 45 MJ/kg (where the oxidizer is obtained from the environment and therefore has no associated mass penalty), which is more than 200 times greater than the energy density of conventional electrical batteries. This implies that even with poor conversion efficiency (< 10%), and with only mild expectations of miniaturizing the energy converter, the available energy to the actuator per unit mass of the energy source (mass of fuel plus mass of energy converter) is still at least one order of magnitude greater than the battery/motor system. Additionally, linear pneumatic actuators have roughly one order of magnitude greater power density than traditional electrical motors. Therefore, the proposed technology offers the potential of greatly increased energetic characteristics over state-of-the-art electrical power supply and actuation systems. 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 [3]. 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 [4]. General Motors presented the “Hyprex” engine in 1957 [5]. 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. [6, 7] 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 [8], 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 [9]. The FPC presented here is intended as a power supply for a mobile pneumatic robotic system of human comparable power, mass and size. It is shown analytically in [2] that the use of a free piston engine as a direct air compressor offers nearly ideal loading characteristics necessary for high efficiency (relative to similar scale combustion based devices), in a simple and small package. A first design of the FPC was presented in [10]. In this previous design, particular emphasis was placed into the combustion portion of the device, and it outlined the main features of the FPC concept and design considerations of that particular prototype. The main features should be restated here, since they constitute the essence of the FPC idea: Inertial Loading – The free piston is not rigidly attached to a crankshaft or any timing linkage alike, so it offers purely inertial loading to the expanding combustion gases. This allows the free piston to load up with kinetic energy resulting from the work done by the ideal adiabatic expansion [10] of the combustion gases. The combustion gases are allowed to expand until they reach atmospheric pressure, all while still contributing to the inertial loading. This full expansion contributes to a higher efficiency than if full expansion were not allowed, as is the case with most small-scale IC engines. As an additional consequence, the FPC has a quiet exhaust, since no high-pressure gases will be exhausted into the atmosphere. Breathe-in Mechanism – When the combustion gases reach atmospheric pressure, the free piston will still be traveling (with maximum kinetic energy), and thus will induce a drop of pressure in the combustion chamber as the motion continues. This pressure drop will cause an intake check valve to open and allow fresh atmospheric air to enter the chamber. This fresh air will both cool down and dilute the combustion products. The breathe-in mechanism ensures a low temperature operation of the device. Start on Demand – Since the intake valves and spark plug are electrically actuated, and since high-pressure injection of air and fuel eliminate the need for a conventional intake and compression stroke, the FPC does not require the implementation of a starter. This allows the engine to start on demand, without the need for a separate starting cycle. This feature highlights the compatibility between the FPC and a pneumatic robotic system, since they can be tied together by implementing a simple control loop to maintain a particular pressure in a supply reservoir. The FPC would receive a signal and start operating as soon as the actuation pressure supply drops, and likewise turn off once reaching the desired pressure. The features outlined above, among others, are discussed more in detail in [10], and become the platform for the work presented in this paper. What follows is an introduction of the new FPC prototype and some new design considerations, a comprehensive analysis of the design evolution, analysis of new experimental data showing combustion and pumping, and a calculation of the total efficiency of the system. 2.0 NEW VERSION OF FPC The previous prototype of the FPC [10] consisted of one combustion cylinder and two pumping cylinders. The three cylinders were aligned in parallel, with the combustion cylinder in the center. The pistons were rigidly attached to each other, and their main functions (combustion and pumping) occurred at the right of the pistons. The combustion would drive the piston assembly to the left, and a set of springs reversed the motion back to the right, thus initiating the pumping phase. Figure 1 shows a schematic of this previous prototype. Propane or other self pumping fuel Fuel Valve Spark Exhaust valve Air valve High pressure air reservoir pneumatic power ports Engine Compressor 1 Inlet and outlet check valves Compressor 2 Return Springs Magnet Magnet Connecting Plate Inlet and outlet check valves (to air reservoir) Breathe-in check valve Figure 1: Schematic of Old Version of FPC. The most appealing aspect of this configuration was that combustion and pumping occurred on separate strokes, so the inertial loading during combustion was not resisted by the compressed air. While this prototype provided some useful and insightful data, it also showed some limitations. These were mostly due to energy losses associated with collisions with the springs, the non-ideality of the springs in storing and returning energy, and the high friction in the cylinders. The rigid linking of the pistons also provided a very slight misalignment, which accounted for even greater friction. Additionally, the exhaust valve, breathe-in check valve, and inlet and outlet check valves were offering severe flow restrictions, which suggested the use of a smaller cylinder bore, or alternatively, larger valves. Figure 2 shows a schematic of the new version of the FPC. This prototype highlights combustion and pumping within the same stroke. The device consists of two cylinders in-line and opposing one another. Both cylinders have a combustion side (back of the piston) and a pumping side (rod side of the piston). 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 2: Schematic of New Version of FPC. The device is completely symmetrical, so its starting position can be on either side. Assuming the piston’s initial position is at the left, the piston is held in place by the magnets while injecting a mixture of pressurized air and propane into the combustion chamber. Once the proper amount of mixture has entered the chamber, the air and propane valves close and a sparkplug initiates combustion. The piston will then travel to the right while serving four functions: (1) pump fresh air into the air reservoir; (2) exhaust the diluted combustion products from previous combustion out of the right cylinder’s combustion chamber; (3) breathe in fresh air into the right cylinder’s pumping chamber; and (4) breathe in fresh air into the combustion chamber after the pressure has dropped below atmospheric, thus cooling down and diluting the combustion products. At the end of the stroke the piston will be held in place by the magnets on the right side, and the cycle can occur on the opposite side in the same fashion (from right to left). The work required to break the magnetic holding force after combustion is retrieved at the end of pumping. The forcedistance profile of the magnets also allows dominantly inertial loading presented to the combustion pressure after a very short distance after break-away has occurred. 3.0 DESIGN Figure 3 shows a picture of the new FPC prototype. Most of the hardware was inherited from the first prototype (valves, magnets, sensors, spark plugs). The cylinders were replaced with two 4-inch stroke, 3⁄4-inch bore BIMBA standard air cylinders, and ported appropriately. Figure 4 shows a closeup of the new cylinder configuration, with the piston fully retracted (and the opposite piston disconnected). By examining Figure 3, it should be noted that the hardware implementation of this prototype mainly differs from the generalized schematic (Figure 2) in that the piston rods are connected to each other through a moving mass, which is needed for inertial loading purposes. Another important difference to note is that the moving mass carries the magnets (2 on each side), which snap onto the ferrous plates at the end of each stroke. Figure 4 shows the magnets pressed against the ferrous plate. Figure 3: Picture of New FPC Figure 4: Close-up Picture of Cylinder This new configuration addresses some of the limitations found in the previous version of the FPC. The cylinder rods were connected to the moving mass with small pieces of plastic tubing in order to avoid a purely rigid connection, which would yield friction due to misalignments. Added to that, the cylinders chosen are much smoother than the previous ones, so the frictional losses in this system are Fuel Valve