Dynamic Characteristics of a Free Piston Compressor

The design and dynamic characterization of a free piston compressor is presented in this paper. The free piston compressor is a proposed device that utilizes combustion to compress air into a high-pressure supply tank. The device is configured such that the transduction from thermal energy to stored energy, in the form of compressed gas, is efficient relative to other small-scale portable power supply systems. This efficiency is achieved by matching the dynamic load of the compressor to the ideal adiabatic expansion of the hot gas combustion products. It is shown that a load that is dominantly inertial provides a nearly ideally matched load for achieving high thermodynamic efficiency in a heat engine. The device proposed exploits this fact by converting thermal energy first into kinetic energy of the free piston, and then compressing air during a separate compressor phase. The proposed technology is intended to provide a compact pneumatic power supply source appropriate for human-scale robots. The combined factors of a high-energy density fuel, the efficiency of the device, the compactness and low weight of the device, and the use of the device to drive lightweight linear pneumatic actuators (lightweight as compared with power comparable electric motors) is projected to provide at least an order of magnitude greater total system energy density (power supply and actuation) than state of the art power supply (batteries) and actuators (electric motors) appropriate for human-scale power output. A thermodynamic analysis reveals relationships between key design variables with regard to efficiency. A dynamic model of the proposed device is developed and simulation results are discussed. This model shows the potential of the proposed device. 1.0 INTRODUCTION 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 [7]. The automotive industry conducted a large amount of research in the 1950’s. Ford Motor Company considered the use of a free piston device as a gasifier in 1954 [5]. General Motors presented the “Hyprex” engine in 1957 [8]. Such endeavors 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. [1, 2] 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 [3], 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 [6]. Following from the motivations outlined in [4], the free piston compressor presented here is intended as a power supply for a mobile robotic system of human comparable power, mass and size. In this paper it is shown that the use of a free piston engine as a direct air compressor offers nearly ideal loading characteristics necessary for high efficiency, in a simple and small package. Indeed the original patent by Pescara [7] intended the free piston engine foremost as a compressor. It is additionally shown that such a device can run at low temperatures and with low noise relative to other internal combustion devices. An outline of this paper is as follows. Section 2 describes the proposed device and its operation. Section 3 presents an idealized thermodynamic analysis of the engine side and the compressor side separately. This analysis yields a set of relationships that are useful for design specifications regarding, among other things, the selection of a propellant, the required mass of combustion gasses, the combustion chamber volume, and the required compressor chamber volume, for nominal operation at maximum efficiency. Section 4 presents a dynamic model of the system that more accurately models the interaction of the engine side and the compressor side, in addition to providing the capability of modeling the effects of heat losses, friction, valve losses, and other influences. 1 Copyright © 2004 by ASME 2.0 THE FREE PISTON COMPRESSOR A schematic of the free piston compressor system is shown in Figure 1. As shown in its original position, the device operates by first opening the air and fuel valves to allow the proper mixture and amount of air and fuel into the combustion chamber of the engine side. Once the proper air/fuel mixture is inside, the valves close and a spark initiates the combustion. Upon combustion, the free piston moves to the right as the combustion gases expand, converting the energy of combustion into kinetic energy of the free piston. The compressor side of the device is configured such that the piston sees a negligible compressive force for a distance greater than required for the combustion chamber to both expand down to atmospheric pressure, and allowed to intake fresh cool air to cool the exhaust products through the intake check valve. After this point the kinetic energy of the free piston is converted into the work required to compress and then pump the gasses in the compressor chamber into the high-pressure reservoir. The cycle is completed when the light return spring moves the piston to the left pushing out the diluted exhaust products of the engine side, and refilling the compressor side with air drawn in through an inlet check valve. Besides advantages regarding efficiency, the free piston compressor offers on-demand start and stop (since there is no compression stroke in the engine side), cool operation (given that the combustion products are greatly diluted with air after expanding down below atmospheric pressure), quiet operation (given that there is no exhaust of high-pressure gasses), and simple. 3.0 THERMODYNAMIC ANALYSIS The thermodynamic analysis below reveals a number of design constraints and key relationships regarding the dependency of efficiency on certain design variables. 3.1 Engine side The engine side of the free piston compressor converts the energy of combustion into kinetic energy of the free piston, while the compressor side then converts this kinetic energy into stored compressed gas in the high-pressure reservoir. Presenting a purely inertial load during the expansion of the combustion products allows the right loading characteristics such that the high-pressure combustion products are allowed to fully expand down to atmospheric pressure. When this is the case, and assuming an adiabatic process in the combustion chamber, the work done on the inertial load will be equal to the following, ( ) ( 0 0 1 0 0 1 e ef atm e ef e e e e V V P V V V P W e e e − − − γ − = γ − γ − γ ) (1) where is the initial combustion pressure, V and V is the initial and final volume of the combustion chamber respectively, and 0 e P 0 e ef e γ is the ratio of specific heats of the combustion gases (products of combustion). Assuming losses associated with friction are negligible, the kinetic energy of the piston will be equal to the work done W , when reaching the position associated with the final volume V . The combustion chamber volume required such that the combustion gases are allowed to fully expand down to atmospheric pressure under adiabatic conditions can be found as: e ef 0 1 0 e atm e ef V P P V e γ       = (2) Intake check valve Spark Exhaust valve Fuel valve Propane or other self pumping fuel Air valve High pressure air reservoir pneumatic power ports Engine side Compressor side Inlet and outlet check valves Figure 1: Schematic of the free piston compressor system. Taking the initial combustion pressure as a specifiable quantity (design specification), the required mass of the fuel/air mixture can be found using the ideal gas law as, 0 e P AFT e e e e T R V P m 0 0 0 = (3) where is the average gas constant associated with the combustion products, and T is the adiabatic flame temperature. e R