Thrust 2: Compactness
Title
2B.1: Free-piston Engine Compressor
Project Leader
Prof. Eric Barth (Vanderbilt)
Statement of Project Goals
The goal is to develop a compact high energy density pneumatic power supply applicable to untethered fluid-power applications. This will be achieved by modeling, designing, building and testing a free-piston engine utilizing spark-ignited fuel that is specifically load matched to the task of compressing air. Target metrics for the device include 100W average continuous output power of 80 to 100 psig compressed air, 1.5 kg dry weight, greater than 1500 kJ/kg energy density, and a footprint 3 inches in diameter and 20 inches in length. This device will be integrated into the Compact Rescue Crawler Test bed by 2012. Fundamental research will result in a generalized design method for the exploitation of free-piston engine dynamics for optimizing the efficiency and power density of the energetic conversion and transduction processes between chemical stored energy, kinetic energy of the free-piston, compression and pumping work, and stored pneumatic potential energy. This model-based design methodology takes a combined system dynamic / thermodynamic perspective that uniquely addresses the role of dynamic elements and effects seen to have a larger role in free-piston engines than more standard kinematic engines. Correspondingly, a generalized control methodology for free-piston engines will be formulated and applied.
Project's Role in Support of the Strategic Plan
This project contributes mainly to the compactness thrust. The compactness is achieved both due to the high gravimetric energy density of the driving fuel, and the compact configuration of the engine which favors dynamic "linkages" over larger kinematic ones. This project will contribute to the Center's goal of breaking the barrier of low energy density power sources for untethered devices. Additionally, given that an adequate level of overall efficiency is required to break the energy barrier and provide an order of magnitude increase in energy density over conventional technology, this project also has some crossover with the efficiency thrust of the Center.
Description and Explanation of Research Approach
The need for an effective portable power supply for 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 capacity of their power sources in between charges (the operation time of the humanoid-size Honda P3, for instance, is only 20 to 25 minutes). Given the low energy density of state-of-the-art rechargeable batteries, operational times of these systems in the 100W range are restrictive (Dunn-Rankin, et al, 2005). This limitation becomes a strong motivation for the development and implementation of a more adequate source of power. Moreover, the power density of the actuators coupled to the power source needs to be maximized such that, on a systems level evaluation, the combined power supply and actuation system is both energy and power dense. Put simply, state-of-the-art batteries are too heavy for the amount of energy they store, and electric motors are too heavy for the mechanical power they can deliver, in order to present a viable combined power supply and actuation system that is capable of delivering human-scale mechanical work in a human-scale self contained robot package, for a useful duration of time (Goldfarb, et al, 2003).
The total energetic merit of an untethered power supply and actuation system is a combined measure of 1) the source energy density of the energetic substance being carried, 2) the efficiency of conversion to controlled mechanical work, 3) the energy converter mass, and 4) 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%) with very little converter mass (e.g. PWM amplifiers); however, the energy density of batteries is relatively low (about 350 kJ/kg specific work for Li-ion batteries after the gearhead), and the power density of electrical motors is not very high (on the order of 50 W/kg), rendering the overall system heavy in relation to the mechanical work that it can output. With regard to the hydrocarbon-pneumatic power supply and actuation approach presented here relative to the battery/motor system, the converter mass is high and the total conversion efficiency is shown to be low. However, the energy density of hydrocarbon fuels, where the oxidizer is obtained from the environment and is therefore free of its associated mass penalty, is in the neighborhood of 45 MJ/kg, which is about 2 orders of magnitude greater than the energy density of state of the art electrical batteries. This implies that even with poor conversion efficiency (one order of magnitude less than battery/motor systems), the available energy to the actuator per unit mass of the energy source is still at least one order of magnitude greater than the battery/motor system. Additionally, linear pneumatic actuators have approximately an order of magnitude better volumetric power density and a five times better mass specific power density (Kuribayashi, 1993) than state of the art electrical motors. Therefore, 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.
The free piston engine compressor presented in this summary serves the function of converting chemically stored energy of a hydrocarbon into pneumatic potential energy of compressed air. More specifically, it extracts the energy via 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.
Figure 1: Schematic of High Inertance Free Liquid Piston Engine Compressor at effective TDC
The use of free piston engines for compressors is not a new idea. In fact, the first free piston machine designed by Pescara in 1928 was used as an air compressor (Pescara, 1928). Free piston engine compressors were used through the mid-twentieth century, such as the Junkers-designed compressor used in German submarines (Nakahara, 2001). Other applications for the technology were investigated, such as gas generators for use in automobiles (Klotsch, 1959; Underwood, 1957) and small power plants. However, the lack of adequate sensing and control technology led to the free piston engine being largely abandoned after 1960 (Johansen, et al, 2002). Modern electronic controls available today have led to a second generation of free piston engine research. Most of this research, however, uses free piston engine technology for hydraulic pumps (Beachley and Fronczak, 1992; Achten, et al, 2000)) and small-scale electrical power generators (Aichlmayr et al, 2002a, 2002b, 2003), not as air compressors. An extensive review of early free-piston engine compressor and gas generator applications as well as the recent resurgence in research in free piston hydraulic pumps and linear alternators has been conducted by Mikalsen and Roskilly (2007).
Despite free piston devices having been studied in the past, none of these previous designs explicitly featured what is perhaps the main advantage of a free piston, which is its capability to offer a dominantly inertial load. Although it is widely recognized that the inertial load presented by a free-piston can be used advantageously to influence the thermal efficiency, previous research fails to explicitly exploit this feature through design. The main focus of this work is to exploit through design the fact that a free piston can present an inertial load to the combustion pressure, and as a result, desirable operational characteristics can be obtained, such as high efficiency, low noise, and low temperature operation. The fundamental research barrier preventing this is a lack of tools regarding the design of "dynamic engines". What is needed is a model-based design approach that combines the system dynamics and thermodynamics that are more intimately coupled in a free piston engine than a traditional kinematic engine. Methodologies associated with system dynamics and controls are not typically applied to engine design, and this research provides an opportunity to formulate: 1) the dynamic analysis of such engines in light of exploiting the intermediate kinetic energy storage of the free piston, and 2) a synthesis method for the design of free-piston engine devices that have a load tailored for certain applications, such as pumping hydraulic fluid, compressing air, and other outputs, while also being "shaped" to benefit the combustion cycle for efficiency, power density, control and/or other metrics. Additionally, this work aims to demonstrate that a free piston compressor stands as a strong candidate for a portable power supply system for untethered human-scale pneumatic robots.
References
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Willhite, J. A. and Barth, E. J. 2009. "Reducing Piston Mass in a Free Piston Engine Compressor by Exploiting the Inertance of a Liquid Piston". 2009 ASME Dynamic Systems and Control Conference & Bath/ASME Symposium on Fluid Power and Motion Control. DSCC2009-2730, pp. 1-6, October 12-14, 2009, Hollywood, CA.
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Willhite, J. A., Yong, C. and Barth, E. J. 2011a. "The high inertance free piston engine compressor part 1: dynamic modeling," ASME Journal of Dynamic Systems, Measurement and Control, submitted January 2011.
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