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Free Piston Enqines: Thermoucoustic Stirling Engine
Free Piston Stirling Engine
Free Piston Enqines: Thermoucoustic Stirlinq Enqine
ABSTRACT
The International Academy of Engineering convened an expert panel to select the technologically outstanding achievements of the 2O century & its no surprise that the I.C. Engine Technology topped the list. But, Pollution concerns, global warming and shrinking fossil fuel reserves have focused world attention on how engines generate electrical and mechanical power in a better way.
The free piston engine is an attempt to combine the high thermal efficiency of a reciprocating engine with high power/weight ratio of a rotary turbine. It is a combination of a reciprocating engine and a rotary turbine.
The quest for increased power from a given cylinder size has resulted in a long process of development. Important steps in this process of development are improvements in the fuels used and in the design of various components for higher efficiencies and lower cost and weight. However, a different approach in the direction of using different cycles of operation or modifications of existing cycle, has also been pursued with great interest.
In a step towards exploiting existing power cycles, scientists at the U.S Department of Energyâ„¢s Los Alamos National Laboratory have developed a remarkably simple, energy-efficient engine which works on Stirling Cycle and has no oscillating pistons, oil seals or lubricants, known as the
Thermoacoustic Stirling Engine.
Sound waves in thermoacoustic engines can replace the pistons and cranks that are typically built into conventional engines & hence in true sense thermoacoustic stirling engine can be termed as advancement in free piston engines.
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CoNvrnoN STIRLING CYcLE
Basic Stirling Cycle:
Stirling Engine, type of engine that derives mechanical power from the expansion of a confined gas at a high temperature. The stirling cycle [Fig. 1] was patented in 1816 by the Scottish clergyman Robert Stirling and was used as a small power source in many industries during the 19th and early 20th centuries. The need for automobile engines with low emission of toxic gases has revived interest in the Stirling engine, and prototypes have been built with up to 500 horsepower and with efficiencies of 30 to 45 percent.
The cycle that provides the work is called the Stirling cycle; it consists in its simplest form of the compression of a fixed amount of so-called working gas (hydrogen or helium) in a cool chamber. This cool compressed gas is transferred to a hot chamber, which is heated by an external burner, where the gas expands and drives a piston that delivers the work. The expanded hot gas is then cooled and returned to the cold chamber, and the cycle begins again. Stirling also conceived the idea of a regenerator (a solid with many holes running through it, which he called the economiser) to store thermal energy during part of the cycle and return it later [Fig. 2]
The engine is able to transform heat into work because the expansion of
the gas at high temperature delivers more work than is required to compress the same amount of gas at low temperature.
The heat for the expansion chamber is provided by an external continuous burner that can operate on gasoline, alcohol, natural gas, propane, butane, or solar energy and the exhaust generated has very low free carbon and toxic gas levels. The Stirling engine runs smoothly because pressure variations in the compression and expansion chambers are sinusoidal, that is, relatively gradual, rather than explosive as in internal-combustion cycles.
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Stirling engines are unique heat engines because their theoretical efficiency is nearly equal to their theoretical maximum efficiency, known as the Carnot Cycle efficiency.
Figure 1: PV & TS Representation Of Stirling Cycle
Figure 2: Stirling Cycle
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WHAT IS TURMOACOUSTICS?
Thermoacoustics is the study of the thermoacoustic effect and the attempt to harness the effect as a useful heat engine. A thermoacoustic prime mover uses heat to create sound. Simply put, thermoacoustic effect is the conversion of heat energy to sound energy or vice versa. Utilizing the Thermoacoustic effect, engines & refrigerators are developed that use heat as an energy source and have no moving parts!
Transformation of Heat Energy into intense Acoustic Energy:
Thermoacoustic device [Fig. 3] consists, in essence, of a gas-filled tube containing a stack (top), a porous solid with many open channels through which the gas can pass. Resonating sound waves (created, for example, by a loudspeaker) force gas to move back and forth through openings in the stack.
If the temperature difference along the stack is made sufficiently large, sound can compress and warm a parcel of gas (a), but it remains cooler than the stack and thus absorbs heat. When this gas shifts to the other side and expands (b), it cools but stays hotter than the stack and thus releases heat. Hence, the parcel thermally expands at high pressure and contracts at low pressure, which amplifies the pressure oscillations of the reverberating sound waves, transforming heat energy into acoustic energy. A device that creates sound from heat is called a thermoacoustic heat engine.
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THERMOACOUSTIC STIRLING ENGINE (ASHE)
Introduction:
The thermoacoustic Stirling heat engine [Fig. 4 & 5] developed by the LANL scientistâ„¢s converts heat to intense acoustic power in a simple device that comprises only pipes and conventional heat exchangers and has no moving parts. The acoustic power can be used directly in acoustic refrigerators or pulse- tube refrigerators to provide heat-driven refrigeration, or it can be used to generate electricity via a linear alternator or other electroacoustic power transducer. Already the engineâ„¢s 30% efficiency and high reliability may make medium-sized natural-gas liquefaction plants (with a capacity of up to a million gallons per day) and residential cogeneration economically feasible.
Figure 3: Working Principle of a Thermoacoustic device
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The power production process is environmentally friendly and up to 30 percent efficient while typical internal combustion engines are 25 to 40 percent efficient.
Because the thermoacoustic Stirling heat engine contains no moving parts and is constructed of common materials, it requires little or no maintenance, can be manufactured inexpensively, and is expected to have many future uses
Figure 4: Thermoacoustic Stirling Engine (TASHE)
Figure 5: Thermoacoustic Stirling Engine (TASHE)
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APPARAT1.Js
Scaled drawing of the TASHE, used in the measurements is shown in Figure 6. Essentially, it is composed of a 1/4-wavelength resonator filled with 30- bar helium. The torus-shaped section contains the heat exchangers, regenerator and other duct work necessary to force the he-hum to execute the Stirling cycle. The rest of the hardware past the resonator junction forms the resonator and variable acoustic load.
Loud Speaker: It is used to generate sound waves in the resonator tube.
Cold Heat exchanger: It is a simple shell & tube type heat exchanger with tubes arranged in parallel to acoustic displacement.
Regenerator: The regenerator is a mesh of fine wires or sintered metal structure sealed within the tube The function of the regenerator is to abstract and hold heat from working gas flowing from hot space to cold space and return it back to working gas flowing from cold space to hot space thus increasing thermal efficiency.
Hot Heat exchanger: It is similar in construction to the cold heat exchanger. Its location is chosen so as to not disturb the flow in the thermal buffer tube.
Thermal buffer tube: The thermal buffer tube (TBT) is a tapered tube & provides a thermal buffer between the hot heat exchanger and room temperature.
Flow Straightener: It ensures that the flow entering the bottom of the TBT is spatially uniform, not a jet flow due either to the geometry of the secondary cold heat exchanger or to flow separation at the resonator junction.
Clockwise farther around the torus are the resonator junction, feedback inertance, and comjliance. The inertance and compliance provided by these components act (respectively) like inductance and capacitance in an analogous electrical circuit (bottom), which introduce phase shifts (between voltage and current in an electrical network and between gas pressure and velocity in an acoustic network). Although pressure and gas velocity are 90 degrees out of phase within the main standing-wave resonator
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Jet Pump: It is used to stop streaming problems known as gudgeon streaming.
Acoustic Load: Here the sound energy is converted to useful work.
a
Resonator
Main cold heat Zexchanger (Or)
f” Regenerator
Hot heat exchanger (h)
E” Thermal buffer tube (TBT)
Secondary cold heat
Vâ„¢exchanger and flow
Straightener
-
Vaijable acoustic load
b
CompIance
/7/ pump
Feedback inertance Resonator junct ion
20cm
”I
Figure 6: Apparatus of TASHE
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How DO ThS MACHINES WORK?
In a nutshell, a thermoacoustic engine converts heat from a high- temperature source into acoustic power while rejecting waste heat to a low- temperature sink. A thermoacoustic refrigerator does the opposite, using acoustic power to pump heat from a cool source to a hot sink. Thermoacoustic Stirling engine designed at Los Alamos National Laboratory (top) weighs 200 kilograms and measures 3.5 meters long. The regenerator (middle, dark red) sits in one of two channels that connect the main helium-filled resonator with a compliance volume (dark blue); the other connection is through a narrow pipe, or inertance tube (dark green). The inertance and compliance provided by these components act (respectively) like inductance and capacitance in an analogous electrical circuit (bottom), which introduce phase shifts (between voltage and current in an electrical network and between gas pressure and velocity in an acoustic network). The phase shift created by the inertance-compliance network at the left creates a small pressure difference across the regenerator, driving gas through it. This flow increases and decreases in phase with the rise and fall of pressure in the main resonator. These conditions ensure that the regenerator provides more gain than loss, thus amplifying the acoustic oscillations within the engine [Fig. 7a]
The thermal energy injected at the hot end of the regenerator is transformed efficiently into acoustic energy, which can be used, for example, to drive a reciprocating electric generator or to power a refrigerator. One such device under development for commercial application is intended to liquefy natural gas. These devices perlorm best when they employ noble gases as their thermodynamic working fluids. Unlike the chemicals used in refrigeration over the years, such gases are both nontoxic and environmentally benign. Another appealing feature of thermoacoustics is that one can easily flange an engine onto a refrigerator, creating a heat-powered cooler with no moving parts at all.
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The sound levels generated in such devices, using specialised speakers, are extreme:
in one case the levels reach 190 dB, about 10 million times as intense as the front row levels at a rock concert and 300 times the intensity needed to ignite human hair. However, the sound levels outside the rigid pressure vessel are acceptable. They are not noisy because the casing is a quarter of an inch thick. You hear only a low frequency hum. A prototype refrigerator has already been built and uses sound to pump heat from a lower temperature to a higher. The engine has an efficiency of 30 per cent, which is comparable with that of a car engine (25-40 per cent).
So far, most machines of this variety reside in laboratories. But prototype thermoacoustic refrigerators have operated on the Space Shuttle and aboard a Navy warship. And a powerlul thermoacoustic engine has recently demonstrated its ability to liquefy natural gas on a commercial scale.
Figure 7a: Apparatus of TASHE
Figure 7b: Equivalent electrical Circuit
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Electrical Analogy:
The pressure and velocity of acoustic waves in a gas have rough
analogies in AC electric circuits: The pressure resembles the voltage, and the velocity the current [Fig. 7b]
The regenerator produces an amount of acoustic power that is proportional to the product of the oscillating pressure of the gas and the oscillating velocity of the gas. The power wasted in the regenerator is proportional to the square of the oscillating velocity. This loss is analogous to the power dissipated in an electrical resistor, which is proportional to the square of the current that flows through it.
Faced with such losses”say, from the resistance of the wires in a transmission line”electrical engineers long ago found an easy solution: Increase the voltage and diminish the current so that their product (which equals the power transferred) remains constant. So if the oscillatory pressure could be made very large and the flow velocity made very small, in a way that preserved their product, we could boost the efficiency of the regenerator without reducing the power it could produce.
Traveling acoustic waves, in contrast, have their pressure and velocity in phase with each other. Peter Ceperley of George Mason University noted 20 years ago that when traveling waves pass through a regenerator, the thermodynamic cycle of compression, heating, expansion, and cooling that the gas undergoes is the same as in a Stirling engine, where mechanical pistons establish the proper phasing of the gas motion. With gas velocity and pressure in phase, a traveling wave acoustic engine can use a reversible, much more efficient heat transfer process. Viscous dissipation and other losses have plagued the experimental implementation of traveling wave engines, and the high expectations for these engines are only now beginning to be realized.
THERMOACOUSTIC MAGNFrOHYDRODYNAMIC ELECFRIC GNRAThR:
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The intense acoustic energy generated in the resonator of thermoacoustic Stirling engine is fed to the Thermoacoustic electric generator which acts as the acoustic load. A thermoacoustic magnetohydrodynamic electrical generator comprises of a magnet having a magnetic field, an elongated hollow housing containing an electrically conductive liquid and a thermoacoustic structure positioned in said liquid, heat exchange means thermally connected to said thermoacoustic structure for inducing said liquid to oscillate at an acoustic resonant frequency within said housing, said housing being positioned in said magnetic field and oriented such that the direction of said magnetic field and the direction of oscillatory motion of said liquid are substantially orthogonal to one another, first and second electrical conductor means connected to said liquid on opposite sides of said housing along an axis which is substantially orthogonal to both the direction of said magnetic field and the direction of oscillatory motion of said liquid, whereby an alternating current output signal is generated in said conductor means at a frequency corresponding to the frequency of said oscillatory motion of said liquid.
Assâ„¢r & LIcNsEs
The thermo-acoustic technology development was led by the Los Alamos National Laboratoryâ„¢s Material Science Technology Division. Praxair, Inc. has acquired the assets and licenses to acoustic heat engines and acoustic refrigerators. Assets acquired by Praxair include pilot plants, commercial demonstration equipment, exclusive patent rights, licenses and development programs. The prototype demonstration and validation previously was conducted by Chart Industries. Praxair will continue to work with these agencies to commercialize thermo-acoustic technology.
CONCLUSION
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At the most efficient operating point, the engine delivers 710 W to its resonator with an efficiency of 0.30 which corresponds to 41% of the Carnot efficiency. At the most powerlul operating point, the engine delivers 890 W to its resonator with an efficiency of 0.22
Thermoacoustic engines and refrigerators were already being considered a few years ago for specialized applications, where their simplicity, lack of lubrication and sliding seals, and their use of environmentally harmless working fluids were adequate compensation for their lower efficiencies. This latest breakthrough, coupled with other developments in the design of high-power, single-frequency loudspeakers and reciprocating electric generators, suggests that thermoacoustics may soon emerge as an environmentally attractive way to:
Power hybrid electric vehicles
Capture solar energy
Refrigerate food
Air condition buildings
Liquefy industrial gases
Residential Co-generation
Navy Warships
Space Shuttles
and serve in other capacities that are yet to be imagined.
In 2099, the International Academy of Engineering probably will again convene an expert panel to select the outstanding technological achievements of the 21st century. We hope the machines that our unborn grandchildren see on that list will include thermoacoustic devices, which promise to improve everyoneâ„¢s standard of living while helping to protect the planet
Free Piston Enqines: Thermoucoustic Stirlinq Enqine :14
RtRt4cEs
1. Mathur. M. I and Sharma. R. P., Internal Combustion Engines.
2. Technical Guidance- Scott Backhaus, U. S. Dept.â„¢s Los Alamos National Laboratory.
3. Backhaus, S., and G. W. Swift. 2000. A thermoacoustic Stirling heat engine. Journal of the Acoustical Society of America 107:3148”3166,
June 2000.
4. 5. L. Garrett and S. Backhaus. The power of sound. American Scientist, 88 (6), 516-525, Nov.-Dec. 2000.
5. Ceperley, P. H. 1979. A pistonless Stirling engine”The traveling wave heat engine. Journal of the Acoustical Society of America 66:1508”1513.
6. 5. Backhaus and G. W. Swift, A thermoacoustic-Stirling heat engine, Nature, 399: 335-338, May 1999.
7. Swift, G. W. 1988. Thermoacoustic engines. Journal of the Acoustical Society of America 88:1145”1180.
8. Swift, G. W. 1997. Thermoacoustic natural gas liquefier. Proceedings of the DOE Natural Gas Conference, Morgantown, West Virginia: Federal
Energy Technology Center.
9. Swift, G. W. 1997. Thermoacoustic engines and refrigerators. In Encyclopedia of Applied Physics 21:245”264, ed. G. L. Trigg. New York:
Wiley-VCH.
10.Yazaki, T., A. Iwata, T. Maekawa and A. Tominaga. 1998. Traveling wave thermoacoustic engine in a looped tube. Physical Review Letters
81:3128”3132.
11 .Journal, The stirling Machine World, USA.

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Free-piston engine
The free-piston engine is a linear, 'crankless' internal combustion engine, in which the piston motion is not controlled by a crankshaft but determined by the interaction of forces from the combustion chamber gases, a rebound device (e.g., a piston in a closed cylinder) and a load device (e.g. a gas compressor or a linear alternator).
The basic configuration of free-piston engines is commonly known as single piston, dual piston or opposed pistons, referring to the number of combustion cylinders. The free-piston engine is in practice restricted to the two-stroke operating principle, since a power stroke is required every fore-and-aft cycle.
First generation free-piston engines
The free-piston engine was first proposed by R.P. Pescara [1] and the original application was a single piston air compressor. The engine concept was a topic of much interest in the period 1930-1960, and a number of commercially available units were developed. These first generation free-piston engines were without exception opposed piston engines, in which the two pistons were mechanically linked to ensure symmetric motion. The free-piston engines provided some advantages over conventional technology, including compactness and a vibration-free design.
Air compressors
The first successful application of the free-piston engine concept was as air compressors. In these engines, air compressor cylinders were coupled to the moving pistons, often in a multi-stage configuration. Some of these engines utilised the air remaining in the compressor cylinders to return the piston, thereby eliminating the need for a rebound device.
Free-piston air compressors were in use among others by the German Navy, and had the advantages of high efficiency, compactness and low noise and vibration [2].
Gas generators
After the success of the free-piston air compressor, a number of industrial research groups started the development of free-piston gas generators. In these engines there is no load device coupled to the engine itself, but the power is extracted from an exhaust turbine. (The only load for the engine is the supercharging of the inlet air.)
A number of free-piston gas generators were developed, and such units were in widespread use in large-scale applications such as stationary and marine powerplants [3]. Attempts were made to use free-piston gas generators for vehicle propulsion (e.g. in gas turbine locomotives) but without success [4][5].
Modern applications
Modern applications of the free-piston engine concept include hydraulic engines, aimed for off-highway vehicles, and free-piston engine generators, aimed for use with hybrid electric vehicles.
Hydraulic free-piston engines
These engines are commonly of the single piston type, with the hydraulic cylinder acting as both load and rebound device using a hydraulic control system. This gives the unit high operational flexibility, and excellent part load performance has been reported for such engines [6][7].
Free-piston engine generators
The use of a free-piston engine with a linear generator is being investigated by a number of research groups, driven by the increasing interest in the hybrid electric vehicle concept in the automotive industry. The first free piston generator was patented in 1959 [8], and since then, a number of variations have been proposed. Examples include the Stelzer engine and the Free Piston Power Pack manufactured by Pempek Systems based on a German patent[9].
These engines are mainly of the dual piston type, giving a compact unit with high power to weight ratio. A challenge with this design is to find an electric machine with sufficiently low weight, and control challenges in the form of high cycle-to-cycle variations have been reported for dual piston engines.[10][11].
Free-piston features and potential advantages
The operational characteristics of free-piston engines differ from those of conventional, crankshaft engines. The main difference is due to the piston motion not being restricted by a crankshaft in the free-piston engine, leading to the potentially valuable feature of variable compression ratio. This does, however, also present a control challenge, since the position of the dead centres must be accurately controlled in order to ensure fuel ignition and efficient combustion, and to avoid excessive in-cylinder pressures or, worse, the piston hitting the cylinder head.
Advantages
Potential advantages of the free-piston concept include
• Simple design with few moving parts, giving a compact engine with low maintenance costs and reduced frictional losses.
• The operational flexibility through the variable compression ratio allows operation optimisation for all operating conditions and multi-fuel operation. The free-piston engine is further well suited for Homogeneous Charge Compression Ignition (HCCI) operation.[12]
• High piston speed around top dead centre (TDC) and a fast power stroke expansion enhances fuel-air mixing and reduces the time available for heat transfer losses and the formation of temperature-dependent emissions such as nitrogen oxides (NOx).[13][14]
Challenges
The main challenge for the free-piston engine is engine control, which can only be said to be fully solved for single piston hydraulic free-piston engines. Issues such as the influence of cycle-to-cycle variations in the combustion process and engine performance during transient operation in dual piston engines are topics that need further investigation.

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