THERMODYNAMIC WORKING FLUIDS FOR STIRLING-CYCLE, RECIPROCATING, THERMAL MACHINES

Abstract: New thermodynamic working fluids with primary application to Stirling-cycle, reciprocating, thermal machines which possess a greater dynamic heat transfer coefficient than either hydrogen or helium and which are both chemically inert and easily liquified. A novel power level control subsystem (Figure 4) based on the thermodynamic properties of these fluids is disclosed for use in conjunction with Stirling-cycle machines; this new approach greatly simplifies procedures for changing the mean system working pressure, and thereby the instantaneous power level in such machines, during their operation.

Description: THERMODYNAMIC V:0RLvIIϊG FLUIDS FOR STIRLING-CYCLE, RECIPROCATING, HI3HHAL MACHINES Technical Field This invention relates to Stirling-cycle engines, also • known as regenerative thermal machines, and more particu¬ larly to a new family of thermodynamic working fluids for such machines. The working fluids of the present invention are specifically selected with regard to whether or not they possess a high dynamic heat transfer coefficient, as defined by knovm empirical relations for heat transfer in turbulent flows, in addition to other requisite ther ophysical proper¬ ties such as chemical inertness and thermal stability. They are disclosed to be improved agents for use in all heat en¬ gines which embody a practical approximation to the well knovm Stirling thermodyn.amic cycle -and which are employed in the production of both mech-anical power (i.e., prime movers, compressors, fluid pumps) and refrigeration (i.e., refriger¬ ators, air conditioners, heat pumps, gas liguefiers) . A Stirling-cycle engine is a machine which operates on a closed regenerative thermodynamic cycle, with periodic compression and expansion of a gaseous working fluid at dif¬ ferent temperature levels, and where the flow is controlled by volume changes in such a way as to produce a net conver¬ sion of heat to work, or vice versa. The regenerator is a device which in prior art takes the forta of a porous mass of metal in an insulated duct. This mass takes up heat from the working fluid during one part of the cycle, temporarily stores it within the machine until a later part of the cy¬ cle, and subsequently returns it to the working fluid prior to the start of the next cycle. Thus the regenerator may be thought of as an oscillatory thermodynamic sponge, alter¬ nately absorbing and releasing heat with complete reversi¬ bility and no loss. A reversible process for a ther odynar.ic system is an ideal process, which once having taken place, can be revers¬ ed without causing a change in either the system or its surroundings. Regenerative processes are reversible in that they involve reversible heat transfer and storage; their importance derives from the fact that idealized reversible O PI heat transfer is closely approximated by the regenerators of actual machines. Thus the Stirling engine is the only prac¬ tical example of reversible heat engine which can be operat¬ ed either as a prime mover or as a heat pump. Background The Stirling-cycle engine was first conceived and re¬ duced to practice in Scotland 164 years ago. A hot-air, closed-cycle prime mover based on the principle was patented by the Reverend Robert Stirling in 1817 as an alternative to the explosively dangerous steam engine. Incredibly, this event occured early in the Age of Steam, long before the in- ventiσμ of the internal combustion engine and several years before the first formal exposition of the Laws of Thermo¬ dynamics. Air was the first and only working fluid in early 19th century machines, whereas hydrogen and helium have been the preferred working fluids for modern machines. In Great Bri¬ tain, Europe, and the United States thousands of regenera¬ tive hot air prime movers in a variety of shapes and sizes were widely used throughout the 19th century. The smaller engines were reliable, reasonably efficient for their time, and most important, safe compared with contemporary recipro¬ cating steam engines. The larger engines were less re¬ liable, however, because they tended to overheat and often succumbed unexpectedly to premature material failure. Toward the end of the 19th century the electric motor •and the internal combustion engine were developed and began to replace not only the Stirling-cycle engines, but also the reciprocating steam engines of that era. These new machines were preferred because they could produce greater power from more compact devices and because they were more amenable to simple power level controls. Both of these limitations of the original Stirling engine are directly attributable to the use of air as the working fluid. That is, the specific power capacity and the overall mechanical complexity of a practical Stirling-cycle machine are direct consequences of the inherent performance characteristics and heat transfer properties of the working fluids used. Since World War II there have been unprecedented ad- ' ξfREA ances in the general technologies of machine design, heat transfer, materials science, system analysis and simulation, manufacturing methods, and Stirling engine development. To¬ day, in comparison to their conventional internal combustion counterparts, all modern Stirling-cycle prime movers are external combustion engines which consistently demonstrate (in the laboratory) higher efficiency, ultifuel capability, lower exhaust emissions, quieter operation, equivalent power density, and superior torque characteristics. Nevertheless, none of these engines is mass produced for any commercial application anywhere in the modern world. The reason for this is that contemporary Stirling en¬ gines have been developed largely by adapting traditional methods and designs from the more familiar internal combust- ion engine technology base. Patchwork adaptation of the old as a shortcut path to the new is a process which inexorably produces a hodge-podge arrangement of excessive mechanical complexity and which inevitably results in high production costs. The use of hydrogen and helium has led to increased power density, but it has also required the use of complex seals, makeup storage vessels, expensive materials, and elab¬ orate controls. Hydrogen, in particular, is difficult to contain and causes ordinary engine materials to become brittle. Despite clearly superior technical performance characteristics, therefore, contemporary Stirling engines are invariably not cost competitive from the standpoint of economical mass production. Disclosure The invention comprises fundamental concepts and mat¬ erial properties which are used in combination to form a new and less complicated technology base for the development of improved Stirling-Cycle machines, specifically including the following: (1) working fluids other than hydrogen, helium, or air, namely certain fluorine compounds exemplified by by sulfur hexafluoride, perfluorobutane, perfluoropropane, and octafluorocyclobutane, which provide an increased dy¬ namic heat transfer coefficient yet are nonflammable, non- toxic, and easily liquefied; and (2) an engine power level control subsystem by which the re-an system working pressure, and thereby the instantaneous power level of the engine, is conveniently varied by the hydraulic injection or ejection of condensed working fluid through a special heat exchanger to be known as the reservoir cooler. It is a primary object of the invention to provide a new and improved family of thermodynamic working fluids for Stirling-cycle, reciprocating, thermal machines, other than the usual hydrogen, helium, or air which possess increased dynamic heat transfer coefficients; have a critical tempera¬ ture somewhat above the minimum ambient temperature of the available heat sink yet somewhat below the designated heat rejection temperature of the cycle as maintained within the engine cooler; and which are also nonflammable, nontoxic, and inexpensive, inert, and of low viscosity. It is a further object of the invention to provide an engine power level control subsystem for use in conjunction with any regenerative thermal machine by means of which the nominal operating pressure of the cycle is conveniently and automatically varied as a direct function of power demand by the hydraulic injection or ejection of condensed working fluid through a special heat exchanger to be known as the reservoir cooler. 3rief Description of Drawings Other objects, advantages, and novel features of the invention will become readily apparent upon consideration of the following detailed description when read in conjunction with the accompanying drawings wherein: FIG. 1 is an illustration of the operational sequence of events during one complete cycle of an idealized single- acting two-piston Stirling engine used in the prime mover mode; FIG. 2(a) and FIG. 2(b) are schematics which illustrate the idealized pressure-volume and temperature-entropy dia- grams of the thenαodynaraic cycle of the working fluid in the s-a e machine depicted by FIG. 1; FIG. 2(c) is a pressure- volume diagram which depicts the working of an actual mach¬ ine; FIG. 3 is a bar graph comparison of the dynamic heat OM transfer coefficient calculated for various gaseous working fluids relative to air; and FIG. 4 is a schematic representation of means for con¬ trolling the instantaneous power level of a Stirling-cycle machine by adjusting the mean operating pressure. Best Mode for Carrying Out Invention Attention is directed to FIG. 1 wherein numeral 1 des¬ ignates an idealized version of a two-piston Stirling-cycle prime mover. A conceptually constant mass of pressurized gaseous working fluid occupies the working volume between the compression piston 2 and the expansion piston 3. The total working volume is comprised by compression space 4, regenerator 5, and expansion space 6. A portion of com¬ pression space 4 is continually cooled by cooler 7, while a portion of expansion space 6 is continually heated by heater 8. Arrows 9 are intended to represent the input of heat by conduction, convection, or radiation. Escape of fluid from the working volume is prevented by the piston seals 10. During the compression stroke (between positions I and II) the working fluid is compressed isother ally by piston 2 at the minimum temperature level of the cycle. Heat is con¬ tinually rejected at this temperature through cooler 7; the pressure rises slightly and the total working volume de- creases to a minimum. During the forward displacement (cold-side to hot-side transfer) stroke (between positions II and III) regenerator 5 yields stored heat to the working fluid as it is transferred to expansion space 6 with the volume remaining constant. The temperature and pressure rise to their maximum levels. During the expansion stroke (between positions III and IV) the working fluid expands isothermally at the maximum temperature level of the cycle, doing work on piston 3. The temperature level is maintained by the input of heater 8; the pressure drops and the total working volume increases to a maximum. During the reverse displacement (hot-side to cold-side transfer) stroke (between positions IV and I) re¬ generator 5 recovers heat from the working fluid as it is transferred to compression space 4 with the volume remain- ^ ing constant. The temperature and pressure return to the starting levels of the cycle. A clearer understanding of the foregoing may be ob¬ tained by referring to the diagrams of FIG. 2(a) and FIG. 2(b) wherein the same complete cycle is presented in terms of the pressure-volume diagram and the temperature- entropy diagram for the working fluid. For each process as depicted by the curves between the indicated position num¬ bers I-II, II-III, III-IV, and IV-I, the area under a curve on the P-V diagram, is a representative measure of the me¬ chanical work added to or removed from the system during the ,,procesp. Similarly, the area under a curve on a T-S dia¬ gram is a measure of the heat transferred to or rejected from the working fluid during the process. Actual machines differ fundamentally from the idealized versions in that the motion of each piston is continuous and smooth, rather than discontinuous and abrupt. This causes the indicated processes of FIG. 2(a) and Fig. 2(b) to overlap one another, and results in P-V diagrams which are smooth continuous curves devoid of sharp corners as shown by FIG. 2{c) . Thus the piston motion of actual mach- ines is smoothly periodic to the point of being sinusoidal, and the working fluid is likewise distributed in a periodic¬ ally time-variant manner throughout the total working volume. The instantaneous phase angle between the relative motions of the two pistons is a critical parameter in the operation of real machines and usually has a value on the order of 90 degrees. The processes of compression and expansion in real machines are not strictly isothermal, which constitutes another major departure from the ideal. The provision of heater 8 adjacent to expansion space 6 and of cooler 7 ad¬ jacent to compression space 4 effects only a crude approx¬ imation to the isothermal condition. Additionally, the presence of these elements tends to increase the unswept or dead volume ratio, which has a critically adverse effect on performance. Moreover, the working fluid is heated or cooled, not only when flowing in the correct direction be¬ tween regenerator 5 and either expansion space 6 or co - pression space 4, but also when flowing in the respective opposite directions. Nevertheless, extant Stirling engines exhibit comparable power densities and significantly great¬ er efficiencies than conventional internal combustion en- gines of all types. One favorable embodiment of the present invention is the utilization of alternative working fluids which provide increased performance, greater safety, and improved relia¬ bility. From a historical standpoint there appear to be only three working fluids of significant interest for appli- cation in regenerative thermal machines: air, helium, and hydrogen. Air was and still is of interest primarily be¬ cause of its universal availability. But helium and hydro¬ gen are the normal working fluids of choice in the prior art because their thermophysical properties are such as to per¬ mit high rates of heat transfer and flow to occur, with relatively low viscous flow losses, compared to air. Under standard conditions of temperature and pressure, for example, relative values for the thermal conductivity, heat capacity, and gaseous viscosity are 6.45/5.47/1.00, 14.31/5.16/1.00, and 0.47/1.06/1.00 for hydrogen, helium, and air respectively. In terms of engine performance, therefore, hydrogen is better than helium, and is also very much less expensive. On the other hand, hydrogen is dangerously combustible in the presence of air or oxygen, tends to destructively embrittle common engine con¬ struction materials, and is the most difficult of all ele¬ ments to confine under pressure. The pre-eminence of hydrogen and helium as working media for modern Stirling-cycle machines derives from the need to maximize the rates of heat transfer and minimize flow losses, or drag, in high performance applications. The complexities of fluid flow phenomena which include un¬ steady heat, mass, and momentum transport are analytically intractable for even a simple boundary geometry. Realistic engineering design and development must therefore rely heavily on empirical methods and equations, which in some cases are well developed and reasonably accurate. It is known, for example, that the rate at which heat is exchanged between a solid wall and a fluid can be des¬ cribed by the equation: Q = h x A dT. Here Q is the heat flow rate, A is the heat transfer surface area, dT is some characteristic temperature difference, and h is a heat transfer coefficient defined empirically for various types of flow. Thus, for a fixed boundary condition and given surface temperatures, maximum rates of heat transfer result from high heat transfer coefficients. It is also well known that the heat transfer coefficient for heat exchange be- tween a fluid and a long tube (length/diameter > 10) may be accurately described by the following empirically derived equation: h = 0.026 x (k/D) x (DxVxd/v)exp0.8 x (CpxvA)exp0.33 Here k is the fluid thermal conductivity, D is the tube dia- meter, V is the velocity of flow, d is the fluid density, v is the absolute viscosity, and Cp is the specific heat capacity. Therefore, it is an important specific teaching of this invention that by means of this relationship,, and by choos- ing a representative flow velocity and tube diameter for purposes of comparison, it can be demonstrated that certain gases other than hydrogen or helium are superior heat trans¬ fer media. Results for some selected fluorine compounds, namely sulfur hexafluoride, perfluoropropane, and octa- fluorocyclobutane are illustrated by the graph of FIG. 3. Although the indicated values were derived from data for these gases at room temperature and atmospheric pressure, a similar calculation made for the nominal operating condi¬ tions of a Stirling cycle prime mover, i.e. a te^erature of 750°C -and a pressure of 20 HPa (2900.74 psia) , indicates that the heat transfer coefficient for sulfur hexafluoride is nearly twice (1.8) that for hydrogen under the saiae conditions. The three alternative working fluids suggested by FIG. 3 are nontoxic, nonflammable, and easily liquefied under pressure at room temperature, which leads to im¬ proved safety and ease of handling. They are also chemi¬ cally and thermally stable, and generally possess a much OMPΪ higher molecular weight corr.pared to hydrogen or helium. According to Graham's Law the rate at which gases tend to diffuse through very small openings is inversely propor- . tional to the square root of their density. Thus these high molecular weight gases present a fa'r less difficult reciprocating seal design problem compared to either hy¬ drogen or helium, and a far greater quantity of makeup fluid can be stored in a given volume as a liquefied gas than as a pressurized gas. Another reason often given for choosing hydrogen or helium over other gases as a working fluid is that their low viscosity automatically result in the lowest possible viscous flow losses. But the empirically derived dimension- less parameter known as the Reynolds Number, which is well known to those skilled in the science of fluid mechanics to equal Density x -Speed x 3ize / Viscosity, is a measure of the ratio of inertial forces in a flow to viscous forces in the flow. This means that at low Reynolds Number flows, viscous effects in the flow dominate the inertial effects in the flow and vice versa. But it is also well known that most common examples of high performance thermal machines, including Stirling-cycle engines, operate generally in a high Reynolds number flow regime where inertial effects are known to dominate viscous effects. And although the fluid viscosity surely is not irrele¬ vant in high Reynolds :; ..ιber flows, an approximate experi¬ mental law for drag in such flows is known to be: Drag = (Speed)exp2 x Density x (Size)exp2. This me-ans that the choice of a working medium which possess a higher heat transfer coefficient will, for a given heat flow rate or power level, require a lower fluid velocity (Speed) and/or a smaller wetted heat transfer surface area (Size) , and will therefore result in a lower fluid drag, not withstand- i ^ the value of the fluid, viscosity, and to a lesser ex- tent, the density of the :.ιediur.ι. It is for these reasons that the invention proposes the uuilization of the indicated compounds, and others, as al¬ ternative working fluid media in the Gtirling-cycle engine. The essence cf this concept is the novel recognition that such working fluids should be selected primarily with re¬ gard to whether or not they exhibit a high dyn-amic heat transfer coefficient, outstanding chemical and physical in¬ ertness, .and the requisite critical properties to facili- tate liquefaction under normal operating conditions. In addition are the enormous advantages of simplicity and safety inherent in the storage and handling of a nontoxic, nonflammable, inert liquid at low pressures, when compared with the storage and handling of dangerously flammable and chemically active hydrogen at high pressures. Thus, it is apparent that this teaching of my invention is away from the teiaching of the prior art and was not at all obvious at that time. Another favorable embodiment of the invention is an engine power level control subsystem, to be used in con¬ junction with the operation of any regenerative thermal machine, by means of which the mean system operating pres¬ sure can be rapidly and automatically varied as a function of power demand. In the case of a modern, but conventional, high-performance, Stirling-cycle engine, the operating power level is normally controlled by simultaneously adjusting both the quantity of heat input to the heater head and the mean working pressure of the cycle, since it is these vari¬ ables which most directly influence the power level. The first of these is accomplished by means of a combustion control subsystem quite similar in function to the familiar accelerator/throttle linkage of the automotive internal combustion engine, except that the time response is much slower due to the large thermal mass of the heat transfer components. The second, however, currently requires a complex pres¬ sure control subsystem consisting of pressurized heavy wall stainless steel hydrogen gas bottles; sophisticated servo- ^ actuated high pressure flow control valves; an array of essential and specially designed check valves, stop valves, bypass valves, relief valves, gauges, and the like; and a high-capacity, hydrogen-compatible compressor as described in prior art patents filed under Subclass 521 of Class 60. (see for example U.S. Patent No. 3,699,770; No. 3,827,241; or no. 4,030,297). In short, the control subsystems, re--- quired to vary the power level of prior art Stirling-cycle engines are both complicated and costly, and they represent a critical stumbling block to the economical use and the widespread acceptance of these engines. This is particular¬ ly true today with respect to prime movers which might be sold in the highly competitive worldwide automotive market. One of the primary technical ramifications of the usual working pressure control subsystem in such applications may be described as an inherent accelerator time lag or "hesi¬ tation". The time response of existing compressor-dependent systems is poor, because the compressor must handle a com¬ paratively large volume of gas in order to effect a small change in the mean system pressure. This is fundamentally unavoidable in such a system, as the bulk modulus of a gas is by nature extremely low. It is therefore an accepted practice to accomplish a rapid transition from low power demand to high power demand by maintaining the gas reser¬ voir at a pressure substantially greater than the maximum working pressure' of the cycle. The force of this greater pressure is then relied upon to rapidly inject additional gas into the working volume whenever an increase in power output is indicated. As will be appreciated by those familiar with the art, the transistion in the reverse direction from a high power demand condition to a low power demand condition is less easily achieved. Since the time required is typically on the order of 5 to 20 seconds, depending on the engine's speed and the degree of change required, additional control valves are sometimes employed to temporarily short circuit the normal flow path between the cylinders to achieve a more rapid decrease in power output. The basic feasibility of this type of power level control subsystem has been demon- strated in the laboratory, but it is an undesirable solution to the general problem because it forces the system design in the wrong direction toward an even greater complexity and cost. Attention is now directed to the schematic illustration of FIG. 4 wherein a novel power level control subsystem is depicted which is deliberately intended to operate in a single-component two—phase mode. This system is similar to the prior art in that it operates on the well-known princi- pie that a change in the steady state power level of a Stirling engine is virtually a direct linear function of a change in the mean operating pressure of the gaseous work¬ ing fluid contained therein. But it is radically differ¬ ent from previous systems in that the working fluid is intended to undergo a change in phase whenever it is added to or withdrawn from the working volume. Thus it is another impo tiant specific teaching of this invention that a rapid transition from a condition of low power demand to some other condition of high power demand may best be accoraplish- ed by the rapid injection of working fluid into the working volume in the form of a virtually incompressible liquid. It may be seen that one preferred embodiment of such a power level control subsystem is comprised by a servo- actuated variable displacement hydraulic pump 30, a power demand control mechanism or accelerator 32, free piston 34, flui reservoir 35, reservoir cooler 36, and a plurality of Stirling cycle engine coolers 24. When an increased demand for power output is indicated by a change in the setting of accelerator 32, pump 30 forces hydraulic fluid 31 into reservoir 35 which in turn forces piston 34, sealed by o- ring 33, to move to the right. This action causes the rapid injection of condensed liquid working fluid 37 into the working volume at points of entry contiguous with coolers 24. The change in mean operating pressure is im ed- iate, because the introduction of nearly incompressible liquid medium in this manner instantly lowers the total working volume available to the gaseous medium. As the in¬ jected liquid is subsequently evaporated by heat transferred within engine coolers 24, the available volume gradually re- turns to its forrαer value, but only as the pressure level is increased by the generation and presence of additional vapor. Thus both the increased mean operating pressure and therefore a higher output power level are sustained. The reverse transition from a condition of high power demand to a condition of lower power demand is also rapidly accomplished by a reverse change in the setting of accel¬ erator 32. This occurs by means of the simple venting of a portion of the gaseous working fluid back to reservoir 35 when pump 30 reverses its direction of flow, since reservoir 35 is maintained at a pressure below the minimum working pressure of the system, and at a temperature less than that of engine coolers 24. Thus it should be apparent to -those skilled in the art that two essential prerequisites for the successful implementation of the proposed power level control system are the presence of an additional heat exchanger shown in FIG. 4 as reservoir cooler 36 and the utilization of a working fluid which possesses a critical temperature, Tc, between the norxαal operating temperature of coolers 24 and that of reservoir cooler 36. Each engine cooler 24 is maintained at a temperature sufficiently low for good Stirling-cycle thermodynamic effi¬ ciency, but at a temperature somewhat above Tc so that its function from the standpoint of power control is always that of an evaporator. On the other h-and, reservoir cooler 36 and therefore revervoir 35 must be maintained at a temper¬ ature somewhat below Tc so that its function is always that of a condenser. Thus the pressure in reservoir 35 is the saturated vapor pressure of the condensed working fluid at that temperature. It is perhaps appropriate at this point to emphasize that the liquid phase is therefore present within the work¬ ing volume of the machine only in the region of each engine cooler dead volume (excluding most of the compression space 22) and for short periods of time during increasing power output transients. A reduction in the overall system com¬ plexity and cost, coupled with a dramatic increase in the system time response, can be expected to result from this approach to the design of a Stirling engine power level control device. From a practical engineering standpoint, the business of effecting and controlling the flow of a high bulk modulus liquid by means of a low capacity positive dis¬ placement pump is preferable to the business of effecting and controlling the equivalent mass flow in the form of a _ I low bulk modulus gas by means of a high capacity compressor. Since the closed cycle Stirling prime mover operates on the basis of the difference in temperature in the working fluid between the hot expansion space and the cold compress- ion space, the development of useful power output is not specific to the source of heat available for use. There¬ fore the design of the heat source can be any one of a large variety of possible types. A rather simple combustion sys¬ tem can be produced, for example, which will cleanly and efficiently burn various kinds of both liquid fuels and gas¬ eous fuels without any modification whatsoever. Thus it will e appreciated by those familiar with the art that a single prime mover may be made to operate on regular or premium gasoline, diesel oil, alcohol, crude oil, lubricat- ing oil, olive oil, vegetable oil, propane, butane, natural gas, and synthetic coal gas. It is important at this point to re-emphasize the-fact that each small segment of a well-designed regenerator transfers heat to and from the working fluid with minimal temperature differences. Thus " all stages in the regenerator are reversible in an actual thermodynamic sense. Therefore, the entire machine cycle is reversible in function; that is, the direction of flow of heat and work can be reversed. The Stirling engine is truly unique in that it is the only prac- tical example of a thermodynamically reversible machine. It should be thoroughly understood, therefore, that many of the design concepts disclosed herein for Stirling prime movers -are also applicable to the design and develop¬ ment of Stirling refrigerators, heat pumps, air condition- ers, and the like. It is another important specific teach- ing of this invention that machines of this kind would be appreciably more efficient than conventional vapor cycle re¬ ciprocating refrigerators or thermally-activated absorption refrigerators, with a substantial savings in size and weight. In addition, a hybrid device obtained from the combination of a Stirling prime mover mechanically coupled to a Stirling heat pump will permit both multifuel and non- fuel powered refrigeration units to be developed and applied to specialized applications. In view of the foregoing it should be readily apparent to those skilled in the art that the operation of the pre¬ sent invention may be accomplished by means of and in the context of an enormous variety of diverse applications. In fact, virtually every market in the world which is currently occupied by the application of a reciprocating internal com¬ bustion prime mover, or by the application of a conventional vapor cycle, absorption, or other type of refrigeration de¬ vice, is subject to improvement by virtue of the diligent application of the teachings of this invention. These include but are by no means limited to the fol¬ lowing: automotive prime movers, marine prime movers, aero¬ nautical prime movers, astronautical prime movers, industrial prime movers, military prime movers, agricultural prime movers, ultifuel prime movers, nonfuel prime movers, portable prime movers, bio edical prime movers, refriger¬ ators, air conditioners, cryogenic cooling engines, residen¬ tial heat pumps, industrial heat pumps, military heat pumps, water coolers, air compressors, other gas compressors, re- mote electric generators, portable- electric generators, - stationary electric generators, hydroelectric power convert¬ ers, nuclear power converters, radioisotope power convert¬ ers, solar power converters, geotherr * .al power converters, •ocean thermal power converters, bio ass power converters, solid waste power converters, small cogeneration power plants, large cogeneration power plants, remote fluid pumps, portable fluid pumps, stationary fluid pumps, remote power tools, portable power tools, outdoor power tools, underwater power tools, toys and novelties. Obviously, many modifications and variations of the present invention may occur to those skilled in the art in the li-ht of the above teachings. Indeed, every potential application of a Stirling-cycle engine which may be accom¬ plished by machines operating on the principles set forth herein is, in and of itself, a unique and special variation of this invention. It is therefore to be unαerstood that within the scope of the appended claims the invention nay be practiced otherwise than as specifically described. I claim: OMPI

Claims: CLAIMS 1. A Stirling-cycle, reciprocating, thermal machine employing a single-phase, gaseous internal working fluid in the working volume of its cycle that has a molecular weight ' substantially greater than that of air, is nontoxic, non¬ flammable, chemically and thermally stable, with a heat -transfer coefficient substantially greater than that of hy¬ drogen at machine operating temperatures and pressures and can be easily liquefied under pressure at room temperature, and including means external of, but capable of interconnec¬ tion with, said volume in which a supply of said flui is stored in the liquid phase. 2. A machine according to Claim 1 in which the working fluid is a fluorine compound. 3. A. machine according to Claim 2 in which the working fluid is sulfur hexafluoride. 4. a machine according to Claim 2 in which the working fluid is a fluorinated hydrocarbon. 5. A machine according to Claim 4 in which the working fluid is perfluoropropan , perfluorobutane, or octafluoro- cyclobutane. 6. A machine according to Claim 2 in which the working fluid is selected from the group consisting of perfluoropro- pane, octafluorocyclobutane, and sulfur hexafluoride. 7. Stirling-cycle, reciprocating, thermal machine in which the internal working fluid is a gas that has a molecu¬ lar weight substantially greater than that of air, is non¬ toxic, nonflammable, chemically and thermally stable, with a heat transfer coefficient substantially greater than that of hydrogen at machine operating temperatures and pressures and can be easily liquefied under pressure at room temperature, and comprising a control system including m ans for varying the main system mean operating pressure by rapid injection of working fluid from, a reservoir into the working volume as a liquid to increase that pressure and by withdrawal of working fluid as a gas from that working volume and its con¬ densation in the reservoir to reduce that pressure. S. A machine according to Claim 7 in which the reser¬ voir is r.'.aintained at a temperature below, and the working OM?I volume at a temperature above, the critical temperature of the working fluid. 9. A machine according to Claim 8 in which the re¬ servoir comprises a cylinder space and a free piston slid- able in said space having condensed working fluid on one side and hydraulic control fluid on the other side. 10. A machine according to Claim 9 which comprises means to augument or diminish the volume of hydraulic fluid in the cylinder in response to changes in the setting of an accelerator.

Inventors: MOSCRIP WILLIAM MATTHEW
Applicants: MOSCRIP WILLIAM M, MOSCRIP WILLIAM MATTHEW
Application year: 1982
Main classification: F02G1
IPC codes: F01B3, F02G1