Stirling Engine Power System And Coupler

Abstract

Stirling engine power system includes a displacer type Stirling engine producing an output of pressure pulses, load means to be driven by the output pulses, and a resonantly operated fluid coupler for coupling the output pulses suitably to the load means to drive the same. In one version of the power system, the load means is a piezoelectric generator driven to generate electrical output energy and the fluid coupler is a mercury-filled tube with a flexible diaphragm at each end to separate the mercury from working gas of the engine and hydraulic fluid of the generator. In another version of the power system, the load means is a hydraulic system including a hydraulic motor load and the fluid coupler is a hydraulic fluid-filled tube with a diaphragm at one end to separate the hydraulic fluid from the engine gas.

Description

BACKGROUND OF THE INVENTION

This invention relates generally to power systems and, more particularly, to a quiet, reliable and high efficiency power system wherein the output capabilities of a Stirling engine are matched with the input requirements of a selected load.

Currently, gasoline or diesel engines are almost universally used to drive rotary electrical generators and various other types of loads. These power units are reasonable in cost but are noisy, relatively unreliable, and require fairly regular and knowledgeable maintenance of their engines. It is evident that the gasoline or diesel engines of the conventional power units are the source of most of the disadvantages thereof. There is, of course, a need in many applications for a power unit which is silent, reliable and durable under rugged use conditions, requiring little and simple maintenance over extended periods of operation.

Where a source of heat or thermal energy such as a gas flame or suitable radioisotope is available, the conventional Stirling engine can be used to drive an electrical generator or other load with low noise emission and requires relatively little maintenance. As is well known, the crankshaft of the conventional Stirling engine is connected to both its power piston and displacer, and can be coupled to drive a suitable load. The actuated displacer transfers working gas between the hot and cold spaces in the engine to produce pressure changes from which net power is derived by a work-producing expansion, with most of the gas in the hot space, followed by compression of mostly cold gas. This is achieved by having the displacer motion lead the power piston by about 90.degree., as set by the crank angle.

Many bearings and seals, some heavily loaded, are needed in the conventional Stirling engine, however. Moreover, the seals must be absolute because leakage of lubricant from the crankcase would foul the heat transfer surfaces in the heat engine. An energy conversion system utilizing the Stirling cycle but requiring less bearings and seals is shown, described and claimed in U.S. Pat. No. 3,400,281 of Marvin J. Malik for Stirling Cycle Drive for an Electrokinetic Transducer, patented Sept. 3, 1968. In this system, the usual power piston in a conventional Stirling engine is replaced with a flexible diaphragm which performs the power piston's function of alternately compressing and expanding the working fluid during the Stirling cycle. The resultant pressure variations are used to drive an electrokinetic transducer which develops an alternating electric potential that is used to energize a motor. The motor, in turn, drives the displacer of the Stirling engine. The electric potential and/or the motor can be used to drive suitable but relatively limited loads.

SUMMARY OF THE INVENTION

Briefly, and in general terms, the present invention is preferably accomplished by providing a simplified Stirling engine having an output of gas pressure pulses, instead of mechanical shaft power, and a resonantly operated fluid coupler for coupling the pressure pulses to load means to drive the same. In one version of the invention, the load means is a piezoelectric generator and the fluid coupler is a mercury-filled tube with a flexible diaphragm at each end to separate the mercury from working gas of the engine and hydraulic fluid of the generator. The coupler provides the required 90.degree. displacer-coupler (piston) phase lag and also matches generator requirements with engine capabilities.

In another version of the invention, the load means is a hydraulic system including a hydraulic motor load and the fluid coupler is a hydraulic fluid-filled tube with a flexible diaphragm at one end to separate the hydraulic fluid from the engine gas. The coupler is connected by check valves to the hydraulic system which preferably includes an accumulator on the high pressure side of the load and a reservoir on the low pressure side thereof to damp the pressure pulses for steady flow of the hydraulic fluid.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be more fully understood, and other advantages and features thereof will become apparent, from the following description of certain exemplary embodiments of the invention. The description is to be taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a block diagram of a Stirling engine power system illustrating the operation of one version of this invention;

FIG. 2 is a diagram of the equivalent mechanical circuit of the system shown in FIG. 1;

FIG. 3 is a graph showing work diagrams illustrating the storage and delivery of kinetic energy by the coupler inertial member of the system shown in FIG. 1;

FIG. 4 is a fragmentary elevational view, shown partially in section, of a fluid coupler connecting with a simplified Stirling engine;

FIG. 5 is a fragmentary sectional view of the fluid coupler shown connected to the simplified Stirling engine in FIG. 4;

FIG. 6 is a sectional view of a piezoelectric generator which can be used with the fluid coupler shown in FIG. 5;

FIG. 7 is a generally perspective view of the stack of piezoceramic discs contained in the piezoelectric generator shown in FIG. 6;

FIG. 8 is a graph showing certain plots of output power density versus applied stress fluctuation and time for a piezoelectric generator similar to that of FIG. 6; and

FIG. 9 is a diagrammatic representation of a Stirling engine power system wherein the load means is a hydraulic system including a hydraulic motor load.

DESCRIPTION OF THE PRESENT EMBODIMENTS

In the following description and accompanying drawings of certain illustrative embodiments of this invention, some specific dimensions and types of materials are disclosed. It is to be understood, of course, that such dimensions and types of materials are given as examples only and are not intended to limit the scope of the invention in any manner.

FIG. 1 is a block diagram of a Stirling engine power system 20 including a simplified Stirling engine 22 having an input 24 of heat and an output 26 of pressure pulses, a piezoelectric generator 28 having an input 30 and an output 32, and a fluid coupler 34 for coupling the pressure pulses to input 30 of the piezoelectric generator which converts the input pressure energy to electricity at its output 32. The engine 22 produces pressure pulses from heat. After a phase shift by inertia in the coupler 34, these pressure pulses can be applied to the generator 28 to produce electricity. Gas in the engine 22 acts as a spring and negative damper, one that produces instead of consuming power. The coupler 34 acts as a mass, and the generator 28 acts as another spring combined with a positive (conventional) damper. The coupler 34 serves as an inertial energy storage device analogous to the flywheel in a conventional engine.

FIG. 2 is a diagram or schematic representation of the equivalent mechanical circuit of the system shown in FIG. 1. The Stirling engine 22 is represented by a negative damper 36 and parallel spring 38, the fluid coupler 34 by an inertial member or mass M, and the piezoelectric generator 28 by a positive damper 40 and parallel spring 42. The coupler 34 provides the required 90 degrees displacer-coupler phase lag by a reasonantly tuned spring-mass-damper system in which the engine 22 (a negative damper 36 or energy source) and generator 28 (a positive damper 40 or energy sink) are springs 38 and 42 acting on the oscillating coupler mass M. The coupler 34 also matches generator requirements with engine capabilities.

FIG. 3 is a graph showing work diagrams including curves 44 and 46 of plots of force on the coupler 34 inertial member or mass M versus displacement thereof, from the Stirling engine 22 and to the piezoelectric generator 28, respectively. It can be seen that engine pressure decreases during the expansion stroke, while the piezoceramic disc stack in the generator 28 requires increasing pressure to compress it. This represents a 180.degree. phase lag between the pressure-time characteristics at the engine 22 and generator 28 ends of the coupler 34. Coupler inertia accomplishes this by the large pressure difference required to accelerate the liquid column or free piston of the coupler 34.

The excess of engine pressure over generator-region pressure during the first half of the expansion stroke is stored as kinetic energy of the inertial member M, which completes generator 28 stack compression on the last half of the expansion stroke. Inertial energy storage and delivery is indicated by the cross hatched areas in FIG. 3. A similar consideration applies for the compression stroke where elastic energy return from the piezoelectric generator 28 provides excess energy during the first half of the compression stroke to complete the last half. For clarity, these energy quantities are not indicated in FIG. 3.

FIG. 4 is a fragmentary elevational view, shown partially in section, of fluid coupler 34 connecting with the simplified Stirling engine 22. The Stirling engine 22 shown here is, of course, only exemplary of a simplified Stirling engine which produces an output of pressure pulses. Other suitable forms of simplified Stirling engines are shown, described and claimed in U.S. Pat. No. Re 27,567 of Arthur R. Baumgardner, Richard P. Johnston, William R. Martini and Maurice A. White for Stirling Cycle Machine With Self-Oscillating Regenerator, patented Jan. 23, 1973 and U.S. Pat. No. 3,604,821 of William R. Martini for Stirling Cycle Amplifying Machine, patented Sept. 14, 1971, for example. The Stirling engine 22 need not be restricted to that specifically shown in FIG. 4 and, in view of description given in the noted patents, a relatively brief description of the engine is believed to be adequate.

The Stirling engine 22 basically includes an engine cylinder 48, displacer 50, displacer drive mechanism 52, and thermal insulation housing 54. The engine cylinder 48 is mounted to displacer drive housing 56 with a bolted flange arrangement 58, and the fluid coupler 34 is suitably attached to the side of the drive housing as shown in FIG. 4. Driveshaft crosshead 60 is supported in lower and upper bearings 62 and 64 which are mounted in the housing 56. The insulation housing 54 is suitably attached to the upper flange 66 of the cooler section of the engine cylinder 48 and contains suitable insulation 70. Electric cartridge heaters 72 are used to heat the upper end of cylinder 48; however, any other suitable heat source can be used. Loops 74 circulating a cooling fluid are used to cool the lower end of the cylinder 48.

Crankshaft 76 is driven by an electric motor 78 which is positioned behind flywheel 80 (and a separating wall) that is suitably affixed to the crankshaft. An offset crankpin 82 is affixed to the flywheel 80 and rotatably mounts the lower end of connecting rod 84. The upper end of the connecting rod 84 is rotatably mounted on wrist pin 86 which is attached to the driveshaft crosshead 60. The upper end 88 of the driveshaft crosshead 60 is pivotably connected axially to the lower end of displacer 50. The upper end of the displacer 50 is supported by a pair of crossed flexures 90 which are centrally attached to the engine cylinder 48 on its axis at the top 92 thereof. The displacer 50 is, for example, 7.00 inches long and 1.44 inches in diameter and made of a thin shell of Inconel 625 with a wall thickness of 0.052 inch and is pressurized internally with argon. A displacer position sensor 68 is mounted at the bottom of the displacer drive housing 56.

The lightweight displacer 50 oscillates (reciprocates) inside the engine cylinder 48 with a five-mil radial clearance gap 94 between the cylinder wall and displacer. This separation is maintained by the flexures 90 at the hot end and the bearing 64 (part of the displacer drive mechanism 52) near the cold end. The gap 94 acts as a gas heater, regenerator and gas cooler. Oscillation of the displacer 50 heats and cools the confined gas (helium, for example) and creates the pressure pulses. The electric motor 78 is used fo startup and fine frequency control. Once oeprating speed is reached, the gas pressure difference applied to the displacer drive piston (upper driveshaft portion of the driveshaft crosshead 60) varies over a cycle in such a way as to apply power to the displacer drive. Thus, if friction is not unduely great, the engine 22 will run itself.

The fluid coupler 34 is suitably attached to the displacer drive housing 56 and communicates with the engine cylinder 48 through gas flow passageway 96. The coupler 34 is a mercury-filled tube 98 with a diffuser 100 at each end. A corrugated diaphragm 102 separates the mercury 104 from the engine gas at one end and from the hydraulic fluid in the piezoelectric generator 28 at the other end. Inertia in the mercury column provides the necessary phase lag between displacer 50 motion and motion of the corrugated metal diaphragm 102 (analogous to a power piston) which makes a positive seal between the engine gas and the mercury 104. The necked-down configuration from diaphragm to diffuser at the ends of the coupler 34 produces the effect of a much larger mass than is actually present. This is similar to a flywheel attached to a speed-increaser gear so that the fluid coupler 34 can be termed a "fluid flywheel."

FIG. 5 is a fragmentary sectional view of the fluid coupler 34. The coupler housing 106 is, for example, made of 300-series austenitic stainless steel and the 10.5 inches long coupler tube 108 is standard 0.5 inch inside diameter tubing with a 0.188 inch wall thickness. The tube 108 can, of course, be suitably coiled to reduce system dimensions. Hoop stresses are approximately 4,000 psi at hydraulic pressures of 3,000 psi. A 5.2 inches long diffuser 110 with a 7.5 degrees half-angle is provided at each end of the coupler tube to minimize flow losses. The mercury 104 inventory in the coupler 34 is 11.5 cubic inches, for example. Mercury was selected as the coupler fluid in this instance primarily because of its high density and low compressibility. High density minimizes the size of the inertia column and low compressibility minimizes lost motion.

The corrugated diaphragms 102 are suitably attached to their respective ends of the coupler housing 106. Each diaphragm 102 is made of 0.0015 inch thick Am 350 stainless steel, and have eight convolutions with a peak-to-peak height of 0.024 inch and a pitch of 0.15 inch, for example. The diaphragms 102 are capable of operating through a swept volume .+-. 0.2 cu. in. from the central unstressed position of their extreme deflected position with a maximum tangential stress of 31,000 psi. The design point swept volume totals 0.21 cu. in. or about one-half the allowable value.

Each diaphragm 102 has a perforated backup plate 112 to limit its travel. The plates 112 can be made of type 7075-T6 aluminum, for example. At the engine/coupler interface, the backup plate 112 is secured by end disc 114 on the gas side of the diaphragm 102. Gas must be distributed over the surface of the diaphragm 102 while minimizing gas dead volume. Eight radial channels 116 can be provided behind the backup plate 112, each connected to eight holes 118 which vent gas to the diaphragm 102. The holes 118 can be, for example, 0.15 inch in diameter. The diaphragm 102 is capable of supporting a 1,060 psi pressure differential across this hole area with a maximum of 20,000 psi flexural stress when bottomed against the backup plate 112. The total dead gas volume between the diaphragm 102 and the engine cold plate is 0.04 cu. in. and the pressure drop in the manifolding is 8 psi. Nominal design deflection of the diaphragm 102 is 0.075 inch, for example.

At the load/coupler interface, the backup plate 112 is secured by end disc 120 on the oil or hydraulic fluid side of the diaphragm 102. The radial channels 116 and vent holes 118 are larger than at the engine/coupler interface because entrained fluid volume is unimporatant, and pressure drops would otherwise be too high. To protect the diaphragms 102 from overpressurization (due to high pressure transients) when against the concave or dished surfaces of backup plates 112, at least one overpressure relief accumulator 122 is provided at each end of the mercury column. There can be, for example, four equiangularly spaced accumulators 122 provided at each end. These can be piston-type accumulators filled with gas through tubing 124 (which is then closed), with a spring 126 loading the piston 128 such that it accepts or is moved at a preset value to prevent damage to the associated diaphragm 102. Ports 130 for fill, bleed and trim valves (not shown) can be located at each end of the coupler 34 for charging and providing the proper amount of mercury. The end disc 120 has a passageway 132 with end ports 134 and 136 which can accommodate respective check valves (not shown here), or the port 134 can be plugged and the port 136 suitably connected to drive the piezoelectric generator 28. A suitable hydraulic fluid or oil is preferably used in the lines connecting with the passageway 132.

FIG. 6 is a sectional view of the piezoelectric generator 28 which can be used with the fluid coupler 34 shown in FIG. 5. The generator 28 requires pulsating mechanical power at relatively high force and low displacement, and which can be provided by the Stirling engine 22 and coupler 34 power source. The generator 28 includes a housing 138 which is, for example, approximately 5.5 inches long and 2.0 inches in diameter. The housing 138 contains a stack 140 of a hundred piezoceramic discs 1.25 inches in diameter and 0.040 inch thick, and a piston 142 positioned against one end of the stack. A fitting 144 is attached to the piston end of the hosuing 138 and a indicator rod 146 is affixed to the piston 142 as shown. The rod 146 cooperates with proximity transducer 148 mounted on the other end of the housing 138. A pressure transducer 150 is also mounted to the fitting 144. The transducers 148 and 150 are used for experimental measurements and can, of course, be omitted. The port 136 of the coupler 34 shown in FIG. 5 is connected by a line (not shown) to the input port 152 of fitting 144. An insulating hydraulic fluid or oil is used in the connecting line and also surrounds the stack 140 in the housing 138 to prevent arcing. The stack 140 has an output lead y.

FIG. 7 is a generally perspective view of the stack 140 of piezoceramic discs 154 contained in the housing 138. The discs 154 are made from a lead zirconatetitanate ceramic (PZT-4 or equivalent) and metallized on both flat surfaces for electrical contact with metal electrode connectors 156. Warm discs 154 are polarized (analogous to magnetizing a permanent magnet) by applying a high voltage to align the dipoles of the disc material. These discs 154 are stacked with alternating polarity and connected electrically in parallel with one side to a load R and the other side to ground as indicated. When the stack 140 is subjected to an axial compressive stress, pulsating at a suitable frequency (such as 60 Hz), electric power is generated. Compression of the stack 140 produces electrical charges delivered as load current in one direction, followed by a reversed polarity and current when compression is relaxed. Only about half the applied strain energy produces a charge. The balance is stored elastically and is utilized for the compression stroke on the engine.

FIG. 8 is a graph showing indicative plots of output power density versus applied stress fluctuation and time for the piezoelectric generator 28. Experimental power density plots follow the predicted curve 158 closely to 8,000 psi stress fluctuation, with decreased performance at higher levels. Optimum load resistance with no inductance was used in the measurements. A continuous run of 20 hours at 6,000 psi stress fluctuation produced the curve 160 which showed only an initial, reversible degradation. When inductance was used in the load circuit to compensate for capacitance inherent in the generator 28, however, power density increased by a factor of 2.5 as compared with a predicted value of 3.0. Piezoelectric generator 28 efficiency was measured with mean values in the 80 to 90 percent range.

FIG. 9 is a diagrammatic representation of a Stirling engine power system 162 which is similar to the system 20 shown in FIG. 1 except that the load means is a hydraulic system 164 instead of the piezoelectric generator 28, and the fluid coupler 166 is a modified version of the fluid coupler 34 shown in FIG. 5. The fluid coupler 166 is similar to the coupler 34 except that the diaphragm 102 and its backup plate 112 at the load end of coupler 34 have been deleted, and hydraulic fluid is used throughout the coupler 166 and hydraulic system 164. The diaphragm 102 and its backup plate 112 are retained at the engine end of the coupler 166. Check valves 168 and 170 are installed in the coupler ports corresponding to ports 134 and 136 of the coupler 34. A conventional high pressure accumulator 172 is connected to the line between check valve 170 and hydraulic motor 174, and a conventional low pressure reservoir 176 is connected to the line between the hydraulic motor 174 and check valve 168. The accumulator 172 and reservoir 176 are provided, of course, to damp the pressure pulses for steady flow of the hydraulic fluid in the lines.

While certain exemplary embodiments of this invention have been described above and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of, and not restrictive on, the broad invention and that the invention is not to be limited to the specific constructions or arrangements shown and described, for various modifications may occur to persons having ordinary skill in the art.

Claims

What is claimed is:

1. A power system comprising;

a Stirling engine including a displacer and a gaseous working fluid, said engine producing an output of pressure pulses;

load means adapted to be driven by said pressure pulses; and

a fluid coupler form of power member for coupling said pressure pulses to said load means to drive the same, said coupler including a tubular element of predetermined dimensions, a fluid of predetermined characteristics filling said tubular element and diaphragm means for separating said gaseous working fluid from said fluid filling said tubular element, and said tubular element filling fluid comprising a liquid column of relatively substantial predetermined inertial mass that provides in combination with characteristics of said engine and said load means a resonantly tuned spring-mass-damper system wherein a 90.degree. phase lag is produced at resonant frequency between said displacer and said coupler liquid column, and high output energy can be directly obtained from a displacer type of Stirling engine operating a resonant power member without interposition of a mechanical crank and flywheel.

2. The invention as defined in claim 1 wherein said load means includes a piezoelectric generator, and said coupler matches generator requirements with engine capabilities through inertial storage and delivery of energy by said liquid column.

3. The invention as defined in claim 1 wherein said load means includes a hydraulic system, and said liquid column includes a portion of hydraulic fluid which is used in said coupler and said hydraulic system.

4. The invention as defined in claim 1 wherein said tubular element includes necked-down end portions providing an intermediate portion having a throat area less than those of said end portions, to produce the operational effect of a much larger liquid mass than is actually present in said tubular element.

5. The invention as defined in claim 2 wherein said piezoelectric generator comprises a housing, a stack of piezoceramic discs contained in said housing and a piston adapted to be positioned against one end of said stack to compress the same in response to said pressure pulses, and said coupler matches generator requirements with engine capabilities through inertial storage and delivery of energy wherein said liquid column stores excess energy as kinetic energy during the first halves of the engine expansion and compression strokes and delivers said stored energy during the second halves of said expansion and compression strokes to complete the same.

6. For use in a power system, a fluid coupler comprising:

a tubular element of predetermined length and size, said tubular element including necked-down end portions to provide an intermediate portion having a throat area less than those of said end portions; and

a diaphragm closing off one of said end portions of said tubular element.

7. The invention as defined in claim 6 further comprising another diaphragm closing off the other end portion of said tubular element.

8. The invention as defined in claim 6 further comprising a perforated backup plate mounted adjacent to said diaphragm to limit its travel.

9. The invention as defined in claim 8 further comprising an overpressure relief accumulator operatively mounted at the diaphragm end of said tubular element to protect said diaphragm from overpressurization when against said backup plate.

10. The invention as defined in claim 6 wherein said tubular element further includes longitudinally tapering diffuser portions respectively connecting said end portions to corresponding ends of said intermediate portion to minimize flow losses.