Patent Grant
7762055
Externally heated engines and, in particular, Stirling cycle engines have always held great promise, because their theoretical thermal efficiency approaches that of the Carnot Cycle. This efficiency is established in turn by the difference between the hot and cold temperatures of the cycle. Recent designers of such engines have sought to maximize efficiency by increasing the temperature of the hot side of the engine. In addition, they have utilized fine molecule gasses, such as helium and hydrogen, at very high pressures, to further optimize the power output of the engine. Their combined efforts have led to commercial failure. The high temperatures have required the use of materials which can withstand these temperatures. The practical problems, and enormous expense, of using materials such as titanium and special alloys of stainless steels have combined to make the engines impractical to manufacture, and expensive to own and operate. High pressure gasses and extreme temperatures have made the engine so complex that it has been placed out of the reach of all but the most sophisticated users.
The present invention takes a completely opposite approach. Through the combined use of several innovations, the design of a high efficiency, low temperature, simple engine becomes possible. Existing designs have used either plain cylinder walls as the heat exchanger, or a variation of the shell and tube type of air to air heat exchanger. Materials are typically steel or titanium, both of which are relatively poor conductors of heat.
To overcome these inefficiencies, the temperature differential between the air outside the cylinder, and the working fluid inside the cylinder must be very large to force the transfer of the necessary amount of heat in the very limited time available. This in turn forces the heat source itself to operate at an even higher temperature, and to be very tightly coupled to the heat exchanger. This tends to expose the external portions of the exchanger to even higher temperatures, which requires still more exotic materials.
Some prior art engines use liquid sodium as a phase change material, to get heat inside the cylinder more effectively. Aside from the great expense involved, there is complex technology needed to manufacture such devices. In addition, liquid sodium is very toxic and very hot, making it extremely dangerous to use. This technology is not suitable for use in a simple, mass-produced device.
An additional problem in the prior art engines concerns the temperature of the air sent to the regenerator. The extreme temperatures traditionally involved in the prior art make the use of common low temperature tubing, such as copper, impossible. This also applies to the materials used in the regenerator. Neither the outside of the regenerator or the material used in the regenerator matrix can be optimized for thermal performance, because the overriding concern is survivability at high temperature.
The problems of high temperatures completely dominate the design of a regenerator to be used in the prior art Stirling engines. This leads to significant thermodynamic losses, as well as greater expense, and reduced lifespan. The outside shell of the regenerator has to be made of high strength metals that will tolerate the high temperatures. This leads to high losses of heat to the environment, heat gained from the environment, and heat conducted from one end of the regenerator to the other. This heat conduction forces operation of the regenerator in a manner that is far from ideal.
The heat exchanger on the cold cylinder must efficiently remove heat from the working fluid, during the compression stroke. As with the hot side, prior art heat exchanger designs have used either the basic cylinder shape itself as the heat sink, or they have used simple finned surfaces or some variation of the shell and tube heat exchanger. In all such designs, the thermal resistance inherent in these approaches forces the heat sink to operate with a large difference in temperature (ΔT) between the interior and exterior of the cylinder.
In other words, the working fluid inside the cold cylinder is forced to be at a temperature considerably above the outside temperature at which the heat is finally dissipated. This greatly reduces the ΔT across the engine, which limits the maximum efficiency and power output of the engine.
Since the Stirling Cycle is a closed thermodynamic cycle, the working fluid must be sealed inside the engine. This leads to several major design problems.
First, the prior art engines are forced to operate at high temperatures and pressures. This places great demands on the seals. To survive the high temperatures and pressures, the only practical approach has been to use sealing rings on the piston, as in conventional internal combustion engines. The piston and ring assemblies suffer leakage, or blow-by. This fluid loss from the engine is a critical problem, as it must continually be replaced to avoid loss of power output, and it disturbs the cycle. This usually means that the crankcase itself must be sealed as well, leading to problems of lost work in the crankcase, as the pistons do unwanted work on the crankcase gas. It also means that the crankcase must be filled with the same working fluid as used in the engine itself.
The piston rings scraping up and down on the walls of the cylinder leads to further problems. The biggest of these is the friction created. In a typical engine this can consume some 20% of the engine's output, a very serious loss.
A further problem is that of lubrication. Liquid oils cannot be simply sprayed onto the cylinder walls, as this would leak into the working area of the engine and contaminate the working fluid. This would lead to problems involving unwanted contamination, corrosion, and loss of efficiency. But without adequate lubrication, the friction losses become even greater.
The present invention solves all these problems found in the prior art designs.
Briefly described, the present invention includes an externally heated engine having at least two pistons. A first piston reciprocates within a first cylinder. The first piston has a first side (working side) and a second side opposite the first side. The first side of the first piston and the first cylinder define a first working chamber containing working fluid, which may consist of any usable gas. The second side of the first piston and the first cylinder define a first opposite chamber containing an opposing fluid. A heater heats the working fluid in the working chamber. Preferably, the chamber is heated by a heat source so that the working fluid has a temperature of no more than 500° Fahrenheit with a temperature difference between the heat source and the working fluid of less than 5° Fahrenheit. The working fluid may be heated with a heat exchanger or heat injector. Heated fluid is delivered to the heat injector and flows through grooves around thermally conductive material, thus injecting heat directly into the engine. The heat is trapped inside the engine by the thermally insulating material. The working fluid flows in the longitudinal direction through the thermally conductive material. The thermally conductive material has passageways so that the working fluid may pass longitudinally through it. The longitudinal passages for the working fluid are narrow and run the entire useable length of the heat injector.
Preferably, the heat injector has grooves for the heated fluid which include multiple, parallel grooves which form a spiral or helical pattern along the entire outside useable length of the heat injector. The spiral grooves could be in sets of 2, 3, 4 or more, running parallel to one another and into which the heated fluid is injected simultaneously. By keeping these grooves very narrow and deep, a very high value of length to depth and thus low temperature differential is achieved, while providing adequate useable cross-sectional area to permit a sufficient volume of heated fluid to flow and provide heat input.
The grooves and passages are separated by a solid layer of the conductive material. The heat injector of the present invention is described more fully below.
Another method of heating the working fluid is through the use of heat pipes. Heat pipes are tubes which rely on phase change of a fluid within the pipes to transfer heat. Through the change from liquid, to gas and back to liquid, heat is transferred from one end of the pipe to the other. The heat pipes may pass through the wall of the first cylinder and fill the space in the cylinder beyond top dead center of the piston. Thin copper fins may be attached to the heat pipes outside of the cylinder. Hot air is swirled through the heat exchanger area, creating a very effective exchange of heat between the hot air and the heat pipes. Instead of the usual 25° to 45° Fahrenheit temperature difference (“ΔT”) between the air and the metal of the heat exchanger, a ΔT of only some 5 degrees will exist.
In the heat pipes, the heat travels along the length of the pipe, directly into the interior volume of the cylinder. As is usual in a heat pipe design, a negligible ΔT exists along the length of the pipe. This means that the heated copper inside the cylinder is within only 5 degrees of the temperature of the hot air outside. The externally heated engine may include a heating medium surrounding the heat pipes which is heated with thermoelectric generators.
Preferably many small heat pipes are used. These have a small diameter, and since there are so many packed into a small volume, there is only a very limited dead volume associated with the heat exchanger. Furthermore, the ΔT between the copper and the working fluid inside the engine is held to an absolute minimum by this design.
The second piston reciprocates within a second cylinder, and has a first side (working side) and a second side opposite the first side. The first side and the cylinder define a working chamber containing working fluid. The second side of the piston and the cylinder define a second opposite chamber containing an opposing fluid. The working fluid in the second cylinder is cooled to a temperature of below 35° Fahrenheit.
Preferably, the engine includes diaphragms associated with the pistons to separate the working chambers from the opposing chambers. The diaphragm provides many benefits as will be described in detail below. Because of the use of the diaphragm, it is beneficial to control the pressure of the opposing fluid. This prevents a large pressure differential across the diaphragm, which, if uncontrolled, could cause it to burst. A second reason is to vary the pressure on the opposing side in concert with the action of the engine's throttle control. That is, as working fluid pressure is raised and lowered, the same is done with the opposing fluid, to avoid doing unwanted work on the gas in the opposing chamber and to protect the diaphragm.
The working fluid pressure is controlled as a means of throttling the engine. As more working fluid is forced into the engine, by increasing its pressure with the control system, the engine will increase its power output, because the greater volume of working fluid will transfer more heat into and out of the engine cycle and thus do more work. Reducing the pressure will have the opposite effect. In this way, engine output can be continuously varied, to match the load conditions. Having too large a throttle setting when the load is reduced would be inefficient because the engine would over-speed, and excess heat would be drawn in and dumped to the chiller.
In order to force the greatest possible percentage of the working fluid in the engine to participate effectively in the thermodynamic process, this air must be swept alternatively all the way through the engine, from hot side to cold side and back again. While steps are always taken (described elsewhere in regard to the heat injector, the heat extractor, and the regenerator) to reduce unswept volume, the piston characteristics are also controlled to reduce unswept volume.
In the two piston engine configuration, the stroke of the engine must be greater in length than the diameter of the bore. That is, the ratio of stroke length/bore diameter must be greater than one. The ratio could be much larger, as high as 2 or 3, or more, until practical limitations prevent further gains. Since a fixed dead volume space exists at the head of the piston even at end of stroke, making the stroke longer reduces this dead volume greatly as a percentage of total volume, and ensures that the swept volume is many times greater than the unswept volume. By doing this, the great majority of working fluid particles can be swept all the way through the engine and efficiency is enhanced.
In displacer type engines described in detail below, the same desirable effect is obtained by making the stroke, and thus the displacement by the displacer, as great as practical. This again ensures that the vast majority of working fluid contributes effectively to the process.
The preferred embodiment of the chiller system comprises a compression/expansion cycle refrigerant based system. It is designed to produce intense cold inside the engine, in the heat extractor, by evaporating (boiling) the refrigerant directly inside the engine in the extractor. Because the heat extractor is of a similar design to the heat injector, with a low temperature differential between the cooling fluid and the working fluid, the engine working fluid can be reduced in temperature to at least 50 degrees F. below zero. This adds perhaps 100 degrees of temperature differential between the hot and cold sides of the engine to the engine design, compared to conventional chilling methods.
Preferably, the chiller employs three compressors, three condensers, and three two speed cooling fans, which are controlled by the chiller controls. However, other numbers of compressors, condensers and cooling fans are possible. Only as much capacity is switched on at any given time as is actually needed by the engine. This greatly improves the engine power budget, and thus efficiency, by not using unneeded power.
As an alternative heat extractor, the externally heated engine may include heat pipes which pass through the cylinder wall of the second cylinder to cool the cylinder. The heat pipes may be cooled with thermoelectric coolers or any other suitable cooling method. The heat pipes may be surrounded by a heat exchanger medium which has a freezing temperature of below 32° Fahrenheit, such as brine, methanol, ethylene glycol, or other fluid with a freezing temperature below 32 degrees Fahrenheit. Instead of heat pipes, small tubes could be used to carry cooling fluid directly to the cylinder through the cylinder wall or preferably into passages in a heat exchanger as described more fully below. Cooling can be accomplished through any number of other ways.
A cold water jacket can surround the cylinder. In this way a certain amount of heat can be drawn out through the cylinder walls themselves. This also ensures that no stray heat can leak into the engine through this path.
In one embodiment, there are a large number of heat pipes installed through the wall of the cylinder, extending between the inside of the cylinder and the area of the cold heat exchanger. This ensures that the ΔT between the chilled cooling water and the interior of the engine is essentially negligible, thus reducing the cold working fluid temperature inside the engine to the lowest possible level.
Inside the cold liquid jacket, the heat pipes are attached to copper fins, to greatly increase the heat transfer between the cold liquid and the heat pipes. The cold liquid is pumped into a lower corner of the jacket, and is made to swirl through the area. This significantly further increases the heat transfer. To cool the liquid, a chiller system or thermoelectric coolers (Tec) may be used.
The operating temperature of the heat extractor is made as low as possible, by holding the operating pressure of the expanding refrigerant to the lowest possible value, consistent with the limits of the design.
By making the cold side very cold, the temperature difference is increased between the cold and hot sides, so that the hot side does not have to exceed 500 degrees Fahrenheit.
A thermoelectric cooler uses electricity to pump heat from the cold side to the hot side, in the manner which is well known. The Tec's in the chiller will be powered by some of the energy produced by the engine. This is accomplished in part by using thermoelectric generators (Teg) on the hot engine exhaust, and in part by using some of the electricity produced by a generator connected to the engine. Bonded-fin, copper heat sinks and forced air cooling are used on the hot side of the Tec's. The heat sinks have a thick copper plate that has been machined to the proper degree of flatness and finish. These can be readily obtained from ERM Thermal Technologies in Ontario, N.Y.
Secondly, only a small portion of the heat pumping capacity of each Tec is used, as this enormously boosts efficiency.
The ΔT between the hot side and the cold side of the Tec's is limited. If desired, the temperature can be limited by a simple, inexpensive, passive use of geothermal cooling. A moderate length of pipe may be buried several feet below the surface, and the cooling air is pumped through this pipe prior to use. As is well known, the temperature of the earth at this depth is approximately a constant 50 degrees F. This means that the hot heat sink is cooled with 50 degree air. In the winter, one could use even colder cooling air to great effect.
The working fluid reciprocates between the cylinders in a closed fluid path. A closed fluid path means that during normal operation, fluid reciprocates between the pistons, compared to an internal combustion engine, for example, which continually intakes combustion air and exhausts combustion byproducts to the atmosphere. The closed fluid path in the present invention does allow for the introduction of additional working fluid when necessary and for pressure control as described below.
A pressure differential is maintained between the working fluid and the opposing fluid in the first cylinder of between 4 PSI and 250 PSI. By maintaining pressurized opposing fluid, a higher working fluid pressure is possible while maintaining the integrity of the diaphragm. In addition, the opposing fluid aids in the compression stroke by reducing the work necessary to compress the working fluid. However, the pressure of the opposing fluid is not so high that it interferes with the power stroke. The externally heated engine may have the working fluid at a pressure of below 10 atmospheres. The externally heated engine may have the working fluid at a pressure of greater than 60 PSI.
A regenerator is provided within the closed fluid path. The regenerator is a temporary repository of heat during certain cycles of the engine. Because the temperatures are lower than in engines of the prior art, the present invention employs a shell made out of polytetrafluoroethylene material. This material does not conduct heat. Thus there is no thermal short circuit around the mesh. In prior art regenerators operating at extremely high temperatures, only all-metallic internal components could be used. Since each layer of such metallic mesh touched both adjacent layers, a continuous, thermally conductive path was established from the hot side of the regenerator to the cold side. This resulted in a continuous loss of high temperature energy over to the cold side.
In the present invention, the regenerator operates at low enough temperatures to allow the introduction of non-metallic layers of mesh. Preferably, non-metallic mesh layers are used after every 10 or so metal layers. These non-conductive layers break up the conductive path, and thus prevent the unwanted loss of energy from the hot side to the cold side of the regenerator. In addition, since the non-metallic mesh layers can be made, for example, of woven fiberglass, they have enough thermal capacity to add slightly to the heat retention capacity of the regenerator, further adding regenerating action without adding unwanted, unswept volume.
Preferably, in addition to the metallic mesh layers and insulating mesh layers in the regenerator, a third type of layer is used. Specifically, a thicker, copper layer, which is solid with a pattern of larger openings can be used. The openings are arranged to break up and redistribute the air flow within the regenerator to ensure that the entire mesh content is fully utilized efficiently. The thicker copper also retains some additional heat, which adds further to the regenerating capacity.
The regenerator does not need stainless steel wire in the mesh as with prior art regenerators, but may include copper wire, which is far more conductive than steel. Silver may be used as an alternative to copper, for even higher performance. The copper mesh may be coated with diamond and may include a high melting point thermal insulating polymer such as polytetrafluoroethylene in the form of an outer cylinder and a center core. The regenerator may include a perforated disk constructed from a diamond copper composite. These choices allow the use of less mesh, with a consequent reduction in pumping losses.
The engine operates in the following manner. The heat applied to the hot side causes the working fluid, such as air, methane or another gas, to rise in pressure, and to expand. This forces the hot and cold pistons outward, thus doing useful work. The working fluid is then passed through the regenerator, on its way to the cold side. In the process it leaves behind much of its heat, which is temporarily stored in the regenerator mesh matrix. The fluid thus arrives in the cold cylinder much reduced in temperature.
Once in the cold cylinder, the fluid is compressed back to its original, smaller volume. This requires the removal of some heat, which is rejected to a recuperator. This heat is thus recovered and reused.
Finally, the fluid passes back through the regenerator to the hot cylinder. On the way it picks up the heat left behind in the regenerator mesh matrix. The fluid thus arrives in the hot cylinder at a much increased temperature and pressure. As further heat is added through the hot heat injector or exchanger, the fluid again enters an expansion process, thus beginning a new cycle of the engine. The first piston and the second piston are arranged to reciprocate such that the volume of the working fluid is compressed and expanded alternately to provide a ratio of the expanded volume to the compressed volume of greater than 2 to 1.
The externally heated engine may include a flexible rolling diaphragm attached to the pistons to create a seal between the piston and the cylinder. The diaphragm may be a standard, Type F, silicone diaphragm made by Dia Corn Inc. This diaphragm has virtually zero friction and zero break-away force. The diaphragm has no metal reinforcement and has a low melting temperature. Leakage is so slow as to be negligible. The unit is low cost, and will give up to a billion cycles in service.
The reason such a diaphragm can be employed in an externally heated engine is because of low temperature and pressure in the present invention. Without this, the high temperatures and pressures make the use of a diaphragm impractical. In prior art designs, a diaphragm would have to be made partly of thin, high temperature metals, with heat shielding. This would greatly increase friction and reduce service life, negating advantages of the diaphragm.
However, with the present invention, the diaphragm makes it possible to eliminate the main source of friction in the engine. That is, the piston rings are eliminated. A prior art Stirling engine will lose at least 20% of its output power to friction. The great majority of this friction is eliminated with the present invention. The diaphragm also eliminates the problem of leakage which is present with traditional piston ring seals. Because there is no leakage, the working fluid and opposing fluid do not mix, so that the working fluid does not become contaminated by the opposing fluid if those two fluids are not the same. The working fluid and opposing fluid need not be the same because of the perfect seal provided by the diaphragm. An opposing fluid such as dry nitrogen could be used, for example, to avoid oxidation and contamination of the volume enclosed in the bonnet. In addition, a light gas, such as helium, may be used as the working fluid, to obtain thermodynamic benefits, while still using a heavy gas such as air or nitrogen as the opposing fluid, thus avoiding the expense and difficulty of sealing the lighter gas on the opposing side, or providing quantities of it to make up for leakage.
Additionally, with the diaphragm, there is no need for lubrication in the cylinders, because the diaphragm is essentially frictionless. By eliminating lubricating oil, the working fluid does not become contaminated with lubricant.
The present invention will now be described, by way of example, with reference to the accompanying drawings, in which:
Chiller 50 cools a portion of the piston assembly 10 as will be described below. Burner 60 and heater 70 provide heat to the piston assembly 10, as will also be described below. The entire assembly is mounted on framework 80. One of ordinary skill in the art will appreciate that there are many equally feasible power transmission methods and physical arrangements of the various elements described. The foregoing description is meant to provide a conceptual overview and should not be viewed as limiting the invention.
Turning to
Referring to
Piston rod 14 is attached at its opposite end 142 to slider assembly 150. Slider assembly 150 contains block 152 adapted for linear motion on rails 154 and 156. Wheels 158 allow sliding motion with respect to the rails 154 and 156. Slider assembly 150 eliminates any lateral forces from being placed on piston rod 142. Link 160 allows for the conversion of linear motion to rotational motion of crank 162.
Each end of the cylinder housing 100 is capped by a bonnet 550 and 560 which contains the opposing fluid. The bonnet 550 contains and supports the bearing 530 which controls the motion of the push rod 16. The bonnet 550 includes a seal 552 to contain the opposing fluid, and has an inlet port 554 through which the opposing fluid is introduced.
The bonnet 550 has limited surface area in the walls. Thus, the amount of force exerted on the walls by the action of the pressurized opposing fluid is limited. In addition, the bonnet 550 is exposed to relatively low temperatures and pressures. The bonnet 550 can be made of lightweight metal, such as aluminum, and need not have thick walls nor have stiffening ribs. The seal 552 can be of a type suitable for low temperature and pressure applications. The seal 552 supports only translational motion, not rotational which eliminates the problems associated with a crankcase in a traditional engine. The second bonnet 560 is attached to the cylinder 114 and includes a seal 562 and inlet 564.
The heat injector assembly 200 is shown in
The grooves 205 for the heated fluid include multiple, parallel grooves which form a spiral pattern along the entire outside useable length of the heat injector 200. By keeping these grooves 205 very narrow and deep, a very high value of length to depth and thus low temperature differential are achieved, while providing adequate useable cross-sectional area to permit a sufficient volume of heated fluid to flow and provide heat input.
The grooves 205 and passages 210 must be separated by a solid portion of the conductive material 212. If the engine operated at high pressure and temperature, then great strength would be needed in this layer, as it must serve as a pressure containment vessel. This would require that the thermally conductive material 212 be made of a relatively thick layer of a material such as stainless steel. This would lead to a very high temperature differential across this layer, as the heat was conducted into the engine through the layer.
However, since the engine operates at low temperatures and pressure, this is not necessary with the present invention. A very thin layer of the thermally conductive material 212 such as copper can be used. This makes the temperature differential negligible across this layer, while still adequately resisting the pressure.
As shown in
As shown in
Pressure containment, mechanical strength and mounting are provided for by an outside cylinder 100 of, preferably, aluminum, with suitable mounting features. The polytetrafluoroethylene 350 insulates this outside cylinder 100 from the mesh 302.
The regenerator includes a center insulating core 360. This is comprised of a solid, relatively large diameter rod of polytetrafluoroethylene or similar material. The center diameter of each layer of mesh 302 is punched out, to fit over this core 360. Since the core 360 is non-conductive, it contributes no heat loss. The regenerator 300 also includes copper disks 362 with holes 363 (
By making the core 360 of the regenerator 300 solid, the total volume of mesh 302 is kept to the proper size—no larger than it needs to be—to prevent unwanted unswept volume in the engine while the outside diameter of the mesh is kept large—the same as the rest of the engine—so that there is no discontinuity in the air flow passage diameters that would lead to very high loss disruptions of the fluid flow.
The heat extractor 400 is shown in
The second piston assembly 500 is shown in
A simplified slider assembly 151 is shown in
In
In
A compressor 820 and pressure reserve 824 delivers pressurized opposing fluid to the cylinders 114 and 504. Pressure reserve 822 delivers high pressure working fluid to the engine. This preloads the engine with the proper amount of working fluid at the proper pressure. Pressure regulators 826, 828 and 830 are also provided to ensure proper operation of the system.
A system controller 1006 controls the operation of the heat injection system. Other heat generation and heat delivery systems are possible and are well within the skill of one of ordinary skill in the art to construct.
As the air is displaced into the heat extractor 1067 of the engine, it passes through the hot heat injector 1068 last, and thus it reaches high temperature and pressure. A regenerator 1071 is provided which is identical to the regenerator described in connection with
Next, the displacer forces the working fluid to the cold side 1066 of the engine. The temperature and pressure are thus greatly reduced. The piston 1080 is timed so that it is positioned ready to compress this low temperature and low pressure working fluid into a smaller volume, without having to do much work. The cycle then repeats.
In
The engine of
The single crank 1092 has two pins 1104 and 1106 on it, located the appropriate number of degrees apart. This correctly times the motions of the displacer 1084 and the piston 1090. The piston 1090 drives rod 1094, which through links 1100 and 1102 drive crank 1092. Links 1100 and 1102 are connected to crank 1092 through pins 1104 and 1106, respectively. Link 1100 is pivotably mounted to sliding block 1108, constructed in a manner similar to that of block 152 of
With only a single crank 1092, and no chains, the engine of
In
The larger diameter section 1156 has two grooves 1160 machined into it. In each groove is fitted a ring of polytetrafluoroethylene 1162 or other low friction material. The rings 1162 are sized for a tight fit inside the cylinder (not shown). These two rings serve the dual purpose of bearings between the cylinder and piston 1150, and also locate the piston properly in the cylinder and to hold it straight and aligned.
Since the volume between the upper ring and the back surface of the diaphragm 1154 varies as the piston 1150 moves back and forth, there would be a compression effect in this variable volume which could damage the diaphragm 1154. To prevent this from happening, holes 1164 are drilled through the piston skirt 1166, which allow the excess pressure to bleed off harmlessly into the hollow center 1168 of the piston 1150.
Since the piston is held straight and aligned by the two rings, a conventional wrist pin 1170 and connecting rod 1172 can be used as shown in
In
While the invention has been described by reference to various specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but will have full scope defined by the language of the following claims.
This application is a continuation of U.S. application Ser. No. 11/198,681 entitled Externally Heated Engine filed Aug. 5, 2005 now U.S. Pat. No. 7,076,941. The present invention relates to externally heated engines. More particularly, the present invention relates to improvements in the efficiencies of externally heated engines operating at relatively low temperatures and pressures.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3996745 | Davoud et al. | Dec 1976 | A |
| 4004421 | Cowans | Jan 1977 | A |
| 4389844 | Ackermann et al. | Jun 1983 | A |
| 7076941 | Hoffman et al. | Jul 2006 | B1 |
| Number | Date | Country | |
|---|---|---|---|
| 20070095064 A1 | May 2007 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11198681 | Aug 2005 | US |
| Child | 11446951 | US |
