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Closed-cycle, solar, hot-air engines Part 1—A 14 -hp engine
Closed-Cycle, Solar, Hot-Air Engines Part I
A 1/4-hp Engine
Erich Farber
Ford L. Prescott
Professor and Research Professor
Professor Emeritus
Department of Mechanical Engineering, University of Florida, Gainesville, Florida
OT-AIR engines of both the closed and open cycle design are not new. The first patent on a hot-air engine was taken out by Henry Wood in 1759. The first working model of a hot-air engine was constructed by Sir George Cayley in 1807. Others followed with variations in design and many of the engines are designated by their designers' names as the Buckett, Stirling, Robinson, Ericsson, Wenham, Bailey, Jahn, Rider, Jenkin, Benier, Genty, and so on, engines. With the advent of the small gasoline engine around the turn of the century hot-air engines were forgotten and the gasoline engines improved. Many areas of the world do not have gasoline readily available or it is expensive and in those areas engines that can operate on other fuels have advantages. The hot-air engine can operate with any source of heat, thus any fuel. The emphasis to utilize new and different sources of energy such as solar energy has revived the interest in hot-air engines. Since World War II a number of air engines have been built and investigated with probably the best known engine of useful size the Philips engine, developed by the research laboratories of the N. V. Philips' Glueilampenfabriken of Eindhofen, Holland. Air engines are designed to approach as well as possible reversible gas cycles such as the Stirling, Ericsson, Braytoa or Joule, and others, utilizing regeneration. This paper describes a relatively simple and inexpensive hot-air engine.
H
If these cycles are combined, an air-engine cycle shown in Fig. l(b) is obtained. This can be considered the ideal cycle for the hot-air engine. This cycle is approximated by the engine under discussion as can be seen from its indicator card, Fig. 2. For the understanding of the engine Fig. 3 can be used. It shows schematically a design that, in its operation, tends to approximate the cycle of Fig. l(b). A cylinder (horizontal) is mounted above the power cylinder (vertical) of the engine. The horizontal or displacer cylinder is heated on one end and cooled on the other. The heating can be done by flame, electricity, ,+Q
I
V=C V=C
4
T=C ' % ~ 5
-Q
STIRLING
CYCLE
÷Q
I
2
T=C
=C
Operating Principle For the operation of a closed-cycle engine, such as like the one discussed here it is necessary that the air in the engine which produces the power by pushing on the piston is alternately heated and cooled. The pressure-volume and temperature-entropy diagrams for both the Stirling and the Ericsson cycles, both ideal reversible cycles are shown in Fig. 1. They are made highly efficient by the process of regeneration which is the storing of heat when not needed and then the release of it when needed. A regenerator to do this is schematically shown in Fig. l(a) also.
4
5
-Q
S
ERICSSON
CYCLE
REGENERATOR
Presented at the Winter Annual Meeting of the A.S.M.E., Nov. 29, 1964, New York City, but not published. 170
FIG. l(a)--Diagrams for the Stirling and Ericsson engine cycles.
Solar Energy
solar energy (concentrated), and the like, while the cooling can be carried out by circulating fluids such as water or air or by evaporating a liquid as water. Because the method of heating and the method of cooling are unimportant the engine is able to work under widely differing environments. If the displacer plunger, with clearance all around it, is moved to the left (the "cold" end) the air in the engine must move to the right of the "hot" end of the cylinder. The bulk of the air, now being in contact with the hot surfaces is heated, increasing the pressure in the engine. Proper timing through the linkage operating the displacer plunger allows this high pressure to push the power piston down. Before the return of the power piston the displacer plunger is moved to the right or hot end of the displacer cylinder moving the air in the engine to the left or cold end. The air, being in contact with the cold surfaces will be cooled, reducing the pressure in the engine. Now it is time for the power piston to move up, the pressure in the piston being low. This high pressure during the downward motion and the low pressure during the upward motion of the power piston produces the useful power of the engine. With proper timing of the motion of the displacer plunger, maximum power can be produced. The walls of the displacer cylinder and the displacer plunger vary cyclically in temperature owing to the alternate passing of hot and cool gases, making these surfaces partial regenerators.
Design of the Engine The engine parts are described as follows: Power-Cylinder Assembly--This part of the engine was taken from a mass-produced 4-cycle, 1½-hp gasoline lawnmower engine, of 2 ~ in. bore and 1½ in. stroke. A 16-in. flywheel was attached to give smoother motion at
h0 OC O~ fl.
I
I
VOLUME
FIO. 2--I-Iot-air engine indicator diagram. low speeds than the much smaller flywheel of the original engine could provide. Displacer-Cylinder Assembly--The displacer plunger is a hollow aluminum cylinder 3 ~ in. in diameter and 1 1 ~ in. long, both outside dimensions. One end of this plunger is attached to the ½-in. diameter steel push rod. The stroke of the plunger is 3 in. The displacer cylinder, made of standard 4-in. steel pipe, giving a side clearance around the plunger of a-~ in. is long enough to give an end clearance of ~ in. A section involving 6 ~ in. of the hot end of the displacer cylinder is removable and exchangeable, and hot ends of different materials can be used such as steel, copper, aluminum, and so on. The cold end of the displacer cylinder is surrounded by a galvanized-iron jacket having ½ in. clearance and forming a reservoir to hold water. The displacerplunger push rod penetrates this end of the displacer cylinder. Timing Linkage Assembly--The timing linkage moves the displacer plunger. The timing is adjusted by rotating the crank connecting the linkage to the crankshaft of the engine.
CooLi Watenrg
÷Q 2 T:C~5
V~~ ~P~5 -4' C¢=C
I[
Displacer
Heat
]4
-Q AIR ENGSNE CYCLE
REGENERATOR
FIG. l(b)--Diagram for ideal hot-air-engine cycle.
Vol. 9, No. 4, 1965
FIG. 3--Schematic diagram of closed-cycle hot-air engine. 171
FIG. 4--Disassembled closed-cycle, hot-air engine.
The various components of the engine can be seen in Fig. 4, the disassembled engine, and in Fig. 5 the assembled engine.
Operation of Engine To operate this hot-air engine it is necessary only to heat the hot end of the displacer cylinder and to keep the cold end relatively cool. As soon as enough of a temperature differential exists between the hot and cold ends of the engine, if started it will take off on its own. This engine was operated with five sources of heat mostly for the purpose of comparison. They were: blow torch; gas torch, welding torch; electric coil, and solar energy. The engine performed well with all these sources giving power corresponding to the maximum temperature of the hot end. The efficiency of the engine varied
FIG, 5--Assembled closed-cycle, hot-air engine. with the magnitude of the heat losses (a function of the source) and was highest for the operation with solar energy. The absolute power output was highest with the large welding torch as source since more energy could be supplied to the engine with this equipment. Solar energy was supplied by a 5-ft parabolic mirror of the type used in any army searchlight. A poorer quality mirror could have been used since such high concentration was not needed, but such a mirror was not available. Since the main purpose of this study was the operation of this engine with solar energy the subsequent performance of the engine is given for this source. The engine is shown in operation in Fig. 6 with the engine mounted right in the searchlight pointed at the sun. The engine in operation with another mirror of the same type is shown in Fig. 7. In this arrangement the engine is fixed in position and only the mirror requires periodic adjustment. The concentration of solar energy on the hot end of the engine is shown in Fig. 8.
Performance of the Engine
Fro. 6--The hot-air engine operating rigidly mounted on the reflector frame so that the solar radiation is concentrated onto the "hot" end of the engine. 172
For performance evaluation of the engine, indicator diagrams were obtained to determine the general shape of the actual diagram for comparison with the ideal and to determine the friction horsepower of the engine. Friction is a critical quantity in hot-air engines since it is a loss of useful power and it should be reduced to a Solar Energy
FIO. 8--The heated end of the engine cylinder is shown at
Fro. 7--The engine as set up in a fixed position in front of a manually adjusted mirror. minimum. Modifications of this engine are contemplated, such as mounting the displacer cylinder and plunger vertically, to reduce this quantity further. Shaping the hot end will also improve the heat-collecting ability of the engine. A comparison of the actual indicator diagram of the engine and the ideal cycle of the engine can be made by comparing Figs. l(b) and 2. The useful work of the engine is the actual power produced and this is probably the most valuable information in judging performance. I t was measured for various speeds of the engine. The brake-horsepower output of the engine varies widely with speed and reaches a m a x i m u m of over 0.2 at about 135 rpm, for the sun as a source as Fig. 9 shows. Values of 0.37 were obtained with a torch source. To get an idea of how well the engine uses the available energy, although in this ease this energy was free, the thermal efficiency, defined as output of the engine divided by the amount of energy intercepted b y the mirror is given in Fig. 10. The m a x i m u m value is about 9 percent. For other sources of heat this efficiency was much lower even though the power output was in one case greater. Discussion
of Results
To establish the usefulness of this engine or another serving the same purpose a number of criteria can be set down for analysis. T h e y are: (a) Versatility of engine (b) Absolute power output of engine (c) Cost of engine (d) Availability and cost of fuel. The foregoing criteria will now be applied to this engine. Vol. 9, No. ~, 1965
the left end of the engine.
0.2
0.1'
I00
200
500
REVOLUTIONS
400
PER
MINUTE
Frc-,. 9--Output of engine with speed. O.lO
0.08
~5 uu_
0.06
,~
0.0
a= t*J I F
0.02
4
I I00
200
300
REVOLUTIONS
400
PER
5OO
MINUTE
FIG. l{)---Efficiency curve of engine with speed.
(a) Since the hot-air engine as described here can operate from any source of heat t h a t produces the required temperatures, this engine is quite flexible as to the environment in which it call work. 173
Cooling can be provided b y liquid or air circulation, liquid evaporation, or in extreme cases radiation. The engine can be classified as a multifuel engine. In addition this engine can utilize other sources of heat such as waste steam, waste or exhaust gases, nuclear, and solar energy. (b) The absolute power of this engine is about ¼ hp. This is a convenient size for small jobs and the engine can easily be moved. For solar-energy applications the output was limited b y the amount of energy intercepted b y the concentrating mirror. If a larger mirror had been used more power could have been obtained. Friction in the engine as designed now is still greater than Imcessary and modifications are planned. Mounting of the displacer cylinder and displacer plunger vertically will reduce the friction in the bushing supporting the displacer plunger. Pressurizing the engine also increases the power output but introduces another problem of making the engine airtight. The m a x i m u m power was obtained at about 135 rpm. The relatively slow rate at which heat is transferred into and out of the air in the engine, reducing its maxim u m and minimum temperatures, made the power fall off at higher rpm. A gas with higher thermal conductivity and under higher pressure would improve this. A further factor is t h a t the friction horsepower is increased with speed. When friction horsepower becomes equal to the power produced b y the engine, all the engine can do is run with no energy to spare. This occurred at about 440 r p m for the runs presented here. At low speed or rpm the power output falls off owing to the decrease in number of cycles gone through per unit time. (c) Since the engine is considerably simpler t h a n a gasoline engine, not requiring valves, ignition systems, not having to be able to withstand high pressures, not having stresses produced due to high speed, it is expected t h a t hot-air engines of this type, if mass-produced, would be less expensive t h a n gasoline engines of this size. (d) Availability and cost of fuel are problems t h a t have to be determined for each locality. In most populated areas of the world where fuels, especially gasoline are unavailable or expensive, solar energy is available in abundance. While gasoline engines are more applicable in countries like the United States, hot-air engines m a y be more applicable in the so-called developing countries. One factor of solar energy is that it is free. For m a n y purposes such as water pumping for irrigation, supplying power to machines, and so on, the fact t h a t there is no sunshine at night is not critical since the operation can then be discontinued. If operation becomes a necessity at such times another source of heat can be substituted without much difficulty. 174
Conclusions Based on the work outlined in this paper it is believed that hot-air engines have certain advantages in the right environment and can be mass-produced as economically as other engines of similar size. T h e y are safer, more versatile, more rugged, and have fewer parts.
REFERENCES 1. W. J. M. Rankine, "The Steam Engine," Charles Griffin & Company, Limited, England, 1908, pp. 345-374. 2. B. Donkin, "Gas, Oil and Air Engines," Charles Griffin & Company, Limited, England, 1911, pp. 563-581. 3. H. Rinia, F. K. DuPre, "Air Engines," Philips Technical Review, 8, no. 5, May 1946, pp. 129-160. 4. De Brey, H. Rinia, F. L. VanWeenan, "Fundamentals for the Development of the Philips Air Engine," Philips Technical Review, 9, no. 4, 1948. 5. F. L. VanWeenen, "Tile Construction of the Philips Air Engine," Philips Technical Review, 9, no. 5, 1948. 6. D. K. Edwards, K. E. Nelson, "Radiation Characteristics in Optimization of Solar Heat-Power Conversion Systems," ASME Paper 61-WA-158, November 1961. 7. T. Finkelstein, "Internally Focusing Solar Power Systems: Part I: Conversion of Solar Radiation into Power," ASME paper 61-WA-297, November 1961. 8. E. H. Parers, "Investigation of the Application of Solar Energy to Hot Air Engines," MS thesis, University of Florida, January 1959.
FIG. 2--1/~-hp, closed-cycle, hot-air engine with solar energy concentrated on hot end. --~ Solar Energy
A 1/4-hp Engine
Erich Farber
Ford L. Prescott
Professor and Research Professor
Professor Emeritus
Department of Mechanical Engineering, University of Florida, Gainesville, Florida
OT-AIR engines of both the closed and open cycle design are not new. The first patent on a hot-air engine was taken out by Henry Wood in 1759. The first working model of a hot-air engine was constructed by Sir George Cayley in 1807. Others followed with variations in design and many of the engines are designated by their designers' names as the Buckett, Stirling, Robinson, Ericsson, Wenham, Bailey, Jahn, Rider, Jenkin, Benier, Genty, and so on, engines. With the advent of the small gasoline engine around the turn of the century hot-air engines were forgotten and the gasoline engines improved. Many areas of the world do not have gasoline readily available or it is expensive and in those areas engines that can operate on other fuels have advantages. The hot-air engine can operate with any source of heat, thus any fuel. The emphasis to utilize new and different sources of energy such as solar energy has revived the interest in hot-air engines. Since World War II a number of air engines have been built and investigated with probably the best known engine of useful size the Philips engine, developed by the research laboratories of the N. V. Philips' Glueilampenfabriken of Eindhofen, Holland. Air engines are designed to approach as well as possible reversible gas cycles such as the Stirling, Ericsson, Braytoa or Joule, and others, utilizing regeneration. This paper describes a relatively simple and inexpensive hot-air engine.
H
If these cycles are combined, an air-engine cycle shown in Fig. l(b) is obtained. This can be considered the ideal cycle for the hot-air engine. This cycle is approximated by the engine under discussion as can be seen from its indicator card, Fig. 2. For the understanding of the engine Fig. 3 can be used. It shows schematically a design that, in its operation, tends to approximate the cycle of Fig. l(b). A cylinder (horizontal) is mounted above the power cylinder (vertical) of the engine. The horizontal or displacer cylinder is heated on one end and cooled on the other. The heating can be done by flame, electricity, ,+Q
I
V=C V=C
4
T=C ' % ~ 5
-Q
STIRLING
CYCLE
÷Q
I
2
T=C
=C
Operating Principle For the operation of a closed-cycle engine, such as like the one discussed here it is necessary that the air in the engine which produces the power by pushing on the piston is alternately heated and cooled. The pressure-volume and temperature-entropy diagrams for both the Stirling and the Ericsson cycles, both ideal reversible cycles are shown in Fig. 1. They are made highly efficient by the process of regeneration which is the storing of heat when not needed and then the release of it when needed. A regenerator to do this is schematically shown in Fig. l(a) also.
4
5
-Q
S
ERICSSON
CYCLE
REGENERATOR
Presented at the Winter Annual Meeting of the A.S.M.E., Nov. 29, 1964, New York City, but not published. 170
FIG. l(a)--Diagrams for the Stirling and Ericsson engine cycles.
Solar Energy
solar energy (concentrated), and the like, while the cooling can be carried out by circulating fluids such as water or air or by evaporating a liquid as water. Because the method of heating and the method of cooling are unimportant the engine is able to work under widely differing environments. If the displacer plunger, with clearance all around it, is moved to the left (the "cold" end) the air in the engine must move to the right of the "hot" end of the cylinder. The bulk of the air, now being in contact with the hot surfaces is heated, increasing the pressure in the engine. Proper timing through the linkage operating the displacer plunger allows this high pressure to push the power piston down. Before the return of the power piston the displacer plunger is moved to the right or hot end of the displacer cylinder moving the air in the engine to the left or cold end. The air, being in contact with the cold surfaces will be cooled, reducing the pressure in the engine. Now it is time for the power piston to move up, the pressure in the piston being low. This high pressure during the downward motion and the low pressure during the upward motion of the power piston produces the useful power of the engine. With proper timing of the motion of the displacer plunger, maximum power can be produced. The walls of the displacer cylinder and the displacer plunger vary cyclically in temperature owing to the alternate passing of hot and cool gases, making these surfaces partial regenerators.
Design of the Engine The engine parts are described as follows: Power-Cylinder Assembly--This part of the engine was taken from a mass-produced 4-cycle, 1½-hp gasoline lawnmower engine, of 2 ~ in. bore and 1½ in. stroke. A 16-in. flywheel was attached to give smoother motion at
h0 OC O~ fl.
I
I
VOLUME
FIO. 2--I-Iot-air engine indicator diagram. low speeds than the much smaller flywheel of the original engine could provide. Displacer-Cylinder Assembly--The displacer plunger is a hollow aluminum cylinder 3 ~ in. in diameter and 1 1 ~ in. long, both outside dimensions. One end of this plunger is attached to the ½-in. diameter steel push rod. The stroke of the plunger is 3 in. The displacer cylinder, made of standard 4-in. steel pipe, giving a side clearance around the plunger of a-~ in. is long enough to give an end clearance of ~ in. A section involving 6 ~ in. of the hot end of the displacer cylinder is removable and exchangeable, and hot ends of different materials can be used such as steel, copper, aluminum, and so on. The cold end of the displacer cylinder is surrounded by a galvanized-iron jacket having ½ in. clearance and forming a reservoir to hold water. The displacerplunger push rod penetrates this end of the displacer cylinder. Timing Linkage Assembly--The timing linkage moves the displacer plunger. The timing is adjusted by rotating the crank connecting the linkage to the crankshaft of the engine.
CooLi Watenrg
÷Q 2 T:C~5
V~~ ~P~5 -4' C¢=C
I[
Displacer
Heat
]4
-Q AIR ENGSNE CYCLE
REGENERATOR
FIG. l(b)--Diagram for ideal hot-air-engine cycle.
Vol. 9, No. 4, 1965
FIG. 3--Schematic diagram of closed-cycle hot-air engine. 171
FIG. 4--Disassembled closed-cycle, hot-air engine.
The various components of the engine can be seen in Fig. 4, the disassembled engine, and in Fig. 5 the assembled engine.
Operation of Engine To operate this hot-air engine it is necessary only to heat the hot end of the displacer cylinder and to keep the cold end relatively cool. As soon as enough of a temperature differential exists between the hot and cold ends of the engine, if started it will take off on its own. This engine was operated with five sources of heat mostly for the purpose of comparison. They were: blow torch; gas torch, welding torch; electric coil, and solar energy. The engine performed well with all these sources giving power corresponding to the maximum temperature of the hot end. The efficiency of the engine varied
FIG, 5--Assembled closed-cycle, hot-air engine. with the magnitude of the heat losses (a function of the source) and was highest for the operation with solar energy. The absolute power output was highest with the large welding torch as source since more energy could be supplied to the engine with this equipment. Solar energy was supplied by a 5-ft parabolic mirror of the type used in any army searchlight. A poorer quality mirror could have been used since such high concentration was not needed, but such a mirror was not available. Since the main purpose of this study was the operation of this engine with solar energy the subsequent performance of the engine is given for this source. The engine is shown in operation in Fig. 6 with the engine mounted right in the searchlight pointed at the sun. The engine in operation with another mirror of the same type is shown in Fig. 7. In this arrangement the engine is fixed in position and only the mirror requires periodic adjustment. The concentration of solar energy on the hot end of the engine is shown in Fig. 8.
Performance of the Engine
Fro. 6--The hot-air engine operating rigidly mounted on the reflector frame so that the solar radiation is concentrated onto the "hot" end of the engine. 172
For performance evaluation of the engine, indicator diagrams were obtained to determine the general shape of the actual diagram for comparison with the ideal and to determine the friction horsepower of the engine. Friction is a critical quantity in hot-air engines since it is a loss of useful power and it should be reduced to a Solar Energy
FIO. 8--The heated end of the engine cylinder is shown at
Fro. 7--The engine as set up in a fixed position in front of a manually adjusted mirror. minimum. Modifications of this engine are contemplated, such as mounting the displacer cylinder and plunger vertically, to reduce this quantity further. Shaping the hot end will also improve the heat-collecting ability of the engine. A comparison of the actual indicator diagram of the engine and the ideal cycle of the engine can be made by comparing Figs. l(b) and 2. The useful work of the engine is the actual power produced and this is probably the most valuable information in judging performance. I t was measured for various speeds of the engine. The brake-horsepower output of the engine varies widely with speed and reaches a m a x i m u m of over 0.2 at about 135 rpm, for the sun as a source as Fig. 9 shows. Values of 0.37 were obtained with a torch source. To get an idea of how well the engine uses the available energy, although in this ease this energy was free, the thermal efficiency, defined as output of the engine divided by the amount of energy intercepted b y the mirror is given in Fig. 10. The m a x i m u m value is about 9 percent. For other sources of heat this efficiency was much lower even though the power output was in one case greater. Discussion
of Results
To establish the usefulness of this engine or another serving the same purpose a number of criteria can be set down for analysis. T h e y are: (a) Versatility of engine (b) Absolute power output of engine (c) Cost of engine (d) Availability and cost of fuel. The foregoing criteria will now be applied to this engine. Vol. 9, No. ~, 1965
the left end of the engine.
0.2
0.1'
I00
200
500
REVOLUTIONS
400
PER
MINUTE
Frc-,. 9--Output of engine with speed. O.lO
0.08
~5 uu_
0.06
,~
0.0
a= t*J I F
0.02
4
I I00
200
300
REVOLUTIONS
400
PER
5OO
MINUTE
FIG. l{)---Efficiency curve of engine with speed.
(a) Since the hot-air engine as described here can operate from any source of heat t h a t produces the required temperatures, this engine is quite flexible as to the environment in which it call work. 173
Cooling can be provided b y liquid or air circulation, liquid evaporation, or in extreme cases radiation. The engine can be classified as a multifuel engine. In addition this engine can utilize other sources of heat such as waste steam, waste or exhaust gases, nuclear, and solar energy. (b) The absolute power of this engine is about ¼ hp. This is a convenient size for small jobs and the engine can easily be moved. For solar-energy applications the output was limited b y the amount of energy intercepted b y the concentrating mirror. If a larger mirror had been used more power could have been obtained. Friction in the engine as designed now is still greater than Imcessary and modifications are planned. Mounting of the displacer cylinder and displacer plunger vertically will reduce the friction in the bushing supporting the displacer plunger. Pressurizing the engine also increases the power output but introduces another problem of making the engine airtight. The m a x i m u m power was obtained at about 135 rpm. The relatively slow rate at which heat is transferred into and out of the air in the engine, reducing its maxim u m and minimum temperatures, made the power fall off at higher rpm. A gas with higher thermal conductivity and under higher pressure would improve this. A further factor is t h a t the friction horsepower is increased with speed. When friction horsepower becomes equal to the power produced b y the engine, all the engine can do is run with no energy to spare. This occurred at about 440 r p m for the runs presented here. At low speed or rpm the power output falls off owing to the decrease in number of cycles gone through per unit time. (c) Since the engine is considerably simpler t h a n a gasoline engine, not requiring valves, ignition systems, not having to be able to withstand high pressures, not having stresses produced due to high speed, it is expected t h a t hot-air engines of this type, if mass-produced, would be less expensive t h a n gasoline engines of this size. (d) Availability and cost of fuel are problems t h a t have to be determined for each locality. In most populated areas of the world where fuels, especially gasoline are unavailable or expensive, solar energy is available in abundance. While gasoline engines are more applicable in countries like the United States, hot-air engines m a y be more applicable in the so-called developing countries. One factor of solar energy is that it is free. For m a n y purposes such as water pumping for irrigation, supplying power to machines, and so on, the fact t h a t there is no sunshine at night is not critical since the operation can then be discontinued. If operation becomes a necessity at such times another source of heat can be substituted without much difficulty. 174
Conclusions Based on the work outlined in this paper it is believed that hot-air engines have certain advantages in the right environment and can be mass-produced as economically as other engines of similar size. T h e y are safer, more versatile, more rugged, and have fewer parts.
REFERENCES 1. W. J. M. Rankine, "The Steam Engine," Charles Griffin & Company, Limited, England, 1908, pp. 345-374. 2. B. Donkin, "Gas, Oil and Air Engines," Charles Griffin & Company, Limited, England, 1911, pp. 563-581. 3. H. Rinia, F. K. DuPre, "Air Engines," Philips Technical Review, 8, no. 5, May 1946, pp. 129-160. 4. De Brey, H. Rinia, F. L. VanWeenan, "Fundamentals for the Development of the Philips Air Engine," Philips Technical Review, 9, no. 4, 1948. 5. F. L. VanWeenen, "Tile Construction of the Philips Air Engine," Philips Technical Review, 9, no. 5, 1948. 6. D. K. Edwards, K. E. Nelson, "Radiation Characteristics in Optimization of Solar Heat-Power Conversion Systems," ASME Paper 61-WA-158, November 1961. 7. T. Finkelstein, "Internally Focusing Solar Power Systems: Part I: Conversion of Solar Radiation into Power," ASME paper 61-WA-297, November 1961. 8. E. H. Parers, "Investigation of the Application of Solar Energy to Hot Air Engines," MS thesis, University of Florida, January 1959.
FIG. 2--1/~-hp, closed-cycle, hot-air engine with solar energy concentrated on hot end. --~ Solar Energy