Heat-Powered Engines (Thermal to Mechanical Conversion)
   You lose some energy every time you either convert it from one form to another, or store it and retrieve it. For this reason it is best to use energy directly whenever possible. If there is water to be pumped for instance, the mechanical energy from wind is best used directly, rather than being converted first to electricity and then running an electric motor to pump the water.
   Modern electrical generators (typically alternators) and motors can be quite efficient. They typically convert between electrical and mechanical energy at rates better than 90%.
   Converting heat energy into mechanical energy however, is a different matter. A French physicist named Carnot developed a formula that gives the maximum efficiency available for a heat engine. Although the steam engine was the target of his work, his formula works for any form of heat engine: The maximum efficiency possible from any heat engine can be calculated by subtracting the exhaust temperature from the maximum temperature, and dividing this difference by the maximum temperature – the maximum temperature being degrees above absolute zero. In simple arithmetic terms this would be written as (tmax-tmin)/tmax.
   Compressing gas increases the temperature, and expanding it cools it. Standard automotive engines rarely have compression ratios of greater than about 9 to 1. The temperatures during combustion are thus allowed to expand and cool as the piston travels downward. The efficiency of such engines is typically about 20%, although it can exceed 30% under ideal loading and RPM. Diesel engines however, with compression ratios often exceeding 20 to 1, can have efficiencies of 40% and higher.
   Of all the various build-it-yourself offerings that have filled books, magazines, and their ads over the years, none have fascinated me as much as mechanical energy. I have seen very few articles on the subject, and fewer still with instructions on how to build your own. And yet, if a person had a technique for converting THERMAL energy into mechanical energy, he could:
   To summarize, a sustainable energy system could consist of (a) One or more energy sources designed to produce heat from whatever energy sources were available in the area, (b) At least one thermal storage unit (an additional unit might even be designed to store the more efficient high temperatures, if an appropriate energy source was available), and ( c )A means of converting the stored thermal energy into mechanical energy.
   Such technology would be to the development of alternative infrastructures as the steam engine was to the industrial revolution.
   The reason I emphasize THERMAL to mechanical conversion is because any form of energy that you might have can be converted into heat. This would of course include wind power, water power, methane, solar, firewood, buffalo chips, junk mail, and obsolete utility bills.
   Another reason is for the sake of energy storage. There are few more simple or economical techniques for storing large amounts of energy than heating something in an insulated container. Contrast this with the complexity and environmentally-destructive materials used in battery technologies. This really came home to me one time as I was reading about a then high-tech battery that used molten lithium salt. When it occurred to me to calculate the energy stored in the heat alone, I found it to exceed the electrical capacity of the battery!
   A further design challenge would be to come up with an engine that could operate on a relatively low temperature difference. Now I recognize that a low temperature difference means low efficiency, but it also increases your options for energy sources.
   Consider this source: I have never lived in a place where there was not a noticeable difference between day and night time temperatures. Two thermal storage units could be used to capture this heat and cold for use in powering an engine.
   Here are some of the thoughts, inputs, and experiments I’ve had involving thermal-to-mechanical conversion.
  1.    1. I've read of freon-based turbines that could power air conditioners and generate electricity, but we are talking kind of high-tech here.
  1.    I have however considered a simple form of steam turbine/generator. This began as a concept for a pressure release valve, consisting of a bolt through a stack of washers sitting on the end of a vertical tube. Any pressure high enough to lift the bolt and washers would be free to escape. If there were spiral grooves cut in the bottom washer, and counter-rotating groves in a seat on the end of the tube, the assembly would spin. If you hadn’t guessed, this assembly could become the rotor of a small generator with a built-in pressure release.

  1.    I read of two sealed cylinders half filled with water connected by pipe that ran through a water motor. The air in one cylinder was expanded by a spray of hot water while the air in the other was being shrunk by a cold shower, forcing water to run through the pipe and motor, as it traveled from one cylinder to the other. Next, the hot cylinder got a cold shower while the cool cylinder got a hot shower, sending the water racing the other direction. The motor was actually connected through some extra plumbing involving check valves, to keep it spinning the same direction with each change of direction. Water was recycled in the system to be sent to the cooler or the heater. This was a very creative and practical system that was apparently too dangerous to big business to allow more than a brief accidental public exposure.
   4. I built a low tech steam engine -- functional, but needs work.
  1.    A rubber band

  1.    I used super glue to glue thin strips of aluminum foil and hard thin plastic together, and sprayed the resulting bi-material strip black. You get a lot of bend when you expose it to radiant heat. I even made a crude flower from this product, which would open up when you brought it next to the woodstove. Approximately speaking, plastics expand and contract with heat about ten times more than metals do, for the same temperature change. This could be an interesting way to get the sunshine to do something mechanical directly. Like most things, I discovered this the hard way. I had built a solar panel with a flat plastic cover, and the whole thing tried to pretzel when it warmed up.
  1.    Concept steam engine with two-cylinders and two moving parts. The cylinders are the closed ends of a single piece of tubing, slotted to clear the valve pin.  One end of the tube is fitted with a connection to the flywheel. The flywheel rocks the cylinder/ piston assembly back-and-forth, alternately exposing pistons to the pressure and exhaust channels in the valve shaft.


a.    Pistons 'A' and 'B' are a single piece,
b.    They inhale and exhale through tubes in the valve pin
c.    They pivot at the center of a stationary valve pin
   We need more experimenters who do not hide what they discover and prove, but give freely of the creativity and gifts that God has given them. Stand on my shoulders. We desperately need some appropriate technology products to fill this need. Don't tell me about perpetual motion, mysterious magnetic things, motors that run generators that run motors, or things that go "Tesla" in the dark. If you actually build something that produces a power output, and you can show me a working model, and if other people can build it and make it work, and if it requires nothing that you cannot find in a small town, then you will have a potential contribution.
Check-Valve-Engine
   Ok, so I have neither made one, seen one, or even heard of one, but this next one is still an interesting idea. I must allow that somebody else somewhere in history may have already invented it, and that I’m merely laying claim to somebody else’s brilliant or stupid idea.    I was contemplating ideas for the simplest possible positive displacement engine, and this is what I came up with. This engine consists of a piston and cylinder, two check valves, a heat sink to cool the air, and a heat source to heat it. While referencing the drawing below, consider the following:
1.    When the piston travels upwards air tries to enter both the source and sink.
2.    The source doesn’t want any however because as it tries to enter it expands and is forced back out.
  1.    When it enters the cool side however (the heat sink), it begins to contract and really sucks, while check #2 keeps any of the hot air from getting sucked over to the sink.
  1.    As the piston begins to withdraw, cool air is drawn through the heat source and begins to expand, but check #1 does not allow this pressure to return through the heat sink. This directs all the
       pressure to the face of the piston via the heat source.
   Passive Solar Air Compressor
   Squarely in the “random idea” camp, consider a passive solar-powered air compressor. Place a series of tubular tanks side-by side in an east-west line. They are connected to each other by check-valves that allow air to flow only from west to east.
   The west-most tank also has a check valve that allows outside air to enter, but does not let it escape – its air can only escape into the tank immediately to its east. The east-most tank has a fitting to allow the use of compressed air.
   Above this row is a shade with a slit in it that allows the sun to heat one tank at a time as the sun progresses across the sky. As each tank heats it dumps its expanding air into the tank east of it, and then cools to receive air from the tank to its west.
   The pressure would increase in each tank in sequence from west to east. This arrangement would be a little more effective if the tanks were placed in a semicircle where each one was the same distance from the slit. If the slit were covered by a fresnel lens, heat could be concentrated, and the tanks could all be in the same plane.
   A Jet Engine In My Living Room
   This one was almost tragic. A major portion of our heating in Colorado was provided by a high-efficiency woodstove in the living room. It was pleasant to have a nice warm room to hide in, but it did little for our bedroom or the rest of the downstairs rooms.
There were some existing ducts I could have tapped into, and distributed heat with the aid of a blower, but that would have been too easy – and besides, how would that be of any use during a power (or economic) failure?
   I reasoned that the stove itself was a great source of energy, and that I could convert some of it into mechanical energy to drive a blower. I was initially considering a low-temperature-difference turbine, but figured the best reliability would be to have no moving parts – don’t laugh, ram-jets do it, if you happen to be current on hyper-sonic fluid dynamics.
   Well, I came up with a configuration that I thought might work (without the benefit of hyper-sonic fluid dynamics), and being a little low on cash I fabricated it out of used flue pipe. My family has had to put up with a lot, and on its maiden voyage the “used” part of the pipe began to smoke.
   Suddenly there was a loud roar accompanied by a scream from my wife as a three foot spear of orange-violet flame was blasting out of the device about 3-1/2 feet above the floor. After a few seconds the flame ceased, but the experiment was still red hot and continued to spew a 5” diameter blast of scorching air for a few more seconds.Apparently the thin crust of creosote in the pipe had ignited and provided the jet fuel.
   Looking on the bright side:
·    No one was hurt.
·    The smoking stopped.
·    The house was a little warmer.
·    The kids were very well behaved for awhile.
·    Something about the configuration worked
·    I haven’t messed with it since.
The Stirling Engine
   In the early nineteenth century a young Scottish minister named Robert Stirling was concerned about the number of people being killed or maimed by exploding steam engines. So he invented an engine that could not explode, and his engines are still the world’s most efficient engines ever developed.
   Referencing the drawing below, the energy is obtained from the temperature difference between the COLD SURFACE and the HOT SURFACE. These two surfaces are separated by a cylinder of an insulating material so that the two surfaces will not cancel each other out by conduction.
   The DISPALCER PISTON is also of an insulating material, and fits loosely so that air can move freely around its outside edges. It is shown here next to the COLD SURFACE. Since the air itself is in this case in contact with the hot surface it is in its expanded state and the power piston is at the top of its stroke.
   In this next drawing the displacer piston has been pushed downward. Since the air was able to flow freely around its perimeter, it took no significant energy to move it.
   The air however, has now been forced into contact with the cold surface and has contracted. This has sucked air through the hole, pulling the power piston to the bottom of its cylinder.
   Presumably our young minister became bored with moving the displacer piston in and out just so he could watch the power piston jump around, so he connected them both to the same crank shaft, with a flywheel to keep the motion smooth.
   By having the displacer piston lead the power piston by 90 degrees, the pressure in the system was always changing just ahead of the power piston. By the time the power piston was at top dead center because the air had just been heated, for instance, the displacer piston was already halfway back down, beginning to create a vacuum. In this way the power piston was always chasing the displacer piston but never quite catching up with it. I am sure we all have situations in our own lives that can relate to this.
   Stirling had yet another trick: If you enlarged the displacer piston so the air could not so easily slip past it, and then drilled holes in it so that air could flow through it, you could force the air past conductive material that would store its heat or cool. In this way, hot air on its way to the cool side would leave some of its heat in the piston itself, and pick it up on its way back to the hot side. This clever REGENERATOR component is what makes a true Stirling engine so efficient.
   There are many clever variations of Stirling engines suitable to various applications, but they all involve moving air between hot and cold surfaces and continually changing pressure on a power piston. True Stirlings also include some form of regenerator.
   This category of engine is the most promising technology for sustainable energy for several reasons:
·    It is the omnivore of the engine world. Unlike internal combustion engines, it doesn’t care what you feed it – as long as it’s hot.
·    It can be configured to operate on low temperature differences. I have seen model Stirling engines animated by setting them on a cup of hot water (I suppose that would animate most of us), and it would change directions when my friend would move it over to a cup of ice water.
·    They can’t explode. They don’t even have steam to hiss and chuff out of whatever.
·    Unlike internal combustion engines, they are not noisy; all the motions are smooth and explosion free.
   There is yet another important feature of these marvelous machines – consider: If you were to apply rotation to this device from some external source, the power piston would compress air while it is next to the normally cold side, and evacuate it while the air while it is next to the normally hot side. Depending upon which direction you turned it, one side would produce heat, and the other side would cool.
   I am saying that a Stirling engine may be used as a heat pump to produce both hot and cold surfaces! Consider a scenario on a hot day where concentrated solar energy is efficiently running a large Stirling engine. This engine is driving a smaller engine which is cooling a living space (or a keg). No point in wasting the heat produced, so it is either recycled to contribute to the solar heat, or stored for later use.
   The thing that disappoints me most about Stirling engines is the fact that I’ve never built one, but lots of people have. The internet has enough sites and Stirling engine drawings and information to thoroughly confuse anybody.
   If I can ever catch up with my to-do list – if I can just catch that displacer piston – I too will have one to share.
   There are three things that must be controlled to make an efficient Stirling engine:
  1. All forms of thermal shorts
  2. Transfer of heat to and from the internal air
  3. All forms of friction.
   The thermal shorts can be minimized by using insulating materials in the displacer piston and cylinder. Surface to air heat transfer is of course related to the amount of surface the air contacts. For this reason the displacer cylinder is typically short and wide to maximize the hot and cold surfaces.
The rate of energy derived will naturally increase with the rpm. When you have a broad flat displacer piston, the mechanical stresses could soon become extreme.
   The greatest source of friction is likely to be along the sides of the power piston. Other sources would include all rotating and shaft sliding bearings, and any mechanical linkage involved in connecting the engine to actual work. A third category could involve the movement of the air itself as it is shuttled back and forth between the hot and cold surfaces.
   Machined parts are expensive, for, so initial experimenting I cast a plastic piston in the actual cylinder. An automotive filler putty (AKA “Bondo” ® ) would probably work, as well as other forms epoxy.
   When the piston is cast, it must include either a connecting rod, or some means of attaching one. For a very light-weight experiment, a flexible wire might be enough to serve as the rod itself.
   To allow some clearance and lubrication first apply a layer of grease to the inside of the cylinder (with a Q-tip), and then stick a layer of graphite to it (Q-tip also) to provide lubrication. The grease is intended to allow the piston to be released. It is then wiped out and replaced by a light machine oil.
   As a group, plastics expand roughly ten times as fast with changes in temperature as metals do, so a plasticpiston in a metal cylinder might get a little snug as temperatures increase. One way to minimize this would be to make the core of the piston out of metal, and just cast the outermost layer with plastic.
   Another thermal consideration would be that plastics have a limited temperature range, so if you have a high-temperature heat source, plastics may be impractical.
   Just how much friction can be tolerated, and how much pressure will be available to move the piston? The atmospheric pressure at sea level is about 14.7 lbs per sq. in. of surface. If we take the value from the Carnot column in the table below, and multiply it by 14.7, we get the maximum number of pounds per square inch change available from the respective temperature change.
   Referencing the first line of the table below (Tmax = 212, and Tmin = 100), the Carnot value is 0.167. If we multiply this times 14.7, we have a change in pressure of 2.45 psi. Our piston with a bore diameter of 1.5” has a surface area of 1.77 square inches. This gives us a total maximum pressure on the piston of: 1.77 times 2.45 = 4.33lbs.
   Pressure does add up quickly with area. When we check of the total pressure against the top and bottom hot and cold surfaces we might be surprised. A 6” diameter circle has an area of 28.3 square inches. 28.3 times our pressure of 2.45 lbs gives us a total of almost 70 lbs! So we need to be careful of this innocent toy.
   The table below provides calculations based upon what I think I understand at this point. When a prototype is developed to where actual measurements can be made and confirmed, this information will be modified, confirmed, or laughed at. This table relates only to the potential energy before the various forms of inefficiency are considered. At best, I wouldn’t expect a real world performance of more than half the power anticipated by the table.
For those who want to try it, consider the table and the comments that follow.

 

ALL DIMENSTIONS ARE IN INCHES

 

 

 

 

 

 

 

 

 

 

ft.lb/sec

 

Displacer

Power

cycles/

 

calories/

Watts

hp

sec

Tmax

Tmin

dia.

stroke

volume

bore

stroke

sec

Carnot

cycle

10.0

0.013

7.4

212

100

6

1.0

28.0

1.50

2.64

5

0.167

5.7

50.0

0.067

36.9

212

100

10

2.2

175.0

3.00

4.13

4

0.167

35.8

200.0

0.268

147.5

212

100

18

3.7

933.4

5.0

7.9

3

0.167

191.1

500.0

0.670

368.7

212

100

24

7.7

3500.2

6.0

20.6

2

0.167

716.4

1000.0

1.340

737.5

212

100

32

8.7

7000.4

8.0

23.2

2

0.167

1432.9

5000.0

6.702

3687.3

212

100

60

12.4

35002.0

15.0

33.0

2

0.167

7164.5

20.0

0.027

14.7

140

32

6

1.8

49.8

1.5

5.1

5

0.180

10.6

50.0

0.067

36.9

212

32

8

0.8

42.2

2.0

3.6

4

0.268

22.3

200.0

0.268

147.5

212

32

12

2.0

224.9

3.0

8.5

3

0.268

118.9

500.0

0.670

368.7

212

32

16

4.2

843.2

4.0

18.0

2

0.268

445.8

1000.0

1.340

737.5

212

32

24

3.7

1686.4

6.0

16.0

2

0.268

891.6

5000.0

6.702

3687.3

212

32

36

8.3

8432.0

9.0

35.5

2

0.268

4457.9

100.0

0.134

73.7

350

100

6

1.3

36.6

1.5

6.4

5

0.309

31.0

200.0

0.268

147.5

350

100

8

1.8

91.5

2.0

9.0

4

0.309

77.4

500.0

0.670

368.7

350

100

12

2.7

304.8

3.0

13.3

3

0.309

257.9

1000.0

1.340

737.5

350

100

18

3.6

914.5

4.5

17.7

2

0.309

773.8

1000.0

1.340

737.5

350

100

18

3.6

914.5

4.5

17.7

2

0.309

773.8

5000.0

6.702

3687.3

350

100

30

6.5

4572.6

7.5

31.9

2

0.309

3868.8

100.0

0.134

73.7

500

100

6

1.1

31.4

1.5

7.4

2

0.417

57.3

200.0

0.268

147.5

500

100

8

1.2

62.7

2.0

8.3

2

0.417

114.6

500.0

0.670

368.7

500

100

12

2.8

313.6

3.0

18.5

1

0.417

573.2

1000.0

1.340

737.5

500

100

18

4.9

1254.5

4.5

32.9

0.5

0.417

2292.6

1000.0

1.340

737.5

500

100

24

2.8

1254.5

6.0

18.5

0.5

0.417

2292.6

5000.0

6.702

3687.3

500

100

30

4.4

3136.2

7.5

29.6

1

0.417

5731.6

100.0

0.134

73.7

800

100

3

0.7

5.0

0.8

6.3

4

0.556

21.5

200.0

0.268

147.5

800

100

4

1.1

13.4

1.0

9.5

3

0.556

57.3

500.0

0.670

368.7

800

100

8

2.0

100.8

2.0

17.8

1

0.556

429.9

1000.0

1.340

737.5

800

100

8

2.0

100.8

2.0

17.8

2

0.556

429.9

1000.0

1.340

737.5

800

100

12

1.8

201.6

3.0

15.8

1

0.556

859.7

5000.0

6.702

3687.3

800

100

24

4.5

2016.1

6.0

39.6

0.5

0.556

8597.4

   One other wild idea before leaving this subject: If you are using a system based upon hot and cold water for energy sources, consider replacing the displacer piston with a chamber as shown below. The chamber is heated and cooled by sprays of alternating hot and cold water, which is recycled. The injector pumps would serve the dual purpose of spraying the chamber and circulating the water, and could be either coupled mechanically or driven by solenoids. If driven electrically by solenoids they could be timed to the top and bottom dead centers of the power piston (as opposed to the 90-degree lead of mechanically coupled systems), and thereby increase the efficiency.

   The internet has lots of videos and discussions related to Stirling engines. If you're serious about this, there is plenty of research that can be done.