Stirling Engine ReferenceGuide and Catalog

Extremely Interesting Model Stirling Engines

The MM-6 is Powered by the Heat of Your Hand

Stirling Enginesby Brent H. Van Arsdell

Reprinted from MacMillan Encyclopedia of EnergyUsed by Permission

The principle that makes Stirling engines possible is

quite simple. When air is heated it expands, and

when it is cooled it contracts. Stirling engines work

by cyclically heating and cooling air (or perhaps

another gas such as helium) inside a leak tight

container and using the pressure changes to drive a

piston. The heating and cooling process works like

this: One part of the engine is kept hot while

another part is kept cold. A mechanism then moves

the air back and forth between the hot side and the

cold side. When the air is moved to the hot side, it

expands and pushes up on the piston, and when the

air is moved back to the cold side, it contracts and

pulls down on the piston. 2

Figure 1.Key to Stirling Engine. The air flows through and around the porous displacer. The displacer looks like a piston but is not.

While Stirling engines are conceptually quite

simple, understanding how any particular engine

design works is often quite difficult because there

are hundreds of different mechanical configurations

that can achieve the Stirling cycle. Figure 1 shows a

schematic of a transparent educational

demonstration engine that runs on the top of a cup

of hot coffee. This engine uses a piece of foam

similar to what would be used as a filter for a

window air conditioning unit to "displace" the air

between the hot side and the cold side. This foam

displacer is carefully mounted so it does not touch

the walls of the cylinder. Figure 2 shows how this

particular engine achieves the Stirling cycle. In this

engine, the air flows through and around the

displacer from the hot side then back to the cold

side, producing a power pulse during both the hot

and cold portion of the cycle. Stirling engines can

be mechanically quite simple since they have no

valves, and no sparkplugs. This can result in

extremely high reliability as there are fewer parts to

fail.

It is worthwhile to compare Stirling engines to other

more familiar engines and note their similarities as

well as their differences. Stirling engines are a type

of heat engine. They turn heat into mechanical

work and in this sense they perform the same

function as other well known heat engines such as

gasoline, diesel, and steam engines. Like steam

engines, Stirling engines are external combustion

engines, since the heat is supplied to the engine

from a source outside the cylinder instead of being

supplied by a fuel burning inside the cylinder.

Because the heat in a Stirling engine comes from

outside of the engine, Stirling engines can be

designed that will run on any heat source from fossil

fuel heat, to geo-thermal heat, to sunshine. Unlike

steam engines, Stirling engines do not use a boiler

that might explode if not carefully monitored.

When operating on sunshine, or geo-thermal heat,

Stirling engines obviously produce no pollution at

all, but they can be exceedingly low emissions

engines even when burning gasoline, diesel, or

home heating oil. Unlike gasoline or diesel engines

that have many thousands of start stop cycles of

combustion each minute, burners in Stirling engines

burn fuel continuously. It's much easier to make a

continuous combustion engine burn very cleanly

than one that has to start and stop. An excellent

demonstration of this principle is to strike a match,

let it burn for a few seconds, then blow it out. Most

of the smoke is produced during the starting and

stopping phases of combustion.

A Brief History

In the early days of the industrial revolution, steam

engine explosions were a real problem. Metal

fatigue was not well understood, and the steam

engines of the day would often explode, killing and

injuring people nearby. In 1816 the Reverend

Robert Stirling, a minister of the Church of

Scotland, invented what he called "A New Type of

Hot Air Engine with Economiser" as a safe and

economical alternative to steam. His engines

couldn't explode, used less fuel, and put out more

power than the steam engines of the day.

The engines designed by Robert Stirling and those

who followed him were very innovative engines,

but there was a problem with the material that was

used to build them. In a Stirling engine, the hot

side of the engine heats up to the average

temperature of the flame used to heat it and remains

at that temperature. There is no time for the

cylinder head to cool off briefly between power

pulses. When Robert Stirling built his first engines,

cast iron was the only readily available material,

and when the hot side of a cast iron Stirling engine

was heated to almost red hot, it would oxidize

fairly quickly. The result was that quite often a hole

would burn through the hot side causing the engine

to quit. In spite of the difficulties with materials,

tens of thousands of Stirling engines were used to

power water pumps, run small machines, and turn

fans, from the time of their invention up until about

1915.

As electricity became more widely available in the

early 1900s, and as gasoline became readily

available as a fuel for automobiles, electric motors 3

Insert figure 2 here

Figure 2. Four phases of the Stirling engine power cycle. 4

and gasoline engines began to replace Stirling

engines.

Regeneration

Robert Stirling's most important invention was

probably a feature of his engines that he called an

"economiser." Stirling realized that heat engines

usually get their power from the force of an

expanding gas which pushes up on a piston. The

steam engines that he observed dumped all of their

waste heat into the environment through their

exhaust and the heat was lost forever. Stirling

engines changed all that. Robert Stirling invented

what he called an "economiser" that saved some heat

from one cycle and used it again to pre-heat the air

for the next cycle.

It worked like this: After the hot air had expanded

and pushed the piston as far as the connecting rod

would allow, the air still had quite a bit of heat

energy left in it. Stirling's engines stored some of

this waste heat by making the air flow through

economiser tubes that absorbed some of the heat

from the air. This pre-cooled air was then moved to

the cold part of the engine where it cooled very

quickly and as it cooled it contracted, pulling down

on the piston. Next the air was mechanically moved

back through the pre-heating economiser tubes to the

hot side of the engine where it was heated even

further, expanding and pushing up on the piston.

This type of heat storage is used in many industrial

processes and today is called "regeneration."

Stirling engines do not have to have regenerators to

work, but well designed engines will run faster and

put out more power if they have a regenerator.

Continued Interest

In spite of the fact that the world offers many

competing sources of power there are some very

good reasons why interest in Stirling engines has

remained strong among scientists, engineers, and

public policy makers. Stirling engines can be made

to run on any heat source. Every imaginable heat

source from fossil fuel heat to solar energy heat can

and has been used to power a Stirling engine.

Stirling engines also have the maximum theoretical

possible efficiency since their power cycle (their

theoretical pressure volume diagram) matches the

Carnot cycle. The Carnot cycle, first described by

the French physicist Sadi Carnot, determines the

maximum theoretical efficiency of any heat engine

operating between a hot and a cold reservoir. The

Carnot efficiency formula is: (T(hot)-

T(cold))/T(hot). T(hot) is the temperature on the

hot side of the engine. T(cold) is the temperature on

the cold side of the engine. These temperatures

must be measured in absolute degrees (Kelvin or

Rankine).

Stirling Applications

Stirling engines make sense in applications that take

advantage of their best features while avoiding their

drawbacks. Unfortunately, there have been some

extremely dedicated research efforts that apparently

overlooked the critical importance of matching the

right technology to the right application.

In the 1970s and 1980s a huge amount of research

was done on Stirling engines for automobiles by

companies such as General Motors, Ford, and

Philips Electronics. The difficulty was that Stirling

engines have several intrinsic characteristics that

make building a good automobile Stirling engine

quite difficult. Stirling engines like to run at a

constant power setting, which is perfect for pumping

water, but is a real challenge for the stop and go

driving of an automobile.

Automobile engines need to be able to change

power levels very quickly as a driver accelerates

from a stop to highway speed. It is easy to design a

Stirling engine power control mechanism that will

change power levels efficiently, by simply turning

up or down the burner. But this is a relatively slow

method of changing power levels and probably is

not a good way to add the power necessary to 5

accelerate across an intersection. It's also easy to

design a simple Stirling engine control device that

can change power levels quickly but allows the

engine to continue to consume fuel at the full power

rate even while producing low amounts of power.

However it seems to be quite difficult to design a

power control mechanism that can change power

levels both quickly and efficiently. A few research

Stirling engines have done this, but they all used

very complex mechanical methods for achieving

their goal.

Stirling engines do not develop power immediately

after the heat source is turned on. It can take a

minute or longer for the hot side of the engine to get

up to operating temperature and make full power

available. Automobile drivers are used to having full

power available almost instantly after they start their

engines.

In spite of these difficulties, there are some

automobile Stirling applications that make sense.

Hybrid electric cars which include both batteries and

a Stirling engine generator would probably be an

extremely effective power system. The batteries

would give the car the instant acceleration that

drivers are used to, while a silent and clean running

Stirling engine would give drivers the freedom to

make long trips away from battery charging stations.

On long trips, the hybrid car could burn either

gasoline or diesel, depending on which fuel was

cheaper.

To generate electricity for homes and businesses,

research Stirling generators fueled by either solar

energy or natural gas have been tested. They run on

Solar power when the sun is shining and

automatically convert to clean burning natural gas at

night or when the weather is cloudy.

There are no explosions inside Stirling engines, so

they can be designed to be extremely quiet. The

Swedish defense contractor Kockums has produced

Stirling engine powered submarines for the Swedish

navy that are said to be the quietest submarines in

the world.

Aircraft engines operate in an environment that gets

increasingly colder as the aircraft climbs to altitude,

so Stirling aircraft engines, unlike any other type of

aircraft engine may derive some performance

benefit from climbing to altitude. The communities

near airports would benefit from the extremely quiet

operation that is possible. Stirling engines make

sense where these conditions are met:

1. There is a premium on quiet.

2. There is a very good cooling source available.

3. Relatively slow revolutions are desired.

4. Multiple fuel capacity is desired.

5. The engine can run at a constant power output.

6. The engine does not need to change power

levels quickly.

7. A warm-up period of several minutes is

acceptable.

Low Temperature Difference Engines

In 1983, Ivo Kolin, a professor at the University of

Zagreb in Croatia, demonstrated the first Stirling

engine that would run on a heat source cooler than

boiling water. After he published his work, James

Senft, a mathematics professor at the University of

Wisconsin, River Falls built improved engines that

would run on increasingly small temperature

differences, culminating in an elegant and delicate

Stirling engine that would run on a temperature

difference smaller than 1 F.

These delicate engines provide value as educational

tools, but they immediately inspire curiosity into the

possibility of generating power from one of the

many sources of low temperature waste heat (less

than 100 C.) that are available. A quick look at the

Carnot formula shows that an engine operating with

a hot side at 100 C. and a Cold Side at 23 C. will

have a maximum Carnot efficiency of [((373 K-296

K )/373 K) *100] about 21 percent. If an engine

could be built that achieved 25 percent of the

possible 21 percent Carnot efficiency it would have

about 5percent overall Carnot efficiency.

That figure seems quite low until one realizes that

calculating Carnot efficiency for an engine that uses 6

a free heat source might not make much sense. For

this type of engine it would probably be more

worthwhile to first consider what types of engines

can be built, then use dollars per watt as the

appropriate figure of merit.

Stirling engines that run on low temperature

differences tend to be rather large for the amount of

power they put out. However this may not be a

significant drawback since these engines can be

largely manufactured from lightweight and cheap

materials such as plastics. These engines could be

used for applications such as irrigation and remote

water pumping.

Cryocoolers

It isn't immediately obvious, but Stirling engines are

a reversible device. If one end is heated while the

other end is cooled, they will produce mechanical

work. But if mechanical work is input into the

engine by connecting an electric motor to the power

output shaft, one end will get hot and the other end

will get cold. In a correctly designed Stirling cooler,

the cold end will get extremely cold. Stirling coolers

have been built for research use that will cool to

below 10 K. Cigarette pack sized Stirling coolers

have been produced in large numbers for cooling

infrared chips down to 80 K. These micro Stirling

coolers have been used in high end night vision

devices, antiaircraft missile tracking systems, and

even some satellite infrared cameras.

BIBLIOGRAPHYBooksSenft, James R. (1993). An Introduction to Stirling Engines. River Falls, Wisconsin: Moriya Press.

Senft, James R. (1996). An Introduction to Low Temperature Differential Stirling Engines. River Falls, Wisconsin: Moriya Press.

Walker, Graham. (1980). Stirling Engines. Oxford: Oxford University Press.

Walker, Graham.; Fauvel, Owen R.; Reader,

Graham.; Bingham, Edward R. (1994). The Stirling Alternative. Yverdon, Switzerland: Gordon and Breach Science Publishers.

West, Colin. (1986). Principles and Applications of Stirling Engines. New York: Van Nostrand Reinhold.

If Stirling Engines Are So Great, Why Don't We Have

Them Already?by Darryl Phillips

Old Stirling designs from the late 1800s were made of cast iron and were very heavy, as much as a hundred pounds per horse. So the engine got a reputation for being too heavy to consider. Of course IC engines from the same period were heavy too, but we tend to forget that. By the time high temperature alloys were available, IC technology had outrun the competitors.

"…These imperfections have been in a great measure removed by time and especially by the genius of the distinguished Bessemer. If Bessemer iron or steel had been known thirty five or forty years ago there is a scarce doubt that the air engine would have been a great success … It remains for some skilled and ambitious mechanist in a future age to repeat it under more favourable circumstances and with complete success…" (written in the year 1876 by Dr. Robert Stirling [1790-1878])

Much work has been done on Stirlings for auto engines. NASA has displayed a blue Dodge pickup at trade shows for several years, with a ten million dollar Stirling under the hood. You may have seen it running up and down the flightline during the afternoon airshow, transporting performers in front of the crowd.

Ford, GM, and the European car makers have all run Stirlings. But the public wants a new engine to feel like the old one, and Stirlings are different. It is difficult to achieve good idle fuel economy at a red light for minutes at a time, then produce instant tire-squealing power when the light turns green.

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Stirlings are not good car engines.

But the mission profile of the aircraft engine is totally different from the car engine. The characteristics that make the Stirling wrong for a car make it right for an aircraft.

The premier publication for Stirling engine development is STIRLING MACHINE WORLD. SMW is a quarterly, distributed worldwide to design engineers, the academic community, and individual Stirling buffs. The following two sections are adapted from a series of Aircraft Stirlings by Darryl Phillips published in 1993 and 1994.

Part #1..…Ten Tips for Stirling Engine Design

1…Always keep in mind that an engine is just a machine that converts energy from one form to another. Energy stored in fuel or from some other source is changed to kinetic energy. The task is to maximize throughput and minimize losses.

2…Minimize temperature losses. The first temperature loss isn't even measurable in an immediate way. It is the difference between the temperature a given fuel could have burned at, and the temperature it actually burns. Having purchased a particular fuel, the user should be able to utilize the maximum potential contained in that fuel. The second loss we encounter is the difference between combustion temperature and heater temp. And so on through out the engine. At each step, strive to eliminate temperature differences. Ideally, the entire temperature spread from combustion to ambient should be seen in the hot to cold temperature swing of the gas itself. Temperature losses are inevitable of course, but they should be minimized to the extent possible.

3…Consider the logarithmic nature of temperature, as opposed to the linear nature of heat energy. A given temperature loss on the cold side hurts much more than the same number of degrees on the hot side. Thinking of temperature in linear degrees distorts the problem, it's helpful to work in decibels of temperature (DBT) rather than degrees. (See Part Two, "The Adventures of Heat and Hot")

4…Minimize aerodynamic losses. This is the biggest problem in many contemporary Stirling designs. Think like an aerodynamicist. Pay close attention to gas velocity. Watch sharp bends, abrupt changes in cross section, anything the higher speed molecules will dislike. Think like an electrical engineer. Move the gas in parallel rather than series. Keep the bore big and the stroke small, probably around 10% of bore. Think torque rather than speed. Study the low delta-T designs, they utilize tiny temperature differences to the best advantage, and they point the way to efficient Stirlings in higher delta regions too.

5…Minimize dead volume. Strive to move 100% of

the gas from heated to cooled space. An ideal Stirling would see average gas temp equal to heater temp at one end of the cycle, and average gas temp equal to cooler temp at the other end. While this is never fully possible in the real world, try, try, try.

6…Use the correct compression ratio. Better yet, forget compression, and use the correct expansion ratio. These are the same number, but expansion better reflects the conversion of heat to kinetic energy. The ideal expansion ratio is the square root of the ratio of average absolute gas temperatures taken at the hottest and coolest points in the cycle. Note that any gas not fully heated or cooled reduces this average. This is the fundamental reason it's so important to heat/cool as near 100% of the gas as possible.

7…Pressurization. It is tempting to double the power of an engine by doubling the internal pressure. But twice the power means twice the heat energy in and out. If exchanger area remains constant, temperature losses will increase, and twice the power won't be obtained. Plus, the higher pressure fluid will suffer greatly increased flow losses and power output will decline further. As pressure rises, containing it requires more material. That material might have been better utilized providing increased heat transfer area. Like all the other design compromises, pressure is a tool to be used. Just remember that excessive pressure is just as harmful as excessive dead volume or excessive flow loss.

8…Regenerators. No doubt the regenerator is the bait that hooked most of us on the Stirling in the first place. The idea of using the same thermal energy more than once is fascinating and very worthwhile. But don't let the regenerator get in the way of the engine. There are reports of Stirlings that ran OK, then ran better when the regenerator was removed. If the above rules are flaunted, nothing in the regenerator will save the design. To the extent the regenerator increases dead volume, or adds flow loss, it is the enemy of the engine. Simplicate the regenerator.

9…Much of the thermal promise of the regenerator can be achieved with the recuperator. Same idea, applied to the incoming combustion air and the exhaust and coolant streams. If energy is only allowed to leave the machine via the crankshaft, the desired efficiency will be attained. And it's easier to deal with dead volume and tortuous flow paths in the combustion gasses than inside the Stirling itself.

10…Build the engine. No amount of computer modeling takes the place of a real engine. The industrial revolution owes its success to the fact that the computer hadn't been invented yet. Else we would still be modeling and simulating the cotton gin, the telegraph, the steam engine, the railroad. Build the engine, the marketplace awaits.

Part 2…..The Adventures of Heat and Hot8

One of the ten tips listed last time involved the logarithmic nature of temperature. So let's begin there. We'll show why degrees are not the best way to measure how hot something is. Then we will introduce a way to graphically illustrate how much usefulness a given source of thermal energy contains, and how most of the energy escapes unused. Plus some thoughts on the creative process.

Creativity is closely coupled with intuition, that is, with our unconscious grasp of a subject. Thermodynamics isn't very intuitive for a couple of reasons. First, we humans exist in a very narrow temperature range, the spread between what "feels very hot" and what "feels very cold" is tiny compared to the range from cryogenics to combustion. Second, while we can feel quality, we have no sensory ability to feel quantity. Thus our language, our definitions, and our thought processes become confused. We use "heat" and "hot" as though they were forms of the same word, when they actually refer to very different phenomena. Improving our intuition of thermal energy, and thus our creativity, is the goal of this discussion.

Thermal energy is of interest because it can do something useful, such as making a Stirling engine run. This energy has two dimensions, quantity and quality. Heat and Hot. They exist together but each has distinct properties. Neither heat nor hot can accomplish anything alone. It takes both, just as voltage and current are both necessary to deliver electrical power. Quantity, heat, is a linear concept. A hundred calories will accomplish twice as much as 50. Quality, hot, is not linear at all. This may be the biggest block to grasping thermodynamics at an intuitive level.

Figure 6 adds decibels of temperature. Each arrow is 6 dB long. Now it's obvious that 6 dB is a 2:1 absolute temperature ratio in electronics. This temperature relationship is equally true at room ambient, or the cryogenic regions, or at the surface of the sun. It's a factor that can be computed mentally in a flash, something most of us can't do with degrees Celsius. The 0 dB pint has been

Hot has traditionally been measured in degrees. We call it temperature. But why do we use linear units to describe a phenomenon that is nonlinear for our purposes? Let's break out of that tradition, and see where it leads.

Degrees in thermodynamics are analogous to volts in electronics. Yet the linear volt is a cumbersome way to describe gain in an amplifier or loss in a cable. Ratios are involved here, not absolute units, and for this purpose the decibel is better suited. Without the decibel, it's fair to say that electronics could never have made the huge strides we've seen in recent history. It's time to grant the same benefits to thermodynamics.

In Figure 4, all the arrows are the same length, that is, each represents an absolute temperature ration of 2:1. But they don't look the same length, do they? Expressing temperature in degrees is the source of severe distortion that makes some arrows appear much longer than others. This same effect exists mentally, distorting our intuition.

Figure 5 shows similar arrows, now with the degrees distorted. Again, each arrow illustrates a 2:1 ratio of absolute temperature. This is a first step, but it doesn't do much for human intuition.

arbitrarily set at 0 degrees C, thus dBT is defined as 20 log T1/T2, with T1 the temperature of interest, and T2 equal to 0 degrees C, both expressed in absolute degrees.

Now we've created a better bridge between thermodynamics and the human mind. The statement that a given engine would run with a 10 degree differential is meaningless unless a reference temperature is cited, but an engine that will run on 0.1 dB will do it anywhere within the limitations of it's material and environment.

A chart for quick conversion from temperature to dBT is shown in Figure 7. Plotted to make Celsius a straight line, it provides an interesting and perhaps surprising

9

illustration of the nonlinearity of our common temperature scales.

The importance of dBT is hard to overstate. Each dB is exactly as useful and important and valuable as any other. Now it becomes easier to see how much capability exists in the spread between T(hot) and T(ambient), and to see where it may be going astray. Question: Is it a better design compromise to accept a 200 degree loss in a heater, or a 40 degree loss in a cooler? There is just no way to answer that without referring to the specific temperatures and doing the math. But in the dBT world, a loss in one place of a 0.4 dB can be directly compared to a loss somewhere else of, say, 1.3 dB. Intuitive.

Carnot faced the same questions of temperature ratio long ago. Using the decibel just gives us another viewpoint of that ratio, hopefully providing a tool more suited to human intuition. But temperature is only a single dimension of the problem, we must look further.

Heat and hot. Heat (calories) is linear, and by using dBT we can express "hot" in a linear manner. Again, it's necessary to remember that they are as distinct as current is from voltage. And just as we can multiply electrical voltage and current to find power, we can multiply heat and hot to find an "area" of energy. This simply wasn't practical with temperature measured in degrees. Each square in Figure 8 is exactly as valuable as every other square. Now we can see the relationship graphically between x axis quantity (heat) and y axis quality (temperature). To produce an area both the x and y axes are needed, neither heat nor hot can do it alone. They can, however, be traded one for the other. That is, a given number of area units will do the same job whether that area is composed of many calories at a small temperature spread, or

fewer calories with a greater delta T. The "area" can be square, or tall, or short and squat and if it has the same number of squares, it can do the same amount of work.

For an example, suppose we are designing a Stirling engine to operate at an ambient of 33 degrees C (91 F, or 1 dBT). This is shown as point A in Figure 8. The fuel we have chosen has a capability, under ideal conditions, of burning at 2450 C (20 dBT), point B. We plan on burning at a fuel flow rate equivalent to point C. Note that the x axis can be scaled to any units of heat and time that may be convenient, such as 10,000 calories per second, or whatever. Thus, A-B-C-D defines an area of 19 dBT by 15 quantity units, or 285 squares. And remember that each of these 285 squares contain precisely the same amount of capability or usefulness.

Now let's examine where this energy is going. See Figure 9 (on the next page).

Loss #1 in this hypothetical design represents fuel that passes out the exhaust without burning. Perhaps due to a bad atomizer design, poor mixture control, or other mechanical flaw. This is fuel purchased and consumed without yielding any benefit. In this example, it represents 10% of total fuel, or 28.5 squares.

The second loss involves the difference between the temperature this fuel could have produced under ideal conditions, and the temperature of combustion actually realized. Perhaps fuel/air mixture is to blame here too. 37.8 squares is 13.2% lost.

Conduction and other thermal losses involving energy that does not flow through the Stirling cycle are covered in loss #3. These might include loss due to simple metallic heat conduction, radiant

10

losses, exhaust temperature rise above ambient, et cetera. Here we're seeing 40.5 squares, or 14.2%.

We've already lost over 37%, about 62% is still available to feed into the engine. Loss #4 is the difference between the combustion temperature (17.2 dBT) we managed to achieve above, and the 14.6 dBT at the inner surface of the heater. This stems from thermal conduction loss within the metallic heater, but also from inefficient transfer from the burner gasses to the heater structure. Here we see a loss of 2.6 dB, which represents 28.6 squares or another 10%.

A similar loss exists in the cold side, between ambient temperature and the actual temp we manage to achieve at the inner surface of the cooler, this is loss #5. Chalk up another 30.8 squares representing 10% of the 285 we purchased from the fuel supplier.

Now, finally, we're into the engine itself! Loss #6 is the difference between the heater surface and the mean temperature of the working fluid, taken at the hottest point in the cycle. To find this mean, we must account for all the gas in the system including any dead volume, not just the gas residing in the hot chamber. This points out the strong need to minimize dead volume since any gas not exposed to the heater won't be heated. Loss #6 represents 48.4 squares, or 17%.

Loss #7 is the mirror image of #6, taken at the coolest time in the cycle. Again, dead volume, or incompletely-swept gasses won't be cooled, and will contribute to the difference between mean gas temperature and the cooler surface itself. Tally another 26.4 squares, amounting to 9.3%.

Of the 285 squares of capability, we've managed to lose 241 of them along the way.

The remaining 44 (15.4%) are available to run the engine. They represent the thermal energy that produces the actual rise and fall in pressure that makes things go around. They must supply energy to overcome all the internal aerodynamic drag, all the friction in bearings and other materials, and if we're lucky leave a little to come out the shaft as rotary power. This message bears emphasis. The last thing is the output power, it only exists as the residual after all the losses are satisfied.

The above is not discouragement. To the contrary, it shows the extreme promise of the Stirling once we identify and minimize each individual loss. Now we have a way to visualize the relative value of one design choice over another, taking into account the relationship of heat and hot.

Of course the numbers above are hypothetical for the purpose of illustration. A particular real-world engine will have different numbers in every department. But the losses are real, and the goal in each design must be to minimize all losses and thus maximize the remainder available as output power.

No attempt has been made to include all the possible loss mechanisms. It may be helpful to break the above categories down further. The intention here is only to demonstrate a way to see how much capability the fuel contains, and where it goes.

Finally let us look at one improvement in the above Stirling design. Again refer to Figure 9. Mentally make just one change. Drop the ambient temperature by 2 dBT, to around -30 degrees C. This moves losses #1, 3, 5, and 7 down 2 dB, and increases the gross engine power from 44 to 66 units, a 50% improvement. (I'm ignoring a number of lesser factors here for simplicity.) This 50% improvement in gross power, that is, power prior to supplying the internal aerodynamic and friction losses, might equate closer to 100% improvement in net shaft horsepower. But where would we find such a cold ambient to operate this engine? Answer: the higher you go, the colder it gets. We'll use this engine to power an aircraft.

And that takes us to the subject next time, the amazing match between the Stirling and the lightplane. Stay tuned.

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The Most Effective Education Starts With Curiosity...

This engine is powered by an ice cream cone or a

Have we made you curious?The MM-4, like all engines produced by American Stirling, is a teaching tool designed to inspire students with a love for physics. This transparent Stirling engine will run on the temperature difference between an ice cream cone and 23 deg. C. room air. To reverse the engine, place the MM-4 on

. The flexible piston allows students to see the effects of the expanding and contracting gas (air or helium) inside the engine.

While the MM-4 requires about 23 deg. C of temperature difference to operate, an engine such as the MM-6 will run about 120 rpm on the heat from your warm hand.

Here’s how it works.

cup of steaming hot coffee

a cup of steaming hot coffee

All American Stirling engines run on the same principles, which we will describe using the MM-4 as an example. Inside the clear cylinder is a large yellow foam "displacer". This displacer looks like a piston, but has a 3.125 mm (1/8") gap around its outside edge. Air never pushes on the displacer - it flows around it.

This engine can be started when either the bottom plate or the top plate is heated or cooled to a temperature at least 23 deg. C warmer or colder than the other plate. A gentle spin on the propeller is necessary to start the engine.

As the yellow foam displacer moves away from the warmer side, most of the air flows from the

cold side to the warmer side and is heated. When the air is heated it expands, which increases the pressure inside the entire engine. This increase in pressure pushes up on the power piston (rubber diaphragm).

Next the energy stored in the propeller moves the displacer to the warm side of the engine and the air once again flows around the displacer to the cold side of the engine. When the air is cooled it contracts, and the pressure drops throughout the entire engine. This drop in pressure pulls down on the power piston and the displacer moves back to the cold side, returning the air to the warm side. Then the cycle starts all over again.

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The Model MM-4 Engine

Coffee Cup Stirling EngineOur Original VersionModel MM-1 $179.00

The MM-1 shown running on a cup of hotcoffee (mug and quarter shown for scale.)

This lovely Stirling engine has jewel like ball bearings and an incredibly slick graphite piston in a glass cylinder. This engine will run on the heat of your warm hand in a 72 degree F (23 degree C) room. You can even run it on a computer monitor, fax machine or bright sunlight shining through a window. This engine truly shows the beauty and magic of our Stirling engines.

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Powered by your warm hand -- the MM-6

This is the classic Coffee Cup Stirling engine that is still our most popular engine. Power this engine at speeds up to about 250 rpm with a cup of your favorite steaming hot coffee. We do mean steaming - this is no place for tepid coffee!

Some customers also enjoy “powering” their engines by cooling down the bottom side of the engine to 32 deg. F (0 deg. C) on a pile of ice chips. This “cold powered” method of running the MM-1 works well in rooms that are 72 deg. F. (23 deg. C) or warmer.

Model MM-1 $179.00 Shipping: $12.00 domestic, $16.00 international

Model MM-6 $359.00 Shipping: $17.00 domestic, $34.00 elsewhere

13 Order online, www.stirlingengine.com or call: 760-742-2727; fax 858-777-3459

Heat of Your Hand EngineModel MM-6 $359.00

Coffee Cup engine kitModel MM-5 $119.00

Model MM-5 $119.00 Shipping: $12.00 domestic, $16.00 international

Sometimes building an engine is more fun than just buying one and seeing it run. It’s exciting to see an engine run for the first time that you built yourself!

Plan on spending two or three enjoyable evenings to assemble your MM-5 kit. To make your job easier we now have an excellent set of online instructions complete with video clips to help you build your engine.

All you need to assemble the engine is a needle nose pliers scissors, a drill bit, some epoxy or silicone rubber caulking, and tiny bit of superglue.

14Order online, www.stirlingengine.com or call: 760-742-2727; fax 858-777-3459

Build this engine with our kit!

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Ordering Instructions...

ill out the order form on the next page and fax it to us. International customers please remember to fill out the mailing label at the bottom of the order form for faster delivery. Make your checks payable to:

Order online at www.stirlingengine.com or f

American Stirling Company 2726 Shelter Island Drive #172San Diego, Calif. 92106

Phone: 760-742-2727Fax: 858-777-3459Email: info74@stirlingengine.comInternet: www.stirlingengine.com

Spare Diaphragm for MM-1 through MM-5The diaphragm is the most fragile part of these model engines. It is also the only part that wears out. The key component is incredibly hard to find, very thin silicone rubber.When our engineer first tried to buy this material he spent three days on the phone. You might want to have a spare.

Spare Diaphragm $14.95Shipping $3.00 domestic, $5.00 international

Schools and Universities in the United States are welcome to order using a purchase order. Please fax your purchase order to (858) 777-3459 then mail the original purchase order to the address shown above. Remember to include the shipping cost on your purchase order.

Copyright American Stirling Company ©2002

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Engines Unit Price Qty. Shipping Sub Total MM-10 Pack of Kits

MM-6 Heat of Your Hand Engine

MM-1 Original Transparent Engine

MM-5 Kit Engine (qty. 1)

990.00

359.00

179.00

119.00

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International CustomersFor quicker delivery please fill in your mailing address in this box:

Order online at www.stirlingengine.comor Fax your order to: 858-777-3459 or Call: 760-742-2727

Order form mailed out with catalog

Shipping is the listed price for the first engine, 50% of the listed price for additional engines

Spare Diaphragm (for MM-1 & MM-5) 14.95

“Around the World by Stirling Engine” by Brent Van Arsdell

17.95

American Stirling Company Order Form

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