The Stirling Engine(1)

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Klaus Hünig
The Stirling Engine
Assembly Instructions
handle
axe-bearing frame
axle bearing
crankshaft
stand
piston rod
fly wheel
working piston
stand
working cylinder
main cylinder lid
main cylinder wall
main cylinder
displacememt piston
main cylinder
wall joint
main cylinder base
AstroMedia
Hands-on Science Series
ISBN 3-935364-36-9 - Order No 228.STM - © Klaus Hünig - SunWatch - Design Nils Rhode
Inside the Stirling engine:
The principle is ingeniously simple and easily explained:
In a sealed cylinder (“main cylinder”), heated or cooled at one end, a piston (“displace-
ment piston”) moves the enclosed air back and forth between the cylinder’s hot and cold
ends.
In this way, the air is alternately heated and cooled, creating a cycle of compression and
expansion, of high and low air pressure.
A piston (“working piston”), connected to the main cylinder, is kept in motion by the
alternating air pressure, and in turn moves a crankshaft and flywheel.
A small portion of the produced energy is used to move the displacement piston and
keep the engine running by itself.
Robert Stirling
Robert Stirling (1790 – 1878) was a priest
of the Presbyterian Church of Scotland with
a passion for tinkering with mechanical
things. He witnessed industrialization’s first
golden age and its ravenous hunger for en-
ergy, supplied by thousands of the steam
engines James Watt had invented in 1776.
Moved by pity for the victims of the count-
less steam boiler explosions, Stirling devel-
oped the concept of a machine that would
produce energy without the use of high pres-
sure.
On September 27th 1816 he applied for a
patent on a hot air engine, which by 1818
he had perfected so far that it could be put
to work in a mine in Ayrshire as a water
pump. With his brother, he continued to im-
prove his design until he reached an effec-
tiveness of 18 % - an efficiency unheard of
at the time. Robert Stirling died on June 6th
1878 at the age of 87.
At the outset of the 20th century there were
about 250.000 Stirling engines in use world-
wide, powering table ventilators, water
pumps or small machine drives and supply-
ing mechanical energy to private households
as well as to workshops. When internal com-
bustion engines and electric motors became
increasingly popular, they gradually pushed
the Stirling engine out of the market.
Today, with growing ecological conscience
and ever-rising fuel costs, the undemand-
ing and quiet Stirling engine is attracting
renewed interest. More efficient, even qui-
eter and less prone to vibrations, contem-
porary engines are environmentally friendly
and can run on any heat source, including
solar energy.
Phase 1:
The displacement piston rises. The
air moves from the cold to the hot segment.
For an instant the outside and inside air pres-
sures are equal. The working piston reaches
its lower still point.
Phase 2:
The displacement piston reaches
its upper still point. All the air is in the hot
segment, heats up and exerts pressure. The
air pressure on the inside is greater than
outside, forcing the working piston upwards.
Phase 3:
The displacement piston de-
scends. The air moves from the hot to the
cold segment. Outside and inside air pres-
sure equal for an instant. The working pis-
ton reaches its lower still point.
Phase 4:
The displacement piston reaches
its lower still point. All the air is in the cold
segment, cools down and loses pressure.
The air pressure outside is greater than in-
side, forcing the working piston down.
The AstroMedia* Stirling engine is of the flat plate type. These engines possess an ex-
tremely flat main cylinder and need only very slight differences in temperature – some can
run just from body heat on the palm of your hand. This approach was first developed by
Professor Ivo Kolin, University of Zagreb.
Fields of application for modern Stirling engines include, for example, solar power units -
where the hot end of the main cylinder lies in the focal point of a parabolic mirror - , block
type power stations for private homes, or, amazingly enough, space craft: Stirling engines
produce electricity in space probes from radioactive material. They are also put to good use
as reverse cycle heating systems (heat pumps) and even as cooling units: when the engine
is put into motion mechanically from the outside, it transfers the heat from one side of the
cylinder to the other, with either a cooling or heating effect.
Industrial Stirling engines use a so-called regenerator, which brings an additional extreme
increase in performance: a wire matrix built into the displacement piston, stores excess
heat from the passing hot air and returns it to the cooled-down air on its back pass.
 Please read
before assembly!
1.
The assembly instructions are divided
into many small steps. This may appear
to be a lot of text, but it does make the
construction understandable and leads to
good results in an easy way. Please read
and understand each step completely
before applying it.
Things needed for assembly:
Two-component-glue
for attaching the transparent cylinder wall to the aluminium
base and lid. A good alternative is white wood glue, though initially not transparent
and taking long to set.
A good
all-purpose glue,
best with a thin application tip for applying small glue drop-
lets. Solvent-based glues have an advantage over water-based ones- they will not
cause the cardboard to warp and will dry much quicker.
Some fine
sandpaper
for roughening gluing surfaces and for sanding off projecting
cardboard edges, if necessary.
Alcohol
for degreasing metal gluing surfaces.
2.
Each piece is marked with a name and
a number. The sections are lettered in al-
phabetical order and follow the assem-
bly process. Generally, each assembly
group has its own letter. It is best to dis-
connect only the pieces needed at the
time - or write the number on the back.
Light, non-resinous machine or
silicone oil
(do not use food oils!). It is a good idea to
use a syringe with needle for the exact application of oil droplets.
A
toothpick
or something similar, for applying tiny glue and oil drops accurately.
A large
cup
of about 4” diameter, on which the main cylinder can rest during assem-
bly.
Small
scissors
and a
cutter
or scalpel with a thin blade for detaching pieces from the
cardboard and for cutting pre-punched lines.
3.
Avoid tearing the pieces from the card-
board; it is better to cut through the con-
nectors with a knife, so that edges will be
smooth.
A
blunt-tipped instrument
for deepening the folds. A
blunt knife
or empty
ball-point
refill
will do.
A
cutting-board
made of thick, completely flat cardboard, or of wood or plastic.
4.
Wherever the cardboard is to be folded,
you will find little pre-punched cuts, which
by themselves allow folding. The folds will
turn out cleaner, though, if you press a
groove into the cardboard with a ruler and
some blunt-tipped instrument before fold-
ing (see the column to the right). Almost
all of the perforated lines will be folded
”forward”; only a few of them ”backward”.
”Backward” means that you fold away
from yourself when facing the printed side
of the cardboard. ”Forward” means fold-
ing toward yourself.
A
set square
for checking right angles. The right-angled corner of a sheet of paper
will also do.
A large
marker pen
(about 17 mm diameter), a round wooden rod or something simi-
lar with a flat end. It will serve to bend several small cardboard pieces and also for
building the latex working piston.
A small, thin pair of
pliers
or strong tweezers for fine-tuning at the end.
A few
paper clips
or clothes pegs, a
pencil
, some
cello tape
, a
rubber band
and a
bit of thin
sewing
thread
.
This kit contains:
5.
Areas marked grey indicate places on
which something is to be glued. When you
want small gluing surfaces to bond
quickly, try this: apply glue liberally to one
side, connect both surfaces for an instant
so that the glue covers both equally, dis-
connect them and blow 2 or 3 times on
both surfaces. Now press the two parts
into their correct position with force – the
bond holds instantly.
Four printed and pre-punched cardboard sheets, 0.5 mm thickness
One round aluminium plate, 126 mm diameter (main cylinder base)
One aluminium plate with two holes, 126 x 126 mm (main cylinder lid)
Two transparent PVC strips, 0.5 mm thick, 18 mm wide (main cylinder wall)
Two thin brass tubes, 18 mm length (casing for the displacement piston rod and
mounting for the working piston rod)
One silicone tube, 110 mm long, (connects the piston rods and holds the axle bearing
discs in place)
6.
Especially the larger, flat surfaces
should be gently pressed, to avoid any
warping. Use a few books, on a plane
surface, for example.
One latex glove (latex seals for the working piston)
One foam disc, 113 x 8 mm with bore hole (displacement piston)
Three bent pieces of spring steel wire, 1 mm thick, with small hook (piston rods)
One bent steel wire, 1.5 mm thick, with two projections, length 117 mm (crankshaft)
Four large PVC discs, with bore hole (axle bearing discs for crankshaft, displacement
and working pistons)
Eight small PVC discs, with bore hole (guide discs for axle bearing discs)
Important Notice: Like any engine, the Stirling engine must be assembled with great care
to ensure its good functioning, especially since it is designed to run on the minimal heat
supplied by a cup of hot water. The two most important conditions for success are that the
main and working cylinders be airtight and that all moving parts run smoothly and with lit-
tle friction. Please pay special attention to these two aspects.
Take plenty of time and have patience, especially with the fine-tuning after assembly is
done. If you do, you will be rewarded with a beautiful model with a very long running time.
Important assembly steps and tests are highlighted just like this paragraph.
Assembly
The construction consists of
64 steps
, di-
vided into sections
A
through
O:
Section A:
The Flywheel
Even though the flywheel will be attached
to the engine only at the very end, it is
needed for the gluing of the main cylinder
wall right at the outset.
black now. Paint the outsides of the main
cylinder, that is, the prettier surfaces, pre-
vent any paint from getting into the centre
hole and allow the paint to dry well.
Now lay the flywheel down in the centre of
the less pretty side, which will now protrude
about 3 mm on all sides. Take a thin pencil
or pen and run a line along the circumfer-
ence so that a circle is visible on the metal.
Use sanding paper to gently roughen the
metal surface both along and inside this line.
This way the glue will form a stronger bond
and the roughened surface will give off more
heat to the air inside the cylinder. Renew
the circle now. It will guide you in applying
the 2-component-glue. While you are at it,
also clean and degrease the other metal
plate, the future lid, and the two PVC strips,
the cylinder wall-to-be.
Step 8:
Put the flywheel on your work sur-
face and connect the two ends of the cylin-
der wall without glue to form a ring. It should
fit tightly around the flywheel. If need be, you
can shorten the PVC strips a little. Take note
of the depth to which the end of the strip
needs to be inserted into the slot, so that
the flywheel sits snugly and is held tight. Glue
the strip into the joint without the flywheel
inside. Check once more that the cylinder
wall edge runs level over its complete length.
Clamp the joint and allow to set.
Step 1:
Mark a vertical pencil line on the
grey front of the two centre pieces of the
flywheel (A1) and (A2) and on the white back
of the outer and inner pieces, (A3) and (A4).
Now separate the pieces from the cardboard
sheet and remove the superficial material
between the spokes. Save these little pieces
for fine-tuning later on (see the chapter on
tuning tips).
Step 9:
Push the flywheel into the cylinder
wall once more so that it is held tightly. You
may have to use little bits of cardboard to
wedge it in place. It should rest just above
the middle and should not touch either of
the edges, top or bottom. The flywheel forces
the wall to take on its final circular shape.
The flywheel will be removed during step 13
after gluing the wall onto the cylinder base.
Step 5:
Glue the middle piece (B2) of the
cylinder wall joint on the centre of the back
side of the outer piece (B1). The middle piece
has the same height but only about a third
of the width of the outer piece, leaving about
6 mm free on each side. Do not allow glue
to run out onto this free area. Now glue the
second outer piece (B3) onto the centre
piece. You now have a square joint with a 6
mm deep slot on each side, where the ends
of the PVC strips will fit nicely (see illustra-
tion 1). Repeat all with parts (B4), (B5) and
(B6) of the second joint.
Tip:
The pencil line marks the fibre direc-
tion of the cardboard material. All paper
and cardboard materials have increased
flexibility in one direction and less of it
when turned 90 degrees. If you take this
fact into account before gluing, you can
achieve a plywood-like resistance to warp-
ing in the finished flywheel.
Step 10:
On the less pretty side of the cylin-
der lid, stick a piece of cello tape over the
small hole in the centre, then lay the base
on your work surface with the cello tape fac-
ing down. Gently round one of the ends of
each of the small brass tubes with sandpa-
per. This will facilitate slipping on the silicone
tubes later on. Put one tube aside and in-
sert the other into the hole of the cylinder
lid. This will be part of the casing for the dis-
placement piston rod. Don’t glue it on yet –
wait until after the next step.
Step 2:
The crankshaft hole in the centre of
the four flywheel parts is only pre-punched,
because of its small diameter. Use the crank-
shaft to punch through the holes from be-
hind. You can also use the cutter to deepen
the cutting line by making careful incisions.
Step 11:
Mix a sufficient amount of 2-com-
ponent-glue and apply a continuous and not-
too-thin line to the main cylinder base. It
should be several millimetres wide and cover
both sides of the pencil marking, where the
cylinder wall will rest. With the flywheel in-
side, set the cylinder wall on the glue line,
turning it clockwise and counter clockwise
a bit to ensure complete contact with the
glue. Now take a close look and inspect the
contact line all around the cylinder wall – if
you find any gaps or insufficient amounts of
glue, close them with drops of glue. If you
want to be sure that the wall is sitting tightly
on its base, weigh it down by carefully lay-
ing a book on top.
Step 3:
Glue the unprinted sides of the two
flywheel centre parts (A1) and (A2) together
in perfect alignment, making the pencil
marks run parallel - giving the two parts the
same direction. After drying, the outside (A3)
and the inside piece (A4) are glued onto the
centre pieces – but with their pencil marks
turned one spoke further. Press and allow
to dry well.
Abbildung 1
Step 6:
Lightly sand the ends of the two
transparent PVC cylinder wall strips at both
ends, and on both sides, for a width of about
4 mm. Without this sanding, the plastic sur-
face would be too smooth for the glue to
bond well.
Section B:
Maincylinder wall and base
Tip:
The next two steps are all about mak-
ing the cylinder wall just large enough so
that the flywheel, serving as a temporary
model, will fit in tightly.
The circular aluminium piece will become
the base of the main cylinder, the other one
the lid.
For production reasons, the metal is liable
to have one good side and one with
scratches. If so, simply turn the less pretty
side to the inside of the cylinder during as-
sembly. The wall of the main cylinder is made
of two strips of transparent PVC, which are
shaped into a ring and glued onto the base.
In order to get this ring onto the metal in a
perfectly round shape, the flywheel is
needed.
Step 7:
Introduce a bit of glue into the two
slots of one of the cylinder wall joints. Now
insert one end of each of the two PVC strips
into each of the slots to a depth of about 5
mm – not going all the way to the end – and
wipe off any extra glue. You should now have
the two strips connected by the joint to a
total length of about 37 cm. Now glue the
second joint to one of the free ends, but do
not yet connect the last end to form a ring.
Check that the strip runs level through all of
its parts without corners at the joints by gen-
tly knocking it on a level surface with its edge.
If necessary, clamp the joints with clothes
pegs or paper clips and allow to dry.
Test:
Before putting it aside to dry, have
another look to ensure that the wall has
not moved and is sitting right in the centre
with equal distance to the base edge all
around.
Step 12:
Use a toothpick to put a small
amount of 2-component-glue in the corner
that the brass tube forms with the cylinder
lid. Turn the tube a bit and also take it up
and down so that the glue covers the entire
contact surface. The cello tape will keep the
tube from looking out of the bottom of the
lid.
Step 4:
Remove any protective foil from the
two aluminium pieces. Degrease and clean
the circular aluminium base with a bit of cloth
and alcohol. If you’ve got solar energy in
mind for your engine (see section O, tuning
tips), you can paint the aluminium plates
 Test:
Before the glue sets, check that the
tube is vertical with the help of a set square
or a right-angled piece of paper. The tube
must be at 90° on all sides. The piston rod
casing has to be exactly vertical on the
cylinder lid, otherwise the displacement
piston in the main cylinder would not run
parallel to the cylinder base and lid later
on.
Section C:
The Displacement Piston
The displacement piston consists of the
foam disc, 8 mm thick, and a cardboard
mounting (C1 to C14) (see ill. 3), which will
fit into the hole in its centre. The mounting
holds the wire piston rod.
Step 19:
After having set, insert the mount-
ing into the hole in the foam disc, which will
stretch to accommodate the block. The pis-
ton rod will rest in the very centre of the block
now.
Test:
Push the displacement piston rod
into the brass tube in the cylinder lid. Turn
everything vertical and spin the piston on
its axis by twisting the wire rod. You will
see at once whether the foam disc really
is perpendicular to the piston rod, or
whether it wobbles instead. If need be,
make adjustments. Take this opportunity
to once again check and perhaps correct
the right angle between brass tube and
cylinder lid.
Abbildung 3
Step 13:
After drying, remove the flywheel
from the main cylinder and do another visual
check: the glue should form an airtight weld
between cylinder wall and base. You can now
remove the cello tape from the cylinder lid
and check, from above, with one of the 3
thin wire piston rods, whether it can move
freely and easily within the tube. Push out
any obstructing glue with the wire.
Step 20:
Remove the displacement piston
from the piston rod casing and glue the card-
board mounting into place by applying a few
drops of glue between the outer cardboard
discs and the foam with a toothpick. Do a
final check on the right angle as described
above – the good functioning of the engine
will, among other things, depend on it.
Step 14:
Since it isn’t needed anymore as a
gluing model, we can finish constructing the
flywheel now. Separate the three parts of the
flywheel axle mounting (A5), (A6) and (A7)
from the cardboard, deepen the fold lines
marked with small cuts and fold them forward.
The two very close lines are fold lines, too,
and are also folded forward. The wedge-
shaped mounting lashes – you can identify
them by their black lines and screws – will
be glued onto the flywheel later. One each
of the other two lashes marked with a dou-
ble line must be glued to its mate, which is
part of another of the 3 mountings. In this
way, you will get something like a three-
pointed star, with gluing lashes, marked by a
line and screw, at its lower edge (see ill. 2).
Step 16:
With the end of one of the 3 piston
rods, remove the cardboard bits from the
hole in discs (C1) to (C10). Discs (C11) and
(C12) have a partially punched slot instead
of a hole, running from the centre to the
edge. Cut the slot free, dislodge the card-
board bits inside and then glue all 12 pieces
together, so that the two slots lie above one
another at one end (see ill. 3). Continue with
the next step right away.
Step 21:
Push out the cardboard bits from
the holes in pieces (D1 to D12), which will
be the mounting for the piston rod casing.
First, glue the large disc (D1) on the cylin-
der lid by threading it onto the brass tube. If
it does not lie flat because of the glue line at
the tube’s base, enlarge the hole in the disc
a little. Over this, glue the rest of the larger
discs (D2 to D8), and then the smaller ones
(D9 to D12). A small, solid block should be
the result; it stabilizes the piston rod casing.
Step 17:
Before the glue sets, insert the long
end of one of the piston rods into the block
of cardboard discs, so that the small hook
at the end rests in the slot. Glue the printed
large disc (C13) right onto this slotted end
of the block and centre it. The hole in the
piece’s centre can be ignored. Try putting
the long wire end, from beneath, into the
brass tube in the centre of the main cylinder
lid. By moving the cardboard block back and
forth, make sure that it rests flat on the metal
and does not wobble when you spin the pis-
ton rod. In this way you can be sure that the
piston rod is perpendicular to its mounting.
Abbildung 2
Step 15:
From below,
insert the long end of
the crankshaft into the centre of the star,
where the parallel pre-punched lines are. You
are making a channel for the wire. Pull the
mounting off again, insert the crankshaft from
the other side and put glue on the gluing
lashes. Now insert the crankshaft into the
hole in the flywheel centre – on the side with
markings for the mounting lashes. Next, push
the mounting onto the crankshaft, against the
flywheel, and glue the 3 pairs of lashes onto
their marked spots. Take care that the fly-
wheel sits at a right angle in respect to the
crankshaft, so that it won’t wobble later on
when turning. After drying, you can remove
the crankshaft. The flywheel will be attached
to the engine only at the very end.
Important:
The long wire end must be per-
pendicular to the block. The piston rod
needs to stand vertically on its mounting.
Step 18:
Now push disc (C14) on the piston
rod without glue, as in ill. 3, and test the over-
all height of the cardboard mounting by
putting it next to the foam disc. Height will
vary depending on the glue amounts used,
but it should not be higher than 7, at the most
7.5 mm – but in any case less than the 8
mm thickness of the foam. If necessary, re-
move the last, small cardboard disc from the
block again with your knife. Then glue disc
(C14) on. The foam disc will be installed on
the mounting in the next step, after it has
dried.
Tip:
Before we complete the main cylin-
der, we need to assemble the displace-
ment piston and the working cylinder along
with the working piston in the next sections.
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