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UNDERST NDING ST RL N ENGINES
by
William
Beale
l lus trate d by
Fred L
e l t s ley
Technical
Reviewers
David M Berchowitz
Michael F
Feeney
Robert C Wagman
Francis E Woodling
ubl:i.
shed by:
Volunteers in Technical
Assis tance,
Inc. VITA)
1315
North Lynp t ree t Suite 200
Arlington, Virg in ia 22209
US
Telephone 703 27j-1800
Cable
VITAINC
Telex
440192
TITAUI
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PREF E
This paper i s one of
a
s e r i e s
publ i shed
by
V olun teers in
Techn ica l A ss is tance to provide
an
i n t roduc t ion
to
spec i f i c
s t a t e o f t h e a r t t echno log ies
of
i n t e r e s t to
people
in deve l -
oping
c oun t r i e s . The papers a re in tended
to
be used as guide -
l i n e s to
help
people choose t echno log ies
t ha t are s u i t a b l e
to
t h e i r
s i t u a t i o n s .
They are
not
in tended
to
provide
cons t ruc-
t ion o r
implementa t ion
d e t a i l s .
People
are
urged
to con tac t
VIT
o r
a s im i l a r
organ iza t ion fo r
fu r the r informat ion
and
te ch n ic al a ss is ta nc e i f they f ind t ha t a p a r t i c u l a r t echno l -
ogy
seems to meet
t h e i r
needs
The papers
in the
se r
ie s were wr
t
t en rev iewed and us-
t r a t ed a lmost e n t i r e l y by
VIT
Volunteer te ch nic al e xp er ts on
a
pure ly vo lun ta ry
ba s i s .
Some
500
vo lun tee rs
were
involved
in the produc t ion
of
the f i r s t 100 t i t l e s i s sued c on t r ibu t -
ing
approximate ly
5 000 hours of t h e i r t ime VIT
s t a f f
i nc l
uded
Lesl
ie
Got
t scha lk as p r imary ed i t o r Ju l ie Berman
handl ing t ype s e t t i ng
and
l ayou t
and
Margare t Crouch as
p r o j e c t manager
Will iam
Beale author
of t h i s
pape r i s
p re s i d e n t of Sunpower
Incorpora ted .
e has
des igned developed manufactured
and
marketed S t i r l i n g engines in
Bangladesh and o th e r
developing
c o u n t r i e s
and
has publ i shed widely in the s o l a r energy
f i e l d .
~ v w r s David
M Berchowi tz
Michael
F Feeney
Rober t
C
Wagman
and
Franc i s
E
Woodling
are
also
s pe c i a l -
i s t s in
the
a rea . A r t i s t Fred Hel t s ley has an engineer ing
background
and
i s a p ro fe ss io na l te ch nic al i l l u s t r a t o r on a
c o n su l t a n t ba s i s .
VIT
i s
a p r i v a t e n on pro fit o rg an iz at io n t ha t suppor ts
people working on t e c h n i c a l problems
in
developing
c oun t r i e s .
VIT o f f e r s informat ion and ass i s tance aimed a t he lping
i nd iv idua l s
and groups
to s e l e c t
and implement t echno log ies
appropr i a t e
to
tb e i r s itu a t io ns .
VIT
main ta ins
an
i n t e rna -
t i o n a l
Inquir y Serv ice
a spec ia l ized
documenta t ion c e n te r
and
a
computer ized
r o s t e r of vo lun tee r t echn ica l
c o n su l t a n t s ;
manages
long te rm
f i e l d p ro j ec t s ;
and
pub l i shes
a
va r i e ty
of
t e c hn ic a l manuals
and
papers . For
more
informat ion about VIT
s e rv i c e s in
g e n e ra l
or
the
technology presented
in t h i s
paper c o n t a c t VIT a t 1815 North Lynn S t r e e t Sui te
200
Arl ing ton
Vi rg in ia 22209
US
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STIRLIN ENGINES FOR EVELOPING O U N ~ R I S
VIT olunteer William eale
INTRO U TION
S t i r l i ng
e n g i n e s
are
ex t e rna l
c o m b u s t i o n
e n g i n e s
t ha t
use
a i r
or o the r g a s p s a s
wo rk i n g f l u id . They can bur n any
so l id
or
l iqu id fue l a s t he i r heat source . T h i s makes them
v e r y
a t t r a c t i v e ,
pa r t i c u l a r l y
in
s i t u a t i on s
where convent ional
fue l s are
e x p e n s i v e
and
h ard
to
ob t a in . B e c a use some
t y p e s
o f
S t i r l i ng e n g i n e
a re so
s i m p l e
to
make
and ye t so e f f e c t i v e ,
t h e y are exce l l en t choices f o r
power genera t ion in d ev elo p in g
coun t r i e s .
This
p a p e r de sc L ibe s
th e bas ic S t i r l i ng engine ,
a s w e l l a s
some
of
th e
most
p r o m i s i n g modern va r i e t i e s . The i n t en t
h e r e
i s to f ami l i a r i ze p e o p le in d e v e lo p in g count r i es wi t h
the
eng ine s
opera t ion
and
r a n g e
o f
app l i ca t i ons .
HISTORY
The S t i r l i ng
e n g i n e
was i n v e n t e d
by
R o b e r t
S t i r l i ng ,
a Scot
t i sh
minis te r in 1 8 1 6 . The ea r l y St i r l ing e n g i n e had
a h i s
to ry
of
good se rv ice and long l i f e u p to 20 yea r s ) .
I t
wa s
use d a s a
r e l a t i ve l y
l o w- p o we r wa te r - pum ping
e n g i n e
from the
m i d d l e
of
the nine teenth cen tu ry to a b o u t
1 9 2 0 ,
when th e
in te rna l
c o m b u s t i o n
e n g i n e and th e e l ec t r i c m o t o r rep laced
t The ho t - a i r e n g i n e
was
known f o r i t s e a s e o t o pera tio ni
i t s
ab i l i ty to u se any b u r n a b l e m a t e r
i a l
a s fue l ; i t s sa fe ,
qu ie t ,
m o d e r a t e l y
e f f i c i e n t
Jpe ra t ion;
and
i t s
durab i l i t y
and
low m a i n t e n a n c e requi rements .
I t
was v e r y l a rge f o ~ i t s s m a l l
power output ,
h o wev er, and
had
a
h i g h p u r c h a s e cos t .
N e v e r
t h e l e s s , i t s lo w
opera ting cos t usua lly
j u s t i f i ed
ch o o s i n g
t
o v e r the steam eng ine - - the
o n l y
a l t e rna t ive a t t h e t ime-
wh i c h bur ne d much more fue l fo r th e same power and
demanded
cons t an t a t t en t ion to
av o i d
d a n g e r o u s explosions o r othe r
f a i l u r e s .
The
o the r
m aj o r d isa dv an ta ge o f th e
ear ly ho t - a i r
e n g i n e was
i t s t e n d e n c y to
f a i l
i f th e hea te r
head
g o t to o hot .
T h i s
was
a r e su l t o f th e r e l a t i v e ly lo w h e a t r e s i s t ance o f th e ca s t
iron hea te r
h e a d .
The
p ro b l em
was
overcome
by
redesigning
th e
burne r , which p r e v e n t e d th e e n g i n e from overhea t ing .
This
i m p ro v em en t
r e su l ted
i n sa fe ,
b u t
even l o w e r , power opera
t i on . Despi te t h i s
i m p ro v em en t ,
th e St i r l ing e n g i n e c o u ld n o t
co m p et e wi th th e cheaper ,
more p o w e r f u l i n t e rna l combust
iOT l
eng ine ,
and
i t
disappeared
from th e
co m m erci al
s c e n e .
The a d v e n t of newer
lA_ der s tand i ng o f th e
s ta in le ss s te e ls
eng i ne s complex
1
and a d v a n c e s
thermodynar.1ic
in the
process
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brought
new a t t en t i on to the
engine
dur ing
and
a f t e r
World
War I . The performance o f the o ld ho t - a i r eng ine was im-
proved
and
i t s s ize
and
cos t
were
reduced. I t s
s impl
i e i ty
of
cons t ruc t ion
and
ope ra t ion ,
and most impor tan t ,
i t s ab i l i t y
to use
rough
fue l s
were r e t a i ned . These
e f fo r t s on
s t i r l i ng
eng ines
were almost exc lus ive ly aimed
a t
d i f f i c u l t
app l i ca
t io n s th a t
were not appropr i a t e
for developing coun t r i e s -
namely,
~ n
advanced
automot ive
eng ine ,
space
power,
and
a r t i f i c i a l hea r t s . Almost no e f f o r t was put in to the
r e l a
t i ve ly easy
t ask
of
des igning an engine fo r
o rd ina ry uses .
The
h igh ly
d ev elo pe d c ountI ies in which
the
S t i r l i ng engine
work
was
being
done did
not
need
s imple eng ine ,
so
t he re
was
no economic i ncen t ive to
des ign
one .
This s i t u a t i on
changed in 1980, when th e U .s .
Agency
fo r
I n t e r na t i ona l
Development
USAID) funded the
development
o f
s imple S t i r l i ng engine
spec i f i c a l l y
in tended for manufac ture
and
use
in developing coun t r i e s . The engine was des igned ,
bu i l t ,
t e s t ed , and de l ive red to Bangladesh , and copies
of
i t
were
bu i l t
and
put
in to
opera t ion
t he r e .
This
demonst ra ted
th e pos s ib i l i t y
of the
eng ine s manufac ture
in
s imple machine
shops o f
the type found in many r eg ions of Afr i ca , Asia , and
Lat in America .
As
r e su l t
of
t h i s and o the r r ecen t deve lopments ,
the for
merly
dim prospec t s
fo r the
app l i c a t i on of S t i r l i ng
eng ines
in
deve loping
coun t r i e s
have
improved
enormously .
Plans
are
now in
motion
to b r ing new des ign of the S t i r l i ng
engine
n t ~ commercia l product ion in much improved form. This
modern
vers ion
wi l l be much more powerful fo r i t s weight
and
much more
e f f i c i e n t ;
a t
the same
t ime , i t wi l l
be as qu i e t ,
easy to use ,
r e l i a b l e ,
and
rugged as
the
o r i g i na l
eng ine .
Addi t iona l
models ,
capable
of genera t ing
e l e c t r i c i t y ,
coo l
ing , pumping
water ,
and serv ing
in
o the r usefu l
ways
not
pos s ib l e
with
the old
ho t - a i r eng ine ,
a re also
coming in to
commercia l produc t ion .
NEEDS SERVED Y THE TE HNOLOGY
Although th e S t i r l i ng
engine
i s an o ld machine, modern
mate r i a l s
and
des ign methods make i t much lfi0re a t t r a c t i v e
than
ever
befo re . The c rank-d r ive S t i r l i ng engine i s d e f i
n i t e l y
usefu l to
anyone
who
has
so l id fu e l .
This
type
o f
S t i r l i n g engine
can burn any
l o ca l
fue l as
i t s
source
of
heat
to produce
e l e c t r i c i t y ,
pump water , or perform ta sk s req l i r
ing mechanica l power
such as
food
process ing .
Very
s imple machines using atmospher ic
can be bu i l t
from l o ca l
mate r i a l s such
People
who are
i nc l ined to t ry
such
chance
of
success .
a i r
as working f lu id
as
metal con ta ine r s .
des igns have
good
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OPER TING PRINCIPLES
SIC
THEORY OF
THE
TE HNOLOGY
The
S t i r l i ng
cycle i s
shown
in the
diagram
in Figure 1. 1 he
bas i c
idea
is t ha t when
gas
in a closed
cyl inder
i s moved
in to the
hot
par t of the cyl inder i t expands
i t s
r s s ~ r
i nc reases
and
i t
can
do
work.
When
the
gas
moves
in to
the
cold pa r t of the cyl inder i t s pressure i s reduced. Once the
gas
reaches the lower pressu re
t
i s compressed back to
i t s
or i g i na l volume. The gas
performs
more work during i t s expan-
s ion than i s requ ired to be put in to
i t
during
i t s
compres-
s ion . Thus
the
en t i r e cycle r e su l t s in the net pos i t ive
ou tpu t o f work.
As shown in
Phase
1
of
Figure 1 the
pis ton i s out bottom
dead cen t e r and the d i sp lace r i s in as fa r as i t can go.
The
g
as i s
in
the
cold
space
and
the
g as pressure is low.
Note
t ha t the
gas is a t the
same
pressure a t any
i n s t an t
in
every
pa r t
of the engine
but
t ha t
t h i s
pressure
is
changing
wi th
t ime . Because
the
pressure i s
low the pis ton can be
moved
in eas i l
to
compress
the
gas a t the low t empera ture .
At
the
end
of
t h i s compression process
the
engine has
reached Phase 2 as
shown
in Figure 1.
w t i s t ime to
increase the
gas
pressure . This
i s not done
by burning a
fue l
ins ide
the gas as
i s
done in
an
in te rna l
combust ion
engine .
The gas i s moved from
the
cold space
through a se r ie s of
hea t
exchangers which cause i t to ente r
the
hot space
a t
a high
t empera ture .
Note t ha t the gas
in
the
hea te r coo le r r egene ra to r and hot and cold spaces is
always
a t
the same
pressure a t any
in s tan t s ince the gas
flow
passages
are
l a rge and do not
r e s t r i c t
the passage of
the
gas .
As shown in
Phase
3 of
Figure
1 the gas is compressed hot
and
a t high
pressu re .
At th i s
poin t
i t i s ready
to
expand and
to work on the pis ton .
As
the
pis ton
moves out of
the cy-
l i nde r
the
d i sp lace r moves with t
in
order to keep as much
of
the gas as
poss ib le
in the hot space so t ha t the pressure
i s kept as
high
as poss ib le to do the
maximum
amount of work
on
the
p i s ton . Th i s expans ion
and ou
tward
movement of the
pis ton r e su l t s in
the
a t t a inmen t of Phase 4
as shown
in
Figure
1.
The next s tep is to reauce the gas pressure by
moving t
from
the hot space
through
the
hea t
exchangers to the
cold
space.
This i s
done
by
moving
the d i sp l ace r
from
i t s pos i t i on
as
shown in PhasE:
4
back to i t s inward pas
i t ion
as shown in
Phase 1.
The cycle
is now complete . Not
ice t ha t
the pis ton
has
expanded
the
gas
by moving
outward when the gas i s hot
and a t high
pressu re
and has
compressed the
gas
when i t
i s
cold and a t low
pressure .
Thus the or ig i na l plan has been
3
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Cyl inder
~ ~ ~ e a t in Disp lace r
Regenera tur
Heater
Head
Phase 1: Pis ton a t
bottom
dead c e n t e r . Disp lace r a t top
dead
cen t e r . All gas
in
cold
space .
-
-
Phase
2: Disp lace r remaining a t top
dead cen t e r .
Pis ton has
compressed
gas
a t lower t empera ture .
/ - - = = r ~ : > > : ~ = = = ~ l
f
. :::-:>}-
~ I ~ ~ ~ ~ ~ \ ~ ~ ~ ~
_
Phase
3: Pis ton remaining a t
sh i f t ed gas through
in to hot
space .
top dead c e n t e r . Disp lace r
has
c oo le r r e ge ne ra to r and
hea te r
Phase
4: Hot
gas expanded. Disp lace r and
pis ton have
reached
bot tom dead
c e n te r
t oge t he r . With pis ton
s t a t iona ry
d i s p l a c e r now forces
gas
through hea t e r r e g e n e r a t ~
o r and coo le r
in to cold
space
thus
r e a t t a i n i n g
phase
1.
Figure 1. The
S t i r l i ng
Engine Cycle
Source : G.
Walker , S t i r l i ng
Engines.
Oxford,
England:
Oxford
Univers i ty Press
1980 .
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accomplished,
and
the
cycle has
produced net work to
the
ou t s i d e .
OL
t h i s four-phase
process
to con t inue
i nde f in i t e ly ,
hea t
must
be con t inua l ly added to the hot
hea t
exchanger
froln
sOlne
outs ide
source l i ke a
f i r e
or a
so l a r
co l l e c to r , and
the
cold
end must be con t inua l ly cooled
by
a
stream
of water or
a i r .
You might now wonder how the movements of the
pis ton
and the
d i sp l a c e r are
accomplished,
s ince they c lea r ly cannot move on
t h e i r own. The answer i s t ha t
the re
are a t l e a s t two
ways
to
make
th e two
components of the sim ple S t i r l i ng engine move as
we
wish:
1 we
can
a t tach them to cranks through connec t ing
_ods as
i s
commonly
done in
automobi le eng ines ; or \2 we
can use gas fo rces in a ca re fu l ly designed way so t ha t they
bounce
on
gas sp r ings , with
the d i sp l ace r
always ahead of the
p i s t o ~ in i t s i n -and-ou t os c i l l a t i on . Of the
two
methods, the
use of cranks
ca l l ed
the c rank-d r ive , or kinemat ic
S t i r l i ng ,
i s the
more eas i ly
unders tood
method.
The
second m ethod,
which
uses
osc i l l a t i ng
motions
of the
p i s ton
and
d i sp l ace r
on
sp r i ngs , i s ca l l ed the f r ee -p i s t on S t i r l i ng .
The c rank-d r ive
S t i r l i ng i s eas i e r
to
unders tand ye t harder.
to
make,
while
th e f r ee -p i s ton
S t i r l i ng
i s harder to understand y et eas i e r
to
make
in a t
l e a s t
some of i t s
forms.
DESIGN V RI \TIONS
This sectiol1 of the paper desc r ibe s a var ie ty of promising
S t i r l i ng engines .
I t
emphasizes t h e i r physica l char-acter- is
t i c s ,
advantages
and d i sadvan tages ,
app l i ca t i ons ,
and fuel
e f f i c i e n c i e s .
TYPES STIRLING ENGINES
rank Drive St i r l ing Engine
A schematic of the c rank-d r ive
S t i r l i ng
engine i s
shown
in
Figure 2, and
a crank-dr- ive St i r - l ing engine pumping water i s
shown in
Figure 3.
while t h i s engine
i s
unusual ly l a rge
fo r
th e
smal l
amount of power 5 ki lowa t t s i t pr-oduces, i t
i s
neve r t he l e s s
very sim ple to make and ope ra t e .
uses no o i l
in the
cr-ankcase;
here
i t
i s
impor tant
to
avoid
get t ing
o i l
in to
the
hot
working pa r t s
of the
engine ,
because
i t could
block
th e flow of a i r
thr-ough
the hea t exchanger-s and
a lso
cause an exp los ion . Any of the fo llow ing th ree types of
bear ings can be used: sea led
r o l l e r
bear ings ,
ba l l bear ings ,
o r u nlu br ic a ted
bushings made
of a
p l s ~ i l ike
Tef lon .
I f
necessa ry , the
ba l l and
rol ler- bea r
ings can
be r-eplaced by
j o u rn a l bea r i ngs and sea l ed in g rea s e .
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Displacer
iston
Cooler Regenerator.
Heater Head
Side
View:
Engine
Assembly
Q
Q
c
Q
c
.
0
t
til
Bottom
View: Engine Assembly
1. Primary
combustion
sect ion
2. Ash separat ion
3. Secondary
a ir manifold
4. Secondary combustion sect ion
5.
Engine heater head
6. Ash removal
Burner
Design
Figure
2.
Schematic
of
the
Crank-Drive
St i r l ing
Engine
Not drawn to
same
scale as
s
ide and bot
tom v
iews of
en ;] ne
assembly.
Source:
Gary J .
Wood e t a l .
Des ign of
a w
Pressure
Ai r
Engine for Third World Use, paper presented at the
17th Annual
In tersoc ie ty Energy
Conference, Los
Angeles, Cal i fornia
August
1982.
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_ ~
Figure A Crank-Drive St i r l i ng Engine
4 Power
Output Pumping Water
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Since the engine i s s l ig h t ly pres su r i zed , up to about 4
atmospheres
a tm , t uses a
s imple
crank sha f t
s ea l
tq keep
th e
a i r in , and a smal l a i r pump to maint-.ain the
pressure
aga ins t slow l eakage
pas t the
s ea l .
The
a i r pump
as wel i as
a l l o th er a cc esso rie s needing power a re d r iv en d i r ec t l y from
the ro t a t i ng engine sha f t .
Other
accesso r i e s
r equ i r ing sha f t power a re the auger feed ing
the fue l , the combust ion
a i r
blower , and the coo ling water
pump and
r ad i a to r
fan . With
t hese
accesso r i e s ,
the
engine i s
able
to
work
wi
t hou t any o the r source o f power, and needs
only fu e l to ope r a t e .
Typica l opera t ing
i n s t r u c t i on s a re as fo l lows :
1. Make sure
the
engine i s in good
opera t ing
condi t ion and
the
hopper
i s fu l l
o f
f u e l .
2. S t a r t a f i r e in
the
burner
with
kindl ing e . g . , wood shav
ing s , d r ied l e aves , and opera te the
a i r
blower by hand
un t i l the i n t e r i o r
of
the burner
i s
su f f i c i en t l y hot to
r ece ive
and i gn i t e
the fue l
from the
fue l
feed .
3 .
Hand
o p ~ r t
the combust ion
a i r blower
and the fue l
auger
unt l the hea t e r
head
of the eng ine
reaches
a
moderate
t empera tu re abou t 300C . The
engine i s
now ready to
s t a r t .
4 .
Turn
the
f lywheel
over ,
and
the
engine
should begin to run
on i t s own power immedia te ly
easy
s t a r t i ng i s one of the
bes t
f ea tu re s
of t h i s eng ine .
5 .
Allow
a sho r t t ime fo r the engine to
pres su r i ze
i t s e l f
and
to dr ive
the
burner un t i l i t i s a t f u l l
opera t ing
pressure
and
t empera ture .
During
t h i s t ime,
the engine wi l l
grad-
ual ly grow s t r onge r and more capable to do work. The load
can be
increased
as the
engine grows s t ronge r . This hap
pens
au tomat ica l ly i f the engine i s
a t t ached
to loads such
as cen t r i f uga l
water
pumps
or
gene ra to r s , bu t
loads
such
as
saws and mil l ing
machines
have the
capab i l i t y to
s t a l l
the engine i f t he i r load i s appl ied
too quick ly . I f
the
engine i s s t a l l ed ,
t can
be r e s t a r t ed immediate ly by
unloading
t
and t u rn ing the f lywhee l aga in .
6 . nc e the eng
in
e i s
up
to
fu l l power n
d do i ng i t s
r k
the opera tor
needs only to
keep the
fue l hopper f u l l
and
mainta in a l oad . I the load i s removed for any reason ,
the
engine wi l l
speed
up, but not to a harmful
degree ;
t
wi l l quick ly reach a speed a t which i t s power ou tpu t drops
to zero , and t wi l l cont inue to run .
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7. When it i s t ime to shu t o ff the eng ine , simply
cu t
o ff the
fue l
and
the
engine wi l l
s lowly
come to a
s top .
I t
can pe
s topped more quickly a t any t ime by
re leas ing
the i n t e rna l
pressu re , which reduces
th e
power
to
a
low va lue .
S ince
the re
a re very
few c r i t i c a l ad jus tmen ts of fue l , a i r ,
o r water f low and the re i s no fue l i n j ec to r or spark system;
the c ran k-d riv e
S t i r l i ng
engine
i s
extremely
r e l i a b l e
and
easy to
run .
But in order to achieve max imum performance it
i s
impor tant to
make co r r e c t
adjus tments of these
f lows
which
someone
with
only a
little exper ience can do
ea s i l y .
Because of i t s ease of
opera t i on ,
du rab i l i t y , l oca l manufac-
t u r a b i l i t y ,
and the ab i l i t y
to
use any l oca l
fue l
as i t s
hea t
source ,
the
modern
c rank-dr ive
S t i r l i ng engine i s remarkably
wel l
su i t ed
fo r
power genera t ion
in
developing coun t r i e s .
P lans fo r
th is S ti r l in g engine wil l
be
ava i l ab l e
from
USAID
in 1984
or 1985.
o m m e r ~ i a l
produc t ion
of the engine , or ver
s ions of it i s
expected
to begin in
1984.
impl
Free -Pis ton Engine
Figure 4
por t r ays
a s low speed f r ee -p i s t on
eng ine ,
which i s a
s imple vers ion of the S t i r l i ng
eng ine .
This engine i s almost
th e u l t ima te
in s impl i c i t y compared to othe r S t i r l i ng engine
des igns . t i s the so -ca l l ed
overdr iven
conf igura t ion in
which
the d isp la ce r is f loa ted by sp r ings and wi l l move spon-
t aneous ly e i t h e r up or down under the i n f luence
of
the
s l i gh t e s t
fo rce or
dis turbance
of the pressu re
ins ide
the
engine . The grea t advantage of th i s arrangement i s th a t the
engine i s
not
only s e l f - s t a r t i ng requi r ing
only
t ha t a good
t empera tu re
d i f fe rence be es t ab l i shed
between
the
hot
and
cold
spaces , b\lt it w il l ad ju s t to
any
load ,
even
a complete
s topping
of
the
p i s t on ,
and
still
cyc le
up
and down. Thus
the
engine
i s
very
forg iv ing and easy to ope ra t e . I t s major
disadvantage i s th a t it is too big fo r i t s smal l pO er ou t
pu t ; t h i s i s because it uses atmos?her ic a i r as working
f lu id
and ope ra te s a t a very low f requency. Counterbalancing t h i s
disadvantage
are the
very
high
lift
capac i ty and
e f f i c i ency
of the s imple pos i t i ve displacement pump which
the engine
can
opera t e .
Dimensions
The d i sp l a ce r
and
p i s ton
d iamete r
can
be the
same. ~ h e
d i sp lace r should be a t l e a s t as long as i t s d iamete r , with
a
maximum le r .g th of
t h ree
t imes i t s diameter ,
and
the end cap
should be domed to allow some
s t rength
aga ins t
co l l apse .
The
gap
between
the
d i sp l ace r
and
the cyl inde r
should be
about
one
to
two
hundred ths of the diameter , with a pre fe rence
fo r
th e sm alle r gap. ~ order to
keep
the d i sp l ace r cente red , it
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should
have
ra i sed
bumps of
the
gap
s l i gh t l y aga ins t
the
cy l inde r in
i t s
cold
t
c ~ : l s s
0 : : on
t h a t
rub
The h ea te r sec t io n l ength should be about
one
four th of th e
d i sp l a c e r d iamete r and th e coo le r
about the
same. This
l eaves one ha l f of the
d i sp l a ce r
to
ac t as a
r egene ra to r
which se rve s to
s t o re the
hea t o f
the a i r
as it
comes from
th e hea t e r
to
the coo le r and r e l e a s e s it
to
the a i r as it
comes
back
from
the
coo le r to
the
hea t e r .
This
ac t
ion
in
c r e a se s the fue l e f f i c i ency
of
the
eng ine .
The
d i sp l a ce r
movement
ava i l ab l e
should be about
one
th i rd o f
its l eng th .
The d i sp l a ce r
dr ive
rod
should cover about 15 pe rcen t of the
a rea of the
d isp la ce r c yl in de r .
The
dr ive
rod
should fit
c l o se l y in i t s s leeve but be f ree to
move.
Mate r i a l s
The
b es t m a te r ia l
fo r
the
d i sp l a ce r
hot
end
i s
anyone
o f the
300 s er ie s s ta in le ss s t e e l s such as
304
316 or 321. These
a re a lso
ca l l ed
18-8 type s t a i n l e s s the kind used in cooking
po t s .
The hot end of the d i sp l ace r cyl inde r must be of
s t a i n
le s s s te e l
a l so or
poss ib ly
ceramic i f
it can
be made 3.ir
t i gh t .
Of
course i f
only
shor t - te rm exper iments
are the aim
then ord ina ry carbon s t e e l shee t can be used fo r both
d i s
p lace r and hea te r
head.
The
d i sp l a ce r i t s e l f can be qu ite th in provided t h a t a
non-re tu rn
valve i s
i n s t a l l ed
in i t s cold
end to
allow the
i n t e r i o r to reach
the
maximum
cycle pressu re and s tay
t he re .
Otherwise
the
d i sp l a ce r
could
col lapse
under
pre s su re .
I t
i s
a lso impor tant
to make the
d isp la ce r sh el l
th ick enough to
p reven t i t s col lapse under
ou t s ide
pre s su re .
The r e s t of the engine can be of s t e e l ca s t iron
aluminum
or
whatever
i s
lo ca lly a v ai la b le
s ince
i t i s not
exposed
to
hea t .
Care should
be t aken
to
make the d i sp l ace r as l i g h t as
i s p r a c t i c a l . Othe rwise
it
wi l l respond too slow ly to gas
pre s su re and wi l l
not
develop
the
lead in motion over
the
pis ton
necessa ry to accomplish the
S t i r l i ng
cyc le .
Energy
Output
A s imple f r ee -p i s t on engine w ith a 60-cm diameter
o pe ra t ing a t
one cyc le
per
second
can
be expected
about 500 w t ~ o f power
50
l i t e r -me t e r / s e c
wate r . Of
course as
with any f i r s t
at tempt
ou tpu t could be much
l e s s .
d i sp l a c e r
to produce
of pumped
t l e ac tua l
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Exhaust
Water
Inlet
Water Discharge
10
Liter -Meter
/Sec
100 Watts
Seal
~ E = B : : : u r n e r
Fuel
Free Cylinder ~ ~ v s
I ~
w ~ I I
Up
and Down to
Actuate Water
Pump
Piston
- H ~
Displacer
Rubber
Disks
check valves
Figure Section of Free yl inder Engine
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any o th er e x te r n al e f f e c t .
t
i s very s i m p l e .
almost
as sim -
p l e
a s t h e
f r e e - c y l i n d e r water
pump and
i s very
f u e l - e f f i
c i e n t i f c a r e f u l l y d e s i g n e d .
F i g u r e shows a t y p i c a l
c r o s s
s e c t i o n o f th e
duplex s t i r l i n g
eng ine des igned
as a
h e a t
d r i v e n food
r e f r i g e r a t o r .
F i g u r e 7 shows
t in
o p e r a t i o n .
The
b a s i c i d e a behind t h e
o p e r a t i o n o f
th e duplex
S t i r l i n g
e n g i n e i s t h a t when d r i v e n i t becomes a h e a t pump.
In t h e
duplex
S t i r l i n g
a
S t i r l i n g
engine
i s
used
to
d r i v e
a
S t i r
l i n g h e a t pump. T h i s can be
done
w ith only t h r e e moving
p a r t s - - t h e hot
d i s p l a c e r t h e p i s t o n which a c t s
as
th e p i s t o n
f o r
both
t h e h e a t engine and t h e h e a t pump and
tIle
cold d i s
p l a c e r . T h i s
combinat ion
o f p a r t s
makes a
simple
and
e f f e c
t i v e
h e a t - d r i v e
h e a t
pump which can
be s c a l e d tO any
s i z e o r
t e m p e r a t u r e
r a n g e
from
very c o l d
te m p e ra tu re s n ec es sa ry
t o
l i q u i f y a i r t o mild t e m p e r a t u r e s u s e f u l f o r space c o o l i n g .
The
duplex s t i r l i n g
engine
w i l l
be commercia l ly
v i l ~ l
w i t h i n
th e next
few y e a r s
probably
as a p o r t a b l e
food-
s t o r i n g f r e e z e r - r e f r i g e r a t o r
in
smal l
s i z e s .
F r e e - P i s t o n
A l t e r n a t o r
Engine
Recent e f
f o r t s
t o
develop the f r e e - p i s t o n
a l
t e r n a t o r eng i n e
have
produced
o u t s t a n d i n g r e s u l t s . While
the
~ n g i n w i l l not
become a
com mercial i tem as q u i c k l y as t h e c r a n k - d r i v e
S t i r
l i n g
e n g i n e t
w i l l f o l l c w w ith only about a
y e a r s
d e l a y .
The one most developed a t t h e moment i s
a 1 kW
o u t p u t
machine
t h a t has
e x c e l l e n t
f u e l
e f f i c i e n c y
promises long
l i f e
and
i s very compact . T h i s machine i s not s i m p l e however c 1d
r e q u i r e s
h i g h l y
s o p h i s t i c a t e d
manufactur ing p r o c e d u r e s and
m a t e r i a l s .
On
t h e
o t h e r
hand
because
t
i s
h e r m e t i c a l l y
s e a l e d t
cannot
be damaged
by
any
s o r t o f rough t r e a t m e n t
a l th o u g h th e
c o n t r o l
system
and o t h 0 r a u x i l i a r i e s a r e not so
i n v u l n e r a b l e .
The
f r e e - p i s t o n
a l
t e r n a t o r eng
i ne
i s
i d e a l l y
su
i
t e d t o t h e
t a s k o f developing
e l e c t r i c i t y
from
s o l a r
e n e r g y
e s p e c i a l l y
when
matched to
a
l o w - c o s t
p l a s t i c
fi l m
c o n c e n t r a t o r
o f th e
type now coming on the market .
Such
machine:; a re being a c
t i v e l y
deve loped
in s i z e s up to 10 kW
and
could be a v a i l a b l e
i n even
l a r g e r
s i z e s in
a few
y e a r s . The one shown in Figure
8 has a
10 kW o u t p u t .
USES
OF
THE
STIRLING ENGINE
I r r i g a t i o n With
Biomass
Both th e c r a n k - d r i v e
engine a r e p r a c t i c a l
S t i r l i n g engine
and
f o r
i r r i g a t i o n with
4
the f r e e - c y l i n d e r
biomass provided
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w
Expansion
z
Space
z
Heater
w
ti
Regenerator
isplacer
w
ooler
)
z
o m ~ i e s s i o n
J
Space
t
Power Pis ton
ompression
t
Warm
Heat
Space
w
Exchanger
:
)
Regenerator
isplacer
z
J
hiller
Expansion
~
Space
Figure 6 Duplex S t i r l i ng Beat Driven Heat Pump
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j
i
. i
\... .-
.
I _ ~
I
I
I
I
igure
uplex
St i r l ing Heat u p
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~ 1 2 c m
Figure 8 A 10 kW
Free Pi s ton
Alterna tor E ng in e D ev el op ed
fo r Sular Power
17
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tha t
ample biomass
is
ava i lab le for fuel as
well
as cheap
labor
to
feed
the engine with fuel
and
tend i t s
opera t ion .
The crank-drive engine
i s p ra c t ica l
from about
5
watts to
tens of
ki lowat t s
of
del ivered
power but
in
power above 3
wil l requi re a wheeled
ca r t
to t ranspor t i t . The
f tee-
cylinder engine makes a
good
i r r iga t ion pump up to about 500
wat ts .
Ei ther
engine can drive
both
shallow
well and deep
well pumps as well as l ow l i f t ditch
pumps. Also
the e lec -
t r i ~
generator
free
piston
can
be
attached to
an
e lec t r ic
purr? for th i s se rv ice .
Elec t r i c i t y Generation--Small
Sizes Sol id Fuel
Both the
crank-drive St i r l ing engine and the
f ree p is ton
a l te rna tor engine are prac t ica l
for
th i s use. The f ree p is ton
a l t e rna to r
eng
ine has the advantage
of
very low
noise
and
long
l i f e but
is harder to repai r in the f ie ld . fhe crank
r i v ~
St i r l ing engine
i s
simple easy
to
r epa i r and
cheaper
and
can
be manufactured in simple repa i r shops;
however
i t
i s not as fuel e f f i c i en t .
E lec t r i c i t y Generat ion--Vil lage Power--Solid
Fuel
Here
again both the crank-dr ive St i r l ing
engine
and the
f ree p is ton al te rna tor engine
would
serve
for any power u ~ to
about
k
St i r l ing engines
of higher
power
output
are not
l ike ly in the
near future
l t h ~ u g h i t
is
always possible to
combine smaller uni ts into
a l a rger
uni t for more
power.
In
th i s appl ica t ion
constant
attendance i s required
to
assure the proper operat ion of the fuel feed and other auxi
l i a r i e s . Useful
by-products
include hot water from the cool
ing system
and
ash
from
the
burner .
Grain Process ing--Grain
Waste as
Fuel
This is an ideal appl icat ion
because of the
avai labi l i ty
of
the biomass by-product
as
fuel for the engine. The USAID-
funded
simple hot a i r engine
referenced
ea r l i e r in th i s
paper
as having been
developed
for manufacture in Bangladesh
i s an exce l len t example. Figure 9 shows th i s
hot a i r
engine
millng r ice .
I t burns
the r ice
husk
produced by
the mill
i t
dr ives . Only a f ract ion
of the
husk
produced
by
the mil l
is
needed to
fuel the
engine so
ample
amounts are l e f t
over
for
the
engine
to
use
while
pumping i r r iga t ion
water
for
the
next
r ice crop.
In
th i s
way so la r
energy
in the form
of biomass
i s used as the prim ary energy input for the r ice-growing
process
and
no outside fuel is necessary .
Solar Power
I t i s important to recognize
high-temperature machine.
I t
tha t
the St i r l ing
engine is
a
cannot
run
well
on the
low
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W
Figure 9 Rice
Husk
Fueled Simple Hot Air Engine
o Rice MiTl
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t empera tu res
ava i l ab l e
from
siITlple f l a t
p l a t e
so l a r
collec
t o r s . I t must use a
concen t r a t i ng
sun- t rack ing
so l a r
l l e ~
t o r . This dev ice adds c on sid erab ly to the cos t and main te
nance r equ i rements
of
the
system.
Also , such a device
does
not
make
use
of the
d i f f u s e component
of so l a r
energy only
the d i r e c t
component . So hazy sun i s
not
good
enough. Br igh t
c l e a r
sk i e s
are needed
be fo re the
concen t ra t ing co l l e c t o r
wi l l
develop
the
high
tempe ra tu re n ec es sa ry
to opera te
the
S t i r l i ng
eng ine .
For a l l th ese reasons S t i r l i ng
sys tems
us ing concen t r a t i ng
sun- t rack ing
so l a r
co l l e c t o r s
wi.ll be
much
more
expens ive and wi l l r equ i r e
more
care in t h e i r
opera t ion than those using fue l
as t h e i r hea t source .
w ith those
r e se rva t i ons
in mind,
t
i s r i gh t to po in t
ou t
t h a t the re are s i t u a t i on s in which
such so l a r -d r i ven
sys tems
are worthy
of cons ide r a t ion : where
i n t ense
sun
i gh t
i s
th e
r u l e
where
t he re i s
no
biomass aVai lab le and none
der ivable
from the e f f e c t s of the eng ine (as t he re would
be
even tua l ly
i f th e
engine were i r r i g a t i ng
a
formerly
dese r t
a r e a and
where
the
cos t
of the
eng ine
co l l e c t o r t r a cke r
mount,
and
maintenance
the reof
i s not proh ib i t i ve .
Such
a s i t u a t i on
could
ex i s t
where s eve ra l
k i lowa t t s of e l e c t r i c i t y
a re
needed, and the cos t of photovol ta ic sys tems i s too h igh .
I t
i s
l i k e l y
t ha t
a so l a r
e l e c t r i c
system based
on
a
f r ee -p i s t on
S t i r l i ng engine wi l l cos t consIderably
l e s s
per wat t
de l ive r ed than wil l
a
pho tovo l t a i c
system
in the ki lowa t t
range
of
power.
A cau t ionary note on so l a r S t i r l i ng systems: a l though the
S t i r l i ng engine wil l
be
commerc ia l ly ava i l ab l e in one or two
yea r s
the
concen t ra t ing co l l e c t o r s and th e i r a ux il ia rie s are
st
some
dis tance
away
from
produc t ion .
For
a l l
t hese
rea sons
so l a r
S t i r l i ng
engine systems
are l i ke ly to
be
much
more
cos t ly
than
o the r
sys tems
except where
noth ing
e l se i s
ava i l ab le as might be
the case in
ext reme dese r t
zones.
More
of ten
than no t a d i r e c t
so l a r
system
i s
le ss p ra ct ic al
than
one t h a t
uses biomass grown
with the
he lp of i r r i g a t i on
provided by the eng ine . By th i s
means,
land
t ha t
would
o tne r
wise
grow nothing could conceivably
be
made
to produce food
as
wel l
as fue l
fo r the
i r r i g a t i ng
pump.
Put s imply , a f i e ld
o f weeds, ha rves ted to be burned in the eng ine
i s
a
much
ea s i e r rou te to
so l a r power
than
an e l abora t e op t i c a l sys tem,
mount ,
and
t r acke r .
And
weeds, unl
ike
the
sun,
do
not
hide
behind c louds or go away a t n igh t .
U L EFFICIENCY AND POWER OUTPUT
The S t i r l i ng engiL2
i s
l i ke ly to burn roughly 10 ki lograms
(kg) per k i lowat t -hour kWh of biomass
fue l
and 6
kg/kWh
of
coa l . This i s le s s than the
ra te
of fue l consumption of smal l
-
-
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steam
engines .
Depending on how wel l an
opera tor
guides the
machine t h i s burning
ra te can eos i ly vary as
much
as 20
pe rcen t up or down; with wel l des igned
and
well a t t .ended
engines t
could
be as l ttl as hal f as much.
The
po\tj =r out.put per
un i t
of weight var i e s grea t ly with
the
des i gn . Genera l ly t r.anges from about .04 Kw/Kg for a s im-
p le c rank dr ive model to about
.07
Kw/Kg
fo r
a commercial
high technology
f ree p i s ton
a l t e rn a to r
engine .
V
OMP RING
THE
LTRRN TIVES
The S t i r l i ng
engine i s capable of accept ing
hea t
from any
S0urce above
about
400C
and conver t ing pa r t of
the
hea t
in to
use fu l work. This makes
i t capable
of a wide
var ie ty
of uses .
Which
of them are p ra c t i c a l and worth
cons idera t ion in
com-
par i son with
the
o the r
sources
of mechanical
energy?
I f convent ional
fue l s and
machines are
avai l ab le
and s a t i s -
f ac to ry
t
i s
probably not
p ra c t i c a l
to con side r
rep lac ing
them with a S t i r l i ng engine . Only
when
pe t ro l
or d ie se l
or
c lean gaseous fue l s
are
scarce
expens ive
or
otherwise un-
a t t r a c t i v e and
when
the s pa rk ig n it io n in te rn a l combust ion
engine
or d ie se l
engine i s
too
shor t
l ived
or too expensive
to mainta in or
purchase
i s i t
sens ib le
to
cons ide r
the
app l i ca t ion of
the
S t i r l i ng
engine .
I f you consider i n t roduc -
ing
the
S t i r l i ng engine you
must
ca re fu l ly eva lua te i t s
a v a i l a b i l i t y proven performance cha r ac t e r i s t i c s and econom-
i c s l e s t disappointment
r e s u l t .
OMPETITORS OF THE ST RL N ENGINE
The
co mp etit io n fo r the S t i r l i ng
engine
i s
the
i n t e rna l
com-
bus t ion
engine
inc luding the spark ign i t ion
engine
running
on
pe t r o l na t u ra l gas
a lcohol
biogas or producer gas and
the d ie se l
engine running on
d ie se l
fue l or a mixture
o f
d ie se l and other gaseous
or
1
iqu id fue l s .
The
va r i ous so la r
c e l l
devices
as
wel l as
the steam
engine
are
a lso
considered
to
be
com peti t ion for the S t i r l i ng engine .
The S t i r l i ng engine is most
l ike ly
to
be the
bes t choice
where
the
power
required
i s between 100
watts and 20
k and
some s o r t of biomass
coal
or peat i s avai l ab le as fue l .
I f
gaseous or
l iqu id
fue l
i s
r ead i l y ava i l ab le
a
proper ly
adapted
i n t e rna l combust ion
engine i s l ike ly to be
cheaper
a t l e a s t in the
shor t run
a1 though
depending on
the r e l a -
t i v e
c os t
of
the
fue l s
the S t i r l i ng engine could be cheaper
in
the
long run due to
lower
maintenance and fue l
cos t s .
Because the S t i r l i ng
engine
has been
re int roduced
only
recen t ly
t i s hard
to
pro jec t the r e l a t i v e
purchase cos t s
of th e
seve ra l
types of S t i r l i ng
machines . I t i s l ik e ly
t h a t
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24/26
the S t i r l i ng engine wi l l c os t
more
than
the
spa rk i gn i t i on
i n t e r n a l
combust
ion
eng ine
and roughly the
same as a s low
speed d i e s e l engine
of
the
same
q u a l i t y .
But the
St i r l ing
engine i s
l i ke ly
to have lower maintenance c os t s than e i t he r
o f
these
because of
i t s
g r e a t
s impl i c i ty .
The Producer Gas
Engine
as a
Compet i tor
o f the S t i r l i ng
Engine
The producec gas engine runs on gas by means of a b iomass to -
gas
conver t e r
ca l l ed a producer
gas
gener.acor.
The
engine
using
the
producer gas
can
be a conver ted pe t ro l
engine
or a
d i e s e l engine
using
mainly
producer
gas but
a lso req uir in g a
smal l
amount
of
d ie se l
fue l as i g n i t e r
fo r
the
producer gas .
Stnce
t h i s combinat ion
can
in fac t do the same
th ing
as a
S t i r l i ng eng i ne t ha t i s
develop
mechanical power from wood
and o the r biomass--one i s compel led to ask whether
the
S t i r -
l ing engine has any advantage over
the
combinat ion of pro-
ducer gas
genera to r and conven t iona l i n t e rna l combust ion
eng ine . In some
cases the
answer i s yes .
The
S t i r l i ng engine has th ree advantages: 1 it
can burn
fue l s with
high dsh con ten t such
as
r i ce husks,
which
the
producer gas
system
cannot ; 2 s ince the combustion
produc ts
do
not
en t e r the S t i r l i ng
eng ine
they requi re no
c leanup
in
c o n t r a s t to the
producer
gas i n t e rna l combust
ion eng
ine ;
and 3 the
S t i r l i ng
eng ine in
c ombin ation w ith a s imple
wrought
fue l burner
i s
a
much
s impler and
more
rnaintenance
f ree system than
the
combinat ion of producer gas ge ne ra to r
cleanup
system, and i n t e rna l combust ion
eng ine .
The S t i r l i ng engine over t akes the producer gas engine
system
i f the fu e l to be used i s not
of
high
qual i ty
such as r i ce
husks , and i f the cos t of mainta in ing the ign i t ion system,
i n j ec t ion system, l u b r i c a t i o n and o the r r ela ti ve ly d el ic a te
components of the i n t e rna l combustion engine and the gas pro-
ducer i s a problem, as it so of ten
i s .
The Steam Engine as a Compet i tor of
the
S t i r l i ng
Engine
t i s l og i ca l to consider the steam engine as na tu r a l compe
t i t i on fo r the S t i r l i ng eng ine as it in
f ac t
was a t the
t ime
Rev. S t i r l i ng
invented it
At
t ha t
t ime
the
steam engine was
the dominant
power
producer whereas the
S t i r l i ng
engine was
more fue l
e f f i c i e n t
and
much sa fe r
s ince it i s
a lmost
im
poss ib le
to cause a S t i r l i ng engine to
blow
up,
and r a the r
easy
to do
with
a steam engine . Also a t t ha t
t ime, the grea t
disadvantage of the S t i r l i ng engine was
the poor t empera tu re
r e s i s t ance of the
cas t
i ron
hea te r head.
Today,
the
s i tu a t io n is
d i f f e r e n t . The steam
engine has
fa l l en in to
d i s us e
and
the
S t i r l i ng
has
l eap t
ahead in pe r
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25/26
formance l i f e
and a l a i l a b i l i t y .
With the
use o f s e r i e s 300
s t a i n l e s s s t e e l
commonly
a v a i l a b l e mater
i a l
t h e r e
i s
no
lo n g e r t h e danger
of h e a t e r head f a i l u r e a t l e a s t below
700C
which
normal SOlid f u e l combuster
produces
on
t
running S t i r l i n g e n g i n e . And i s f e a s i b l e t o
make
the
h e a t e r
head o f
ceramic e s p e c i a l l y in very l o w - p r e s s u r e
e n g i n e s
such as the s imple
f r e e - p i s t o n water
pumper.
T h e r e f o r e fo r low power appl i c a t
i o n s
below
s e v e r a l t e n s of
k i l o w a t t s the
S t i r l i n g engine
i s l i k e l y
to
be
much
more
f u e l
e f f i c i e n t
much
e a s i e r
t o o p e r a t e much s a f e r and
r e q u i r e
much
l e s s
maintenance. I t
i s
a l s o l i k e l y to
c o s t l e s s
s i n c e
th e
S t i r l i n g engine
has so
few
p a r t s and such
s imple ones in
comparison
t o
the steam e n g i n e . For example the
S t i r l i n g
engine needs no v a l v e s whereas th e steam
engine r e q u i r e s
many
each
one
of which must work u n f a i l i n g l y
in
h o t
c o r r o s i v e environment.
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I LIOGR PHY
Josh i Deep; Seckler
David; and J a in
B.C. Socia l Fores t ry
Wood
Gas i f i e r s
and L if t I r r ig at io n:
Synerg is t ic
Rela
t ions
Between
Technology
and Natura l
Resources
in
Rural Ind ia . January
1983,
p.
1-16.
M imeographed)
Nat iona l
Academy of
Sciences . S t i r l i ng Engines. Energy for
Rural
Development.
Washington, D.C.:
Nat ional
Academy
Press ,
1981,
pp.
149-158.
Nat
iona l Academy
of
Sc i e n ~ s Externa l Combust ion
Eng
ines -
Rankine and
S t i r l i ng Engines
as Smal l -Scale
Power
Sources fo r
Developing Count r i es . Energy fo r Rural
Development.
Washington,
D.C.:
Nat ional
Academy Press
1976,
Appendix
4, pp.
246-269.
Ross , A. S t i r l i ng Cycle Engines .
Phoenix,
Arizona,
1977.
Ur ie l i
I and Berchowitz, D M
S t i r l i ng
Cycle Engine
Analys i s . Br i s t o l England:
dam
Hilger
1984.
Walker ,
G.
S t i r l i ng
Cycle
Machines. Oxford, England:
Oxford
Univers i ty
Press 1973.
Walker , G
S t i r l i ng
Engines .
Univers i ty Press 1980.
Oxford,
England:
Oxford
Wood,
J . Gary; Chagnot, Bruce
J . ;
and Penswick,
Design
of
a
w Pressure
Air Engine
fo r
Use.
Paper
presented
a t
the 17th
Annual
Energy Conference, Los Angeles ,
Cal i fo rn ia
Lawrence B.
Third
World
In te r soc ie ty
August 1982.