Understanding Stirling Engines

8/12/2019 Understanding Stirling Engines

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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


2/26

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


3/26

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


4/26

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


5/26

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


6/26

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 .

- 4 -


7/26

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-drive 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 .

5


8/26

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.

- 6 -


9/26

_ ~

Figure A Crank-Drive St i r l i ng Engine

4 Power

Output Pumping Water


10/26

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 .

8


11/26

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

9


12/26


13/26

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


14/26


15/26

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

3


16/26

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


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


17/26

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

5


18/26

j

i

. i

\... .-

.

I _ ~

I

I

I

I

igure

uplex

St i r l ing Heat u p

6


19/26

~ 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


20/26

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


21/26

W

Figure 9 Rice

Husk

Fueled Simple Hot Air Engine

o Rice MiTl

9


22/26

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

-

-


23/26

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


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


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


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


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.


26/26

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.

Documents

Understanding Stirling Engines