TECHNICAL PAPER #1
 
 
                             UNDERSTANDING
                            STIRLING ENGINES
 
 
 
 
                                  By
                             William Beale
 
 
                             Illustrated By
                            Fred L. Heltsley
 
                          Technical Reviewers
                          David M. Berchowitz
                           Michael F. Feeney
                           Robert C. Wagman
                         Francis E. Woodling
 
 
 
 
 
                               VITA
 
                            Published By
                  VOLUNTEERS IN TECHNICAL ASSISTANCE
     1600 Wilson Boulevard, Suite 500, Arlington, Virginia 22209 USA
              Telephone: (703) 276-1800, Fax: (703) 243-1865
                  Telex: 440192 VITAUI, Cable: VITAINC
          Internet: vita@gmuvax.gmu.edu, Bitnet. vita@gmuvax
 
 
 
                     Understanding Stirling Engines
                         ISBN: 0-86619-200-X
             [C]1984, Volunteers in Technical Assistance
 
 
 
 
 
                           PREFACE
 
 
This paper is one of a series published by Volunteers in
Technical Assistance to provide an introduction to specific
state-of-the-art technologies of interest to people in developing
countries. The papers are intended to be used as guidelines
to help people choose technologies that are suitable to
their situations. They are not intended to provide construction
or implementation details. People are urged to contact
VITA or a similar organization for further information and
technical assistance if they find that a particular technology
seems to meet their needs.
 
The papers in the series were written, reviewed, and illustrated
almost entirely by VITA Volunteer technical experts on
a purely voluntary basis. Some 500 volunteers were involved
in the production of the first 100 titles issued, contributing
approximately 5,000 hours of their time. VITA staff
included Leslie Gottschalk as primary editor, Julie Berman
handling typesetting and layout, and Margaret Crouch as
project manager.
 
William Beale, author of this paper, is president of Sunpower
Incorporated. He has designed, developed, manufactured, and
marketed Stirling engines in Bangladesh and other developing
countries, and has published widely in the solar energy
field. Reviewers David M. Berchowitz, Michael F. Feeney,
Robert C. Wagman, and Francis E. Woodling are also specialists
in the area. Artist Fred Heltsley has an engineering
background and is a professional technical illustrator on a
consultant basis.
 
VITA is a private, nonprofit organization that supports
people working on technical problems in developing countries.
VITA offers information and assistance aimed at helping
individuals and groups to select and    implement technologies
appropriate to their situations. VITA maintains an international
inquiry Service, a specialized documentation center,
and a computerized roster of volunteer technical consultants;
manages long-term field projects; and   publishes a variety of
technical manuals and papers.
 
 
               STIRLING ENGINES FOR DEVELOPING COUNTRIES
 
                    by VITA Volunteer William Beale
 
 
I. INTRODUCTION
 
Stirling engines are external combustion engines that use air
or other gases as working fluid. They can burn any solid or
liquid fuel as their heat source. This makes them very
attractive, particularly in situations where conventional
fuels are expensive and hard to obtain. Because some types of
Stirling engine are so simple to make and yet so effective,
they are excellent choices for power generation in developing
countries.
 
This paper describes the basic Stirling engine, as well as
some of the most promising modern varieties. The intent here
is to familiarize people in developing countries with the
engine's operation and range of applications.
 
 
HISTORY
 
The Stirling engine was invented by Robert Stirling, a Scottish
minister, in 1816. The early Stirling engine had a history
of good service and long life (up to 20 years). It was
used as a relatively low-power water-pumping engine from the
middle of the nineteenth century to about 1920, when the
internal combustion engine and the electric motor replaced
it. The hot-air engine was known for its ease of operation;
its ability to use any burnable material as fuel; its safe,
quiet, moderately efficient operation; and its durability and
low maintenance requirements. It was very large for its small
power output, however, and had a high purchase cost. Nevertheless,
its low operating cost usually justified choosing it
over the steam engine--the only alternative at the time--
which burned much more fuel for the same power and demanded
constant attention to avoid dangerous explosions or other
failures.
 
The other major disadvantage of the early hot-air engine was
its tendency to fail if the heater head got too hot. This  ;as
a result of the relatively low heat resistance of the cast
iron heater head. The problem was overcome by redesigning the
burner, which prevented the engine from overheating. This
improvement resulted in safe, but even lower, power operation.
Despite this improvement, the Stirling engine could not
compete with the cheaper, more powerful internal combustion
engine, and it disappeared from the commercial scene.
 
The advent of newer stainless steels and advances in the
understanding of the engine's complex thermodynamic process
brought new attention to the engine during and after World
War II. The performance of the old hot-air engine was improved
and its size and cost were reduced. Its simplicity of
construction and operation, and most important, its ability
to use rough fuels were retained. These efforts on Stirling
engines were almost exclusively aimed at difficult applications
that were not appropriate for developing countries--namely,
the advanced automotive engine, space power, and
artificial hearts. Almost no effort was put into the relatively
easy task of designing an engine for ordinary uses.
The highly developed countries in which the Stirling engine
work was being done did not need a simple engine, so there
was no economic incentive to design one.
 
This situation changed in 1980, when the U.S. Agency for
International Development (USAID) funded the development of a
simple Stirling engine specifically intended for manufacture
and use in developing countries. The engine was designed,
built, tested, and delivered to Bangladesh, and copies of it
were built and put into operation there. This demonstrated
the Possibility of the engine's manufacture in simple machine
shops of the type found in many regions of Africa, Asia, and
Latin America.
 
As a result of this and other recent developments, the formerly
dim prospects for the application of Stirling engines
in developing countries have improved enormously. Plans are
now in motion to bring a new design of the Stirling engine
into commercial production in a much improved form. This
modern version will be much more powerful for its weight and
much more efficient; at the same time, it will be as quiet,
easy to use, reliable, and rugged as the original engine.
Additional models, capable of generating electricity, cooling,
pumping water, and serving in other useful ways not
possible with the old hot-air engine, are also coming into
commercial production.
 
NEEDS SERVED BY THE TECHNOLOGY
 
Although the Stirling engine is an old machine, modern
materials and design methods make it much more attractive
than ever before. The crank-drive Stirling engine is definitely
useful to anyone who has solid fuel. This type of
Stirling engine can burn any local fuel as its source of heat
to produce electricity, pump water, or perform tasks requiring
mechanical power such as food processing.
 
Very simple machines using atmospheric air as working fluid
can be built from local materials such as metal containers.
People who are inclined to try such designs have a good
chance of success.
 
II. OPERATING PRINCIPLES
 
BASIC THEORY OF THE TECHNOLOGY
 
The Stirling cycle is shown in the diagram in Figure 1 . The

use1x4.gif (600x600)


basic idea is that when gas in a closed cylinder is moved
into the hot part of the cylinder, it expands, its pressure
increases, and it can do work. When the gas moves into the
cold part of the cylinder, its pressure is reduced. Once the
gas reaches the lower pressure, it is compressed back to its
original volume. The gas performs more work during its expansion
than is required to be put into it during its compression.
Thus, the entire cycle results in the net positive
output of work.
 
As shown in Phase 1 of Figure 1, the piston is out (bottom
dead center), and the displacer is in as far as it can go.
The gas is in the cold space, and the gas pressure is low.
(Note that the gas is at the same pressure at any instant in
every part of the engine, but that this pressure is changing
with time.  Because the pressure is low, the piston can be
moved in easily to compress the gas at the low temperature.
At the end of this compression process, the engine has
reached Phase 2, as shown in Figure 1.
 
Now it is time to increase the gas pressure. This is not done
by burning a fuel inside the gas as is done in an internal
combustion engine. The gas is moved from the cold space
through a series of heat exchangers, which cause it to enter
the hot space at a high temperature. Note that the gas in the
heater, cooler, regenerator, and hot and cold spaces, is
always at the same pressure at any instant, since the gas
flow passages are large and do not restrict the passage of
the gas.
 
As shown in Phase 3 of Figure 1, the gas is compressed, hot,
and at high pressure. At this point it is ready to expand and
to work on the piston. As the piston moves out of the cylinder,
the displacer moves with it, in order to keep as much
of the gas as possible in the hot space so that the pressure
is kept as high as possible to do the maximum amount of work
on the piston. This expansion and outward movement of the
piston results in the attainment of Phase 4, as shown in
Figure 1.
 
The next step is to reduce the gas pressure by moving it from
the hot space through the heat exchangers to the cold space.
This is done by moving the displacer from its position, as
shown in Phase 4, back to its inward position, as shown in
Phase 1. The cycle is now complete. Notice that the piston
has expanded the gas by moving outward when the gas is hot
and at high pressure, and has compressed the gas when it is
cold and at low pressure. Thus, the original plan has been
accomplished, and the cycle has produced net work to the
outside.
 
For this four-phase process to continue indefinitely, heat
must be continually added to the hot heat exchanger from some
outside source like a fire or a solar collector, and the cold
end must be continually cooled by a stream of water or air.
 
You might now wonder how the movements of the piston and the
displacer are accomplished,  since they  clearly cannot move on
their own. The answer is that there are at least two ways to
make the two components of the simple Stirling engine move as
we wish: (1) we can attach them to cranks through connecting
rods, as is commonly done in automobile engines; or (2) we
can use gas forces in a carefully designed way so that they
bounce on gas springs, with the displacer always ahead of the
piston in its in-and-out oscillation. Of the two methods, the
use of cranks called the crank-drive, or kinematic Stirling,
is the more easily understood method. The second method,
which uses oscillating motions of the piston and displacer on
springs, is called the free-piston Stirling. The crank-drive
Stirling is easier to understand yet harder to make, while
the free-piston Stirling is harder to understand yet easier
to make in at least some of its forms.
 
III. DESIGN VARIATIONS
 
This section of the paper describes a variety of promising
Stirling engines. It emphasizes their physical characteristics,
advantages and disadvantages, applications, and fuel
efficiencies.
 
TYPES OF STIRLING ENGINES
 
Crank-drive Stirling Engine
 
A schematic of the crank-drive Stirling engine is shown in
Figure 2, and a crank-drive Stirling engine pumping water is

use2x6.gif (600x600)


shown in Figure 3. While this engine is unusually large for

use3x7.gif (540x540)


the small amount of power (5 kilowatts) it produces, it is
nevertheless very simple to make and operate. It uses no oil
in the crankcase; here it is important to avoid getting oil
into the hot working parts of the engine, because it could
block the flow of air through the heat exchangers and also
cause an explosion. Any of the following three types of
bearings can be used: sealed roller bearings, ball bearings,
or unlubricated bushings made of a plastic like Teflon. If
necessary, the ball and roller bearings can be replaced by
journal bearings and sealed in grease.
Since the engine is slightly pressurized, up to about
atmospheres (atm), it uses a simple crank shaft seal to keep
the air in, and a small air pump to maintain the pressure
against slow leakage past the seal. The air pump as well as
all other accessories needing power are driven directly from
the rotating engine shaft.
 
Other accessories requiring shaft power. are the auger feeding
the fuel, the combustion air blower, and the cooling water
pump and radiator fan.. With these accessories, the engine is
able to work without any  other source of power, and needs
only fuel to operate.
 
Typical operating instructions are as follows:
 
1. Make sure the engine is in good operating condition and
   the hopper is full of fuel.
 
2. Start a fire in the burner with kindling (e.g., wood shavings,
   dried leaves), and operate the air blower by hand
   until the interior of the burner is sufficiently hot to
   receive and ignite the fuel from the fuel feed.
 
3. Hand operate the combustion air blower and the fuel auger
   until the heater head of the engine reaches a moderate
   temperature (about 300[degrees]C). The engine is now ready to
   start.
 
4. Turn the flywheel over, and the engine should begin to run
   on its own power immediately (easy starting is one of the
   best features of this engine).
 
5. Allow a short time for the engine to pressurize itself and
   to drive the burner until it is at full operating pressure
   and temperature. During this time, the engine will gradually
   grow stronger and more capable to do work. The load
   can be increased as the engine grows stronger. This happens
   automatically if the engine is attached to loads such
   as centrifugal water pumps or generators, but loads such
   as saws and milling machines have the capability to stall
   the engine if their load is applied too quickly. If the
   engine is stalled, it can be restarted immediately by
   unloading it and turning the flywheel again.
 
6. Once the engine is up to full power and doing its work,
   the operator needs only to keep the fuel hopper full and
   maintain a load. If the load is removed for any reason,
   the engine will speed up, but not to a harmful degree; it
   will quickly reach a speed at which its power output drops
   to zero, and it will continue to run.
 
7. When it is time to shut off the engine, simply cut off the
   fuel and the engine will slowly come to a stop. It can be
   stopped more quickly at any time by releasing the internal
   pressure, which reduces the power to a low value.
 
 
Since there are very few critical adjustments of fuel, air,
or water flow, and there is no fuel injector or spark system,
the crank-drive Stirling engine is extremely reliable and
easy to run. But in order to achieve maximum performance it
is important to make correct adjustments of these flows,
which someone with only a little experience can do easily,
 
Because of its ease of operation, durability, local manufacturability,
and the ability to use any local fuel as its heat
source, the modern crank-drive Stirling engine is remarkably
well suited for power generation in developing countries.
Plans for this Stirling engine will be available from USAID
in 1984 or 1985. Commercial production of the engine, or versions
of it, is expected to begin in 1984.
 
Simple Free-Piston Engine
 
Figure 4 portrays a slow-speed free-piston engine, which is a

use4x10.gif (600x600)


simple version of the Stirling engine. This engine is almost
the ultimate in simplicity compared to other Stirling engine
designs. It is the so-called overdriven configuration in
which the displacer is floated by springs and will move spontaneously
either up or down under the influence of the
slightest force or disturbance of the pressure inside the
engine. The great advantage of this arrangement is that the
engine is not only self-starting (requiring only that a good
temperature difference be established between the hot and
cold spaces), but it will adjust to any load, even a complete
stopping of the piston, and still cycle up and down. Thus,
the engine is very. forgiving and easy to operate. Its major
disadvantage is that it is too big for its small power output;
this is because it uses atmospheric air as working fluid
and operates at a very low frequency. Counterbalancing this
disadvantage are the very high lift capacity and efficiency
of the simple positive displacement pump which the engine can
operate.
 
 
Dimensions
 
The displacer and piston diameter can be the same. The
displacer should be at least as long as its diameter, with a
maximum length of three times its diameter, and the end cap
should. be domed to allow some strength against collapse. The
gap between the displacer and the cylinder should be about
one to two hundredths of the diameter, with a preference for
the smaller gap. In order to keep the displacer centered, it
should have raised bumps of the gap thickness that rub
slightly against the cylinder in its cold section.
 
The heater section length should be about one fourth of the
displacer diameter, and the cooler about the same. This
leaves one half of the displacer to act as a regenerator,
which serves to store the heat of the air as it comes from
the heater to the cooler, and releases it to the air as it
comes back from the cooler to the heater. This action increases
the fuel efficiency of the engine.
 
The displacer movement available should be about one third of
its length.
 
The displacer drive rod should cover about 15 percent of the
area of the displacer cylinder. The drive rod should fit
closely in its sleeve but be free to move.
 
Materials
 
The best material for the displacer hot end is any one of the
300 series stainless steels, such as 304, 316, or 321. These
are also called 18-8 type stainless, the kind used in cooking
pots. The hot end of the displacer cylinder must be of stainless
steel also, or  possibly ceramic if it can be made airtight.
Of course, if only short-term experiments are the aim,
then ordinary carbon steel sheet can be used for both displacer
and heater head.
 
The displacer itself can be quite thin, provided that a
non-return valve is installed in its cold end to allow the
interior to reach the maximum cycle pressure and stay there.
Otherwise, the displacer could collapse under pressure. It is
also important to make the displacer shell thick enough to
prevent its collapse under outside pressure.
 
The rest of the engine can be of steel, cast iron, aluminum,
or whatever is locally available, since it is not exposed to
heat. Care should be taken to make the displacer as light as
is practical. Otherwise, it will respond too slowly to gas
pressure and will not develop the lead in motion over the
piston necessary to accomplish the Stirling cycle.
 
Energy Output
 
A simple free-piston engine with a 60-cm diameter displacer,
operating at one cycle per second, can be expected to produce
about 500 watts of power (50 liter-meter/sec) of pumped
water. Of course, as with any first attempt, the actual
output could be much less.
 
 
Free-Cylinder Engine
 
Another excellent candidate for use in developing countries
is the free-cylinder engine. It shares many of the virtues of
the crank-drive Stirling engine and is even simpler to make.
Moreover, because it is hermetically sealed, it is impervious
to damage from outside contaminants. However, because it is
basically a reciprocating output machine, it must have some
sort of power-transforming device such as a ratchet drive and
gearbox to provide rotary motion if such is needed.   There are
many uses for simple reciprocating motion, such as water
pumping, and in these uses the free-cylinder engine is an
excellent choice.
 
Figure 5 shows a model of a typical free-cylinder engine used

use5x13.gif (600x600)


as a water pump. The power is taken from the oscillating
cylinder, which moves in reaction to the opposite motion of
the heavy piston inside. The displacer is driven by the gas
pressure on its rod, which is attached to the piston.
 
The free-cylinder engine, like all Stirling engines, can
operate on any heat source. Use of a ratchet drive permits
the high-frequency short-stroke free-cylinder engine to drive
any load requiring a rotating shaft, and thus greatly enhances
its utility.
 
The free-cylinder engine used as a reciprocator can easily
drive not only fluid pumps but air or gas compressors as
well. It can also drive refrigerant pumps for food preservation.
 
Starting the free-cylinder engine can be automatic if the
engine is in a vertical position; otherwise, a slight jar is
necessary to start the initial motions, after which the
engine will run vigorously as long as the temperatures prescribed
for both the hot end and the cold end of the cylinder
are maintained. The temperature required is usually from 400
to 700[degrees]C on the hot end and up to 100[degrees]C on the cooling
jacket.
 
Since there are only two moving parts inside the cylinder,
the free-cylinder engine is even easier to make than the
crank-drive Stirling engine described earlier in this paper.
Moreover, Since the cylinder is hermetically sealed, the
engine does not need an air pump or a sliding seal to contain
its working gas. Therefore, the engine can operate at a high
pressure, say up to 15 atm, making it very compact and cheap
for its power.
 
Duplex Stirling Engine
 
The duplex Stirling engine is a heat-driven cooling machine;
that is, it takes in heat and produces cold without producing
any other external effect. It is very simple, almost as   simple
as the free-cylinder water pump, and is very fuel-efficient
if carefully designed. Figure 6 shows a typical cross-section

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of the duplex Stirling engine designed as a heat-driven
food refrigerator. Figure 7 shows it in operation.

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The basic idea behind the operation of the duplex Stirling
engine is that when driven, it becomes a heat pump. In the
duplex Stirling, a Stirling engine is used to drive a Stirling
heat pump. This can be done with only three moving
parts--the hot displacer, the piston which acts as the piston
for both the heat engine and the heat pump, and the cold displacer.
This combination of parts makes a simple and effective
heat-drive heat pump, which can be scaled to any size or
temperature range, from very cold temperatures necessary to
liquify air to mild temperatures useful for space cooling.
 
The duplex Stirling engine will be commercially available
within the next few years, probably as a portable, foodstoring
freezer-refrigerator in small sizes.
 
 
Free-Piston Alternator Engine
 
Recent efforts to develop the free-piston alternator engine
have produced outstanding results. While the engine will not
become a commercial item as quickly as the crank-drive Stirling
engine, it will follow with only about a year's delay.
The one most developed at the moment is a 1-kW output machine
that has excellent fuel efficiency, promises long life, and
is very compact. This machine is not simple, however, and
requires highly sophisticated manufacturing procedures and
materials. On the other hand, because it is hermetically
sealed, it cannot be damaged by any sort of rough treatment,
although the control system and other auxiliaries are not so
invulnerable.
 
The free-piston alternator engine is ideally suited to the
task of developing electricity from solar energy, especially
when matched to a low-cost plastic film concentrator of the
type now coming on the market. Such machines are being actively
developed in sizes up to 10 kW, and could be available
in even larger sizes in a few years. The one shown in Figure 8

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has a 10-kW output.
 
 
USES OF THE STIRLING ENGINE
 
Irrigation with Biomass
 
Both the crank-drive Stirling engine and the free-cylinder
engine are practical for irrigation with biomass, provided
that ample biomass is available for fuel as well as cheap
labor to feed the engine with fuel and tend its operation.
The crank-drive engine is practical from about 500 watts to
tens of kilowatts of delivered power, but in power above 3 kW
it will require a wheeled cart to transport it. The free-cylinder
engine makes a good irrigation pump up to about 500
watts. Either engine can drive both shallow well and deep
well pumps, as well as low-lift ditch pumps. Also, the electric
generator free piston can be attached to an electric
pump for this service.
 
Electricity Generation--Small Sizes--Solid Fuel
 
Both the crank-drive Stirling engine and the free-piston
alternator engine are practical for this use. The free-piston
alternator engine has the advantage of very low noise and
long life, but is harder to repair in the field. The crank-drive
Stirling engine is simple, easy to repair, and cheaper,
and can be manufactured in simple repair shops; however, it
is not as fuel efficient.
 
Electricity Generation--Village Power--Solid Fuel
 
Here again, both the crank-drive Stirling engine and the
free-piston alternator engine would serve for any power up to
about 100 kW. Stirling engines of higher power output are not
likely in the near future, although it is always possible to
combine smaller units into a larger unit for more power.
 
In this application, constant attendance is required to
assure the proper operation of the fuel feed and other auxiliaries.
Useful by-products include hot water from the cooling
system and ash from the burner.
 
Grain Processing--Grain Waste as Fuel
 
This is an ideal application because of the availability of
the biomass by-product as fuel for the engine. The USAID-funded,
simple hot-air engine, referenced earlier in this
paper as having been developed for manufacture in Bangladesh,
is an excellent example. Figure 9 shows this hot-air engine

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millng rice. It burns the rice husk produced by the mill it
drives. Only a fraction of the husk produced by the mill is
needed to fuel the engine, so ample amounts are left over for
the engine to use while pumping irrigation water for the next
rice crop. In this way, solar energy, in the form of biomass,
is used as the primary energy input for the rice-growing
process, and no outside fuel is necessary.
 
Solar Power
 
It is important to recognize that the Stirling engine is a
high-temperature machine.  It cannot run well on the low
temperatures available from simple flat plate solar collectors.
It must use a concentrating, sun-tracking solar collector.
This device adds considerably to the cost and maintenance
requirements of the system. Also, such a device does
not make use of the diffuse component of solar energy, only
the direct component. So hazy sun is not good enough. Bright,
clear skies are needed before the concentrating collector
will develop the high temperature necessary to operate the
Stirling engine. For all these reasons, Stirling systems
using concentrating, sun-tracking solar collectors will be
much more expensive and  will require more care in their
operation than those using fuel as their heat source.
 
With those reservations in mind, it is right to point out
that there are situations in which such solar-driven systems
are worthy of consideration: where intense sunlight is the
rule, where there is no biomass available and none derivable
from the effects of the engine (as there would be eventually
if the engine were irrigating a formerly desert area), and
where the cost of the engine, collector, tracker, mount, and
maintenance thereof is not prohibitive. Such a situation
could exist where several kilowatts of electricity are
needed, and the cost of photovoltaic systems is too high. It
is likely that a solar electric system based on a free-piston
Stirling engine will cost considerably less per watt
delivered than will a photovoltaic system in the kilowatt
range of power.
 
A cautionary note on solar Stirling systems: although the
Stirling engine will be commercially available in one or two
years, the concentrating collectors and their auxiliaries are
still some distance away from production. For all these
reasons, solar Stirling engine systems are likely to be much
more costly than other systems except where nothing else is
available, as might be the case in extreme desert zones.
 
More often than not, a direct solar system is less practical
than one that uses biomass grown with the help of irrigation
provided by the engine. By this means, land that would otherwise
grow nothing could conceivably be made to produce food
as well as fuel for the irrigating pump. Put simply, a field
of weeds, harvested to be burned in the engine, is a much
easier route to solar power than an elaborate optical system,
mount, and tracker. And weeds, unlike the sun, do not hide
behind clouds or go away at night.
 
FUEL EFFICIENCY AND POWER OUTPUT
 
The Stirling engine is likely to burn roughly 10 kilograms
(kg) per kilowatt-hour (kWh) of biomass fuel, and 6 kg/kWh of
coal. This is less than the rate of fuel consumption of small
steam engines. Depending on how well an operator guides the
machine, this burning rate can easily vary as much as 20
percent, up or down; with well-designed and well-attended
engines, it could be as little as half as much.
 
The power output per unit of weight varies greatly with the
design. Generally, it ranges from about .04 Kw/Kg for a simple
crank-drive model to about .07 Kw/Kg for a commercial
high-technology free-piston alternator engine.
 
IV. COMPARING THE ALTERNATIVES
 
The Stirling engine is capable of accepting heat from any
source above about 400[degrees]C and converting part of the heat into
useful work. This makes it capable of a wide variety of uses.
Which of them are practical and worth consideration in comparison
with the other sources of mechanical energy?
 
If conventional fuels and machines are available and satisfactory,
it is probably not practical to consider replacing
them with a Stirling engine. Only when petrol or diesel or
clean gaseous fuels are scarce, expensive, or otherwise unattractive,
and when the spark-ignition internal combustion
engine or diesel engine is too short lived or too expensive
to maintain or purchase, is it sensible to consider the
application of the Stirling engine. If you consider introducing
the Stirling engine, you must carefully evaluate its
availability, proven performance characteristics, and economics,
lest disappointment result.
 
COMPETITORS OF THE STIRLING ENGINE
 
The competition for the Stirling engine is the internal combustion
engine, including the spark-ignition engine running
on petrol, natural gas, alcohol, biogas, or producer gas, and
the diesel engine running on diesel fuel, or a mixture of
diesel and other gaseous or liquid fuels. The various solar
cell devices as well as the steam engine are also considered
to be competition for the Stirling engine.
 
 
The Stirling engine is most likely to be the best choice
where the power required is between 100 watts and 20 kW, and
some sort of biomass, coal, or peat is available as fuel. If
gaseous or liquid fuel is readily available, a properly
adapted internal combustion engine is likely to be cheaper,
at least in the short run, although, depending on the relative
cost of the fuels, the Stirling engine could be cheaper
in the long run, due to lower maintenance and fuel costs.
 
Because the Stirling engine has been reintroduced only
recently, it is hard to project the relative purchase costs
of the several types of Stirling machines. It is likely that
the Stirling engine will cost more than the spark-ignition
internal combustion engine, and roughly the same as a slowspeed
diesel engine of the same quality. But the Stirling
engine is likely to have lower maintenance costs than either
of these because of its great simplicity.
 
The Producer Gas Engine as a Competitor of the Stirling
Engine
 
The producer gas engine runs on gas by means of a biomass-to-gas
converter called a producer gas generator. The engine
using the producer gas can be a converted petrol engine or a
diesel engine using mainly producer gas but also requiring a
small amount of diesel fuel as igniter for the producer gas.
Since this combination can in fact do the same thing as a
Stirling engine--that is, develop mechanical power from wood
and other biomass--one is compelled to ask whether the Stirling
engine has any advantage over the combination of producer
gas generator and conventional internal combustion
engine. In some cases, the answer is yes.
 
The Stirling engine has three advantages:   (1) it can burn
fuels with high ash content such as rice husks, which the
producer gas system cannot; (2) since the combustion products
do not enter the Stirling engine, they require no cleanup,
in contrast to the producer gas internal combustion engine;
and (3) the Stirling engine, in combination with a simple
wrought fuel burner, is a much simpler and more maintenance-free
system than the combination of producer gas generator,
cleanup system, and internal combustion engine.
 
The Stirling engine overtakes the producer gas engine system
if the fuel to be used is not of high quality, such as rice
husks, and if the cost of maintaining the ignition system,
injection system, lubrication, and other relatively delicate
components of the internal combustion engine and the gas producer
is a problem, as it so often is.
 
The Steam Engine as a Competitor of the Stirling Engine
 
It is logical to consider the steam engine as natural competition
for the Stirling engine, as it in fact was at the time
Rev. Stirling invented it. At that time the steam engine was
the dominant power producer, whereas the Stirling engine was
more fuel efficient, and much safer since it is almost impossible
to cause a Stirling engine to blow up, and rather
easy to do with a steam engine. Also at that time, the great
disadvantage of the Stirling engine was the poor temperature
resistance of the cast iron heater head.
 
Today, the situation is different. The steam engine has
fallen into disuse, and the Stirling has leapt ahead in performance,
life, and availability. With the use of series 300
stainless steel, a commonly available material, there is no
longer the danger of heater head failure, at least below
700[degrees]C, which a normal solid fuel combuster produces on a
running Stirling engine. And it is feasible to make the
heater head of ceramic, especially in very low-pressure
engines such as the simple free-piston water pumper.
 
Therefore, for low-power applications below several tens of
kilowatts, the Stirling engine is likely to be much more fuel
efficient, much easier to operate, much safer, and require
much less maintenance. It is also likely to cost less, since
the Stirling engine has so few parts and such simple ones in
comparison to the steam engine. For example, the Stirling
engine needs no valves, whereas the steam engine requires
many, each one of which must work unfailingly in a hot,
corrosive environment.
 
 
 
                     BIBLIOGRAPHY
    
 
Joshi, Deep; Seckler, David; and Jain, B.C. "Social Forestry,
     Wood Gasifiers and Lift Irrigation: Synergistic Relations
     Between Technology and Natural Resources in
     Rural India." January 1983, p. 1-16. (Mimeographed)
 
National Academy of Sciences. "Stirling Engines." Energy for
     Rural Development. Washington, D.C.: National Academy
     Press, 1981, pp. 149-158.
 
National Academy of Sciences. 'External Combustion Engines--
     Rankine and Stirling Engines as Small-Scale Power
     Sources for Developing Countries." Energy for Rural
     Development. Washington, D.C.: National Academy Press,
     1976, Appendix 4, pp. 246-269.
 
Ross, A. Stirling Cycle Engines. Phoenix, Arizona, 1977.
 
Urieli, I., and Berchowitz, D.M. Stirling Cycle Engine
     Analysis. Bristol, England: Adam Hilger, 1984.
 
Walker, G. Stirling Cycle Machines. Oxford, England: Oxford
     University Press, 1973.
 
Walker, G. Stirling Engines. Oxford, England: Oxford
     University Press, 1980.
 
Wood, J. Gary; Chagnot, Bruce J.; and Penswick, Lawrence B.
     "Design of a Low Pressure Air Engine for Third World
     Use." Paper presented at the 17th Annual Intersociety
     Energy Conference, Los Angeles, California, August 1982.
 
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