Motor_Stirling.pdf

Stirling Engine Reference

Guide and Catalog

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

Extremely Interesting

Model Stirling Engines

Figure 1.

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

Stirling Engines

another gas such as helium) inside a leak tight

container and using the pressure changes to drive a

piston. The heating and cooling process works like

this: One part of the engine is kept hot while

another part is kept cold. A mechanism then moves

the air back and forth between the hot side and the

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

expands and pushes up on the piston, and when the

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

pulls down on the piston.

by Brent H. Van Arsdell

Reprinted from MacMillan Encyclopedia of Energy

Used by Permission

The principle that makes Stirling engines possible is

quite simple. When air is heated it expands, and

when it is cooled it contracts. Stirling engines work

by cyclically heating and cooling air (or perhaps

While Stirling engines are conceptually quite

simple, understanding how any particular engine

design works is often quite difficult because there

are hundreds of different mechanical configurations

that can achieve the Stirling cycle. Figure 1 shows a

schematic of a transparent educational

demonstration engine that runs on the top of a cup

of hot coffee. This engine uses a piece of foam

similar to what would be used as a filter for a

window air conditioning unit to "displace" the air

between the hot side and the cold side. This foam

displacer is carefully mounted so it does not touch

the walls of the cylinder. Figure 2 shows how this

particular engine achieves the Stirling cycle. In this

engine, the air flows through and around the

displacer from the hot side then back to the cold

side, producing a power pulse during both the hot

and cold portion of the cycle. Stirling engines can

be mechanically quite simple since they have no

valves, and no sparkplugs. This can result in

extremely high reliability as there are fewer parts to

fail.

combustion each minute, burners in Stirling engines

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

continuous combustion engine burn very cleanly

than one that has to start and stop. An excellent

demonstration of this principle is to strike a match,

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

of the smoke is produced during the starting and

stopping phases of combustion.

A Brief History

In the early days of the industrial revolution, steam

engine explosions were a real problem. Metal

fatigue was not well understood, and the steam

engines of the day would often explode, killing and

injuring people nearby. In 1816 the Reverend

Robert Stirling, a minister of the Church of

Scotland, invented what he called "A New Type of

Hot Air Engine with Economiser" as a safe and

economical alternative to steam. His engines

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

power than the steam engines of the day.

It is worthwhile to compare Stirling engines to other

more familiar engines and note their similarities as

well as their differences. Stirling engines are a type

of heat engine. They turn heat into mechanical

work and in this sense they perform the same

function as other well known heat engines such as

gasoline, diesel, and steam engines. Like steam

engines, Stirling engines are external combustion

engines, since the heat is supplied to the engine

from a source outside the cylinder instead of being

supplied by a fuel burning inside the cylinder.

Because the heat in a Stirling engine comes from

outside of the engine, Stirling engines can be

designed that will run on any heat source from fossil

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

steam engines, Stirling engines do not use a boiler

that might explode if not carefully monitored.

The engines designed by Robert Stirling and those

who followed him were very innovative engines,

but there was a problem with the material that was

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

side of the engine heats up to the average

temperature of the flame used to heat it and remains

at that temperature. There is no time for the

cylinder head to cool off briefly between power

pulses. When Robert Stirling built his first engines,

cast iron was the only readily available material,

and when the hot side of a cast iron Stirling engine

was heated to almost red hot, it would oxidize

fairly quickly. The result was that quite often a hole

would burn through the hot side causing the engine

to quit. In spite of the difficulties with materials,

tens of thousands of Stirling engines were used to

power water pumps, run small machines, and turn

fans, from the time of their invention up until about

1915.

When operating on sunshine, or geo-thermal heat,

Stirling engines obviously produce no pollution at

all, but they can be exceedingly low emissions

engines even when burning gasoline, diesel, or

home heating oil. Unlike gasoline or diesel engines

that have many thousands of start stop cycles of

As electricity became more widely available in the

early 1900s, and as gasoline became readily

available as a fuel for automobiles, electric motors

Insert figure 2 here

Figure 2.

Four phases of the Stirling engine power cycle.

and gasoline engines began to replace Stirling

engines.

to run on any heat source. Every imaginable heat

source from fossil fuel heat to solar energy heat can

and has been used to power a Stirling engine.

Regeneration

Stirling engines also have the maximum theoretical

possible efficiency since their power cycle (their

theoretical pressure volume diagram) matches the

Carnot cycle. The Carnot cycle, first described by

the French physicist Sadi Carnot, determines the

maximum theoretical efficiency of any heat engine

operating between a hot and a cold reservoir. The

Carnot efficiency formula is: (T(hot)-

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

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

the cold side of the engine. These temperatures

must be measured in absolute degrees (Kelvin or

Rankine).

Robert Stirling's most important invention was

probably a feature of his engines that he called an

"economiser." Stirling realized that heat engines

usually get their power from the force of an

expanding gas which pushes up on a piston. The

steam engines that he observed dumped all of their

waste heat into the environment through their

exhaust and the heat was lost forever. Stirling

engines changed all that. Robert Stirling invented

what he called an "economiser" that saved some heat

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

for the next cycle.

It worked like this: After the hot air had expanded

and pushed the piston as far as the connecting rod

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

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

this waste heat by making the air flow through

economiser tubes that absorbed some of the heat

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

the cold part of the engine where it cooled very

quickly and as it cooled it contracted, pulling down

on the piston. Next the air was mechanically moved

back through the pre-heating economiser tubes to the

hot side of the engine where it was heated even

further, expanding and pushing up on the piston.

This type of heat storage is used in many industrial

processes and today is called "regeneration."

Stirling engines do not have to have regenerators to

work, but well designed engines will run faster and

put out more power if they have a regenerator.

Stirling Applications

Stirling engines make sense in applications that take

advantage of their best features while avoiding their

drawbacks. Unfortunately, there have been some

extremely dedicated research efforts that apparently

overlooked the critical importance of matching the

right technology to the right application.

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

was done on Stirling engines for automobiles by

companies such as General Motors, Ford, and

Philips Electronics. The difficulty was that Stirling

engines have several intrinsic characteristics that

make building a good automobile Stirling engine

quite difficult. Stirling engines like to run at a

constant power setting, which is perfect for pumping

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

driving of an automobile.

Continued Interest

Automobile engines need to be able to change

power levels very quickly as a driver accelerates

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

Stirling engine power control mechanism that will

change power levels efficiently, by simply turning

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

method of changing power levels and probably is

not a good way to add the power necessary to

In spite of the fact that the world offers many

competing sources of power there are some very

good reasons why interest in Stirling engines has

remained strong among scientists, engineers, and

public policy makers. Stirling engines can be made

Plik z chomika:

Inne pliki z tego folderu:

Inne foldery tego chomika: