The Stirling Engine Essay

The Stirling Engine
Robert Stirling invented the Stirling
engine in 1816. At the time he was a Scottish minister. Stirling engines
were the safest engines made during that time period that would not explode
like a steam engine could. The Stirling engine would not explode because
the pressures could not be elevated to that to such a high level. The machine
simply stopped when the heater section failed from thermal stress or imperfections
in the material or manufacturing process. So from that day on there was
a better, safer way to produce power, far superior than that of a steam
engine.


A Stirling Engine is a mechanical device,
which operates on a closed looping thermodynamic cycle. Different temperature
levels cause compression and expansion of the air or steam, which causes
the piston arms to move back and forth keeping up with the changes in the
internal pressure. The flow of the steam is controlled by changes in the
volume of the hot and cold spaces, without the use of valves.

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The Stirling cycle can be is still applied
to day in common appliances such as a refrigerator. The heat cycle when
applied to a heat cycle can produce cool air. This will occur when work
is done on the Stirling device, and the heat energy is discarded into the
room. The heat energy that came from the Stirling cycle would be take from
the inside of the refrigerator therefore making it cooler on the inside.


In 1876 Rev. Stirling wrote in a letter
about his brother James, who had just died, “…These imperfections have
been in great measure removed by time and especially by the genius of the
distinguished Bessemer. If Bessemer iron or steel had been known thirty
five or forty years ago there is scarce a doubt that the air engine would
have been a great success…It remains for some skilled and ambitious mechanist
in a future age to repeat it under favorable circumstances and with complete
success….” Rev. Dr. Robert Stirling (1790-1878) from “Stirling Engines”
by G. Walker.


Robert Stirling applied for his first patents
for this engine and the economizer in 1816, only after a few months of
getting nominated as a minister at the Church of Scotland. Sir George Caley
had devised air engines previous to this time and other devices called
air engines were known as early as 1699. Steam engines began to carry a
bad name along with it because they were so dangerous. Air engines were
so safe and they operated on completely different principles. The”economizer” or regenerator has come to be recognized as a most important
part of the patent that Robert Stirling received. This patent was so outstanding
because of the fact that it predated much of the study of thermodynamics.


Some historians believe that the reason for Robert Stirling’s efforts at
such a device were driven by his concern for the working people of his
parishes as steam engines were being used extensively in that area and
time period. Because of the lack of strength in the materials available
to construct, they would frequently explode sending shrapnel, boiling water,
and steam at the people working nearby.


After the years the gasoline internal
combustion engine has taken over. The reason this occurred was because
of the time it takes for a Stirling engine to heat up enough to get moving.


Lately with all of the problems with the environment a need for automobile
engines with low emission of toxic gases has revived interest in the Stirling
engine. Some Stirling engines have been built with up to 500 horsepower
and with efficiencies of 30 to 45 percent. The common internal-combustion
engine would have efficiency in the range of 20 to 25 percent.


One of the fastest moving technologies
is that of composites. These materials have a type of plastic make up along
with other properties. The strength of this type of material is superior
in nature and for the purposes of a Sterling engine, I feel that it would
be perfectly suited. They have already begun to experimentally replace
certain motor parts in the internal combustion engine, such as the pistons.


The advantages of this would be that the weight of internal parts would
be significantly lighter and therefore able to run at higher speeds. Another
advantage would be that the heat developed inside the engine would have
less affect on the components because composites react, (shrink and expand)
less that a metal would.


Lubrication also plays a huge part in engine
performance. If all parts slide together easy than the total force put
into overcoming friction would be reduced. Bearings and other parts would
also play a large part in reducing friction.


The design technology of sterling engines
over the years has changed very slowly. Over the years the materials have
slowly began to change, what was once a wood wheel is now machined out
of steal. The main theory of the Sterling cycle has remained the same,
but because of its incapability to be convenient when placed into a car,
the Sterling engine has never become a huge success. This probably explains
why the manufacturing process has never moved into a mass production or
and assembly line operation of larger sized engines. The model industry
that produces them, as toy is probable the only type of company that does
mass-produce them. Because the parts for the Stirling engine are fairly
easy to machine, the materials are common, and fairly inexpensive, an assembly
line type of production would be fairly easy and inexpensive to set up.


These are the 2 basic design types; the
two-piston type Stirling engine is shown to the left. A space above a hot
piston is always heated and the space above a cold piston is always cooled.


The displacer type Stirling engine (the one to the right) has a space above
a displacer piston, and it is always heated by a heat source. A space below
the displacer piston is cooled always. The displacer piston displaces hot
air and cold air.


The 1900’s brought on a time of industrialization
and few things were still made by hand. In this age Stirling engines could
be built bye assembly lines but Henry Ford had already began production
of his model T Ford, which used an internal combustion engine. This pretty
much killed the need for the Stirling engine. During this these old times
prototypes were almost non-existent except for that of a seldom made model.


The reason this occurred is if a “prototype” was made it was probably put
into use not just set aside whit the actual models were getting manufactured.


Before this time if somebody wanted something they had to build it for
him or her self, or they would have to find someone to build it for them.


This explains why there were very few Sterling engines that were similar
back then. Most parts were built by hand which explains the wooden flywheel.


Because the cost of the part presently used to make it are so cheep and
the parts are adequate to the demands of this engine.


If I was the designer of the Stirling
engine and I had the technology of today to help me in my work I would
have to make it out of composite products or at least a good portion of
it. The Sterling engine in itself if very safe and the simple facts that
a cleaner fuel that is being burned will also reduce emissions making it
a better engine for the environment. To increase power and make it run
faster I would add piston rings to the piston to give it a tighter seal.


To reduce friction in the flywheel I would make sure bearings were placer
on the axle and make sure they were oiled properly. Another was to make
the engine move faster would be to add some wings or flattened metal sheets
to the cool piston case in order to increase surface area allowing it to
cool even faster.

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