25-07-2014, 01:58 PM
STIRLING ENGINE
STIRLING ENGINE.pdf (Size: 991.56 KB / Downloads: 332)
Introduction
"…These imperfections have been in a 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 a scarce 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 more favorable
circumstances and with complete success…"
(Written in the year 1876 by Dr. Robert Stirling
Presentation of Stirling Engines
Stirling thermodyna
The Stirling engine
fixed mass of gas called the "working fluid" (air, hydrogen or helium). The principle
is that of thermal expansion and contraction of this fluid due to a temperature
differential.
So the ideal Stirling cycle consists of four thermodynamic
acting on the working fluid:
volume processes.
Each one of which can be separately analysed:
1-2: isothermal compression process
fluid, while an equal amount of heat Q
cooling source. The working fluid cools and contracts at constant
TC.
2-3: constant volume displacement process with heat addition
absorbed by the working fluid and temperature is raised from T
work is done.
3-4: isothermal expansion process
while an equal amount of heat Q
source. The working fluid heats and expands at constant temperature T
4-1: constant volume displacement process with heat rejection
rejected by the working fluid
work is done.
Figure
Engines
thermodynamic cycle
cycle is a closed cycle and it contains, most commonly a
ansion thermodynamics distinct processes
two constant-temperature processes and two constant
ch process. Work W1-2 is done on the working
Q1-2 is rejected by the system to the
onstant process. Work W3-4 is done by the working fluid
Q3-4 is added to the system from the heating
and temperature decrease from T
4 : A pressure/volume graph of the ideal Stirling
cycle
7
constantis
temperature
addition. Heat Q2-3 is
TC to TH. No
fluid,
TH.
rejection. Heat Q4-1 is
TC to TH. No
Engine configurations
Mechanical configurations of Stirling engines are classified into three
important distinct types: Alpha, Beta and Gamma arrangements.
These engines also feature a regenerator (invented by Robert Stirling). The
regenerator is constructed by a material that conducts readily heat and has a high
surface area (a mesh of closely spaced thin metal plates for example).
When hot gas is transferred to the cool cylinder, it is first driven through the
regenerator, where a portion of the heat is deposited. When the cool gas is transferred
back, this heat is reclaimed. Thus the regenerator “pre heats” and “pre cools” the
working gas, and so improve the efficiency.
But many engines have no apparent regenerator like beta and gamma engines
configurations with a “loose fitting” displacer, the surfaces of the displacer and its
cylinder will cyclically exchange heat with the working fluid providing some
regenerative effect.
Gamma Stirling
A gamma Stirling is simply a beta Stirling in which
mounted in a separate cylinder alongside the displacer piston cylinder, but is still
connected to the same flywheel. The gas in the two cylinders can flow freely between
them and remains a single body. This configuration produces a lowe
ratio but is mechanically simpler and often used in multi
Gamma type engines have a displacer and power piston, similar to Beta machines, but
in different cylinders. This allows a convenient complete separation betw
exchangers associated with the displacer cylinder and the compression and expansion
work space associated with the piston.
Reasons to use a Stirling Engine
There are several reasons to use a Stirling Engine:
One reason is that for this kind of engine it’s almost impossible to explode.
You don’t have to produce steam in a high pressure boiler. And inside the
cylinder there are no explosions needed to run the pistons like in an Otto or
Diesel engine. There are no ignitions, no carburetion because you only need
one kind of gas and no valve train because there are no valves. This was a big
advantage to the steam engines in the days when Stirling invented his engine
because it was much less dangerous to work next to a Sterling Engine than to a
common steam engine.
Inside the pistons can be used air, helium, nitrogen or hydrogen and you don’t
have to refill it because it uses always the same body of gas.
Analyze from Economic point
As said above the Stirling engine is a kind of external combustion engine, and
it can use a variety of fuels. It ca
material, including gasoline, diesel, propane, sunshine and salad oil; even cow dung
can be run on as fuels.
A cup of coffee cannot become a cup of gasoline, but it can be also used as a
Stirling engine driver. There is a famous experiment that a Stirling engine can easily
run on a cup of coffee. The Stirling engine is a kind of piston engine. In the
heating sealed chamber, the expansion of gases inside the engine promotes the pistons
work. After the expanded gases cooling down in the air
process is taking on. As long as a certain value of the temperature difference exists, a
Stirling Engine can be formed.
Applications of the Stirling power
In the ages of 1970s and 1980s several automobile companies like “General
Motors” or “Ford” were researching about Stirling Engine. This device is good for a
constant power setting, but it is a challenge for the stop and go of the automobile.
A good car can change the power quickly. One possibility to obtain this
important characteristic is design a power control mechanism that will turn up or
down the burner. This is a slow method of changing power levels because is not
enough to accelerate crossing an intersection.
The best solution in spite of these difficulties in automobiles is hybrid electric
cars where Stirling Engine could give enough power to make long trips where could
get burn gasoline or diesel, depending on which fuel was cheaper. The batteries could
give the instant acceleration that
drivers are used to. This invention
makes the car silent and clean
running.
On March 20, 2002 I
delivered one of our KY-2000
Stirling engines to the Mechanical
Engineering department at San
Diego State University. While I was
there I had the opportunity to see
their hybrid diesel/electric car. It has
a 60 hp diesel engine and a 200 hp
electric motor. The extra power
(above 60 hp) for the electric motor
is of course supplied by the batteries
when needed.“Brent Van Arsdell",
March 21, 2002.
Conclusion
Stirling engines qualify for “free energy” designation when they allow us to
tap previously inaccessible sources of naturally occurring energy. Stirling cycle
engines are very efficient for a given temperature difference between the heat source
and the heat sink. Actually, steam engines (the Rankine cycle) fall into this category,
too. But depending upon what kind of hardware and its maintenance you prefer, one
or the other will be preferred. Steamers have fewer parts and higher power density.
Other fluids, such as a variety of refrigerants, can be used instead of water. Stirlings
avoid fluid containment problems, as they can run with air as the working fluid, and
will have less maintenance issues.
Stirling Engines are very flexible. There are a lot of different types of engines.
They can be very small and run with only a small temperature difference, they are
very quiet, for example to use them in submarines or they can be used as a CHP plant.
Another good point is that they can be constructed in a way that they produce
no emissions. That means, in combination with solar or geothermal heat, they can be
used as a renewable energy source to produce electricity.
As is showed above, the Stirling engine has strong economic practicality.
Above all, the original cost is quite lower than for any other engines. Even a few
calories can drive it and keep it running. The next point is that the auxiliary costs are
low, because the Stirling engine costs little on environment protection. The fuels it
uses can be clean, so it costs little to handle with pollution governance. What is more,
the profit of the Stirling engine is far beyond the cost. And the high efficiency can
bring the maximum utilization. And last but not least, the Stirling engine is consistent
with the requirements of sustainable development. It is the main development way in
the future, so the Stirling engine does not only meet the economic needs at present
time, but also in the future.
The Stirling engine is an interesting device like it is showed in this document
with various applications and high development. Its advantages are really beneficial
for the environment because it is possible produce electricity with the power of sun
with high efficiency (theorically like the Carnot Cycle). It is a huge advantage to the
economy because is possible to burn the cheapest fuel and it is working instead of the
more expensive one. And this engine is comfortable for the people because is quiet
and not noisy like an internal combustion engine.