Stirling Engine

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Introduction to Stirling engine




Stirling engine was invented by a Scottish Minister Robert Stirling in 1816. The safety of these
engines was far more than normal Steam Engines during that period and stirling engines were
considered the safest engines as ever made and they still have the potential to be much more
efficient than a gasoline or diesel engine. But they are only used in very sophisticated
applications like submarines and auxiliary power generators for yachts. A stirling engine can be
defined as a mechanical device operating on closed thermodynamic cycle. Difference of
temperatures causes compression and expansion of gases or steam forcing the piston arm to
move back and forth by keeping up with the changes in internal pressure. In case of steam,
volume changes through hot and cold spaces control the flow of steam without using the valves.
Its practical use was confined to low power applications in the first century. Talking about its
components, the heat driving the stirling engine must be transmitted from some heat source to
the working fluid by heat exchangers and there should be a sink where the heat is rejected. No
combustion takes place inside the engine. A stirling engine has one heat source, one heat sink,
and almost five heat exchangers according to the classification. Heat source can be provided
from combustion of fuel, concentrated solar energy, geothermal energy, nuclear energy etc. and
heat sink is typically the environment at ambient temperature. There are 3 major types of Stirling
engines. Alpha, Beta and Gamma. These types are classified according to their pistons and
cylinders. Stirling engines work on stirling cycle. An idealized stirling cycle has four main
processes including isothermal expansion, constant volume heat removal, isothermal
compression and constant volume heat addition. And its thermal efficiency equals to that of
Carnot cycle.
Now the question arises that why stirling engines aren’t more common? Here are some key
characteristics that make the use of stirling engines almost impractical for many applications
including automobiles and generators etc. The simple reason is that the heat source is external
and thus it takes a little while for the engine to respond to changes in the amount of heat being
applied to the cylinder due to which it takes time for the heat to be conducted through the
cylinder walls and into the gas inside the engine. This means that the engine needs some time to
heat up enough to produce some useful power plus the engine is unable to alter its power
completely. As there are certain variations in involved in the engine speed and sometimes there
is a need of abrupt change in case of emergency, these shortcomings relate to the matter of its
inability to replace an internal combustion engine for an automobile. But as the time passed, the
problems which gasoline engines are creating for environment, stirling engines are gain focus
interest as there are no exhaust gases leaving the body of the engine. Some stirling engines with
almost 500 HP have also been built with efficiencies of 30 to 45 percent. A comparison of
stirling and internal combustion engines indicates that stirling engines have various choices of
heat sources plus they are quieter and reliable with low maintenance. They become highly
preferable for applications where cost/unit energy is appreciated more than capital cost per unit
power and they have become cost competitive up to 100W on this basis. In contrast to internal
combustion engine of same rating, a stirling engine has high capital investment and are larger in
size and weight. However, they beat internal combustion engines in efficiency and their low
maintenance cost counters the capital investment. For low power applications, a stirling engine is
more preferable than an internal combustion engine. They are competitive with internal
combustion engines in applications like water pumping, astronautics and electrical generation
units. However are always beaten by the price of material, low power density and per unit power
Concluding the discussion, it would be wise to say that everything has its pros and cones. One
can make a choice depending upon the requirements. Talking about the design technology of
stirling engines, one can tell that there has been very slow progress. The main theory of the
engine has remained same, but due to being incapable of being placed inside a car body, these
engines has never been a huge success. However, the parts of stirling engine are very easy to
manufacture, economical and common. Plus assembly line of production is very easy and
inexpensive to set up. There is a large room for advancement in this case. The composites are
now in market which are replacing certain internal combustion engine parts and making them
lighter and more effective. Same things can be done for stirling engines to make them even better
choice for the consumers.


S. Backhaus; G. Swift (2003). "Acoustic Stirling Heat Engine: More Efficient than Other No-
Moving-Parts Heat Engines". Los Alamos National Laboratory. Archived from the original on
2008-08-01. Retrieved 2009-01-19.
E.H. Cooke-Yarborough; E. Franklin; J. Geisow; R. Howlett; C.D. West (1974). "Harwell
Thermo-Mechanical Generator". Proceedings of the 9th IECEC. San Francisco: American
Society of Mechanical Engineers. pp. 1132–1136
A.J. Organ (2007). The Air Engine: Stirling Cycle Power for a Sustainable Future. Woodhead
Publishing. ISBN 1-84569-231-4.
Hasci, James (14 July 2008). "Modified Stirling Engine With Greater Power Density".Create the
Future Design Contest.
Korzeniewski, Jeremy (July 8, 2009). "Blast from the Past: NASA's Stirling-powered AMC
Spirit". Retrieved March 18, 2012.

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