01-06-2013, 04:52 PM
Stirling Engine
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INTRODUCTION
The first Stirling engine was developed by the Scottish priest Robert Stirling in 1816. Robert Stirling wanted to create a safe alternative to the high pressuresteam engines which were spreading in those days.
High pressure steam engines caused dangerous boiler explosions which often severely injured persons and even caused deaths. Robert Stirling’s idea was to build an engine that didn’t need high pressures to work, so that the risk of explosions was reduced.
Not only the safety played a significant role in the development of the Stirling engine; the fact that the fuel consumption of the Stirling engine was lower than of steam engines was important, as well. The first Stirling engine was used in 1818 to run a water pump in a mine.
Towards the end of the 19th century the Stirling engine was used as a source of energy in private houses. At the end of the 20th century approximately a quarter million Stirling engines was used all around the world as energy sources for fans or small appliances like sewing machines.
In the 1930s the Dutch company Philips developed the Stirling enginefurther so that it could function as an easy to handle and portable power source for radios. In this connection the Philips Stirling engine was developed and contained various reforms compared to the previous model.
WORKING
The pressure chamber contains a small amount of air that is held captive inside the engine. One end of the engine is warm, and the other end is cool. The displacer moves the air inside the engine back and forth repeatedly, from warm, to cold, and to warm again.
The air inside the engine expands when it gets warm, and pushes outward on the drive mechanism. When this same air is moved to the cool side of the engine, it contracts. This pulls in on the drive mechanism.
The drive mechanism pushes and pulls on the crankshaft. This causes the crankshaft and flywheel to rotate. The rotation of the crankshaft causes the displacer to rise and fall inside the pressure chamber.
The crankshaft is fashioned so that the cycle will repeat. The air heats, expands, and pushes the crankshaft through the expansion phase. This moves the displacer and causes the air to enter the cool side of the engine. The air cools, contracts, and pulls the crankshaft through the contraction phase.
MERITS & DEMERITS
The Merits:
a) The silence of operation : there is no expansion in the atmosphere like in the case of an internal combustion engine, combustion is continuous outside of the cylinders. In addition, its design is such as the engine is easy to balance and generates few vibrations.
b) The high efficiency : it is function of the temperatures of the hot and cold sources. As it is possible to make it work in cogeneration (mechanical and caloric powers), the overall efficiency can be very high.
c) The multitude of possible “hot sources” : combustion of various gases, wood, sawdust, waste, solar or geothermic energy...
d) The ecological aptitude to respond to the environmental requirements on air pollution. It is easier to achieve a complete combustion in this type of engine.
e) Reliability and easy maintenance: : the technological simplicity makes it possible to have engines with a very great reliability and requiring little maintenance.
f) An important lifetime because of its “rusticity”.
g) The very diverse uses because of its autonomy and adaptability to the needs and the different kinds of hot sources (from mW to MW).
The Demerits:
a) The price : its cost is probably the most important problem, it is not yet competitive with other means well established. A generalization of its employment should solve this problem inherent in any novelty.
b) The ignorance of this type of engine by the general public. Only a few fans know it exists. It is therefore necessary to promote it.
c) The variety of models prevents standardization and, consequently, lower prices.
d) The problems of sealing are difficult to solve as soon as one wishes to have high pressures of operation. The choice of “ideal” gas would be hydrogen for its lightness and its capacity to absorb the calories, but its ability to diffuse through materials is a great disadvantage.