Steam Engine
Steam Engine, mechanical device used
to transfer the energy of steam into mechanical energy for a variety of
applications, including propulsion and generating electricity. The basic
principle of the steam engine involves transforming the heat energy of steam
into mechanical energy by permitting the steam to expand and cool in a cylinder
equipped with a movable piston. Steam that is to be used for power or heating
purposes is usually generated in a boiler. The simplest form of boiler is a closed
vessel containing water, which is heated by a flame so that the water turns to
saturated steam. The ordinary household-heating system usually has a boiler of
this type, but steam-generating plants used for power purposes are more complex
in design and are equipped with various auxiliary devices. The efficiency of a
steam engine is generally low, and therefore, in most power generation
applications, the steam engines have been replaced by steam turbines (see Turbine).
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HISTORY
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Parts of a Steam Engine
Harnessing the power of steam marked a
significant step in technology. The introduction of the steam engine led to
many new inventions, most notably in transportation and industry. Steam engines
transfer the energy of heat into mechanical energy, often by allowing steam to
expand in a cylinder equipped with a movable piston. As the piston moves up and
down (or alternatively, from side to side), an attached arm converts this
motion into parallel motion that drives a wheel. Models of the steam engine
were designed as early as 1690, but it was not until 70 years later that James
Watt arrived at the design of the modern steam engine.
The first piston engine was
developed in 1690 by the French physicist and inventor Denis Papin and was used
for pumping water. Papin's engine, which was little more than a curiosity, was
a crude machine in which the actual work was done by air rather than steam
pressure. It consisted of a single cylinder that also served as a boiler. A
small amount of water was placed in the bottom of the cylinder and heated until
steam was formed. The pressure of this steam raised a piston fitting in the
cylinder, and, after it was raised, the source of heat was removed from the
bottom of the cylinder. As the cylinder cooled, the steam condensed and air
pressure on the upper side of the piston forced the piston down.
In 1698, the English engineer
Thomas Savery built a steam engine that used two copper vessels alternately
filled with steam from a boiler. Savery's engine was used for pumping water,
but could only raise water about 6 m (20 ft) without using pressures which
risked explosion, and was quickly abandoned. The first practical steam engine,
the so-called atmospheric engine, was built by the English inventor Thomas
Newcomen in 1712. This device had a vertical cylinder and a piston that was
counterweighted. Steam admitted to the bottom of the cylinder at very low
pressure acted with the counterweight to move the piston to the top of the
cylinder. When the piston reached this point, a valve opened automatically and
sprayed a jet of cold water into the cylinder. The water condensed the steam,
and atmospheric pressure forced the piston back to the bottom of the cylinder.
A rod attached to the arm of the pivoted beam that connected piston and
counterweight moved up and down as the piston moved, actuating a pump.
Newcomen's engine was not efficient, but it was sufficiently practical to be
used extensively for pumping water from coal mines.
In the course of making
improvements to the Newcomen engine, the Scottish engineer and inventor James
Watt produced a series of inventions that made possible the modern steam
engine. Watt's first important development was the design of an engine that
incorporated a separate condensing chamber for the steam. This engine, patented
in 1769, greatly increased the economy of the Newcomen machine by avoiding the
loss of steam that occurred in alternate heating and cooling of the engine
cylinder. In Watt's engine, the cylinder was insulated and remained at steam
temperature. The separate condenser chamber, which was water-cooled, was
equipped with a pump to maintain a vacuum so that the steam was drawn from the
cylinder to the condenser. The pump was also used to remove the water from the
condenser chamber.
Another radical departure in the
design of the early Watt engines was the use of steam pressure instead of
atmospheric pressure to perform the actual work of the engine. Watt also
devised a method in which the reciprocating pistons of engines drove a
revolving flywheel. He accomplished this first by a system of gearing (see Gear),
and later by means of a crankshaft, as in modern engines. Watt's other
improvements and inventions included application of the principle of double
action, whereby steam was admitted to each end of the cylinder alternately to
drive the piston back and forth. He also equipped his engines with throttle valves
to control speed and also with governors in order to maintain automatically a
constant speed of operation.
The next important development
in the field of steam engines was the introduction of practical noncondensing
engines. Although Watt had recognized the principle of the noncondensing
engine, he had been unable to perfect machines of this type, probably because
he used steam at extremely low pressure. At the beginning of the 19th century
the British engineer and inventor Richard Trevithick and the American inventor
Oliver Evans devised successful noncondensing engines using the high-pressure
steam. Trevithick used this model of steam engine to power the first railroad
locomotive ever made (see Locomotive). Both Trevithick and Evans also
built steam-powered carriages for road travel.
At about the same time,
the first compound steam engines were built by the British engineer and
inventor Arthur Woolf. In the compound engine, steam at high pressure is used
in one cylinder and then, after it has expanded and consequently lessened in
pressure, is piped to another cylinder, in which it expands still further.
Woolf's original engines were of the two-cylinder type, but later types of
compound engines used triple and even quadruple expansion. The advantage of
compounding two or more cylinders is that less energy is lost in the heating of
the cylinder walls; as a result, the engine is more efficient.
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MODERN STEAM ENGINES
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Steam Engine: Figures 1a-d
In a steam engine, the slide valve
alternately routes steam to the back and front of the cylinder to drive the
piston. The diagram at right shows some of the important components of a steam
engine, while Figures 1a through 1d depict a steam engine’s complete cycle of
operation.
The operation of a typical
modern steam engine is depicted in Figures 1a-d, which show a steam engine's
cycle of operation. In Fig. 1a, when the piston is at the left end of the
cylinder, steam admitted to the valve chest flows through the port into the
cylinder at the left-hand side of the piston. The position of the slide valve
also allows the used steam at the right-hand end of the cylinder to escape
through the exhaust port. The piston's motion drives a flywheel, which in turn
drives the rod that controls the slide valve. The relative positions of the
piston and the slide valve are governed by the relative positions of where the
crankshaft and the slide valve rod are fastened to the flywheel.
In the second position,
shown in Fig. 1b, the steam at the left side of the cylinder has expanded and
moved the piston to the center point of the cylinder. At the same time, the
valve has moved to the closed position so that the cylinder is entirely sealed
and neither the steam in the cylinder nor the steam in the valve chest can
escape.
As the piston continues to
move toward the right under the pressure of the expanding steam, as shown in
Fig. 1c, the port at the left end of the cylinder is connected to the exhaust
by the valve, and, at the same time, the valve chest, which contains steam, is
connected to the right end of the cylinder. In this position, the engine is
prepared for the second stroke of its double-action cycle. Finally, in the
fourth position (Fig. 1d), the valve again covers the ports from both ends of
the cylinder, and the piston moves toward the left, driven by the expansion of
steam at the right end of the cylinder.
The type of valve illustrated
in the figure is the simple slide valve, which is the basis for most valves
used on modern steam engines. Such valves have the advantage of being
reversible; their position relative to the piston can be varied by varying the
position of the eccentric that drives them, as shown in Fig. 2. Moving the
eccentric through 180° makes it possible to reverse the direction of rotation
of the engine.
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The slide valve, however,
has a number of drawbacks, the most important of these being the friction
caused by steam pressure on the back of the valve. To avoid the wear caused by
this pressure, steam-engine valves are frequently made in a cylindrical form
entirely enclosing the piston, so that the pressure is equal all around the
valve and friction is minimized. The development of this type of valve is attributed
to the American inventor and manufacturer George Henry Corliss. In other types
of slide valves, the moving portion of the valve is designed so that steam does
not press directly on the back of the valve.
The linkage between the
piston of the engine and the valve supplying steam to the engine is very
important in determining the power and efficiency of an engine. By varying the
point in the engine cycle at which steam is admitted to the cylinder, it is
possible to vary the amount of compression and expansion in the cylinder and
hence to vary the power output of the engine. A number of different types of
valve gears for linking the piston to the valve have been developed that permit
not only reversing of the engine, but also a range of control of the admission
time and cutoff of the steam. Valve gears are of particular importance in steam
locomotives in which the effort required from the engine varies widely; the
effort is at a maximum when the locomotive is starting and less when it is
running at full speed.
An important adjunct to all
types of reciprocating steam engines is the flywheel, which is driven by the
piston crank. Because of its inertia, the flywheel, usually a heavy metal
casting, makes continuous the individual surges of power of the steam expanding
within the cylinder, and permits the engine to provide a uniform flow of power.
In single-cylinder steam
engines, the engine can stop when the piston is at one end of the cylinder or
the other. If the cylinder is in this position, the engine is said to be on
dead center and is impossible to start. To eliminate the dead-center points,
steam engines are frequently equipped with two or more coupled cylinders,
arranged in such a way that no matter what the position of the pistons, the
engine is able to start. The simplest way of coupling two cylinders in an
engine is to arrange the two cranks on the flywheel as shown in Fig. 3. For
better balance, it is also possible to use a three-cylinder engine in which the
various cranks are set at an angle of 120°. The coupling of engines not only
eliminates difficulties in starting but also produces a power plant that
operates more reliably.
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The cylinder of a compound
engine, unlike that of a single-cylinder engine of the ordinary type, can be
kept close to a uniform temperature, which makes the engine more efficient.
Further improvement in the
design of steam engines is afforded by the uniflow engine, which uses the
piston itself as a valve and in which all portions of the cylinder remain at
approximately the same temperature when the engine is operating. In the uniflow
engine, steam moves in only one direction while entering the cylinder of the
engine, expanding, and then leaving the cylinder. This unidirectional flow is
accomplished by employing two sets of inlet ports at either end of the
cylinder, together with a single set of outlet ports in the cylinder wall at
the center. The flow of steam into the two sets of inlet ports is controlled by
separate valves. The inherent advantages of the uniflow system are such that
engines of this type were usually chosen for use in large installations,
although the initial cost of the engines is considerably higher than that of
conventional steam engines. One virtue of the uniflow engine is that it permits
the efficient use of high-pressure steam in a single cylinder engine without
the necessity of compounding.
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