Ernest Victor Lallier.

An elementary manual of the steam engine; containing also a chapter on the theory, construction and operation of internal combustion engines for the operating engineer online

. (page 2 of 17)
Online LibraryErnest Victor LallierAn elementary manual of the steam engine; containing also a chapter on the theory, construction and operation of internal combustion engines for the operating engineer → online text (page 2 of 17)
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whose radius is the distance between the eccentric and
the concentric centers.

In Figs. 9 to 15 are shown a number of diagrams
illustrating the action of the eccentric in controlling the
movements of the slide valve relative to the position
of the piston at various points of the stroke and also
indicating the necessity for the presence of LAP on
a slide valve in producing economical operation of a
steam engine. All of the diagrams show the piston
moving forward on the same stroke. The position of
each of the moving parts is indicated by the arrows.

In Fig. 9 the piston is at the beginning of its stroke,
and the plain slide valve, without lap or lead, is shown



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i6



ELEMENTARY STEAM ENGINEERING.




Figs. 9-15.



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ACTION OF THE ECCENTRIC. 17

just ready to open the port and allow the admission of
steam. Reference to the circles at the right-hand side
of the diagram will show that the small crank, represent-
ing the eccentric, is placed at right angles to the main
engine crank. When the piston has reached one half
of its stroke. Fig. 10, the engine crank is vertical, the
eccentric is horizontal, and it has brought the valve
over as far as it will go during that stroke. The steam
port is shown completely opened as well as the exhaust
port. When the piston has continued to the end of
the stroke. Fig. 11, the position of the crank has been
advanced one fourth of a revolution. The piston, con-
necting rod, and crank are again on dead center and
the slide valve has again closed both ports, while the
one on the right-hand side is just ready to open for the
return stroke. It is clearly seen that with such a con-
struction there can be no cut-ofF and no expansion of
steam. The steam must, of necessity, follow during
the entire stroke and cannot be cut off previous to the
end of the stroke, for if the position of the eccentric
crank were such as to allow the valve to close one
steam port before the end of the stroke, it would, at
the same time, open the opposing port for the admis-
sion of steam. In Fig. 12 there has been added to
the valve, both inside and outside, an amount of mate-
rial producing the LAP, and the valve and piston are
placed as before, the valve being barely opened in order
to allow the admission of steam to the cylinder. We
find that the eccentric crank has been advanced in
the direction of rotation an amount represented by the
angle between its original point, as shown by the dotted



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l8 ELEMENTARY STEAM ENGINEERING.

line A| and its present position as shown at B. Tliis
amount is called the angular advance of the eccen-
tric and is equal to the amount required by the lap
and the lead. As the engine rotates and the piston has
reached its quarter stroke. Fig. 13, the steam port is
entirely opened, as is also the exhaust port, allowing a
free exhaust. On reaching the half stroke. Fig. 14,
it will be noticed that the valve is now traveling in a
direction opposite to that of the piston and that the port
has just been closed. Consequently, from this point
on during the remainder of the stroke the piston will
be propelled by the expansive force of the steam only.
At three quarters of the stroke. Fig. 15, both ports are
shown closed. The piston is still moving forward, due
to the expansion, and the exhaust now being closed,
compression is taking place and will continue during
the remainder of the stroke. At the end of the stroke
the conditions shown in Fig. 12 will again exist, but, of
course, in the reverse direction.

From the foregoing it will be noted that with lap
added, with the piston on center, and all parts moving
forward, it is necessary to give the eccentric sufficient
angular advance to produce the required lead at the
beginning of the stroke. It will also be seen that in-
creasing the outside lap will allow of cutting off steam
at an earlier period of the stroke, thereby increasing
the expansion, while decreasing the outside lap pro-
duces reverse results.

It may also be noted that the addition of inside lap
to the exhaust valve causes an earlier closing of the
exhaust valve, therefore increased compression. This



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ACTION OF THE ECCENTRIC.



19



also would delay the release of exhaust steam. Con-
sequently in designing an engine, the amount of lap,
both inside and outside, must be carefully considered
in their relation to the general periods of cut-off| ex-
pansion, and compression.

As the eccentric, in the position shown, controls the
valve in such manner as to admit steam as required to
produce a forward movement of the engine, a little
thought will make it clear that in order to operate the
engine in the opposite direction, it will be necessary to
place the eccentric in a reverse position. It is incon-
venient to do this while the engine is in operation.
On locomotive, marine, or similar engines, two eccen-
trics are employed. One is set in the proper position
for operating ahead, the other is in a position for re-
verse. These are placed in active operation in con-
nection with the valve rod by means of a link motion,
Fig. 16, so arranged that when the link is in the central




Fig. 16.

position neither eccentric has any control over the valve
movement. But when one or the other is shifted into
position by the link, it changes the valve position so
as to enable the engine to run forward or backward, as
desired.



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20 ELEMENTARY STEAM ENGINEERING.

As the steam must be ready to propel the piston as
soon as it passes the dead center, or the beginning of
the stroke, the lap of the valve enables us to open the
port slightly before the beginning of the stroke, likewise
to close it before the end of the stroke.

liAP represents the part added to the valve which
makes it wider than the port opening.

LEAD represents the space the valve is open when
the piston commences to move forward.

VALVE TRAVEL is the distance the valve m^ves
during the stroke.

The TRAVEL of a slide valve equals the outside
lap plus the width of the steam port m,ultiplied by
two.

Clearance. — This is the space between the piston
and the head of the cylinder at the end of the stroke.
It also includes the volume of the steam passages or
ports between the cylinder and the valves. To a cer-
tain extent clearance is a drawback on account of the
quantity of steam which occupies the space; still, on
the whole, it is an advantage, for when steam is used
expansively, the steam in the clearance space expands
as well as the rest, and adds slightly to the power de-
veloped. A convenient method of measuring the clear-
ance volume is to place the piston at the end of the
stroke, fill the clearance space with water, and, drawing
it off in a measure, calculate its volume in cubic inches.

Steam Jacketing. — As it is an advantage to main-
tain a constant temperature in the engine cylinder in
order to avoid condensation, the engine casting is so
made that the cylinder proper is enclosed in an external
shell thus forming an annular space between the outer



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ACTION OF THE ECCENTRIC. 21

shell and the cylinder which is filled with steam. Such
a construction is called a steam jacket.

Wire Drawing. — When the main valves are not
sufficiently large or are partly closed, so that the steam
does not enter with sufficient rapidity to maintain the
pressure behind the piston, it is said to be wire drawn.

Relief valve. — Some condensation of steam may
take place in the cylinder at any time, also water may be
carried into the cylinder with the live steam; and, par-
ticularly when starting an engine cold, after a period
of rest| condensation is likely to take place. To enable
us to get rid of this water, relief or drain valves are
connected to the engine cylinder and are opened by
the engineer for a short time, usually on starting.
After the engine is warmed up they are again closed to
prevent any excessive escape of steam. As water is
practically incompressible, should a quantity collect in
the cylinder serious damage might be caused. When
an engine is operating under such conditions that this
is apt to occur, relief valves similar in form to spring
safety valves are sometimes placed in the cylinder
head. These will provide means of escape for the
water without injury to the engine.

QUESTIONS.

1. How is the slide valve operated?

2. Sketch and describe the eccentric.

3. What is the travel of a valve?

4. What is meant by lap and lead?

5. What is the angular advance of the eccentric?

6. What effect has the angular advance of the eccentric on the
running of the engine ?



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22 ELEMENTARY STEAM ENGINEERING.

7. Define clearance; steam jacketing; wire drawing of steam.

8. What is a relief valve ?

9. How may the direction of rotation be changed ?

10. What is the travel of a valve?

11. If the eccentric center is offset 2 inches, what will be the
lengtii of the valve travel?



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CHAPTER in.
THE GOVERNOR.

If the eccentric is considered as a small cranky fas-
tened to the engine shaft by means either of a key or
iet screw or both, it may be so adjusted relative to
the main crank that its action transmitted through the
various rods will open the inlet port into the cylinder
just before the piston is ready to start on the forward
stroke, thus admitting a supply of steam.

It is evident that a given quantity of steam will do
but a certain amount of work. If the engine is running
at a certain speed with a certain load and a greater
number of machines are put on or the load increased
in any way without varying the amount of steam sup-
plied, or if, on the other hand, the load is decreased
and the same quantity of steam is allowed to enter the
cylinder, it is readily seen that the engine will slow
down in the first instance and increase in speed in the
second.

It is necessary, therefore, to change the quantity of
steam delivered to the cylinder in accordance with the
work that the engine is called upon to do at any given
time. For this ptupose a governor is employed.

The principle of the governor is as follows:

Let us assume a vertical rod a. Fig 17, connected to
the main shaft of the engine by means of a belt oper-
ating through a pulley and two beveled gears p and p^

23



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24



ELEMENTARY STEAM ENGINEERING.



SO that the rod a revolves at the same speed as the
engme shaft. Suspended from pivots in a block c at




. Pig. 17.

the upper part of rod a are two rods e, e, on the extrem-
ities of which are fastened the weights d^ d.

In accordance with the well-known principles of cen-
trifugal force and gravity, as the rod a increases or
decreases in speed, the weights d^ d will rise or fall.
Pivoted at e, e aire two rods also pivoted to the collar
/, which is free to rise and fall upon the rod a. Any
change in speed of the engine shaft will cause a corre-
sponding change in the speed of the rod a, also a change



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THE GOVERNOR. 2$

in the position of the weights cf, d and in the vertical
position of the collar /. If now the main steam pipe g^
supplying the engine, be considered as having a valve
at n, the valve rod being connected to the bell crank i,
which in turn is pivoted to the collar /, it will be seen
that as the speed of the engine increases, due to a
decrease of load, the collar / will rise and, through the
various levers and the valve n, will partially close the
opening of steam pipe g. On the other hand a de-
crease in speed, due to an increase of load, will cause
the collar / to drop, thereby opening the valve n. In
the one case less steam will be admitted to the cylinder
and in the other, more, thus providing for the change of
load, and maintaining a constant speed of the engine.

An engine governor is constantly in operation, chang-
ing with the load. A governor of this type is called a
THROTTLING GOVERNOR. The pivoted rods may be
replaced by springs serving the same purpose. Its dis-
advantage is that the steam admission is regulated at
a point some distance from the cylinder in which it
is used. Consequently the regulation of the engine is
not as close as might be desired. An improvement
upon this type is that of the Corliss engine, a descrip-
tion of which will be given later.

The previous illustration of the governor is correct
in principle, but it is merely a general description of
the governor action and its application to the regula-
tion of steam engines by varying the supply of steam to
meet the conditions existing at any given time. Gov-
ernors vary in detail of construction with almost every
make of engine. They are all built, in general, along



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26 ELEMENTARY STEAM ENGINEERING.

similar lines and operate on the principles of throttling
and automatic cut-off. In the former case, as already
mentionedy the operation takes place at some little dis-
tance from the cylinder, in consequence of which the
action is slow and not sufficiently sensitive for high
speed or economical work. Modem practice in this
line has made it necessary to bring the action of the
governor as close to the cylinder as possible and pref-
erably to have it operate directly on the steam valve, the
length of ports between the valve and the cylinder being
made as short as possible. An excellent illustration
of this type is that of the Corliss engine. Fig. 64, where
two steam valves, one at either end of the cylinder, are
immediately controlled by the governor. In case of a
slide-valve engine, as illustrated in the diagram showing
its action. Fig. 15, a somewhat different problem is pre-
sented, due to the fact that under certain conditions close
regulation of the valve would not allow sufficient open*
ing to introduce the required quantity of steam into the
cylinder. This difficulty is overcome by what is called
the riding cut-off. In this instance the valve and ports
are allowed a generous opening for the admission of
steam and two eccentrics are employed, one being per-
manently located in position and regulating the general
movement of the valve for the admission of steam, and
the other being a movable eccentric, the position of
which may be varied by the governor. This second
eccentric controls a secondary valve riding on top of
the slide valve proper and serving to cut off the steam
at the required moment.
The automatic type of governor, according to its



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THE GOVERNOR.



27



position, may be divided into two classes, namely: A
fly -ball or pendulum governor ^ and the shaft governor.
In these cases the centrifugal force, due to revolving a
weight around a central pivot, is the operating power.
In the fly-ball governor, similar in general construc-
tion to the illustration previously given, the weights and
rods, by which they are suspended, form a cone. The
path through which they move is called the cone of revo-
lution. When the governor is revolving about its axis
at a constant speed the weights revolve in a circle, hav-
ing the radius r. Fig. 18. The distance from this plane




^'^



Fig. 18.



to the intersection of the rod is called the height and
is equal to h. If the balls revolve fast, the centrifugal
force increases, r becomes greater, and h diminishes.'
The centrifugal force is expressed by the formula

F = ; that is, the centrifugal force varies directly as

the weight of the balls and the square of their velocity;
and inversely as the radius.
While the pendulum is revolving, centrifugal force



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28



ELEMENTARY STEAM ENGINEERING.



acts horizontally outward and tends to make the ball fly
from the center; gravity tends to make the ball drop
downward.

From certain mathematical calculations it is deter-
mined that the height hy Fig. i8, is independent of the
weight of the balls or the length of the rod; it depends
upon the number of revolutions.

The height varies inversely as the square of the num-
ber of revolutions.




Fig. 19.

As the speed increases the cone becomes shorter
because the weights rise. A constantly increasing
speed would produce a relatively diminishing height
of cone.

Such a governor is too sensitive for high-speed engines.
When, however, it is desired to employ such a governor
for fairly high speed, weights are added to the spindle,
usually above the collar /, Fig. 17.



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THE GOVERNOR. 29

Shaft governors, as their aame implies, are
fastened directly to the shaft, or to some comiecting part,
and revolve with it. They are of two general forms,
known as centrifugal and inertia governors.

The principle of the first type is illustrated in Fig. 19.
An arm whose length is r is pivoted at some point on the
fly-wheel, having attached to it a weight w. At a dis-
tance r' from the pivot a spring is attached so as to act
at right angles to the arm.

The centrifugal force of the weight, when the engine
is running, will be balanced by the pull of the spring
and the pull on the arm. But as the latter acts directly
through the supporting pivot it may be neglected. When
the engine increases in speed, the weight is thrown out
toward the rim of the wheel and, by means of a lever
through which it is connected to the eccentric, it shifts
the eccentric's position so as to produce an early cut-off.
Remember that the amount of valve travel is governed
by the throw of the eccentric, which, in turn, depends
upon the distance out of the true center that the shaft
hole is bored in the eccentric. Also bear in mind that
were this hole in the true center, the eccentric would
impart no movement to the slide valve.

The INERTIA GOVERNOR, Fig. 20, consists essen-
tially of a bar with a weight at either end pivoted at the
pin c. A spring d tends to hold the weights and the bar
in the starting position against the stop e. The eccentric
rod is connected to the pin / which takes the place of
the eccentric. As the speed increases, the centrifugal
force of the weights is increased until it overcomes the
tension of the spring d^ and the bar swings about its



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30



ELEMENTARY STEAM ENGINEERING.



supporting pivot c, changing the position of the pin /
relative to the center of the shaft, and producing the same
effect as was produced by the shifting of the eccentric in




Fig. 20.

the previous description. If there should occur a sudden
change of load the weights will tend for a moment to
continue ahead at the same speed at which they were
traveling. They may, therefore, run ahead or lag be-
hind. The result of this action is to change the position
of the eccentric pin in its relation to the center of the
shaft. In such a governor the inertia of the cross bar and
weights materially supplements the centrifugal force and
increases its sensitiveness.



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THE GOVERNOR. 31

QUESTIONS.

z. How is the speed of an engine controlled?

2. Make a sketch embodying the principles of governor opera-
tion.

3. What objection is there to the throttling governor ?

4. In a pendulum governor, what effect has the speed of rota-
tion on the position of the weights?

5. What effect has the weight of the balls on the height?

6. How does the height vary?

7. What is a shaft governor?

8. Name two forms of shaft governors*

9. Describe a centrifugal governor,
zo. Describe an inertia governor.



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CHAPTER IV.
ENGINE CALCULATIONS.

The horse power of an engine is calculated on the
basis of a period of one minute of time. One mechanical
horse power equals 33,000 foot pounds of work done in
one minute.

If, for example, a certain engine is doing 33,000 foot
pounds of work and is requiring two minutes' time to do
that work, it is working at the rate of one half of one
horse power. If it does the work in thirty seconds or
one half minute, it will then be able to do twice the work
in one minute and is working at the rate of two horse
power.

In the steam engine we may determine the horse power,
providing we know the pressure of steam, area of the
piston, and the number of feet through which the piston
travels per minute. The distance that the piston travels
from one end of the cylinder to the other is called the
stroke. The diameter of the cylinder, which equals
the diameter of the piston, and the length of the strokes
are usually given in inches.

The pressure of the steam is given in pounds per
square inch and it should be remembered that there are
two strokes per revolution. With these facts in mind
we may lay down the following rule:

The pressure of steam per square inch times the
number of square inches in the area of the piston
equals the total weight or load to be carried*

32



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ENGINE CALCULATIONS. 33

Ttvice the number of revolutions multiplied by the
length of the stroke in feet eqtial the distance through
which the load is to be carried. The length of the stroke
is calculated in feet, as we are determining the number
of foot pounds of work. Then, the total load times the
distance through which it is carried in one minute
equals the nuniber of foot pounds. In reducing this to
a convenient formula, we have the following:

S^ = H.P.
33,000

In the formula the pressure on each square inch is
indicated by the letter P, the length of the stroke in
feet is indicated by the letter L, the area of the piston
in square inches by the letter A, and the number of
strokes per minute by the letter N. The number of
foot pounds per horse power is indicated by the figures
33,000.

Find the horse power of an engine whose cylinder is
ten inches in diameter, the length of stroke two feet,
the pressure per square inch fifty pounds, with engine
making one hundred revolutions per minute.

PLAN so (lb.) X 2 (ft.) X 78'S (sq. in.) X 200 (strokes)
33,000 33>ooo (ft. lb.)

^ 1,570,000 (ft. lb.) ^p^
33,000

The diameter of the cylinder and the length of stroke
are expressed in inches. An engine having a cylinder
twelve inches in diameter and a stroke of two feet
would be written 12" X 24".



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34 ELEMENTARY STEAM ENGINEERING.

When the exhaust steam is allowed to escape directly
to the atmosphere, after having been used in the
engine cylinder, the engine is said to be running non-
condensing.

In some causes the exhaust steam is run to a condenser
and after having been condensed is sent to the boiler to
be used over again. Such an engine is of advantage
because the use of the condenser enables us to produce
a partial vacuum on the one side of the piston. The
reduction of pressure thus obtained is an immediate
gain in power. When the steam enters one side of the
cylinder, let us say at a pressure of one hundred pounds
per square inch, this is not entirely available in produc-
ing work. Some of it is used in doing the work neces-
sary to overcome friction in the engine itself. Some
more of it is required to force the exhaust steam out
into the air if it is a non-condensing engine ; the exhaust
steam offers resistance, because in order to get out of
the cylinder it has to displace the atmospheric air; also
considerable friction is produced in passing through
the pipes, fittings, valves, and other obstructions which
it meets between the cylinder and the outer air. This
resistance is called back pressure^ and the amount of
work required to overcome this back pressure must be
subtracted from the pressure of the steam entering the
cylinder in order to determine how much we have left
as effective pressure to do the external work. If, for
instance, the back pressure, due to friction in the pipes,
etc., plus that due to the atmosphere equals seventeen
pounds per square inch and the pressure entering the
cylinder equals one hundred pounds per square inch,



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ENGINE CALCULATIONS. 35

then seventeen of the origmal one hundred pounds must
be used to overcome the back pressure, leaving us only
eighty-three pounds as actual effective pressure to do
the external work.

If the engine is connected to a condenser and, instead
of exhausting steam direct to the atmosphere, exhausts
it to the condenser where it is changed into water, the
volume of this water will be far less than the volume of
steam which formed it. Consequently a partial vacuum
will be formed on the exhaust side of the piston and
the back pressure will fall from the previous seventeen
pounds per square inch to perhaps seven pounds per
square inch, giving us a total gain in effective pressure


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Online LibraryErnest Victor LallierAn elementary manual of the steam engine; containing also a chapter on the theory, construction and operation of internal combustion engines for the operating engineer → online text (page 2 of 17)