Then (113.0976 × 500 × 40)
————————————————————— = 68.544 H. P.
33000
Q. What is a single valve engine?
A. It is an engine in which a single valve controls the admission and
distribution of steam to both ends of the cylinder, or a common slide
valve engine.
Q. What is a four valve engine?
A. An engine which has separate steam and exhaust valves for each end
both top and bottom of cylinder, such as a Corliss engine.
Q. Into what three classes are engines divided with reference to the
manner in which they are governed?
A. Throttling engines, Automatic cut-off and Governor engines.
Q. What is an Automatic cut-off engine?
A. An engine in which the amount of steam supplied is automatically cut
off at various points in the stroke, in accordance with the load and
pressure. In Throttling engines the volume admitted is constant and the
pressure varied. In Automatic cut-off engines, steam is admitted at
the highest available pressure, and the volume is varied to meet the
requirements of the load. In Governor engines, the steam is admitted
and cut off by the governor.
Q. What is a throttle governed engine?
A. An engine in which the amount of steam supplied is regulated by
changing the pressure at which it enters the cylinder in accordance
with the variation of the load.
Q. What is a Governor engine?
A. An engine in which the supply of steam is regulated by the governor.
Q. Into what classes may the Automatic cut-off engine be divided?
A. Into two classes: The four valve engine, in which the cut-off is
usually effected by a detaching mechanism or trip under the control of
the governor; the single valve engine, in which the point of cut-off is
varied by changing the amount of travel of the valve.
Q. Give examples of the single valve type.
A. High speed, self-contained engines which have shaft governors.
Q. What are their advantages?
A. High rotative speed, compactness, portability, light weight and
simplicity.
Q. Are they more economical than the four valve engine?
A. No; the four valve engines are the more economical.
Q. Give a prominent example of the four valve engine.
A. The Reynold’s Corliss.
Q. What is meant by an engine running “over”?
A. The top of the drive wheel running away from the cylinder.
Q. What is meant by an engine running “under”?
A. The top of the drive wheel running towards the cylinder.
Q. Which way are engines generally run?
A. “Over.”
Q. What advantages do engines have in running “over”?
A. The pressure of the cross-head on engines running over, is always
downward upon the guides; for when the pressure is on the head end
of the piston, the pressure against the connecting rod which is
pointing upward, reacts by pressing the cross-head down upon the lower
guide, and when the pressure is on the crank end of the cylinder, the
cross-head will be dragging the crank, and as the crank is below the
center line, it will pull the cross-head down upon the lower guide,
while if the engine is running under, the pressure of the cross-head
will be upon the top guide, both on the outward and inward strokes, and
unless the cross-head is nicely adjusted to its guides and the guides
are perfectly parallel, the cross-head will be lifted when subjected
to thrust, and will fall on the center by its own weight, causing the
engine to pound.
Q. At what point in the stroke is the pressure on the cross-head
greatest with a uniform pressure in the cylinder?
A. When the crank is at right angles to the guide.
Q. How does the relative length of the connecting rod affect this
pressure?
A. The longer the connecting rod as compared with the crank, the less
will be the pressure on the guides.
Q. What is the usual ratio of connecting rod to crank?
A. The connecting rod is from four to six times the length of the crank.
Q. Are there any objections to a long connecting rod?
A. A long connecting rod makes a long engine, and makes extra cost in
the bed or frame and the room occupied. The longer rod is heavier, and
brings extra weight upon the cross-head, guides and crank-pin. The long
rod also lacks stiffness unless excessively heavy.
Q. What determines the length of the crank?
A. The stroke.
Q. What limits the stroke?
A. The piston’s speed limits the length of stroke allowable with a
given rotative speed, or the number of revolutions per minute with a
given stroke.
Q. What is the practical limit of piston’s speed?
A. Engines of from four to six foot stroke can run at from seven to
eight hundred feet piston’s speed per minute. Those of shorter stroke
should not run over six hundred feet.
Q. Why do high speed engines have a short stroke in comparison with the
diameter of their cylinders?
A. So that they can run at a high rate of speed without exceeding the
limit of piston’s travel.
Q. What is the office of the fly-wheel?
A. It maintains a uniformity of motion of the crank, notwithstanding
the unequal moving force upon the crank-pin.
Q. Is the force upon the crank-pin unequal, even when the pressure from
the cylinder is uniform throughout the stroke?
A. Yes. No matter what the pressure on the piston is, it has no effect
in turning the engine when the crank is in line with the guides, which
is termed “on the center.” As the crank gets away from the centers, the
effect of a given pressure becomes greater, and reaches its maximum
when the crank is nearly at right angles with the guides.
Q. How does the fly-wheel counteract the jerky motion of the crank
which would result from this?
A. By its tendency to resist an excessive moving force, and by its
momentum keeps the engine in motion when the moving force is deficient.
Q. What would you do if the cylinder gets worn or cut from too tight
rings or lack of oil?
A. Rebore the cylinder.
Q. What would you do if the crank-pin heats, gets worn or cut?
A. If bent it should be turned true again; if not bent it can be filed
and polished perfectly true by hand.
Q. What would you do if the main bearings heat?
A. Loosen the caps and apply plenty of good oil. If this does not stop
it take off the caps, examine the oil holes to ascertain why the oil
does not reach the bearing. If the bearings have become rough and cut,
the shaft will have to be smoothed again.
Q. Would any harm result from starting an engine with the drip cocks
closed?
A. Yes, the condensed steam filling the space would smash the cylinder
or piston head.
Q. What do you mean by atmospheric pressure?
A. The weight of the atmosphere, which is 14.7 lbs. per square inch at
sea level.
Q. How hot can you get water with exhaust steam under atmospheric
pressure?
A. 212° Fahr.
Q. Does atmospheric pressure have any influence on the boiling point?
A. It does.
Q. Would you run an engine with throttle wide open, or partly open?
A. Wide open on governor engines, as it is more economical.
Q. How many pounds of water required per horse-power for the best
engines?
A. From 25 to 30 pounds.
Q. At what temperature has iron the greatest tensile strength?
A. About 600 degrees.
Q. How much water is consumed (in pounds) per hour per indicated horse
power?
A. From 25 to 60 pounds.
Q. How much steam will be evaporated from one cubic inch of water under
atmospheric pressure?
A. About one cubic foot, approximately.
Q. How much coal is consumed per hour per indicated horse-power?
A. From two to seven pounds.
Q. How much does one cubic foot of fresh water weigh?
A. 62½ pounds.
Q. How much does one cubic foot of iron weigh?
A. 486⁶/₁₀ pounds.
Q. What does one square foot of half inch boiler iron weigh?
A. Twenty pounds.
Q. For steam purposes, how much wood is required to equal one ton of
coal?
A. About 4000 pounds of wood.
Q. Of what does coal consist?
A. Carbon, nitrogen, sulphur, hydrogen, oxygen and ash.
Q. What are their relative proportions?
A. There are different proportions in different specimens of coal. The
average per cent is carbon eighty, nitrogen one, sulphur two, hydrogen
five, oxygen seven, ash five.
Q. Of what is air composed?
A. It is composed of nitrogen and oxygen in the proportion of
seventy-seven of nitrogen and 23 of oxygen.
Q. Of what does water consist?
A. Hydrogen and oxygen in the proportion of one of hydrogen to eight of
oxygen by weight.
Q. What are the different kinds of heat?
A. Latent heat, sensible heat, and sometimes total heat.
Q. What is meant by latent heat?
A. Heat that does not affect the thermometer and which expends itself
in changing the nature of a body, such as turning ice into water or
water into steam.
Q. Under what circumstances do bodies get latent heat?
A. When they are passing from a solid to a liquid state, or from a
liquid to a gaseous state.
Q. How can latent heat be recovered?
A. By bringing the body back from a state of gas to a liquid, or from a
liquid to a solid.
Q. If the power is in coal, why should we use steam?
A. Because steam has some properties which make it an invaluable agent
for applying the energy of the heat to the engine.
Q. What is steam?
A. It is an invisible elastic fluid generated from water by the
application of heat.
Q. What are its properties which make it so valuable to us?
A. First. The ease with which we can condense it.
Second. The small space which it occupies when condensed.
Third. Its great expansive power.
Q. What do you understand by the term “horse-power”?
A. A horse-power is equivalent to raising 33,000 pounds one foot per
minute.
Q. What do you understand by “lead” on an engine valve?
A. Lead on a valve is the admission of steam into the cylinder before
the piston completes its stroke.
Q. What are considered the greatest improvements on the stationary
engine in the past forty years?
A. The Corliss valve gear, the governor, the compound and triple
expansion.
Q. What is meant by triple expansion engine?
A. A triple expansion engine has three cylinders using the same steam
expansively in each one.
Q. What is the clearance of an engine as the term is applied at the
present time?
A. Clearance is the space between the cylinder head and the piston head
with the ports included.
Q. What is the principal which distinguishes a non-condensing from a
condensing engine?
A. Where no condenser is used, and the exhaust steam is open to the
atmosphere, it is a non-condensing engine.
Q. Why do you condense steam?
A. To form a vacuum and thus remove the atmospheric and back pressure
that would otherwise be on the piston, thereby getting more useful work
out of the steam.
Q. What is meant by vacuum?
A. A space void of all pressure.
Q. How can you maintain a vacuum?
A. By the steam used being constantly condensed by the cold water or
cold tubes, and the air pump constantly clearing the condenser.
Q. Why does condensing the used steam form a vacuum?
A. Because a cubic foot of steam at atmospheric pressure shrinks into
about one cubic inch of water.
Q. What is a condenser as applied to an engine?
A. The condenser is that part of an engine into which the exhaust steam
enters and is condensed.
Q. About how much gain is there by using the condenser?
A. Seventeen to twenty-five per cent. where cost of water is not
figured.
Q. What do you understand by the use of steam expansively?
A. Where steam admitted at a certain pressure is cut off and allowed to
expand to a lower pressure.
Q. How many inches of vacuum gives the best result in a condensing
engine?
A. About 25 inches.
Q. What is meant by a horizontal compound tandem engine?
A. One cylinder being back of the other with two pistons on the same
rod.
Q. What do you understand by lap?
A. Outside lap is that portion of the valve which extends beyond the
ports when valve is placed on the center of its travel; inside lap is
that portion of valve which projects over the ports on inside or toward
the middle of the valve.
Q. Of what use is lap?
A. It gives expansion to the steam in the cylinder.
Q. What is the dead center of an engine?
A. The point where the center of shaft, center of wrist-pin and center
of piston rod are in the same straight line.
Q. From what cause do belts have power to drive shafting?
A. By friction and cohesion.
Q. When would you oil an engine?
A. Before starting it and as often while running as is necessary.
Q. What is the tensile strength of American boiler iron?
A. 40,000 to 60,000 pounds per square inch.
Q. What are the principal defects found in boiler iron?
A. Imperfect welding, brittleness, low ductility.
Q. What is the advantage of steel as a material for boiler plate?
A. Tensile strength, ductility, homogeneity, malleability and freedom
from laminations and blisters.
Q. What are the disadvantages of steel as a material for boiler plate?
A. It requires greater care in working than iron and is subject to
flaws induced by the pressure of gas bubbles in the ingots from which
the plates are made.
Q. How far apart should stay bolts be put in a boiler?
A. They vary from 4 to 6 inches apart, depending on thickness of
plates, size of stay bolts and amount of steam pressure to be carried.
[Illustration: J. I. Case Portable Skid Engine.]