FOUNDATION AND EXHAUST
The reader will remember from what has already been said that a
gas-engine is a motor which, more than any other, is subjected to
forces, suddenly and repeatedly exerted, producing violent reactions on
the foundation. It follows that the foundation must be made particularly
resistant by properly determining its shape and size and by carefully
selecting the material of which it is to be built.
=The Foundation Materials.=--Well-hardened brick should be used. The top
course of bricks should be laid on edge. It is advisable to increase the
stability of the foundation by longitudinally elongating it toward the
base, as shown in the accompanying diagram (Fig. 54).
As a binding material, only mortar composed of coarse sand or river sand
and of good cement, should be used. Instead of coarse sand, crushed
slag, well-screened, may be employed. The mortar should consist of 2/3
slag and 1/3 cement. Oil should not in any way come into contact with
the mortar; it may percolate through the cement and alter its resistant
qualities.
As in the construction of all foundations, care should be taken to
excavate down to good soil and to line the bottom with concrete, in
order to form a single mass of artificial stone. A day or two should be
allowed for the masonry to dry out, before filling in around it.
When the engine is installed on the ground floor above a vaulted cellar,
the foundation should not rest directly on the vault below or on the
joists, but should be built upon the very floor of the cellar, so that
it passes through the planking of the ground floor without contact.
[Illustration: FIG. 54.--Method of building the foundation.]
When the engine is to be installed on a staging, the method of securing
it in place illustrated in Fig. 55 should be adopted.
Although a foundation, built in the manner described, will fulfill the
usual conditions of an industrial installation, it will be inadequate
for special cases in which trepidation is to be expected. Such is the
case when engines are to be installed in places where, owing to the
absence of factories, it is necessary to avoid all nuisance, such as
noise, trepidations, odors, and the like.
[Illustration: FIG. 55.--Elevated foundation.]
=Vibration.=--In order to prevent the transmission of vibration, the
foundation should be carefully insulated from all neighboring walls. For
this purpose various insulating substances called "anti-vibratory" are
to be recommended. Among these may be mentioned horsehair, felt packing,
cork, and the like. The efficacy of these substances depends much on the
manner in which they are applied. It is always advisable to interpose a
layer of one of these substances, from one to four inches thick, between
the foundation and the surrounding soil, the thickness varying with the
nature of the material used and the effect to be obtained. Between the
bed of concrete, mentioned previously, and the foundation-masonry and
between the foundation and the engine-frame, a layer of insulating
material may well be placed. Preference is to be given to substances not
likely to rot or at least not likely to lose their insulating property,
when acted upon by heat, moisture or pressure.
Here it may not be amiss to warn against the utilization of cork for the
bottom of the foundation; for water may cause the cork to swell and to
dislocate the foundation or destroy its level.
The employment of the various substances mentioned does not entail any
great expense when the foundations are not large and the engines are
light. But the cost becomes considerable when insulating material is to
be employed for the foundation of a 30 to 50 horse-power engine and
upwards. For an engine of such size the author recommends an arrangement
as simple as it is efficient, which consists in placing the foundation
of the engine in a veritable masonry basin, the bottom of which is a bed
of concrete of suitable thickness. The foundation is so placed that the
lateral surfaces are absolutely independent of the supporting-walls of
the basin thus formed. Care should be taken to cover the bottom with a
layer of dry sand, rammed down well, varying in thickness with each
case. This layer of sand constitutes the anti-vibratory material and
confines the trepidations of the engine to the foundation.
As a result of this arrangement, it should be observed that, being
unsupported laterally, the foundation should be all the more resistant,
for which reason the base-area and weight should be increased by 30 to
40 per cent. The expense entailed will be largely offset by saving the
cost of special anti-vibratory substances. In places liable to be
flooded by water, the basin should be cemented or asphalted.
When the engine is of some size and is intended for the driving of one
or more dynamos which may themselves give rise to vibrations, the
dynamos are secured directly to the foundation of the engine, which is
extended for that purpose, so that both machines are carried solidly on
a single base.
The foregoing outline should not lead the proprietor of a plant to
dispense with the services of experts, whose long experience has brought
home to them the difficulties to be overcome in special cases.
It should here be stated, as a general rule, that the bricks should be
thoroughly moistened before they are laid in order that they may grip
the mortar.
After having been placed on the foundation and roughly trimmed with
respect to the transmission devices, the engine is carefully leveled by
means of hardwood wedges driven under the base. This done, the bolts are
sealed by very gradually pouring a cement wash into the holes, and
allowing it to set. When the holes are completely filled and the bolts
securely fastened in place, a shallow rim, or edge of clay, or sand is
run around the cast base, so as to form a small box or trough, in which
cement is also poured for the purpose of firmly binding the engine frame
and foundation together. When, as in the case of electric-light engines,
single extra-heavy fly-wheels are employed, provided with bearings held
in independent cast supports, the following rule should be observed to
prevent the overheating due to unlevelness, which usually occurs at the
bushings of these bearings: That part of the foundation which is to
receive such a support should rest directly on the concrete bed and
should be rigidly connected at the bottom with the main foundation. When
the foundation is completely blocked up, the fly-wheel bearing with its
support is hung to the crank-shaft; and not until this is effected is
the masonry at the base of the support completed and rigidly fixed in
its proper position.
For very large engines, the foundation-bolts should be particularly well
sealed into the foundation. In order to attain this end the bricks are
laid around the bolt-holes, alternately projected and retracted as shown
in Fig. 54. Broken stone is then rammed down around the fixed bolt; in
the interstices cement wash is poured.
=Air Vibration, etc.=--Vibration due chiefly to the transmission of
noises and the displacement of air by the piston should not be confused
with the trepidations previously mentioned.
The noise of an engine is caused by two distinct phenomena. The one is
due to the transmitting properties of the entire solid mass constituting
the frame, the foundation, and the soil. The other is due to vibrations
transmitted to the air. In both cases, in order to reduce the noise to a
minimum, the moving parts should be kept nicely adjusted, and above all,
shocks avoided, the more harmful of which are caused by the play between
the joint at the foot of the connecting-rod and the piston-pin, and
between the head of the connecting-rod and the crank-shaft.
Although smooth running of the engine may be assured, there is always an
inherent drawback in the rapid reciprocating movement of the piston. In
large, single-acting gas-engines, a considerable displacement of air is
thus produced. In the case of a forty horse-power engine having a
cylinder diameter and piston-stroke respectively of 13-3/4 inches and
21-3/5 inches, it is evident that at each stroke the piston will
displace about 2 cubic feet of air, the effect of which will be doubled
when it is considered that on the forward stroke back pressure is
created and on the return stroke suction is produced.
The air motion caused by the engine is the more readily felt as the
engine-room is smaller. If the room, for example, be 9 feet by 15 feet
by 8 feet, the volume will be 1,080 cubic feet. From this it follows
that the 2 cubic feet of air in the case supposed will be alternately
displaced six times each second, which means the displacement of 12
cubic feet at short intervals with an average speed of 550 feet per
minute. Such vibrations transmitted to halls or neighboring rooms are
due entirely to the displacement of the air.
In installations where the air-intake of the engine is located in the
engine-room, a certain compensation is secured, at the period of
suction, between the quantity of air expelled on the forward stroke of
the piston and the quantity of air drawn into the cylinder. From this it
follows that the vibration caused by the movement of the air is felt
less and occurs but once for two revolutions of the engine.
This phenomenon is very manifest in narrow rooms in which the engine
happens to be installed near glass windows. By reason of the elasticity
of the glass, the windows acquire a vibratory movement corresponding in
period with half the number of revolutions of the engine. It follows
from the preceding that, in order to do away with the air vibration
occasioned by the piston in drawing in and forcing out air in an
enclosed space, openings should be provided for the entrance of large
quantities of air, or a sufficient supply of air should be forced in by
means of a fan.
The author ends this section with the advice that all pipes in general
and the exhaust-pipe in particular be insulated from the foundation and
from the walls through which they pass as well as from the ground, as
metal pipes are good conductors of sound and liable to carry to some
distance from the engine the sounds of the moving parts.
=Exhaust Noises.=--Among the most difficult noises to muffle is that of
the exhaust. Indeed, it is the exhaust above all that betrays the
gas-engine by its discharge to the exterior through the exhaust-pipe.
The most commonly employed means for rendering the exhaust less
perceptible consists in extending the pipe upward as far as possible,
even to the height of the roof. This is an easy way out of the
difficulty; but it has a bad effect on the operation of the engine. It
reduces the power generated and increases the consumption, as will be
explained in a special paragraph.
Expansion-boxes, more commonly called exhaust-mufflers, considerably
deaden the noise of explosion by the use of two or three successive
receptacles. But this remedy is attended with the same faults that mark
the use of extremely long pipes. The best plan is to mount a single
exhaust-muffler near the discharge of the engine in the engine-room
itself, where it will serve at least the purpose of localizing the
sound.
[Illustration: FIG. 56.--Exhaust-muffler.]
The employment of pipes of sufficiently large cross-section to
constitute expansion-boxes in themselves will also muffle the exhaust. A
more complete solution of the problem is obtained by causing the
exhaust-pipe, after leaving the muffler, to discharge into a masonry
trough having a volume equal to twelve times that of the engine-cylinder
(Fig. 56). This trough should be divided into two parts, separated by a
horizontal iron grating. Into the lower part, which is empty, the
exhaust-pipe discharges; in the upper part, paving-blocks or hard stones
not likely to crumble with the heat, are placed. Between this layer of
stones and the cover it is advisable to leave a space equal to the
first. Here the gases may expand after having been divided into many
parts in passing through the spaces left between adjacent stones. The
trough should not be closed by a rigid cover; for, although efficient
muffling may be attained, certain disadvantages are nevertheless
encountered. It may happen that in a badly regulated engine, unburnt
gases may be discharged into this trough, forming an explosive mixture
which will be ignited by the next explosion, causing considerable
damage. Still, the explosion will be less dangerous than noisy. It may
be mentioned in passing that this disadvantage occurs rarely.
A second arrangement consists in superposing the end of the exhaust-pipe
upon a casing of suitable size, which casing is partitioned off by
several perforated baffle-plates. This casing is preferably made of
wood, lined with metal, so that it will not be resonant. The size of the
casing, the number of partitions and their perforations, and the manner
of disposing the partitions have much to do with the result to be
obtained. Here again the experience of the expert is of use.
Various other systems are employed, depending upon the particular
circumstances of each case. Among these systems may be mentioned those
in which the pipe is forked at its end to form either a yoke (Fig. 57)
or a double curve, each branch of which terminates in a muffler (Fig.
58).
[Illustration: FIG. 57.]
[Illustration: FIG. 58.--Two types of exhaust-mufflers.]
It should be observed that, under ordinary conditions, noises heard as
hissing sounds are often due to the presence of projections, or to
distortion of the pipes near the discharge opening. Consequently, in
connecting the pipes, care should be taken that the joints or seams have
no interior projections. Occasionally, water may be injected into the
exhaust-muffler in order to condense the vapors of the exhaust, the
result being a deadening of the noises; but in order to be truly
efficient this method should be employed with discretion, for which
reason the advice of an expert is of value.