THE SELECTION OF AN ENGINE
Explosion-engines are of many types. Gas-engines, of the four-cycle
type, such as are industrially employed, will here be principally
considered.
=The Otto Cycle.=--The term "four-cycle" motor, or Otto engine, has its
origin in the manner in which the engine operates. A complete cycle
comprises four distinct periods which are diagrammatically reproduced in
the accompanying drawings.
=The First Period.=--_Suction:_ The piston is driven forward, creating a
vacuum in the cylinder, and simultaneously drawing in a certain quantity
of air and gas (Fig. 2).
[Illustration: FIG. 2.--First cycle: Suction.]
=The Second Period.=--_Compression:_ The piston returns to its initial
position. All admission and exhaust valves are closed (Fig. 3). The
mixture drawn in during the first period is compressed.
=The Third Period.=--_Explosion and Expansion:_ When the piston has
reached the end of its return stroke, the compressed mixture is ignited.
Explosion takes place at the dead center. The expansion of the gas
drives the piston forward (Fig. 4).
[Illustration: FIG. 3.--Second cycle: Compression.]
[Illustration: FIG. 4.--Third cycle: Explosion and expansion.]
=The Fourth Period.=--_Exhaust:_ The piston returns a second time. The
exhaust-valve is opened, and the products of combustion are discharged
(Fig. 5).
[Illustration: FIG. 5.--Fourth cycle: Exhaust.]
These various cycles succeed one another, passing through the same
phases in the same order.
=Valve Mechanism.=--It is to be noted that in modern motors valves are
used which are better adapted to the peculiarities of explosion-engines
than were the old slide-valves used when the Otto engine was first
introduced. The slide-valve may now be considered as an antiquated
distributing device with which it is impossible to obtain a low
consumption.
In old-time gas-engines rather low compressions were used. Consequently
a very low explosive power of the gaseous mixture, and low temperatures
were obtained. The slide-valves were held to their seats by the pressure
of external springs, and were generously lubricated. Under these
conditions they operated regularly. Nowadays, the necessity of using
gas-engines which are really economical has led to the use of high
compressions with the result that powerful explosions and high
temperatures are obtained. Under these conditions slide-valves would
work poorly. They would not be sufficiently tight. To lubricate them
would be difficult and ineffective. Furthermore, large engines are
widely used in actual practice, and with these motors the frictional
resistance of large slide-valves, moving on extensive surfaces would be
considerable and would appreciably reduce the amount of useful work
performed.
[Illustration: FIG. 6.--Modern valve mechanism.]
By reason of its peculiar operation, the slide-valve is objectionable,
the gases being throttled at the time of their admission and discharge.
As a result of these objections there are losses in the charge; and
obnoxious counter-pressures occur. The necessity of using elements
simple in their operation and free from the objections which have been
mentioned, has naturally led to the adoption of the present valve. This
valve is used both for the suction of the gas and of the air, as well as
for the exhaust, with the result that either of these two essential
phases in the operation of the motor can be independently controlled.
The valves offer the following advantages: Their tightness increases
with the pressure, since they always open toward the interior of the
cylinder (Fig. 6). They have no rubbing surfaces, and need not,
therefore, be lubricated. Their opening is controlled by levers provided
with quick-acting cams; and their closure is effected by coiled springs
almost instantaneous in their action (Fig. 7). Each valve, depending
upon the purpose for which it is used, can be mounted in that part of
the cylinder best suited for its particular function. The types of
valved motors now used are many and various. In order to attain economy
in consumption and regularity in operation they should meet certain
essential requirements which will here be reviewed.
Apart from proportioning the areas properly and from providing a
suitable means of operation, it is indispensable that the valves should
be readily accessible. Indeed, the valves should be regularly examined,
cleaned and ground. It follows that it should be possible to take them
apart easily and quickly.
[Illustration: FIG. 7.--Controlling mechanism of valve.]
It is necessary that the exhaust-valve be well cooled; otherwise the
valve, exposed as it is to high temperatures, will suffer derangement
and may cause leakage. The water-jacket should, therefore, surround the
seat of the exhaust-valve, care being taken that the cooling water be
admitted as near to it as possible (Fig. 8). The motor should control
the air-let valve or that of the gaseous mixture. Hence these valves
should not be actuated simply by springs, because springs are apt to
move under the influence of the vacuum produced by suction.
[Illustration: FIG. 8.--Water-jacketed valve.]
The mixture of gas and air should not be admitted into the cylinder at
too low a pressure; otherwise the weight of the mixture admitted would
be lower than it ought to be, inasmuch as under these conditions the
valve will be opened too tardily and closed prematurely. At the
beginning as well as at the end of its stroke the linear velocity of the
piston is quite inadequate to create a vacuum sufficient to overcome the
resistance of the spring. It is, therefore, generally the practice
separately to control the opening or closing of the one or the other
valve (gas-valve or mixture-valve). Consequently these valves must be
actuated independently of each other. Nowadays they are mechanically
controlled almost exclusively,--a method which is advocated by
well-known designers for industrial motors in particular. Valves which
are not actuated in this manner (free valves) have only the advantage of
simplicity of operation. Nevertheless, this arrangement is still to be
found in certain oil and benzine engines, notably in automobile-motors.
In these motors it is necessary to atomize the liquid fuel by means of
aspired air, in order to produce an explosive, gaseous mixture.
=Ignition.=--In the development of the gas-engine, the incandescent tube
and the electric spark have taken the place of the obsolete naked flame.
The last-mentioned mode of exploding the gaseous mixture will not,
therefore, be discussed.
The hot tube of porcelain or of metal has the indisputable merit of
regularity of operation. The methods by which this operation is made as
perfect as possible are many. Since certainty of ignition is obtained by
means of the tube, it is important to time the ignition, so that it
shall occur exactly at the moment when the piston is at the dead
center. It has been previously stated that premature or belated ignition
of the explosive mixture appreciably lessens the amount of useful work
performed by the expansion of the gas. If ignition occur too soon, the
mixture will be exploded before the piston has reached the dead center
on its return stroke. As a result, the piston must overcome a
considerable resistance due to the premature explosion and the
consequent pressure. Furthermore, by reason of the high temperature of
explosion, the gaseous products are very rapidly cooled. This rapid
cooling causes a sudden drop in the pressure; and since a certain
interval elapses between the moment of explosion and the moment when the
piston starts on its forward stroke, the useful motive effort is the
more diminished as the ignition is more premature.
=Incandescent Tubes.=--In Figs. 9 and 10 two systems most commonly used
are illustrated. In these two arrangements, in which no valve is used,
the length or height to which the tube is heated by the outer flame is
so controlled that the gaseous mixture, which has been driven into the
tube after compression, reaches the incandescent zone as nearly as
possible at the exact moment when ignition and explosion should take
place. The temperature of the flame of the burner, the richness of the
gaseous mixture, and other circumstances, however, have a marked
influence on the time of ignition, so that the mixture is never fired at
the exact moment mentioned.
[Illustration: FIGS. 9-10.--Valveless hot tubes.]
These considerations lead to the conclusion that motors in which the
mixture is exploded by hot tubes provided with an ignition-valve are
preferable to valveless tubes. By the use of a special valve, positively
controlled by the motor itself, the chances of untimely ignition are
lessened, because it is necessary simply to regulate the temperature and
the position of the tube in order that ignition may be surely effected
immediately upon the opening of the valve, at the very moment the
cylinder gases come into contact with the incandescent portion of the
tube (Fig. 11). Many manufacturers, however, do not employ the
ignition-valve on motors of less than 15 to 20 horse-power, chiefly
because of the cheaper construction. The total consumption is of less
moment in a motor of small than of great power, and the loss due to the
lack of an ignition-valve not so marked. In a high-power engine,
premature explosion may be the cause of the breaking of a vital part,
such as the piston-rod or the crank-shaft. For this reason, a valve is
indispensable for engines of more than 20 to 25 horse-power. A breakage
of this kind is less to be feared in a small motor, where the parts are
comparatively stout. The gas consumption of a well-designed burner does
not exceed from 3.5 to 5 cubic feet per hour.
[Illustration: FIG. 11.--Ignition-tube with valve.]
=Electric Ignition.=--Electric ignition consists in producing a spark in
the explosion-chamber of the engine. The nicety with which it can be
controlled gives it an undeniable advantage over the hot tube. But the
objection has been raised, perhaps with some force, that it entails
certain complications in installing the engine. Its opponents even
assert that the power and the rapidity of the deflagration of the
explosive mixture are greater with hot-tube ignition. This reason may
have caused the hot-tube system to prevail in England, where
manufacturers of gas-engines are very numerous and not lacking in
experience.
Electric ignition is effected in gas-engines by means of a battery and
spark-coil, or by means of a small magneto machine which mechanically
produces a current-breaking spark.
[Illustration: FIG. 12.--Electric ignition by spark-coil and battery.]
[Illustration: FIG. 13.--Spark-plug.]
=Electric Ignition by Battery and Induction-Coil.=--The first system is
the cheaper; but it exacts the most painstaking care in maintaining the
parts in proper working condition. It comprises three essential
elements--a battery, a coil, and a spark-plug (Fig. 12). The battery may
be a storage-battery, which must, consequently, be recharged from time
to time; or it may be a primary battery which must be frequently
renewed and carefully cleaned. The induction-coil is fitted with a
trembler or interrupter, which easily gets out of order and which must
be regulated with considerable accuracy. The spark-plug is a
particularly delicate part, subject to many possible accidents. The
porcelain of which it is made is liable to crack. It is hard to obtain
absolutely perfect insulation; for the terminals deteriorate as they
become overheated, break, or become foul (Fig. 13). In oil-engines,
especially, soot is rapidly deposited on the terminals, so that no spark
can be produced. In benzine or naphtha motors, such an accident is less
likely to happen. In automobile-motors, however, the spark-plug only too
often fails to perform its function. The one remedy for these evils is
to be found in the most painstaking care of the spark-plug and of the
other elements of the ignition system.
[Illustration: FIG. 14.--Magneto ignition apparatus.]
[Illustration: FIG. 15.--General view and details of a magneto ignition
apparatus.]
=Ignition by Magnetos.=--Magneto apparatus, on the other hand, are
noteworthy for the regularity of their operation. They may be used for
several years without being remagnetized, and require no exceptional
care. Magneto ignition devices are mechanically actuated, the necessary
displacement of the coil being effected by means of a cam carried on a
shaft turning with half the motor speed (Figs. 14 and 15). At the moment
when it is released by the cam, the coil is suddenly returned to its
initial position by means of a spring. This rapid movement generates a
current that passes through terminals, which are arranged within the
cylinder and which are immediately separated by mechanical means. Thus a
much hotter circuit-breaking spark is produced, which is very much more
energetic than that of a battery and induction-coil, and which surely
ignites the gaseous mixture in the cylinder. The terminals are generally
of steel, sometimes pointed with nickel or platinum (Fig. 16). The only
precaution to be observed is the exclusion of moisture and occasional
cleaning. For engines driven by producer-gas magneto-igniters are
preferable to cells and batteries. In general, electrical ignition is to
be recommended for high-pressure engines.
[Illustration: FIG. 16.--Contacts of a magneto-igniter.]
[Illustration: FIG. 17.--Device for regulating the moment of ignition.]
In order to explain more clearly modern methods of ignition a diagram is
presented, showing an electric magneto-igniter applied to the
cylinder-head of a Winterthur motor, and also a sectional view of the
member varying the make-and-break contacts which are mounted in the
explosion-chamber (Figs. 18 and 19)
1. The magneto _A_ consists of horseshoe-magnets, between the poles of
which the armature rotates. At its conically turned end, the
armature-shaft carries an arm _B_, held in place by a nut.
[Illustration: FIG. 18.--Winterthur electric ignition system.]
2. The igniter _C_ is a casting secured to the cylinder-head by a
movable strap and provided with two axes _D_ and _M_, of which the one,
_D_, made of bronze, is movable, and is fitted with a small interior
contact-hammer, a percussion-lever, and an exterior recoil-spring; the
other, _M_, is fixed, insulated, and arranged to receive the current
from the magneto _A_, by means of an insulated copper wire _E_.
3. The spring _F_ comprises two continuous coils contained in a brass
casing, and actuating a steel striking or percussion-pin.
4. The controlling devices of the magneto include a stem or rod _G_,
slidable in a guide _H_, provided with a safety spring and mounted on an
eccentric spindle, the position of which can be varied by means of a
regulating-lever (_I_). The rod is operated from the distributing-shaft,
on the conical end of which a cam _J_ carrying a spindle is secured.
[Illustration: FIG. 19.--Contacts of the Winterthur system.]
_Regulation of the Magneto._--The position assumed by the armature when
at rest is a matter of importance in obtaining a good spark on breaking
the circuit. The marks on the armature should be noted. The position of
the armature may be experimentally varied, in order to obtain a spark of
maximum intensity, by changing the position of the arm B on the
armature-shaft.
_Control of the Magneto._--The controlling gear should enable the
armature to oscillate from 20 to 25 degrees. The time at which the
breaking of the circuit is effected can be regulated by shifting the
handle (_I_). In starting the engine, the circuit can be broken with a
slight retardation, which is lessened as the engine attains its normal
speed.
_Igniter._--It is advisable that there should be a play of 1/2 mm.
(0.0196 in.) between the lever _Z_ when at rest and the striking-pin.
The axis _D_ of the circuit-breaking device should be easily movable;
and the hammer which it carries at its end toward the interior of the
cylinder should be in perfect contact with the stationary spindle _M_,
which is electrically insulated. This spindle _M_ should be well
enclosed, in order to prevent any leakage that might cause a
deterioration of the insulating material.
The subject of ignition is of such extreme importance that the author
will recur to it from time to time in the various chapters of this book.
Too much stress cannot be laid upon proper timing; otherwise there will
be a needless waste of power. Cleanliness is a point that must be
observed scrupulously; for spark-plugs are apt to foul only too readily,
with the result that short-circuits and misfires are apt to occur. In
oil and volatile hydrocarbon engines the tendency to fouling is
particularly noticeable. In the chapter devoted to these forms of
motors the author has dwelt upon the precautions that should be taken to
forestall a possible derangement of the ignition apparatus. As a general
rule the ignition apparatus installed by trustworthy manufacturers will
be found best suited for the requirements of the engine.
The apparatus should be fitted with a device by which the ignition can
be duly timed by hand during operation (Fig. 17).
[Illustration: FIG. 20.--Design of the piston.]
=The Piston.=--Coming, as it does, continually in contact with the
ignited gases, the piston is gradually heated to a high temperature. The
rear face of the piston should preferably be plane. Curved surfaces are
not to be recommended because they cool off badly. Likewise, faces
having either inserted parts or bolt-heads are to be avoided, since they
are liable to become red-hot and to ignite the mixture prematurely (Fig.
20).
[Illustration: FIG. 21.--Piston with lubricated pin.]
Among the parts of the piston which rapidly wear away because constant
lubrication is difficult, is the connection with the piston-rod (Fig.
21). It is important that the bearing at the piston-pin be formed of two
parts which can be adjusted to take up the wear. The pin itself should
be of case-hardened steel. For large engines, some manufacturers have
apparently abandoned the practice of locking the pin, by set-screws, in
flanges cast in one piece with the piston. Indeed, the piston is often
fractured by reason of the expansion of the pins thus held on two sides.
It seems advisable to secure the pin by means of a single screw in one
of the flanges, fitting it by pressure against the opposite boss. The
use of wedges or of clamping-screws, introduced from without the piston
to hold the pin, should be avoided. It may happen that the wedges will
be loosened, will move out, and will grind the cylinder, causing
injuries that cannot be detected before it is too late. The strength of
the piston-pin should be so calculated that the pressure per square inch
of projected surface does not exceed 1,500 to 2,850 pounds per square
inch. It should be borne in mind that the initial pressure of the
explosion is often equal to 400 to 425 pounds per square inch. Some
manufacturers mount the pin as far to the back of the piston as
possible, so as to bring it nearer the point of application of the
motive force of the explosion. Other manufacturers, on the other hand,
mount the pin toward the front of the piston. No great objection can be
raised against either method. In the former case the position of the
rings will limit that of the pin.
The number of these rings ought not to be less than four or five,
arranged at the rear of the piston. It is to be observed that makers of
good engines use as many as 8 to 10 rings in the pistons of fair-sized
motors.
Piston-rings of gray pig-iron can be adjusted with the greatest nicety
in such a manner that, by means of tongues fitting in their grooves,
they are held from turning in the latter, whereby their openings are
prevented from registering and allowing the passage of gas. As a general
rule, a large number of rings may be considered a distinguishing feature
of a well-built engine. In order to prevent a too rapid wear of the
cylinder, several German manufacturers finish off the front of the
piston with bronze or anti-friction metal in engines of more than 40 to
50 horse-power. It is to be observed, however, that this expedient is
not applicable to motors the cylinders of which are comparatively cold;
otherwise the bronze or anti-friction metal will deteriorate.
=Arrangement of the Cylinder.=--The cylinder shell or liner, in which
the piston travels, and the water-jacket should preferably be made in
separate pieces and not cast of the same metal, in order to permit a
free expansion (Figs. 22 and 23). If for want of care or of proper
lubrication, which frequently occurs in gas-engines, the cylinder should
be injured by grinding, it can be easily renewed, without the loss of
all the connecting parts.
[Illustration: FIG. 22.--Head, jacket and liner of cylinder, cast in one
piece.]
[Illustration: FIG. 23.--Cylinder with independent liner and head.]
For the same reason, the cylinder and its casing should be independent
of the frame. In many horizontal engines, the cylinders overhang the
frame throughout the entire length, by reason of the joining of their
front portions with the frames. Although such a construction is attended
with no serious consequences in small engines, nevertheless in large
engines it is exceedingly harmful. Indeed, in most modern single-acting
engines, the pistons are directly connected with the crank-shaft by the
piston-rod, without any intermediate connecting-rod or cross-head. The
vertical reaction of the motive effort on the piston is, therefore,
taken up entirely by the thrust of the cylinder, which is also vertical
(Fig. 24). This thrust, acting against an unsupported part, may cause
fractures; at any rate, it entails a rapid deterioration of the cylinder
joint.
[Illustration: FIG. 24.--Single-acting engines.]
[Illustration: FIG. 25.--Engine with inclined bearings.]
=The Frame.=--Gas-engines driven as they are, by explosions, giving rise
to shocks and blows, should be built with frames, heavy, substantial,
and broad-based, so as to rest solidly on the ground. This essential
condition is often fulfilled at the cost of the engine's appearance; but
appearance will be willingly sacrificed to meet one of the requirements
of perfect operation. For engines of more than 8 to 10 horse-power,
frames should be employed which can be secured to the masonry foundation
without a separate pedestal or base. Some manufacturers, for the purpose
of lightening the frame, attach but little importance to the foundation
and to strength of construction, and employ the design illustrated in
place of the crank-shaft bearing (Fig. 25); others, in order to
facilitate the adjusting of the connecting-rod bearings, prefer the
second form (Fig. 26). It is evident that, in the first case, a part of
the effort produced by the explosion reacts on the upper portion of the
connecting-rod bearing, on the cap of the crank-shaft bearing, and
consequently on the fastening-bolts. In the second case, if the
adjustment be not very carefully made, or if the rubbing surfaces are
insufficient, the entire thrust due to the explosion will be received
by the meeting parts of the two bushings, thus injuring them and causing
a more rapid wear. In the construction of large engines, some
manufacturers take the precaution of forming the connecting-rod bearings
of four parts, adjustable to take up the wear, so that the effort is
exerted against the parts disposed at right angles to each other. A form
that seems rational is that shown in Fig. 27, in which the reaction of
the thrust is taken up by the lower bearing, rigidly supported by the
braced frame, in the direction opposite to that of the explosive effort.
[Illustration: FIG. 26.--Engine with straight bearings.]
[Illustration: FIG. 27.--Engine with correctly designed bearings.]
The sum of the projecting surfaces of the two bearings should be so
calculated that a maximum explosive pressure of 405 to 425 pounds per
square inch will not subject the bearings to a pressure higher than 425
to 550 pounds per square inch.
=Fly-Wheels.=--In gas-engines particularly, the fly-wheel should be
secured to the crank-shaft with the utmost care. It should be mounted as
near as possible to the bearings; otherwise the alinement of the shaft
will be destroyed and its strength impaired. If the fly-wheel be
fastened by means of a key or wedge having a projecting head, it is
advisable to cover the end of the shaft by a movable sleeve. The
fly-wheel should run absolutely true and straight even if the explosion
be premature. In well-built engines the fly-wheels are lined up and
shaped to the rim. The periphery is slightly rounded in order the better
to guide the belt when applied to the wheel.
[Illustration: FIG. 28.--Single fly-wheel engine with external bearing.]
Furthermore, fly-wheels should be nicely balanced; those are to be
preferred which have no counter-weights cast or fastened to the hub, the
spokes, or the rim. The system of balancing the engine by means of two
fly-wheels, mounted on opposite sides, is used chiefly for the purpose
of equalizing the inertia effects. Special engines, employed for driving
dynamos, and even industrial engines of high power, are preferably
fitted with but a single fly-wheel, with an outer bearing, since they
more readily counteract the cyclic irregularities or variations of speed
occurring in a single revolution (Fig. 28). If in this case a pulley be
provided, it should be mounted between the engine and the outer
bearing. The following advantages may be cited in favor of the single
fly-wheel, particularly in the case of dynamo-driving engines:
1. The single fly-wheel permits a more ready access to the parts to be
examined.
2. It involves the employment of a third bearing, thus avoiding the
overhang caused by two ordinary fly-wheels.
3. It avoids the torsional strain to which the two-wheel crank is
subjected when starting, stopping, and changing the load, the peripheral
resistance varying in one of the fly-wheels, while the other is
subjected to a strain in the opposite direction on account of the
inertia.
4. Two fly-wheels, keyed as they are to projecting ends of the shaft,
will be so affected at the rims by the explosions that the belts will
shake.
The third bearing which characterizes the single-fly-wheel system, is
but an independent support, resting solidly on the masonry bed of the
engine. The bearing with its independent support is sufficiently rigid,
and is not subjected to any stress from the crank at the moment of
explosion, the reaction of the crank affecting only the frame bearings.
With such fly-wheels, reputable firms guarantee a cyclic regularity
which compares favorably with that of the best steam-engines. For a duty
varying from a third of the load to the maximum load, these engines,
when driving direct-current dynamos for directly supplying an
electric-light circuit, will insure perfect steadiness of the light;
and the effectually aperiodic measuring instruments will not indicate
fluctuations greater than 2 to 3 per cent. of the tension or intensity
of the current. The coefficient of the variations in the speed of a
single revolution will thus be not far from 1/60.
[Illustration: FIG. 29.--Curved spoke fly-wheel.]
=Straight and Curved Spoke Fly-Wheels.=--The spokes of fly-wheels are
either straight or curved. In assembling the motor parts it too often
occurs that curved spoke fly-wheels are mounted with utter disregard of
the direction in which they are to turn. It is important that curved
spokes should be subjected to compression and not to traction. Hence the
fly-wheels should be so mounted that the concave portions of the spokes
travel in the direction of rotation, as shown in the accompanying
diagram (Fig. 29). If a single fly-wheel be employed on an engine of the
type in which the speed is governed by the "hit-and-miss" system, the
fly-wheel should be extra heavy to counteract the irregularities of the
motive impulses when the engine is not working at its full load, or in
other words, when no explosion takes place at every cycle.
[Illustration: FIG. 30.--Forged crank-shafts.]
=The Crank-Shaft.=--The crank-shaft should be made of the best mild
steel. Those shafts are to be preferred the cranks of which are not
forged on (Fig. 30), but cut out of the mass of metal; furthermore, the
brackets or supports should be planed and shaped so that they are square
in cross-section.
[Illustration: FIG. 31.--Correct design of crank-shaft.]
Such a design involves fine workmanship and speaks well for the
construction of the whole engine. Moreover, it enables the bearings to
be brought nearer each other, reduces to a minimum that part of the
crank-shaft which may be considered the weakest, and permits a rational
and exact counterbalancing of the moving parts, such as the crank and
the end of the connecting-rod. The best manufacturers have adopted the
method of fastening to the cranks balancing weights secured to the
brackets, especially for high-speed engines or for engines of high
power. The projecting surface of the crank-pin should, as a rule, be
calculated for a pressure of 1,400 pounds per square inch.
[Illustration: FIG. 32.--Crank-shaft with balancing weight.]
=Cams, Rollers, etc.=--The cams, rollers, thrust-bearings, as well as
the piston-pin in particular, should be made of good steel,
case-hardened to a depth of at least .08 of an inch. Their hardness and
the degree of cementation may be tested by means of a file. This is the
method followed by the best manufacturers.
=Bearings.=--All the bearings and all guides should be adjustable to
take up the wear. They are usually made of bronze or of the best
anti-friction metal.
=Steadiness.=--The steadiness of engines may be considered from two
different standpoints.
[Illustration: FIG. 33.--Inertia governor.]
1. _Variation of the Number of Revolutions at Different Loads._--This
depends chiefly on the sensitiveness of the governor, which should be of
the "inertia" or of the "ball" (or centrifugal) type. The first form is
rarely employed, except in small engines up to 10 horse-power, and is
applicable only to engines in which the "hit and miss" system is
employed (Fig. 33). The second form is more widely used, and is
applicable to engines having "hit-and-miss" or variable admission
devices. In the first form, the governor simply displaces a very light
member, whatever may be the size of the engine, for which reason the
dimensions are very small. In the second form, on the other hand, the
governor acts either on a conical sleeve or on some other regulating
member offering resistance. Evidently, in order to overcome the
reactions to which it is subjected, it must be as heavy and powerful as
a steam-engine governor. Sufficient allowance is made in a good engine
for variation in the number of revolutions between no load and full
load, not greater than two per cent. if the admission be of the
"hit-and-miss" type, and five per cent. if it be of the variable type.
2. _Cyclic Regularity._--This term means simply that the speed of the
engine is constant in a single revolution. In practice this is never
attained. Allowance is made in engines used for driving direct-current
dynamos for a variation of about 1/60; while in industrial engines a
variation of 1/25 is permissible. Cyclic variation depends only on the
weight of the fly-wheel; whereas variation in the number of revolutions
is determined chiefly by the governor.
=Governors.=--Diagrams are here presented of the principal types of
governors--the inertia governor, the ball or centrifugal governor
controlling an admission-valve of the "hit-and-miss" type (Fig. 34), and
the ball or centrifugal governor controlling a variable gas-admission
valve (Fig. 35).
In distinguishing between the operation of the two last-mentioned types,
it may be stated that the former bears the same relation to the
hit-and-miss gear as it does, for example, to the valve gear of a
Corliss steam-engine. In other words, it is an apparatus that
_indicates_ without _inducing_, admission or cut-off. The second type,
on the other hand, operates by means of slides and the like, as in the
Ridder type of engine, in which it controls the displacement of the
cut-off or distribution slide-valve and is subjected to variable forces,
depending on the pressure, lubrication, the condition of the
stuffing-boxes, and the like.
In gas as well as in steam engines, designs are to be commended which
shield the delicate mechanism from strains and stresses that are likely
to destroy its sensitiveness, as is the case in the automatic cut-off of
the Corliss steam-engine.
[Illustration: FIG. 34.--"Hit-and-miss" governor.]
Governors should be provided with means to permit the manual variation
of the speed while the engine is in operation.
For small motors, one of the most widely used admission devices is that
of the "hit-and-miss" type. As its name indicates, this admission
arrangement allows a given quantity of gas to enter the cylinder for a
number of consecutive intervals, until the engine is about to exceed its
normal speed. Thereupon the governor cuts off the gas entirely. The
result is that, in this system, the number of admissions is variable,
but that each admitted charge is composed of a constant proportion of
gas and air.
The governors employed for the "hit-and-miss" type are either "inertia"
or "centrifugal" governors.
Inertia governors (Fig. 33) are less sensitive than those of the
centrifugal type. They are generally applied only to industrial engines
of small power, in which regularity of operation is a secondary
consideration.
Centrifugal governors employed for gas-engines with "hit-and-miss"
regulation are, as a general rule, noteworthy for their small size,
which is accounted for by the fact that, in most systems, merely a
movable member is placed between the admission-controlling means and the
valve-stem (Fig. 34). It follows that this method of operation relieves
the governor of the necessity of overcoming the resistance of the weight
of moving parts, more or less effectually lubricated, and subjected to
the reaction of the parts which they control.
In engines equipped with variable admission devices for the gas or the
explosive mixture, the governor actuates a sleeve on which the
admission-cam is fastened (Fig. 35). Or, the governor may displace a
conical cam, the reaction of which, on contact with the lever, destroys
the stability of the governor. These conditions justify the employment
of powerful governors which, on account of the inertia of their parts,
diminish the reactionary forces encountered.
The centrifugal governor should be sufficiently effectual to prevent
variations in the number of revolutions within the limits of 2 to 3 per
cent. between no load and approximately full load. Under equivalent
conditions, the inertia governor can hardly be relied upon for a
coefficient of regularity greater than 4 to 5 per cent.
[Illustration: FIG. 35.--Variable admission governor.]
The manner of a governor's operation is necessarily dependent on the
admission system adopted. And the admission system varies essentially
with the size, the purpose of the engine, and the character of the fuel
employed.
[Illustration: FIG. 36.--Vertical engine.]
[Illustration: FIG. 37.--Section through an engine of the vertical or
"steam-hammer" type.]
=Vertical Engines.=--For some years past there seems to have been a
tendency in Europe to use horizontal instead of vertical engines,
especially since engines of more than 10 or 15 horse-power have been
extensively used for industrial purposes. The vertical type is used for
1 to 8 horse-power engines, with the cylinder in the lower part of the
frame, and the shaft and its fly-wheel in the upper part (Fig. 36). The
only merit to be attributed to this arrangement is a great saving of
space. It is evident, however, that beyond a certain size and power,
such engines are unstable. In America particularly, many manufacturers
of high-power engines (50 to 100 horse-power or more) prefer the
vertical or "steam-hammer" arrangement, which consists in placing the
cylinder in the upper part, and the shaft in the lower part of the frame
as close to the ground as possible (Figs. 37 and 38). The problem of
saving space, as well as that of insuring stability, is thus solved, so
that it is easily possible to run up the speed of the engine. There is
also the advantage that the shaft of a dynamo can be directly coupled
up with the crank-shaft of the engine, thus dispensing with a belt,
which, at the least, absorbs 4 to 6 per cent. of the total power. It
should, nevertheless, be borne in mind that the direct coupling of
electric generators to engine-shafts implies the use of extremely large
and, therefore, of extremely costly dynamos. Furthermore, by reason of
this arrangement, groups of electro-generators can be disposed in a
comparatively small amount of space. Some English manufacturers are also
beginning to adopt the "steam-hammer" type of engine for high powers,
the result being a marked saving in material and lowering of the cost of
installation.
[Illustration: FIG. 38.--Side and end elevations of a vertical or
"steam-hammer" engine.]
=Power of the Engine.=--The first thing to be considered is that the
power of a gas-engine is always given in "effective" horse-power, and
that the power of a steam-engine is always given in "indicated"
horse-power in contracts of sale. In England and in the United States,
the expression "nominal" horse-power is still employed. It may be
advisable to define these various terms exactly, since unscrupulous
dealers, to the buyer's loss, have done much to confuse them.
"Indicated" horse-power is a designation applied to the theoretical
power produced by the action of the motive agent on the piston. The work
performed is measured on an indicator card, by means of which the
average pressure to be considered in the computation of the theoretical
power is ascertained.
The "effective" or brake horse-power is equal to the "indicated"
horse-power, less the energy absorbed by passive resistance, friction of
the moving parts, etc.
The "effective" work is an experimental term applied to the power
actually developed at the shaft. This work is of interest solely to the
engine user.
In a well-built motor, in which the passive resistance by reason of the
correct adjustment and simplicity of the parts, is reduced to a minimum,
the "effective" horse-power is about 80 to 87 per cent. of the
"indicated" horse-power, when the engine runs under full load. This
reduced output is usually called the "mechanical efficiency" of the
engine.
"Nominal" horse-power is an arbitrary term in the sense in which it is
used in England and America, where it is quite common. The manufacturers
themselves do not seem to agree on its absolute value. A "nominal"
horse-power, however, is equal to anything from 3 to 4 "effective"
horse-power. The uncertainty which ensues from the use of the term
should lead to its abandonment.
In installing a motor, the determination of its horse-power is a matter
of grave importance, which should not be considered as if the motor were
a steam-engine or an engine of some other type. It must not be forgotten
that, especially at full load, explosion-engines are most efficient, and
that, under these conditions, it will generally be advisable to
subordinate the utility of having a reserve power to the economy which
follows from the employment of a motor running at a load close to its
maximum capacity. On the other hand, the gas-engine user is unwilling to
believe that the stipulated horse-power of the motor which is sold to
him is the greatest that it is capable of developing under industrial
conditions. Business competition has led some firms to sell their
engines to meet these conditions. It is probably not stretching the
truth too far to declare that 80 per cent. of the engines sold with no
exact contract specifications are incapable of maintaining for more than
a half hour the power which is attributed to them, and which the buyer
expects. It follows that the power at which the engine is sold should be
both industrially realized and maintained, if need be, for an entire
day, without the engine's showing the slightest perturbation, or
faltering in its silent and regular operation. To attain this end, it is
essential that the energy developed by the engine in normal or constant
operation should not exceed 90 to 95 per cent. of the maximum power
which it is able to yield, and which may be termed its "utmost power".
As a general rule, especially for installations in which the power
fluctuates from the lowest possible to double this, as much attention
must be paid to the consumption at half load as at full load; and
preference should be given to the engine which, other things being
equal, will operate most economically at its lowest load. In this case
the consumption per effective horse-power is appreciably higher.
Generally, this consumption is greater by 20 to 30 per cent. than that
at full load. This is particularly true of the single-acting engines so
widely used for horse-powers less than 100 to 150.
In some double or triple-acting engines, according to certain writers,
the diminution in the consumption will hardly be proportional to the
diminution of the power, or at any rate, the difference between the
consumption per B.H.P. at full load and at reduced load will be less
than in other engines. It should be observed, however, that this
statement is apparently not borne out by experiments which the author
has had occasion to make. To a slight degree, this economy is obtained
at the cost of simplicity, and consequently, at the cost of the engine.
At all events, the engines have the merit of great cyclic regularity,
rendering them serviceable for driving electric-light dynamos; but this
regularity can also be attained by the use of the extra heavy fly-wheels
which English firms, in particular, have introduced.
=Automatic Starting.=--When the gas-engine was first introduced,
starting was effected simply by manually turning the fly-wheel until
steady running was assured. This procedure, altogether too crude in its
way, is attended with some danger. In a few countries it is prohibited
by laws regulating the employment of industrial machinery. If the engine
be of rather large size one, moreover, which operates at high
pressure--such a method of starting is very troublesome. For these
reasons, among others, manufacturers have devised automatic means of
setting a gas-engine in motion.
Of such automatic devices, the first that shall be mentioned is a
combination of pipes, provided with cocks, by the manipulation of which,
a certain amount of gas, drawn from the supply pipe, is introduced into
the engine-cylinder. The piston is first placed in a suitable position,
and behind it a mixture is formed which is ignited by a naked flame
situated near a convenient orifice. When the explosion takes place the
ignition-orifice is automatically closed, and the piston is given its
motive impulse. The engine thus started continues to run in accordance
with the regular recurrence of the cycles. In this system, starting is
effected by the explosion of a mixture, without previous compression.
Some designers have devised a system of hand-pumps which compress in the
cylinder a mixture of air and gas, ignited at the proper time by
allowing it to come into contact with the igniter, through the
manipulation of cocks (Fig. 39).
These two methods are not absolutely effective. They require a certain
deftness which can be acquired only after some practice. Furthermore,
they are objectionable because the starting is effected too violently,
and because the instantaneous explosion subjects the stationary piston,
crank, and fly-wheel to a shock so sudden that they may be severely
strained and may even break. Moreover, the slightest leakage in one of
the valves or checks may cause the entire system to fail, and,
particularly in the case of the pump, may induce a back explosion
exceedingly dangerous to the man in charge of the engine.
These systems are now almost generally supplanted by the compressed-air
system, which is simpler, less dangerous, and more certain in its
effect.
The elements comprising the system in question include essentially a
reservoir of thick sheet iron, capable of resisting a pressure of 180 to
225 pounds and sufficient in capacity to start an engine several times.
This reservoir is connected with the engine by piping, which is disposed
in one of two ways, depending upon whether the reservoir is charged by
the engine itself operatively connected with the compressor, or by an
independent compressor, mechanically operated.
[Illustration: FIG. 39.--Tangye starter.]
In the first case, the pipe is provided with a stop-cock, mounted
adjacent to the cylinder, and with a check-valve. When the engine is
started and the gas cut off, the air is drawn in at each cycle and
driven back into the reservoir during the period of compression. When
the engine, running under these conditions by reason of the inertia of
the fly-wheel, begins to slow down, the check-valve is closed and the
gas-admission valve opened, so as to produce several explosions and to
impart a certain speed to the engine in order to continue the charging
of the reservoir with compressed air. This done, the valve on the
reservoir itself is tightly closed, as well as the check-valve, so as to
avoid any leakage likely to cause a fall in the reservoir's pressure.
In the second case, which applies particularly to engines of more than
50 horse-power, the charging pipe connected with the reservoir is
necessarily independent of the pipe by means of which the motor is
started. The reservoir having been filled and the decompression cam
thrown into gear, starting is accomplished:
1. By placing the piston in starting position, which corresponds with a
crank inclination of 10 to 20 degrees in the direction of the piston's
movement, from the rear dead center, immediately after the period of
compression;
2. By opening the reservoir-valve;
3. By allowing the compressed air to enter the cylinder rapidly, through
the quick manipulation of the stop-cock, which is closed again when the
impulse is given and reopened at the corresponding period of the
following cycle, this operation being repeated several times in order to
impart sufficient speed to the motor;
4. By opening the gas-valve and finally closing the two valves of the
compressed-air pipe.
The pipes and compressed-air reservoirs should be perfectly tight. The
reservoirs should have a capacity in inverse ratio to the pressure under
which they are placed, _i.e._, they increase in size as the pressure
decreases. If, for example, the reservoirs should be operated normally
at a pressure of 105 to 120 pounds per square inch, their capacity
should be at least five or six times the volume of the engine-cylinder.
If these reservoirs are charged by the engine itself, the pressure will
always be less by 15 to 20 per cent. than that of the compression.