OIL AND VOLATILE HYDROCARBON ENGINES
Although this book is devoted primarily to a discussion of street-gas
and producer-gas engines employed in various industries, a few words on
oil and volatile hydrocarbon engines may not be out of place.
Oil-engines are those which use ordinary petroleum as a fuel or
illuminating oil of yellowish color, having a specific gravity varying
from 0.800 to 0.820 at a temperature of 15 degrees C. (59 degrees F.),
and boiling between 140 and 145 degrees C. (284 to 297 degrees F.).
Volatile hydrocarbon engines are those which employ light oils obtained
by distilling petroleum. These oils are colorless, have a specific
gravity that varies from 0.680 to 0.720, and boil between 80 degrees and
115 degrees C. (176 to 257 degrees F.). Among these "essences," as they
are called in Europe, may be mentioned benzine and alcohol.
In general appearance, and the way in which they are controlled,
oil-engines differ but little from gas-engines. Their usual speed,
however, is 20 to 30 per cent. greater than that of gas-engines. Except
in some engines of the Diesel and Banki types, the compression does not
exceed 43 to 71 pounds per square inch. In volatile hydrocarbon engines,
on the other hand, the speed is very high, often running from 500 to
2,000 revolutions per minute, while the speed of gas or oil engines
rarely exceeds 250 or 300 revolutions per minute.
=Oil-Engines.=--Oil-engines are employed chiefly in Russia and in
America. Because of the high price of oil in other countries they are to
be found only in small installations in country regions and are used
mainly for driving locomobiles and launches. The improvements which have
been made of late years in the construction of gas-engines supplied by
suction gas-producers for small as well as for large powers, have
hindered the general introduction of oil-engines.
The characteristic feature in the design of many of the oil-engines of
the four-cycle type now in use (to which type we shall confine this
discussion) is to be found in the controlling mechanism employed. The
underlying principle of this mechanism lies not in acting upon the
admission-valve, but in causing the governor to operate the
exhaust-valve in such a manner that it is held open whenever the engine
tends to exceed its normal speed. Some engines, however, are built on
the principle of the gas-engine, with an admission-valve so controlled
by the governor that it is open during normal operation and closed
whenever the speed becomes excessive.
The necessity of producing a mixture of air and oil capable of being
ignited in the engine-cylinder has led to the invention of various
contrivances, which cannot be used if illuminating-gas or producer-gas
be employed. These contrivances are the atomizer, the carbureter, the
oil-pump, the air-pump, the oil-tank, and the oil-lamp. In some
oil-engines all of the elements may be found, but for the purpose of
simplifying the construction and of avoiding unnecessary complications,
manufacturers devised arrangements which rendered it possible to discard
some of them, particularly those of delicate construction and operation.
It is not the intention of the author to enter into a detailed
description of these various devices, since the limitations of this book
would be considerably surpassed. The reader is referred to books on the
oil-engine, published in the United States, England, and France.[B]
Most of the observations which have been made on the construction and
installation of gas-engines, as well as the precautions which have been
advised in the conduct of an engine, apply with equal force to
oil-engines. It will therefore be unnecessary to recur to this phase of
the subject so far as oil-engines are concerned. One point only should
be insisted upon--the necessity of very frequently cleaning the valves
and moving parts of the engine.
Illuminating-oil when burnt produces sooty deposits, particularly if
combustion be incomplete, which deposits foul the various parts and
cause premature ignitions and faulty operation.
The use of oil in atomizers, carbureters, and lamps is accompanied with
the employment of pipes and openings so small in cross-section that the
slightest negligence is attended with the formation of partial
obstructions that inevitably affect the operation of the engine.
=Volatile Hydrocarbon Engines.=--Only those engines will here be treated
which have become of importance in the development of the automobile.
Some designers have attempted to employ the volatile hydrocarbon engine
for industrial and agricultural purposes, and have devised
electro-generator groups, hydraulic groups, and so-called "industrial
combinations" in which belt and pulley transmission is employed. These
applications in particular will here be rapidly reviewed.
The high speed at which engines of this class are driven renders it
possible to operate a centrifugal pump directly and to mount both the
engine and machine which it actuates on the same base. The hydrocarbon
engine has the merit of being very light and of taking up but little
room. Its cost is considerably less than that of an oil or producer-gas
engine of corresponding power. On the other hand, its maintenance is
much more expensive, and the hydrocarbons upon which it depends for fuel
anything but cheap. Furthermore, the engines wear away rapidly, on
account of their high speed. For this reason it is advisable to base
calculations on a life of three to four years, while oil and gas engines
may generally be considered to be still of service at the end of
thirteen years. On the following page a comparison of costs for
installation and maintenance is drawn between the oil and hydrocarbon
engine on the basis of ten horse-power.
=Comparative Costs.=--A 10 horse-power oil-engine, in the matter of
first cost of installation, is about 35 per cent. more expensive than a
volatile hydrocarbon engine of equal power. On the other hand, the
operating expenses of the oil-engine are less by 25 per cent. than they
are for the volatile hydrocarbon engine.
The engines which are here discussed usually have their cylinders
vertically arranged, as in steam-engines of the overhead cylinder type.
The crank-shaft and the connecting-rods are enclosed in a hermetically
sealed box filled with oil, so that the movement of the parts themselves
ensures the liberal lubrication of the piston. The suction-valve is
generally free, although latterly designers have shown a tendency to
connect it with the cam-shaft, with the result that it has become
possible to reduce the speed appreciably without stopping the engine.
The carbureter is operated by the suction of the engine. If the fuel
employed is alcohol, it must be heated.
=Tests of High-speed Engines.=--High-speed engines present various
difficulties which must be contended with in controlling their
operation. Their high speed renders it impossible to take indicator
records as in the case of most industrial engines. Indicator cards,
moreover, at best give but very crude data, which relate to each
explosion cycle only, and which are therefore inadequate in determining
the exact conditions of an engine's operation. Oil, benzine, and other
so-called carbureted-air engines are particularly difficult to control
because of many phenomena which cannot be recorded. In order to test the
operation of high-speed engines, two different types of instruments are
at present employed: the manograph and the continuous explosion
recorder.
=The Manograph.=--The manograph, which is the invention of Hospitalier,
is an optical instrument in which a series of closed diagrams are
superimposed upon a polished mirror similar in form to Watt diagrams.
Because the images persist in affecting the retina of the eye an
absolutely continuous, but temporary, gleam is seen. Still, it is
possible to obtain a photograph or a tracing of these diagrams.
=The Continuous Explosion Recorder for High-speed Engines.=--The author
has devised an explosion and pressure recorder, which is mounted upon
the explosion chamber to be tested and which communicates with the
chamber through the medium of a cock _r_ (Fig. 136). The instrument is
somewhat similar in form to the ordinary indicator. Its record, however,
is made on a paper tape which is continuously unwound. The cylinder _c_
is provided with a piston _p_, about the stem of which a spring _s_ is
coiled. A clock train contained in the chamber _b_ unwinds the strip of
paper from the roll _p'_ and draws it over the drum _p''_, where the
pencil _t_ leaves its mark. The tape is then rewound on the spindle
_p'''_. A small stylus or pencil _f_ traces "the atmospheric line" on
the paper as it passes over the drum _p''_. In order to obviate the
binding of the piston _p_ when subjected to the high temperature of the
explosions, the cylinder _c_ is provided with a casing _e_ in which
water is circulated by means of a small rubber tube which fits over the
nipple _e'_. This recorder analyzes with absolute precision the work of
all engines, whatever may be their speed. It gives a continuous graphic
record from which the number of explosions, together with the initial
pressure of each, can be determined, and the order of their succession.
Consequently the regularity or irregularity of the variations can be
observed and traced to the secondary influences producing them, such as
the section of the inlet and outlet valves and the sensitiveness of the
governor. It renders it possible to estimate the resistance to suction
and the back pressure due to expelling the burnt gases, the chief causes
of loss in efficiency in high-speed engines. Furthermore, the influence
of compression is markedly shown from the diagram obtained.
[Illustration: FIG. 136.--R. Mathot's continuous explosion recorder.]
[Illustration: FIG. 137.--12 H.P. Oil-engine.]
[Illustration: FIG. 138.--6 H.P. Volatile Hydrocarbon Engine.]
[Illustration: FIG. 139.--Effect of size of section and exhaust ports.]
The recorder is mounted on the engine; its piston is driven back by each
of the explosions to a height corresponding with their force; and the
stylus or pencil controlled by the lever _t_ records them side by side
on the moving strip of paper. The speed with which this strip is unwound
conforms with the number of revolutions of the engine to be tested, so
that the records of the explosions are placed side by side clearly and
legibly. Their succession indicates not only the number of explosions
and of revolutions which occur in a given time, but also their
regularity, the number of misfires. The atmospheric pressure of the
explosions is measured by a scale connected with the recorder-spring. By
employing a very weak spring which flexes at the bottom simply by the
effect of the compression in the engine-cylinder, it is possible to
ascertain the amount of the resistance to suction and to the exhaust. It
is simply sufficient to compare the explosion record with the
atmospheric line, traced by the stylus _f_. By means of this apparatus,
and of the records which it furnishes, it is possible analytically to
regulate the work of an engine, to ascertain the proportion of air, gas,
or hydrocarbon, which produces the most powerful explosion, to regulate
the compression, the speed, the time of ignition, the temperature, and
the like (Figs. 137, 138 and 139).
In order to explain the manner of using this recorder several specimen
diagrams are here given.
I. _Determination of the Amount of Compression._--A spring of average
power is employed, the total flexion of which corresponds almost with
the maximum compression so as to obtain a curve of considerable
amplitude. The engine is first revolved without producing explosions,
driving it from the dynamo usually employed in shops, at the different
speeds to be studied. The compression of the mixture varies in inverse
ratio to the number of revolutions of the shaft, owing to the
resistances which are set up in the pipes and the valves and which
increase with the speed. The accompanying cut (Fig. 140) shows two
distinct records taken in two different cases, namely:
A.--Speed of engine, 950 revolutions per minute; amount of compression,
68.9 pounds per square inch.
B.--Speed of engine, 1,500 revolutions per minute; amount of
compression, 61 pounds per square inch, or 11.5 per cent. less.
[Illustration: FIG. 140.]
II. _Determination of the Resistance to Suction and Exhaust._--Influence
of the tension of the spring of the suction valve and of the section of
the pipe. Effect of the section of the exhaust-valve and of the length
and shape of the exhaust-pipe:
A very light spring is utilized, the travel of which is limited by a
stop so as to obtain on a comparatively large scale the depressions and
resistance respectively represented by the position of the corresponding
curve, above or below the atmospheric line (Fig. 141).
[Illustration: FIG. 141.]
C.--Tension of the suction-valve: 2.9 pounds. Resistance to suction: 1/7
of an atmosphere (2.7 pounds).
D.--Tension of the suction-valve: 2.17 pounds. Resistance to suction:
2/7 of an atmosphere (5.4 pounds).
E.--A chest is used for the exhaust. Resistance to exhaust: 2/7 of an
atmosphere (5.4 pounds).
F.--The exhausted gases are discharged into the air, the pipe and the
chest being discarded. Resistance to the exhaust is zero (Fig. 142).
[Illustration: FIG. 142.]
The depression graphically recorded is partly due to the inertia of the
spring of the explosion-recorder, which spring expands suddenly when the
exhaust is opened.
III. _Comparison of the Average Force of the Explosions by Means of
Ordinates._--A powerful spring is employed. The paper band or tape of the
recorder is moved with a small velocity of translation so as to
approximate as closely as possible the corresponding ordinates
representing the explosions (Fig. 143).
[Illustration: FIG. 143.]
G.--Pure alcohol. Explosive force, 369.72 to 426.6 pounds per square
inch.
H.--Carbureted alcohol. Explosive force, 397.6 to 510.8 pounds per
square inch.
I.--Volatile hydrocarbon. Explosive force, 483.48 to 531.92 pounds per
square inch.
IV. _Analysis of a Cycle by Means of Open Diagrams Representing the Four
Periods._--A powerful spring is employed, and the paper is moved with
its maximum speed of translation. The four phases of the cycle are
easily distinguished as they succeed one another graphically from right
to left in other words, in a direction opposite to that in which the
paper is unwound. A diagram is made which reproduces exactly the values
of the corresponding pressures at different points in the travel of the
piston (Fig. 144). The periods of the cycle are reproduced as faithfully
as if the ordinary indicator which gives a closed curved diagram had
been employed. There is no difficulty in reading the record, since the
paper is not in any way connected with the engine-piston. Some attempts
have been made to secure open diagrams in which the motion of
translation given to the paper is controlled by the engine itself; but
these apparatus as well as the ordinary indicators cannot be used when
the speed of the engine exceeds 400 to 500 revolutions per minute.
[Illustration: FIG. 144.]
J.--Speed, 1,200 revolutions; carbureted alcohol; average force of the
explosions, 426.6 pounds per square inch. Average compression, 92.43
pounds per square inch. Pressure at the end of the expansion, 21.33
pounds per square inch.
V. _Analysis of the Inertia of the Recorder. Selection of the Spring to
be Employed._--Given the rapidity with which the explosions succeed one
another in automobile engines, it is readily understood that the inertia
of the moving parts of the recorder will be graphically reproduced (Fig.
144). The effect of this inertia is a function of the weight of the
moving parts and of the extent of their travel.
The moving masses are represented by the piston and its rod, the spring
and the levers of the parallelogram stylus. The effects due to inertia
have been considerably lessened by reducing the weight of the various
parts to a minimum. A hollowed piston, a hollowed rod and short and
light levers have been adopted. The traditional pencil has been
displaced by a silver point which traces its mark upon a metallically
coated paper. For the heavy springs with their long travel, light but
powerful springs with small amplitudes have been substituted. Since the
perfect lubrication of the recorder-cylinder is of great importance, a
simple oiling device certain in its action has been adopted. The recess
of the piston forms a cup that can be filled with oil whenever the
spring is changed.
At each explosion the violent return of the piston splashes oil against
the cylinder walls and thus insures perfect lubrication. It should be
observed that if the directions given are not followed, particularly in
the choice of a spring suitable for each experiment, inertia effects
will be produced. These can easily be detected on the record and cannot
be confused with the curves which interpret the phenomena occurring in
the cylinder of the engine. At a height equal to the end of the piston's
stroke, the cylinder of the recorder is provided with a water-jacket
which keeps the temperature down to a proper point and prevents the
binding of the piston.
The explosion-chamber of automobile engines being rather small in
volume, should not be sensibly increased in order that the record
obtained may conform as nearly as possible with actual working
conditions on the road. In order to attain this end the cylinder of the
recorder is so disposed that the piston travels to the height of the
connecting-cock. As a result of this arrangement the field of action of
the gases is reduced to a minimum. Since these gases have no winding
path to follow, they are subjected neither to loss of quantity nor to
cold.
FOOTNOTES:
[B] Hiscox, Gas and Oil Engines, Norman W. Henley Pub. Co., New York.
Parsell and Weed, Gas and Oil Engines, 1900, Norman W. Henley Pub. Co.,
New York. Goldingham, 1900, Spon & Chamberlain, London. Dugald Clerk,
1897, Longmans, London. Grover, 1902, Heywood, Manchester. Aime. Witz,
1904, Barnard, Paris. H. Gueldner, 1903, Springer, Berlin.