Gas-Engines and Producer-Gas Plants Part 17

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 surpa.s.sed. 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 cla.s.s 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 c.o.c.k _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 pa.s.ses 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 a.n.a.lyzes 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.

[Ill.u.s.tration: FIG. 136.--R. Mathot's continuous explosion recorder.]

[Ill.u.s.tration: FIG. 137.--12 H.P. Oil-engine.]

[Ill.u.s.tration: FIG. 138.--6 H.P. Volatile Hydrocarbon Engine.]

[Ill.u.s.tration: 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 a.n.a.lytically 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.

[Ill.u.s.tration: 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).

[Ill.u.s.tration: 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).

[Ill.u.s.tration: 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).

[Ill.u.s.tration: 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. _a.n.a.lysis 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.

[Ill.u.s.tration: 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. _a.n.a.lysis 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 ma.s.ses 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 subst.i.tuted. 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-c.o.c.k. 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 quant.i.ty nor to cold.

FOOTNOTES:

[B] Hisc.o.x, Gas and Oil Engines, Norman W. Henley Pub. Co., New York.

Pa.r.s.ell 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. Guldner, 1903, Springer, Berlin.