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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).

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).

Fig. 3.—Second cycle: Compression.

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).

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.

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.

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.

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.

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.

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.

Fig. 12.—Electric ignition by spark-coil and battery.

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.

Fig. 14.—Magneto ignition apparatus.

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.

Fig. 16.—Contacts of a magneto-igniter.

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.

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.

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).