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