=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 ill.u.s.trated 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.

[Ill.u.s.tration: FIG. 26.--Engine with straight bearings.]

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: FIG. 29.--Curved spoke fly-wheel.]

=Straight and Curved Spoke Fly-Wheels.=--The spokes of fly-wheels are either straight or curved. In a.s.sembling 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.

[Ill.u.s.tration: 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 ma.s.s of metal; furthermore, the brackets or supports should be planed and shaped so that they are square in cross-section.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: 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 princ.i.p.al 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.

[Ill.u.s.tration: 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 quant.i.ty 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.

[Ill.u.s.tration: 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.

[Ill.u.s.tration: FIG. 36.--Vertical engine.]

[Ill.u.s.tration: 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 s.p.a.ce. 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 s.p.a.ce, 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 s.p.a.ce. 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.

[Ill.u.s.tration: 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 pa.s.sive 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 pa.s.sive 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.