separate inward stroke of the piston, and a fresh charge is drawn into the cylinder through the inlet port by a separate outward stroke. Generally speaking, one event occurs during each of the four strokes of this cycle; that is, considering the stroke by which the charge is drawn into the cylinder as the first stroke, the mixture is compressed during the second stroke, burned during the third stroke, and the exhaust gases are expelled during the fourth stroke, after which the conditions are the same as at first and the cycle is complete. This type is sometimes known as the four-stroke Otto-cycle engine, and is in more general use than the two-cycle engine as it is much more economical in fuel.

Two-Cycle Engines. A two-cycle engine is one in which only two strokes of the piston, corresponding to one revolution of the crank-shaft, are required to complete the cycle. In this cycle an explosion occurs on each downward stroke of the piston, the fresh charge being admitted and the exhaust gases expelled at or near the end of this stroke. Hence, for the same number of revolutions of the crank-shaft, there are twice as many explosions in the cylinder of a two-cycle engine as in that of a four-cycle engine. However, this does not mean that the power developed by a two-cycle engine is twice as great as that produced by a four-cycle engine of the same size and speed, for, on account of the inefficient scavenging, or cleaning, of the cylinder after the explosion and the lower compression pressure in the two-cycle engine, the

explosions are not so powerful as in the four-cycle engine. It is generally estimated that a two-cycle engine of a certain size and speed will develop about 1.65 times as much power as a four-cycle engine of the same size and speed. This type is sometimes known as the two-stroke Otto-cycle engine.

Application of Four-Cycle Principle.-In the four-cycle engine, the first outward stroke is the suction stroke, the gas being driven into the cylinder by the pressure of the atmosphere or other pressure, because of the partial vacuum produced by the movement of the piston. This stroke fills the cylinder with a mixture of fuel and air at very nearly the pressure of the atmosphere. On the return stroke of the piston, all the openings leading from the cylinder are closed and the mixture is compressed. As the piston nears the end of this second stroke, which is known as the compression stroke, the igniter, or device by means of which the charge is fired, is operated in time to produce full ignition of the mixture at the end of the stroke. The pressure rises to three or four times that due to compression, and drives the piston forwards on its next outward stroke, which is known as the power, or expansion, stroke. Just before this stroke is completed, or as it is, the exhaust valve is opened, permitting the burned gas and uncombined air to escape to the atmosphere, and during the following inward stroke practically all of this waste material is expelled; this last is known as the exhaust stroke.

Graphic Representation of Four-Stroke Cycle.-The four strokes of the engine and the corresponding indicator diagram are shown in Fig. 1. Here,

p denotes the piston; r is the connecting-rod; k. the crankpin; q, the crankshaft. In the indicator diagram, the ordinates or vertical distances represent pressures, and the abscissas or horizontal distances denote the distance the piston has proceeded on its stroke. The pressures are measured from the line OV, which represents the pressure of the atmosphere. The line Ovba is the suction line, and the line b2c2 is the compression line. At c2, the charge is ignited, c2d2 is the explosion line; d2e2, the expansion line; c2f2, combined expansion and exhaust; and f2w0 is the exhaust line. The pressures represented by the two lines v and w are slightly exaggerated, in order that the lines may be distinguished from the atmospheric line OV, which they follow very closely.

In the suction stroke, the crank-shaft turns in the direction of the arrow and the piston moves from the line a to the position shown. The space between the end of the cylinder, when at the line ai, and the cylinder head is called the clearance space or the combustion chamber. In this stroke, the inlet valve is open and the mixed air and gas is being drawn into the cylinder. The pressure within the cylinder drops slightly below the atmosphere, as shown by the line v. The valve remains open until the piston gets to the right-hand end of its stroke. The numbers at the left of the diagram represent the pressures, and those at the bottom the volumes, corresponding to the cross-lines opposite which they are written.

When the piston starts on its return stroke, the inlet valve is closed and the mixture is trapped within the cylinder and compressed. The rise of pressure during compression is shown in the indicator diagram by the line baca. When the compression has proceeded to c2, a spark is produced by the igniter and combustion begins. The rise of pressure from c2 to d2 is therefore due to the compression and the combustion of the gas, but the maximum pressure is lessened somewhat by expansion. The flame spreads rapidly, and during the short time at the end of the stroke when the piston is practically at rest the pressure rises to d2. This stroke is called the compression stroke.

It has been found that by compressing the charge before igniting it, a greater amount of power can be obtained from a given quantity of fuel than by simply burning it at atmospheric pressure. In other words, the efficiency of the internal-combustion engine is increased by compressing the charge before igniting it. Compressing the charge heats it; hence, on account of the danger of preigniting the charge the compression pressure is limited to from 60 to 75 lb. per sq. in., as shown by a pressure gauge.

In the expansion stroke, during which the pressure of the heated gases drives the piston toward the right, the pressure falls as the piston moves forwards, as shown by the drop in the line d2e2. When the expansion stroke has been nearly completed, the exhaust valve is opened and from er to V the drop of pressure is due both to expansion and to the escape of the gas through the exhaust valve. By the time the end of the stroke is reached, the pressure has fallen very nearly to that of the atmosphere, and the expanding gas has done its work.

During the next stroke, the piston is returning, the exhaust valve is open, and the gases are driven from the cylinder to prepare it for the reception of a new charge. There is a small rise of pressure during this stroke, due to the driving of the gas from the cylinder, indicated by the line w. At the end of the exhaust stroke, the exhaust valve closes, and the succeeding outward stroke begins a new cycle with the suction of a fresh charge of gas and air.

The series of operations that take place during the four-stroke cycle is as follows:

FIRST REVOLUTION

First Stroke.-Outwards; suction; inlet valve open; pressure falls below atmosphere.

Second Stroke.-Inwards; compression; both valves closed; pressure rises; ignition before end of stroke, followed by explosion and rapid rise of pressure. SECOND REVOLUTION

Third Stroke.-Outwards; expansion; both valves closed; pressure falls; exhaust valve opens near end of stroke.

Fourth Stroke.-Inwards; exhaust; exhaust valve open; pressure rises very little above that of the atmosphere.

Application of Two-Cycle Principle.-Fig. 2 (a) illustrates the operation of a typical two-cycle engine, in which is the piston; q, the crank-shaft; a, the crank; k, the crankpin; r, the connecting-rod; e, the exhaust port; o, the inlet, or transfer, port; b, the passage leading from the crank-chamber to the cylinder; s, the inlet valve; d, a deflector on the end of the piston; and i, the part of the igniting device at which the spark is produced. The diagram of

pressures in the cylinder is shown in (b), while the diagram for the pressures in the crank-case is shown in (c).

The difference between the diagrams of this engine and that of the fourcycle engine should be carefully noted. When the piston is moving toward the cylinder head, it is compressing the mixture of gas and air, while at the same time it is drawing a new charge into the crank-case through the valve s. That portion of the diagrams given during this stroke is shown by full lines. In reality, the first part of the cycle must always be the suction into the crankcase before any mixture is taken into the cylinder. The line Vfgh is identical with the compression and explosion line of the four-stroke cycle and covers the same series of operations; namely, compression to f, where ignition takes place, increase of the rate at which the pressure rises from f to g, and the explosion line gh. While the piston is compressing the charge in the cylinder, the crank-case is drawing more fuel through the valve s, the pressure in the crank-case falling below the atmosphere, as shown by the line v below O'V'.

It should be noted that the diagram for the pressures in the crank-case have a different scale of pressures from the scale of the diagram for the pressures in the cylinder.

The next stroke moves the piston away from the head end, making the expansion stroke for the cylinder and the compression stroke for the crankcase, the inlet valve s being closed. Before the exhaust port e is uncovered, the portion of the indicator diagram from h to j for the cylinder and from to c' for the crank-case is drawn.

When the piston is very near the end of the outward stroke, both the inlet and the exhaust ports o and e are open; the exhaust gases escape from the exhaust port e and the fresh charge enters through the by-pass b and port o, and is thrown by means of the deflecting plate d toward the cylinder head. The momentum of the column of exhaust gas as it leaves the cylinder is so

great that, unless there is considerable resistance in the exhaust passage, the pressure falls below that of the atmosphere, as shown by the small loop w, and is raised slightly, as shown by the loop y, when the fresh charge enters from the crank-case. If the engine is properly proportioned, none of the new mixture will escape at the exhaust port e, as it will be closed before the fresh charge has reached it. During this part of the stroke, the pressure in the crank-case rises from c' to c and then drops to V', when the transfer port is opened. The following inward stroke compresses the new mixture in the cylinder and draws a new charge into the crank-case, thus beginning a new cycle. The series of operations taking place during the two-stroke cycle are as follows:

CYLINDER

FIRST STROKE, INWARDS

Compression: pressure rises; ignition near end of stroke, followed by explosion and rapid rise of pressure.

CRANK-CASE

Suction: inlet valve open; pressure falls below atmosphere.

SECOND STROKE, OUTWARDS

Expansion: pressure falls; exhaust followed by entrance of fresh mixture from crank-case.

Compression: pressure rises to from 4 to 8 lb.; charging cylinder; pressure falls to atmospheric pressure.

GAS-ENGINE FUELS

Gaseous Fuels. Of the gases described on page 308 and the following pages, those generally employed for power purposes are: Natural gas, used at and within piping distance (150 to 200 mi.) of the wells where it is produced; water, or illuminating, gas, used in those cities having gas plants, although its application is limited by reason of its relatively high price; producer, or fuel, gas, used generally at iron and steel works; by-product gas, used at by-product coke ovens, which are usually built in connection with steel works or in some large city where there is a market for the coke as well as the gas. Gaseous fuels are suitable for use in stationary engines but not in haulage motors. They are rarely used in internal-combustion engines at coal mines, although natural gas, if the price is low by reason of the wells being in the coal fields, is sometimes used instead of coal under the boilers.

Alcohol. Of the two kinds of alcohol, methyl, or wood, alcohol, CH4O, and grain, or ethyl, alcohol, C2H6O, the former is not suited for use in internalcombustion engines as it apparently liberates acetic acid, which corrodes the cylinders or valves.

Grain alcohol has a specific gravity of .795, or 64° Baumé. It is obtained by distillation from vegetable substances containing sugar or starch, such as corn, wheat, rye, or other grains, potatoes, molasses, etc. When pure, it absorbs water more rapidly than it loses its own substance by evaporation. When diluted with 15% of water, it evaporates as if a single liquid and not a mixture.

The revenue laws of most countries require that grain alcohol must be denatured or rendered unfit for the manufacture of liquors before being sold as a fuel, by the addition of some poisonous or harmful ingredient such as wood alcohol, petroleum distillates, coloring matter, or the like. In France, the denaturing is accomplished by adding to 26 gal. of grain alcohol, 17 oz. of heavy benzine, and 10% of wood alcohol. The alcohols used are each of 90° strength or purity. To reduce the cost of the mixture below 38c., it is generally mixed with an equal volume of benzol containing 85% of benzine. In Germany, a fuel costing from 15 to 17 c. a gallon is made by adding to the grain alcohol 15% of benzol, no wood alcohol being used.

Gasoline.-Gasoline is produced by the distillation of petroleum, being among the first of the hydrocarbons to be given off in the manufacture of kerosene, or illuminating oil. Its boiling point varies from 158° to 176° F., its specific gravity from .66 to .67, and its density according to the Baumé scale from 80° to 78°.

Commercial gasoline is not a simple substance but a mixture of lighter and heavier products. It is rated according to its density by the Baumé scale. Owing to evaporation and other causes, the density of the gasoline as actually purchased is likely to be somewhat greater than its nominal rating and may test as low as 68°. The vapor of gasoline that forms over the liquid

consists chiefly of pentane, C5H12, having a specific gravity of .628; but the liquid gasoline consists of a mixture of hexane and heptane, the composition varying with the specific gravity of the gasoline.

A gasoline with a specific gravity of .683 and a boiling point of 154° F. has shown the following composition by analysis: hexane, 80%; heptane, 18%; pentane, 2%. The chemical composition is 83.8% carbon and 16.2% hydrogen; and the chemical formula is 41.86C6H14+6.48C7H16+C5H12. This formula will aid in the calculation of the fuel value.

Commercial gasoline evaporates very readily at ordinary temperatures, but quite slowly in cold weather, and leaves small percentages of a heavier oil, which evaporates slowly or not at all. The vapor tension varies considerably with the temperature, but at 60° F. the vapor of commercial gasoline represents about 130 volumes of the liquid and sustains a water pressure of from 6 to 8 in. An explosive mixture of gasoline vapor and air is composed of the vapor of 1 part of liquid gasoline to from 8,000 to 10,000 parts of air by volume. The volume of the vapor will vary, but an average proportion will be 2.15 of gasoline vapor to 100 parts of air.

Kerosene. Kerosene, or illuminating oil, the principal product of the distillation of petroleum and sometimes used in internal-combustion engines, boils at 302° to 572° F., has a specific gravity of .753 to .964, and a density of 56° to 32° Baumé.

Commercial kerosene varies in specific gravity (at 59° F.) from .760 to .820. Exceptionally light kerosene, such as the Pennsylvania light oil, has a specific gravity below .760. The boiling point of kerosene of .760 specific gravity is 302° F. and of kerosene of .820 specific gravity 536° F. Kerosene begins to give off vapor at from 100° to 120° F., and this vapor is mainly nonane, CH20. Liquid kerosene is a mixture of decane, C10H22, with a little hexadecane, C16H34. The boiling points of these three liquids are as follows: nonane, CH20, 277° F.; decane C10H22, 316° F; hexadecane, C16H34, 536° F. Average kerosene consists chiefly of decane. For the chemical action that takes place when kerosene is burned, that corresponding to the combustion of decane may be taken without appreciable error.

Fuel, or Compound, Oils.-Oils that are lighter than about 70° Baumé evaporate so rapidly that a large part is often lost before they reach the consumer. To reduce this loss on the part of the light oils and to make a market for the less salable heavy oils, the two are sometimes mixed and offered as fuel, or compound, oil or by some trade name. These mixtures are not to be confused with the fuel oil produced directly from wells and described on 395 and the following pages, which crude petroleum. As the demand for the difficultly salable heavy oils varies, so will vary the composition of the artificial fuel oils into which they enter.

Rating of Oil and Gasoline.-In selecting gasoline, it is usually sufficient to know its density by Baumé's scale, this being the rating at which it is sold in the general market. For instance, "Gasoline 72 Baumé" means that the density of the gasoline is 72° of Baumé's hydrometer. Kerosene is generally rated by its flashing point. This point is the number of degrees of temperature to which it must be heated before the vapors given off from the surface of the oil will take fire from a flame held over the containing vessel. Thus, oil of 150° test is oil that will flash or take fire when heated to a temperature of 150° F. Kerosene, at ordinary temperatures, should extinguish a lighted taper when the taper is plunged into it.

Baumé Hydrometer.-The Baumé hydrometer shown in the figure consists of a glass tube, near the bottom of which are two bulbs. The lower and smaller bulb is loaded with mercury or shot, so as to cause the instrument to remain in a vertical position when placed in the liquid in the vessel a. The upper bulb b is filled with air, and its volume is such that the whole instrument is lighter than an equal volume of water.

a

The point to which the hydrometer sinks when placed in water is usually marked, the tube being graduated above and below in such a manner that the specific gravity of the liquid can be read directly. It is customary to have two instruments: one with the zero point near the top of the stem, for use in liquids heavier than water; and the other with the zero point near the bulb, for use in liquids lighter than water.

Comparative Value of Liquid Fuels. So far as their heating value per pound goes, there is not much to choose between kerosene and gasoline, each