CLASSIFICATION BY CYLINDER ARRANGEMENT
Fig. 2.—Plate Showing Heavy, Slow Speed Internal Combustion Engines Used Only for Stationary Power in Large Installations Giving Weight to Horse-Power Ratio.
Fig. 3.—Various Forms of Internal Combustion Engines Showing Decrease in Weight to Horse-Power Ratio with Augmenting Speed of Rotation.
Fig. 4.—Internal Combustion Engine Types of Extremely Fine Construction and Refined Design, Showing Great Power Outputs for Very Small Weight, a Feature Very Much Desired in Airplane Power Plants.
Of all the types enumerated above engines having less than eight cylinders are the most popular in everything but aircraft work. The four-cylinder vertical is without doubt the most widely used of all types owing to the large number employed as automobile power plants. Stationary engines in small and medium powers are invariably of the single or double form. Three-cylinder engines are seldom used at the present time, except in marine work and in some stationary forms. Eight- and twelve-cylinder motors have received but limited application and practically always in automobiles, racing motor boats or in aircraft. The only example of a fourteen-cylinder motor to be used to any extent is incorporated in aeroplane construction. This is also true of the sixteen- and eighteen-cylinder forms and of twenty-four-cylinder engines now in process of development.
The duty an engine is designed for determines the weight per horse-power. High powered engines intended for steady service are always of the slow speed type and consequently are of very massive construction. Various forms of heavy duty type stationary engines are shown at [Fig. 2]. Some of these engines may weigh as much as 600 pounds per horse-power. A further study is possible by consulting data given on [Figs. 3] and [4]. As the crank-shaft speed increases and cylinders are multiplied the engines become lighter. While the big stationary power plants may run for years without attention, airplane engines require rebuilding after about 60 to 80 hours air service for the fixed cylinder types and 40 hours or less for the rotary cylinder air-cooled forms. There is evidently a decrease in durability and reliability as the weight is lessened. These illustrations also permit of obtaining a good idea of the variety of forms internal combustion engines are made in.
CHAPTER II
[Operating Principles of Two- and Four-Stroke Engines]—[Four-cycle Action]—[Two-cycle Action]—[Comparing Two- and Four-cycle Types]—[Theory of Gas and Gasoline Engine]—[Early Gas-Engine Forms]—[Isothermal Law]—[Adiabatic Law]—[Temperature Computations]—[Heat and Its Work]—[Conversion of Heat to Power]—[Requisites for Best Power Effect].
OPERATING PRINCIPLES OF TWO- AND FOUR-STROKE CYCLE ENGINES
Before discussing the construction of the various forms of internal combustion engines it may be well to describe the operating cycle of the types most generally used. The two-cycle engine is the simplest because there are no valves in connection with the cylinder, as the gas is introduced into that member and expelled from it through ports cored into the cylinder walls. These are covered by the piston at a certain portion of its travel and uncovered at other parts of its stroke. In the four-cycle engine the explosive gas is admitted to the cylinder through a port at the head end closed by a valve, while the exhaust gas is expelled through another port controlled in a similar manner. These valves are operated by mechanism distinct from the piston.
Fig. 5.—Outlining First Two Strokes of Piston in Four-Cycle Engine.
Fig. 6.—Outlining Second Two Strokes of Piston in Four-Cycle Engine.
The action of the four-cycle type may be easily understood if one refers to illustrations at [Figs. 5] and [6]. It is called the “four-stroke engine” because the piston must make four strokes in the cylinder for each explosion or power impulse obtained. The principle of the gas-engine of the internal combustion type is similar to that of a gun, i.e., power is obtained by the rapid combustion of some explosive or other quick burning substance. The bullet is driven out of the gun barrel by the pressure of the gas evolved when the charge of powder is ignited. The piston or movable element of the gas-engine is driven from the closed or head end to the crank end of the cylinder by a similar expansion of gases resulting from combustion. The first operation in firing a gun or securing an explosion in the cylinder of the gas-engine is to fill the combustion space with combustible material. This is done by a down stroke of the piston during which time the inlet valve opens to admit the gaseous charge to the cylinder interior. This operation is shown at [Fig. 5], A. The second operation is to compress this gas which is done by an upward stroke of the piston as shown at [Fig. 5], B. When the top of the compression stroke is reached, the gas is ignited and the piston is driven down toward the open end of the cylinder, as indicated at [Fig. 6], C. The fourth operation or exhaust stroke is performed by the return upward movement of the piston as shown at [Fig. 6], D during which time the exhaust valve is opened to permit the burnt gases to leave the cylinder. As soon as the piston reaches the top of its exhaust stroke, the energy stored in the fly-wheel rim during the power stroke causes that member to continue revolving and as the piston again travels on its down stroke the inlet valve opens and admits a charge of fresh gas and the cycle of operations is repeated.
Fig. 7.—Sectional View of L Head Gasoline Engine Cylinder Showing Piston Movements During Four-Stroke Cycle.
The illustrations at [Fig. 7] show how the various cycle functions take place in an L head type water cooled cylinder engine. The sections at A and C are taken through the inlet valve, those at B and D are taken through the exhaust valve.
The two-cycle engine works on a different principle, as while only the combustion chamber end of the piston is employed to do useful work in the four-cycle engine, both upper and lower portions are called upon to perform the functions necessary to two-cycle engine operation. Instead of the gas being admitted into the cylinder as is the case with the four-stroke engine, it is first drawn into the engine base where it receives a preliminary compression prior to its transfer to the working end of the cylinder. The views at [Fig. 8] should indicate clearly the operation of the two-port two-cycle engine. At A the piston is seen reaching the top of its stroke and the gas above the piston is being compressed ready for ignition, while the suction in the engine base causes the automatic valve to open and admits mixture from the carburetor to the crank case. When the piston reaches the top of its stroke, the compressed gas is ignited and the piston is driven down on the power stroke, compressing the gas in the engine base.
Fig. 8.—Showing Two-port, Two-cycle Engine Operation.
When the top of the piston uncovers the exhaust port the flaming gas escapes because of its pressure. A downward movement of the piston uncovers the inlet port opposite the exhaust and permits the fresh gas to bypass through the transfer passage from the engine base to the cylinder. The conditions with the intake and exhaust port fully opened are clearly shown at [Fig. 8], C. The deflector plate on the top of the piston directs the entering fresh gas to the top of the cylinder and prevents the main portion of the gas stream from flowing out through the open exhaust port. On the next upstroke of the piston the gas in the cylinder is compressed and the inlet valve opened, as shown at A to permit a fresh charge to enter the engine base.
Fig. 9.—Defining Three-port, Two-cycle Engine Action.
The operating principle of the three-port, two-cycle engine is practically the same as that previously described with the exception that the gas is admitted to the crank-case through a third port in the cylinder wall, which is uncovered by the piston when that member reaches the end of its upstroke. The action of the three-port form can be readily ascertained by studying the diagrams given at [Fig. 9]. Combination two- and three-port engines have been evolved and other modifications made to improve the action.
THE TWO-CYCLE AND FOUR-CYCLE TYPES
In the earlier years of explosive-motor progress was evolved the two types of motors in regard to the cycles of their operation. The early attempts to perfect the two-cycle principle were for many years held in abeyance from the pressure of interests in the four-cycle type, until its simplicity and power possibilities were demonstrated by Mr. Dugald Clerk in England, who gave the principles of the two-cycle motor a broad bearing leading to immediate improvements in design, which has made further progress in the United States, until at the present time it has an equal standard value as a motor-power in some applications as its ancient rival the four-cycle or Otto type, as demonstrated by Beau de Rocha in 1862.
Thermodynamically, the methods of the two types are equal as far as combustion is concerned, and compression may favor in a small degree the four-cycle type as well as the purity of the charge. The cylinder volume of the two-cycle motor is much smaller per unit of power, and the enveloping cylinder surface is therefore greater per unit of volume. Hence more heat is carried off by the jacket water during compression, and the higher compression available from this tends to increase the economy during compression which is lost during expansion.
From the above considerations it may be safely stated that a lower temperature and higher pressure of charge at the beginning of compression is obtained in the two-cycle motor, greater weight of charge and greater specific power of higher compression resulting in higher thermal efficiency. The smaller cylinder for the same power of the two-cycle motor gives less friction surface per impulse than of the other type; although the crank-chamber pressure may, in a measure, balance the friction of the four-cycle type. Probably the strongest points in favor of the two-cycle type are the lighter fly-wheel and the absence of valves and valve gear, making this type the most simple in construction and the lightest in weight for its developed power. Yet, for the larger power units, the four-cycle type will no doubt always maintain the standard for efficiency and durability of action.
The distribution of the charge and its degree of mixture with the remains of the previous explosion in the clearance space, has been a matter of discussion for both types of explosive motors, with doubtful results. In [Fig. 10], A we illustrate what theory suggests as to the distribution of the fresh charge in a two-cycle motor, and in [Fig. 10], B what is the probable distribution of the mixture when the piston starts on its compressive stroke. The arrows show the probable direction of flow of the fresh charge and burnt gases at the crucial moment.
In [Fig. 10], C is shown the complete out-sweep of the products of combustion for the full extent of the piston stroke of a four-cycle motor, leaving only the volume of the clearance to mix with the new charge and at D the manner by which the new charge sweeps by the ignition device, keeping it cool and avoiding possibilities of pre-ignition by undue heating of the terminals of the sparking device. Thus, by enveloping the sparking device with the pure mixture, ignition spreads through the charge with its greatest possible velocity, a most desirable condition in high-speed motors with side-valve chambers and igniters within the valve chamber.
Fig. 10.—Diagrams Contrasting Action of Two- and Four-Cycle Cylinders on Exhaust and Intake Stroke.
THEORY OF THE GAS AND GASOLINE ENGINE
The laws controlling the elements that create a power by their expansion by heat due to combustion, when properly understood, become a matter of computation in regard to their value as an agent for generating power in the various kinds of explosive engines. The method of heating the elements of power in explosive engines greatly widens the limits of temperature as available in other types of heat-engines. It disposes of many of the practical troubles of hot-air, and even of steam-engines, in the simplicity and directness of application of the elements of power. In the explosive engine the difficulty of conveying heat for producing expansive effect by convection is displaced by the generation of the required heat within the expansive element and at the instant of its useful work. The low conductivity of heat to and from air has been the great obstacle in the practical development of the hot-air engine; while, on the contrary, it has become the source of economy and practicability in the development of the internal-combustion engine.
The action of air, gas, and the vapors of gasoline and petroleum oil, whether singly or mixed, is affected by changes of temperature practically in nearly the same ratio; but when the elements that produce combustion are interchanged in confined spaces, there is a marked difference of effect. The oxygen of the air, the hydrogen and carbon of a gas, or vapor of gasoline or petroleum oil are the elements that by combustion produce heat to expand the nitrogen of the air and the watery vapor produced by the union of the oxygen in the air and the hydrogen in the gas, as well as also the monoxide and carbonic-acid gas that may be formed by the union of the carbon of gas or vapor with part of the oxygen of the air. The various mixtures as between air and gas, or air and vapor, with the proportion of the products of combustion left in the cylinder from a previous combustion, form the elements to be considered in estimating the amount of pressure that may be obtained by their combustion and expansive force.
EARLY GAS ENGINE FORMS
The working process of the explosive motor may be divided into three principal types: 1. Motors with charges igniting at constant volume without compression, such as the Lenoir, Hugon, and other similar types now abandoned as wasteful in fuel and effect. 2. Motors with charges igniting at constant pressure with compression, in which a receiver is charged by a pump and the gases burned while being admitted to the motor cylinder, such as types of the Simon and Brayton engine. 3. Motors with charges igniting at constant volume with variable compression, such as the later two- and four-cycle motors with compression of the indrawn charge; limited in the two-cycle type and variable in the four-cycle type with the ratios of the clearance space in the cylinder. This principle produces the explosive motor of greatest efficiency.
The phenomena of the brilliant light and its accompanying heat at the moment of explosion have been witnessed in the experiments of Dugald Clerk in England, the illumination lasting throughout the stroke; but in regard to time in a four-cycle engine, the incandescent state exists only one-quarter of the running time. Thus the time interval, together with the non-conductibility of the gases, makes the phenomena of a high-temperature combustion within the comparatively cool walls of a cylinder a practical possibility.
THE ISOTHERMAL LAW
The natural laws, long since promulgated by Boyle, Gay Lussac, and others, on the subject of the expansion and compression of gases by force and by heat, and their variable pressures and temperatures when confined, are conceded to be practically true and applicable to all gases, whether single, mixed, or combined.
The law formulated by Boyle only relates to the compression and expansion of gases without a change of temperature, and is stated in these words:
If the temperature of a gas be kept constant, its pressure or elastic force will vary inversely as the volume it occupies.
It is expressed in the formula P × V = C, or pressure × volume = constant. Hence, C/P = V and C/V = P.
Thus the curve formed by increments of pressure during the expansion or compression of a given volume of gas without change of temperature is designated as the isothermal curve in which the volume multiplied by the pressure is a constant value in expansion, and inversely the pressure divided by the volume is a constant value in compressing a gas.
But as compression and expansion of gases require force for their accomplishment mechanically, or by the application or abstraction of heat chemically, or by convection, a second condition becomes involved, which was formulated into a law of thermodynamics by Gay Lussac under the following conditions: A given volume of gas under a free piston expands by heat and contracts by the loss of heat, its volume causing a proportional movement of a free piston equal to 1⁄273 part of the cylinder volume for each degree Centigrade difference in temperature, or 1⁄492 part of its volume for each degree Fahrenheit. With a fixed piston (constant volume), the pressure is increased or decreased by an increase or decrease of heat in the same proportion of 1⁄273 part of its pressure for each degree Centigrade, or 1⁄492 part of its pressure for each degree Fahrenheit change in temperature. This is the natural sequence of the law of mechanical equivalent, which is a necessary deduction from the principle that nothing in nature can be lost or wasted, for all the heat that is imparted to or abstracted from a gaseous body must be accounted for, either as heat or its equivalent transformed into some other form of energy. In the case of a piston moving in a cylinder by the expansive force of heat in a gaseous body, all the heat expended in expansion of the gas is turned into work; the balance must be accounted for in absorption by the cylinder or radiation.
THE ADIABATIC LAW
This theory is equally applicable to the cooling of gases by abstraction of heat or by cooling due to expansion by the motion of a piston. The denominators of these heat fractions of expansion or contraction represent the absolute zero of cold below the freezing-point of water, and read -273° C. or -492.66° = -460.66° F. below zero; and these are the starting-points of reference in computing the heat expansion in gas-engines. According to Boyle’s law, called the first law of gases, there are but two characteristics of a gas and their variations to be considered, viz., volume and pressure: while by the law of Gay Lussac, called the second law of gases, a third is added, consisting of the value of the absolute temperature, counting from absolute zero to the temperatures at which the operations take place. This is the Adiabatic law.
The ratio of the variation of the three conditions—volume, pressure, and heat—from the absolute zero temperature has a certain rate, in which the volume multiplied by the pressure and the product divided by the absolute temperature equals the ratio of expansion for each degree. If a volume of air is contained in a cylinder having a piston and fitted with an indicator, the piston, if moved to and fro slowly, will alternately compress and expand the air, and the indicator pencil will trace a line or lines upon the card, which lines register the change of pressure and volume occurring in the cylinder. If the piston is perfectly free from leakage, and it be supposed that the temperature of the air is kept quite constant, then the line so traced is called an Isothermal line, and the pressure at any point when multiplied by the volume is a constant, according to Boyle’s law,
pv = a constant.
If, however, the piston is moved very rapidly, the air will not remain at constant temperature, but the temperature will increase because work has been done upon the air, and the heat has no time to escape by conduction. If no heat whatever is lost by any cause, the line will be traced over and over again by the indicator pencil, the cooling by expansion doing work precisely equalling the heating by compression. This is the line of no transmission of heat, therefore known as Adiabatic.
Fig. 11.—Diagram Isothermal and Adiabatic Lines.
The expansion of a gas 1⁄273 of its volume for every degree Centigrade, added to its temperature, is equal to the decimal .00366, the coefficient of expansion for Centigrade units. To any given volume of a gas, its expansion may be computed by multiplying the coefficient by the number of degrees, and by reversing the process the degree of acquired heat may be obtained approximately. These methods are not strictly in conformity with the absolute mathematical formula, because there is a small increase in the increment of expansion of a dry gas, and there is also a slight difference in the increment of expansion due to moisture in the atmosphere and to the vapor of water formed by the union of the hydrogen and oxygen in the combustion chamber of explosive engines.
TEMPERATURE COMPUTATIONS
The ratio of expansion on the Fahrenheit scale is derived from the absolute temperature below the freezing-point of water (32°) to correspond with the Centigrade scale; therefore 1⁄492.66 = .0020297, the ratio of expansion from 32° for each degree rise in temperature on the Fahrenheit scale. As an example, if the temperature of any volume of air or gas at constant volume is raised, say from 60° to 2000° F., the increase in temperature will be 1940°. The ratio will be 1⁄520.66 = .0019206. Then by the formula:
Ratio × acquired temp. × initial pressure = the gauge pressure; and .0019206 × 1940° × 14.7 = 54.77 lbs.
By another formula, a convenient ratio is obtained by (absolute pressure)/(absolute temp.) or 14.7⁄520.66 = .028233; then, using the difference of temperature as before, .028233 × 1940° = 54.77 lbs. pressure.
By another formula, leaving out a small increment due to specific heat at high temperatures:
| I. | Atmospheric pressure × absolute temp. + acquired temp. | = |
| Absolute temp. + initial temp. |
absolute pressure due to the acquired temperature, from which the atmospheric pressure is deducted for the gauge pressure. Using the foregoing example, we have
| 14.7 × 460.66° + 2000° | = 69.47 - 14.7 = 54.77, the gauge pressure, |
| 460.66 + 60° |
460.66 being the absolute temperature for zero Fahrenheit.
For obtaining the volume of expansion of a gas from a given increment of heat, we have the approximate formula:
| II. | Volume × absolute temp. + acquired temp. | = heated volume. |
| Absolute temp. + initial temp. |
In applying this formula to the foregoing example, the figures become:
| I. × | 460.66° + 2000° | = 4.72604 volumes. |
| 460.66 + 60° |
From this last term the gauge pressure may be obtained as follows:
III. 4.72604 × 14.7 = 69.47 lbs. absolute - 14.7 lbs. atmospheric pressure = 54.77 lbs. gauge pressure; which is the theoretical pressure due to heating air in a confined space, or at constant volume from 60° to 2000° F.
By inversion of the heat formula for absolute pressure we have the formula for the acquired heat, derived from combustion at constant volume from atmospheric pressure to gauge pressure plus atmospheric pressure as derived from Example I., by which the expression
| absolute pressure × absolute temp. + initial temp. |
| initial absolute pressure |
= absolute temperature + temperature of combustion, from which the acquired temperature is obtained by subtracting the absolute temperature.
Then, for example,
| 69.47 × 460.66 + 60 | = 2460.66, and 2460.66 - 460.66 = 2000°, |
| 14.7 |
the theoretical heat of combustion. The dropping of terminal decimals makes a small decimal difference in the result in the different formulas.
HEAT AND ITS WORK
By Joule’s law of the mechanical equivalent of heat, whenever heat is imparted to an elastic body, as air or gas, energy is generated and mechanical work produced by the expansion of the air or gas. When the heat is imparted by combustion within a cylinder containing a movable piston, the mechanical work becomes an amount measurable by the observed pressure and movement of the piston. The heat generated by the explosive elements and the expansion of the non-combining elements of nitrogen and water vapor that may have been injected into the cylinder as moisture in the air, and the water vapor formed by the union of the oxygen of the air with the hydrogen of the gas, all add to the energy of the work from their expansion by the heat of internal combustion. As against this, the absorption of heat by the walls of the cylinder, the piston, and cylinder-head or clearance walls, becomes a modifying condition in the force imparted to the moving piston.
It is found that when any explosive mixture of air and gas or hydrocarbon vapor is fired, the pressure falls far short of the pressure computed from the theoretical effect of the heat produced, and from gauging the expansion of the contents of a cylinder. It is now well known that in practice the high efficiency which is promised by theoretical calculation is never realized; but it must always be remembered that the heat of combustion is the real agent, and that the gases and vapors are but the medium for the conversion of inert elements of power into the activity of energy by their chemical union. The theory of combustion has been the leading stimulus to large expectations with inventors and constructors of explosive motors; its entanglement with the modifying elements in practice has delayed the best development in construction, and as yet no really positive design of best form or action seems to have been accomplished, although great progress has been made during the past decade in the development of speed, reliability, economy, and power output of the individual units of this comparatively new power.
One of the most serious difficulties in the practical development of pressure, due to the theoretical computations of the pressure value of the full heat, is probably caused by imparting the heat of the fresh charge to the balance of the previous charge that has been cooled by expansion from the maximum pressure to near the atmospheric pressure of the exhaust. The retardation in the velocity of combustion of perfectly mixed elements is now well known from experimental trials with measured quantities; but the principal difficulty in applying these conditions to the practical work of an explosive engine where a necessity for a large clearance space cannot be obviated, is in the inability to obtain a maximum effect from the imperfect mixture and the mingling of the products of the last explosion with the new mixture, which produces a clouded condition that makes the ignition of the mass irregular or chattering, as observed in the expansion lines of indicator cards; but this must not be confounded with the reaction of the spring in the indicator.
Stratification of the mixture has been claimed as taking place in the clearance chamber of the cylinder; but this is not a satisfactory explanation in view of the vortical effect of the violent injection of the air and gas or vapor mixture. It certainly cannot become a perfect mixture in the time of a stroke of a high-speed motor of the two-cycle class. In a four-cycle engine, making 1,500 revolutions per minute, the injection and compression in any one cylinder take place in one twenty-fifth of a second—formerly considered far too short a time for a perfect infusion of the elements of combustion but now very easily taken care of despite the extremely high speed of numerous aviation and automobile power-plants.
Table I.—Explosion at Constant Volume in a Closed Chamber.
| Diagram Curve Fig. 8. | Mixture Injected. | Temp. of Injection Fahr. | Time of Explosion. Second. | Observed Gauge Pressure. Pounds. | Computed Temp. Fahr. | ||||||||
| a | 1 | volume | gas | to | 14 | volumes | air. | 64° | 0.45 | 40. | 1,483° | ||
| b | 1 | „ | „ | „ | 13 | „ | „ | 51° | 0.31 | 51. | 5 | 1,859° | |
| c | 1 | „ | „ | „ | 12 | „ | „ | 51° | 0.24 | 60. | 2,195° | ||
| d | 1 | „ | „ | „ | 11 | „ | „ | 51° | 0.17 | 61. | 2,228° | ||
| e | 1 | „ | „ | „ | 9 | „ | „ | 62° | 0.08 | 78. | 2,835° | ||
| f | 1 | „ | „ | „ | 7 | „ | „ | 62° | 0.06 | 87. | 3,151° | ||
| g | 1 | „ | „ | „ | 6 | „ | „ | 51° | 0.04 | 90. | 3,257° | ||
| h | 1 | „ | „ | „ | 5 | „ | „ | 51° | 0.05 | 5 | 91. | 3,293° | |
| i | 1 | „ | „ | „ | 4 | „ | „ | 66° | 0.16 | 80. | 2,871° | ||
In an examination of the times of explosion and the corresponding pressures in both tables, it will be seen that a mixture of 1 part gas to 6 parts air is the most effective and will give the highest mean pressure in a gas-engine. There is a limit to the relative proportions of illuminating gas and air mixture that is explosive, somewhat variable, depending upon the proportion of hydrogen in the gas. With ordinary coal-gas, 1 of gas to 15 parts of air; and on the lower end of the scale, 1 volume of gas to 2 parts air, are non-explosive. With gasoline vapor the explosive effect ceases at 1 to 16, and a saturated mixture of equal volumes of vapor and air will not explode, while the most intense explosive effect is from a mixture of 1 part vapor to 9 parts air. In the use of gasoline and air mixtures from a carburetor, the best effect is from 1 part saturated air to 8 parts free air.
Table II.—Properties and Explosive Temperature of a Mixture of One Part
of Illuminating Gas of 660 Thermal Units per Cubic Foot with Various
Proportions of Air without Mixture of Charge with the Products of a
Previous Explosion.
| Propor- tion, Air to Gas by Volumes. | Pounds in One Cubic Foot of Mixture. | Specific Heat. Heat Units Required to Raise 1 Lb. 1 Deg. Fahrenheit. | Heat to Raise One Cubic Foot of Mixture 1 Deg. Fahr. | Heat Units Evolved by Combus- tion. | Ratio Col. 6/5 | Usual Combus- tion Efficien- cy. | Usual Rise of Temperature due to Explosion at Constant Volume. | |||||
| Constant Pressure. | Constant Volume. | |||||||||||
| 6 | to | 1 | .074195 | .2668 | .1913 | .014189 | 94. | 28 | 6644. | 6 | .465 | 3090 |
| 7 | to | 1 | .075012 | .2628 | .1882 | .014116 | 82. | 5844. | 4 | .518 | 3027 | |
| 8 | to | 1 | .075647 | .2598 | .1858 | .014059 | 73. | 33 | 5216. | 1 | .543 | 2832 |
| 9 | to | 1 | .076155 | .2575 | .1846 | .014013 | 66. | 4709. | 9 | .56 | 2637 | |
| 10 | to | 1 | .076571 | .2555 | .1825 | .013976 | 60. | 4293. | .575 | 2468 | ||
| 11 | to | 1 | .076917 | .2540 | .1813 | .013945 | 55. | 3944. | .585 | 2307 | ||
| 12 | to | 1 | .077211 | .2526 | .1803 | .013922 | 50. | 77 | 3646. | 7 | .58 | 2115 |
The weight of a cubic foot of gas and air mixture as given in Col. 2 is found by adding the number of volumes of air multiplied by its weight, .0807, to one volume of gas of weight .035 pound per cubic foot and dividing by the total number of volumes; for example, as in the table, 6 × .0807 = .5192⁄7 = .074195 as in the first line, and so on for any mixture or for other gases of different specific weight per cubic foot. The heat units evolved by combustion of the mixture (Col. 6) are obtained by dividing the total heat units in a cubic foot of gas by the total proportion of the mixture, 660⁄7 = 94.28 as in the first line of the table. Col. 5 is obtained by multiplying the weight of a cubic foot of the mixture in Col. 2 by the specific heat at a constant volume (Col. 4), Col. 6/Col. 5 = Col. 7 the total heat ratio, of which Col. 8 gives the usual combustion efficiency—Col. 7 × Col. 8 gives the absolute rise in temperature of a pure mixture, as given in Col. 9.
The many recorded experiments made to solve the discrepancy between the theoretical and the actual heat development and resulting pressures in the cylinder of an explosive motor, to which much discussion has been given as to the possibilities of dissociation and the increased specific heat of the elements of combustion and non-combustion, as well, also, of absorption and radiation of heat, have as yet furnished no satisfactory conclusion as to what really takes place within the cylinder walls. There seems to be very little known about dissociation, and somewhat vague theories have been advanced to explain the phenomenon. The fact is, nevertheless, apparent as shown in the production of water and other producer gases by the use of steam in contact with highly incandescent fuel. It is known that a maximum explosive mixture of pure gases, as hydrogen and oxygen or carbonic oxide and oxygen, suffers a contraction of one-third their volume by combustion to their compounds, steam or carbonic acid. In the explosive mixtures in the cylinder of a motor, however, the combining elements form so small a proportion of the contents of the cylinder that the shrinkage of their volume amounts to no more than 3 per cent. of the cylinder volume. This by no means accounts for the great heat and pressure differences between the theoretical and actual effects.
CONVERSION OF HEAT TO POWER
The utilization of heat in any heat-engine has long been a theme of inquiry and experiment with scientists and engineers, for the purpose of obtaining the best practical conditions and construction of heat-engines that would represent the highest efficiency or the nearest approach to the theoretical value of heat, as measured by empirical laws that have been derived from experimental researches relating to its ultimate volume. It is well known that the steam-engine returns only from 12 to 18 per cent. of the power due to the heat generated by the fuel, about 25 per cent. of the total heat being lost in the chimney, the only use of which is to create a draught for the fire; the balance, some 60 per cent., is lost in the exhaust and by radiation. The problem of utmost utilization of force in steam has nearly reached its limit.
The internal-combustion system of creating power is comparatively new in practice, and is but just settling into definite shape by repeated trials and modification of details, so as to give somewhat reliable data as to what may be expected from the rival of the steam-engine as a prime mover. For small powers, the gas, gasoline, and petroleum-oil engines are forging ahead at a rapid rate, filling the thousand wants of manufacture and business for a power that does not require expensive care, that is perfectly safe at all times, that can be used in any place in the wide world to which its concentrated fuel can be conveyed, and that has eliminated the constant handling of crude fuel and water.
REQUISITES FOR BEST POWER EFFECT
The utilization of heat in a gas-engine is mainly due to the manner in which the products entering into combustion are distributed in relation to the movement of the piston. The investigation of the foremost exponent of the theory of the explosive motor was prophetic in consideration of the later realization of the best conditions under which these motors can be made to meet the requirements of economy and practicability. As early as 1862, Beau de Rocha announced, in regard to the coming power, that four requisites were the basis of operation for economy and best effect. 1. The greatest possible cylinder volume with the least possible cooling surface. 2. The greatest possible rapidity of expansion. Hence, high speed. 3. The greatest possible expansion. Long stroke. 4. The greatest possible pressure at the commencement of expansion. High compression.
CHAPTER III
[Efficiency of Internal Combustion Engines]—[Various Measures of Efficiency]—[Temperatures and Pressures]—[Factors Governing Economy]—[Losses in Wall Cooling]—[Value of Indicator Cards]—[Compression in Explosive Motors]—[Factors Limiting Compression]—[Causes of Heat Losses and Inefficiency]—[Heat Losses to Cooling Water].
EFFICIENCY OF INTERNAL COMBUSTION ENGINES
Efficiencies are worked out through intricate formulas for a variety of theoretical and unknown conditions of combustion in the cylinder: ratios of clearance and cylinder volume, and the uncertain condition of the products of combustion left from the last impulse and the wall temperature. But they are of but little value, except as a mathematical inquiry as to possibilities. The real commercial efficiency of a gas or gasoline-engine depends upon the volume of gas or liquid at some assigned cost, required per actual brake horse-power per hour, in which an indicator card should show that the mechanical action of the valve gear and ignition was as perfect as practicable, and that the ratio of clearance, space, and cylinder volume gave a satisfactory terminal pressure and compression: i.e., the difference between the power figured from the indicator card and the brake power being the friction loss of the engine.
In four-cycle motors of the compression type, the efficiencies are greatly advanced by compression, producing a more complete infusion of the mixture of gas or vapor and air, quicker firing, and far greater pressure than is possible with the two-cycle type previously described. In the practical operation of the gas-engine during the past twenty years, the gas-consumption efficiencies per indicated horse-power have gradually risen from 17 per cent. to a maximum of 40 per cent. of the theoretical heat, and this has been done chiefly through a decreased combustion chamber and increased compression—the compression having gradually increased in practice from 30 lbs. per square inch to above 100; but there seems to be a limit to compression, as the efficiency ratio decreases with greater increase in compression. It has been shown that an ideal efficiency of 33 per cent. for 38 lbs., compression will increase to 40 per cent. for 66 lbs., and 43 per cent. for 88 lbs. compression. On the other hand, greater compression means greater explosive pressure and greater strain on the engine structure, which will probably retain in future practice the compression between the limits of 40 and 90 lbs. except in super-compression engines intended for high altitude work where compression pressures as high as 125 pounds have been used.
In experiments made by Dugald Clerk, in England, with a combustion chamber equal to 0.6 of the space swept by the piston, with a compression of 38 lbs., the consumption of gas was 24 cubic feet per indicated horse-power per hour. With 0.4 compression space and 61 lbs. compression, the consumption of gas was 20 cubic feet per indicated horse-power per hour; and with 0.34 compression space and 87 lbs. compression, the consumption of gas fell to 14.8 cubic feet per indicated horse-power per hour—the actual efficiencies being respectively 17, 21, and 25 per cent. This was with a Crossley four-cycle engine.
VARIOUS MEASURES OF EFFICIENCY
The efficiencies in regard to power in a heat-engine may be divided into four kinds, as follows: I. The first is known as the maximum theoretical efficiency of a perfect engine (represented by the lines in the indicator diagram). It is expressed by the formula
| T1 - T0 |
| T1 |
and shows the work of a perfect cycle in an engine working between the received temperature + absolute temperature (T1) and the initial atmospheric temperature + absolute temperature (T0). II. The second is the actual heat efficiency, or the ratio of the heat turned into work to the total heat received by the engine. It expresses the indicated horse-power. III. The third is the ratio between the second or actual heat efficiency and the first or maximum theoretical efficiency of a perfect cycle. It represents the greatest possible utilization of the power of heat in an internal-combustion engine. IV. The fourth is the mechanical efficiency. This is the ratio between the actual horse-power delivered by the engine through a dynamometer or measured by a brake (brake horse-power), and the indicated horse-power. The difference between the two is the power lost by engine friction. In regard to the general heat efficiency of the materials of power in explosive engines, we find that with good illuminating gas the practical efficiency varies from 25 to 40 per cent.; kerosene-motors, 20 to 30; gasoline-motors, 20 to 32; acetylene, 25 to 35; alcohol, 20 to 30 per cent. of their heat value. The great variation is no doubt due to imperfect mixtures and variable conditions of the old and new charge in the cylinder; uncertainty as to leakage and the perfection of combustion. In the Diesel motors operating under high pressure, up to nearly 500 pounds, an efficiency of 36 per cent. is claimed.
Fig. 12.—Graphic Diagram Showing Approximate Utilization of Fuel Burned in Internal-Combustion Engine.
The graphic diagram at [Fig. 12] is of special value as it shows clearly how the heat produced by charge combustion is expended in an engine of average design.
On general principles the greater difference between the heat of combustion and the heat at exhaust is the relative measure of the heat turned into work, which represents the degree of efficiency without loss during expansion. The mathematical formulas appertaining to the computation of the element of heat and its work in an explosive engine are in a large measure dependent upon assumed values, as the conditions of the heat of combustion are made uncertain by the mixing of the fresh charge with the products of a previous combustion, and by absorption, radiation, and leakage. The computation of the temperature from the observed pressure may be made as before explained, but for compression-engines the needed starting-points for computation are very uncertain, and can only be approximated from the exact measure and value of the elements of combustion in a cylinder charge.
TEMPERATURES AND PRESSURES
Owing to the decrease from atmospheric pressure in the indrawing charge of the cylinder, caused by valve and frictional obstruction, the compression seldom starts above 13 lbs. absolute, especially in high-speed engines. Col. 3 in the following table represents the approximate absolute compression pressure for the clearance percentage and ratio in Cols. 1 and 2, while Col. 4 indicates the gauge pressure from the atmospheric line. The temperatures in Col. 5 are due to the compression in Col. 3 from an assumed temperature of 560° F. in the mixture of the fresh charge of 6 air to 1 gas with the products of combustion left in the clearance chamber from the exhaust stroke of a medium-speed motor. This temperature is subject to considerable variation from the difference in the heat-unit power of the gases and vapors used for explosive power, as also of the cylinder-cooling effect. In Col. 6 is given the approximate temperatures of explosion for a mixture of air 6 to gas 1 of 660 heat units per cubic foot, for the relative values of the clearance ratio in Col. 2 at constant volume.
Table III.—Gas-Engine Clearance Ratios, Approximate Compression,
Temperatures of Explosion and Explosive Pressures with a Mixture
of Gas of 660 Heat Units per Cubic Foot and Mixture of Gas
1 to 6 of Air.
| Clearance Per Cent. of Piston Volume. | Ratio
| Approximate Compression from 13 Pounds Absolute. | Approximate Gauge Pressure. | Absolute Temperature of Compression from 560 Deg. Fahrenheit in Cylinder. | Absolute Temperature of Explosion. Gas, 1 part; Air, 6 parts. | Approximate Explosion Pressure Absolute. | Approximate Gauge Pressure. | Approximate Temperature of Explosion, Fahrenheit. | ||||||
| 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | ||||||
| Lbs. | Deg. | Deg. | Lbs. | Lbs. | Deg. | |||||||||
| .50 | 3. | 57. | 42. | 822. | 2488 | 169 | 144 | 2027 | ||||||
| .444 | 3. | 25 | 65. | 50. | 846. | 2568 | 197 | 182 | 2107 | |||||
| .40 | 3. | 50 | 70. | 55. | 868. | 2638 | 212 | 197 | 2177 | |||||
| .363 | 3. | 75 | 77. | 62. | 889. | 2701 | 234 | 219 | 2240 | |||||
| .333 | 4. | 84. | 69. | 910. | 2751 | 254 | 239 | 2290 | ||||||
| .285 | 4. | 50 | 102. | 88. | 955. | 2842 | 303 | 288 | 2381 | |||||
| .25 | 5. | 114. | 99. | 983. | 2901 | 336 | 321 | 2440 | ||||||
FACTORS GOVERNING ECONOMY
In view of the experiments in this direction, it clearly shows that in practical work, to obtain the greatest economy per effective brake horse-power, it is necessary: 1st. To transform the heat into work with the greatest rapidity mechanically allowable. This means high piston speed. 2d. To have high initial compression. 3d. To reduce the duration of contact between the hot gases and the cylinder walls to the smallest amount possible; which means short stroke and quick speed, with a spherical cylinder head. 4th. To adjust the temperature of the jacket water to obtain the most economical output of actual power. This means water-tanks or water-coils, with air-cooling surfaces suitable and adjustable to the most economical requirement of the engine, which by late trials requires the jacket water to be discharged at about 200° F. 5th. To reduce the wall surface of the clearance space or combustion chamber to the smallest possible area, in proportion to its required volume. This lessens the loss of the heat of combustion by exposure to a large surface, and allows of a higher mean wall temperature to facilitate the heat of compression.
LOSSES IN WALL COOLING
In an experimental investigation of the efficiency of a gas-engine under variable piston speeds made in France, it was found that the useful effect increases with the velocity of the piston—that is, with the rate of expansion of the burning gases with mixtures of uniform volumes: so that the variations of time of complete combustion at constant pressure, and the variations due to speed, in a way compensate in their efficiencies. The dilute mixture, being slow burning, will have its time and pressure quickened by increasing the speed.
Careful trials give unmistakable evidence that the useful effect increases with the velocity of the piston—that is, with the rate of expansion of the burning gases. The time necessary for the explosion to become complete and to attain its maximum pressure depends not only on the composition of the mixture, but also upon the rate of expansion. This has been verified in experiments with a high-speed motor, at speeds from 500 to 2,000 revolutions per minute, or piston speeds of from 16 to 64 feet per second. The increased speed of combustion due to increased piston speed is a matter of great importance to builders of gas-engines, as well as to the users, as indicating the mechanical direction of improvements to lessen the wearing strain due to high speed and to lighten the vibrating parts with increased strength, in order that the balancing of high-speed engines may be accomplished with the least weight.
From many experiments made in Europe and in the United States, it has been conclusively proved that excessive cylinder cooling by the water-jacket results in a marked loss of efficiency. In a series of experiments with a simplex engine in France, it was found that a saving of 7 per cent. in gas consumption per brake horse-power was made by raising the temperature of the jacket water from 141° to 165° F. A still greater saving was made in a trial with an Otto engine by raising the temperature of the jacket water from 61° to 140° F.—it being 9.5 per cent. less gas per brake horse-power.
It has been stated that volumes of similar cylinders increase as the cube of their diameters, while the surface of their cold walls varies as the square of their diameters; so that for large cylinders the ratio of surface to volume is less than for small ones. This points to greater economy in the larger engines. The study of many experiments goes to prove that combustion takes place gradually in the gas-engine cylinder, and that the rate of increase of pressure or rapidity of firing is controlled by dilution and compression of the mixture, as well as by the rate of expansion or piston speed. The rate of combustion also depends on the size and shape of the explosion chamber, and is increased by the mechanical agitation of the mixture during combustion, and still more by the mode of firing.
VALUE OF INDICATOR CARDS
Fig. 13.—Otto Four-Cycle Card.
To the uninitiated, indicator cards are considerable of a mystery; to those capable of reading them they form an index relative to the action of any engine. An indicator card, such as shown at [Fig. 13], is merely a graphical representation of the various pressures existing in the cylinder for different positions of the piston. The length is to some scale that represents the stroke of the piston. During the intake stroke, the pressure falls below the atmospheric line. During compression, the curve gradually becomes higher owing to increasing pressure as the volume is reduced. After ignition the pressure line moves upward almost straight, then as the piston goes down on the explosion stroke, the pressure falls gradually to the point of exhaust valve, opening when the sudden release of the imprisoned gas causes a reduction in pressure to nearly atmospheric. An indicator card, or a series of them, will always show by its lines the normal or defective condition of the inlet valve and passages; the actual line of compression; the firing moment; the pressure of explosion; the velocity of combustion; the normal or defective line of expansion, as measured by the adiabatic curve, and the normal or defective operation of the exhaust valve, exhaust passages, and exhaust pipe. In fact, all the cycles of an explosive motor may be made a practical study from a close investigation of the lines of an indicator card.
Fig. 14.—Diesel Motor Card.
A most unique card is that of the Diesel motor ([Fig. 14]), which involves a distinct principle in the design and operation of internal-combustion motors, in that instead of taking a mixed charge for instantaneous explosion, its charge primarily is of air and its compression to a pressure at which a temperature is attained above the igniting point of the fuel, then injecting the fuel under a still higher pressure by which spontaneous combustion takes place gradually with increasing volume over the compression for part of the stroke or until the fuel charge is consumed. The motor thus operating between the pressures of 500 and 35 lbs. per square inch, with a clearance of about 7 per cent., has given an efficiency of 36 per cent. of the total heat value of kerosene oil.
COMPRESSION IN EXPLOSIVE MOTORS
That the compression in a gas, gasoline, or oil-engine has a direct relation to the power obtained, has been long known to experienced builders, having been suggested by M. Beau de Rocha, in 1862, and afterward brought into practical use in the four-cycle or Otto type about 1880. The degree of compression has had a growth from zero, in the early engines, to the highest available due to the varying ignition temperatures of the different gases and vapors used for explosive fuel, in order to avoid premature explosion from the heat of compression. Much of the increased power for equal-cylinder capacity is due to compression of the charge from the fact that the most powerful explosion of gases, or of any form of explosive material, takes place when the particles are in the closest contact or cohesion with one another, less energy in this form being consumed by the ingredients themselves to bring about their chemical combination, and consequently more energy is given out in useful or available work. This is best shown by the ignition of gunpowder, which, when ignited in the open air, burns rapidly, but without explosion, an explosion only taking place if the powder be confined or compressed into a small space.
Fig. 15.—Diagram of Heat in the Gas Engine Cylinder.
In a gas or gasoline-motor with a small clearance or compression space—with high compression—the surface with which the burning gases come into contact is much smaller in comparison with the compression space in a low-compression motor. Another advantage of a high-compression motor is that on account of the smaller clearance of combustion space less cooling water is required than with a low-compression motor, as the temperature, and consequently the pressure, falls more rapidly. The loss of heat through the water-jacket is thus less in the case of a high-compression than in that of a low-compression motor. In the non-compression type of motor the best results were obtained with a charge of 16 to 18 parts of gas and 100 parts of air, while in the compression type the best results are obtained with an explosive mixture of 7 to 10 parts of gas and 100 parts of air, thus showing that by the utilization of compression a weaker charge with a greater thermal efficiency is permissible.
It has been found that the explosive pressure resulting from the ignition of the charge of gas or gasoline-vapor and air in the gas-engine cylinder is about 41⁄2 times the pressure prior to ignition. The difficulty about getting high compression is that if the pressure is too high the charge is likely to ignite prematurely, as compression always results in increased temperature. The cylinder may become too hot, a deposit of carbon, a projecting electrode or plug body in the cylinder may become incandescent and ignite the charge which has been excessively heated by the high compression and mixture of the hot gases of the previous explosion.
FACTORS LIMITING COMPRESSION
With gasoline-vapor and air the compression should not be raised above about 90 to 95 pounds to the square inch, many manufacturers not going above 65 or 70 pounds. For natural gas the compression pressure may easily be raised to from 85 to 100 pounds per square inch. For gases of low calorific value, such as blast-furnace or producer-gas, the compression may be increased to from 140 to 190 pounds. In fact the ability to raise the compression to a high point with these gases is one of the principal reasons for their successful adoption for gas-engine use. In kerosene injection engines the compression of 250 pounds per square inch has been used with marked economy. Many troubles in regard to loss of power and increase of fuel have occurred and will no doubt continue, owing to the wear of valves, piston, and cylinder, which produces a loss in compression and explosive pressure and a waste of fuel by leakage. Faulty adjustment of valve movement is also a cause of loss of power; which may be from tardy closing of the inlet-valve or a too early opening of the exhaust-valve.
The explosive pressure varies to a considerable amount in proportion to the compression pressure by the difference in fuel value and the proportions of air mixtures, so that for good illuminating gas the explosive pressure may be from 2.5 to 4 times the compression pressure. For natural gas 3 to 4.5, for gasoline 3 to 5, for producer-gas 2 to 3, and for kerosene by injection 3 to 6.
The compression temperatures, although well known and easily computed from a known normal temperature of the explosive mixture, are subject to the effect of the uncertain temperature of the gases of the previous explosion remaining in the cylinder, the temperature of its walls, and the relative volume of the charge, whether full or scant; which are terms too variable to make any computations reliable or available.
For the theoretical compression temperatures from a known normal temperature, we append a table of the rise in temperature for the compression pressures in the following table:
Table IV.—Compression Temperatures
from a Normal Temperature of
60 Degrees Fahrenheit
| 100 | lbs. | gauge | 484° | 60 | lbs. | gauge | 373° |
| 90 | lbs. | gauge | 459° | 50 | lbs. | gauge | 339° |
| 80 | lbs. | gauge | 433° | 40 | lbs. | gauge | 301° |
| 70 | lbs. | gauge | 404° | 30 | lbs. | gauge | 258° |