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.
[Illustration: Fig. 5.--Outlining First 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.
[Illustration: Fig. 6.--Outlining Second Two Strokes of Piston in
Four-Cycle Engine.]
[Illustration: 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.
[Illustration: 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.
[Illustration: 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.
[Illustration: Fig. 10.--Diagrams Contrasting Action of Two- and
Four-Cycle Cylinders on Exhaust and Intake Stroke.]
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.
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 x V = C, or pressure x 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 deg. C. or -492.66 deg. = -460.66 deg. 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_.
[Illustration: 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 deg.) to
correspond with the Centigrade scale; therefore 1/492.66 = .0020297, the
ratio of expansion from 32 deg. 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 deg. to 2000 deg.
F., the increase in temperature will be 1940 deg.. The ratio will be
1/520.66 = .0019206. Then by the formula:
Ratio x acquired temp. x initial pressure = the gauge pressure; and
.0019206 x 1940 deg. x 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 x 1940 deg. = 54.77 lbs.
pressure.
By another formula, leaving out a small increment due to specific heat
at high temperatures:
Atmospheric pressure x absolute temp. + acquired temp.
I. -------------------------------------------------------- =
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 x 460.66 deg. + 2000 deg.)/(460.66 +
60 deg.) = 69.47 - 14.7 = 54.77, the gauge pressure, 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:
Volume x absolute temp. + acquired temp.
II. ------------------------------------------ =
Absolute temp. + initial temp.
heated volume. In applying this formula to the foregoing example, the
figures become:
460.66 deg. + 2000 deg.
I. x ------------------------- = 4.72604 volumes.
460.66 + 60 deg.
From this last term the gauge pressure may be obtained as follows:
III. 4.72604 x 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
deg. to 2000 deg. 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 x 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 x 460.66 + 60)/14.7 = 2460.66, and 2460.66 -
460.66 = 2000 deg., 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.
=====+================================+======+=======+========+=========
Dia- | | Temp.| Time | Ob- | Com-
gram | | of | of | served |puted
Curve| Mixture Injected. |Injec-|Explo- | Gauge |Temp.
Fig. | | tion | sion |Pressure|Fahr.
8. | | Fahr.|Second.| Pounds |
-----+--------------------------------+------+-------+--------+---------
_a_ |1 volume gas to 14 volumes air. |64 deg| 0.45 | 40. |1,483 deg.
_b_ |1 " " " 13 " " |51 deg| 0.31 | 51.5 |1,859 deg.
_c_ |1 " " " 12 " " |51 deg| 0.24 | 60. |2,195 deg.
_d_ |1 " " " 11 " " |51 deg| 0.17 | 61. |2,228 deg.
_e_ |1 " " " 9 " " |62 deg| 0.08 | 78. |2,835 deg.
_f_ |1 " " " 7 " " |62 deg| 0.06 | 87. |3,151 deg.
_g_ |1 " " " 6 " " |51 deg| 0.04 | 90. |3,257 deg.
_h_ |1 " " " 5 " " |51 deg| 0.055 | 91. |3,293 deg.
_i_ |1 " " " 4 " " |66 deg| 0.16 | 80. |2,871 deg.
-----+--------------------------------+------+-------+--------+---------
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.
[A] Proportion, Air to Gas by Volumes.
[B] Pounds in One Cubic Foot of Mixture.
[C] Specific Heat. Heat Units Required to Raise 1 Lb. 1 Deg.
Fahrenheit. Constant Pressure.
[D] Specific Heat. Heat Units Required to Raise 1 Lb. 1 Deg.
Fahrenheit. Constant Volume.
[E] Heat to Raise One Cubic Foot of Mixture 1 Deg. Fahr.
[F] Heat Units Evolved by Combustion.
[G] Ratio Col. 6/5
[H] Usual Combustion Efficiency.
[I] Usual Rise of Temperature due to Explosion at Constant Volume.
=======+========+======+======+========+======+=======+=====+=====
[A] | [B] | [C] | [D] | [E] | [F] | [G] | [H] | [I]
-------+--------+------+------+--------+------+-------+-----+-----
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
x .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 x 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._