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
(T_{1} - T_{0})/T_{1} and shows the work of a perfect cycle in an engine
working between the received temperature + absolute temperature (T_{1})
and the initial atmospheric temperature + absolute temperature (T_{0}).
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.
[Illustration: 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 deg. 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.
[A] Clearance Per Cent. of Piston Volume.
[B] Ratio (_V_/_V_{c}_) = (_P_ + _C_ Vol.)/Clearance
[C] Approximate Compression from 13 Pounds Absolute.
[D] Approximate Gauge Pressure.
[E] Absolute Temperature of Compression from 560 Deg. Fahrenheit in
Cylinder.
[F] Absolute Temperature of Explosion. Gas, 1 part; Air, 6 parts.
[G] Approximate Explosion Pressure Absolute.
[H] Approximate Gauge Pressure.
[I] Approximate Temperature of Explosion, Fahrenheit.
=====+======+======+=====+======+======+=====+=====+=====
[A] | [B] | [C] | [D] | [E] | [F] | [G] | [H] | [I]
-----+------+------+-----+------+------+-----+-----+-----
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 deg.
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 deg. to 165 deg. F.
A still greater saving was made in a trial with an Otto engine by raising
the temperature of the jacket water from 61 deg. to 140 deg. 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
[Illustration: 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.
[Illustration: 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.
[Illustration: 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 4-1/2 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 deg. | 60 lbs. gauge 373 deg.
90 lbs. gauge 459 deg. | 50 lbs. gauge 339 deg.
80 lbs. gauge 433 deg. | 40 lbs. gauge 301 deg.
70 lbs. gauge 404 deg. | 30 lbs. gauge 258 deg.
-------------------------------+------------------------------
CHART FOR DETERMINING COMPRESSION PRESSURES
A very useful chart (Fig. 16) for determining compression pressures in
gasoline-engine cylinders for various ratios of compression space to
total cylinder volume is given by P. S. Tice, and described in the
Chilton Automobile Directory by the originator as follows:
[Illustration: Fig. 16.--Chart Showing Relation Between Compression
Volume and Pressure.]
It is many times desirable to have at hand a convenient means for at
once determining with accuracy what the compression pressure will be in
a gasoline-engine cylinder, the relationship between the volume of the
compression space and the total cylinder volume or that swept by the
piston being known. The curve at Fig. 16 is offered as such a means. It
is based on empirical data gathered from upward of two dozen modern
automobile engines and represents what may be taken to be the results as
found in practice. It is usual for the designer to find compression
pressure values, knowing the volumes from the equation
P_{2} = P_{1} (V_{1}/V_{2})^{1.4} 1
which is for adiabatic compression of air. Equation (1) is right enough
in general form but gives results which are entirely too high, as
almost all designers know from experience. The trouble lies in the
interchange of heat between the compressed gases and the cylinder walls,
in the diminution of the exponent (1.4 in the above) due to the lesser
ratio of specific heat of gasoline vapor and in the transfer of heat
from the gases which are being compressed to whatever fuel may enter the
cylinder in an unvaporized condition. Also, there is always some piston
leakage, and, if the form of the equation (1) is to be retained, this
also tends to lower the value of the exponent. From experience with many
engines, it appears that compression reaches its highest value in the
cylinder for but a short range of motor speeds, usually during the
mid-range. Also, it appears that, at those speeds at which compression
shows its highest values, the initial pressure at the start of the
compression stroke is from .5 to .9 lb. below atmospheric. Taking this
latter loss value, which shows more often than those of lesser value,
the compression is seen to start from an initial pressure of 13.9 lbs.
per sq. in. absolute.
Also, experiment shows that if the exponent be given the value 1.26,
instead of 1.4, the equation will embrace all heat losses in the
compressed gas, and compensate for the changed ratio of specific heats
for the mixture and also for all piston leakage, in the average engine
with rings in good condition and tight. In the light of the foregoing,
and in view of results obtained from its use, the above curve is
offered--values of P_{2} being found from the equation
P_{2} = 13.8 (V_{1}/V_{2})^{1.26}
In using this curve it must be remembered that pressures are absolute.
Thus: suppose it is desired to know the volumetric relationships of the
cylinder for a compression pressure of 75 lbs. gauge. Add atmospheric
pressure to the desired gauge pressure 14.7 + 75 = 89.7 lbs. absolute.
Locate this pressure on the scale of ordinates and follow horizontally
across to the curve and then vertically downward to the scale of
abscissas, where the ratio of the combustion chamber volume to the total
cylinder volume is given, which latter is equal to the sum of the
combustion chamber volume and that of the piston sweep. In the above
case it is found that the combustion space for a compression pressure of
75 lbs. gauge will be .225 of the total cylinder volume, or .225 / .775
= .2905 of the piston sweep volume. Conversely, knowing the volumetric
ratios, compression pressure can be read directly by proceeding from the
scale of abscissas vertically to the curve and thence horizontally to
the scale of ordinates.
CAUSES OF HEAT LOSS AND INEFFICIENCY IN EXPLOSIVE MOTORS
The difference realized in the practical operation of an internal
combustion heat engine from the computed effect derived from the values
of the explosive elements is probably the most serious difficulty that
engineers have encountered in their endeavors to arrive at a rational
conclusion as to where the losses were located, and the ways and means
of design that would eliminate the causes of loss and raise the
efficiency step by step to a reasonable percentage of the total
efficiency of a perfect cycle.
An authority on the relative condition of the chemical elements under
combustion in closed cylinders attributes the variation of temperature
shown in the fall of the expansion curve, and the suppression or
retarded evolution of heat, entirely to the cooling action of the
cylinder walls, and to this nearly all the phenomena hitherto obscure in
the cylinder of a gas-engine. Others attribute the great difference
between the theoretical temperature of combustion and the actual
temperature realized in the practical operation of the gas-engine, a
loss of more than one-half of the total heat energy of the combustibles,
partly to the dissociation of the elements of combustion at extremely
high temperatures and their reassociation by expansion in the cylinder,
to account for the supposed continued combustion and extra adiabatic
curve of the expansion line on the indicator card.
[Illustration: Fig. 17.--The Thompson Indicator, an Instrument for
Determining Compressions and Explosion Pressure Values and Recording
Them on Chart.]
The loss of heat to the walls of the cylinder, piston, and clearance
space, as regards the proportion of wall surface to the volume, has
gradually brought this point to its smallest ratio in the concave
piston-head and globular cylinder-head, with the smallest possible space
in the inlet and exhaust passage. The wall surface of a cylindrical
clearance space or combustion chamber of one-half its unit diameter in
length is equal to 3.1416 square units, its volume but 0.3927 of a cubic
unit; while the same wall surface in a spherical form has a volume of
0.5236 of a cubic unit. It will be readily seen that the volume is
increased 33-1/3 per cent. in a spherical over a cylindrical form for
equal wall surfaces at the moment of explosion, when it is desirable
that the greatest amount of heat is generated, and carrying with it the
greatest possible pressure from which the expansion takes place by the
movement of the piston.
[Illustration: Fig. 18.--Spherical Combustion Chamber.]
[Illustration: Fig. 19.--Enlarged Combustion Chamber.]
The spherical form cannot continue during the stroke for mechanical
reasons; therefore some proportion of piston stroke of cylinder volume
must be found to correspond with a spherical form of the combustion
chamber to produce the least loss of heat through the walls during the
combustion and expansion part of the stroke. This idea is illustrated in
Figs. 18 and 19, showing how the relative volumes of cylinder stroke and
combustion chamber may be varied to suit the requirements due to the
quality of the elements of combustion.
Although the concave piston-head shows economy in regard to the relation
of the clearance volume to the wall area at the moment of explosive
combustion, it may be clearly seen that its concavity increases its
surface area and its capacity for absorbing heat, for which there is no
provision for cooling the piston, save its contact with the walls of the
cylinder and the slight air cooling of its back by its reciprocal
motion. For this reason the concave piston-head has not been generally
adopted and the concave cylinder-head, as shown in Fig. 19, with a
flat piston-head is the latest and best practice in airplane engine
construction.
[Illustration: Fig. 20.--Mercedes Aviation Engine Cylinder Section
Showing Approximately Spherical Combustion Chamber and Concave Piston
Top.]
The practical application of the principle just outlined to one of the
most efficient airplane motors ever designed, the Mercedes, is clearly
outlined at Fig. 20.
HEAT LOSSES TO COOLING WATER
The mean temperature of the wall surface of the combustion chamber and
cylinder, as indicated by the temperatures of the circulating water, has
been found to be an important item in the economy of the gas-engine.
Dugald Clerk, in England, a high authority in practical work with the
gas-engine, found that 10 per cent. of the gas for a stated amount of
power was saved by using water at a temperature in which the ejected
water from the cylinder-jacket was near the boiling-point, and ventures
the opinion that a still higher temperature for the circulating water
may be used as a source of economy. This could be made practical in the
case of aviation engines by adjusting the air-cooling surface of the
radiator so as to maintain the inlet water at just below the boiling
point, and by the rapid circulation induced by the pump pressure, to
return the water from the cylinder-jacket a few degrees above the
boiling point. The thermal displacement systems of cooling employed in
automobiles are working under more favorable temperature conditions than
those engines in which cooling is more energetic.
For a given amount of heat taken from the cylinder by the largest volume
of circulating water, the difference in temperature between inlet and
outlet of the water-jacket should be the least possible, and this
condition of the water circulation gives a more even temperature to all
parts of the cylinder; while, on the contrary, a cold-water supply, say
at 60 deg. F., so slow as to allow the ejected water to flow off at a
temperature near the boiling-point, must make a great difference in
temperature between the bottom and top of the cylinder, with a loss in
economy in gas and other fuels, as well as in water, if it is obtained
by measurement.
From the foregoing considerations of losses and inefficiencies, we find
that the practice in motor design and construction has not yet reached
the desired perfection in its cycular operation. Step by step
improvements have been made with many changes in design though many have
been without merit as an improvement, farther than to gratify the
longings of designers for something different from the other thing, and
to establish a special construction of their own. These efforts may in
time produce a motor of normal or standard design for each kind of fuel
that will give the highest possible efficiency for all conditions of
service.