Combustion—High Pressure System—Low Pressure System—Mixed
System—Burners for Furnaces, Locomotives, etc. 363
Gas, Oil and Steam Engines
CHAPTER I
HEAT AND POWER
(1) The Heat Engine.
Heat engines, of which the steam engine and gas engine are the most
prominent examples, are devices by which heat energy is transformed into
mechanical power or motion. In all heat engines, this transformation of
energy is accomplished by that property of heat known as “expansion,” by
which an increase or decrease of temperature causes a corresponding
increase or decrease in volume of the material subjected to the varying
temperatures. The substance whose expansion and contraction actuates the
heat engine is known as the “working medium,” and may be either a solid,
liquid, or a gas. The extent to which the working medium is expanded
depends not only upon the change of temperature but also on its
composition.
In all practical heat engines, the heat energy is developed by the
process of combustion, which is a chemical combination of the oxygen of
the air with certain substances, such as coal or gasoline, known as
“fuels.” The heat producing elements of the fuels are generally
compounds of carbon and hydrogen, which when oxydized or burnt by the
oxygen form products that are unlike either of the original components.
It is due to this chemical change that heat energy is evolved, for the
heat represents the energy expended by the sun in building up the fuel
in its original form, and as energy can neither be created nor
destroyed, heat energy is liberated when the fuel is decomposed. The
heat energy thus liberated is applied to the expansion of the working
medium to obtain its equivalent in the form of mechanical power.
During the period of expansion, the heat obtained by the combustion is
absorbed by the working medium in proportion to its increase in volume,
and as this increase is proportional to the mechanical effort exerted by
the engine, it will be seen that the output of the engine in work is a
measure of the heat applied to the medium. The quantity of heat absorbed
by the medium represents the energy required to set the molecules of the
medium into their new positions in the greater volume, or to increase
their paths of travel. In the conversion of heat, each heat unit applied
to the medium results in the production of 778 foot pounds of energy,
providing that there are no heat or frictional losses.
In explanation of these terms or units, we wish to say, that the unit of
heat quantity, called the =BRITISH THERMAL UNIT= is the quantity of heat
required to raise one pound of water, one degree Fahrenheit, and the
=FOOT POUND= is the work required to raise one pound through the
vertical distance of one foot. As the British Thermal Unit = 778 foot
pounds it is equivalent to the work required to raise 778 pounds one
foot or one pound 778 feet, or any other product of feet and pounds
equal to the figure 778.
As liquids expand more than solids with a given temperature, and gases
more than either, the mechanical work returned for a given amount of
thermal energy (the =EFFICIENCY=) will be greater with an engine using
gas as a working medium than one using a solid or liquid working medium.
The steam engine and the gas engine are both examples of heat engines
using gaseous working mediums, the medium in the steam engine being
water vapor and in the gas engine, air and the gaseous products of
combustion. For this reason the working medium will be considered as a
gas in the succeeding chapters.
Practically the only way of obtaining mechanical effort from an
expanding gas is to enclose it in a cylinder (c) fitted with a freely
sliding plunger or piston (p) as shown in Fig. 1. Two positions of the
piston are shown, one at M indicated by the dotted lines, and one at N
indicated by the full lines. It will be assumed that the space between
the cylinder head P and the piston at M represents the volume of the gas
before it is heated and expanded, and that the volume between O and N
represents the volume after heating and expansion have occurred. The
vessel B represents a chamber containing air that is periodically heated
by the lamp L, and which is connected to the working cylinder C by the
pipe O.
[Illustration:
=Figs. 1–2–3.= Showing Expansion in an External Combustion Engine, the
Cycle of Operations in an Internal Combustion Engine, and the
Pressure Diagram of the Latter Engine Giving the Pressures at
Various Points in the Stroke.
]
With the piston at M, the lamp L is lighted and placed under the retort
B which results in the immediate expansion of the air in B. The expanded
air passes through O into the cylinder, and if sufficient heat is
supplied, exerts pressure against the piston since it occupies much more
than its original volume. Providing that the friction of the device and
the load on the shaft S are low enough the pressure on the piston will,
move it to the position N in the direction of the arrow, thus
accomplishing mechanical work. The motion of the piston revolves the
crank to which it is connected by the rod X from D to E. During the trip
from M to N the volume of gas has greatly increased being supplied
continuously with heat from the lamp. As a considerable amount of heat
has been radiated from the cylinder during the piston travel, and a
considerable portion of the mechanical work lost through the friction of
the piston on the cylinder walls, and by the crank, not all of the heat
units are represented at the crank as mechanical effort.
Because of the limiting length of the cylinder, and the temperature
limits of the lamp it is not possible to expand the working medium and
increase the temperature indefinitely, therefore there must be a point
where the application of heat must cease and the temperature be reduced
in order to bring the gas back to its original volume and the piston to
its original position so that the expansion may be repeated. This
condition results in a very considerable loss of heat and power in
addition to the losses previously mentioned, as the heat taken from the
medium to reduce it to its original volume is thrown away as far as the
production of power is concerned. To return the piston to its former
position without expending energy on the engine, the volume and pressure
may be reduced either by allowing the gas to escape to the atmosphere by
means of a valve, or by removing the lamp and cooling the air by the
application of water, but in any case the heat of the air is lost and
the efficiency of the engine reduced.
To increase the efficiency of the engine and reduce the loss just
mentioned, nearly all heat engines, either steam or gas, have the
working medium at the highest temperature for only a small portion of
the stroke, after which no heat is supplied to the cylinder. As the
pressure forces the piston forward the volume increases, and as no more
heat is supplied, both the pressure and the temperature continue to
decrease until the end of the stroke is reached, thus utilizing the
greater part of the heat in the expansion. Since the temperature at the
end of the stroke is comparatively low, very little heat is rejected
when the valve is opened for the return stroke. This loss would be the
least when the temperature of the gas at the end of the stroke was equal
to the temperature of the surrounding air. With both the internal and
external temperatures equal, there would be no difference between the
pressure of the gas in the cylinder and that of the surrounding air.
[Illustration:
=Fig. 1-a.= Fairbanks-Morse Two Cylinder, Type “R E” Stationary Engine
Direct Connected to a Dynamo.
]
It will be seen from the example just given that the heat engine
performs mechanical work by dropping the working medium from a high to a
low temperature, as it receives the medium at a high temperature from
the lamp and rejects it at atmospheric temperature after delivering a
small percentage of useful work. This may be compared to a water wheel
which receives the working medium (water) at a high pressure and rejects
it at a lower pressure. Carrying this comparison still further, it is
evident that an increase in the range of the working temperatures (high
and low) would increase the output of the heat engine in the same way
that an increase in the range of pressures would increase the output of
the water wheel. The temperature at which the engine receives the
working medium and the temperature at which it is rejected determines
the number of heat units that are available for conversion into
mechanical energy, and therefore, if the range be increased by either
raising the upper limit of temperature or by reducing the lower limit,
or by the combined increase and decrease of the limits, the available
heat will be increased.
Based on the temperature range, the maximum possible efficiency of the
heat engine may be expressed by the ratio—
=Reception Temperature—Rejection Temperature=
E = ———————————————————————————————————————————
=Reception Temperature=
This maximum defined by Carnot establishes a limit that can be exceeded
by no engine, whatever the construction or working medium.
According to the methods adopted in applying the heat of combustion to
the working medium, heat engines are divided into two general classes,
(1) External combustion engines, (2) Internal combustion engines. The
expressions “Internal” or “External” refer to the point at which
combustion takes place in regard to the working cylinder, thus an
internal combustion engine is one in which the combustion takes place in
the working cylinder, and an external combustion engine is one in which
the combustion takes place outside of the working cylinder. The steam
engine is an example of an external combustion engine, the fuel being
burned in the furnace of a boiler which is independent of the engine
cylinder proper. As the fuel is burned directly in the cylinder of a gas
engine it is commonly known as an internal combustion engine.
An external combustion engine, such as the steam engine is subject to
many serious heat losses because of the indirect method by which the
heat is supplied to the working cylinder, aside from the losses in the
cylinder. Much of the heat goes up the smoke stack and much is radiated
from the boiler settings and the steam pipes that lead to the engine.
The greatest loss however is due to the fact that the range of
temperatures in the working cylinder is very low compared to the
temperatures attained in the boiler furnace, for it is practically
impossible to have a greater range than 350°F to 100°F with a steam
engine, while the furnace temperatures may run up to 2500°F and even
beyond.
High temperatures with a steam engine result in the development of
enormous pressures, a temperature of 547°F corresponding to an absolute
pressure of 1000 pounds per square inch. This pressure would require an
extremely heavy and inefficient engine because of the terrific strains
set up in the moving parts. The pressures established by air as a
working medium are very much lower than those produced by air or any
permanent gas at the same temperature, and for this reason it is
possible to exceed a working temperature of over 3000°F in the cylinder
of a gas engine without meeting with excessive pressures. This high
working temperature is one of the reasons of the extremely high
efficiency of the gas engine.
In order to compete with the gas engine from the standpoint of
efficiency, the steam engine builders have resorted to super-heating the
steam after it has left the boiler in order to increase the temperature
range in the cylinder. By applying additional heat to the steam after it
has passed out of contact with the water it is possible to obtain up to
600°F without material increase in the pressure, but the practical gains
have not been great enough to approach the gas engine with its 3000°F.
After reaching his maximum temperature at this comparatively low
pressure, the steam engineer has still to eliminate a number of other
losses that do not obtain with the gas engine.
Since the radiation losses of a burning fuel are proportional to the
time required for burning, it is evident that the rate of combustion has
much to do with the efficient development of the heat contained in it,
and it is true that rapid combustion develops more useful heat from a
given fuel than slow. In the gas engine the combustion is practically
instantaneous with a low radiation loss, but in the steam engine the
rate is slow, and with the excess of air that must necessarily be
supplied, a great part of the value of the fuel is lost before reaching
the water in the boiler. The temperature of the medium determines the
efficiency of the engine and as rapid combustion increases the
temperature it is evident that the gas engine again has the best of the
problem.
In the case of the gas engine where the fuel (in gaseous form) is drawn
directly into the working cylinder in intimate contact with the working
medium (air) and in the correct proportions for complete combustion,
each particle of fuel, when ignited, applies its heat to the adjacent
particle of air instantly and increases its volume with a minimum loss
by radiation.
A gas engine is practically a steam engine with the furnace placed
directly in the working cylinder with all intervening working mediums
removed, the gases of combustion acting as the working medium. It
derives its power from the instantaneous combustion of a mixture of fuel
and air in the cylinder, the expansion of which causes pressure on the
piston. Under the influence of the pressure on the piston, the crank is
turned through the connecting rod and delivers power to the belt wheel
where it is available for driving machinery. Whether the fuel be of
solid, liquid, or gaseous origin it is always introduced into the
cylinder in the form of a gas.
[Illustration:
Fig. 1-b. The English Adams Automobile Motor (End View), Showing the
Magneto Driven by Spiral Gears at Right Angles to the Crank-Shaft.
]
(3) Combustion In the Cylinder.
As the working medium in an internal combustion engine is in direct
contact with the fuel it must not only be uninflammable but it must also
be capable of sustaining combustion and must have a great expansion for
a given temperature range. Since atmospheric air possesses all of these
qualifications in addition to being present in all places in unlimited
quantities it is natural that it should be used exclusively as the
working medium for gas engines. Unlike the vapor working medium in a
steam engine the medium in the gas engine not only acts in an expansive
capacity but also as an oxydizing agent for burning the fuel, and
therefore must bear a definite relation to the quantity of the fuel in
the cylinder to insure complete combustion.
In the gas engine the use of gaseous fuel is imperative since there must
be no solid residue existing in the cylinder after combustion and also
for the reason that the fuel must be in a very finely subdivided state
in order that the combustion shall proceed with the greatest possible
rapidity. In addition to the above requirements the introduction of a
solid fuel into the cylinder would involve almost unsurmountable
mechanical problems in regard to fuel measurement for the varying loads
on the engine. This limits the fuel to certain hydrocarbon or compounds
of hydrogen and carbon in gaseous form of which the following are the
most common examples:
(a) =CARBURETED AIR= consisting of a mixture of atmospheric air and the
vapor of some hydrocarbon (liquid) such as gasoline, kerosene or
alcohol.
(b) =OIL GAS= formed by the distillation of some heavy, nonvolatile oil,
or the distillation of tar or paraffine.
(c) =NATURAL GAS= obtained from natural accumulations occurring in
subterranean pockets in various parts of the country.
(d) =COAL GAS=, made artificially by the distillation of coal, commonly
called “illuminating” gas.
(e) =PRODUCER GAS=, some times known as “fuel gas,” produced by the
incomplete combustion of coal in a form of furnace called a “producer.”
(f) =BLAST FURNACE GAS=, the unconsumed gas from the furnaces used in
smelting iron, somewhat similar to producer gas but lower in heat value.
It should be noted that there is no essential difference between engines
using a permanent gas or an oil as in either case the fuel is sent into
the cylinder in the form of a vapor. In the case of oil fuel, the vapor
is formed by an appliance external to the engine proper. In this book,
the heat action of an engine using one form of fuel applies equally to
the engine using another. The selection of a particular fuel for use
with a gas engine depends not only upon its value in producing heat, but
also upon its cost, the ease with which it meets the peculiar conditions
under which the engine is to work, and its accessibility.
Neglecting for the moment, all of the items that do not affect the
operation of the engine from a power producing standpoint, the principal
requirement of a fuel is the production of a high temperature in the
cylinder since the output is directly proportional to the temperature
range. Since a very considerable mass of air is to be raised to this
high temperature, the heat value, or =CALORIFIC VALUE= of the fuel in
British Thermal units is of as much importance as the temperature
attained in the combustion. The calorific value of different fuels vary
widely when based either on the cubic foot or pound, and a considerable
variation exists even among fuels of the same class owing to the
different methods of production or to the natural conditions existing at
the mine or well from which they originated. The principal elements of
gas engine fuels, carbon and hydrogen, exist in many different
combinations and proportions, and require different quantities of air as
oxygen for their combustion because of this difference in chemical
structure.
Since complete combustion is never obtained under practical working
conditions, the actual evolution of heat and the actual temperatures are
always much lower than those indicated by the =CALORIMETER= or heat
measuring device. Besides the loss of heat due to imperfect combustion,
there are many other losses such as the loss by radiation, connection,
and slow burning, the latter being the principal cause of low combustion
temperatures. From the statements in the foregoing paragraphs it will be
seen that the theoretical or absolute calorific value of a fuel is not
always a true index to its efficiency in the engine.
Complete combustion results in the carbon of the fuel being reduced to
carbon dioxide (CO_{2}) and the hydrogen to water (H_{2}O), with the
liberation of atmospheric nitrogen that was previously combined with the
fuel, and some oxygen. The reduction of the fuel to carbon dioxide and
water produces every heat unit available since the latter compounds
represent the lowest state to which the fuel can be burned. Carbon
however may be burned to an intermediate state without the production of
its entire calorific contents when there is not sufficient oxygen
present to thoroughly consume the fuel. Incompletely consumed carbon
produces a gas, carbon monoxide, as a product of combustion, and a
quantity of solid carbon in a finely subdivided state known as “soot.”
Unlike the products of complete combustion, both the carbon monoxide and
soot may be burned to a lower state with a production of additional heat
when furnished with sufficient oxygen, both the soot and the monoxide
being reduced to carbon dioxide during the process.
[Illustration:
Fig. F-2. Sunbeam Engine with Six Cylinders Cast “En Bloc” (in one
piece). At the Right and Under the Exhaust Pipe is the Compressed
Air Starting Motor that Starts the Motor Through the Gear Teeth
Shown on Flywheel. _From “Internal Combustion.”_
]
As the soot and monoxide have a calorific value it is evident that much
of the heat of the fuel is wasted if they are exhausted from the
cylinder without further burning at the end of the stroke. To gain every
possible heat unit it is necessary to furnish sufficient oxygen or air
to reduce the fuel to its lowest state. As the free oxygen and nitrogen
contained in the fuel are without fuel value, their rejection from the
cylinder occasions no loss except for that heat which they take from the
cylinder by virtue of their high temperature.
With complete combustion the =TEMPERATURE= attained increases with the
rate of burning, while the number of heat units developed remain the
same with any rate of combustion. Because of the conditions under which
the fuel is burned in the gas engine the fuel is burned almost
instantaneously with the result that high temperatures are reached with
fuels of comparatively low calorific value. With a given gas the rate of
combustion is increased with an increase in the temperature of the gas
before ignition and remains constant for all mixtures of this gas in the
same proportion when the initial temperature is the same. The rate of
combustion also varies with the composition of the gas, hydrogen burning
more rapidly than methane. As a rule it might be stated that the rate of
burning decreases with the specific gravity of the gas, the light gases
such as hydrogen burn with almost explosive rapidity, while the heavier
gases such as carbon dioxide are incombustible or have a zero rate of
combustion. In practice an increased rate of burning is obtained by
heating the charge before ignition by a process that will be explained
later.
Another factor governing the output of an engine with a given size
cylinder is the amount of air required to burn the fuel. The quantity of
air necessary for the combustion of the fuel determines the amount of
fuel that can be drawn into a given cylinder volume, and as we are
dependent upon the fuel for the expansion it is evident that with two
fuels of the same calorific value, the one requiring the least air will
develop the most power. Since the air required to burn hydrogen gas is
only one fourth of that required to burn the same amount of methane it
is clear that more hydrogen can be burned in the cylinder than methane.
This great increase in output due to the hydrogen charge is however,
considerably offset by the greater calorific value of the methane.
Should the air be in excess of that required for complete combustion, or
should a great quantity of incombustible gas, such as nitrogen be
present in the mixture, the fuel will be completely burned, but the
speed of burning will be reduced owing to the dilution. As the air is
increased beyond the proper proportions the explosions become weaker and
weaker as the gas becomes leaner until the engine stops entirely.
Because of the fact that it is impossible in practice to so thoroughly
mix the gas and air that each particle of gas is in contact with a
particle of air, the volume of air used for the combustion is much
greater than that theoretically required. A =SLIGHT= excess of air,
making a lean mixture, increases the efficiency of combustion although
it reduces the temperature and pressure attained in the cylinder. This
is due to the fact that while the temperature of the mixture is lower
than with the theoretical mixture the temperature of the burning gas
itself is much higher. A mixture that is too lean to burn at ordinary
temperatures will respond readily to the ignition spark if the
temperature or pressure is raised.
(4) Compression.
In the practical gas engine the gas is not ignited at the beginning of
the suction stroke by which it is drawn into the cylinder, but is
compressed in the front end of the cylinder by the return stroke of the
piston, and then ignited. The process of compression adds greatly to the
power output of a given sized cylinder and increases the efficiency of
the fuel and expansion. In order to understand the relation that the
compression bears to the expansion let us refer to Fig. 2 in which C is
the working cylinder, P the piston and G the crank. While the piston is
moving towards the crank in the direction of the arrow A it draws the
mixture, indicated by the marks x x x x x, into the cylinder, the
quantity being proportional to the position of the piston. In this
particular case let us assume that the area of the piston is 50 square
inches and that the entire stroke (B) of the piston is 12 inches. To
prevent confusion due to considerations of heat loss we will further
assume that the cylinder is constructed of non-conducting material.
With the piston at the position H, midway between J and I, the volume D
is filled with the explosive mixture at atmospheric pressure and a
temperature of 500° absolute. Since D = 6 inches and the area of the
piston is 50 square inches, the volume D is equal to 6 × 50 = 300 cubic
inches, and the entire volume is 2 × 300 = 600 cubic inches. On igniting
this mixture (at atmospheric pressure) the temperature will rise
immediately, say to 1000°F with the piston at H. According to a law
governing the expansion of gases, known as Gay-Lussac’s Law, the
expansion (v × T)/t = V where v = the initial volume of the gas before
ignition = 300 cubic inches; t = the temperature before ignition 500°
absolute; V = the volume of the gas after expansion; and T = temperature
after ignition = 1000° absolute. Inserting the values in numerical form
we have as the final volume:—(300 × 1000)/500 = 600 cubic inches = the
volume after expansion, or twice the original volume of gas. This means
that the expansion is capable of driving the piston from H to I before
the pressure is reduced again to atmospheric pressure. As the volume is
expanded to twice that of the original volume at atmospheric pressure
(14.7 pounds per square inch), the pressure against the piston before it
starts moving will be 2 × 14.7 = 29.4 pounds per square inch.
Let us now consider the case in which the charge is compressed before
ignition occurs and compare the expansion and pressure established with
that produced by ignition at atmospheric pressure. To produce the
compression the piston will travel through the entire stroke to the
position I on the suction stroke filling the entire cylinder volumes of
600 cubic inches with the mixture. On the return stroke the piston stops
at H, reducing the original volume of 600 cubic inches to 300 cubic
inches, doubling the pressure of the gas. The initial and final
temperatures will be considered as being the same as those in the first
example, 500° and 1000°. From Gay-Lussac’s Law—(v × T)/t = V and
substituting the numerical values (600 × 1000)/500 = 1200 cubic inches,
or the expanded volume will be four times the compressed volume, or four
times the initial volume of the first case where the gas was ignited at
atmospheric pressure.
It should be noted however, that while the expansion has been greatly
increased by the compression, that this is not all gain, as equivalent
work has been expended in compressing the charge. With the exception of
doubling the fuel taken into the cylinder, and consequently doubling the
output for a certain cylinder capacity, there has been no increase in
fuel efficiency except that due to conditions other than the mere
reduction in volume. In the second case the volume was increased four
fold which resulted in a piston pressure of 4 × 14.7 = 58.8 pounds per
square inch before the piston increased the volume by moving from H to
I.
The work done by the engine on the charge in compressing is converted
into heat energy causing a rise in the temperature of the gas. This
would not be a loss as it would reappear as mechanical energy on the
return stroke of the piston through its expanding effect on the gas.
This heat would, in effect, be added to the temperature due to ignition,
and the sum would produce its equivalent expansion. The temperature due
to the combustion may be determined by reversing Gay-Lussac’s Law—
t I Pt
— = — or T = ——
p P p
Where t = initial temperature; T = temperature combustion; P = pressure
after combustion; p = pressure before combustion.
Because of the fact that the act of compressing the charge in the
cylinder before ignition increases the temperature of the working
medium, the compression will increase the speed of combustion and
efficiency of the fuel as the rate of combustion increases with the
initial temperature. This increased temperature due to initial
compression of course results in a greater temperature range and output
due to the increased rate of burning, and this rate of combustion may be
varied for different fuels by changing the compression pressure. In a
previous paragraph it was explained that the fuel efficiency was
increased by a slight dilution or excess of air, and that while the
temperature and pressure of the mixture were reduced by the dilution the
temperature of the fuel was increased, provided that the inflammability
was not decreased.
Compression affords a means of using dilute mixtures without loss of
inflammability, as the heat gained by the compression restores the
inflammability lost by the effects of dilution. Increased compression
pressures increases the possible range of dilution, so that extremely
lean gases and mixtures may be used with success with appropriately high
compression. As an example of this fact we can refer to the engine using
blast furnace gas, a fuel that is so lean that it cannot be ignited
under atmospheric pressure. By increasing the piston speed, the heat of
the compression can be made more effective as the gas lies in contact
with the cylinder walls for a shorter time which of course reduces the
heat to the jacket water.
(5) Efficiency and Heat Losses.
Up to the present time we have considered an engine in which there is no
heat loss or loss from friction, but in the actual engine such losses
are large and tend to materially reduce the values of heat and pressure
to be obtained from a fuel with a given calorific content. Applying the
rule for heat engines given in a previous section where the efficiency
is—
T – t
E = —————
T
We have the theoretical efficiency of a gas engine, neglecting friction,
loss to the cylinder walls, and loss through the rejection of heat with
the exhaust gas, equal to—
1960 – 520
E = —————————— = 73.5 percent.
1960
In substituting the numerical values in the above calculation it was
assumed that the temperature of the burning mixture would be 1500° F
above zero, and that the exhaust temperature would be as low as 60.
Since the calculation is made from absolute zero, which is 460° below
the zero marked on our thermometers, the temperature of the burning
charge, T = 1500 + 460° = 1960° above absolute zero. Similarly the
absolute temperature of the exhaust would be, t = 60 + 460 = 520°
absolute. The application of the absolute temperatures will be seen from
the calculation for efficiency. The value given, 73.5 per cent, it
should be understood is the theoretical efficiency and is at least 20
per cent above the best results obtained in practice. The best record
that we have had to date, is that established by a Diesel engine which
returned 48.2 per cent of the calorific value of the fuel in the form of
mechanical energy. In order that the reader may have some idea of the
losses that occur in the engine, and their extent we submit the
following table. These are the results of actual tests obtained from
different sources and represent engines built for different services and
of various capacities:
═════════════════════════╤══════════╤══════════╤═══════════╤═══════════
LOSSES—DATA │Automobile│Stationary│Stationary │
│ Motor │ Engine │ Engine │
─────────────────────────┼──────────┼──────────┼───────────┼───────────
Horse-power │ 30. │ 200 │ 1000 │
│ │ │ │Loss at per
Heat lost to jacket water│ 35.8% │ 31.0% │2970 B.T.U.│Horse-power
│ │ │ │in B.T.U.’s
Heat lost in exhaust │ 24.6% │ 30.0% │2835 B.T.U.│
Friction loss │ 8.6% │ 6.5% │810 B.T.U. │
Heat lost by radiation │ 15.4% │ 8.2 │540 B.T.U. │
Heat available as power │ │ │2700 B.T.U.│
Efficiency (per cent) │ 15.6% │ 24.3 │ │
Fuel │ Gasoline │ │ Producer │
│ │ │ Gas │
─────────────────────────┴──────────┴──────────┴───────────┴───────────
The remarkable efficiency of the Diesel engine is due principally to the
extremely high compression pressure, which was from 500 to 600 pounds
per square inch. When this is compared to the 60 to 70 pounds
compression pressure used with automobile engines it is easy to see
where the Diesel gains its efficiency. It is evident that as much
depends on the manner in which the fuel is used in the engine as on the
calorific value of the fuel.
(6) Expansion of the Charge.
When an explosive mixture is ignited in the cylinder with the piston
fixed in one position thus making the volume constant, the increase of
temperature is accompanied by an increase of pressure. If the piston is
now allowed to move forward increasing the volume, the increase of
volume decreases the pressure. Since in the operation of the gas engine
the piston continuously expands the volume on the working stroke it is
evident that there is no point in the stroke where the pressures are
equal, and that the pressure is the least at the end of the stroke, it
being understood of course that no additional heat is supplied to the
medium after the piston begins its stroke.
This distribution of pressure in the cylinder in relation to the piston
position is best represented graphically by means of a diagram as shown
by Fig. 3, in which K is the cylinder and P the piston. Above the
cylinder is shown the diagram HGDE the length of which (HE) is equal to
the stroke of the piston shown by (BC). Intersecting the line HI are
vertical lines, A, a, b, c, C, which represent certain positions of the
piston in its stroke. The height of the diagram H G represents to scale
the maximum explosion pressure in pounds per square inch, and the line
HG is drawn immediately above the piston position B which is at the
inner end of the stroke. To the left of the line HS is drawn a scale of
pressures ML divided in pounds per square inch so that the pressures may
be read off of the pressure curve GD. The line JI represents atmospheric
pressure, and the divisions on ML, of course, begin from this line and
increase as we go up the column. As an example in the use of the scale
we find that the point F is at 50 pounds pressure above the atmospheric
line JI.
We will consider that the clearance space AB is full of mixture at the
point B, and that it is moved toward the left to the point C filling the
space AC full of mixture at atmospheric pressure. The location of the
piston on the diagram is shown by D and E. The opening through which the
gas was supplied to the cylinder is now closed, and the piston starts on
its compression stroke, moving from C to A. As the volume is reduced
from AC to AB, there is an increase of pressure which is shown
graphically by the rising line EF. This line rises gradually from the
line JI in proportion to the reduction in volume until the piston
reaches the end of the compression stroke at B, at which point the
compression is at a maximum. The extent of this pressure is shown by the
length of HF which on referring to the scale of pressure at the left
will be found to be 50 pounds per square inch.
Ignition now occurs and the pressure increases instantly from the
compression pressure at F to the maximum pressure at G which on
referring to the scale will be found to equal 200 pounds. The actual
increase of pressure due to ignition above the compression pressure will
be shown by the length of the line FG which is equal to 150 pounds. As
the pressure is now established against the piston it will begin to move
forward with an increase of volume and a corresponding decrease in
pressure, until it reaches the point C. This point at the end of the
stroke is indicated on the diagram by D which by reference to the scale
will be found equal to 25 pounds above atmosphere. An exhaust valve is
now opened allowing the gas to escape to the atmosphere which reduces
the pressure instantly from D to E on the atmospheric line. Expansion
along the line GD is not complete as the pressure is not decreased to
atmospheric pressure in the cylinder which means that there is a
considerable loss of heat in the exhaust. In practice the expansion is
never complete, but ends considerably above atmospheric pressure as
shown.
[Illustration:
Fig. F-3. Front Elevation of Curtiss “V” Type Aeronautical Motor. This
is the Front View of the Motor Shown in the Frontispiece. See
Chapter V for Description of this Type of Motor.
]
Complete expansion is shown by the dotted line GE which terminates at E
on the atmospheric line. By following the vertical lines up from the
points a, b, c, and d, the pressures corresponding to these piston
positions can be found by measuring the distance of the curve from the
atmospheric line, on the given lines a, b, c or d. To find the pressure
at the position a, for instance, follow upwards along the line a to the
point c on the curve, the length of the line ef from the curve to the
atmospheric represents the pressure, which by reference to the scale ML
will be found equal to 125 pounds. The pressure at any other point can
be found in a like manner. Compression pressures may be found at any
point by measuring from the atmospheric line to the compression curve FE
along the given line. It will be noted that the combustion is so quick
that the pressure rises in a straight line along GH, indicating that
combustion was complete before the piston had time to start on the
outward stroke. The expansion curves GE and GD are similar to the
compression curve FE. With the actual engine the shape of the ideal card
as shown by Fig. 3 is sometimes considerably deformed owing to the
effects of defective valves, leaks, or improperly timed ignition.
Pressure curves of actual engines are of the greatest value as they show
the conditions within the cylinder at a glance and make it possible to
detect losses due to leaks, poor valve settings, etc. These curves are
traced by means of the =INDICATOR= which is an instrument consisting of
a small cylinder which is connected to the cylinder of the engine, and
an oscillating drum that is driven to and fro by the engine piston. The
piston in the indicator cylinder is provided with a spring that governs
its movements and communicates its motion to a recording pencil through
a system of levers. The spring is of such strength that a pressure of so
many pounds per square inch in the cylinder causes the pencil to draw a
line of a definite length, this line being equivalent to the pressure
line GH in Fig. 3. A piece of paper is wrapped about the indicator drum,
and the drum is attached to the piston in such a manner that it turns a
certain amount for every piston position, the complete stroke of the
piston turning the drum through about three-quarters of a revolution.
Rotation of the drum traces the horizontal lines of the diagram and the
movement of the piston draws the vertical lines, so the combined
movements of the drum and piston records the pressures and piston
positions as shown by Fig. 3.
Since the movement of the indicator piston represents the pressures in
the cylinder to scale it is possible to compute the power developed in
the cylinder as the output in mechanical units is equal to the product
of the average force acting on the piston multiplied by the speed of the
piston in feet per minute. This product of the force and velocity (known
as “foot pounds per minute”) divided by 33,000 (one horse-power = 33,000
foot pounds) gives the output of the engine, in horse-power.
As the pressure on the piston fluctuates throughout the stroke, it would
be wrong to consider the force, in the calculation for power as being
equal to the explosion pressure, and so the effective pressure is taken
as being the average of all the pressures from the point of explosion to
the exhaust. The average pressure or “mean effective pressure” as it is
called is computed from the indicator diagram by dividing it into a
number of equal parts along the horizontal line, adding the lengths of
the pressure lines such as CH, CF, etc., and dividing the total length
by the number of the lines. After the average height of the diagram is
thus determined, the average length is multiplied by the scale of the
indicator or the pressure that is shown by it per inch.
[Illustration:
Fairbanks-Morse Gasoline Pumping Engine. Pump is Gear Driven From the
Engine Crank-Shaft at Reduced Speed.
]
Knowing the mean effective pressure, the total pressure on the piston,
or the force is found by multiplying the area of the piston in square
inches by the average pressure per square inch. This product is
multiplied by the piston speed in feet per minute and is divided by the
product of the number of strokes to the explosion and the quantity
33,000. Should there be more than one cylinder the result is multiplied
by the number of cylinders, and this is multiplied by 2 in the case of a
double acting engine. Stated as a formula this rule becomes:
A × P × 2R × L × N × O
H.P. = ——————————————————————
33000 × C
When A = Area of piston in square inches.
P = Average or mean effective pressure per square inch. About 75
pounds for Gasoline Engines. See Table on Page 31.
R = Revolutions per minute.
L = Stroke of piston in feet.
N = Number of cylinders.
O = 2 when engine is double acting, that is when explosions occur
on both sides of the piston.
C = Number of strokes per explosion. C = 4 in a four cycle engine,
and 2 in a two cycle.
It should be specially noted that the area of the piston is given in
square inches and the stroke of the piston in feet. The number of
revolutions per minute, R, is multiplied by two in order to obtain the
number of strokes, as there are two strokes per revolution. When the
engine governs its speed by dropping explosions to meet varying loads,
the quantity C should be omitted and the explosions counted.
* * * * *
Due to the fact that the incoming charge of the mixture is expanded by
the heat of the passages, a full charge computed at atmospheric
temperature is never obtained in the cylinder and for this reason the
gas should be kept as cold as possible before entering the passages in
order to obtain the maximum output. Friction due to restricted passages
and valve openings also reduces the amount of mixture available. Small
exhaust valves and pipes prevent the gases from escaping freely to the
atmosphere and produces a back pressure on the piston which cuts down
the effective pressures. All of these items are recorded by the
indicator and makes it possible to make alterations that will increase
the output of the engine.
Because of the reduced atmospheric pressures at high altitudes the
output and compression are reduced for every foot of elevation above sea
level. As the weight of the atmosphere is reduced, less mixture is drawn
into the cylinder. Taking the output of the engine as 100 per cent at
sea level, it is reduced to less than 62 per cent at an elevation of
15,000 feet.
CHAPTER II
FUELS AND COMBUSTION
(7) Combustion.
The phenomenon called combustion by which we obtain the heat energy
necessary for the operation of the internal combustion engine is a
chemical combination of the air with the fuel. This process results in
heat and some light which is equal in quantity to the energy required to
separate the fuel compound into its elements or to build it up in its
present form from the original elements. If the process is comparatively
slow, the compound is called a fuel, if it is instantaneous it is called
an explosive. Some substances produce mechanical force through an
instant, without the evolution of much heat, due to the disintegration
of an unstable compound. The effect of the latter type of which dynamite
is an example is static, that is to say, it is not capable of producing
power, but only pressure. For this reason, compounds having an
instantaneous effect without the ability to produce the pressure through
a distance, or an expansion, are not considered as suitable fuels for a
heat engine.
A fuel is essentially a substance which is capable of generating heat,
which is a form of energy, and not static pressure. The heat engine is
an instrument which transforms this energy into power which is again
dissipated into heat through the friction of the engine itself and by
the load that it drives. This is an illustration of the physical law
that “energy can neither be created nor destroyed,” that is, the heat
energy developed by the fuel is converted into mechanical energy which
is again transformed into heat energy through friction.
It should be understood that fuel belongs to that class of substances
that will not burn nor evolve energy under any temperature, pressure, or
shock, without an outside supply of oxygen. This is the characteristic
property of all fuels used with the internal combustion engine. Each
element, such as carbon and hydrogen, in a compound fuel, develops a
certain definite amount of heat during their complete combustion, and at
the close of the process certain compounds are formed that represent the
lowest chemical form of the compound. To restore the products of
combustion to their original form as fuel would require an expenditure
of energy equal to that given out in the combustion.
While all substances that are capable of oxydization or combustion can
be made to liberate heat energy, it does not follow that all of them can
be successfully used as fuels. A fuel suitable for the production of
power must be cheap, accessible and of small bulk, and must burn
rapidly. Such fuels must also be products of nature that require no
expenditure of energy in their preparation or completion.
[Illustration:
Fig. F-4. Fairbanks-Morse Producer Plant and Engine, Connected for
Operation.
]
In practical work, the natural fuels are coal, mineral oils, natural
gas, and wood, which are compounds of the elements carbon and hydrogen.
When these fuels are burned to their lowest forms the products of
combustion consist of carbon dioxide and water, the first being the
result of the oxydization of carbon, and the latter a compound of oxygen
and hydrogen. In solid fuels, such as coal, a portion of the compound
consists of free carbon and the remainder of a compound of carbon and
hydrogen known as a =HYDROCARBON=. In liquid fuels there is little, if
any, free carbon, the greater proportion being in the form of a
hydrocarbon compound. Natural gas is a hydrocarbon compound.
It should be noted that a definite amount of oxygen is required for the
complete combustion of the fuel elements, and that a smaller amount of
oxygen than that called for by the fuel element results in incomplete
combustion, which produces a product of higher form than that produced
by the complete reduction. The product of incomplete combustion
represents a smaller evolution of heat than that of the complete
process, but if reburned in a fresh supply of oxygen the sum of the
second combustion together with that of the first will equal the heat of
the complete oxydization. When pure carbon is incompletely burned the
product is carbon monoxide (CO) instead of carbon dioxide (CO_{2}).
Carbon completely burned to carbon dioxide produces 14,500 British
thermal units per pound of carbon, while the incomplete combustion to
carbon monoxide evolves only 4,452 British thermal units, or less than
one-third of the heat produced by the complete combustion. Theoretically
one pound of carbon requires 2.66 pounds of oxygen to burn it to carbon
dioxide. On supplying additional oxygen, the carbon monoxide may be
burned to carbon dioxide and the remainder of the heat may be recovered,
or 10,048 British thermal units. When a hydrocarbon, either solid,
liquid or gaseous is burned with insufficient oxygen, solid carbon is
precipitated together with lower hydrocarbons, and tar. In an internal
combustion engine the precipitated solid carbon is evident in the form
of smoke.
Since the carbon and hydrogen elements of a fuel exist in many different
proportions and conditions in coal and oil, different amounts of oxygen
are required for the consumption of different fuels. It should also be
borne in mind that a greater quantity of air is required for the
combustion of a fuel than oxygen, as the air is greatly diluted by an
inert gas, nitrogen, which will not support combustion. Because of the
impossibility of obtaining perfectly homogenous mixtures of air and the
fuel, a greater quantity of air is used in practice than is
theoretically required.
In a steam engine the fuel can be used in any form, solid, liquid, or
gaseous, but in an internal combustion, it must be in the form of a gas
no matter what may have been the form of the primary fuel. Fortunately
there is no fuel which may not be transformed into a gas by some process
if not already in a gaseous state. The petroleum products are vaporized
by either the heat of the atmosphere or by spraying them on a hot
surface. Coal is converted into a gas by distilling it in a retort or by
incomplete combustion. The heat energy developed by a gas when burning
in the open air depends on its chemical combustion, but its mechanical
equivalent in power when burned in the cylinder of the engine depends
not only upon its composition but upon the conditions under which it is
burned as stated in the chapter devoted to the subject of heat engines.
(8) Gaseous Fuels.
While the calorific values of the different gases given in the
accompanying table are approximately correct for gases burning in the
open air at atmospheric pressure they develop widely different values in
the cylinder of an engine because of the effects of compression and
preheating. The table serves, however, as an index to the relative
values of the fuels under ordinary conditions without compression. While
natural gas has nearly eight times the calorific value of producer gas
in the open air, its actual heat value in the cylinder is only about 45
per cent greater. While acetylene has an exceedingly high calorific
value and explodes five times as fast as gasoline gas, it develops only
20 per cent more power in the same cylinder. Another item affecting the
value of a gas is the rate at which it burns, which is in part a
characteristic of the fuel and partly a factor of the conditions under
which it is burnt. This subject is treated of in the chapter devoted to
the heat engine.
The calorific value of a gas may either be computed from its chemical
composition or by burning it in an instrument known as a =calorimeter=.
A gas calorimeter consists of a small boiler or heating tank which is
carefully covered with some non-conducting material so as to prevent a
loss of heat to the atmosphere. The gas under test is burned in the
boiler whose extended surface catches as much of the heat as possible
and transfers it to the water in the boiler. The weight of the water
heated and its temperature are taken when a certain amount of the gas
has been burned (say 100 cubic feet), and from this data, the heat units
per cubic foot of gas are computed.
FUEL GASES.
══════════════╤══════╤═════════════════════════╤═══════════╤══════
GAS │B.T.U.│ Cubic Feet of Air │ Usual │Ratio
│ per │Required to Burn 1 Cubic │Compression│of Gas
│Cubic │ Foot of Gas │ Lbs. per │to Air
│ Foot │ │ Sq. Inch │
│ │ │ │
──────────────┼──────┼────────────┬────────────┼───────────┼──────
│ │ Actual │Theoretical │ │
──────────────┼──────┼────────────┼────────────┼───────────┼──────
Natural Gas │ 1000.│ 12.60│ 9 │ 130│1–12.6
Natural Gas │ 1000.│ │ │ 110│1–6
Coal Gas │ 650.│ 9.00│ 5.85 │ 80│1–9
Producer │ 140.│ 1.20│ 1.85 │ 160│1–1.2
Anthracite │ │ │ │ │
Producer │ 160.│ 3.20│ 2.20 │ │
Bituminous │ │ │ │ │
Water Gas │ 290.│ 3.60│ 2.20 │ │
(Uncarb.) │ │ │ │ │
Water Gas │ 500.│ 8.50│ 5.15 │ │1.8
(Carb.) │ │ │ │ │
Blast Furnace │ 94.│ 1.10│ 0.70 │ 170│
Gas │ │ │ │ │
Acetylene │ 1500.│ 20.00│ 12.60 │ │
Gasolene Vapor│ 520.│See Table of Liquid Fuels│ 70│1.12
Gasolene Vapor│ 520.│See Table of Liquid Fuels│ 70│1.8
Gasolene Vapor│ 520.│See Table of Liquid Fuels│ 70│1.6
Kerosene Vapor│ │See Table of Liquid Fuels│ 60│1.8
Coke Oven Gas │ 520.│ 7.│ 5.4 │ │
Alcohol │ │ │ │ 180│
──────────────┴──────┴────────────┴────────────┴───────────┴──────
══════════════╤═════════╤═══════════╤═══════════╤══════╤══════╤═════════
GAS │Explosion│Temperature│ Ignition │Weight│Candle│ Mean
│Pressure │ of │Temperature│ per │Power │Effective
│ in Lbs. │Combustion │ F° │Cubic │ │Pressure
│ per Sq. │ F° │ │Foot, │ │
│ In. │ │ │ Lbs. │ │
──────────────┼─────────┼───────────┼───────────┼──────┼──────┼─────────
│ │ │ │ │ │
──────────────┼─────────┼───────────┼───────────┼──────┼──────┼─────────
Natural Gas │ 375│ │ 1100│.0459 │ │ 94.00
Natural Gas │ 245│ │ 1000│ │ │ 72.00
Coal Gas │ 285│ │ 1200│.035 │ 18.00│ 85.00
Producer │ 360│ │ 1450│.065 │ │ 88.00
Anthracite │ │ │ │ │ │
Producer │ │ │ 1350│ │ │
Bituminous │ │ │ │ │ │
Water Gas │ │ │ │.044 │ │
(Uncarb.) │ │ │ │ │ │
Water Gas │ │ │ │ │ 22.00│
(Carb.) │ │ │ │ │ │
Blast Furnace │ │ │ 1560│.080 │ │ 77.5
Gas │ │ │ │ │ │
Acetylene │ │ │ │ │ │
Gasolene Vapor│ 245│ 1865│ 1550│ │ │ 79.00
Gasolene Vapor│ 360│ 2950│ 925│ │ │ 82.00
Gasolene Vapor│ 410│ 3160│ 910│ │ │ 84.50
Kerosene Vapor│ 285│ │ 945│ │ │ 85.00
Coke Oven Gas │ │ │ │.042 │ │
Alcohol │ 450│ │ │ │ │
──────────────┴─────────┴───────────┴───────────┴──────┴──────┴─────────
As a British thermal unit is the amount of heat required to raise the
temperature of one pound of water through one Fahrenheit degree (at
about 39.1° F.), the total heat per cubic foot of gas as observed by the
calorimeter is equal to the weight of the water multiplied by its rise
in temperature in degrees, divided by the number of cubic feet of gas
burned in the calorimeter. Since a British thermal unit is equal to 778
foot pounds in mechanical energy, its mechanical equivalent is equal to
the number of British thermal units multiplied by 778.
Another difference between the actual and theoretical results obtained
is that due the perfect combustion in the calorimeter and the imperfect
combustion in the engine. Since some gases require more air for their
combustion than others, less of the first gas will be taken into the
cylinder on a charge than the latter, which tends still further to
balance the heating effect of rich and lean gases in the cylinder.
(9) Gasifying Coal.
=Coal Gas= or =Illuminating Gas= is generated by baking the coal in a
closed retort or chamber out of contact with the air so that no
combustion takes place either complete or incomplete. The hydrocarbon
gases and tars are set free from the coal as permanent gases and are
then piped to a gas holder after going through various purifying
processes to remove the tars, oils, moisture and dust. The free or solid
part of the coal remains in the retort in the form of =coke=, which is
again burned for fuel.
Because of its high carbon content, coal gas burns with a
yellowish-white flame and is extensively used for lighting purposes,
hence the name =illuminating gas=. In many ways coal gas is an ideal
fuel for power purposes as it has a high calorific value (650–750 B.T.U.
per cubic ft.), is supplied by the illuminating company at practically a
constant pressure, and is uniform in quality. Its only drawback is its
comparatively high cost.
This gas is always obtained from the city service mains as its
preparation is too expensive and complicated for the gas engine owner.
Because of its cost, the use of coal gas is restricted to small engines.
(10) Water Gas.
Water gas is made by blowing air through a thick bed of some coal that
is low in hydrocarbons until the coal becomes incandescent, the gases
that are formed are allowed to escape to the atmosphere. At this point a
jet of steam is blown into the incandescent bed, which is broken up into
its elements, oxygen and hydrogen, by the heat of the fuel. As there is
no air present the oxygen combines with the carbon of the fuel to form
carbon monoxide while the hydrogen goes free. Both of these gases,
carbon monoxide and hydrogen, are collected and supplied to the engine.
The production of water gas is intermittent, as the steam blast cools
down the fuel bed, and requires further blowing before more steam can be
passed. While this gas has a lower heating value than coal gas, it is
much cheaper to make and all of the coal is consumed in the process.
Water gas is high in hydrogen and is too “snappy” for gas engines; the
hydrogen places a limit on the allowable compression.
For each thousand feet of =water gas= generated, approximately 24 pounds
of water are required.
By the introduction of hydrocarbons or vaporized oil, illuminating value
is given to water gas, this process is called =carburetion=. Carbureted
gas is not usually used for power, as it is expensive, and is not
proportionately high in heating value.
(11) Blast Furnace Gas.
Many steel companies are utilizing the unconsumed gas of the blast
furnaces for power.
Blast furnace gas is of very low calorific value, rarely if ever,
exceeding 85 B.T.U. per cubic foot. This allows of very high
compression, which greatly increases the actual power delivered by the
engine.
A smelter produces approximately 88,000 cubic feet of gas per ton of
iron smelted.
Blast furnace gas is so lean that it cannot be burned satisfactorily
under a boiler; the high compression of the gas engine makes its use
possible.
(12) Producer Gas.
Producer gas which is generated by the incomplete combustion of fuels in
a deep bed is the most commonly used gas for engines having a capacity
of 50 horsepower and over, because of the simplicity and economy of its
production. While producer gas has been obtained from practically every
solid fuel, of which coal, coke, wood, lignite, peat, and charcoal are
examples, the fuel most generally used is either coal or coke. While
producer gas is much lower in calorific value than either natural or
illuminating gas it gives admirable results in the gas engine and is a
much cheaper fuel than coal gas in units above 50 horse-power capacity.
The fuel is completely burned to ash in the producer without the
intermediate coke product that exists in the manufacture of coke.
A producer consists of three independent elements as shown by Fig. F-6;
the =PRODUCER= or generator (A), the steam boiler (B), and the
=SCRUBBER= or purifier (C). The incandescent fuel (F) in the form of a
cone lies on the grate bars (G) at the lower end of the producer. Above
the burning fuel is a deep bed of coal (D) which reaches to the top of
the producer at which point it is admitted to the bed through the
charging valve or gate (H). The gas resulting from the combustion in the
producer is drawn out of the tank through the gas outlet pipe (E) by the
suction of the engine. The air for the combustion is drawn up through an
opening in the ash pit (J) by the engine.
When the oxygen of the air strikes the incandescent fuel on the grate it
combines with a portion of it forming carbon dioxide (CO_{2}) which is
an incombustible gas, but on passing through the burning fuel above this
point, one atom of the oxygen in the CO_{2} recombines with the fuel
forming the combustible gas—carbon monoxide (CO). Because of the
distilling effect of the heat in the bed, the volatile hydrocarbons of
the coal are set free and mingle with the CO formed by the combustion.
The producer gas consists, therefore, principally of CO, with a certain
proportion of the volatile hydrocarbons of the coal such as marsh gas,
ethylene, and some oil vapor.
Since the hydrocarbons are easily condensed on coming into contact with
the coal walls of the piping, to form trouble making tars and oils, they
must either be washed out of the gas in the purifier or passed again
through the high temperature zone to convert them into permanent gases.
In the usual producer, the hydrocarbons are reheated, as they form a
considerable percentage of the heat of the gas. After the volatile
constituents are reheated, the gases pass through the boiler (B), which
absorbs the heat of the gas in generating steam, and from this point the
gases enter the scrubber where the dust and the residual tars are
removed. The scrubber, which is a sort of filter, is an important factor
in the generating plant, for if the dust and dirt were allowed to pass
into the cylinder of the engine it would only be a question of a short
time until the valves and cylinder would be ground to pieces.
When the steam from the boiler is allowed to flow into the ash pit of
the producer and up through the incandescent fuel, the heat separates
the water vapor into its two elements, oxygen and hydrogen. The oxygen
set free combines with the carbon in the coal forming more carbon
monoxide, while the hydrogen which is unaffected by the combustion adds
to the heat value of the gas. The last additions to the combustion due
to the disassociation of the steam are really what is known as “water
gas.” A limited amount of steam may be admitted continuously in this
manner without lowering the temperature of the fuel below the gasifying
point, and its presence is beneficial for it not only provides more CO
and hydrogen but produces it without introducing atmospheric nitrogen.
The steam is also a great aid in preventing the formation of clinkers on
the grate bars. Since the air used in burning the fuel in the first
reaction contains about 79 per cent of nitrogen, which is an inert gas,
the producer gas is greatly diluted by this unavoidable admixture, which
accounts for its low calorific value.
[Illustration:
Fig. F-6. Diagram of Suction Gas Producer Showing the Generator,
Boiler and Washer.
]
While the air required for the combustion of the fuel is drawn through
the producer by the suction of the engine in the example shown (=SUCTION
PRODUCER=), there is a type in common use called a =PRESSURE PRODUCER=
in which the air is supplied under pressure to the ash pit by a small
blower, which causes a continuous flow of gas above atmospheric
pressure.
Gas producers are divided into two classes: suction producers and
pressure producers. The suction producer presents the following
advantages:
1. The pipe line is always less than atmospheric pressure, hence no
leaks of gas to the air are possible.
2. The regulation of the gas supply is automatic.
3. No gas storage tank is required.
4. The production of gas begins and stops with the engine.
5. Uniform quality of gas.
The suction producer is limited to power application and cannot be used
where the gas is to be used for heating, as in furnaces, ovens, etc., or
where the engine is at a distance from the producer, unless pumped to
its destination.
The pressure producer does not yield a uniform quality of gas, hence
requires a storage tank where low quality gas will blend with gas of
higher calorific values and produce a gas of fairly uniform quality.
The pressure producer is adapted to the use of all grades of fuels, such
as bituminous coal and lignite.
Anthracite coal contains little volatile matter and is an ideal fuel for
the manufacture of producer gas, while bituminous coal with its high
percentage of volatile matter and tar, requires more efficient
scrubbing, as these substances must be removed from the gas.
On starting the producer shown by Fig. 6, the producer is filled with
the proper amount of kindling and coal, and a blast of air is sent into
the ash pit by a small blower, the products of combustion being sent
through the by-pass stack (K) until the escaping gas becomes of the
quality required for the operation of the engine. The by-pass valve is
now closed, and the gas is forced through the scrubber to the engine
until the entire system is filled with gas. When good gas appears at the
engine test cock the engine is started, and the blower stopped, the gas
now being circulated by the engine piston. The volume of gas generated
by the producer is always equal to that required by the engine so that
no gas receiver or reservoir is required. Because of the friction of the
gas in passing through the fuel, scrubber and piping, its pressure at
the engine is always considerably below that of the atmosphere, which of
course reduces the amount of charge taken into the cylinder. Because of
the weak gas and the low pressure in the piping, it is necessary to
carry a much higher compression with producer gas than with natural or
illuminating gas.
The efficiency of a producer is from 75 to 85 per cent, that is, the
producer will furnish gas that has a calorific value of an average of 80
per cent of the calorific value of the fuel from which it is made, the
remaining 15 to 20 per cent being consumed in performing the combustion.
This is far above the efficiency of the furnace in a steam boiler, as an
almost theoretically exact amount of air can be supplied in the producer
to effect the combustion, while in the boiler furnace about ten times
the theoretical amount is passed through the fuel bed to burn it.
Heating up this enormous volume of air to the temperature of the
products of combustion consumes a large amount of fuel and reduces the
efficiency of the furnace considerably. Because of the reduction in the
air supply, a gas fired furnace is always more efficient than one fired
with coal. Producer gas with 300,000 British thermal units per thousand
cubic feet, and oil having 130,000 British thermal units per gallon will
result in 1,000 cubic feet of gas being equal to about 2.30 gallons of
fuel oil.
If the gas is to be used for heating ovens or furnaces in connection
with the generation of power, the character of the fuel will be
determined to a great extent by the requirements of the ovens and by the
type of producer used, as each fuel will give the gas certain
properties. Thus gas used for firing crockery will not be suitable for
use in open hearth steel furnaces, as the impurities in the various
fuels may have an injurious effect on the manufactured product. The cost
of the fuel, cost of transportation, heat value, purity, and ease of
handling are all factors in the selection of a fuel.
The size and condition of a fuel is also of importance. Exceedingly
large lumps and fine dust are both objectionable.
Wet fuel reduces the efficiency of the producer, as the water must be
evaporated, this causing a serious heat loss.
With careful attention a producer gas engine will develop a horse-power
hour on from 1 to 1¼ pounds of anthracite pea coal, and in many
instances the consumption has been less than this figure. The efficiency
in dropping from full load to half load varies by little, one test
showing a consumption of 1.1 pounds of coal per horse-power hour at full
load and 1.6 pounds of coal at half load. Producer gas power is nearly
as cheap as water power, in fact the producer gas engine has displaced
at least two water plants to the writer’s knowledge. According to an
estimate made by a well known authority, Mr. Bingham, it is possible for
a producer gas engine to generate power for only .1 of one cent more per
K.W. hour than it is generated at Niagara Falls.
According to the United States Bureau of Mines,
“The tests in the gas producer have shown that many fuels of so low
grade as to be practically valueless for steaming purposes, such as
slack coal, bone coal and lignite, may be economically converted into
producer gas and may thus generate sufficient power to render them of
high commercial value.
“It is estimated that on an average each coal tested in the producer-gas
plant developed two and one-half times the power that it would develop
in the ordinary steam-boiler plant.
“It was found that the low-grade lignite of North Dakota developed as
much power when converted into producer gas as did the best West
Virginia bituminous coals burned under the steam boiler.
“Investigations into the waste of coal in mining have shown that it
probably aggregates 250,000,000 to 300,000,000 tons yearly, of which at
least one-half might be saved. It has been demonstrated that the
low-grade coals, high in sulphur and ash, now left underground, can be
used economically in the gas producer for the ultimate production of
power, heat and light, and should, therefore, be mined at the same time
as the high-grade coal.
“As a smoke preventer, the gas producer is one of the most efficient
devices on the market, and furthermore, it reduces the fuel consumption
not 10 to 15 per cent, as claimed for the ordinary smoke preventing
device offered for use in steam plants, but 50 to 60 per cent.”
(13) Producer Gas From Peat.
The production of gas from peat having a low water content (up to about
20 per cent) for use in suction gas engines has already met with
considerable success in Germany, but for a number of years efforts have
been made to utilize peat with a water content as high as 50 to 60 per
cent and thus eliminate the costly process of drying the raw material.
Difficulties have been encountered in preventing a loss of heat through
radiation and other causes, and in getting rid of the dust and tar
vapors carried over by the gases to the engine; but great strides have
been made recently in overcoming these obstacles. Peat with a water
content up to 60 per cent has been found to be a suitable fuel. Owing to
its great porosity and low specific gravity it presents a large
combustion surface in the generator, so that the oxygen in the air used
as a draft can easily unite with the carbon of the peat.
[Illustration:
Fig. F-7. German Producer for Generating Producer Gas from Peat.
]
One of the great difficulties is to eliminate the tar vapors that clog
up many of the working parts of the engine. The passing of the gas
through the wet coke washers and dry sawdust cleansers does not appear
to have thoroughly remedied the evil. Efforts were therefore made to
remove the tar-forming particles of the gas in the generator itself or
to render them harmless. That of the Aktien-Gesellschaft Gorlitzer
Maschinenbau Ansalt und Eissengiesserei of Gorlitz, was displayed at the
exposition at Posen in 1911. The gas from the generating plant was
employed in a gas suction engine of 300 horse-power used to drive a
dynamo for developing the electric energy for the exposition. The fuel
used was peat with a water content of about 40 per cent. The efficiency
and economy results obtained were very promising.
The advantages claimed for the Gorlitz engine are that the sulphurous
gases and those containing great quantities of tar products are drawn
down by the suction of the engine through burning masses of peat and
thus rid of their deleterious constituents. The air for the combustion
purposes is well heated before entering the combustion chamber, thereby
producing economical results. It is claimed also that the gas produced
by its system is so free from impurities that the cleaning and drying
apparatus may be of the simplest kind.
In _Stahl und Eisen_, an abstract is given of a paper by Carl Heinz
describing a peat gas producer, built by the Goerlitzer
Maschinenbauanstalt. We are indebted to _Metallurgical and Chemical
Engineering_ for the translation of this paper:
Air and fuel enter the producer at the top, and the gas exit is in the
center of the bottom so that the air is forced to pass through the
center of the producer, decomposing the volatile matter into gases of
calorific value. The moisture which is present in the peat fuel in
considerable quantities must be taken into consideration. For its
decomposition which passing through the hot-fire zone only a certain
amount of heat is available. It is, therefore, important that the heat
from the gasification be fully utilized.
There are two kinds of heat losses in a gas producer, due to radiation
and to the sensible heat of escaping gases. Both these amounts of heat,
however, are utilized according to the special design of this producer.
The air circulates first through the lower conduit and comes so in
contact with the warm scrubber water. A part of the air which has been
preheated is carried upwards through the pipe =A= in the center of the
producer where it is thoroughly preheated by the hot gases and enters
then the air superheater =B= in which the temperature rises to a still
higher degree.
The other part of the air passes through the feet of the producer into
an air jacket which envelops the whole shell of the producer and enters
finally the producer by the reversing valve =C= on top of the producer.
In this way the outer surface of the producer is maintained at a
temperature hardly higher than that of the surrounding air. The escaping
gases are cooled down so far that the gas outlet into the scrubber may
be touched by hand. All ordinary heat losses are thus made use of in the
gasification process.
If there is a large excess of moisture in peat, the process is somewhat
modified by regulating both air supplies in such a way that the
gasification in the upper part of the fuel-bed takes place in two
directions, one downwards and the other upwards.
It seems that a content of 80 per cent moisture and 20 per cent dry fuel
in the peat is about the limit permitting evaporation of the water, but
it is, of course, impossible to obtain in this case a gas of calorific
value.
The modification of the process for very wet fuel is as follows:
When the fire on top of the fuel bed appears to disappear, the heater
opens the stack and valve =D=. Valve =C= is then closed, to prevent air
from entering on top. The preheated air enters by =D= causing a down
draft combustion due to the suction of the gas engine and an upward
combustion due to the draft in the stack. The moisture is evaporated and
escapes through the stack. When the fire has burned through at the top,
the valve is switched over. The bad smelling gases rising from the
scrubber enter the producer together with air and are there consumed.
In commercial use at the exhibition in Posen the whole plant worked
continuously day and night and cleaning of the gas engines was necessary
only every three months. Slagging of ashes is done during the operation
of the producer, without any nuisance from dust.
The highest percentage of moisture in peat gasified was 50 per cent. The
fuel consumption per horse-power hour is 2.2 lb. (1 kg.) of peat.
Careful tests made by Prof. Baer, of Breslau, showed that with a cost of
peat of $1 per ton the kw-hour at the switchboard costs 0.15 cent.
(14) Crude Oil Producers.
The development of the crude oil gas producer, for which there is great
demand, in oil regions remote from the coal field, has been exceedingly
slow but it is believed that definite progress has recently been made
along this line. The most recent notes on this subject relate to the
Grine oil producer. In this type steam spray is used for atomizing the
oil which is introduced into the upper part of the generator where
partial combustion takes place. The downdraft principle is then applied
and the hydrocarbon broken up and the tar fixed by passing through a bed
of incandescent coke. Mr. Grine reports that a power plant using one of
these producers has been in operation a year in California. With crude
oil as a fuel costing 95 cents per barrel, or 2.3 cents per gallon, the
plant is reported to develop the same amount of power per gallon of
crude as is ordinarily developed by the standard internal combustion
engine operating on distillates at 7 cents per gallon. Including the
cost of fuel, labor, supplies, interest, depreciation and taxes, Mr.
Grine states the cost per b.h.p. hour to be 0.76 cents for a plant of
100 h.p. rating.
(15) Operation of Producers.
A good producer operator is simply a good fireman, he must know how to
keep a uniform bed of coal and how to draw the fire. While there are
many thousands of men running producer plants without previous
mechanical training, there are now but few steam engineers running steam
engines of the same capacity but what have had at least two years’
training and sufficient mechanical knowledge to pass an examination and
obtain a license. While a considerable amount of skill is necessary to
obtain the best efficiency from a producer, it is a knack that is easily
acquired in a short time by “sticking around” the plant. Skill in
operating a producer consists chiefly in keeping the right sort of a
fire without damage to the lining by poking down ashes and clinkers.
When a new plant is installed, the manufacturer generally sends an
instructor to operate the plant for a short time so that with a few days
running in his hands any man with ordinary intelligence can overcome the
difficulties which arise from time to time.
While there are many types of producers, the main difference will be
found in the character of the draft, that is whether it is up, down, or
crossways. Down draft producers are generally used with bituminous
coals, as the tars and oils that emanate from the coal are drawn through
the fire which converts them into a permanent gas, and avoids the
difficulty of removing great quantities of the tar from the producer. An
up draft producer will not do this as the gas is drawn directly into the
mains without coming into contact with the fire. This would result in
considerable expense due to the frequent cleaning. Anthracite coal which
does not contain much tar can be used successfully in an up draft
producer.
A compromise between the up draft and down draft producer is had in the
=DOUBLE ZONE= producer, which “burns the candle at both ends” as it
were, a fire being at both the top and bottom of the producer. Nearly
any class of fuel may be used with this type.
It should be remembered that a hot fire and fuel are required for the
manufacture of gas, and that the ash pit and grate must be kept clear of
the ashes and clinkers that not only reduce the temperature of the fire,
but also reduce the gas available at the cylinder by increasing the
friction. Shaking down and cleaning out will in nearly every instance
start a bucking producer into operation.
When operating under full load a much hotter fire is required than when
operating under a reduced load, or the producer will not furnish the
necessary gas. According to the size of the producer, the depth of the
incandescent fuel will run from 30 inches in the large sizes to 15
inches in the smaller. After being charged up, suction producers will
continue to give gas in sufficient quantities with the bed at half this
depth. This is only possible with a hot producer, and when no fuel is
being fed, as the feeding of a cold charge will reduce the output. A
steady depth of fire should be kept to maintain a uniform quality of
gas.
In suction producers careful watch should be kept for leaks, as the gas
being below atmospheric pressure gives no outward signs of dilution. If
water seals are used in the system they should be given careful
attention. When using coals that are rich in tar or hydrocarbons, or
with fuels that have much fine dust, considerable trouble is had with
some types of producers due to “caking” or to the adhesion of the coal
particles to the walls of the producers or to their adhesion to one
another. In the latter case the “stickiness” of the fuel prevent the
proper feed. This difficulty may often be overcome by a change in the
rate of feeding or by regulating the depth of the incandescent bed.
Porosity of the fuel, and the rate at which the air is supplied to the
producer determines the depth of the incandescent bed. Particular care
should be taken that the blast or draft occurs evenly over the fire
surface, and that no holes occur in the fire which will cause more rapid
combustion in one spot than in another. Neglect of this precaution not
only causes a waste of fuel but often results in the fuel “arching” and
preventing further feed. The producer should be so proportioned that at
full load, the rate of combustion does not exceed 24 pounds of fuel per
square foot of producer area per hour.
In his researches, Professor Bone (Iron and Steel Institute, May, 1907)
has shown that up to 0.32 lbs. of steam per lb. of coal can be
completely decomposed in a producer, but that, from 0.45 lbs. to 0.55
lbs. should be used, approximately 80% more.
Now, in considering the question of the proper proportion of steam for
the production of gas for power purposes we must bear in mind that as
much heat as possible should be utilized in the producer itself. Some
manufacturers of plant go so far as to state that as much as 1 lb. of
steam per lb. of coal should be used, but we are safe in saying that 0.5
lb. to 0.7 lb. should be the figure for a power plant. The common
practice is to use a blast saturation of 55% whenever the clinkering
character of the coal renders it possible. This figure corresponds to
about .57 of steam per lb. of coal gasified.
It is of the utmost importance that the proportion of steam and air
should be constant, and the best figure being determined, it should not
be varied to any degree. It is equally important that the fuel depth
should be left constant. By this I mean that not only should the coal in
the producer be kept at a specific level, but the position of the fire
on the ash bed should be kept as near as possible a fixed point. Ashes
should be drawn at regular intervals, or, if desired, continuously by
mechanical means.
Further, the supply of air and steam should be regularly distributed, so
that the velocity of the gases through the fuel shall be as nearly as
possible regular across its whole area.
In some cases the by-products of a producer, such as ammonia, tar, etc.,
have a commercial value, and if a large amount of gas is generated it
will sometimes pay to select a fuel that is rich in these particular
substances.
(16) Coal.
Coal which is the basis of producer gas, is composed generally speaking
of the combustible matter, moisture, ash and sulphur. The combustible
element may be subdivided into the =HYDROCARBONS, OR VOLATILES=, and the
solid fixed carbon. The exact composition of coal is generally given by
what is known as =PROXIMATE= analysis, which analysis divides the
constituents of the coal into five groups, viz.: =MOISTURE=,
=VOLATILES=, =FIXED CARBON=, =ASH=, and =SULPHUR=. Ultimate analysis
resolves the coal into its ultimate chemical elements, such as hydrogen,
carbon, nitrogen, sulphur, etc., and being a difficult and tedious
process it is not much used.
The proximate analysis gives all the necessary information and takes
less time to perform.
VALUES OF COAL
══════════════════╤══════════════════════════════════════════╤═════════
Location of Mine │ PROXIMATE ANALYSIS │Calorific
│ │Value in
│ │ B.T.U.
│ │ per Lb.
│ │ of Coal
──────────────────┼────────┬────────┬───────┬───────┬────────┼─────────
│Moisture│Volatile│ Fixed │ Ash │Sulphur │
│ │ Matter │Carbon │ │ │
──────────────────┼────────┼────────┼───────┼───────┼────────┼─────────
=ANTHRACITE= │ │ │ │ │ │
Northern Pa. │ 3.39│ 4.41│ 83.30│ 8.17│ .73│ 13,200
──────────────────┼────────┼────────┼───────┼───────┼────────┼─────────
Eastern Pa. │ 3.70│ 3.07│ 86.42│ 6.18│ .63│ 13,440
──────────────────┼────────┼────────┼───────┼───────┼────────┼─────────
Western Pa. │ 3.12│ 3.76│ 81.60│ 10.61│ .53│ 12,875
──────────────────┼────────┼────────┼───────┼───────┼────────┼─────────
=SEMI-ANTHRACITE= │ 1.25│ 8.15│ 83.30│ 6.27│ 1.63│ 13,900
──────────────────┼────────┼────────┼───────┼───────┼────────┼─────────
=SEMI-BITUMINOUS= │ │ │ │ │ │
Pennsylvania │ .80│ 15.60│ 77.40│ 5.35│ .85│ 14,900
──────────────────┼────────┼────────┼───────┼───────┼────────┼─────────
Pennsylvania │ 1.55│ 16.45│ 71.50│ 8.63│ 1.87│ 14,200
──────────────────┼────────┼────────┼───────┼───────┼────────┼─────────
Pocahontas Va. │ 1.00│ 21.00│ 24.40│ 3.02│ .58│ 15,100
──────────────────┼────────┼────────┼───────┼───────┼────────┼─────────
West Virginia │ .90│ 17.83│ 77.70│ 3.30│ .27│ 15,230
──────────────────┼────────┼────────┼───────┼───────┼────────┼─────────
=BITUMINOUS= │ │ │ │ │ │
Youghiogheny Pa. │ 1.00│ 36.50│ 59.00│ 2.59│ .86│ 14,400
──────────────────┼────────┼────────┼───────┼───────┼────────┼─────────
Sample No. 2 │ 1.20│ 30.18│ 59.00│ 8.84│ .78│ 14,400
──────────────────┼────────┼────────┼───────┼───────┼────────┼─────────
Hocking Valley │ 6.5│ 35.06│ 48.80│ 8.05│ 1.59│ 12,100
──────────────────┼────────┼────────┼───────┼───────┼────────┼─────────
Kentucky │ 4.00│ 34.00│ 54.70│ 7.00│ .03│ 12,800
──────────────────┼────────┼────────┼───────┼───────┼────────┼─────────
Indiana │ 8.00│ 30.20│ 54.20│ 7.60│ │ 12,500
──────────────────┼────────┼────────┼───────┼───────┼────────┼─────────
Illinois │ 10.50│ 36.15│ 37.00│ 12.90│ 3.45│ 10,500
──────────────────┼────────┼────────┼───────┼───────┼────────┼─────────
Colorado │ 6.00│ 38.01│ 47.90│ 8.09│ │ 12,200
──────────────────┼────────┼────────┼───────┼───────┼────────┼─────────
=LIGNITE= │ 9.00│ 42.26│ 44.30│ 3.27│ 1.18│ 11,000
──────────────────┴────────┴────────┴───────┴───────┴────────┴─────────
The =CALORIFIC VALUE= of a fuel may be calculated from its analysis, or
may be determined by means of the =CALORIMETER= from a sample of the
coal; the latter method is the most reliable. Table gives approximately
the calorific values, and the proximate analysis of several
representative coals from various sections of the country. The values
given in the table are not exact, as the coal from each locality varies
considerably in quality, but the figures will indicate what may be
expected from each type of coal.
Connellsville, Pa., Coke has a calorific value of approximately 13,000
B.T.U.’s per pound, contains no volatile matter, and has an approximate
content of 10% ash. Coke is a valuable fuel for the gas producer, but is
rather expensive. It is clean and the absence of volatile matter reduces
the “scrubbing” problem to a minimum.
Small coal such as buckwheat and pea contain a much higher percentage of
moisture than given in the table, running from 5% to 10% higher than the
given values.
Bituminous coal is high in hydrocarbons or volatiles which condense
easily and form tar. If the tar is not removed or converted into a
permanent gas, it will clog the passages of the producer and the engine
and cause trouble.
The removal of the tar and ash from a gas is called =SCRUBBING=, and is
performed by a device much resembling a filter. Anthracite coal and coke
are low in volatiles or hydrocarbons, and therefore do not cause trouble
with tar deposits.
A high percentage of volatile matter also causes trouble by the tar
cementing the particles of fuel together. This interferes with the
proper action of the producer.
Fuels having a high percentage of ash call for perfect filtering or
“scrubbing” as such fuels will fill the gas passages with dust. Dust
should be kept out of the engine at all costs, for the dust even in a
quantity will cause wear in the cylinder.
Depending on the quality of the fuel, bituminous coal will produce about
4½ pounds of ammonia and 12 gallons of tar with about 5% of sulphur.
Anthracite coal will produce approximately six pounds of tar, and two
pounds of ammonia with traces of sulphur.
Loose Anthracite coal requires approximately 40 cubic feet of storage
space per ton of 2240 pounds and weighs about 56 pounds per cubic foot
(market sizes).
Loose Bituminous coal requires approximately 45 cubic feet of storage
space per ton of 2240 pounds, and weighs about 52 pounds per cubic foot
in market sizes.
Dry coke requires approximately 85 cubic feet of storage space per ton
of 2240 pounds, and weighs about 26 pounds per cubic foot.
(17) Fuel Oils.
Crude oil, a natural product, is the base of the fuels most commonly
used in internal combustion engines, especially in the smaller sizes.
From this compound the following derivatives are obtained by the process
of distillation, a separation possible because of the different boiling
points of the various oils. As each derivative or =DISTILLATE= has a
different boiling point, the temperature of the crude oil is maintained
at the boiling point of that product that is desired, and the resulting
vapor is condensed. The following list is not anywhere near complete for
there are several hundred distinctly different distillates, but it
contains those that are of the most interest to the engine man.
1. Crude Oil.
2. Gasoline.
3. Naphtha.
4. Solar Oil.
5. Kerosene.
The specific gravity of the crude oil as obtained in the field will
range from 12° to 56° Beaumé scale. The crude from Pennsylvania will
average 40° Beaumé while that from Texas will average 20°. The
accompanying table will give the calorific values and general properties
of the principle liquid fuels. It should be noted that the weight or
density of the liquids is given in terms of specific gravity or Beaumé
scale, in which the =SPECIFIC GRAVITY= of the fuel is the ratio of its
weight per unit volume to the weight of an equivalent volume of water.
The specific gravity of a liquid is generally determined by an
instrument known as a =HYDROMETER= which consists of a glass tube sealed
at both ends carrying a graduated scale on the upper portion of the
stem, and a ballast weight of shot or mercury at the bottom.
The hydrometer is floated in the liquid to be tested, and the lower the
specific gravity, the lower the hydrometer sinks, and vice versa. The
specific gravity of the liquid is read directly from the graduation on
the stem that are on a level with the surface of the liquid under test.
As in the case of thermometers, hydrometers are all graduated in two
different scales, the specific gravity scale and the Beaumé scale. The
specific gravity scale reads at 1.00 when floated on distilled water,
and the Beaumé at 10.00 when floated on the same liquid.
A difference in temperature affects the density of a liquid, hence all
hydrometers are graduated for a standard temperature of 60°F unless
otherwise specified. For a difference of 10°F there is a variation of
one degree gravity in the Beaumé scale, and for a difference of 20°F in
temperature there is a change of one degree on the specific gravity
scale. If the temperature differs from 60°F, the corresponding
correction should be made in the reading.
To convert the Beaumé reading (B) to terms of the specific gravity scale
(S) use the following formula:
140
S = ——————— = specific gravity.
130 + B
140
B = ——— = Beaumé scale.
S
PROPERTIES OF OILS
Degrees Specific Weight B.T.U.’S B.T.U.’S
Beaumé Gravity per gal. per lb. per gal.
Gasoline 67.2 .7125 5.932 21120 125,284
Heavy naphtha 64.6 .7216 6.011 20527 123,388
Kerosene 48.8 .7848 6.538 20018 130,877
W. Virginia crude 40.0 .8251 6.874 19766 135,871
Penn. fuel oil 31.9 .8660 7.215 19656 141,818
Kansas crude 29.0 .8816 7.345 19435 142,750
Fuel oil 22.7 .9176 7.645 19103 146,042
California crude 22.5 .9248 7.710 18779 144,786
California crude 15.2 .9646 8.036 18589 149,381
Alcohol, 95% 41.9 .816 6.798 10500 71,380
It will be noted that the petroleum products contain an enormous amount
of heat energy, nearly 25% more than that of the same weight of pure
carbon. It will also be noted that the lighter products such as
gasoline, kerosene, etc., have more heat per pound but less per gallon
than the heavier oils. This is rather confusing at first, but as will be
seen after deliberation that the heavier fuel is the most economical
since the least is used per horse-power, and is bought by the gallon.
The calorific values given in the table are obtained by a calorimeter,
and are burnt in the open air, and consequently have a different heating
value when under compression in the cylinder of the engine.
In all cases the liquids are vaporized before being introduced in the
cylinder, the more volatile liquids such as gasoline being converted
into vapor at atmospheric temperature, and the heavier non-volatiles by
being sprayed into a heated vessel or preheated air. The percentage of
liquid fuel contained in a cubic foot of air vapor mixture depends on
the temperature, the boiling point of the liquid and upon the pressure
and humidity.
Gasoline consists principally of compounds of the methane series, the
one representative of gasoline being Hexane (C_{6}H_{14}). It requires
15.5 pounds of air for combustion theoretically and about 10 per cent.
more in practice. The formation of gasoline vapor produces a drop in
temperature of 50°F, and should be heated 100°F above the atmosphere for
the best results. The volume of air required for the combustion is about
192 cubic feet. With alcohol at 20 cents per gallon and gasoline at 12½
cents the number of B.T.U.’s for one cent in the case of alcohol is 3594
and 9265 in the case of gasoline. In the engine the difference is not so
great owing to the difference in compression pressures.
(18) Tar for Fuel.
Because of the increasing interest in the Diesel type engine and the low
grade fuels that it has made possible, we quote the specifications laid
down by Dr. Rudolph Diesel, the inventor, before the English Institution
of Engineers.
(1.) Tar-oils should not contain more than a trace of constituents
insoluble in xylol. The test on this is performed as follows:—25 grammes
(0.88 oz. av.) of oil are mixed with 25 cm.^3 (1.525 cub. in.) of xylol,
shaken and filtered. The filter-paper before being used is dried and
weighed, and after filtration has taken place it is thoroughly washed
with hot xylol. After re-drying the weight should not be increased by
more than 0.1 gr.
(2.) The water contents should not exceed 1 per cent. The testing of the
water contents is made by the well-known xylol method.
(3.) The residue of the coke should not exceed 3 per cent.
(4.) When performing the boiling analysis, at least 60 per cent. by
volume of the oil should be distilled on heating up to 300° C. The
boiling and analysis should be carried out according to the rules laid
down by the Trust. (German Tar Production Trust on Essen-Ruhr.)
(5.) The minimum calorific power must not be less than 8,800 cal. per
kg. For oils of less calorific power the purchaser has the right of
deducting 2 per cent of the net price of the delivered oil, for each 100
cal. below this minimum.
(6.) The flash-point, as determined in an open crucible by Von Holde’s
method for lubricating oils, must not be below 65° C.
(7.) The oil must be quite fluid at 15° C. The purchaser has not the
right to reject oils on the ground that emulsions appear after five
minutes’ stirring when the oil is cooled to 8°.
Purchasers should be urged to fit their oil-storing tanks and oil-pipes
with warming arrangements to redissolve emulsions by the temperature
falling below 15° C.
(8.) If emulsions have been caused by the cooling of the oils in the
tank during transport, the purchaser must redissolve them by means of
this apparatus.
Insoluble residues may be deducted from the weight of oil supplied.
Coal tar oil is the distillate of the tar obtained from gas works, from
which all valuable commercial materials such as aniline have been
removed. Coal oil tar is also known as creosote oil and anthracene oil,
the heat value of which is not quite 16,000 B.T.U. per pound.
(19) Residual Oils.
Residual oil is the residue left after the lighter oils have been
distilled from the petroleum, which before the advent of the Diesel
engine were useless. Residual oil which was hardly fluid at ordinary
temperatures has been successfully used in the Diesel and semi-Diesel
types of engines, by preheating it before admission to the inlet valves.
The enormously increased demand for gasoline has resulted in a great
increase of the formerly useless residual oil so that it is possible
that the demand for gasoline will make the production of the residual
great enough so that it can be seriously considered as a fuel.
(20) Gasoline.
Gasoline is by the far the most widely used fuel for internal combustion
engines because of its great volatility and the ease with which it forms
inflammable mixtures with the air at ordinary temperatures. Another
point in its favor is the fact that it burns with a minimum of sooty or
tarry deposits, without a disagreeable smell with moderate compression
pressures and without preheating through a wide range of air ratios.
Gasoline is a product of crude oil from which it is obtained by a
process of distillation, and as it forms but a small percentage of the
crude oil it is rapidly becoming more and more expensive as the demand
increases. Some Pennsylvania crude oils will yield as much as 20 per
cent of their weight in gasoline, while the low grade Texas and
California crudes very seldom contain more than 3 per cent.
When considered as a term applying to some specific product, the word
“Gasoline” is a very flexible expression as it covers a wide range of
specific gravities, boiling points, and compositions, the latter items
depending on the demand for the fuel and the taste of the manufacturer.
Since the specific gravity of gasoline is a factor that determines its
suitability for the engine, at least in regard to its evaporating power
or volatility, it is graded according to its density in Beaumé degrees
as determined by the hydrometer. According to this scale gasoline will
range from 85° to 60° Beaumé, and even lower, although 60° is supposed
to mark the lowest limit and to form the dividing line between gasoline
and naphtha.
The density of the gasoline in Beaumé degrees is an index to the
volatility, for the higher the degree as indicated on the hydrometer,
the higher is the volatility at a given temperature, consequently a high
degree gasoline will give a better mixture at a low temperature than one
of a low degree. In cold weather all gasoline should be tested with a
hydrometer when purchased to insure a grade that will be volatile enough
for easy starting when the engine is cold. In cold weather the gasoline
should not be lower than 68°, and for the best results should be above
72°, at least for starting the engine. Good gasoline should evaporate
rapidly and should produce quite a degree of cold when a small amount is
spread on the palm of the hand, and it should leave neither a greasy
feeling nor a disagreeable odor after its evaporation.
The high gravity gasoline is of course the most expensive, as there is
less of it in a gallon of the crude oil from which it is made; gasoline
of 76° Beaumé being approximately 15c. per gallon in carload lots, while
naphtha of 58° Beaumé brings 8½c. per gallon.
The calorific value of gasoline increases as the gravity Beaumé
decreases per gallon; 85° gasoline having approximately 113,000 B.T.U.
per gallon while 58° naphtha has an approximate value of 122,000 B.T.U.
per gallon. The calorific value remains nearly constant per pound for
all gravities.
It should be remembered that heat is absorbed in evaporating gasoline as
well as in evaporating water, and that effects of cold weather are
greatly increased by the amount of heat absorbed, (or cold produced) by
the vaporization of the fuel. While the heat absorbed by evaporating a
given quantity of gasoline is only .45 per cent of that absorbed by an
equal amount of water, it is a fact that this heat must be supplied from
some source to prevent a reduction in the vapor density. In starting the
engine, the heat of evaporation is supplied by the atmosphere, and
should the temperature of the air be below that required for a given
vapor density, the engine will refuse to start.
By the use of two tanks and a three way valve, it is possible to use two
grades of fuel: one tank containing high gravity gasoline, and the other
low gravity; the high gravity being used for starting the engine in cold
weather, and the cheaper, low gravity, being used for continuous running
after the engine is warmed up—the change of fuels being made by throwing
over the three way valve.
The =VAPOR DENSITY= of gasoline vapor is the ratio of the weight of the
vapor compared with the weight of an equal volume of dry air at the same
temperature. If the weight of a cubic foot of gasoline vapor is divided
by the weight of a cubic foot of air at the same temperature the result
will be the vapor density of the gasoline vapor. Compared to air, the
gasoline vapor is quite heavy so that if a small quantity of gasoline is
poured on the top of a table, the vapor will flow over the edge of the
table and drop to the floor where it will remain until it has united
with the air by the process of diffusion. Experiments have shown that
pure, dry gasoline vapor has a density of about 3.28, or in other words
weighs 3.28 times as much as an equal volume of dry air. This weight of
course is the weight of pure vapor which is considerably heavier than
the mixture of vapor and air that is used in the cylinder of the engine.
Dampness, or the presence of water vapor in the air reduces the quantity
of gasoline vapor taken up by the air, but only by a small amount, the
maximum difference being only about 2 per cent. Since it is very likely
that the water vapor is broken up into its original elements, oxygen and
hydrogen, by the heat of the combustion it is likely that there is no
heat loss due to the vapor passing out through the exhaust. The
principal trouble due to dampness is the mixture of water and liquid
gasoline caused by the condensation of the water vapor.
All gasolines and oils contain water to a more or less degree, hence
provision should be made for the draining of the water which collects in
the bottom of the tank. Water in liquid fuels is the cause of much
trouble.
Water in gasoline may be detected by dropping scrapings from an
indelible pencil into a sample of the suspected fluid. If water is
present in any quantity the gasoline will assume a violet color.
In filling a supply tank with gasoline, a chamois filter or chamois
lined funnel should always be used, as the chamois skin allows the
gasoline to pass but retains the water and impurities contained therein.
There are many funnels of this type now on the market.
The rate at which gasoline burns depends on the amount of surface
presented to the air by the fluid, for a given quantity of gasoline
burns faster in a wide shallow vessel than in a deep jar. Since a spray
of minute particles presents an enormously greater surface than the
liquid its burning speed is correspondingly greater, and as a true vapor
has an almost limitless area, its speed is much greater than that of the
spray, the combustion under the latter condition being almost
instantaneous. Besides the question of subdivision of the liquid, the
rate of combustion also depends on the intimacy of contact of the vapor
with the air and on the pressure applied to the vapor as previously
explained under the head of “=COMPRESSION=” in another chapter.
=CARBURETING AIR=, or producing an explosive mixture of gasoline vapor
and air is accomplished by two different methods, first by passing the
air over the surface of the liquid, or by passing it through the liquid
in bubbles; second by spraying the liquid into the air. The latter is
the method most generally in use at the present time, the spray being
formed by the suction of the intake air upon the open end of the spray
nozzle. The vapor density of the mixture thus formed depends on the
suction of the air and upon the nozzle opening, either of which may be
varied in the modern carburetor to vary the richness of the mixture.
As a suggestion to the users of gasoline we append the following
remarks.
Gasoline vapor will readily combine with air to form explosive mixtures,
at ordinary temperature. This property at once makes it the most
suitable fuel and the most dangerous to handle.
Never fill tanks or expose gasoline to the air in the presence of an
open flame, or do not attempt to determine the amount of gasoline in a
tank with the aid of a match. There are a number of people who have
successfully accomplished this feat, and a very great number who have
not.
Be very sparing in the use of matches around a gasoline engine; there
are such things as =leaks=.
Always carefully replace the stopper or filler cap in a gasoline tank
after filling. Never use the same funnel for water and gasoline, and
avoid any possibility of water finding its way into the tank.
If you do succeed in igniting a quantity of free gasoline, do not
attempt to extinguish the fire with water. Pouring water on burning
gasoline spreads the fire. Extinguish it with earth or sand, or by the
use of one of the dry powder extinguishers now on the market.
Water may be removed from gasoline by placing a few lumps of desiccated
calcium chloride in the tank, the amount depending on the quantity of
water.
Calcium chloride, has a great capacity for absorbing water, and in a
short space of time will absorb all of the moisture contained in the
tank.
The best way to introduce the chloride is to wrap the lumps in a sheet
of wire gauze and lower into tank with a wire, the wire allowing it to
be easily removed when saturated with water.
(21) Benzol.
Benzol has been used to some extent in Europe as a fuel, its use being
due to the rapidly increasing cost of gasoline.
Benzol is a distillate of coal tar, and is a by-product of the coke
industry. In England benzol brings approximately the same price as
gasoline (called petrol), but benzol proves economical for the reason
that it develops more power per gallon.
Benzol is not as volatile as gasoline, but is sufficiently volatile to
allow of easy motor starting.
Benzol is also used for denaturing alcohol.
(22) Alcohol.
Alcohol is of vegetable origin, being the result of the destructive
distillation of various kinds of starchy plants or vegetables. Starch is
the base of alcohol.
As a fuel, alcohol has much in its favor, as it causes no carbon
deposit, has smokeless and odorless exhaust, can stand high compression,
and requires less cooling water than gasoline, as the heat loss is less
through the cylinder walls, and for this reason it is more efficient
fuel than gasoline.
At the present time the price of alcohol prohibits its general use. In
order that alcohol equal gasoline in price per horse-power hour, it
should sell for 10c. per gallon, the price of gasoline being 15c. per
gallon.
Alcohol can be used in any ordinary gasoline engine with readjustment of
carburetor and the compression.
The nozzle in the carburetor has to be of larger bore for alcohol than
for gasoline, and the compression for alcohol in the neighborhood of 180
pounds per square inch.
The inlet air should be heated to about 280°F for alcohol fuel;
approximately 6% of the heat of the alcohol is required for its
vaporization. Alcohol is much safer to handle than gasoline owing to its
low volatility.
90% alcohol has a calorific value of 10,100 B.T.U. per pound, its
specific gravity being .815.
=WOOD=, or =METHYL= alcohol is made by distilling the starch contained
in the fibres of some species of wood (Poisonous).
=GRAIN=, or =ETHYL= alcohol is the result of the distillation of the
starch contained in grains, potatoes, molasses, etc. =ETHYL=, or =GRAIN=
alcohol rendered unfit for drinking by the addition of certain
substances, is called =DENATURED ALCOHOL=. The process of denaturing
does not affect the calorific value of alcohol to any extent.
(23) Kerosene Oil.
Kerosene is a fractional distillate of crude oil which has a
considerably higher vaporizing temperature than gasoline. It does not
form an inflammable mixture with the air at ordinary temperatures, but
is vaporized in practice by spraying it into a chamber heated to above
200°F. Kerosene forms a greater percentage of crude oil than gasoline
and as there has been less demand for it up to the present time it is
much cheaper. Pennsylvania crude oil produces only 20 per cent of
gasoline while the kerosene contents will average nearly 42 per cent
according to figures at hand.
Kerosene has a very high calorific value per gallon, 8.5 gallons of
kerosene having the same heating effect as 10 gallons of gasoline.
Because of its high calorific value and its low cost per gallon, many
types of engines have been developed for its use during the last few
years, several of which have been very successful. Before the advent of
the modern kerosene engine much difficulty was experienced with the fuel
because of its high vaporizing temperature and its tendency to carbonize
in the cylinder, but as the price of gasoline continued to rise, the
inventive genius of the gas engine builder overcame these troubles so
that the kerosene engine is now as reliable as any form of prime mover.
[Illustration:
Kerosene Vaporizer on Fairbanks-Morse Engine. The Engine is Started on
Gasoline and When Hot, the Kerosene Feed is Turned on.
]
Any gasoline engine will run on kerosene, after a manner, if the engine
is thoroughly heated to insure the vaporization of the kerosene, and if
the fuel is heated in the carburetor. Such an arrangement is make-shift,
however, and is not productive of good results in continuous service. If
kerosene is to be used as a regular fuel, a kerosene engine should be
used to avoid vaporizing and carbonizing difficulties as well as the
sooty, offensive exhaust, and the loss of fuel represented by the soot.
Many kerosene engines are arranged to start on gasoline, and, after
becoming heated, have the running feed of kerosene admitted through a
three way valve. The gasoline feed is then stopped.
The above arrangement admits of easy starting in all weathers and
temperatures.
In the Diesel engine there is no evaporating of fuel, and no deposits of
carbon because of the high temperature of the combustion chamber. With
engines that draw the mixture of vapor and air into the cylinder there
are several methods of applying heat to the liquid, and the combustion
of the vapor thus formed is perfected by the injection of water into the
combustion chamber. It has been found by experiment that a small amount
of water vapor introduced into the cylinder of a kerosene engine makes
the engine run more smoothly and prevents a smoky exhaust and carbon
deposits in the cylinder. The water is introduced into the cylinder
through an atomizer in the form of a mist or fog, the particles of water
being in a very finely subdivided state.
[Illustration:
Kerosene Vaporizer on Fairbanks-Morse Vertical Engine. Started on
Kerosene Directly by Heating Vaporizer with Torch.
]
The deposits of free carbon (soot) caused by the “cracking” or
decomposition of the kerosene vapor before ignition, due to the high
temperature of the cylinder, are burnt to carbon dioxide by the oxygen
of the water which is also set free by the heat of the cylinder. This
produces an odorless gas (CO_{2}) which indicates complete combustion.
Besides the increase of fuel efficiency due to the water vapor, the
cylinder is more thoroughly cooled and is more efficiently lubricated
because of the reduction in temperature.
CHAPTER III
WORKING CYCLES
(24) Requirements of the Engine.
In order that an internal combustion engine shall operate and develop
power continuously the following routine of events must occur in the
cylinder in the following order, no matter what the type of engine.
(1) The cylinder must be filled with a combustible mixture of air and
gaseous fuel at as nearly atmospheric pressure as possible.
(2) The mixture must be compressed in order to develop the value of the
fuel.
(3) Ignition must take place at the end of the compression stroke or at
the highest point of compression.
(4) Complete combustion of the fuel must follow the ignition of the
charge, with an increase of temperature and pressure which will act on
the piston to the end of the power stroke.
(5) After the piston has completed the working stroke the products of
combustion must be ejected from the cylinder completely to make way for
the admission of the new combustible mixture.
With the exception of the Diesel engine which (1) fills the cylinder
with pure air without the fuel, and (2) injects the fuel after
compression, all internal combustion engines not only perform each of
these operations but proceed with events in the order given as well. The
accomplishment of the five acts is called a “cycle of events,” or a
“=CYCLE=,” and the series is performed in different ways in different
types of engines. In the operation of the engine, the series of events
occur over and over again, always in the same order, 1–2–3–4–5,
1–2–3–4–5, 1–2–3–4–5, etc. The five events are generally given in terms
of the number of strokes of the piston taken to accomplish the complete
routine, thus a two stroke cycle engine performs the series in two
strokes, and a four stroke cycle engine in four strokes, and so on.
In order to obtain the benefits of high compression, perfect scavenging
of the products of combustion from the cylinder and perfect mixtures, a
great variety of engines have been developed in which the number of
strokes taken to accomplish the five events varies. In some engines the
cycle is accomplished in two strokes, in other engines it is
accomplished in six strokes, but in the great majority of cases the
cycle is performed in either two or four strokes, and as these are by
far the most common routines, we will confine our description to engines
of these types.
(25) Four Stroke Cycle Engine.
The four stroke cycle engine, some times improperly called the “four
cycle” engine is the most widely used type for all classes of service,
except possibly for marine work. Its extended use is due to its superior
scavenging, high efficiency and reliability, although it is somewhat
more complicated than the two stroke cycle type. Its ability to function
properly under a wide variation of speed has driven the two stroke cycle
type out of the automobile field, and its many admirable characteristics
have cut a wide swath in the marine field, the stronghold of the two
stroke cycle type.
A four stroke cycle engine performs the cycle of events in four strokes
or two revolutions, only one of the strokes being a power of working
stroke. In a single cylinder engine the explosion in the working strokes
supplies enough power to the fly-wheel to carry the engine and its load
through the remaining three strokes. Thus the energy stored in the fly
wheel is sufficient to carry not only the load during the idle strokes
but to “inhale” and compress the charge as well. Due to the long
interval that exists between explosions, they are corresponding heavy
and are productive of heavy strains in the engine and are the cause of
considerable vibration.
To reduce the ill effects of the heavy intermittent blows, the majority
of automobile and stationary engines are provided with two or more
cylinders, the power being equally divided among them. In a four
cylinder engine, there are four times as many impulses as in a single
cylinder engine and the blow dealt by the individual cylinder is only
one-quarter as great. While a single cylinder engine has an impulse only
once in every other revolution, the four cylinder has two impulses in
one revolution. Besides the advantages gained by increasing the
impulses, the mechanical balance of a multiple cylinder engine is always
better than that of a single and is also much lighter in weight since
less material is required to resist shocks of the explosions.
[Illustration:
Fig. 4. Diagrammatic View of Four Stroke Cycle Engine with the Piston
in Various Positions Corresponding with the Five Events. Diagram
A—Suction. Diagram B—Compression. Diagram C—Ignition. Diagram
D—Working Stroke. Diagram E—Release. Diagram F—Scavenging Stroke.
]
Engines with more than four cylinders have “overlapping” impulses, that
is some cylinder on the engine is always delivering power, for before
one cylinder reaches the end of the stroke, another has fired its charge
and has started to deliver power. Thus the impulses “overlap” one
another, and the result is an even and smooth application of power and a
minimum of strain is imposed on the engine.
Aeronautical and speed boat engine builders have carried the multiple
cylinder idea to an extreme because of the nature of their work. Eight
cylinder aeronautical engines are very common and there are several
built having sixteen cylinders. The latter type of engine gives eight
impulses per revolution. To avoid a great multiplicity of cylinders, and
to save on floor space, the great majority of heavy duty stationary
engines are built double acting, that is an explosion occurs alternately
in either end of the cylinder. In effect, a double acting cylinder is
the same thing as a two cylinder single acting engine, as it gives twice
the number of impulses obtained with a single acting cylinder.
The order in which the events occur in a four stroke cycle engine is as
follows:
=STROKE 1.= First outward stroke of the piston causes a partial vacuum
in the combustion chamber thus drawing a charge of combustible gas into
the cylinder through the open inlet valve. The exhaust valve is closed.
See diagram A in Fig. 4. (Suction Stroke.)
=STROKE 2.= Inlet valve closes at the end of the suction stroke and the
piston starts on the inward stroke compressing the charge in the
combustion chamber. See diagram B. (Compression Stroke.) At the end of
the compression stroke, or a little before, the spark “S” occurs causing
the ignition of the charge. See diagram C.
=STROKE 3.= Working Stroke. As the pressure is now established in the
cylinder, the piston moves down on the working stroke forcing the crank
around against the load and supplying sufficient energy to the fly wheel
to carry the engine through the three idle strokes. See diagram D. When
the piston reaches the end of the working stroke, or a little before,
the exhaust valve opens to reduce the pressure and to allow the greater
part of the burnt gas to escape. See diagram E.
=STROKE 4.= Scavenging Stroke. The exhaust valve remains open and the
inwardly moving piston expels the remainder of the burnt gas through the
exhaust valve, clearing the cylinder for the next fresh charge of
mixture. See diagram F. The next stroke is the suction stroke explained
under “Stroke 1.”
In all of the diagrams the crank is supposed to turn in a right handed
direction as indicated by the arrow, the piston moving in the direction
shown by the arrow under the piston head. The valves are operated by
cams on an intermediate shaft known as the “cam shaft.” As the valves go
through their series of movements in two revolutions of the crank shaft,
and as the cam shaft must perform all of these operations in one
revolution, it is evident that the cam shaft must run at exactly
one-half the crank-shaft speed. This change of speed is accomplished by
means of gearing between the cam shaft and crank-shaft from which the
cam shaft is driven.
In some engines, notably the Diesel engine, pure air is drawn into the
cylinder on stroke No. 1 instead of the entire mixture. Fuel is supplied
in this type immediately after the end of the compression stroke.
While an electric spark is shown as the igniting medium in the diagrams,
the ignition is sometimes performed by a hot tube, or simply by the heat
of the compression as in the Diesel engine.