BURNING FUEL IN THE ENGINE CYLINDER
ONE OF the handicaps of steam power is that the heat produced by the
combustion of fuel is not used directly to drive the piston. A large
part of the heat energy in coal goes up the chimney or is wasted
by radiation from the furnace walls. With anthracite coal in the
furnace the loss may be as low as 22 per cent in the very best types
of boilers. In the engine other serious losses occur, so that in the
best condensing reciprocating steam engines the power delivered is
only 10 to 16 per cent of that stored in the fuel. In locomotives the
efficiency is as low as 4 to 6 per cent. In turbines the steam is used
to better advantage and the actual power delivered may run up to 20 per
cent of that in the fuel.
In internal-combustion engines the furnace and boiler losses are
largely overcome by burning the fuel right in the cylinder, where
the heat energy of the combustion may be utilized directly upon the
piston. There are other losses, however, so that the most efficient
gasoline engines deliver only 28 per cent of the energy in the fuel,
and coal-gas engines may run up to 31 per cent.
A wide range of fuels may be used in an internal-combustion engine.
They may be either gaseous, liquid, or even solid. In stationary
engines hydrogen, coal gas, natural gas, blast-furnace gas, and
producer gas are employed to advantage. Volatile fuels, such as
alcohol, bensol, gasoline, and kerosene, are turned into a mist or
vapor and then burned as a gas. In engines of the Diesel type any
liquid fuel ranging down to thick crude oils may be employed, and
in one type of engine, which, however, has not proved commercially
practicable, coal dust is burned in the cylinder.
BURNING AIR IN THE AUTOMOBILE ENGINE
Owing to the widespread use of automobiles, the general public is
better acquainted with internal-combustion engines than with steam
engines. Nevertheless, there are many elementary facts in connection
with gasoline engines that are not generally known. The average
motorist probably does not realize that he burns far more air in his
motor than gasoline, and he probably does not understand why it is that
the mixture of gasoline vapor and air must be compressed before it is
ignited, or why only one out of four strokes of the piston is a power
stroke.
If a volume of gasoline vapor be thoroughly mixed with an equal volume
of air, the mixture will not explode. Only when there is an excess of
air will combustion take place, and the most intensive explosion takes
place when there are nine parts of air to one part of gasoline vapor.
Where gasoline is vaporized in a carburetor the best mixture is one
part of gasoline-saturated air to eight parts of pure air. Hence it is
mainly air that is burned in an automobile engine.
The energy of combustion is much greater when the mixture is
compressed. The particles of gasoline and air are forced into more
intimate contact by the compression. The same thing is true of
gunpowder. If gunpowder is ignited in the open air, it will burn
quickly, but not with explosive violence. If, on the other hand, the
powder is compressed in a cartridge and is then ignited, an explosion
takes place. Nearly twice as much gas was required in the early
noncompression-type motors as is required to-day in the compression
motors of the same power.
In an internal-combustion engine the cylinder serves as a furnace. This
“furnace” must be charged with fuel, and after the fuel has been burned
the “ashes”--i. e., the products of combustion--must be removed. The
piston serves as the furnace stoker.
In the ordinary four-cycle engine the action of the piston is as
follows: On the first or down stroke of the piston a mixture of air and
gasoline vapor is drawn into the cylinder; on the next or rising stroke
the charge is compressed. Then the charge is ignited by an electric
spark, and the rapid combustion of the charge produces gases which
drive the piston down. On the fourth stroke the piston rises again and
pushes the burnt gases out of the cylinder. The piston receives energy
intermittently, or only once out of four strokes, and a flywheel has
to keep it going the rest of the time. It is just like propelling a
bicycle with a single pedal and pushing the pedal every other time it
comes out. It can be done as long as the wheel is moving fast enough to
carry itself along between power strokes. Naturally a single-cylinder
motor cannot be slowed down very much without stalling, and it will not
start of itself because it needs outside help in stoking its furnace
before it acquires the power to do this job alone.
[Illustration: FIG. 51--SECTIONAL VIEW OF A TWO-CYCLE ENGINE]
There is a two-cycle type of motor in which the burnt gases are removed
from the cylinder, not by the piston, but by the injection of the
unburnt fuel mixture. On the upward or compression stroke the piston
not only compresses the mixture in the cylinder, but draws in a fresh
charge of fuel into the crank case, and when the piston is driven
down by the combustion of the fuel in the cylinder, the charge in the
crank case is compressed until, near the end of its stroke, the piston
uncovers a port in the cylinder through which the fuel from the crank
case is forced in. (See Figure 51.) Just before this occurs the piston
uncovers an exhaust port in the opposite side of the cylinder, and
the burnt gases start to flow out before the fresh charge of fuel
pours into the cylinder. The incoming gas is directed upward so as to
completely scavenge the cylinder of all burnt gases.
There are no bothersome poppet valves in this engine. The piston itself
acts as a valve, opening and closing the inlet and exhaust ports as it
slides by them. Every other stroke of the piston is a power stroke, so
that the engine acts like a one-pedal bicycle, which receives a push
every time the pedal comes around. Unfortunately, the burnt gases are
never completely scavenged, and the fresh charge of fuel is always
more or less diluted by the product of combustion remaining from the
previous charge. This represents just so much loss of power, and
may interfere with the ignition of the charge. For this reason the
four-cycle engine is generally considered more reliable and efficient,
and is far more generally used, particularly on motor vehicles.
To overcome the intermittent character of the internal combustion
engine a number of cylinders are used, which come into play
successively. In the four-cylinder motor one piston is always on the
power stroke. In recent years the number of cylinders has progressively
increased from four to six, eight, and twelve cylinders, while in
racing power boats the number of cylinders has gone up as high as
twenty-four.
COOLING THE CYLINDERS
The temperature of the combustion in a gasoline engine may be over
3,000 degrees Fahrenheit. Evidently it would melt the cylinder walls
were not special provisions taken to keep them cool. The usual method
is to surround the cylinders with water which absorbs the heat, and
then cool the water by passing it through a radiator. Air driven
by a fan through the radiator carries off the heat. Of course this
represents just so much wasted energy and lowers the efficiency of the
motor, but it is the most convenient way of getting rid of the intense
heat of combustion. Smaller motors, such as are used on motorcycles,
have their cylinders cooled by direct action of air on the outer
surface of the cylinder. The heat radiated depends upon the surface
exposed and the velocity of the air current. The radiating surface
of the cylinders is increased by forming them with external ribs or
flanges, so that the air that flows over them, while the machine is
in motion, carries off enough heat to keep the temperature of the
cylinders within safe limits. In some systems, particularly in the
case of stationary engines, to insure a good circulation, the air is
positively driven against the flanges by means of a fan.
There are serious disadvantages in using water to cool an engine.
Besides the bother of keeping the water system supplied with water
there is the danger of clogging the radiator with lime deposits, and
in extremely cold weather the water is liable to freeze and burst
the water jackets or the radiator; for this reason, air cooling is
recommended by some for automobile engines. In certain air-cooled
automobile engines the cylinders are air-jacketed; that is, they are
surrounded with casings through which the air is sucked by a powerful
fan driven by the engine shaft, so that the air is bound to flow if the
engine is turning. The cylinders are ribbed, so that they present a
large radiating surface.
[Illustration:
Copyright, Kadel & Herbert
GIANT CAPRONI HYDROAEROPLANE WHICH WAS WRECKED AT LAGO MAGGIORE
Seating capacity 100 persons; power, eight 450-h.p. Liberty motors]
[Illustration: A BOAT OF STONE--THE CONCRETE STEAMSHIP “FAITH”]
[Illustration: THE “VIRGINIAN,” THE LARGEST LOCOMOTIVE IN THE WORLD
Length over all, 97 ft.; weight of engine, 684,000 lbs.; of tender,
214,300 lbs.; tractive power, simple, 176,600 lbs.; compound, 147,200]
Air-cooled engines have hotter cylinders than water-cooled engines
and hence in cylinders of the same size less fuel is drawn into the
air-cooled cylinder. But the fuel is used more efficiently because less
of its heat energy is wasted; so that air-cooled engines show a slight
fuel economy over water-cooled engines. On the other hand, when the
air-cooled engine is overloaded or is run at very high speeds it is
liable to become overheated and may ignite the incoming charge of gas
prematurely.
THE SELF IGNITING DIESEL ENGINE
As has already been pointed out, compression of gas generates heat,
and one of the reasons for cooling the cylinders of an internal
combustion engine is to prevent the gas from exploding prematurely on
the compression stroke of the piston. In the Diesel engine, instead
of avoiding such a critical temperature it is deliberately sought,
because it is the heat of compression, instead of an electric spark,
which ignites the charge. Pure air is drawn into the cylinder on the
suction stroke. This air is under ordinary atmospheric pressure when
it enters, but on the return stroke of the piston it is compressed to
about 500 pounds per square inch. This raises the temperature up to
the neighborhood of 1000 degrees Fahrenheit. Into this highly heated
air a spray of oil is injected by air at still higher pressure and
immediately the oil flashes into flame and the gases resulting from the
combustion drive the piston down.
This is a very economical type of motor. Any liquid fuel may be used,
from light gasoline to heavy crude oils, or the oils that remain after
the more volatile fuels have been distilled from them. The heavy oils
have more heat value in them than is to be found in light volatile
fuels, such as alcohol, gasoline, and kerosene, and because the oils
do not throw off any inflammable vapors unless highly heated, they can
be stored more safely. This is of highest importance on submarines and
Diesel engines are commonly used for propulsion on the surface or to
drive the generators which charge the storage batteries for submerged
travel.
In order to inject the fuel into the cylinder against the pressure of
air therein, a powerful air pump is required. This does not need to be
of very large capacity, but it must compress the air to from 700 to
1,000 pounds per square inch. It may seem at first as if the work done
by the engine in compressing the air in the cylinder and in operating
the injector pump represents so much loss, but a moment’s consideration
will show that it is all recovered. The air in the cylinder acts like
a spring, rebounding when the piston starts down on its power stroke
and adding its energy to that of the burning gases, while the air from
the injector pump also enters the cylinder and helps to push the piston
down.
Diesel engines are made to operate on the two-cycle as well as the
four-cycle principle.
In one type of engine, known as the semi-Diesel, a lower compression is
used in the cylinder. This is not high enough to ignite the oil that
is sprayed in, and so a hot tube or bulb is employed at the top of the
cylinder against which the fuel jet is directed. This bulb is heated to
a dark cherry red by means of a torch until the engine has developed
enough heat to keep the bulb at the requisite temperature.
THE DOUBLE-ACTING JUNKER ENGINE
[Illustration: FIG. 52.--THE JUNKER ENGINE IN WHICH TWO PISTONS
RECIPROCATE TOWARD AND AWAY FROM EACH OTHER]
An interesting modification of the Diesel engine is the Junker engine
(Figure 52) in which the cylinder consists of a tube open at each end.
In this there are two pistons which reciprocate toward and away from
each other. Air is compressed between them as they approach each other
and fuel is injected into this air, ignites and forces them apart.
Both pistons are connected to the same crank shaft, one pushing down
and the other pulling up. The pulling piston has a yoke on the end of
the piston rod from which a pair of connecting rods run down at either
side of the cylinder to a pair of cranks on the crank shaft. Between
these cranks on the opposite side of the shaft is the crank to which
the pushing piston is connected. The advantages of this arrangement are
that the moving masses are perfectly balanced, the construction of the
cylinder is very simple and especially adapted to high pressures, and
the reaction of the gases, instead of being directed against a fixed
part of the engine, is directed against a moving piston, thus reducing
the strain on the structure. Of course the power is not doubled or
increased, because each piston moves only half as far as it would for
a given expansion of gas were it operating in a cylinder closed by a
cylinder head.
The efficiency of Diesel engines, although greater than that of the gas
and gasoline engines, is still very low. The semi-Diesel will yield
about 30 per cent of the energy in the fuel, the Junker engine about 34
per cent, and the best Diesel, four-cycle engine in large units, about
36 per cent. This is a wonderfully high efficiency compared with that
of a locomotive, and yet it seems pitiably low when we consider that
nearly two-thirds of the energy stored in the fuel is thrown away.
SILENCING THE NOISY EXHAUST
The waste of energy is clearly evident in the exhaust pipe of an
internal combustion engine. The gases after doing their work on the
piston rush out with such velocity as to produce a sharp explosive
sound. The noise of the exhaust is highly objectionable in automobiles
and must be overcome, but instead of utilizing the boisterous energy of
the escaping gases and getting a little more useful work out of them,
means are provided for hushing their noise. This is done by passing
them through a series of baffles which reduces the pressure of the
gases before they are discharged into the atmosphere. By letting the
gases expand gradually instead of bursting suddenly into the atmosphere
the noise of the discharge is reduced. However, this must not be done
at the expense of the engine power. Unless the gases pass quickly
through the muffler they will choke and retard the exhaust and make
the engine do useless work in driving them through. Even the best of
mufflers will use up nearly 5 per cent of the engine power. The energy
that escapes at the exhaust is not the only loss. All the heat that
radiates from the engine represents just so much wasted energy. In
our water and air-cooled systems we deliberately abstract heat from
the burning gases and throw it away. If we had materials that would
stand the intense heat of burning gases and enable us to conserve all
the heat developed in the cylinder we could use our fuels much more
economically.
Many attempts have been made to utilize the wasted heat of internal
combustion engines, but they have not met with any considerable degree
of success, with the exception of the invention of William Joseph
Still. This engine, which was the result of many years of patient work,
was first made public in a paper read before the Royal Society of Arts
in London, May 26, 1919, and when it was shown that the new engine had
developed an efficiency of 38 to 41 per cent it was realized that here
was a remarkable advance over any other machine for turning heat into
power.
COMBINED STEAM AND OIL ENGINE
The Still engine is a combined steam and gas or oil engine. The heat
that is ordinarily thrown away in the water jacket and in the exhaust
is utilized to produce steam, and this steam is employed to return the
piston to the top of the cylinder after the gases generated by the
combustion of fuel have driven it down. While in previous experiments
some steam had been developed from the exhaust gases it had not been
found possible to generate steam in the water jacket because of the
low temperature that had to be maintained. In the standard internal
combustion engine the cylinder walls are made of cast iron, thick
enough to withstand the heavy pressures to which they are subjected
should the charge be ignited prematurely. Around them the water
circulates under practically no pressure and so the wall of the water
jacket is made comparatively thin. Because of the thickness of the
cylinder wall the water has to be maintained at a comparatively low
temperature so as to keep the interior of the cylinder from growing
too hot. In the Still engine the cylinder wall is from one-third to
one-fourth the thickness of the standard cylinder wall, while the
water-jacket wall is of thick steel. The cylinder wall is formed with
ribs which extend to the water-jacket wall, so that the latter will
take care of any excessive loads due to premature ignition. Figure 53
shows how the cylinder and jacket of the Still engine compare with
those of the ordinary motor. The thin cylinder wall and the ribs
furnish a far better conduction of heat to the water which circulates
between the ribs. The necessary cooling of the cylinder can be
maintained with water at a much higher temperature than in the ordinary
engine.
[Illustration: FIG. 53.--SECTION THROUGH THE CYLINDER AND WATER JACKET
OF THE STILL ENGINE]
[Illustration: FIG. 54.--DIAGRAMMATIC VIEW OF THE STILL ENGINE]
A diagrammatic representation of a Still engine of the two-cycle
heavy-oil type is shown in Figure 54. This is shown with an auxiliary
boiler heated by an oil burner. The piston of this engine has a sleeve
which fits into an annular steam cylinder. The latter is an extension
of the combustion cylinder. Surrounding the steam and combustion
cylinders are the steam and water jackets. The drawing shows the piston
at the end of its downward stroke, having just been driven down by
the gases of combustion. In this position a scavenging-air port at
the left is uncovered and the burnt gases are swept out by a blast of
air, passing out of the exhaust. This consists of a bank of tubes or
flues which pass through a water heater. The gases leave the cylinder
with a temperature of 900 degrees Fahrenheit and they issue from the
exhaust with a final temperature of only 150 degrees Fahrenheit. Steam
now enters the annular steam cylinder from the steam jacket, through
the port shown at the lower right-hand side, forcing the piston up and
compressing the charge of air in the cylinder. Then oil is sprayed in
through a nozzle, not shown in the drawing, and is ignited by the heat
of the compressed air as in a Diesel engine and the piston is driven
down again by the gases of the combustion. The water surrounding the
combustion cylinder comes from the lower part of the boiler and flows
by the exhaust flues. It is maintained at a temperature of 350 degrees
in the water jacket, and here is turned into steam by the heat of the
combustion. Thence it passes into the steam dome of the boiler. On
the downward stroke of the piston a slide valve, shown at the lower
right-hand side, moves down and connects the steam port with an exhaust
port through which the steam flows out into a condenser. Water from the
hot well of the condenser enters the system at the combustion exhaust.
The arrows show the course of the steam and water and the temperature
at various points is given. As may be noted there are two systems of
steam and water circulation: one from the bottom of the boiler through
combustion exhaust heater to the water jacket and back to the top of
the boiler; and the other from the steam dome of the boiler to the
steam jacket, to the steam cylinder, to the steam exhaust, to the
condenser and back again from the condenser into the combustion exhaust
heater, whence it enters the boiler by way of the water jacket.
LATENT HEAT
When water turns into steam a certain amount of heat is absorbed, which
does not show in the thermometer. This is known as latent heat. If
cold water is placed in a kettle and is then heated the thermometer
will gradually rise until the water reaches the boiling point. Then
there will be no further rise of temperature, although heat is still
applied to the kettle, until all the water is turned into steam, after
which the thermometer will begin to show a rise of temperature, showing
that the steam is beginning to grow sensibly hotter. In the engine,
shown in Figure 54, a boiler pressure of 120 pounds gauge pressure
is maintained, and therefore water will not boil until it reaches
350 degrees Fahrenheit. The water in the water jacket registers 350
degrees and is therefore far hotter than it could possibly be in the
open atmosphere. However, the heat from the combustion does not raise
the temperature of the water, but expends its energy in converting
the water into steam. The cylinder wall is kept at a temperature
of at least 350 degrees all the time, so that the air that enters
the cylinder gathers heat from the cylinder as well as from its own
compression, and its temperature at the end of the compression stroke
is higher than it would be in a cold cylinder, thereby insuring the
ignition of the fuel when it is sprayed in.
The Still engine can also be used as a common gasoline or gas engine of
either two or four cycle and the efficiency in such types is from 31 to
33 per cent. This is much better than the best airplane engines, which
show an efficiency under 27 per cent. Although 41 per cent for the best
Still heavy-oil engine is a remarkable accomplishment, yet it does not
begin to compare with the efficiency of the best water turbines and
Pelton wheels, which turn into useful power from 75 to 87 per cent of
the kinetic energy in the water that drives them.