COOLING
To enable the parts of a motor to work well, there must be freedom of
motion between all that move in contact with each other. This necessary
freedom of motion is provided for to a certain extent by proper
lubrication, but this is not all-sufficient. The necessity for some
additional friction- and heat-reducing system can be better realized
when it is understood that the temperature of the explosion in the
cylinders of a gasoline engine is well over 2,600 degrees, Fahrenheit.
The melting point of pure iron is less than 2,800 degrees. Therefore
were there no escape for this heat, and could the motor be induced
to run under these severe conditions, the cylinders would soon reach
a temperature dangerously near the melting point. Long before this
point could be reached, however, the intense heat would have expanded
the pistons so that they would become stuck in their cylinders, and
no more explosions could occur. An ominous knock in one or more of
the cylinders, followed by a sudden laboring and final cessation of
operation on the part of the motor, is sometimes the first intimation
that the driver may have that his engine is over-heated; but serious as
a "stuck" piston may seem, it is fortunate that the motor stops of its
own accord, for to continue to run under these conditions of constantly
increasing heat would be to wreak far more serious and permanent damage
upon the moving parts than the broken rings or scored cylinders that
usually result from a lack of lubrication or cooling medium.
A large amount of the heat resulting from each explosion is carried
out through the exhaust pipe in the form of the burned gases, while
other portions radiate into the surrounding air. These outlets are
not sufficient, however, to carry away all the heat that is necessary
to enable the motor to run efficiently, for proper piston lubrication
is exceedingly difficult to obtain at high temperatures. There must,
therefore, be more positive and direct means for carrying off this
undesired heat, and to accomplish this result every internal combustion
motor is provided with a cooling system of either the air or liquid
(usually water) type. Motorcycle power plants and a few of the small
and medium-sized automobile engines employ the air-cooling system; the
great majority of automobile engines, stationary plants, and marine
motors use water as the cooling medium.
Let us consider first the air-cooled system. The area presented
by the outside of a smooth cylinder is not large enough to enable
sufficient radiation to take place. That is, the heat is concentrated
on a comparatively small surface, and this is much more difficult to
keep cool than is the same amount of heat distributed over a greater
area--for the cylinder will be exposed to a larger quantity of fresh
air in the latter case. Therefore many air-cooled engines are provided
with a series of grooves and flanges on the outer surface of the
cylinder. The heat is conducted to all parts of this surface--flanges
as well as grooves--and the area of the surface that is exposed to the
cooling air is greatly increased thereby.
These grooves and flanges may extend circumferentially around the
cylinder, as is the case with many motorcycle engines, or they may
extend longitudinally. Another form of air-cooling system consists of
pins or spines projecting radially from the surface of the cylinder.
The motion of the car through the air is generally sufficient to
create a circulation of the cooling medium, but in order that this
circulation may continue while the car is at rest a high-speed fan
is provided that draws the air from the front toward the rear of the
motor. This serves also to supplement the air circulation produced by
the motion of the car, and keeps the motor much cooler than would be
the case were the machine run without the fan. This fan is generally
attached to a bracket at the front of the motor, and is driven either
by a belt or geared shaft. In some designs, however, the fan blades are
included in the flywheel at the rear of the motor and the air is thus
sucked over the cylinders.
One of the most effective air-cooling systems for use on an automobile
motor consists of the above-mentioned longitudinal flanges and grooves
enclosed in a thin jacket or casing surrounding each cylinder.
These jackets are open at the top and bottom of the cylinders, and
connect with large pipes, or troughs, through which air is forced.
The trough into which the top of the jacket spaces open is connected
with the discharge end of a large fan. The air is thus driven into
the top trough, through each jacket, and into the lower trough, the
farther termination of which is connected with the suction end
of a fan included in the flywheel. The two fans serve to set up a
rapid circulation of air which, by means of the troughs and jackets,
is concentrated upon the surfaces of the grooves and flanges of
each cylinder and none is wasted on parts of the motor that it is
unnecessary to cool. Furthermore, the rear cylinders receive as much
air as do the forward ones, for the trough serves to distribute the
circulation equally along the grooves and flanges of each.
Inasmuch as the heat from an air-cooled motor is radiated directly
into the current of air itself, the surface is very susceptible to
temperature changes from the interior. Thus, if the car is run for
a great distance on the low gear, and the cylinders become hot in
consequence, a larger amount of heat will immediately be radiated from
the cooling surfaces than is the case when the motor is running slowly.
A "coast" down a short hill, however, will serve to cool the motor
rapidly, for if the engine is run from the momentum of the car with
the spark turned off, cool air will be drawn into the cylinders, and
this, in addition to the circulation of cold air on the outside, will
reduce the temperature of the engine rapidly. This is a feature of the
operation of an air-cooled motor that is not possessed to so large an
extent by those of the water-cooled type.
It is, perhaps, hardly accurate to apply the term "water-cooled" to
the ordinary type of automobile motor. Water is merely the medium
that transfers the heat from the cylinders to the cooling surface of
the radiator. As air is used to cool this heated water, we see that
the only difference between the two systems lies in the point of
application of the actual heat-absorbing medium--which is air in both
cases. Thus in the air-cooled motor the air is carried directly to the
surfaces to be cooled; while in the other type, the heat is transferred
by means of the water to the point where it may be effectually
discharged into the air.
Each cylinder of a water-cooled motor is surrounded by a space known
as the water jacket. This space is generally cast with the cylinder,
although in some designs of motors the jackets are formed by the
subsequent application of a copper casing that serves to retain the
water. The water jackets are connected with each other by means of
piping and water-tight joints so that the water will pass successively
from one to the other. If the water remained in these spaces, it would
soon be warmed to a temperature far above the boiling point, steam
would be formed, a high pressure generated, and infinite harm would
result--both to motor and to passengers. The piping, however, does not
end with the connections between the cylinders, but extends to and from
the radiator.
This radiator is a large, perforated structure placed either forward of
the motor to form the end of the bonnet-covering, or in front of the
dash between it and the rear cylinder of the engine. The radiator is a
mass of small cellular or tubular passages, each one of which possesses
an exceedingly large outer surface in proportion to the amount of water
that it can contain. When the hot water reaches the radiator it is
distributed to these many cells or tubes, and is thus spread over a
large cooling surface. A large fan is usually located directly behind
the radiator, and as this serves to draw the air rapidly through the
openings between the cells or tubes, cooling is greatly facilitated.
There are several types of radiators in general use. Some consist of
a number of flat cells placed in such a manner that regular-shaped
air openings will be formed. Each side of each flat water cell abuts
on an air passage. Such a radiator is known as the honeycomb, or
cellular, the former term being applied to those whose cells resemble
a honeycomb. The tubular radiator consists of a number of vertical,
parallel tubes through which the water passes, and which are placed
a sufficient distance apart to provide ample air passages between
them. Each tube is covered at frequent intervals with fluted, circular
flanges that serve to increase the radiating surface in much the same
manner as do the grooves and flanges on the cylinders of the air-cooled
motor. All air passages in any radiator extend directly through the
width of the radiator, while the water circulates from top to bottom in
a vertical direction.
The reason for this circulation of the water will be apparent if we
call to mind a bit of our elementary physics. When water is heated, it
expands and rises, and for this reason, we always find the surface of
the water in a teakettle warmer than is that at the bottom--although
the latter is closer to the fire. As the water is circulated through
the radiator, it is cooled by the passage of the large amount of air
through the openings between the cells or tubes. The water thus cooled
sinks to the bottom of the radiator and is replaced by the water
just heated by the motor. The cooled water is conducted to the bottom
portion of the end cylinder, and passes to the others in succession,
gradually rising as it is heated, until it is again forced to the
radiator at the top.
There are two methods of circulating the water through the cylinder
jackets and radiator. The most common method consists of the
introduction of a pump in the lower portion of the circulating
system. In the case of automobile motors, this pump is driven by
gears connected with the crank shaft of the engine. Such a pump will
be either of the gear or centrifugal type, and will suck the cooled
water from the lower portion of the radiator, and force it through the
jackets. The second method is known as the thermo-syphon system because
the circulation is automatic and depends upon the cooling of the water
in the radiator. When the cooled water sinks, a syphon action is formed
that tends to draw the hot water from the cylinder jackets, and the
automatic circulation will thus continue as long as the successive
heating and cooling take place.
Inasmuch as the pump is driven by the crank shaft of the engine, its
speed will be proportional to that of the motor. The same holds true
of the fan that serves to draw the air through the radiator. It will
thus be seen that both the water and the air are forced at a more rapid
rate when the motor runs at high speed, and that therefore the extra
heat generated by the more frequent explosions in the cylinders will be
counteracted to a certain extent. The increased number of explosions
and the higher speed at which the fan turns also cause quicker heating
and cooling of the water by the thermo-syphon system, thus forming a
more rapid circulation. Inasmuch as the force exerted upon the water by
its cooling and heating is not as great as that formed by a high-speed
and efficient pump, the pipes and connections of the thermo-syphon
system must be of ample size in order to keep the resistance to the
passage of the water as low as possible. Care must also be taken in the
design of this system so to construct and connect the pipes and jackets
that the hot water will be allowed to rise and the cool to descend,
and thus to make possible the syphon conditions on which principle the
circulation is based.
The ability of the radiator to carry off the heat from the water
depends upon the rapidity with which the air passes through the
passages provided for the purpose. The amount of air passing through
is determined by the speed of the suction fan and the rapidity of
travel of the car itself against the wind. It has been shown that, when
the motor runs at a high number of revolutions, the fan turns faster
and the rapidity of circulation is increased. But if the car itself
does not increase its speed in proportion to the higher revolutions of
the motor, the maximum amount of air will not be forced through the
radiator passages, and the excess heat will not be carried off entirely
from the cylinders. This is a condition that prevails when the motor
is run on low gear. The speed of the motor is increased, while that of
the car is reduced; additional heat is generated in the cylinders, but
the speed of the air is not increased in proportion. Therefore a motor
that is driven a long distance on the low gear will have a tendency to
overheat.
Water under atmospheric pressure cannot be brought to a temperature
above 212 degrees Fahrenheit without being converted into steam.
Therefore, when the heat from a water-cooled motor cannot be carried
away sufficiently fast, the water in the circulating system will begin
to boil. As long as water remains in the jackets, the temperature of
these spaces cannot well rise above 212 degrees, and consequently there
is small danger that a water-cooled motor will become overheated to
the point at which the pistons will "seize" in the cylinders. The
moment the water in the circulating system begins to boil, however,
exceedingly rapid evaporation naturally takes place, and the water will
soon entirely disappear in the form of steam and vapor. To run the
motor under these conditions will mean that pistons and rings will soon
become stuck in their cylinders, although liberal quantities of oil
will sometimes delay this inevitable result.
But even when the cooling water is not brought to the boiling point
there is a vapor that is constantly dispelled from it whenever its
temperature is brought above that of the air. The water system of
an automobile must therefore be replenished at irregular intervals,
depending upon the amount and nature of the running to which the car
has been subjected. The older cars were provided with an extra water
tank, generally located under the seat, and connected directly with
the water jackets and the radiator. The usual water-cooling system of
the present-day car, however, is self-contained--that is, there is no
separate tank for the storage of the water. The water is poured into
the top of the radiator, and from this high point it reaches every part
of the circulating system. Whenever the radiator will accommodate a
couple of quarts, or more, it is well to fill it, for _too much_ water
_cannot be used_ on the modern design of cooling system. It is true
that a motor runs at its highest efficiency when its temperature is
as great as that at which proper lubrication of the pistons can be
obtained--for a gasoline engine is a "heat engine," and the greater
its unnecessary heat losses, the less will be the power developed by
it. But a motor cannot be kept at the proper temperature by reducing
the amount of cooling water in its circulating system. The best method
is to lessen the rapidity with which the water is cooled, and this
may be accomplished by placing a leather flap, a cardboard, or other
obstruction over a portion of the radiator to reduce the number of
openings through which the air may pass. It should only be necessary
to do this in the coldest weather, however, for the cooling system
of every motor is designed to maintain the proper temperature on all
except the hottest or coldest days.
It has been stated in a preceding paragraph that continued running on
the low gear is the most frequent cause of overheating a motor. This
is true, but it is not the only cause. Obstructions in the circulating
system that reduce the flow of water will have this effect, as will
also deposits on the interior of the cylinders that serve to prevent
the proper transfer of heat to the water in the jacket spaces. Removal
of the carbon will remedy the latter trouble, but to clear out the
circulating system is more or less of a complicated matter. Stoppage in
the pipes or radiator cells may be caused by a lime deposit from "hard"
water that may have been used in the circulating system. There are
preparations intended to remove this deposit, but such should not be
used without first advising with the maker of the car or an experienced
repair man. A series of battered cells in the radiator may reduce the
number of cooling spaces that should be traversed by the water, and
thus the hot water cannot be distributed over as great an air area as
is necessary to maintain the motor at the proper temperature. Such a
condition will be apparent from a marked difference in temperature
between the affected portion of the radiator and the remainder. If a
deposit has been formed on a certain series of cells, or if they have
been obstructed in any other manner, the hot water cannot circulate
through this section of the radiator, and it will remain comparatively
cool.
Water is a liquid that remains in its fluid stage only through a
temperature range of 180 degrees--at atmospheric pressure. At 212
degrees it boils and turns to vapor, while at 32 degrees it freezes and
becomes a solid. In neither of these stages does it form a desirable
cooling medium for a gasoline motor. Of the two, however, its solid
stage is the more harmful to the motor. Not only will it cease to
flow when it becomes ice, but the expansion of the water during the
formation of the solid is liable to burst its retainer--whether it be
the cells of the radiator, the pump, pipes, or even the cylinder walls
themselves. It is the radiator that is the most liable to suffer from
such a cause, however, for each cell contains so small an amount of
water that the liquid will be brought to the freezing point before the
larger volume in the jacket spaces approaches this temperature. Of
course the water will be kept well above the freezing point when the
motor is running, and it is only when the machine has stood idle for
several hours that care must be taken to prevent the formation of ice
in the circulating system.
Aside from keeping the car in a warm place whenever the motor is to be
at rest more than two hours, there is only one method of preventing
the cooling water from freezing, and that is by the introduction of
some chemical that lowers the point at which the liquid will turn to a
solid. There are several ingenious heaters available that are attached
to the circulating pipes and that serve to keep all of the jacket water
warm; the use of these producing the same conditions as though the car
were kept in an artificially-heated garage.
One of the most common liquids used in the cooling water to prevent
freezing is alcohol. If equal parts of wood alcohol and water are used
in the cooling system, the resulting mixture will not freeze until
it reaches a temperature colder than 25 degrees below zero. A weaker
mixture--one having 25 per cent. of wood alcohol--will freeze at
about zero, and it therefore depends upon the prevailing cold-weather
temperature as to the proper proportion that should be used. It must
be remembered that the boiling point of alcohol is much lower than is
that of water, and that therefore a mixture that will not freeze in
exceedingly cold weather is liable to boil away on the first moderate
day on which the car is run. The above-mentioned 50 per cent. mixture
of wood alcohol and water will boil at 135 degrees, while the 25 per
cent. solution will withstand a temperature 40 degrees higher before it
is transformed into vapor. As the lower temperature will be reached
easily if the motor is run for some time in comparatively moderate
weather, it will be seen that the stronger mixture should be used only
where winters are very severe. It must also be borne in mind that,
as alcohol boils more readily than does water, it follows that it
will evaporate more easily, as well. Therefore, in order to maintain
a uniform proportion of wood alcohol to water, the former should be
replenished more often than is the latter.
Glycerine is another substance that is often mixed with the cooling
water to prevent the latter from freezing. A 50 per cent. mixture of
this and water has a freezing point of about zero, or slightly lower,
and boils at practically the same temperature as water--210 degrees.
Combinations of wood alcohol and glycerine may be used--equal parts of
each being the usual proportion--and thus various freezing and boiling
points may be obtained.
The radiator is one of the most delicate parts of the motor car's
construction, and yet it is the most exposed to flying sticks and
stones that may be thrown up by the rapid travel of the car. The
car owner may do well to follow the practice of many racing drivers
who place a heavy wire mesh screen in front of the radiator as a
protection against obstacles that may be struck by the front of the
car. It would seem that sticks and stones would be thrown toward the
rear of the car, and would therefore avoid the radiator by a wide
margin, but experience has proved that, at high speed, such loose
pieces are frequently forced forward and are _run_ into by the front of
the car.