COOLING
We have already explained the necessity for cooling the cylinders of a
petrol engine by means of a water-jacket, and we now proceed to show
how the circulation system may be arranged. There are two forms of
circulation in use: (1) Natural; (2) Forced.
[Illustration: FIG. 60.—THERMO-SYPHON WATER COOLING SYSTEM.]
=Natural or Thermo-Syphon Circulation.=—This system is shown in
Fig. 60, and may be explained as follows:—The heat generated by the
successive explosions within the cylinder causes the water at the
top of the cylinder jacket A to get hot. As a column of hot water is
lighter than one of cold water of equal height, the heated water rises
up the pipe B and flows into the top of the radiator D, while colder
water from the bottom of the radiator flows up the pipe C and into the
cylinder jacket A. It is important that the height of the water in the
radiator D should be at such a level that the outlet from the pipe B is
submerged.
[Illustration: FIG. 61.—FORCED WATER CIRCULATION BY MEANS OF A PUMP
(P).]
In the radiator the water falls through a series of tubes E, having
_gills_ or fins on the outside for the purpose of dissipating the heat.
The cooling of the water is also assisted by the fan F, which is driven
from the fan pulley G and draws air past the radiator tubes at high
speed. Sometimes the water in the radiator is made to fall through a
series of cells which are formed of cast aluminium; such a radiator is
called a _honeycomb_ radiator. It is important that the pipe C should
not have any sharp bends and it should not rise very much in height,
but the outlet pipe B may have a considerable rise with advantage. Both
the inlet and outlet pipes should be of large diameter with this system
of circulation, and the radiator should be so arranged that there is
a good _head_ of water above the cylinders. In the drawings H is the
front cross-member of the chassis, K is the starting-handle clutch, and
L is the starting handle.
=Forced or Pump Circulation.=—With this system the water is positively
circulated through the jackets; it is drawn from the bottom of the
radiator by the pump P (Fig. 61), which is mechanically driven from
the valve shaft of the engine, and delivered under pressure to the
jacket A. The outlet of the pipe B need not be drowned, and the pipe C
may be arranged in any way most convenient to the chassis. Sometimes
when a pump is fitted the pipes are arranged so that the system may be
operated as a thermo-syphon in the event of a breakdown of the pump. It
is not uncommon to experience trouble due to leakage at the pump gland,
which results in gradual loss of water from the system, and therefore
the thermo-syphon or natural circulation has much to recommend it. Also
it may be said that the pump represents an additional _complication_
to the engine and means increased first cost. Every moving part we add
to the engine is of course an additional potential source of trouble,
but the addition of a really first-class water circulating pump of the
type shown in Fig. 58 cannot be said to be anything but a reasonable
precaution. The weight and size of every part of a motor-car engine and
chassis have been so much reduced recently, owing to competition with
American firms, that many manufacturers who adopted the thermo-syphon
principle experienced great trouble with it owing to the small size
of radiator fitted, as well as faulty arrangement of the connexions.
Considering any one engine, it follows that if a certain size of
radiator and a given quantity of water in the circulating system
will keep the engine cool when a pump is used to give a positive
circulation, then a larger radiator and greater quantity of water will
be required for natural circulation. Thermo-syphon circulation also
means a _high_ radiator and bonnet, which many people object to on
the score of appearance, without considering its utility. With natural
circulation greater care must be exercised to keep the radiator well
filled, but this often leads to other difficulties on bad roads owing
to the water splashing from the overflow pipe and finding its way on to
ignition appliances. Before starting an engine it is always advisable
to remove the radiator filling cap and examine the water level; if
it should happen that at any time while the engine is running the
circulating system runs quite dry, owing to a breakdown or leakage,
do not attempt to pour water into the radiator, but simply raise both
sides of the bonnet and leave the engine to cool down first. Again,
when filling the radiator for a forced circulating system, it is
desirable to give the engine a turn or two with the starting handle
occasionally to operate the pump and prevent air locks; very often the
radiator appears to be full, but as soon as the engine commences to run
the water disappears owing to the system not being full, due to the
above-mentioned cause. In cold or frosty weather all the water should
be drained off from the circulating system when the car is in the
garage, unless the garage is heated or some anti-freezing solution is
used. Glycerine or alcohol added to the water will prevent it freezing,
but as an additional precaution in cold countries one often sees
travelling rugs strapped over the radiator and bonnet.
Occasionally one gets trouble due to the water boiling in the jackets,
and on this account reasonable care should always be exercised in
unscrewing the radiator filling cap if the presence of steam is
suspected. An engine may have been running well for a long time without
trouble and then develop symptoms of overheating in the circulation
system. This overheating may be either _local_ or _general_. Local
overheating may result from some partial seizure of the piston in the
cylinder due to dirt on the walls, or from the presence of grease on
the _outside_ of the cylinder walls, in the jacket space. If grease is
suspected or there is _furring_ up in the passages of the jacket due
to bad water supply, the trouble may be cured by adding some common
washing soda to the water in the radiator and running the engine with
the car at standstill for half-an-hour or so. After this drain off all
the water and sludge, allow the engine to cool down, and then fill up
again with clean water.
General overheating may result from leaky pistons and pistons rings,
or from the use of _too weak_ a mixture in the carburettor, or from
overloading the engine. If the mixture supplied to the engine is very
weak, the overheating will be very marked on the _exhaust side_ of the
engine. Local overheating causes the engine to “_knock_” badly.
In arranging the jackets and the pipes care must be taken to arrange
that a cock is placed at the _lowest_ point in the system, so that
the whole may be completely emptied, and the inlet pipe to the jacket
should enter at the very bottom of the jacket chamber for the same
reason. It may be thought that all that is necessary is to provide
plenty of space in the jackets round the cylinders and plenty of water
in the whole system, but experience shows that it is very important
not to make the jacket space _too large_, so as to ensure positive
circulation and avoid _local circulation_ in any one portion of the
jacket. When cylinders are cast in pairs the back pair have a tendency
to discharge their hot water into the front pair and so back to the
inlet pipe again, hence this should be guarded against in arranging the
outlet pipes.
Pipes suitable for use with multi-cylinder engines are shown in Fig.
62, in which (_a_) is an outlet pipe for a monobloc casting, and
(_b_) and (_c_) are inlet and outlet pipes respectively for engines
having separate cylinders. It is advisable to modify the diameter of
the branches by the insertion of metal orifice plates at the flanges
to ensure an equitable distribution of the water among the several
cylinders.
[Illustration: FIG. 62.—FORMS OF WATER PIPING.]
The weight of water carried in the circulation system for a fifteen
horse-power engine would be about 30 lb. with pump circulation, whereas
60 lb. would be required for thermo-syphon cooling. It is not desirable
to cool the engine too much. The jacket water temperature may be
allowed to reach 180° Fahrenheit at full load, but if this is exceeded
there is liability to boiling. Given two similar engines of equal power
and equally loaded, one of which was operated with a jacket temperature
of 100° Fahrenheit and the other at 180° Fahrenheit, the hotter engine
would show a gain in economy of from five to ten per cent. in fuel
consumption. In considering the type of radiator to adopt, one would
not recommend the honeycomb variety (except for appearance) owing to
the difficulty of cleaning the passages after it has been in use some
time; and the gilled tube would be more efficient than the plain tube.
The amount of tube required depends of course on its diameter, but a
rough approximation would be twelve feet of gilled tube or eighteen
feet of plain tube (of half-inch diameter) per brake horse-power.