THE CARBURETTOR AND CARBURATION
A carburettor is a contrivance for supplying an explosive mixture of
air and petrol vapour to a petrol engine. Petrol, although a liquid
fuel, is a combination of carbon and hydrogen which, when supplied with
the necessary air, can be burnt and thus evolve heat, which heat is
turned into work inside the engine cylinder. What we have to supply to
the engine is really a mixture of _air_ and _petrol vapour_ in certain
proportions, such a mixture being often spoken of as _carburetted
air_ on account of the carbon contained in it. About two parts of
petrol vapour (by volume) are required to every one hundred parts of
mixture, or fifteen _pounds_ of air to every pound of petrol vapour
(by weight). This carburetted air must be of the required strength and
form a homogeneous mixture in the form of a vapour. The problem of
_carburation_ consists in forming a mixture of the correct strength
and character. Air may be carburetted by passing it over the surface
of liquid petrol in a _surface carburettor_, or by drawing it over
or among wicks saturated with liquid petrol as in the _wick type of
carburettor_, but both these methods have been largely superseded by
the use of what is now known as a _jet_ or _spray type_ of carburettor,
in which the petrol is sprayed from a fine jet and mixes with air which
is passing up rapidly round the outside of the jet. In all cases,
however, the liquid petrol must be _vaporized_ before entering the
engine, and to do this _heat_ must be supplied to the mixture, just
as water has to be heated before it can be vaporized and turned into
steam. Under ordinary circumstances sufficient heat can be obtained
from the incoming air to effect vaporization of the liquid petrol if
it issues in the form of a very finely divided spray, but when the
demand for mixture, from the engine, is great the air cannot supply the
requisite heat without its temperature falling below the vaporization
point; hence most carburettors of up-to-date pattern are fitted with
a _mixing chamber_ surrounded by a hot-water jacket. The essential
features of the carburetting plant are shown diagrammatically in Fig.
38, in which A is the _petrol tank_ fitted with the _petrol tap_ G, to
which is coupled the _petrol pipe_ F. Some form of _petrol filter_ as
indicated at B should be placed between the tank and the carburettor
C. The throttle valve of the carburettor is shown at H, the _extra-air
valve_ at E, and the engine induction pipe at D.
[Illustration: FIG. 38.—GENERAL ARRANGEMENT OF THE CARBURETTING PLANT,
SHOWING PETROL TANK (A), PETROL FILTER (B), CARBURETTOR (C), AND
EXTRA-AIR VALVE (E).]
The carburettor proper may be constructed in a variety of forms, but
the elements of which it is composed are: (1) the float chamber A, (2)
the petrol jet B, (3) the choke tube C, (4) the mixing chamber D, and
(5) the throttle valve E, as shown in Fig. 39.
[Illustration: FIG. 39.—SECTIONAL DRAWING OF A CARBURETTOR OF THE _JET_
TYPE.]
=The Float Chamber= is generally cylindrical in form and the liquid
enters at the bottom, the flow being regulated by a pointed rod called
a _needle valve_. A hollow metal float which can slide freely up and
down the needle valve stem operates two levers which are pivoted on
the float chamber cover. It is well known that when a body is immersed
in a liquid the liquid exerts an upward pressure on the body equal to
the weight of liquid displaced by the body. The float being hollow
and made of very thin sheet metal, displaces a very large quantity
of liquid in proportion to its own weight, and is therefore very
_buoyant_. The buoyancy of the float will, of course, depend on the
_density_ of the liquid in the float chamber, and it will naturally
sink deeper down into petrol than it would into a heavier spirit such
as paraffin or benzol. The action of the float is as follows:—Supposing
the petrol to be turned off and the needle valve lifted up off its
seating, then on turning on the petrol supply the petrol will run into
the float chamber, and as the level of the liquid rises the float will
rise too, lifting up the outer ends of the levers and depressing the
needle valve down on to its seating by means of the collar which is
rigidly attached to the spindle of the needle valve. If at any time the
level of the liquid in the chamber falls, the float will fall also,
thus allowing the outer ends of the levers to drop and raise up the
needle valve from its seating; this allows more petrol to enter the
chamber and raises the float again, thus keeping a constant level in
the chamber.
The height of the orifice in the top of the petrol jet above the bottom
of the float chamber determines the height at which we require the
liquid to stand in the chamber. As a general rule the level of the
liquid in the float chamber should be slightly below the top of the jet
orifice to prevent the liquid oozing over and causing _flooding_ or
continuous dripping of petrol from the jet, even when the engine is not
running. The height of the collar on the needle valve spindle must be
adjusted until the float closes the valve down on its seating when the
liquid has risen to the desired height in the float chamber. Hence, if
a carburettor has been adjusted to work with petrol, it will require
to have some slight extra weight added to the float when working with
heavier spirits to cause it to sink to the required depth in these
denser spirits.
=The Petrol Jet and Choke Tube.=—The _petrol jet_ generally consists
of a short tube of fine bore, one end of which contains a very small
orifice for the purpose of spraying the petrol into the choke tube.
When the engine is at rest it is easily seen that the pressure of the
air in the choke tube is atmospheric, and that the pressure above the
liquid in the float chamber is also atmospheric, but when the engine is
running it draws air up the choke tube at a very high speed and thus
causes a partial vacuum round the petrol jet, and therefore the petrol
spurts out of the jet under the pressure difference which then exists
and issues in the form of a fine spray which is readily vaporized. The
choke tube is purposely made of rather small diameter, in order to
get a high air speed, which results in a low pressure round the jet
and ensures a good driving force to spray the petrol out of the jet.
The speed of the engine is controlled by the position of the throttle
valve or disc E, which regulates the amount of air flowing up the choke
tube, and therefore incidentally checks the quantity of petrol issuing
from the jet by regulating the vacuum in the neighbourhood of the jet
orifice. At low engine speeds there is very little suction or vacuum
effect on the jet, but at high engine speeds with full throttle opening
the maximum suction of the engine is exerted upon the jet. Thus at
low speeds with this type of carburettor we do not get enough petrol
out of the jet, and at high speeds we get too much, which results in
_too weak_ a mixture at low speeds and _too rich_ a mixture at high
speeds. One reason for this is that the air flows out of the choke
tube _faster_ than it flows into it, owing to the fact that its volume
increases as the pressure decreases, and hence the pressure round the
jet falls very rapidly indeed as the air velocity increases and causes
too much petrol to issue from the jet in proportion to the quantity of
air flowing through the tube. The _choke tube_ is often a plain piece
of pipe, as shown in Fig. 40, instead of being tapered as in Fig. 39.
=The Mixing Chamber and Throttle Valve.=—The throttle valve is usually
a plain flat disc of metal mounted on a spindle which can be rotated
and thus regulate the size of the air passage to the engine. It is
placed above the petrol jet and situated in the mixing chamber, which
is simply a short length of pipe (of the same bore as the engine
induction pipe) surrounded by a hot-water jacket, the supply of hot
water being drawn from the engine cooling system. The heat from this
jacket should be sufficient to make up for the fall in temperature
that would otherwise result due to the vaporization of the petrol as
explained above.
[Illustration: FIG. 40.—PLAIN FORM OF CHOKE TUBE.]
=Recent Improvements in Carburettors.=—Another defect of this simple
type of carburettor becomes apparent in the larger sizes required for
multi-cylinder engines. To pass the requisite quantity of petrol to
keep the engine running at high speeds without creating too great a
suction effort and thereby hampering the engine, necessitates the use
of a jet of larger calibre, so that the liquid is no longer _sprayed_
but issues in the form of a fine stream which is not readily vaporized.
This has been overcome by the use of _multiple-jet carburettors_
which have several jets each surrounded by its own choke tube, but
all controlled by one throttle valve and supplied from one common
float chamber. In this case the total cross-sectional area of all the
jet orifices together could be made sufficient to pass the necessary
quantity of fuel, but the bore of each individual jet orifice would
be comparatively small and spraying would result as before. Another
very successful device is shown in Fig. 41, in which A is the petrol
jet which, in this case, has no special orifice and is surrounded by a
larger tube B containing small holes for the inlet of air and outflow
of petrol. As the petrol issues from the jet it strikes against the
pointed cone on the end of the screw C, and is thus very successfully
_atomized_ and broken into small particles which can be readily
vaporized.
[Illustration: FIG. 41.—PETROL JET, SPECIALLY ARRANGED FOR _ATOMISING_
THE PETROL.]
[Illustration: FIG. 42.—COMPENSATED PETROL JET. A IS THE MAIN JET AND B
THE COMPENSATING JET SUPPLIED THROUGH THE ORIFICE C.]
There are several devices for keeping the strength of the mixture
constant at all engine speeds irrespective of the amount of vacuum in
the choke tube. One of the best of these is illustrated in Fig. 42,
and consists in the use of a _compensating jet_. The main petrol jet A
is of sufficient size to supply the requirements of the engine under
full speed and with the resulting high vacuum; it is fed directly
from the float chamber in the usual manner. The compensating jet B
surrounds the main jet and is supplied with petrol through an orifice
C, so arranged that it offers a greater resistance to flow than the
passage up the centre of the main jet. At all engine speeds up to a
certain predetermined maximum the compensating jet will supply most of
the petrol, but as the demand increases the main jet will also begin
to supply, and simultaneously the compensating jet will commence to go
out of action owing to its supply of petrol becoming partly or wholly
exhausted due to the restriction of the orifice C.
The simple jet-in-tube carburettor has been greatly improved by the
addition of an _automatic extra-air valve_, of which a simple form
is shown in Figs. 43 and 44. It consists of a small mushroom type
valve A, with its seating B so arranged that it can be screwed into
the induction pipe of the engine. The valve is held up against its
seating by a light spring C, so that at high engine speeds when there
is a good vacuum in the induction pipe the pressure of the atmosphere
will open the valve against the tension of the spring and allow air to
pass into the induction pipe, thus reducing the amount of vacuum and
simultaneously weakening the mixture.
[Illustration: FIG. 43.—AUTOMATIC SPRING CONTROLLED EXTRA-AIR VALVE.]
[Illustration: FIG. 44.—PLAN VIEW OF AUTOMATIC EXTRA-AIR VALVE.]
The points of a =good carburettor=:—
These may be set out in the following order—
(1) Complete _atomization_ and _vaporization_ of the liquid fuel at
all engine speeds.
(2) The supply of an _adequate quantity_ of gas of the _correct
proportions_ with all throttle openings and at all temperatures.
(3) Sufficient _mechanical strength_ and _durability_ to withstand
road shocks and to ensure freedom from breakdowns without undue
_weight_ or _complications_.
(4) Ability to continue working correctly when the car is on an
incline or affected by the camber of the highway.
(5) Moderate first cost.
=Pressure Feed and Gravity Feed.=—In Fig. 38 we showed a gravity-fed
system or one in which the petrol is fed from the tank to the float
chamber of the carburettor by the action of gravity only. For this
system to be successful at all times the carburettor must be placed low
down to obtain a good _head_ for the flow of petrol in the connecting
pipes, as there is a practical limit to the height at which the petrol
tank can be fixed. Also the pipes must have a continuous run down
towards the float chamber to prevent air-locks in them, and they must
be kept away from the hot exhaust system. When all these points can be
secured this system is perfect. An alternative system is to force the
petrol into the float chamber by maintaining an air pressure (of 2 or 3
lb. per square inch) on the surface of the liquid in the petrol tank.
With this arrangement the carburettor may, if desired, be situated
_above_ the level of the petrol tank in a more _accessible_ position,
but it necessitates the fitting of a small _air pump_ on the engine and
the use of a _hand air pump_ for starting.