Francis John Kean

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among wicks saturated with liquid petrol as in the wick type of carburettor, but both these methods have been largely superseded by the use of what is now known as a jet or spray type of carburettor, in which the petrol is sprayed from a fine jet and mixes with air which is passing up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.

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.

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.

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.

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.

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.