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.
Fig. 40.—Plain form
of Choke Tube.