The carburetor or mixing chamber, as it is sometimes called, is a device used for obtaining an explosive mixture of gasoline and air. It consists, as shown by the accompanying drawing, of two principal parts, an air pipe and gasoline pipe, the latter running through the wall and discharging into the center of the former.
Fig. 21—Simple drawing of a Carburetor.
In order to make sure that the amount of gasoline flowing out of the gasoline jet shall be just the right amount at all times it is necessary to provide a little gasoline tank, which forms a part of the carburetor casting itself, which is known as a float chamber, so that the amount of gasoline in the main tank will not affect the amount discharged at the nozzle. You can see why this is necessary if you think of a water tank or a dam. If the water was almost up to the top of the dam and you should bore a hole through the wall somewhere near the bottom, the water would flow out faster than if the water was low. By putting this little gasoline tank in the carburetor itself and keeping a certain height of gasoline in this smaller reservoir, which always automatically shuts off the supply at the right time, you can make the pressure, and therefore the flow of the liquid, always the same. The illustration will show this plainly. For instance, when the gasoline gets low the little float will gradually drop down until the ball on the end of the float stem will open the valve in the gasoline pipe. The gasoline will then flow in from the tank until the proper amount has filled the float chamber and caused the float to bob up to its former position, carrying the ball, which closes the gasoline off, up with it. By this means the requisite amount of gasoline is always kept in the float chamber.
The amount of air entering the mixing chamber is controlled by changing the size of the hole through which the air enters and the quantity of gasoline admitted is regulated by means of a needle valve in the gasoline pipe.
Although many carburetors, in fact most of them, do not look like this drawing, yet their action is the same, and by careful study you will find that the same principles enter into their construction. [Fig. 22] shows an actual sectional drawing of a carburetor used on a four-cylinder motor. In this particular carburetor, however, the float chamber and float surround the mixing chamber, and the float valve, instead of being directly under the float, is at the right hand side and is operated by means of a lever. The needle valve, which is the little round rod having a “T” handle, running up through the center of the mixing chamber, controls the amount of gasoline flowing from the gasoline chamber to the nozzle. The air comes up through the bottom and around the gasoline jet. At the left you will notice a small valve which opens downward, which you do not find on the other carburetor. It is known as an auxiliary air valve and allows a certain amount of air to be added to the mixture, a small quantity of which is sometimes needed to keep the mixture just right. The throttle valve, which looks like a damper in a stovepipe and which controls the amount of gasoline vapor going in to the engine, will be seen in the upper pipe.
Fig. 22—A Typical Four-cylinder Carburetor.
THE IGNITION SYSTEM
The ignition system is the name applied to the batteries, coils, commutator and spark plug which, acting as a whole, produce an electric spark hot enough and at the right time to fire the charge in the cylinder. There are three ways in which an electric current may be obtained which have been found to be of practical use in automobile construction. First, by a dry battery; second, by a storage battery; and third, by magneto or dynamo. You can look up the construction of these things in any good book on electricity, so that I will not explain them further here. As the object of the whole system is to produce a sufficiently hot spark to fire the mixture at the right time, it is sometimes necessary to raise the pressure of the electric current. When either a dry battery or storage battery or a certain type of magneto is used, it is necessary to put it through what is known as an induction coil in order to raise the pressure so it will be high enough to jump across the two points in the cylinder.