Carburetion is the term applied to the process of converting the liquid fuel into an explosive mixture. It comprises the vaporization of the fuel and the mixing of gasoline and air in the proper proportion to produce the explosive mixture drawn into the cylinders. The function of the air is to supply oxygen for combustion.
Air is a quantity of infinite variables, since its oxygen content for a given volume is proportionate to its temperature. The higher the temperature, the smaller the quantity of oxygen it contains, without any change of carburetor adjustment. It is accordingly advisable to use the air at the lowest temperature at which vaporization is possible.
The best proportion of air to gasoline varies between sixteen to seventeen parts of air to one of gasoline; however, this will be dependent upon the quality of the fuel.
Since the power of a gasoline motor is derived from the fuel which enters the cylinders during the suction stroke, it is of utmost importance that the mixture of fuel and air which is used by the motor, shall always be of exact proportions, so that when it is ignited by the spark, it will give the maximum force of explosion for a given quantity of fuel. If there is too much fuel or too little, within relative narrow limits, the action of the engine becomes objectionable.
Vaporization of fuel may be accomplished in two ways, by heat or vacuum; vaporization due to pressure reduction is distinguished from vaporization caused by the supplying of heat. In the vacuum method, vaporization is only partly complete, no matter how far the process of reduction is carried, since the part of the liquid, which vaporizes, does so through the abstraction of heat from the remainder, which becomes constantly colder until finally the temperature is so low, that vaporization ceases until heat is supplied from some outside source. When vaporization is brought about entirely by heat from an outside source, the degree to which it may be carried depends wholly on the amount of heat supplied, since the temperature of the liquid is being constantly raised to or maintained at the proper point.
In practice neither of the above processes are carried to the limit, but both act together. The reduced pressure, due to "motor suction," causes vaporization with a lowering of the temperature, and the heat of the air tends to cause vaporization through a transfer of heat from itself to the liquid. Each of these vaporizing actions assist the other; the air supplying heat to the liquid as it is cooled by vaporization under reduced pressure, and the reduction in temperature, due to pressure reduction, facilitating the transfer of heat from the air to the liquid.
The instrument which serves to carry out the above functions is known as the carburetor. Gasoline is stored in a tank, generally located under the driver's seat, and from here it is fed through a small pipe to a compartment of the carburetor called the float chamber, passing through a needle valve and strainer, which regulates the amount of gasoline entering the carburetor. To allow metal floats to be sustained in the gasoline they are made air tight and hollow, so that the needle valve can pass through them onto the valve seating. Somewhere near the top of the needle valve, small weighted arms are pivoted, the ends of which rest idly on the top of the float. The function of this float and chamber is to maintain a constant level of gasoline in its own chamber and the chamber in which the jet is located. The level in the latter chamber must be constant so as to prevent the gasoline flooding over the jet, which will cause faulty running of the engine. The action of the float is very much like an automatic water cistern, with its ball valve, for, as more gasoline enters the first chamber, the float rises, and with it the weights resting on it so that the needle valve is pushed down on its seating and shuts off the supply.
The other part of the carburetor called the choke tube, or mixing chamber, accommodates the jet and allows a stream of air to pass around it when the piston descends on the suction stroke. The float chamber maintains a constant level of gasoline in the jet. When the air rushes up around the jet, it draws with it a certain amount of sprayed gasoline, and, when the mixture of air and gasoline impinges on the sides of the inlet manifold, it becomes a gaseous mixture and enters the engine in this state through the inlet valve.
Just below the manifold flange of the carburetor, and in the choke tube, is located a throttle, which can be opened or closed by the operator from the lever on the steering gear or foot throttle. The more gas the engine can receive, the faster it runs, and the faster it runs, the more gasoline it is likely to draw from the jet. This would give an unduly rich mixture, unless means were provided for admitting more air, and thus giving the mixture approximately correct proportions. For this purpose an auxiliary air valve is provided, communicating with the choke tube just above the jet. As the engine speed increases, this auxiliary air valve opens and permits more air to enter, thus maintaining the proper mixture. It is a general assumption that more air must be admitted at high speeds, but this is not really correct; for, while we are admitting more air, we are merely endeavoring to keep the proportions of air and gasoline the same. Increased engine speed means increased suction on the jet, and naturally the liquid gasoline is drawn through faster than the air, so that more air must be admitted to compensate for the excessive suction. But apart from speed, temperature and humidity have their effect on carburetion. In the summer, the air parts require to be opened wider than in winter, as the atmosphere is less dense; also, when the air is damp and the barometer low, more air will be necessary.