A carbureter is a device which receives and mixes gasoline and air in proper proportions, and in which a vapor is formed for gasoline engines.
The product of the carbureter is a mixture of gasoline vapor and air, not a gas. A gas, as explained, is of such a character that it remains fixed and will not stratify or condense.
Functions of a Carbureter.—The function of a carbureter is to supply air and gasoline by means of its adjustable features so as to make the best mixture. The proportions of air and gasoline will vary, but generally the average is fifteen parts of air to one of gasoline vapor.
If there is too much gasoline, proportionately, a waste of fuel results, as a great amount of soot is formed under those conditions. If there is an excess of air the mixture, when ignited, will not have such a high temperature, hence the expansive force is less, and the result is a decrease of power.
While it is possible to get a rapid evaporation from gasoline by heating it, experience has shown that it is more economical to keep the gasoline cool, or at ordinary temperatures, provided the carbureter is properly constructed, because the vapor, if heated, when drawn into the engine, will be unduly expanded, and less fuel in that case is drawn in at each charge, and less power results.
Rich Mixtures.—There are conditions under which rich mixtures are advantageous. This is a mixture in which there is a larger percentage of gasoline than is necessary for instantaneous combustion. For ordinary uses such a mixture would not be economical.
At low speeds, however, or when carrying heavy loads, it is desirable, for the reasons that at a slow speed the combustion is slower.
Rich mixtures are objectionable at high speeds because, as the combustion is slow, incomplete combustion within the power stroke results, the temperature of the gas at the end of the stroke is very high, and this will seriously affect the exhaust valves. Furthermore, there is likelihood of the gas continuing to burn after it is discharged from the cylinder.
Lean Mixtures.—Such a mixture is one which has a less amount of gasoline than is necessary to make a perfectly explosive compound. For high speeds a lean mixture is desirable, principally because it burns more rapidly than a rich mixture.
Types of Carbureters.—There are two distinct types of carbureters, one which sprays the gasoline into a conduit through which air is passing, and the other in which a large surface of gasoline is placed in the path of the moving air column, which was originally used, but has been absolutely replaced by the jet carbureters on account of their better control features.