The most serious result of a rich mixture, however, is in the production of _carbon_, and the _carbonization_ of the engine. The flame of a rich mixture is smoky; the smoke of this flame, as is the case with smoke from all other sources, is composed of fine particles of carbon, or soot. These particles of carbon will deposit on all parts of the combustion space: on the top of the piston, on the valves, on the spark plugs, and on the inner wall of the cylinder head. At first it is gummy, but it rapidly hardens and forms a crust that must be scraped off with a steel tool. Carbon in an engine will reduce the power through causing _preignition_, or, in other words, by setting fire to the fresh charge before the proper point in the stroke. The heat of the combustion will cause the carbon deposit to become so heated that it will glow, these glowing particles being sufficient to ignite the incoming fresh charge. The remedy for this condition is to remove the carbon, which is usually done by taking off the cylinder head and scraping away the deposit. It may be added that carbon is also formed by the use of too much lubricating oil, as will be explained in the chapter on lubrication. Thus it is seen that if the engine is to run properly, and is to be kept in good condition, the proportions of the mixture must be very carefully maintained. The mixture is formed in a _carburetor_, or _mixer_. This is, roughly, in the form of a tube through which air is sucked during the inlet stroke; projecting into it is a fine tube called a _spray nozzle_ through which the fuel enters. In action it is somewhat similar to the atomizer that is used for spraying the nose and throat. By forcing the fuel to flow rapidly through this small tube it comes out in the form of spray, and the tiny drops are picked up by the current of air and are carried into the cylinder. It is much easier to form a mixture of gasoline than of kerosene or distillate, because gasoline vaporizes more readily at ordinary temperatures. If saucers of gasoline and kerosene are placed in the sun, the gasoline will evaporate rapidly and completely, leaving only a faint oily deposit. The kerosene, on the other hand, will evaporate slowly, and much of it will not evaporate at all. To make kerosene and distillate evaporate completely, they must be heated, just as water must be heated to make it evaporate. In the case of a carburetor for gasoline, the current of air needs only to be warmed; the spray of gasoline will evaporate on coming into contact with the warmed air, and much of it will enter the cylinder as vapor. In order to evaporate kerosene and distillate much more heat must be provided, and it is usually considered necessary to heat not only the current of air, but the liquid fuel as well. Methods of doing this will be explained in the next chapter. [Illustration] [Illustration: FIG. 21.—PRINCIPLE OF CARBURETOR]