the air fuel mixture leaves the carburettor, it passes into the manifold. An ideal manifold would have the following characteristics: (I) be free from puddling; (2) have low resistance to maintain high volumetric efficiency; and (3) distribute the unvaporized and unatomized liquid fuel evenly to the cylinders. Depressions in the manifold, which permit puddles of raw fuel to collect, may entirely destroy the performance of a carburettor in correctly proportioning fuel to the engine demand. Also puddles may, upon sudden acceleration, be drawn into the engine, causing poor operation. The resistance of a manifold is controlled largely by its size and the character of the necessary bends. While large manifolds are desirable to maintain high volumetric efficiencies, they also reduce the veloci ties, permitting more puddling. Thus, a compromise must always be effected between these two points.
Present-day gasoline includes as many as possible of the crude oil fractions. The chemical characteristics are continually changed as new cracking processes and other developments are adopted. A large portion of gasoline can be evaporated only by the use of external heat. This necessitates the use of hot air or hot spots to vaporize a sufficient portion of the fuel to secure ade quate distribution. The use of a hot spot is the most desirable way of obtaining good distribution. The liquid fuel in the mani fold tends to collect on the walls and absorb heat from the hot spot. This film of fuel also acts as an insulator for the air, keep ing the heat away from it and maintaining high volumetric effi ciency. Perfect distribution can be obtained only by having all the fuel in a finely atomized or vaporized state. To do this, how ever, the heat added would greatly decrease the volumetric effi ciency. Thus, a compromise must always be effected between the hot spot size and volumetric efficiency.
Superchargers.—To increase the power output for a given sized engine, superchargers are sometimes resorted to. By this means a greater weight of air-fuel mixture is forced into the cylinders than would be the case with simple induction. The supercharger is a pump capable of handling large volumes of air at comparatively low pressure difference, say about 6 lb., and is located either between the carburettor and the engine or else between the carburettor and the air intake. The super charger finds its widest application in racing-car practice, where the cylinder size is limited and maximum power must be obtained. A few cars in Europe are equipped with superchargers, but in the United States superchargers on passenger cars are seldom in stalled.
Gunpowder was the fuel used in what is thought to have been the first internal-combustion engine. Christian Huygens, a Dutch experimenter, has been credited with trying out that idea about the year 168o; but, for good reasons, his example was not fol lowed in later and more successful engines. Gas was burned as
fuel in the first useful internal-combustion engine, built by Lenoir in 186o. Volatile liquids began to be used for internal combustion engines about 189o, and from the very outset liquid fuels were employed almost exclusively as sources of energy for the automobile engine. Gasoline from petroleum has always made up the bulk of automobile fuel. Other liquids, such as benzol and alcohol, and even paraffin, or kerosene and other fuel oils, have also been used, but only in a small portion of the whole. Each American motor car uses, on the average, about 67ogal. of gasoline a year; but trucks and buses, which represent less than i8% of all the cars in service, account for more than one-fourth of the consumption. The average passenger car uses about 56ogal. a year; the average truck, about 1,200 gallons.