The adoption of the present system of feeding a number of cylinders in succession through a common intake manifold was based upon the idea that the fuel mixture would consist of air impregnated or carbureted with hydrocarbon vapor, but if the original designers of internal-combustion engines had supposed that the fuel would not be vaporized, existing instead as a more or less fine spray in suspension in the incoming air, it is doubtful that they would have had the courage to construct an engine with this type of fuel intake.
That present fuel does not readily change to hydrocarbon vapor in the intake manifold is indicated by tables of vapor density of the different paraffin series of hydrocarbon compounds. This means that for certain vaporization of fuels containing elements of this density the intake manifold must be heated to a temperature slightly greater than the hand can bear continuously, and the present gasoline contains a large percentage of ingredients much less volatile than this.
To verify the belief that the fuel cannot evaporate in the intake manifold at ordinary temperatures, evaporation experiments were made using a large tank having a conical bottom, and further definite proof that the mixture remains wet was obtained by viewing it through a glass-sectioned manifold. Critical air velocities and the range of manifold velocities are then discussed at length and five specific points of design which seem to apply universally are stated and commented upon. The application of heat and the improved operation secured are then considered and illustrated, the different ways of applying heat being enumerated as the heating of the fuel before it issues from the carbureter nozzle; the heating of the air charge; and the heating of the intake-manifold walls.