Manifolds that have been designed as if they were intended to handle a fixed gas and that depend upon the application of excessive heat have not produced satisfactory results. Although heat in a limited amount aids vaporization, it is an agent that must be used with caution. As present-day fuels are composed of volatile constituents blended with the heavier ends, only a part at best can be vaporized and manifolds should be designed so that they will distribute wet mixtures of fog, instead of dry gases, uniformly at varying engine speeds and varying throttle positions. The four elements in the mixture furnished to the engine are air, water vapor, gasoline vapor and liquid particles of gasoline or fog. Liquid particles of considerable volume can be held in the airstream without depositing if the velocity is kept relatively high. But when rapid reversals of the direction of mixture-flow occur, as in the manifold of a multi-cylinder engine, the particles separate out by gravity and deposit.
An efficient manifold must not only conduct the fuel mixture but must distribute it uniformly in the same condition as that in which it left the carbureter. Manifolds that are designed for maximum power sacrifice performance at low speeds, so that the tendency recently has been to reduce the size of the intake and increase the speed of the mixture to obtain more satisfactory low-speed performance. Efficient operation of a manifold is indicated by increased torque, increased acceleration, economy and ease of starting the engine, while the secondary results to be noted are uniform firing, even when starting cold, and a consequent reduction in the dilution of crankcase oil, lessened vibration and lessened carbonization. Although many engineers have assumed that distribution is equal when the power developed by the several cylinders is equal, this assumption may be in error; the air-to-fuel ratio may vary considerably, yet the power developed remain the same. The most accurate means of determining the distribution is an analysis of samples of exhaust gas taken simultaneously from the individual cylinders. To obtain equal distribution the fuel can best be supplied to the cylinders as a fog and this can be done without the use of heat. An analysis of conventional types of manifold construction shows that almost invariably the lighter fuel-fractions are furnished to certain cylinders and the heavy ends to others.
In the Swan method of distribution the forces, due to the suction of the several cylinders, acting upon the carbureter are all equalized at the top of the riser or metering-point of the system. The result is equal flow in all branches. Also successive flows are always in a different direction from this metering-point to minimize inertia effects.
The manifold is a straight-line type of square or rectangular section and the turns are abrupt and at substantially right-angles. The flat floor of the square section provides the maximum area for exposing the liquid fuel to the airstream. Among the advantages claimed for a manifold of this type are (a) straight-line manifold (b) the holding of the atomized fuel from the carbureter in an atomized condition in the airstream to the cylinders and (c) low first cost, which in many cases is appreciably less than in manifolds of conventional type. Although heat is not necessary, to realize equal distribution, a certain amount is required to offset the rapid reduction of temperature above the throttle-valve at closed-throttle positions, which normally would cause the temperature to drop below the dew-point. In the Swan manifold the application of a fixed amount of heat of the maximum intensity is entirely automatic in its action because of the difference in the velocity of the mixture at open and part-throttle positions. Curves of brake horsepower, torque and fuel consumption are given to show the comparative operation at various loads of Swan manifolds and those of the conventional type.


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