The Gasoline Motor

by Harold Whiting Slauson

The first form of carburetors, or "vaporizers," as they were called then, employed a flat, woven lamp wick over which the gasoline flowed. This spread the fuel out over a comparatively large surface and rendered evaporation rapid and simple. The chamber containing this wick was placed in the line of the intake pipe of the motor and was connected with the cylinders on the descent of the pistons on the suction stroke through the medium of the various inlet valves. In a four-cycle motor, the piston acts as a suction pump on alternate down-strokes and serves to draw the charge into the cylinder. This suction created the necessary current of air to facilitate evaporation of the gasoline on the wick, and by regulating the size of the passages, the proper proportion of air and gasoline vapor could be obtained.

The modern, high-speed automobile motor, with its varying demands upon the carburetor, created the necessity for a more delicate, flexible, and compact vaporizer than was to be found in the "lamp wick" type. Consequently the wick was replaced by a small, slender, hollow tube having a cone-shaped opening at its upper end through which the gasoline from the feed pipe was made to pass. Fitting into the upper end of this tube, and pointed to the same angle, was a cone-shaped "needle" that could be moved in and out of the opening. If this needle was unscrewed slightly so that it did not form a tight fit with the end of the tube, a small ring would be formed through which the gasoline must pass when sucked by the alternate down strokes of the pistons. This tube and needle constitute, under various guises, the "needle valve" with which practically every modern carburetor is equipped.

When the gasoline, rushing through the small tube, strikes the restricted opening of the needle valve, it is broken up into a fine spray which, under proper conditions, will become vaporized almost as soon as it comes in contact with a current of air. This air current is induced by the same pump-like effect of the pistons as that which sucks the gasoline through the needle valve, and thus it occurs only when the charge is desired in the cylinders.

But the carburetor is not merely to provide a compact device for vaporizing the gasoline, for it must also furnish a means of regulating the proportion of gas to air. Gasoline vapor is only highly inflammable when mixed with the proper quantity of air, and if this proportion is varied above one limit or below another, ignition of the charge will not occur in the cylinders. In fact, the allowable variation in the proportion of gasoline vapor to air is restricted between very narrow limits, and should not change more than four or five per cent. from one extreme to the other. The proportion of gasoline vapor to air by weight is about one to eleven, although this will vary somewhat with the different grades of fuels.

The point to be emphasized, however, is the fact that the proper proportion of air to gasoline vapor, however it may vary with different grades, should be kept constant at all speeds of the motor whenever that particular grade of fuel is used. By volume, about 97½ per cent. of the mixture should be air and the remainder gasoline vapor, and it is the device that will the most nearly maintain this proportion under all conditions of speed, temperature, and air pressure that will prove to be the most delicate and flexible carburetor.

A carburetor may be adjusted for different motors, or for different operating conditions of the same motor, by means of the needle valve. The farther end of the slim rod on which the needle point is mounted terminates in a thread and finger nut that projects through the shell of the carburetor. By turning this nut in one direction, the needle valve is screwed up toward the cone-shaped end of the tube and the orifice through which the gasoline may pass is thus reduced in size. This will decrease the amount of gasoline sprayed into the air passage and will consequently change the composition of the mixture. This, however, should not be confused with throttling the motor. When the needle valve is tightened, the volume of the mixture passing to the cylinders is the same, for it is only the proportion of gasoline vapor in that mixture that is changed.

Throttling consists in restricting the size of the opening through which the mixture passes, and thus limits the amount of the charge that reaches the cylinders at each suction stroke of the piston. Throttling is used to reduce the power—and consequently the speed—developed by the motor, while a decrease in the amount of gasoline supplied to the air through the needle valve may serve to increase the power through an improvement in the nature of the mixture.

Since the gasoline vapor, by volume, forms only about three per cent. of the explosive mixture admitted to the cylinders, a slight variation in the size of the needle valve opening will result in a marked change in the composition of the charge and may make all the difference between poor and perfect running of the motor. Consequently the needle valve nut should be moved but the small fraction of a turn for each adjustment. A motor which may refuse absolutely to run at one position of the needle valve may give perfect results if the nut is unscrewed but the eighth of a turn.

In view of the marked difference in the results obtained from the use of mixtures that are "just right," and those which vary but a slight percentage in the proportion of gasoline vapor to air, it may be well to examine, superficially, the effects of "rich" and "weak" charges, and therefrom to obtain a list of "symptoms" which may aid us to diagnose motor trouble properly.

We all know that air—or oxygen—is required to support combustion. "Snuffing" a candle is merely covering its end so that air cannot reach the flame. For the same reason, gasoline in a covered tank cannot burn, no matter how great the heat applied to it. The heat of the electric spark in the cylinder, although intense, does not cover a sufficiently large area to ignite any charge except that composed of the proper proportion of gasoline vapor and air. If there is too much gasoline vapor, making a "rich" mixture, there will not be sufficient air in the charge to support the entire combustion of the gas, and the burning will be slow—if it takes place at all. The same conditions will prevail if there is an insufficient supply of air for a given quantity of gasoline vapor, and consequently a rich mixture may be obtained by reducing the air flow as well as by adding to the amount of gas admitted to the mixing chamber.