The Gasoline Motor

by Harold Whiting Slauson

A rich mixture will cause irregular explosions in the cylinders, and will often emit a black, pungent smoke at the exhaust. The motor will probably overheat easily, due to the slow-burning properties of the mixture and the resulting fact that a large portion of the cylinder walls uncovered by the pistons will be exposed to the flame. In some instances, the cylinders will miss fire at regular intervals, thus changing the synchronism of the impulses with a well-defined and periodic "skip" in the sound of the explosions.

While these are by no means certain symptoms of a rich mixture, the first test to be made should be to tighten the needle valve adjustment slightly when the motor is running and to note any resulting improvement in the regularity of the explosions. It may sometimes be difficult to distinguish between the symptoms of a rich and a weak mixture, but the readjustment of the needle valve as just described will at least serve to locate the trouble or to eliminate one or the other possibility from consideration.

When a mixture is "starved", or when there is an insufficient supply of gasoline vapor to unite with the air admitted to the cylinders, the charge will not be highly inflammable and may not be ignited by the small spark formed at the plug. Even when ignition does take place, the resulting power impulse will be weak because of the comparatively small amount of pressure-producing gas in the mixture. The explosions may occur regularly for a while, but there will be a marked decrease in the power developed by the motor, and owing to the fact that weak mixtures may be slow-burning, "back-firing" will often result in some engines to which such a charge has been fed.

On the other hand, if a motor will run at all on a weak mixture, it will produce better results than would be the case were the charge too rich in gasoline vapor. Consequently the needle valve should be closed as much as is consistent with smooth running of the motor, but the moment a loss of power or irregular explosions occur, the mixture should be enriched.

At low speeds of the motor, the pumping action of the pistons is not as great as is the case at high revolutions, and consequently the suction drawing the gasoline through the needle valve is diminished. For this reason, the needle valve opening must be made larger or the air passage restricted for slow speeds of the motor, and it was consequently necessary, on the old, non-automatic vaporizers, to increase the gasoline supply whenever the revolutions of the motor were to be reduced. The modern carburetor is sufficiently automatic in its action to provide the proper mixture within wide ranges of speed change of the motor, but even nowadays it is often found necessary to increase the gasoline supply or to reduce the amount of air admitted to the intake pipe whenever it is desired to throttle the motor down to a very low number of revolutions per minute.

The automatic action of the ordinary carburetor is obtained by increasing the air supply at higher speeds of the motor. Consequently the motorist will realize that whenever the needle valve is to be set, such regulation should be made when the motor is well throttled, for if an ample gasoline supply is obtained at low speeds, the mixture will certainly be sufficiently rich at increased revolutions. If, on the other hand, the carburetor should be set to supply a proper mixture at high speeds, the mixture would be impoverished when the motor is throttled, and irregular running would result.

The air for the operation of the motor at ordinary speeds is supplied through a fixed opening in the carburetor connected with the chamber into which the gasoline spray is introduced. In addition to this, most carburetors are supplied with an "auxiliary air opening" which serves to furnish the additional air necessary for the mixture at high speeds of the motor. The fixed opening, being restricted in size, cannot admit the increased quantity of air demanded by the higher speeds of the motor. The auxiliary opening is provided with some form of automatic valve which may consist either of a series of ball "checks," a spring-actuated "mushroom valve," or a series of special valves, each of which opens at successively increased speeds of the motor.

All of these devices operate on the same principle, however, and allow the increased suction of the motor to add to the size of the air passage automatically—either by the farther opening of a single valve, or by the successive opening of different valves. Some carburetors are provided with an adjustment by means of which the "delicacy," or ease of opening, of the auxiliary air valve may be regulated. This may be done by means of a nut and screw which will increase or decrease the tension of the controlling spring. If this spring is set with a high tension, the auxiliary valve will act only when the motor is exerting great suction, or at fast speeds.

The regulation of the auxiliary valve is an adjustment that should be made only after the needle valve has been set properly for slow speeds of the motor. When this condition is obtained, the throttle should be opened and the further adjustment of the carburetor for high speeds of the motor should then be made through the auxiliary air valve. In other words, the needle valve should be set so that the motor runs properly at low speeds, while the adjustment of the auxiliary air valve should be made only to secure smooth operation at a high number of revolutions.

It is not to be understood that less gasoline is actually required at high speeds of the motor because the supply often needs to be cut down at the needle valve under these conditions. The actual amount required at high speeds is, of course, greater than is the case at slow, on account of the greater number of explosions in the former instance. But the suction of the motor generally increases the gasoline flow beyond the demands of the cylinders at high speeds, and it is for this reason that the automatic auxiliary air supply is provided to furnish the additional air required to support combustion. In fact, at heavy loads, when the total amount of gasoline consumed must be great, a secondary jet of fuel is brought into action in some carburetors. This is known as the "multiple-jet" type and is found on some of the large engines that must possess a speed and power variation between wide ranges. The action of these various jets is entirely automatic and is dependent upon the speed and fuel requirements of the motor.