Fig. 23B illustrates the most usual forms of auxiliary air inlets. In the first type, the valve disk slides on the valve stem, and enlarges the size of the main air inlet. All of the air thus passes the spray nozzle. In the second type, the inlet for the auxiliary air is separate from the main air inlet, the two currents meeting in the mixing chamber, and the extra air diluting the rich mixture that is formed at the spray nozzle. This action is more correct in theory than that of the preceding type, and better practical results are obtained from it. These air valves are defective in opening and closing too abruptly, and in tending to vibrate rather than to remain open a fixed distance. The air inlet illustrated in the fourth diagram was designed to overcome these faults. When the engine is not operating, the air inlets are closed by a hollow piston that is held up by a spring. The upper part of the piston rod carries a metal disk that is attached by a flexible leather washer to the walls of an upper chamber. The portion of the chamber above the disk is tightly closed, except for a small hole in the cover that provides the only communication between the atmosphere and the air confined in the chamber. When the engine runs at speed, the atmospheric pressure against the upper side of the disk is greater than the pressure against the lower side, and the disk is therefore forced downward against the action of the spring. The movement of the disk moves the piston, and as this latter slides downward it uncovers the openings and admits air. The small size of the opening in the cover prevents air from entering or leaving the chamber above the disk rapidly, and the movement of the piston is therefore steady and free from jerks. The third diagram illustrates two positions of a mechanically operated auxiliary air inlet, controlled by a governor.
CHAPTER VI
IGNITION
The charge of explosive mixture in the combustion space is ignited, or set on fire, by an electric spark, and the apparatus for producing and controlling this spark is called the ignition system. It is with this part of the mechanism of an automobile that a novice has the greatest difficulty, for an electric current is usually regarded as being surrounded by an air of mystery. It does its work silently and without visible reason, and when it fails the average man is under the necessity of beginning at the beginning and examining all of the parts of the system because he has so little understanding of the why of it that he is unable to locate trouble in any but a rule-of-thumb method. The principles of electricity may be involved, but the production of a current, its handling, and the uses to which it may be put are not difficult to understand.
Speaking broadly, the parts of the ignition system are the source of current, the arrangement in the combustion space at which the spark is produced, the device by which the instant when the spark passes may be controlled, and the circuit by which these parts are connected. Before going into a description of these, however, it is necessary to understand something of the nature and action of an electric current.
Every generator of electricity has two terminals, or poles, and the flow of current from one to the other is due to what may be explained as a difference in pressure between them. This difference in pressure is similar to that existing when two tanks, one full of water and the other empty, are connected by a pipe. The water will flow from the full to the empty tank as long as there is a difference in level, which is the same thing as a difference in pressure, the flow ceasing when the water in one tank is level with that in the other. When a path is provided between the two poles of the generator, the current will flow from one to the other, always in the same direction, leaving by the positive pole and returning by the negative.
Because of this tendency to flow, the current may be made to perform work, for it will light a lamp, ring a bell, or do anything else within its power in order that it may flow from the positive pole of the generator to the negative. In this there is also a similarity to the two tanks, for if a water wheel is placed in the pipe connecting them, the flow of water from one to the other will operate it.
The path over which the current flows may be formed of any conductor of electricity, such as carbon or any metal; substances by which the current will not flow are called nonconductors or insulators, and those in most common use are rubber, china and glass, wood, wood fiber, mica, etc.
While all metals are conductors, some are better than others, the difference being in the resistance that they offer. A comparison illustrating resistance may be made between the friction presented to the flow of water by a small pipe and by a larger one, the water flowing more easily through the latter than the former. In flowing, the current must overcome the resistance of the conductor, and in so doing will lose part of its strength and will heat the conductor, there being more loss of current and greater heat as the resistance increases.
It is obvious that in order to obtain a current of the greatest strength the conductor by which it flows must present the least possible resistance, and for this reason copper is used almost universally to convey the current from one place to another. A copper wire will carry safely a current that would heat an iron wire of the same size to the melting point. The resistance of a small conductor is much greater than that of a large one, so that the size of a conductor must always be considered in relation to the current that is to be conveyed.
A current of electricity may be measured, just as water flowing through a pipe may be measured, and by the same measurements of pressure and volume. The pressure under which the current flows is measured in volts, and the quantity that passes in amperes; there is also a term for resistance, that being measured in ohms.