Motor-car principles; the gasoline automobile

by Roger B. Whitman

CARBURETOR PARTS

Fig. 23B.—Types of Auxiliary Air Inlets.

Practically all the carburetors on the market are combinations of a few forms of float valves, auxiliary air inlets, and spray nozzles. In addition to the forms shown in Figs. 20 and 21, the most usual float valves may be seen in Fig. 23A. In the first two types shown in this diagram, the float valve stems are separate from the floats, and are sufficiently heavy to shut off the flow of gasoline by their weight. In the third type, the gasoline enters the float chamber from the top, and as the valve stem is attached to the float, the rising of the float results in the shutting off of the gasoline. The fourth type is in use on carburetors with central mixing chambers, the float being hinged to one wall of the float chamber. The loose valve stem is supported by the hinge, and rises to a seat in the valve when the gasoline is at the proper depth on the float chamber.

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.

Because of the desire of the current to flow from the positive to the negative pole, the circuit must be so guarded that there may be no leakage, for if it can return to the generator without doing its work it will do so, taking any path that offers less resistance to its passage. Leakage is prevented by insulating the wires, which may be done by wrapping them with silk or cotton thread, or by coating them with rubber. The wire by which the current flows from the generator to its work is called the lead (pronounced leed), and the conductor by which it flows back, the return. The greatest care must be taken that there is no leakage from the lead wire in order that there may be enough current to perform the work; but when the current has done what was expected of it, any conductor that does not present too much resistance will serve to return it to its source.

While wire is sometimes used on ignition circuits for the return as well as the lead, the most usual method is to utilize the metal of the engine to return the current to its source. The engine being made of iron, it is a conductor, and while this metal is not so good a conductor as copper, yet there is so much of it in an engine that its resistance need not be considered. This method is called grounding the return, the term coming from telegraphy, in which the dampness of the earth forms one of the conductors.

A grounded circuit requires careful insulation of the lead wire, for as it is very likely to come into contact with the metal of the engine, a break in the insulation would permit the current to leak and to return to the generator without doing its work, this condition being called a short circuit. Grounding the circuit saves wire and reduces complication; simplicity is important in automobile work, and a reduction of the parts much to be desired.

In the foregoing the word flow has been used in describing the action of an electric current in a wire, and there are so many points of similarity with the flow of water in a pipe that the term conveys a better idea than would technical expressions. Electricity, however, has neither substance nor weight, and cannot be said to flow or to move in the strict sense of the words; but as flow is in general use, and is the most descriptive of the commonly understood words that are applied to it, it is made use of here.

A current of electricity may be generated either chemically or mechanically, and always at the expense of something else. A chemical generator produces a current at the expense of a metal, usually zinc, which is eaten by the chemicals in proportion to the amount of current delivered; a mechanical generator produces a current at the expense of the power that drives it.

Chemical generators are of two kinds, primary and secondary cells. While primary cells are of many forms, that called the dry cell is universally used for the ignition of automobile engines, being adapted to the purpose because there is nothing to spill, and may be used in any position. It consists of a zinc cup lined with some material of the nature of blotting paper, in which, but not touching it, is a stick of carbon, the space between being filled with absorbent material and broken bits of carbon moistened with the proper chemical solution. The top of the cell is sealed to prevent evaporation, and thumb nuts on the projecting end of the carbon stick and the zinc cup form the terminals to which the wires are attached. The carbon being the positive pole and the zinc the negative, the current flows in that direction whenever a circuit is provided.

The dry cell gives a current at a pressure of about one and a half volts when new, the voltage dropping as the cell is used until it is exhausted, when the trifling cost of a new cell should not be considered in replacing it.

A better source of current than the dry cell is the secondary or storage cell, which is a reservoir rather than a generator. When a current of electricity is passed to it, usually from a lighting circuit, a chemical change takes place, the action being called the charging of the cell. When the change is complete, the cell is disconnected, and will then deliver a current because the parts of the cell that underwent a chemical change tend to return to their previous condition. The current that it gives off is practically equal to that by which it was charged, and may be used steadily or intermittently until the cell is exhausted. It is made of prepared lead plates, which are placed in a hard-rubber or celluloid jar, with a cover to prevent the spilling of the solution with which the plates are covered.

The charging of storage cells must be done by an expert, for mishandling will ruin them, and it is far better to put this matter, as well as repairs, in the hands of a man who knows his business than to attempt it without the proper knowledge and appliances.

One point that the automobilist must watch, however, is the position and quality of the solution in the cells, called the electrolyte. The cells should always be full enough to have the plates well covered, and any loss by leakage or splashing should be made up, for the plates must not be exposed to the air.

Electrolyte is made by adding one part of chemically pure sulphuric acid to from three to four parts of distilled or rain water, adding the acid slowly and stirring constantly. The combination of acid and water generates heat, and care must be taken not to pour the water into the acid, for the generation of heat would then be so sudden that the mixture would boil and splash, burning whatever it touched. By means of an instrument called a hydrometer, the density of the mixture may be ascertained, and when the proportions are correct the scale should read 1,200. Hydrometers are made with the instrument inside of a glass tube, one end of which has a spout, the other being fitted with a rubber bulb. By inserting the spout in the vent hole of the cell cover, the tube may be filled with electrolyte, the hydrometer showing whether or not it is of the correct density.

The cells should be tested every little while, and if the density is not correct acid or water should be added to make it so. Never probe inside of the cells with anything but a glass or hard-rubber rod, for metal would cause a short circuit between the two plates it touched, and probably ruin the cell.

The terminals of a storage cell are marked in order to distinguish them, the symbols being the plus (+) sign for the positive pole and minus (-) for the negative, these being universally used in this way on all electrical work. In some makes of storage cells the positive pole in addition is painted red and the negative black.

A storage cell when fully charged gives a current at a pressure of about two and a half volts; the voltage drops as the cell is used, and when it gets down to 1. 8 volts it must be recharged. When a storage cell is discharged, it should be recharged immediately, for otherwise it will deteriorate.

One dry or storage cell does not give a sufficient current to supply the required spark, and several must therefore be used. The most usual method of connecting dry cells, and the only method of connecting storage cells, to form a battery, is to connect the negative pole of the first cell to the positive pole of the second, using a short length of insulated wire of sufficient size, the negative of the second to the positive of the third, and so on, until all of the cells necessary to supply the required current are used. This leaves the positive pole of the first cell and the negative pole of the last cell free to be connected into the circuit. The current obtained will have a pressure that is as much greater than the pressure of one cell as there are cells in the battery; for instance, if there are four cells in the battery, of one and a half volts each, the voltage of the battery will be six volts. This method is called connecting in series (Fig. 24).

Fig. 24.—Battery Connections.

When the voltage of one cell is sufficient, and it is desired to increase the volume, or amperage, of the current, the cells may be connected in parallel, which is done by connecting together all of the negative poles by one wire, and all of the positive poles by another, the two wires leading to the circuit. This increases the amperage as many times as there are cells.

If it is desired to connect the cells so that both the voltage and amperage are increased, the cells may be divided into two or more groups, with the same number of cells in each. The cells of each group are connected in series, and the free poles of the groups connected in parallel, which is called connecting in series-multiple (Fig. 24). This is a most satisfactory method of connecting the cells for an ignition circuit, for it gives a uniform current, and increases the life of the cells. The voltage of the current is equal to the voltage of one cell multiplied by the number of cells in one group, and the amperage that of one cell multiplied by the number of groups.

The action of a mechanical generator, which when driven produces a current of electricity, is due to magnetism, that being understood to be the power sometimes possessed by iron or steel to attract other pieces of iron or steel. Magnetism may be manifested either by permanent or electro-magnets. If a piece of steel is magnetized—that is, made a magnet—it continues to be a magnet, for steel retains that property, while iron does not. A magnet made of steel is called a permanent magnet.

If a piece of iron is wound with insulated wire through which an electric current passes, the iron will become a magnet, and will remain so as long as the current flows. This is called an electro-magnet.

The two ends of a magnet are the positive and negative poles, and the positive pole of one magnet will repel the positive pole and attract the negative pole of another magnet. Between the two poles of a magnet there is a constant flow of what are known as magnetic lines of force, which may be made visible by laying a piece of paper over the two poles and scattering iron filings on it. The filings will form into curves from one pole to the other, showing the presence of these lines.

If a wire is moved across these lines of force, so that it cuts them, a current of electricity will be set up in it, and if the wire is wound into a coil and moved across the lines, the current will be as much stronger as there are turns of wire cutting them. A mechanical generator consists of a coil of insulated wire, arranged so that as it revolves in the magnetic field each turn of wire cuts across the lines of magnetic force.

The magnet producing the lines of force that compose the magnetic field is called the field, and the revolving coil of wire the armature.

Mechanical generators for producing the current for an ignition circuit are of two types, magnetos and dynamos, the former being in much more general use than the latter. In a magneto the field is a permanent magnet, and in a dynamo it is an electro-magnet. The magnets forming the field of a magneto are bent into horseshoe shape, so that the poles are close together, and the armature revolves between them. The field of a dynamo is of similar shape, wound with insulated wire, the current that flows through it being generated by the dynamo itself; the field is strongly magnetized only when the dynamo is running.

The armature of either type is a piece of soft iron, with grooves cut in it lengthways, in which the wire is wound; the winding thus being contrary to that of thread on a spool. The current generated in the wire of the armature is taken off by means of brushes, and connection is made through binding posts set in the base.

As the care of a mechanical generator requires a knowledge of electrical engineering, it is better to return it to the makers in case of injury, rather than to attempt to repair it without the necessary experience. (See Appendix.)

Practically all ignition circuits are provided with two sources of current, so that in case of the failure of one the other will be available, and either may be connected into the circuit by means of a switch. The simplest form of switch is a piece of flat spring brass, pivoted at one end so that the other may be swung from side to side to make contact with one or the other of two brass knobs, or switch points. One pole of each generator, usually the negative, is grounded, and the other connected to one of the points. The flat spring, or switch blade, is connected to the ignition circuit, so that when it is in contact with one of the points the corresponding source is called on to supply the current. The blade may rest between the points, not touching either, in which case the circuit is broken, and no current flows.

CHAPTER VII
IGNITION—(Continued)

While there are various methods by which an electric spark may be produced, but two are in use for automobile work, one being the high-tension or jump-spark system, and the other the low-tension or make-and-break system. The difference between these is in the pressure of the current, which in the first is great enough to enable it to jump from one terminal to another a short distance away, forming a spark as it passes. The pressure of the current of the low-tension system is not sufficient to permit it to do this, but when two terminals of the system are brought into contact so that the current may flow, a spark will form as they are separated.