air without appreciable noise. Whenever a cut-out valve is provided it is usually planned to open out so that any explosion of gas in the muffler which might result if the motor missed several explosions and then fired the gas in the silencer will tend to open the valve and relieve the excessive pressure in the muffling device. If some means were not provided to relieve the pressure, it might burst the muffler asunder. The average cut-out valve, therefore, performs three useful functions: First, it permits the exhaust gases to be discharged directly to the air at such times that maximum motor power is desired; second, it provides audible indication of irregular engine action; third, it is a safety or relief valve to prevent excessive pressures from damaging the mufller.

CHAPTER VI

Automobile Power Plant Ignition Systems Outlined-Chemical Current Producers-Mechanical Generators of Electricity-Essentials of Battery Ignition Systems-Functions of Timers and Distributors-Operating Principles of Induction Coil-Spark Plug Construction and Action Defined— Advantages of Two-Spark Ignition-Typical Battery Ignition Groups— Low-tension Ignition Systems-High-tension Magneto Forms-Typical Double Ignition Systems.

ONE of the most important auxiliary groups of the gasoline engine comprising the automobile power plant and one absolutely necessary to insure engine action is the ignition system or the method employed of kindling the compressed gas in the cylinder to produce an explosion and useful power. The ignition system has been fully as well developed as other parts of the automobile, and at the present time practically all ignition systems follow principles which have become standard through wide acceptance.

During the early stages of development of the automobile various methods of exploding the charge of combustible gas in the cylinder were employed. On some of the earliest engines a flame burned. close to the cylinder head and at the proper time for ignition, a slide or valve moved to provide an opening which permitted the flame to ignite the gas back of the piston. This system was practical only on the primitive form of gas engines in which the charge was not compressed before ignition. Later, when it was found desirable to compress the gas a certain degree before exploding it, an incandescent platinum tube in the combustion chamber, which was kept in a heated condition by a flame burning in it, exploded the gas. The naked flame was not suitable in this application because when the slide was opened to provide communication between the flame and the gas the compressed charge escaped from the cylinder with enough pressure to blow out the flame at times and thus cause irregular ignition. When the flame was housed in a platinum tube it was protected from

the direct action of the gas, and as long as the tube was maintained at the proper point of incandescence regular ignition was obtained.

Some engineers utilized the property of gases firing themselves if compressed to a sufficient degree, while others depended upon the heat stored in the cylinder head to fire the highly compressed gas. None of these methods were practical in their application to motor car engines because they did not permit flexible engine action which is so desirable. At the present time, electrical ignition systems in which the compressed gas is exploded by the heating value of the minute electric arc or spark in the cylinder are standard, and the general practice seems to be toward the use of mechanical producers of electricity rather than chemical batteries.

Two general forms of electrical ignition systems may be used, the most popular being that in which a current of electricity under high tension is made to leap a gap or air space between the points of the sparking plug screwed into the cylinder. The other form, which has been almost entirely abandoned in automobile practice, but which is still used to some extent on marine engines, is called the lowtension system because current of low voltage is used and the spark is produced by moving electrodes in the combustion chamber.

The essential elements of any electrical ignition system, either high or low tension, are: First, a simple and practical method of current production; second, suitable timing apparatus to cause the spark to occur at the right point in the cycle of engine action; third, suitable wiring and other apparatus to convey the current produced by the generator to the sparking member in the cylinder.

The various appliances necessary to secure prompt ignition of the compressed gases should be described in some detail because of the importance of the ignition system. It is patent that the scope of a work of this character does not permit one to go fully into the theory and principles of operation of all appliances which may be used in connection with gasoline motor ignition, but at the same time it is important that the elementary principles be considered to some extent in order that the reader should have a proper understanding of the very essential ignition apparatus. The first point considered will be the common methods of generating the electricity, then the appliances to utilize it and produce the required spark in the cylinder.

Current Production by Chemical Action.-The simplest method of current generation is by various forms of chemical current producers which may be either primary or secondary in character. A simple form of cell is shown in section at Fig. 171, A, and as the action of all devices of this character is based on the same principles it will be

Fig. 171.-Simple Primary Cells Used to Produce Electric Current. A-Form to Show Principle of Current Production by Chemical Action. B-Dry Cell, the Type Suitable for Automobile Service.

well to consider the method of producing electricity by the chemical action of a fluid upon a metal. The simple cell shown consists of a container which is filled with an electrolyte which may be either an alkali or acid solution. Immersed in the liquid are two plates of metal, one being of copper, the other zinc. A wire is attached to each plate by means of suitable screw terminals.

If the ends of the plates which are not immersed in the solution are joined together a chemical action will take place between the electrolyte and the zinc plate; in fact, any form of cell consists of dissimilar elements which are capable of conducting electricity immersed in a liquid which will act on one of them more than the other. The chemical action of electrolyte on the zinc liberates gas bubbles which are charged with electricity and which deposit them

selves on the copper plate. The copper element serves merely as a collecting member and is termed the "positive" plate, while the zinc which is acted upon by the solution is termed the "negative" member. The flow of current is from the zinc to the copper plate through the electrolyte and it is returned from the copper plate to the zinc element by the wiring which comprises the external circuit.

While in the cell shown zinc and copper are used, any other combination of metals between which there exists a difference in electrical condition when one of them is acted upon by a salt or acid may be employed. Any salt or acid solution will act as an electrolyte if it will combine chemically with one of the elements and if it does not at the same time offer too great a resistance to the passage of the electric curent. The current strength will vary with the nature of the elements used, and will have a higher value when the chemical action is more pronounced between the negative member and the electrolyte.

As the vibrations which obtain when the automobile is driven over highways makes it difficult to use cells in which there is a surplus of liquid, a form of cell has been devised in which the liquid electrolyte is replaced by a solid substance which cannot splash out of the container even if the cell is not carefully sealed. A current producer of this nature is depicted in section at Fig. 171, B. This is known as a dry cell and consists of a zinc can in the center of which a carbon rod is placed. The electrolyte is held close to the zine or negative member by an absorbent lining of blotting paper, and the carbon rod is surrounded by some depolarizing material. The top of the cell is sealed with pitch to prevent loss of depolarizer.

The depolarizer is needed that the cell may continue to generate current. When the circuit of a simple cell is completed the current generation is brisker than after the cell has been producing electricity for a time. While the cell has been in action the positive element becomes covered with bubbles of hydrogen gas, which is a poor conductor of electricity and tends to decrease the current output of the cell. To prevent these bubbles from interfering with current generation some means must be provided for disposing of the gas. In dry cells the hydrogen gas that causes polarization is combined with oxygen gas evolved by the depolarizing medium and the combi