The _ignition_ system is the name applied to the batteries, coils,
commutator and spark plug which, acting as a whole, produce an
_electric spark hot enough_ and at the _right time_ to _fire_ the
charge in the cylinder. There are three ways in which an electric
current may be obtained which have been found to be of practical use
in automobile construction. First, by a dry battery; second, by a
storage battery; and third, by magneto or dynamo. You can look up the
construction of these things in any good book on electricity, so that I
will not explain them further here. As the object of the whole system
is to produce a sufficiently hot spark to fire the mixture at the right
time, it is sometimes necessary to raise the pressure of the electric
current. When either a dry battery or storage battery or a certain type
of magneto is used, it is necessary to put it through what is known as
an _induction coil_ in order to _raise_ the _pressure_ so it will be
high enough to _jump across_ the two points in the cylinder.
An induction coil consists merely of a bundle of soft wires around
which is wound two separate coils of wire. The first, known as the
_primary_ winding, is of coarse wire, and the second, known as the
_secondary_ winding, is of fine wire. When the current flowing through
the primary coil is rapidly made and broken, another current of very
high voltage is created in the secondary circuit.
[Illustration: Fig. 23--Wiring Diagram, showing electric circuit for
one cylinder. The sketch is only diagrammatic--not a reproduction of
any actual wiring system.]
When the current starts to flow through the primary winding, the
bundle of wires immediately becomes a magnet and attracts the vibrator.
As soon as this occurs, however, the flow of the current is interrupted
and the vibrator resumes its former position and the action is
repeated. Thus you can see that the coil automatically makes and breaks
its own circuit. The rapidity with which this is done may be changed at
will by adjusting the vibrator screw.
[Illustration: Fig. 24--The Four-cylinder, Four-cycle Diagram, showing
the order in which the various cylinders do their work.]
Having thus obtained an electric current of sufficient strength
to fire the mixture it is necessary to supply a device which will
_automatically_ open and close the electric circuit at the _proper_
time. Such a device is called a _commutator_ and consists of two parts;
one a _rotating_ part, actuated by the engine, which makes a metallic
contact with one or more points on a stationary part, the points being
so located that contact occurs at the proper time for igniting the
charge in any particular cylinder.
[Illustration: Fig. 25--Four-cylinder Wiring Diagram.]
In the _four-cylinder_ wiring diagram (Fig. 25) various parts of an
ignition system may be easily seen. The electric current is furnished
either by a set of dry cells or by a storage battery. _Four_ individual
coils and a _four point_ commutator are used. The commutator is driven
by means of bevel gears from the engine itself. A single switch
controls the whole circuit, it being provided with two points, making
it possible for either battery to be used at will.
[Illustration: Fig. 26--Types of Spark Plugs.]
[Illustration: Fig. 27--A Single Cylinder Commutator. Diagrammatic
sketch.]
[Illustration: Fig. 28--A Two-point Switch.]
[Illustration: Fig. 29--A Four-cylinder Commutator.]
[Illustration: Fig. 30--A Two-cylinder Commutator.]