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).
[Illustration: 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.