BATTERY IGNITION SYSTEMS
While the greater number of tractor engines use magneto ignition, many
use battery and coil systems, which are the same in general principle
as magneto systems, but produce magnetism in a different manner.
Copper is a _nonmagnetic_ metal; that is, magnetism will not flow
through it, nor can it be magnetized. If a pile of iron filings is
stirred with a copper wire there will be no effect, as might be
expected; but if a current of electricity flows through the wire, the
iron filings will cling to it, as shown in Figure 54, as if it were a
real magnet.
[Illustration: FIG. 54.—MAGNETISM IN A COPPER WIRE]
It is one of the principles of electricity that when a current flows
through a wire, the wire is surrounded by magnetism, which continues as
long as the current flows; when the circuit is broken and the current
stops flowing, the magnetism dies away. The magnetism produced is
feeble and can be very greatly increased by winding the wire around an
iron bar. The magnetism produced by the current then flows into the
bar, and that, like the core of the winding of a magneto, throws out
magnetism of its own. This is indicated in Figure 55. By changing the
intensity of the electric current, or by cutting it off, the strength
of the magnetism can be made to change, and this change of strength can
produce a sparking current.
[Illustration: FIG. 55.—MAGNETISM FROM ELECTRICITY]
The principle employed is illustrated in Figure 56. A is a coil of
wire wound around one end of an iron bar and connected with a battery;
B is an entirely separate coil of wire wound around the other end of
the bar, with its ends separated by a short distance. By closing the
battery switch the current will be permitted to flow in coil A, and
the bar will become magnetized; the magnetism that it throws out will
be felt by coil B. When the switch is opened the current stops flowing
and the magnetism dies out of the bar; these changes in strength will
create an electric current in coil B, which will form a spark as it
passes across the space between the ends.
[Illustration: FIG. 56.—PRINCIPLE OF SPARK COIL]
In ignition coils, coil B is wound on top of coil A. Coil A, called the
_primary winding_, consists of a few layers of coarse wire. The more
turns of wire there are in coil B, called the _secondary winding_, the
more intense will be the current that it produces, and the intensity
is also increased by keeping the windings close to the iron core. The
secondary winding is, therefore, made of exceedingly fine wire, and has
a very great number of turns.
To obtain a spark, a current is permitted to flow through the primary
winding to create magnetism, and the flow is then stopped to cause
the magnetism to die away. The secondary winding is affected by each
of these changes in magnetic strength. The bar loses magnetism more
rapidly than it gains it, however; it is therefore the dying out of the
magnetism that has the greater effect on the secondary winding, and
that causes it to produce a sparking current.
To use this principle for ignition, the engine is fitted with a
revolving switch, which closes the circuit as a piston is on the
compression stroke, and then breaks the circuit at the instant when
a spark is desired. Combined with the revolving switch, or _timer_,
is a distributor like the distributor of a magneto, which passes the
sparking current to the cylinder that is ready to receive it.
[Illustration: FIG. 57.—“ATWATER-KENT” IGNITION SYSTEM]
To produce an intense sparking current, it is necessary to break the
circuit as abruptly as possible, in order to cause the magnetism to die
away suddenly. Figure 57 shows how this is done in the Atwater-Kent
system.
The parts of the circuit breaker are carried on a plate, in the center
of which revolves a shaft with a notch in it. Against the side of this
shaft rests the hooked end of the sliding catch; as the notch comes
under this hooked end, the sliding catch is drawn forward, only to be
snapped back by its spring as the notch moves from under it. The lifter
is a bit of metal, pivoted at one end, with its free end lying between
the sliding catch and the flat steel spring that carries one of the
contact points.
A, Figure 57, is a diagram of the system. B shows the position of the
parts as the notch carries the sliding catch forward, and C shows their
positions as the spring snaps the sliding catch back to its place. It
will be seen that in thus moving back it strikes the lifter, which
in turn moves the contact spring, and so closes the circuit; but the
circuit is instantly broken as the parts spring back to position. The
movement of the parts is so rapid that to the eye they seem to be
standing still. The circuit is closed only for an instant, but that
is sufficient to magnetize and demagnetize the coil, and to produce a
sparking current.
The operation of this system depends on the very great swiftness with
which the circuit is made and broken; there is not sufficient time
for the core to get thoroughly magnetized, but such magnetism as is
produced changes strength so quickly that it gives a sufficiently
intense current to create an ignition spark.
In other battery systems of like principle, the circuit is closed for
a long enough time to allow the core to become fully magnetized, the
circuit then being suddenly broken. In some of these systems the timer
breaks the circuit, while in others it is broken by the magnetism,
through a _vibrator_.
A _vibrator coil_ system is illustrated in Figure 58. The timer is a
ring made of some kind of insulating material, with a plate of metal
set in it and forming one of the timer contacts. The other contact is
the revolving brush, driven by the engine; the circuit is closed when
the brush touches the metal plate.
[Illustration: FIG. 58.—VIBRATOR COIL IGNITION SYSTEM]
Opposite the end of the core is a flat steel spring, or _vibrator
blade_, resting against the tip of a screw; when the core is magnetized
it draws the end of the blade to it, and separates it from the screw.
The battery current flows from the timer contact to the screw, then to
the vibrator blade and to the primary winding of the coil. The core
then becomes magnetized, and draws the blade away from the screw,
which breaks the circuit; this causes the magnetism to die away, and
a sparking current is produced in the secondary winding of the coil.
The vibrator blade, no longer held down by the magnetism, springs
back against the screw; the circuit is again made, and the action is
repeated. The movement of the vibrator blade is very rapid, being some
hundreds of vibrations a second.
[Illustration: FIG. 59.—SPARK PLUG]
A spark plug is illustrated in Figure 59. It consists of a metal shell
screwed into the cylinder, enclosing an _insulator_ of porcelain, mica,
or some similar material. Through the insulator passes the center
electrode, which is a rod of metal, with its lower end separated by a
short distance from the shell or from a wire attached to the shell.
This separation is the gap across which the sparking current passes,
and at which the spark occurs.
Spark plugs receive the pressure of the power stroke, and must be
strongly made in order to withstand it. A leaky spark plug will cut
down the power of the engine, just as a leaky valve will.