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.
MAKE-AND-BREAK SYSTEM
In the make-and-break system, the =igniter=, which is the device
in the combustion space at which the spark occurs, is made with
two metal points, one of which is stationary and the other movable,
the latter being acted on by a cam, through a =tappet=. As the
cam revolves, the tappet is lifted and the movable point brought
into contact with the stationary; when the nose of the cam passes
from under the tappet, the movable point is snapped away from the
stationary through the action of a spring (Fig. 25). These points
are so connected into the circuit that when they are in contact the
current flows, the circuit being broken when they separate. Shortly
before the spark is desired the movable point is brought into contact
with the other so that the circuit is completed and the current
flows, and they are separated at the instant when the spark is
required to ignite the mixture, this occurring as they separate and
break the circuit.
[Illustration: FIG. 25.—MAKE-AND-BREAK IGNITION.]
The current that is supplied by the battery is not capable of
producing this spark, for it flows with large volume and small
pressure—i.e., high amperage and low voltage—and it is necessary
to transform it to a current of higher voltage and lower amperage,
which is done by means of a =primary induction coil=. A coil of this
description consists of a =core= made of a bundle of soft iron wires,
around which is the =primary winding=, formed by winding several
layers of insulated copper wire on it. The core becomes a magnet when
a current of electricity flows through the winding, and ceases to be
a magnet when the flow ceases, the magnetization occurring slowly on
the completion of the circuit, and demagnetization instantly on the
breaking of the circuit.
The influence of a magnet, as, for instance, its attraction for a
piece of iron, is felt throughout a field the extent of which depends
on the strength of the magnetization. When a loop of wire forming
a closed circuit is placed in the =magnetic field=, a current of
electricity will be set up in it whenever the strength of the field
changes. If the strength of the field does not change, no current
will be set up; but the greater the change in strength, whether
from strong to weak or from weak to strong, the greater will be the
strength of the current. This principle is applied in the case of a
coil, for the wire that forms the winding is in the magnetic field of
the core, and a current is set up in it whenever the strength of the
field changes. The strongest current will be set up, or =induced=,
when the strength of the field changes from no magnetism to a degree
when the core is magnetized to its fullest capacity or the reverse,
and when the change occurs in the least possible time. The current
that is set up will last only during the time when the change in
strength occurs, ceasing to exist when the strength of the field is
uniformly strong or weak. The more rapid the change, the greater will
be the pressure of the current, but the shorter the period of its
existence.
It is this induced current that forms the spark in the combustion
space, the battery current serving only to establish the condition
that will produce it. When the two points of the igniter come into
contact and complete the circuit, the flow of the battery current
through the winding of the coil will gradually magnetize the core,
and on the igniter points being separated, this flow through the
winding will cease, the strength of the magnetic field dropping
instantly from full intensity to practically nothing. This change
induces a powerful current in the winding, which current in passing
over the circuit causes a spark between the points of the igniter
as they separate. The induced current is set up so instantaneously
on the breaking of the battery circuit that the spark is usually
considered as being that of the battery current, but, as stated,
the battery current serves only to magnetize the core, the spark
resulting from the current induced by its demagnetization on the
breaking of the circuit.
A magneto is a machine that when driven generates an electric
current, and is in almost universal use for this system of ignition.
In many cars the system is so arranged that the driver has his choice
between ignition by battery and coil, as described, and ignition by
magneto, but the mechanical production of a current is so reliable
that many automobile builders rely on the magneto alone. Magnetos of
the type used for ignition are described in the appendix.
JUMP-SPARK SYSTEM
The jump-spark or high-tension system of ignition depends on the
production of a current at such a pressure that it can jump from one
point to another through high resistance, such as is presented by air
at ordinary or low temperature, and especially when compressed, the
spark occurring as it passes.
This high pressure is obtained through the use of a =secondary
induction coil=, which consists of a core and primary winding similar
to those of the primary induction coil already described, and in
addition has a =secondary winding=, formed by a great length of
insulated copper wire, which to save bulk and weight is very fine,
wound over the primary winding. The change in the strength of the
magnetic field produced by the core affects this secondary winding,
just as it affects the primary winding; that is, a current is set
up in it at every change in the strength of the magnetic field, the
pressure of the current depending on the greater or less extent of
the change, and the rapidity with which it occurs. The magnetization
of the core takes place but slowly, for the particles of iron
must absorb it one from the other, but the demagnetization occurs
instantly; the current that is induced during the demagnetization
is therefore the greater, and is the current that has sufficient
pressure to jump across the gap.
The ignition system employing this coil consists of two distinct
parts: the =primary circuit=, which magnetizes the core, and the
=secondary circuit=, which leads to the combustion space the current
that is induced in the secondary winding.
The primary circuit includes the battery or generator that supplies
the current, the primary winding of the coil, a =timer=, and a
=vibrator=.
THE TIMER
The spark must occur in the combustion space only at the instant
when the mixture is in a condition to be ignited, and it is obvious
that only then is the secondary current required. As the secondary
current results from the flow of the primary current through the
primary winding, a device must be included in the primary circuit to
permit the current to flow and magnetize the core at this instant.
This device is called a =timer=, or =commutator=, and is nothing more
than a revolving switch operated by the engine, that completes the
primary circuit at such times as the secondary current is required,
and breaks the circuit when the secondary current has done its work
in the production of the spark. Because ignition occurs but once in
each cylinder during two revolutions of the crank shaft, the timer
completes the circuit but once in that interval, and is placed on the
half-time shaft that operates the exhaust valves. In its simplest
form, a timer consists of a disk of hard rubber, wood fiber, or
other insulating material in which is set a metal plate, mounted on
the half-time shaft and revolving with it. A flat spring carried on
a plate of hard insulating material bears against the edge of the
disk, and during one revolution of the latter the metal plate set
in it will be brought into contact with the spring (Fig. 26, first
diagram). If one wire of the primary circuit is connected to the
metal plate of the disk and the other wire to the spring, it will be
seen that the circuit will be complete when the revolution of the
disk brings the two together, and broken when the further revolution
of the disk separates them.
[Illustration: _ONE CYLINDER_ _TWO CYLINDER_ _FOUR CYLINDER_ FIG.
26.—TYPES OF TIMERS.]
There is a great variety in the methods by which this result is
secured, the chief essential being that the contact must be made as
positively as possible in order that the resistance to the current
in passing from one part of the timer to the other may be as low
as possible, and broken equally positively. The number of contacts
that the timer makes during one revolution depends on the number of
cylinders of the engine, the revolving part touching two, three,
four, or more springs or contact points on the stationary part as
it turns. A timer for a two-cylinder engine is shown in the second
diagram on Fig. 26, the third diagram being of a form of timer in
which the revolving part is not connected into the circuit, but
serves to bring together a spring and a contact point, the circuit
being completed when they touch. A modern form of timer consists of
a ring of hard rubber on the inside face of which are set the metal
plates, while the moving part revolves within it, and may be a flat
spring, or a plunger, or a roller, contact being maintained by a
coil spring. The difficulty of attaching a wire to the moving part
of the timer is overcome by having its contact in connection with
the half-time shaft, the current flowing from the contact on the
stationary part to that on the moving part, and from there flowing by
the half-time shaft to the ground connection of the battery, for, as
all parts are of metal, the current can pass from the shaft to the
ground connection through the bearings and the engine.
If the spark were always required to pass in the combustion space at
the same point in the stroke of the piston, this could be secured
by setting the stationary part of the timer in such relation to the
moving part that contact would be made when the half-time shaft
reached any desired point in its revolution. As it is necessary to
change the point at which ignition occurs, however, the timer must be
so arranged that it closes the circuit earlier or later, according
to conditions. To effect this, the stationary part of the timer is
so made that it may rotate around the shaft for about a half turn,
and it is controlled by a lever on the steering column, which holds
it in any desired position. If the stationary part is rotated in the
direction opposite to that in which the half-time shaft revolves,
contact will be made and the circuit closed at an earlier point
in its revolution than if it is rotated in the same direction. It
must be remembered that the revolution of the half-time shaft, in
operating the exhaust valve, revolves exactly in accordance with the
crank shaft and the movement of the piston, and that as the moving
part of the timer is attached to it, contact can be made at any
desired point in the stroke of the piston by the position in which
the stationary part is placed.
THE VIBRATOR
The secondary current occurs whenever the core is magnetized and
demagnetized, and it is clear that the oftener the magnetization
occurs the more constant will be its presence. The timer closes and
opens the primary circuit, which results in the production of but two
impulses of the secondary current, and as these are not sufficient
to produce the necessary spark, additional means are employed to
make and break the circuit while the timer makes contact. This is
accomplished by means of a =vibrator=, or =trembler=, which consists
of a flat steel spring secured at one end, so that it may vibrate
after the manner of a tuning fork. About midway between its ends the
point of an =adjusting screw= touches it, and when the flat spring,
or =blade=, vibrates, it springs away from this screw and returns
to it, both screw and blade being tipped with platinum to offset
corrosion. If one wire of the circuit is attached to the blade and
the other to the screw, the circuit will be complete when the blade
touches the screw, and broken when it springs away from it. In the
early type of vibrator the blade was set in vibration by mechanical
means. At the end of the blade was a weight, which rested on the edge
of a cam, the blade then being out of contact with the adjusting
screw (Fig. 27). During the revolution of the cam the weight dropped
into a deep notch, thus setting the blade into vibration and the
circuit being made and broken. This type has been almost entirely
superseded by the =magnetic vibrator=, which is more rapid and
accurate. In this the blade is so placed that its free end is close
to the end of the core of the coil (Fig. 28). When the timer makes
contact and the current passes through the primary winding, the core
becomes magnetized and attracts to it the free end of the steel
blade, this movement drawing the blade away from the adjusting screw
with which it was in contact. The vibrator is so connected into
the primary circuit that the current flows from the battery to the
adjusting screw, thence to the blade and through the primary winding;
when the attraction of the blade by the core draws it away from
the adjusting screw, the circuit is broken, and the magnetism then
ceasing to exist, the elasticity of the blade causes it to spring
back to the screw, to make contact, and to be again attracted by the
magnetism.
[Illustration: FIG. 27.—MECHANICAL VIBRATOR. When the weight drops
into the notch on the cam, the flat spring touches the adjusting
screw and the current passes.]
[Illustration: FIG. 28.—MAGNETIC VIBRATOR. When the magnetism of the
core attracts the vibrator the contact is broken at the adjusting
screw.]
This action continues as long as the timer holds the circuit closed,
and a wave of secondary current is set up in the secondary winding
for every make and break that results. By means of the adjusting
screw, the vibrations may be made long and slow, or short and quick,
with a corresponding difference in the secondary current. As the
object to be attained is the production of a spark that will ignite
the mixture rapidly, the vibrator must be adjusted with this end in
view; but it is also advisable to obtain the spark with the least
possible expenditure of current, and with the least wear of the
parts. The passing of the battery current through the primary winding
establishes the condition that exists in the make-and-break ignition
system, and the demagnetization of the core will set up an induced
current that will result in a spark at the points where the circuit
is broken; that is, at the vibrator and timer contacts. This spark
can be greatly overcome in the timer by packing it with vaseline,
which, being an insulator, breaks down the spark in flowing between
the contacts of the stationary and moving parts as they separate.
If the vibrator contacts presented no resistance—an impossible
condition—there would be no spark, but as they will always be
slightly burned and corroded, the spark must be kept as small as the
requirements of the secondary current will justify by the adjustment
of the screw. In other words, adjust the vibrator so that it gives
the best results in the production of the secondary spark with as
little sparking as possible at the vibrator contacts.
THE SPARK PLUG
The secondary circuit includes the secondary winding of the coil
and the =spark plug=, which is the device in the combustion space
at which the spark occurs. The spark plug provides two points of
metal at a fixed distance apart, which are so connected into the
circuit that the current must jump from one to the other in order to
complete its circuit. The secondary current, as well as the primary,
may make use of the metal of the engine as a ground return, and one
of the points of the spark plug is therefore in contact with the
metal, being supported on a metal sleeve that is screwed into the
cylinder. The other point must of course be insulated from it, for
if it were not the current could pass from one to the other without
jumping the space between them. The insulation is secured by a tube
of porcelain or mica set in the sleeve, the lower end of a metal rod
that passes through it being the required distance from the wire or
projection on the sleeve (Fig. 29). This device must be strong enough
to resist the pressure in the combustion space during the compression
and power strokes, and must be unaffected by the intense heat.
[Illustration: FIG. 29.—_A_, Spark plug; _B_, spark plug in section.]
THE CIRCUIT
Fig. 30 shows the primary and secondary circuits of the jump-spark
system, as applied to a one-cylinder engine, the light lines
representing the primary circuit and the heavy lines the secondary.
Each circuit forms a complete path by which the current passing
over it may return to the point at which it started. In practice
the wiring is simplified by the use of but one ground return for
both circuits, the primary and secondary windings of the coil being
connected at one point, and the secondary current returning either
through the timer as it makes contact or through the battery.
[Illustration: FIG. 30.—IGNITION CIRCUIT. Wiring diagram for
one-cylinder engine, with coil, timer, and two batteries; coil with
magnetic vibrator.]
The flow of the primary circuit is from the battery to the switch,
to the vibrator, which is built into the coil, through the primary
winding and the timer contacts, whence it passes by the metal of the
engine and ground wire back to the battery. The secondary circuit
consists of a wire from the secondary winding of the coil to the
spark plug, and back to its winding by the metal of the engine and a
ground wire, which, as has been explained, may be the separate ground
wire shown in the diagram, or the return that is common to both
circuits.
The timer makes contact as the half-time shaft revolves, and the
vibrator operates as long as the circuit is closed; while this
occurs, the secondary current is set up, and the spark passes.
Included in the primary circuit and built into the base of the coil
is a =condenser= which, while of such delicate construction that it
is most inadvisable for the automobilist to attempt to examine or
repair it, is of such advantage that it should be understood. A
current flowing in a circuit possesses momentum, and if the circuit
is broken, the momentum will cause the current to flow across the
break for a greater or less time, according to the pressure. This
will produce a spark, and a spark at the vibrator as it breaks the
circuit will not only burn away the contacts, but will prevent the
instant demagnetization of the core. The passing of a spark between
the vibrator contacts indicates that the current is still flowing
through the primary winding, and that the core is still magnetized.
The function of the condenser is to prevent sparking at the vibrator
by absorbing the momentum of the current, and, what is more
important, to cause the instant demagnetization of the core by the
instant breaking of the circuit.
While it is usual to provide a multicylinder engine with one coil
for each cylinder, it is possible to equip it with but one, in such
a case a =secondary distributer= being used. This consists of two
parts: a primary timer, and a somewhat similar device by which the
secondary current induced in the coil is switched from cylinder to
cylinder as it is required. For a four-cylinder engine the timer
has four contacts, as in the usual type; but the primary winding is
connected to all of them, so that as the moving part of the timer
revolves, the core of the single coil is magnetized four times to two
revolutions of the crank shaft. A single wire leads the secondary
current that is thus induced to the distributer, by which it is
switched to the several cylinders. The moving part of the distributer
rotates in unison with the moving part of the timer, making contact
as it does so with contact pieces, each of which is connected to one
of the spark plugs.
When the timer makes its contact the distributer is in position to
pass the induced current to the proper cylinder, the connections
being made in accordance with the firing order of the engine. While
this system has advantages in that there is but one vibrator to
adjust, the fact that the coil is operating for the greater part
of the time makes it more liable to injury through heating and the
consequent breaking down of its insulation.