THE IGNITION SYSTEM
It was the application of the electric current to the ignition system
of the gasoline engine that first enabled these new forms of power
plants to be designed with sufficient compactness and to possess
enough flexibility to render their use practical on self-propelled
vehicles. Without the electric ignition system, the speed and power of
the vehicle could not well be controlled, and the explosions would be
uncertain and irregular, at best.
Those of us who are familiar with the electric gas lighters that
were in popular use a few years ago are furnished with a convincing
demonstration of the operation of the first electric ignition systems.
By pulling a chain, a wire, or arm was rubbed across a metal point
until the contact thus formed was suddenly broken. This arm and the
stationary point formed the two terminals of an electric circuit, which
caused a flash of blue flame when the contact was broken as the one
was "wiped" across the other. The flame thus formed at the instant the
contact was broken contained sufficient heat to ignite the gas escaping
from the burner to which the device was attached.
Sparks will be formed in the same manner if we hold two wires,
connected to the opposite poles of a set of batteries, in both hands
and wipe the bare ends across each other. If an arrangement producing
this effect is introduced into the gas engine cylinder at the portion
in which the charge is compressed, the flash resulting when the
terminals are separated will serve to ignite the explosive mixture.
The movable terminal is connected to a rod which passes through the
cylinder walls and is attached to a mechanism actuated by a cam
revolved by the engine. This mechanism is termed the "make-and-break"
ignition system for the reason that contact of these terminals is
alternately made and broken to produce the flash of electricity that
explodes the surrounding charge.
In order to produce a flash of sufficient size when the contact is
broken, the nature of the current, obtained from the dry cells or
storage battery is changed somewhat by conducting it through a coil of
wire surrounding a bundle of bare copper wires. This is known as a
spark coil, and while it is generally used with battery ignition of the
make-and-break type, magnetos may be designed which produce the proper
kind of current direct, without the aid of the coil.
An ordinary set of six dry cells, connected in series--or like with
unlike poles--will produce a current of between twenty and twenty-five
amperes at a pressure of about nine volts--assuming each battery, when
new, to deliver twenty-five amperes at a pressure of one and one-half
volts. The "series" wiring gives the entire set the combined voltage
of all with the average amperage of one. For the benefit of those who
have forgotten their elementary physics, let it be remembered that the
ampere is the measure of current _amount_, or flow, while the voltage
is concerned only with the _pressure_ of the current. By the use of
various arrangements of windings of wires, the voltage may be raised
with a corresponding decrease in the amperage--and vice versa. Thus, if
a coil is used that doubles the original number of amperes produced by
the battery, the voltage will be halved.
The make-and-break type of ignition has been used successfully for many
years, but with the perfection of the magneto, it has been largely
supplanted, in automobile practice, at least, by the "jump spark," or
"high-tension" system. Because of the fact that the latter system is
less expensive to construct and is highly efficient, it will be found
also on the majority of the older cars not equipped with a magneto.
It was found, after the general adoption of the make-and-break
ignition system, that a flame was not necessary for the combustion of
a properly-mixed charge in the engine cylinder. In fact, a tiny spark,
scarcely one-sixteenth of an inch long and no larger around than a
pin, was discovered to be sufficient to produce the ignition of the
charge. Although, of small volume, such a spark generates intense heat,
and it is upon this quality, rather than upon area, that the charge
depends for its ignition--although it is claimed that a large flame
will produce more complete, rapid, and consequently more efficient,
combustion. But the jump spark possesses the advantage of requiring no
moving parts projecting through the cylinder walls into the combustion
chamber, and its greater simplicity over that of the make-and-break
system has resulted in its almost universal adoption by automobile
manufacturers.
It has been stated in a preceding paragraph that the voltage produced
by the average battery set will not exceed nine or ten, and even the
pressure generated by the ordinary magneto is not greater than this.
But air is not a good conductor of electricity and forms a very high
resistance to the passage of a current. It is only when the high
resistance of an air gap is encountered in its circuit, however, that
a spark will be formed by the current, and consequently the form
of electricity used in this system must have resistance-overcoming
properties. But it is only by raising the voltage of the current that
even a short air gap can be bridged by the spark. In fact, a pressure
of somewhat over fifty thousand volts is required to produce a spark
less than an inch long in the air.
Although only called upon to jump a gap about a sixteenth of an inch
across, the ordinary high-tension current is capable of bridging a
space eight or ten times this width in order that ample pressure will
always be assured for the formation of the spark. Furthermore, the
warm gases in which the spark is formed in the cylinder increase the
resistance ordinarily encountered and it is consequently necessary to
raise the voltage above the amount that would be needed were the plug
exposed to the open air.
These conditions make advisable a pressure of from twelve thousand to
thirty thousand volts in the ordinary jump spark system, and it is
from this voltage that the term "high tension" is obtained. The nine
or ten volts delivered by the batteries are transformed to this larger
amount by means of an induction coil--or what is more generally termed
merely the "coil." This is in reality a "step-up" transformer, since
it transforms the current from one of low voltage to another of two or
three thousand times its original pressure.
This transformer consists of two coils of wire, one surrounding the
other. The inner coil is composed of a comparatively few number of
turns of rather coarse wire wound around a soft iron core, and is
termed the "primary" winding, since the current from the batteries is
led directly through it. The outer coil is composed of many turns of a
very fine wire, all of which are thoroughly insulated from each other
and from the inner winding. This outer coil is termed the "secondary"
winding and is the one from which the high-tension, or transformed,
current is taken.
This secondary current is "induced" from the primary winding through
which the battery current passes and possesses a voltage that has
increased over its original amount in the same proportion that the
number of turns in the secondary winding bears to those in the
primary. Therefore, if the original battery voltage is ten and there
are a thousand times as many turns in the secondary winding as in the
primary, the resulting high-tension current will have a pressure of ten
thousand volts.
The principle of the coil is dependent entirely upon that peculiar
electric property known as "induction." Around every wire through which
an electric current passes are invisible "lines of force" similar
to those that emanate from an electro-magnet. These lines of force
surround the wire throughout its length, and arrange themselves in a
spiral formation. Insulation has no effect on these lines of force,
and they may be collected from wires which are separated from each
other by several thicknesses of current-confining material. It is, of
course, necessary to use insulated wires in the construction of these
coils, for otherwise the current would merely pass to adjoining turns
and would not travel the entire length of the winding--and therefore as
great a number of lines could not be collected.
If an additional layer or layers of wire is wound around the first
series of turns, the lines of force will be collected, or "induced,"
by this second coil, and will constitute the secondary current. The
induction effect is greatly increased if the primary current is allowed
to accumulate, or "pile up," and discharge, alternately, for this
surging of the current creates a sort of "overflow" from the original
containing wires.
Ohm's Law, which states that the number of amperes in an electric
circuit is equal to the voltage divided by the number of ohms of
resistance encountered, shows that the current will be changed by its
passage through the primary winding. The induced current is further
changed, and when collected by the secondary winding and sent through
its long coils, we have the high-tension circuit mentioned in the
preceding paragraph.
If the reader remembers that it is but one hundred and ten volts that
is used to operate our electric lights and that five hundred will run a
trolley car, he may wonder why it is not dangerous to handle as great
a pressure as the thirty thousand volts that are used in connection
with the ignition system of a motor car. But it is the combination
of great voltage with high amperage that is dangerous, and if it is
remembered that, as the former is increased, the latter is reduced
correspondingly, it will be realized that the ordinary high-tension
ignition current possesses a _quantity_, or flow, of scarcely one
one-hundredth of an ampere.
If we liken the electric current to a flow of water in a pipe, we have
the amperes corresponding to the quantity of the flow, or the number
of gallons that will be delivered at the outlet in a given time.
Continuing this analogy, the voltage of the electric current will be
the pressure, or "head" in the water system, and the current from the
batteries before the coil is reached will correspond to a moderate
flow of water at a comparatively low pressure. After the coil has
transformed the current to the high voltage, we have the conditions
of a very small opening in the water pipe containing a tremendous
pressure. Such a stream will possess but small flow, but its high
pressure will enable it to be "squirted" to a far greater distance than
would be the case were its volume larger and its "head" less. Although
the pressure is high, its quantity is so low that the stream can do but
little damage and would scarcely more than tickle the flesh of a person
against whom it is directed.
Thus it is with the ignition current. It can "tickle," rather
viciously, sometimes, as many persons will aver, but the _amount_
of electricity involved is so slight as to render the high pressure
harmless. Nevertheless, it is well to avoid allowing the fingers or the
arm to become a part of the high-tension circuit, for the result may be
startling as well as annoying.
But in order that the high voltage shall be induced in the secondary
coil, the primary circuit must be alternately made and broken between
one stroke and the next. Consequently proper "piling up," or "surging,"
of the current will be effected. This is accomplished by means of an
"interrupter" that either vibrates rapidly or "snaps" once at the
formation of each spark. The former is the more common type used with
battery ignition and is known as a vibrating coil. A circuit breaker is
generally incorporated in the mechanism of a magneto, and consequently
when such an instrument is used, the vibrator on the coil is dispensed
with. It is the vibrator on each coil that forms the "buzz" that can
be heard whenever the box cover is removed, and that often furnishes a
simple test for determining the condition of the ignition system of the
particular cylinder with which that coil is connected.
The vibrator is a flat, spring steel piece that rests near one end of
the soft iron core around which the primary coil is wound. The springy
nature of the vibrator ordinarily holds it against a small, adjustable
contact point that should be set about an eighth of an inch from the
end of the above-mentioned soft iron core. The primary coil is so wired
that its current passes through the vibrator steel and the contact
point against which it rests. As soon as the current travels through
the coil surrounding the soft iron core, however, the latter becomes
magnetized and draws the steel vibrator toward it. This breaks the
circuit, the magnetism of the iron core disappears, and the vibrator
returns to its original position against its contact point. But this
action again forms the circuit, and the same operation is repeated as
long as the current is allowed to flow toward the coil.
This is the same principle on which an electric bell is rung, but the
vibrator of the coil makes and breaks the circuit much more rapidly
on account of the less weight of the moving parts. This vibration of
the coil interrupter is so rapid--hundreds a second probably--that the
resulting spark is practically continuous and shows no effect of the
breaks in the circuit.
Even though it is the primary current, of low voltage, that is
interrupted by the vibrator, the frequency of these interruptions
causes a slight sparking, or arcing, at the contact points. These are
therefore subjected to rather a high degree of heat, as well as a large
amount of wear, and it is necessary that they be made of a material
that will resist both. Platinum has been found to be unusually suitable
for this purpose, but owing to its high cost, only a small amount in
the form of two points, or "buttons," is used. One of these points is
placed in the vibrator steel, and the other is embedded in the end of
the screw against which the first rests. Thus the actual contact is
made against these heat-and-wear-resisting platinum points, and it is
evident that upon their proper action depends the formation of the
spark in the cylinder with which that particular vibrator is connected.
Notwithstanding the fact that platinum possesses high heat-resisting
properties, the constant arcing at the contact points will eventually
form a sort of corrosion in which minute particles of the material
are carried from one point to the other in the direction in which the
current flows. If the current is reversed, the corrosion will take
place in the other direction, and consequently the platinum point
that formerly lost a part of its material will gradually be "built up"
again. This corrosive action is known as "pitting," and while it may be
reduced to a certain extent by reversing the terminals of the battery,
as described, the platinum will occasionally require additional
attention.
A coil having badly pitted contact points on the vibrator will "stick"
and will cease to form a spark regularly. It is often difficult to
distinguish between trouble arising from badly-pitted contact points
and that caused by weak or nearly-exhausted batteries, as either
ailment produces the same symptoms of irregular running and "jerking"
in the motor. For this reason, a volt and ampere meter for measuring
the pressure and amount of the current delivered by the batteries
should form a part of every automobile owner's tool equipment.
It is the amperage, rather than the voltage, that is reduced through
continued use of the batteries, and when this quantity falls below
nine or ten, the cells should be discarded--or recharged, in the case
of a storage battery. But if the ignition occurs irregularly when the
batteries are delivering the proper amount of current, it is probable
that the trouble lies in the pitted condition of the platinum contact
points of the vibrator of the coil. Fine emery cloth rubbed over the
surfaces of contact should serve to remedy matters. It should be made
certain that the resulting surfaces on the platinum points are not only
rubbed smooth, but level, as well, in order that the entire area of
each will rest in contact and the current will not be concentrated at a
small portion.
It is probable that there will be a screw adjustment on the vibrator
by means of which the force with which the latter rests against its
contact point may be regulated. If the vibrator is set too tight, an
undue amount of current will be required to magnetize the core of the
coil sufficiently to pull the vibrator away from its contact point, and
the batteries will soon "run out." On the other hand, the tension of
the vibrator should be sufficient to enable it to spring away from the
core of the coil as soon as the circuit is broken, for otherwise the
vibrator will lag and will not be as "lively" as is necessary to obtain
the best results.
The contact screw should be set so that the vibrator rests _about_
three-thirty-seconds of an inch from the end of the magnetic core.
After the tension of the vibrator has been set to approximately the
proper amount, the ear must be trusted for the correct adjustment of
the contact screw. When the switch is thrown on and the motor turned
until current flows through the coil, the resulting buzz emanating
from the vibrator should be decided and forceful. If this buzz is
exceedingly high-pitched, it is an indication that the vibrator has
been set too tight, and its tension should be loosened if unscrewing
the contact point slightly does not lower the tone. It must be
remembered that the tension of the vibrator can be changed by turning
the contact screw. If this screw is turned down so that it forces the
vibrator toward the iron core, the tension will be greater than will be
the case if the contact point is turned to the left.
If the buzz of the vibrator is pitched lower than was formerly the
case, it is an indication that the contact point should be screwed
down, or that the tension of the vibrator should be tightened. It is
probable that turning the contact screw to the right will produce the
proper result. While these changes in the position of the contact
screw are being made, the switch should be left turned on so that the
variations in the pitch of the vibrator buzz may be detected. When an
evenly-pitched, vigorous buzz has been secured, the switch should be
thrown on and off several times to make certain that the response of
the vibrator is instant and positive. The switch should then be left on
and the vibrator allowed to buzz for several seconds in order that it
may be determined whether the pitch of the sound will change, or not.
If there is a change noticeable, the contact screw should be readjusted
until the pitch of the buzz remains constant as long as the circuit is
closed.
The coil and batteries or magneto by no means form the entire ignition
system, although the generation of the spark depends entirely upon
them. The spark must be regulated to occur at the proper point in the
stroke of the piston, as a continuous spark would not only waste the
current, but would cause the ignition of the charge during the upward
stroke and would result in an impulse in the reverse direction that
would prevent the motor from running for more than half a turn.
The device by which the time of the occurrence of the spark is
regulated is called the timer. This consists, in its essentials, of a
hard rubber disc provided with a copper or brass segment. A metal pin,
roller, or ball rests against the outer edge of the disc, and as the
latter is revolved, the electrical circuit is completed whenever the
two metal portions come in contact with each other. The hard rubber
being a non-conductor of electricity, prevents the flow of the current
at all other times. The disc of the timer, known as the "commutator,"
is so geared that it revolves in unison with the motor.
Inasmuch as the explosion occurs in each cylinder only at every second
stroke of a four-cycle motor, the commutator on this type of engine
is geared to revolve at one-half the speed of the crank shaft. In
the two-cycle motor, on the other hand, the explosion occurs in each
cylinder at every revolution, and consequently the commutator should
turn at crank shaft speed.
Although the spark is intended to occur approximately at the extreme
upper end of the compression stroke, a few degrees variation both above
and below this point is necessary in order to obtain the desired speed
and power flexibility of the gasoline motor. At high speeds, the spark
should be timed to occur before the piston reaches the extreme top
of its stroke, while at slower revolutions of the motor the ignition
should take place, in some instances, just after the piston has
started to descend. This variation In timing is obtained by swinging
the contact piece of the timer--known as the brush--either forward
or backward through an arc corresponding to the range of advance and
retard.
If this brush is swung in a direction opposite to that of the
revolution of the commutator, the metal portions will meet sooner, with
the result that the spark will occur earlier, or will be "advanced."
If, however, the brush is swung to a point farther along in the
direction of rotation of the commutator, the spark will occur later,
or will be "retarded." These variations of position of the brush are
generally obtained by means of a lever attached to the steering post or
wheel.
It is evident that the current must pass from the brush to the metal
segment of the commutator in order to complete the circuit through
the timer and thus form the spark. It is the primary current, or
low-tension current from the battery or magneto, that passes through
the timer, and as this is of low voltage and is therefore easily
discouraged, it is necessary that the contact points be kept clean in
order that its travel may be made easy. Timers are generally protected
from dirt, but the particles that will naturally be worn off from the
metal and rubber commutator and brush should be cleaned out before its
accumulation becomes deposited on the contact points and interferes
with perfect electrical connection.
A few years ago, the majority of battery ignition systems employed a
separate coil for each cylinder of the motor. Each coil in this system
is connected with an individual brush that operates against the same
commutator as do the brushes for the other cylinders. With such a
system, the primary circuit leads from one terminal of the battery to
the primary winding of the coil, through this and the vibrator to the
brush of the timer reserved for that particular coil and cylinder, and
thence through the switch to the other terminal of the battery. This
order may be reversed, or the timer, switch, and coil may be placed
in any consecutive position, provided the current passes through all
in its travel from one terminal of the battery to the other. The
secondary, or high-tension current is led from the terminal of the
secondary winding on the coil to the spark plug of the proper cylinder.
There should be a "ground" wire to serve for the return of the
secondary current. This may lead from any part of the primary circuit
to a clean metal connection on the motor.
The multiple coil system is still used to a large extent, but an
elaboration of it will be found on many of the modern cars. This
consists of the use of but a single coil for all of the cylinders of
the motor. This is done by means of a distributor, which is a sort of
"glorified timer" consisting of a commutator provided with as many
segments as there are cylinders in the motor. This distributor receives
the current from a single coil and delivers it to the proper cylinder
as the various connections are made. The timer still performs its
function of completing the circuit from the source of current only at
the proper instant, and leaves the distributor to serve the purpose of
a "switch" to "sidetrack" the current and deliver it at the various
cylinders in turn.
If it should ever become necessary to remove any part of the timer,
or to change the length of the spark control rods, the greatest care
should be taken to make certain that the motor is properly timed when
the various portions are replaced. This can best be done by setting the
spark lever in its central position, removing a plug from one of the
cylinders, and introducing a rod or long screw driver into the opening
for the purpose of determining the exact top of the stroke of the
piston. When the flywheel is turned, the top of the stroke should be
marked on the rod or screw driver as the latter is forced upward by the
piston.
If the spark plug is laid with its large nut resting on the cylinder
head, and the switch is thrown, the time of the occurrence of the spark
can be readily observed as the motor is turned slowly by hand. This
spark should occur in this particular plug just as the piston of that
cylinder reaches the top of its stroke, as indicated by the change in
the direction of the movement of the rod or screw driver. If the spark
occurs too soon or too late, the commutator should be moved backward or
forward to remedy the respective trouble. Although if the timer is set
properly for one cylinder it is probable that the spark in the others
is also timed correctly, it is well to test each to make certain that
there has been no uneven wear in the contact segments of the commutator
or the brush.