IGNITION. HIGH TENSION
This system is used to the largest extent, so that we ought to have a
full explanation of the devices which are required to do the work. While
magnetos are used with the low tension system, for the reasons stated,
they are especially necessary with the _Jump Spark_ method.
Magnetos.--The most important element in this system is the magneto, so
we shall try and make the subject as explicit as possible. As stated, a
magneto is a special type of dynamo which will now be explained. For
this purpose it will be necessary to show the elementary operation of an
alternating current dynamo.
Alternating Current.--In Fig. 42 A is a bar of soft iron, around which
is a coil of wire B, the wire being insulated, so that it will not touch
the bar. There is no magnetism in this bar, and this simple form of
structure is shown, merely to represent what is called the _field_ of a
dynamo.
The object of the coil of wire is to make a magnet of the bar, for the
moment a current is sent over the wire, a magnet is formed, and the
magnetism leaves the bar the moment the current ceases to flow. If this
bar should be of hard steel it would retain the magnetism.
[Illustration: _Fig. 42. Illustrating Alternating Current._]
[Illustration: _Fig. 43. Alternating Current. Second position._]
Now, the primary difference between the magneto and the dynamo, is that
this field bar is a permanent magnet in the magneto, whereas the field
is only a temporary magnet in the dynamo. This should always be kept in
mind.
The end of a magnet, whether it is a temporary one, or permanent, has a
magnetic field of force at the ends as well as at all parts of it,
exterior to the surface of the bar. Such a field is indicated, and in
the dynamo, no such field exists unless a current is passing over the
wire B, which is called the _field winding_.
The U-shaped piece of metal C represents the armature. It is shown
hinged to the top of two posts, for clearness in understanding, and is
adapted to turn to the right, and in turning the loop passes the end of
the field bar B, and passes through the magnetic field which is
indicated by the dotted lines D.
[Illustration: _Fig. 44. Alternating Current. Third position._]
Now, if the loop is simply permitted to remain in the position shown in
Fig. 42, a current would flow through the loop, this transference of the
current being called induction, and this characteristic of the flow of
electricity will be explained and its utility explained.
Cutting Lines of Force.--The loop will now be turned to the right so
that it passes the magnetic field and goes beyond it in its revolution.
This motion of passing the armature through the magnetic field is called
_cutting_ the _lines of force_. While the loop was lying within the
magnetic field, and also when it was moving through the field, the
current set up in the loop flowed in the direction of the darts F, or to
the right, through the pivots D.
In Fig. 43 the loop is shown as having made a quarter turn, and it is
now vertical, or at right angles to its former position. The loop in
thus passing away loses its force, until it reaches the position shown
in Fig. 44, when there is a surging back of the current to the opposite
direction, as indicated by the arrows.
[Illustration: _Fig. 45. Alternating Current. Fourth position._]
When the loop reaches the lowest position, shown in Fig. 45, it again
begins to get the influence of the magnetic field, and a reversal back
to its former direction takes place, this surging movement back and
forth being due to the reversal of the polarity in the coil brought
about by the position in which it is placed relative to the magnetic
field.
It is now an easy matter to connect the ends of the loop with wire
conductors. This is shown in Fig. 46, where a small metal wheel G is
placed on each end of the spindle, and in having a strip of metal
bearing H on the wheel. These are not commutator brushes, but are merely
wiping brushes to take the current from the turning parts. Wires I
connect with these wiping bars, and through them the current is
transmitted to perform the work.
[Illustration: _Fig. 46. Making the Circuit._]
Plurality of Loops.--The dynamo may have a plurality of loops, which are
called _coils_, and there may be a single magnet or any number of
magnets. Instead of driving these coils past the face of the magnet, or
magnets, the latter may be driven past the coils. In fact with most of
the alternating current machines the fields are the rotating parts and
the armatures, or the coils, are fixed.
The voltage is increased if the coils have a large number of turns on
the armature, and also if the armature, or the turning part, is speeded
up. Voltage will also be higher if larger or more powerful magnets are
used in the magnetos.
The Electro-Magnet.--The permanent magnet, such as is used in the
magneto, is distinguished by the fact that it contains a permanent
charge of magnetism, but this is not an _electro-magnet_. This is a
magnet made of soft iron, so it will be readily demagnetized. While not
shown in the diagrams, an iron core may be placed within the loop or
coil, and this is done in all dynamos, because the iron core acts as a
carrier of the magnetism, concentrating it at the center, because it is
a much better conductor than air.
[Illustration: _Fig. 47. The Dynamo._]
[Illustration: _Fig. 48. The Magneto._]
The Dynamo Form.--Consult the diagram, Fig. 47. The iron heads A
represent the bar in the previous diagrams, and B the wire around the
bar. C is the armature, which in this case represents a number of loops,
or coils, and D is the commutator, which is used in the direct current
machine to correct the alternations referred to in the previous
diagrams, so as to send the current in one direction only, the
commutator brushes E being used to carry off the current for use.
The Magneto Form.--The metal loop F, in Fig. 48, being a permanent
magnet, the armature, G, formed of a plurality of loops, has no field
wires to connect with it, as in the case of the dynamo.
Advantage of the Magneto.--The magneto has a pronounced advantage over
the dynamo, as a source of power for ignition purposes, in the
particular that the strength of the magnetic field is constant. In a
dynamo this varies with the output, because when used on an automobile
where the speed is irregular, the voltage will vary. The voltage of the
magneto is a constant one, and is thus better adapted to meet the needs
of ignition.
Induction Coil.--The induction coil is a device which is designed to
produce a very high voltage from a low tension, so that a current from
it will leap across a gap and make a hot spark.
We stated in a previous section that a current leaps across from one
conductor to another, so that electricity can be transferred from a
wire to another not touching it, by means of induction.
Look at Fig. 49, which represents two wires side by side. The current is
flowing over one wire A, and by bringing wire B close to A, but not
touching it, a current will be induced to leap across the gap and the
wire B will be charged. If the ends of the wire B are brought together,
so as to form a circuit, and a current detector is placed in the circuit
it will be found that a current is actually flowing through it, but it
is now moving in a direction opposite to the current flowing through A.
[Illustration: _Fig. 49. Current by Induction._]
Changing the Current.--But we have still another thing to learn. If the
two wires are not of the same thickness it would not prevent the current
from leaping across, but another astonishing thing would result.
First, we shall use a wire B double the thickness of wire A. If now, we
had an instrument to test the voltage and the amperage, it would be
found that the voltage in B is less than that in A, and also that the
amperage is greater.
Second, if the conditions are reversed, and the wire A is thicker than
B, the latter will have an increase of voltage, but a lower ampere flow
than in A.
Now this latter condition is just what is necessary to give a high
tension. Voltage is necessary to make a current leap across a gap. By
this simple illustration we have made an induction coil which may be
used for making a high tension jump spark.
Construction of a Coil.--Two wires side by side do not have the
appearance of a coil, and even though such an arrangement might make a
high tension current, it would be difficult to apply. To put the device
in such a shape that it can be utilized, a spool is made, as shown in
Fig. 50.
[Illustration: Fig. 50. Induction Coil.]
This spool A has a number of layers of thick, insulated wire B first
wound around it, the layers being well insulated from each other, and
the opposite ends brought out at one end or at the opposite ends, as
shown at C, D. On this is a layer of finer wire, also insulated, this
wire E having its terminals also brought out at the ends of the spool,
and after the whole is thus wound, the outside of the coil is covered
with a moisture proof material.
The Primary Coil.--The winding of thick wire is called the _primary_
coil. The current from the battery or the electric generator is led to
this inner coil.
[Illustration: _Fig. 51. Typical Induction Coil._]
The Secondary Coil.--The fine wire wrapping represents the secondary
coil, which is raised to a high voltage, and this actuates the sparking
mechanism.
In the art it is customary to illustrate the various contrivances by
certain conventional forms. Fig. 51 shows the manner of designating an
induction coil in a diagram, in which the heavy zig-zag line indicates
the primary, and the lighter zig-zag lines the secondary coil.
[Illustration: _Fig. 52. Contact Maker._]
Contact Maker.--A simple little device used in the primary circuit of an
induction coil, is known as a _contact maker_. This, as shown in Fig.
52, is merely a case A, through which is a shaft B that carries within
the shell a cam C. A spring finger D has its free end normally bearing
against the cam, and when the nose on the cam moves out the spring
finger, the latter is moved outwardly so it contacts with a plug E in
the side wall of the case, although it is insulated therefrom. This
contact establishes a current through the plug, spring finger and case.
The diagram, Fig. 53, illustrates the principles of construction and
arrangement of a high tension jump spark ignition, in which the
electrical source is a battery actuating an induction coil.
High Tension With Battery and Coil.--The battery A has one side
connected up by wire B with one terminal of the primary C in the
induction coil, and the other side of the battery has a wire D leading
to the contact maker. A switch E is placed in the line of this wire.
[Illustration: _Fig. 53. Typical Circuiting, Jump Spark Ignition._]
The other terminal of the primary has a wire F leading to the insulated
contact plug G of the contact maker. This completes the generating
circuit. The cam H is on a shaft I, which travels one half the speed of
the engine shaft.
One side of the secondary coil J has a wire K leading to the spark plug,
while the other terminal of the secondary has a wire L which is grounded
on the engine M.
When the nose of the cam pushes over the spring finger and closes the
cam, the circuit through the finger flows through the primary coil and
excites the secondary. When the cam again immediately breaks the circuit
a high tension current is momentarily induced in the secondary, so that
the current leaps the gap in the spark plug and makes the spark.
[Illustration: _Fig. 54. Metallic Core, Induction Coil._]
Metallic Core for Induction Coil.--In the previous description of the
induction coil it was stated that the spool might be made of wood. These
coils are also provided with metal cores, which can be used to make what
is called a vibratory coil.
The Condenser.--A necessary addition to the circuiting provided by an
induction coil, is a _condenser_. This is used in the primary circuit to
absorb the self-induced current of the primary and thus cause it to
oppose the rapid fall of the primary current.
The condenser is constructed of a number of tinfoil sheets, of suitable
size, each sheet having a wing at one end, and these sheets are laid on
top of each other, with the wings of the alternate sheets at opposite
ends. Very thin sheets of waxed paper are placed between the tin foil
sheets so that they are thus insulated from each other.
The wings at the ends are used to make connections for the conducting
wires. The device is not designed to conduct electricity, but to act as
a sort of absorbent, if it might so be termed. The large surface affords
a means where more or less of the current moves from the conductor at
one end to the conductor at the other end, and as it is designed to
absorb a portion of the current in the line it is merely bridged across
from one side of the circuit to the other.
[Illustration: _Fig. 55. Condenser._]
The diagram, Fig. 55, represents the conventional form of illustrating
it in sketching electrical devices.
Operation of a Vibrator Coil.--The illustration, Fig. 56, shows the
manner in which a vibrator coil is constructed and operated. The coil
comprises a metal core A, the primary winding B being connected at one
terminal, by a wire C, with a post D, and the other terminal by a wire E
with one side of a battery F. A switch G is in the line of this
conductor.
[Illustration: _Fig. 56. Vibrator Coil and Connections._]
The post D holds the end of a vibrating spring H, which has a hammer H´
on its free end, which is adapted to contact with the end of the metal
core A, but is normally held out of contact, so that it rests against
the end of an adjusting screw I which passes through a post J.
The post J is connected up with the battery by a wire K, and a wire L
also runs from the wire K to the conductor C, through a condenser M.
The secondary coil N, has the outlet wires O, P, which run to the spark
plug Q on the engine.
The operation is as follows: When the switch G closes the circuit, the
battery thus thrown in the primary coil magnetizes the core A, and the
hammer H´ is attracted to the end of the core, thus breaking the circuit
at the contact screw I. The result is that the core is immediately
demagnetized, and the spring H draws the hammer back to be again
attracted by the core which is again magnetized, so that the hammer on
the vibrator arm H goes back and forth with great rapidity.
From the foregoing explanations it will be understood how the primary
induces a high tension current in the secondary, and in order that the
spark may occur at the right time, a _timer_ for closing and opening the
primary circuit must be provided. By this means an induced high tension
current is caused to flow at the time the spark is needed in the cycle
of the engine operation.
_The Distributer._--The distributer is a timing device which controls
both the primary and the secondary currents, and it also has reference
to the revolving switch on the shaft of a magneto whereby the current is
distributed to the various cylinders in regular order.
Fig. 57 shows a form of distributer which will illustrate the
construction. A is the shaft which is driven at one half the engine
speed. It is usually run by suitable gearing direct from the shaft of
the magneto.
[Illustration: _Fig. 57. The Distributer._]
Its outer end rests in a bearing plate B, of insulating material, which
plate serves as the disk to hold the contact plates, 1, 2, 3, 4, to
correspond with the four cylinders to which the current is to be
distributed.
Wires 5, 6, 7, and 8, run to the respective spark plugs C from these
contact plates. The projecting end of the shaft A carries thereon a
contact finger D, which is designed to contact with the respective
plates, and an insulating ring E is interposed between the shaft and
finger so as to prevent short circuiting of the high tension current.
On the side of the finger is a hub F, integral therewith, and a wiper
attached to a post bears against the hub so as to form continuous
contact. A wire leads from the post to one terminal of the secondary
coil.
[Illustration: _Fig. 58. Circuiting with Distributer._]
Circuiting With Distributer.--The diagram Fig. 58 shows the complete
connections of a system which comprises a magneto, induction coil,
condenser, and a distributer. The magneto A has on its armature shaft B
two revolving disks C, D, one of which must be insulated from the shaft,
and one end of the coil E of the armature is connected with one of these
disks, and the other end of the coil is attached to the other disk.
Alongside of these disks is another disk F which has projecting points G
to engage with and make temporary contact with a spring finger which
actuates the interrupter I, this being a contact breaker which breaks
the primary current at the time a spark is required.
One terminal of this interrupter is connected by a wire J with one end
of the primary winding K, of the induction coil, and the other end of
the primary has a wire L which runs to the disk C.
The other terminal of the interrupter has a wire M leading to a
condenser N, and from the other side of the condenser is a wire O
leading to the wire J before described. The wiper of the other disk D
has a wire connection with the wire M.
The distributer shaft P is so mounted that it may receive its motion
from the shaft of the magneto, and for this purpose the latter shaft has
a gear Q one half the diameter of the gear R on the distributer shaft.
The distributer S has been described with sufficient clearness in a
preceding diagram, to show how the wires T lead therefrom and connect up
with the spark plugs U. One terminal of the secondary coil V is
connected by a wire W with the wiper X which contacts with the hub of
the distributer finger X´, and the other terminal of the primary is
grounded at Y, which represents the metal of the engine.