IGNITION
In order that a gas engine may run properly, the mixture must be set on
fire, or _ignited_, at exactly the right time; if ignition occurs too
early or too late, there will be a loss of power.
The greatest pressure will be obtained at the instant when all of the
mixture is burning, and this should take place just as the piston
begins to move outward on the power stroke. A little time is required
for the mixture to burn; there is a brief interval between the instant
when it is set on fire and the instant when it is all in flame. Thus it
is clear that if the mixture is all to be burning as the piston starts
the power stroke, it must be set on fire before that time, or, in
other words, toward the end of the compression stroke.
The point at which ignition should occur depends on the speed of the
engine and should change when the speed changes. The time required
for the flame to spread throughout the mixture does not change; let
us say that, with the engine running at 1200 revolutions a minute,
the mixture can be ignited when the piston is ¼ inch from the end
of the compression stroke, and will all be in flame by the time the
piston starts on the power stroke. If the engine is slowed down to
600 revolutions a minute and no change is made in the ignition, the
mixture will all be in flame before the piston reaches the end of the
compression stroke; pressure will then be produced before the piston
is in position to perform the power stroke. The pressure will try to
make the engine run backwards; it will sometimes be sufficient to make
the engine stop. If the momentum of the flywheel is sufficient to
force the piston to the end of the stroke against the pressure, this
condition will cause a loss of power. This is called _preignition_, or
ignition that occurs too soon. One effect of it is to produce a hard,
metallic knocking, due to the oil being squeezed out of the bearings by
the great pressure, which permits the bearing and shaft to strike. The
remedy is to make ignition occur later in the stroke.
If the engine is speeded up above 1200 revolutions, the piston will
have had time to move some distance on the power stroke before the
mixture is all in flame; the combustion space will then be too large to
permit the mixture to produce its greatest pressure, and again there
will be a loss of power. The remedy in this case is to make ignition
occur earlier in the compression stroke.
When ignition is made to occur early in the compression stroke, it is
said to be _advanced_; when it is made to occur late in the stroke, it
is said to be _retarded_.
To get the best results, the engine should be run with ignition
advanced as far as is possible without causing knocking.
The charge of mixture is always set on fire by an electric spark, and
the parts that produce and control this spark are called the _ignition
system_.
An ignition system consists of: First, the apparatus that produces the
electric current, which is usually a _magneto_; second, a _timer_,
which controls the instant at which the spark occurs; third, the _spark
plugs_, which project into the cylinders, and at which the sparks take
place; fourth, a _switch_, by which the sparking current can be turned
on or off, and fifth, the wires, or _cables_, by which the parts are
connected.
The electric current that gives the spark is always produced by
magnetism. In a magneto, magnetism is obtained from the heavy steel
magnets that are part of it; there is a constant flow of magnetism from
one end of these to the other. To obtain an electric current, a coil
of wire is placed in the magnetism, and the strength of the magnetism
is made to change; it alternately becomes weak and strong. Whenever a
change in strength takes place, an electric current flows in the wire,
and it continues to flow as long as the magnetism continues to change
in strength. When the change in strength is very great, that is, when
the magnetism changes from very weak to very strong, or from very
strong to very weak, the electric current is more powerful than when
there is only a little change in strength. A more powerful current is
also produced by a change that takes place suddenly than by a change
that takes place slowly.
The electrical principle that produces a current in this manner is
called _induction_; the current produced is known as an _induced_
current.
A magneto has two or more magnets, and between their ends, or _poles_,
there revolves a piece of iron called the _armature_. A piece of iron
placed between the poles of a magnet becomes a magnet itself; the
armature is so shaped that, as it revolves, its magnetism continually
changes in strength, and it is the changes in the strength of the
magnetism of the armature that produce the sparking current.
[Illustration: FIG. 42.—ARMATURE]
The iron armature of the Bosch magneto, which is the best known type,
is shown in Figure 42. It has a central bar with two heads, the wire
being wound around the central bar, or _core_. The shafts on which it
revolves are attached to the ends of the heads.
Figure 43 shows different positions of the armature between the poles
of the magnet, and illustrates the changes in the magnetism of the
central bar. There is a continual flow of magnetism from one pole of a
magnet to the other; if a piece of iron lies between them the magnetism
will use it as a bridge, but often its easiest path will be through the
air. In A, Figure 43, the armature lies crossways, and its central bar
or core forms a perfect bridge for the magnetism. Practically all of
the magnetism flows through it, and it then becomes a powerful magnet
itself. It sets up its own flow of magnetism, which flows through the
core to one head, through the air to the other head, and so back to the
core.
[Illustration: FIG. 43.—FLOW OF MAGNETISM THROUGH ARMATURE CORE]
In B, the armature has revolved a little. Most of the magnetism is
still flowing through the core, but some of it is finding an easier
path by flowing through the heads and across the air space to the other
pole. The magnetism of the core is, therefore, a little weaker than it
is in A.
In C, the heads alone form bridges between the poles, and none of the
magnetism flows by the core because that no longer forms a path. The
core is no longer producing magnetism; in moving from A to C there has
thus been a complete change in the strength of the magnetism of the
core, for from full strength it has died away to nothing.
By a further movement, as in D, the core again acts as a bridge, and
another change in strength occurs, this time from nothing to full
strength again. In moving from D to B, there are slight changes in
strength, but not enough to produce a sparking current; it is only in
passing from B to D that a sparking current can be produced.
In this type of magneto the space between the heads is wound full of
wire, which of course revolves with the armature; the more turns of
wire there are, the more intense will be the current, so very fine wire
is used to get the greatest possible number of turns.
In the Bosch magneto the first few layers are of coarse wire, and are
the _primary winding_. The remainder, called the _secondary winding_,
is very fine wire, and the two are connected so that one forms an
extension of the other.
It has been explained that it is most important to have the spark occur
at exactly the right instant in the stroke. On a magneto the instant of
sparking is controlled by a _timer_, or _circuit breaker_, which is a
switch that is automatically operated at the time when the magneto is
producing a current sufficiently intense to form a spark.
Figure 44 illustrates one complete revolution of the armature, and
it will be seen that it passes twice from position B to position
D. This shows that it gives a sparking current twice during each
revolution. The circuit breaker must therefore operate twice during
each revolution. It is placed at the end of the magneto; in some makes
it revolves with the armature and is operated by stationary cams, while
in others it is stationary, and is operated by a cam on the armature
shaft. In either case the effect is the same.
[Illustration: FIG. 44.—ONE COMPLETE REVOLUTION OF THE ARMATURE]
[Illustration: FIG. 45.—CONNECTIONS OF BOSCH MAGNETO]
Figure 45 shows the way in which the winding on the armature of a Bosch
magneto is connected with the circuit breaker and with the armature.
The circuit breaker shown is not the kind used on the Bosch, and serves
only to illustrate the principle. It consists of a lever pivoted at one
end, with the other end resting against the tip of a screw. A cam bears
against the lever and can move it to break the contact with the screw.
The cam is so set that it moves the lever at the time when the current
is most intense.
The coarse wire, or primary winding, on the armature is connected with
the lever and with the screw of the circuit breaker; when the lever is
touching the screw, any current produced in the primary winding has a
complete path, or _circuit_, in which to flow.
The fine wire, or secondary winding, is wound on top of the primary,
and its inmost end is connected to the outmost end of the primary
so that one forms a continuation of the other. The outmost end of
the secondary leads to the spark plug; any current produced in the
secondary winding flows to the spark plug, and, if intense enough,
will jump across the small gap in the plug, and return to the
secondary by way of the primary.
Referring to Figure 43, a weak current is produced in the primary while
the armature revolves from D to B; at that time the circuit breaker is
closed, so the current can flow in the path thus provided for it. A
current also tries to flow in the secondary, but is too weak to jump
across the gap in the spark plug. As the armature comes closer to the
point C, Figure 43, the primary current becomes more intense, and the
electricity in the secondary increases its endeavor to jump the gap in
the spark plug, but is still unable to do so.
As the armature passes over the point C, the circuit breaker opens.
The primary current, which is then most intense, finds its path taken
away from it, and it seeks another, which it finds by flowing into the
secondary winding. This flow of primary current, added to the pressure
already existing in the secondary, forms a current sufficiently
intense to jump across the gap in the spark plug, and in so jumping it
produces the ignition spark.
As the armature passes to position D, Figure 43, the circuit breaker
closes, and the action is repeated.
[Illustration: FIG. 46.—“K-W” INDUCTOR]
A magneto of this type is thus seen to give two sparks to every
revolution of the armature.
K-W and Dixie magnetos operate on the same general principle as the
Bosch, with the difference that the wire windings are separate from
the armature, and do not revolve. The revolving part, which is called
an _inductor_, consists of blocks of iron, so shaped that, as they
revolve, they alternately lead the magnetism to the core of the winding
and then away from it. The result is that the core gains magnetism and
then loses it, and these continual changes in strength produce sparking
currents in the winding.
The inductor of a K-W magneto is shown in Figure 46. It consists of
a shaft on which are mounted two blocks of iron at right angles. The
section of shaft that joins them is the core of the winding; the wire
is wound on it just as thread is wound on a spool, but with a space
between, so that the shaft may revolve inside of the coil.
Figure 47 shows the inductor in three positions of its revolution
between the poles of the magnet. When it is in the first position,
magnetism can flow from one pole of the magnet to the other by going
into one end, A, of one block, through the core, and out of one end,
C, of the other block. This makes a magnet of the core and it forms
magnetism of its own. When the inductor turns to the second position
magnetism can get across without flowing through the core, for the
blocks now give it a path. As the flow through the core ceases, the
core’s magnetism dies away, which gives the change in strength that is
needed to produce a sparking current.
[Illustration: FIG. 47.—“K-W” INDUCTOR IN THREE POSITIONS]
When the inductor is in the third position, the core again becomes the
path for the magnetism and is magnetized; these changes continue as
long as the inductor turns.
[Illustration: FIG. 48.—“DIXIE” INDUCTOR]
While an armature type of magneto, like the Bosch, produces two sparks
to every revolution, the K-W produces four, for there are four periods
during every revolution when there is sufficient change in the strength
of the magnetism of the core to produce a sparking current.
In these magnetos the revolving shaft is parallel to the ends of the
magnets, but in the Dixie magneto it is at a right angle, as shown
in Figure 48. The shaft is of some metal, such as brass or bronze,
through which magnetism will not flow; otherwise the shaft would form a
continuous path. The inductor blocks are mounted on the shaft, and act
as extensions of the poles of the magnet. The core on which the wire is
wound is a separate piece, placed under the arch of the magnets, with
ends that extend down and form a tunnel in which the inductor revolves.
Figure 49 shows an end view of the inductor, the magnets being cut
away so that the core may be seen. As inductor block A is an extension
of one pole of the magnet, magnetism tries to flow from it to block
B, which is an extension of the other pole of the magnet. When the
inductor is in position 1, Figure 49, magnetism can flow from block
A through the core to block B, the core then being magnetized. In
position 2, magnetism can flow from one block to the other by going
through the ends of the core instead of through the core itself; the
core then loses its magnetism, but regains it when the inductor moves
to position 3.
[Illustration: FIG. 49.—THREE POSITIONS OF “DIXIE” INDUCTOR]
In practically all makes of magnetos the circuit breaker is at the
end of the armature or inductor shaft, and is operated by it. The
Bosch circuit breaker is illustrated in Figure 50, the parts being
mounted on a plate attached to the shaft and revolving with it. The
lever is L-shaped, pivoted at the angle, with one end resting on the
tip of a screw. When the shaft revolves, the other end of the lever is
dragged over a block of metal that acts as a cam; this makes it move
on its pivot and separates it from the screw. By turning the screw the
distance of separation may be adjusted.
[Illustration: FIG. 50.—“BOSCH” CIRCUIT BREAKER]
In the circuit breaker of the K-W magneto it is the cam that revolves,
while the lever is stationary, as shown in Figure 51. It will be
noticed that the cam will move the lever only twice during each
revolution; the magneto can produce four sparks during a revolution,
but with this arrangement of the cam only two of them are used.
[Illustration: FIG. 51.—“K-W” CIRCUIT BREAKER]
It has been said that an intense sparking current is produced when
there is a great change in the strength of the magnetism, and when the
change in strength occurs suddenly. There cannot be any alteration in
the change in strength, for the greatest magnetic strength of the core
is what is given it by the magnet, and changing from this to nothing
is the greatest change possible. The suddenness with which the change
takes place, however, depends on the speed at which the magneto runs. A
4-cylinder engine requires two sparks to each revolution of the crank
shaft; the armature of a Bosch magneto for this engine will therefore
run at the same speed as the crank shaft.
The K-W magneto, giving four sparks to the revolution, could run at
half of the speed of the crank shaft, but then the change in the
strength of the magnetism would take place slowly, and the sparking
current would not be sufficiently intense. By using only two of the
sparks the magneto is run at the same speed as the crank shaft; the
change in strength then takes place more suddenly, and a more intense
sparking current is produced.
The circuit breaker of a magneto for a 1-cylinder engine has only one
cam, so that a single spark is produced during each revolution of the
armature; the armature makes one revolution to every two revolutions of
the crank shaft.
However many cylinders an engine may have, the magneto must be revolved
from one point of sparking to the next in the interval between ignition
in one cylinder and ignition in the next cylinder to fire. A magneto
is driven by the crank shaft through gears or by a chain, which are so
proportioned and set that the magneto is at a point of sparking at the
instant when a piston is in position for ignition.
A magneto for an engine with more than one cylinder is provided with
a _distributor_, which passes the sparking current to the particular
cylinder that is ready for ignition. A distributor is a revolving
switch built into the magneto, with as many _points_, or _contacts_, as
the engine has cylinders. At the instant when the magneto produces a
sparking current, the revolving distributor arm is in position to pass
the current to one of the contacts, and the current flows to the spark
plug with which it is connected.
An electric current must have a complete path, or circuit, in order
to be able to flow. In a magneto ignition system this path is partly
of wire and partly of the metal of the engine. The diagram in Figure
45 indicates that the current returns to the magneto from the circuit
breaker lever and the spark plug by wire, but in actual construction it
returns by the metal of the engine. This is called a _ground return_;
the circuit is said to be _grounded_.
[Illustration: FIG. 52.—“BOSCH” MAGNETO IN SECTION]
Figure 52 is a side view of a Bosch magneto, partly broken away to
show the interior. As can be seen, one end of the primary winding is
screwed to the armature, and is thereby connected with the metal of the
magneto; as the magneto is attached to the engine the primary winding
is thus in contact with that also. The other end of the primary winding
leads to the insulated block of the circuit breaker, Figure 50. This
block is _insulated_ from the disk; that is, while it is attached to
the disk, it is kept from touching it by means of pieces of hard rubber
or mica. Through these an electric current cannot pass.
The lever is grounded; that is, it is in contact with the metal of
the magneto. When the lever touches the screw of the insulated block,
current can flow; when they are separated, the circuit is broken.
One end of the secondary winding, Figure 52, is attached to the outer
end of the primary. The other end leads to the _slip ring_, which is a
metal rim on a hard rubber wheel attached to the armature and revolving
with it. Sparking current flowing to the slip ring is led off by a
carbon brush and passed to the distributor.
Should a spark plug wire fall off while the engine is running, the
current would lose its path and would seek another; it is quite
powerful enough to make a path for itself by breaking through the
windings. As this would injure the magneto, such a thing is prevented
by providing a _safety spark gap_, which acts like a safety valve in
giving the current a path when the regular path is interrupted. It
consists of two points of metal, one attached to the metal of the
magneto and the other connected with the slip ring brush; it is a more
difficult path than the one through the spark plug, but easier than
breaking down the windings.
Figure 53 is a section of the K-W magneto. As the coil does not
revolve, no slip ring is necessary; the sparking current flows directly
to the distributor.
[Illustration: FIG. 53.—“K-W” MAGNETO IN SECTION]
To start an engine, the crank shaft must be turned at sufficient
speed to drive the magneto fast enough to produce a spark. With large
engines this is often a difficult matter, so it is very usual to equip
a magneto with an _impulse starter_. One part of this is attached to
the magneto shaft and the other to the engine shaft that drives the
magneto; the two are connected by a spring. When starting, a catch
holds the armature and prevents it from turning. The drive shaft
turns, however, and in so doing winds up the spring. At a certain
point the catch is automatically released, and the spring then throws
the armature over at a speed that gives a good spark. A spark is thus
assured, even though the engine is being cranked very slowly.