IGNITION AND IGNITION DEVICES
We have already stated that the charge of explosive mixture is ignited
in the cylinder at the end of the compression stroke by means of an
electric spark. The electric spark takes place as the result of an
electric discharge across the _gap_ between the _electrodes_ of the
_sparking plug_.
[Illustration: FIG. 45.—SECTIONAL DRAWING OF A SPARKING PLUG.]
=The Sparking Plug.=—Two views of a typical sparking plug are shown in
Figs. 45 and 46, in which A is the _high tension_ electrode which is
periodically charged with electricity at high voltage (or electrical
pressure) from a high tension magneto or a high tension coil, and
B_{1}, B_{2} are electrodes which, being in metallic contact with the
cylinders and framework of the engine, are thus at zero potential. The
electric discharge occurs across the gap C_{1}, C_{2} in the form of a
spark or flash. The electrode A is heavily _insulated_ from the metal
casing D of the sparking plug by porcelain insulators E and F. The
locknuts G and H serve to keep the plug gas-tight and hold the several
portions together mechanically. The terminal K is used for clamping the
wire (or lead) which brings the supply of high tension electricity.
The high tension electric current may be supplied either by (1) a
magneto machine or (2), a coil and accumulator ignition system.
[Illustration: FIG. 46.—A SPARKING PLUG.]
[Illustration: FIG. 47.—OUTSIDE VIEW OF A HIGH TENSION MAGNETO.]
[Illustration: FIG. 48.—END VIEW OF A HIGH TENSION MAGNETO, SHOWING
HIGH TENSION DISTRIBUTOR AND LOW TENSION CONTACT BREAKER.]
=The High Tension Magneto.=—In Figs. 47, 48 and 49 we show a modern
high tension magneto suitable for a four-cylinder engine. It consists
of the stationary magnets A, the driving spindle B, the high tension
electrode D, the high tension distributor C, and the low tension
contact breaker E. The armature, condenser, and distributor gear wheels
are not shown in the drawings, but are situated _inside_ the machine
in the space between the high tension electrode D and the low tension
contact breaker E. As the spindle B is rotated by gearing driven from
the engine crankshaft the armature attached to it generates a high
tension current and a low tension current. The high tension current
passes to the high tension electrode D and thence across the machine to
the carbon brush H of the high tension distributor C. The low tension
current passes through the platinum-tipped contact screws F_{1}, F_{2}
of the low tension contact breaker. Twice during each revolution of the
armature these contacts are separated owing to the fibre block attached
to the bell crank lever G passing over the stationary cams T_{1},
T_{2}; this constitutes the _make-and-break_ device for interrupting
the primary current. The momentary interruption of the primary current
in this way causes a very great increase in the electrical pressure (or
voltage) of the secondary or high tension current which is sufficient
to bring about the spark discharge across the gap between the
electrodes of the sparking plug. Since there are two of these cams on
the low tension contact breaker it will be understood that the armature
can supply current for two sparks in every revolution it makes. If
we bear this fact in mind we will have no difficulty in determining
the relative speeds of the magneto armature and the engine crankshaft
for any type of engine. A four-stroke engine requires one spark in
every two revolutions made by the crankshaft, so that a four-cylinder
engine of this type requires two sparks per revolution, and the magneto
armature must run at crankshaft speed. A six-cylinder engine working on
the four-stroke cycle would require three sparks per revolution, but
the armature of the magneto only supplies two, therefore it must be
driven at one-and-a-half times the crankshaft speed.
[Illustration: FIG. 49.—END VIEW OF A HIGH TENSION MAGNETO, SHOWING THE
EARTHING TERMINAL (P).]
The high tension distributor consists of the carbon brush H driven by
gearing from the magneto armature and the metal segments M_{1}, M_{2},
M_{3}, M_{4}, which are mounted in a block of insulating fibre. There
must be as many segments on the distributor as there are cylinders on
the engine, one segment for each sparking plug; but the armature cannot
supply more than _two_ sparks per revolution, and therefore if the
distributor has four segments it must be driven at half the armature
speed, and if it has six segments it must be driven at one-third of
the armature speed. Each metal segment is electrically connected to
a sparking plug lead such as L_{1}, L_{2}, L_{3}, L_{4}. The high
tension electrode D is attached to a light carbon brush which presses
on a gunmetal collector ring at the high tension end of the armature
winding. A special terminal is provided at P, so that when a wire
is attached to it and connected to the frame of the engine (usually
through a switch) the low tension windings are _short-circuited_ or
closed on themselves, and the make-and-break has no effect, because
there is always the path through the switch until it is opened again.
Under these circumstances the voltage of the high tension circuit is
not sufficient to cause the spark discharge, and the ignition is then
said to be _switched off_. The instant at which the spark occurs may be
_advanced_ or made earlier by moving the rocker arm K, which carries
the stationary cams T_{1}, T_{2} _backwards_, whereas if it is moved
_forward_ the ignition is _retarded_ and occurs later in the stroke.
_Normal_ ignition occurs when the lever is midway in its range of
movement and corresponds to the position of the piston when the crank
is on the top dead-centre, whereas advanced ignition occurs just before
the piston has completed the compression stroke, and retarded ignition
will take place after the crank has passed the dead-centre and when
the piston has moved down a little on the power (or explosion) stroke.
Advancing the ignition increases the speed, and retarding the ignition
reduces the speed, except when the engine is overloaded, and then it
may pick up speed a little or run better if the ignition is slightly
retarded—but the exact behaviour will depend on the temperature of the
metal walls and piston within the cylinder.
[Illustration: FIG. 50.—AN IGNITION COIL, SHOWING THE TREMBLER
MECHANISM.]
We have mentioned that normal ignition occurs when the crank is exactly
on the dead-centre and the piston at the top of its stroke. If we set
the magneto when the engine is at rest so that ignition _ought_ to
occur on dead-centre when the arm K is in its mid position the actual
sparking will be _late_ on account of the _time lag_ of the electric
current. The current takes time to flow and in that brief element of
time the crank has moved a few degrees off the dead-centre, at high
speeds. Hence the ignition _must be advanced_ if the charge is to be
correctly fired when the engine is running fast. If the ignition is
too far advanced it will cause the engine to “_knock_,” especially
under heavy loads. If the ignition is _retarded_ the charge is not
fired at the commencement of the stroke so that a portion of the power
theoretically available in the fuel is lost to exhaust at the end of
the stroke. Retarded ignition always causes overheating of the exhaust
system.
If the arm K is fixed mechanically in its mid position so that the
ignition can neither be advanced nor retarded, we have what is known as
_fixed_ ignition.
[Illustration: FIG. 51.—IGNITION COIL CASE.]
[Illustration: FIG. 52.—LOW TENSION CONTACT BREAKER FOR SINGLE CYLINDER
COIL IGNITION SYSTEM (WIPE CONTACT).]
=An Ignition Coil= suitable for a single cylinder engine is shown in
Figs. 50 and 51, in which A and B are the low tension terminals and
C is the high tension terminal. The trembler blade is shown at D,
with the adjusting screw F and the platinum-tipped contacts G_{1},
G_{2}. The iron core of the coil projects a little above the case,
as shown at E in Fig. 50. The strength and character of the spark
may be varied considerably by slightly screwing F up or down. When
current is supplied to the low tension terminals of the coil it flows
through the primary winding and magnetizes the iron core, completing
its circuit by passing across the platinum contacts. When the trembler
blade is attracted to the iron core the primary circuit is broken
by the temporary separation of the platinum contacts, and therefore
the magnetism ceases, the trembler is released, and the circuit is
completed again. Thus the trembler blade is set rapidly vibrating and
making and breaking the primary circuit as long as the roller attached
to the rotating arm H of the low tension contact breaker shown in
Fig. 52 is in contact with the metal segment K, and this results in
the production of a _succession_ of sparks at the sparking plug which
is connected to the terminal C of the high tension winding. This
is very useful especially when starting an engine, but with modern
high-speed engines the trembler has only time to give one spark at
high engine speeds, and therefore the magneto has the advantage except
for easy starting. This has led to the introduction of _dual_ ignition
systems, and in particular to that system in which the main ignition
is by magneto, but there is a supplementary coil fitted to supply
high tension current to the ordinary high tension magneto distributor
when the engine is at rest, the coil being cut out after the engine
has got up speed. But this has been largely superseded by the use of
electric motors for starting the engine, although the _magneto_ is
still relied upon for the ignition of the charge in the cylinders. The
contact breaker and coil just described would be very suitable for a
single cylinder petrol engine, or a non-trembler coil might be used
in conjunction with a contact breaker of the quick break type used on
magnetos and illustrated in Fig. 48. In the case of a multi-cylinder
engine having coil ignition we may use separate coils without a high
tension distributor, or a single coil _and_ a high tension distributor
having as many segments as there are engine cylinders and arranged
similarly to the magneto distributor of Fig. 48. When no high tension
distributor is fitted there must be a separate coil for each cylinder,
and the high tension wire runs direct from the coil to the sparking
plug, so that the character of the spark as well as the exact instant
at which it occurs may not be the same in each of the cylinders. If
there is a high tension distributor it should be mounted on the same
driving spindle as the low tension contact breaker, in order that the
ignition may be _synchronized_, i.e., the spark will occur at the
_same_ point in the piston’s stroke for all the cylinders. The ignition
may be advanced or retarded by moving the casing of the low tension
contact breaker relative to the roller arm, thus causing it to make
contact either earlier or later in the revolution.
At one time it was thought that _two-point ignition_ gave increased
power and efficiency. Two-point ignition means _simultaneous_ firing of
the charge from more than one plug. Sometimes two high tension leads
were led from each distributor segment and connected to the two plugs
in the corresponding cylinder—this constituted the _parallel_ system.
Another system employed a special plug with _both_ electrodes insulated
from the engine frame; this was coupled in _series_ with an ordinary
plug so that the spark jumped the gaps in succession. It is quite
evident, however, that if the gas is thoroughly mixed up and in a
state of violent agitation as the result of rapid compression, a single
well-placed spark will fire it successfully and so no gain results from
simultaneous ignition at another and less favoured point.
[Illustration: FIG. 53.—WIRING DIAGRAM FOR FOUR CYLINDER ENGINE WITH
HIGH TENSION MAGNETO IGNITION.]
=Wiring Diagram for Magneto Ignition System.=—The electrical connexions
are extremely simple in the case of a high tension magneto ignition
system. In Fig. 53 we show a four-cylinder engine fitted with high
tension magneto. The only wires required are the four high tension
cables from the high tension distributor to the sparking plugs and
the _earthing_ wire leading from the short circuiting terminal to the
frame of the engine through a switch as indicated. The firing order
of the cylinders may be _either_ 1, 3, 4, 2 or 1, 2, 4, 3, as desired
(provided the cranks are arranged in the usual manner, that is, in the
order shown in Fig. 21). In determining the order of firing of the
respective cylinders the engine should be turned round very slowly by
hand and careful note made of the order in which the firing strokes
occur. To determine the _firing_ stroke the piston should be moving
downwards and the position of the valves noted; if _both_ valves are
_shut_ then this is the firing stroke, but if the inlet valve is
opening it is the suction stroke.
=Wiring Diagram for a Coil Ignition System.=—The electrical connexions
for a coil ignition system are slightly more difficult to follow out;
they are shown in Fig. 54 for the same engine illustrated in Fig. 53.
In the diagram we show four separate trembler pattern coils, each of
which can give a succession of sparks as long as contact is being made
on any one segment of the low tension contact breaker connected to it.
All the low tension terminals of the coils are connected together to
a common busbar, which is supplied with current from the accumulator
direct. The current flows from the busbar through the low tension
windings of each of the coils in turn, as it comes into operation
through the engine-driven contact breaker, and returns to the battery
through the frame of the engine. High tension cables lead from the
high tension terminal of each coil direct to the sparking plugs, and
therefore the ignition is not necessarily synchronized.
[Illustration: FIG. 54.—WIRING DIAGRAM FOR FOUR CYLINDER ENGINE WITH
TREMBLER COIL IGNITION.]
When the switch in the low tension circuit is opened the ignition is
_off_, because the current is then permanently interrupted; when the
switch is closed the ignition is _on_. To economize current a quick
make-and-break device should be used instead of the _wipe_ form of
contact breaker illustrated, and a non-trembler coil used. It is very
important to fully _retard_ the ignition lever when starting an engine
having coil ignition, because it is very liable to backfire and injure
the operator’s wrist; with magneto ignition this is less liable to
happen.
=Timing the Ignition.=—Various instructions are given from time to time
for correctly timing magneto ignition, but the following will be found
to give satisfactory results. First ascertain the firing order of the
cylinders as explained above, and then bring No. 1 piston on to the top
dead-centre. Rotate the driving spindle of the magneto until the carbon
brush H of the high tension distributor makes contact on the segment
connected to the lead marked (1). If the leads are not marked it will
be necessary to determine which is No. 1 by observing the direction of
rotation of the brush. Next adjust the position of the driving spindle
very carefully by turning it to and fro, so that when the ignition
lever K (see Fig. 48) is in its mid position the platinum contacts
F_{l}, F_{2} are fully separated, the brush H still being on segment
No. 1. Then push the magneto gear wheel into mesh with the engine gear
wheel which is to drive it, and firmly bolt down the magneto to its
bracket. Similar instructions may be followed out for the coil ignition
system.