ELECTRIC IGNITION
The petrol motor, which to-day is busily engaged all over the world
in driving thousands upon thousands of self-propelled vehicles or
automobiles, belongs to the important class of internal-combustion
engines. Combustion means the operation of burning, and an
internal-combustion engine is one in which the motive power is produced
by the combustion of a highly explosive mixture of gases. In the
ordinary petrol motor this mixture consists of petrol and air, and it
is made by means of a device called a “carburetter.” By suction, a
quantity of petrol is forced through a jet with a very fine nozzle, so
that it is reduced to an extremely fine spray. A certain proportion of
air is allowed to enter, and the mixture passes into the cylinder. Here
it is compressed by the rising piston so that it becomes more and more
heated, and at the right point it is ignited. Combustion takes place
with such rapidity that it takes the form of an explosion, and the
energy produced in this way drives forward the piston, which turns the
crank-shaft and so communicates motion to the driving-wheels.
The part played by electricity in this process is confined to the
ignition of the compressed charge of petrol and air. This may be done
in two ways; by means of an accumulator and a small induction coil, or
by means of a dynamo driven by the engine. At one time the first method
was employed exclusively, but to-day it is used as a rule only for
starting the car engine, the second or magneto method being used when
the engine has started up.
In accumulator ignition the low-tension current from the accumulator
passes through an induction coil, and is thus transformed to
high-tension current. This current goes through a sparking plug, which
is fixed in the head of the cylinder. The sparking plug contains two
metal points separated by a tiny air gap of from about 1/30 to 1/50
inch. This gap provides the only possible path for the high-tension
current, so that the latter leaps across it in the form of a spark.
The spark is arranged to take place when the piston is at the top of
its stroke, that is, when the explosive mixture is at its maximum
compression, and the heat of the spark ignites the mixture, the
resulting explosion forcing down the piston with great power. In
practice it is found better as a rule to cause the spark to pass very
slightly before the piston reaches the extreme limit of its stroke. The
reason of this is that the process of igniting and exploding the charge
occupies an appreciable, though of course exceedingly small amount of
time. Immediately on reaching the top of its stroke the piston begins
to descend again, and if the spark and the top of the stroke coincide
in time the explosion does not take place until the piston has moved
some little distance down the cylinder, and so a certain amount of
power is lost. By having the spark a little in advance of the piston,
the explosion occurs at the instant when the piston begins to return,
and so the full force of the explosion is utilized.
In magneto ignition the current is supplied by a small dynamo. This
generates alternating current, and it is driven by the car engine.
The current is at first at low pressure, and it has to be transformed
to high-tension current in order to produce the spark. There are
two methods of effecting this transformation. One is by turning
the armature of the dynamo into a sort of induction coil, by giving
it two separate windings, primary and secondary; so that the dynamo
delivers high-tension current directly. The other method is to send
the low-tension current through one or more transformer coils, just
as in accumulator ignition. Accumulators can give current only for a
certain limited period, and they are liable consequently to run down at
inconvenient times and places. They also have the defect of undergoing
a slight leakage of current even when they are not in use. Magneto
ignition has neither of these drawbacks, and on account of its superior
reliability it has come into universal use.
In the working of quarries and mines of various kinds, and also in
large engineering undertakings, blasting plays a prominent part. Under
all conditions blasting is a more or less dangerous business, and it
has been the cause of very many serious accidents to the men engaged in
carrying it out. Many of these accidents are due to the carelessness
resulting from long familiarity with the work, but apart from this
the danger lies principally in uncertainty in exploding the charge.
Sometimes the explosion occurs sooner than expected, so that the men
have not time to get away to a safe distance. Still more deadly is
the delayed explosion. After making the necessary arrangements the
men retire out of danger, and await the explosion. This does not take
place at the expected time, and after waiting a little longer the men
conclude that the ignition has failed, and return to put matters right.
Then the explosion takes place, and the men are killed instantly or at
least seriously injured. Although it is impossible to avoid altogether
dangers of this nature, the risk can be reduced to the minimum by
igniting the explosives by electricity.
Electrical shot firing may be carried out in different ways, according
to circumstances. The current is supplied either by a dynamo or by a
battery, and the firing is controlled from a switchboard placed at a
safe distance from the point at which the charge is to be exploded, the
connexions being made by long insulated wires. The actual ignition is
effected by a hot spark, as in automobile ignition, or by an electric
detonator or fuse. Explosives such as dynamite cannot be fired by
simple ignition, but require to be detonated. This is effected by
a detonator consisting of a small cup-shaped tube, made of ebonite
or other similar material. The wires conveying the current project
into this tube, and are connected by a short piece of very fine wire
having a high resistance. Round this wire is packed a small quantity
of gun-cotton, and beyond, in a sort of continuation of the tube, is
placed an extremely explosive substance called “fulminate of mercury,”
the whole arrangement being surrounded by the dynamite to be fired.
When all is ready the man at the switchboard manipulates a switch, and
the current passes to the detonator and forces its way through the
resistance of the thin connecting wire. This wire becomes sufficiently
hot to ignite the gun-cotton, and so explode the fulminate of mercury.
The explosion is so violent that the dynamite charge is detonated, and
the required blasting carried out. Gunpowder and similar explosives
do not need to be detonated, and so a simple fuse is used. Electric
fuses are much the same as detonators, except that the tube contains
gunpowder instead of fulminate of mercury, this powder being ignited
through an electrically heated wire in the same way. These electrical
methods do away with the uncertainty of the slow-burning fuses formerly
employed, which never could be relied upon with confidence.
Enormous quantities of explosives are now used in blasting on a large
scale, where many tons of hard rock have to be removed. One of the most
striking blasting feats was the blowing up of Flood Island, better
known as Hell Gate. This was a rocky islet, about 9 acres in extent,
situated in the East River, New York. It was a continual menace to
shipping, and after many fine vessels had been wrecked upon it the
authorities decided that it should be removed. The rock was bored
and drilled in all directions, the work taking more than a year to
complete; and over 126 tons of explosives were filled into the borings.
The exploding was carried out by electricity, and the mighty force
generated shattered nearly 300,000 cubic yards of solid rock.