MAGNETO IGNITION
The third form of ignition we have to deal with is the electric.
There are a great number of different types made and used, but for
gas-engine use perhaps that known as the magneto ignition is the most
satisfactory. With this form, neither accumulators, dry batteries, or
spark coils are required, and consequently a greater simplicity is
arrived at than would otherwise be the case.
In fig. 19 we show diagrammatically the ordinary form of magneto
machine. Virtually it is a small dynamo which is fixed to the side of
cylinder casting, and is operated in the manner shortly to be described.
As we do not propose to enter into more than a brief explanation of why
and how this apparatus generates current to produce the required spark,
perhaps a simple analogy will make matters most intelligible to any
reader not well acquainted with electrical phenomena. We know that when
a current of electricity is flowing in a wire, and the wire be suddenly
broken, a spark will occur at the point of breakage. This fact may be
observed in an ordinary electric bell when ringing; at the tip of the
contact breaker a number of tiny sparks may be seen to occur, due to the
rapid make and break of the current flowing in the circuit. Precisely
the same action takes place in our magneto-igniter, but, instead of a
multitude of tiny sparks, we produce one at a time, at definite
intervals, viz., at the commencement of each explosion stroke.
[Illustration: FIG. 19.]
In the later form of magneto machines there is a soft iron sleeve
between the magnet poles and the armature. The former is connected to a
system of levers by which a reciprocating motion is imparted to it by
means of a suitably arranged cam on the side shaft. It has been found
that better results are obtained by causing the magnetic field to move
relative to the armature winding than to move the latter through a
stationary field. Reference to the diagrams, figs. 20 and 21, will make
this clear.
In fig. 19 the cam C is shown just on the point of allowing the lever L
to fly back into its normal position, due to the action of the springs
comprising a dashpot S. As the cam rotates, it pushes the lever L to the
left, the sleeve (or virtually the armature A) is also rotated through a
portion of a revolution comparatively slowly; but as soon as L is
released, the sleeve (or armature) flies back again almost
instantaneously and for the moment is generating a current in the same
manner as would any ordinary continuous current dynamo.
[Illustration: FIG. 20.]
[Illustration: FIG. 21.]
At the instant the maximum current is being generated, the circuit is
broken by means of the contact breaker D, fig. 19, which we show in
detail in fig. 22. The latter is mounted on the end of the combustion
chamber, and consists of two parts, D and P.
[Illustration: FIG. 22.]
D is an easy fit in the hole bored to receive it, and has a mushroom
valve head and seating, as shown, so that it moves readily when struck
by the projection E on the rod R (fig. 19); but yet, acting in the
manner of a non-return valve, it allows no gas to escape when the
explosion takes place in the cylinder. D is therefore in direct metallic
communication with the engine frame and earth.
P is a fixed metal pin, carefully insulated from all contact with the
engine frame and earth. To this pin one end of the armature winding is
connected, whilst the other end is connected to the engine frame.
Thus a closed circuit is formed, and when the current is generated it
flows from one terminal of magneto through wire to pin P, on to D,
through D to earth (_i.e._, engine frame), and so back to other terminal
on magneto.
And as the circuit is broken between D and P, we obtain a spark, as
previously explained, which may be timed to take place by adjusting the
position of cam C on side shaft relatively to the position of piston.
It may be said that the position of the magneto-igniter is immaterial;
it will be fixed in different positions on different types of engines,
and so long as the operating mechanism is simple and effective, _i.e._,
as direct as is practicable, it works well, and requires little
attention. The timing of the spark will be dealt with in the chapter on
Cams and Valve Settings.