PUTTING THE KICK IN THE SPARK
Motors are a good deal like men. Very many of them, when everything
is wide open and they are hitting it up at top speed, have excellent
performance, but spit and sputter and knock a bit when compelled to
travel at a lesser pace.
This is not moralizing, but the natural deduction from the complaints
which are constantly received. A great many drivers want to know why it
is they cannot throttle down the engine; they say it will pull well at
medium or high speed, but skips at the lower speeds.
Where magneto ignition is used, this often may be overcome by adjusting
the interrupter points for a little wider gap. That makes the
interrupter points open a little earlier and causes the spark to occur
when the magneto armature is in a more favorable position, and the
spark therefore has more kick in it.
The interrupter points usually are adjusted for normal running
speed, or a little higher than medium speed. Increasing the number
of revolutions per minute of the armature causes a distortion of the
magnetic field, which tends to follow the armature. This distortion
is not so great at lower speeds. To overcome the distortion, which in
effect twists the field around so that the points do not open at the
period of greatest current potency, we must change the time when the
points open and cause the spark; therefore, to get the best spark it is
necessary to have the points open earlier. Widening the gap between the
points accomplishes this.
If the engine misses at high speed, close the points a little to make
the opening come later and when in better armature position, the action
being just the reverse of the foregoing paragraph.
Widening the gap of the spark plug also will help to overcome skipping
at low speeds. At low speeds a full charge of gas is not drawn in and
compression is low and temperature low in consequence; the mixture is
not so near the self-ignition point when the spark occurs as it is at
higher speeds. Therefore a better spark is needed to ignite it. By
widening the gap in the plugs the secondary current is held back until
it acquires high enough potentiality to jump the greater gap; so that
when it does occur an exceptionally powerful spark is obtained.
This may be demonstrated with the use of a vibrating coil. Set one
of the cylinders on top of compression ready to ignite. Turn on the
current and while the vibrator will buzz the spark inside the cylinder
is not strong enough to ignite the charge. Now detach the spark-plug
wire and hold it close to the outer plug terminal and turn on the
current. The extra gap will increase the strength of the spark in that
cylinder and it will ignite the charge, starting the engine. This has
led to the use in some cases, of outside spark gap attachments.
Widening the gap and getting a longer spark is not what ignites the
charge. It is simply that the greater gap causes the current to build
up to a higher pressure, or voltage, before it will jump the gap, so
that when it does occur it is of greater intensity.
There are several other things about sparking not generally understood
by the novice. The general opinion is that the greater insulating
surface of a plug there is inside the cylinder the less likelihood
there is of short circuiting and consequently the more certain will be
the spark occurrence. This is only partly true; it is true so far as
short-circuiting is concerned; but where the surface is too great and
soot or carbon is deposited upon the insulator surface, the carbon acts
as a condenser and absorbs the secondary current. The effect is that
it will not build up a sufficiently high voltage to jump the gap. Very
heavy insulation on the secondary wires absorbs current in the same way.
The fact that the spark jumps across the points of the plug when it
is in the open air does not necessarily mean that it jumps when the
points are under compression in running conditions. A current which
will cause a spark to jump a half-inch gap in the open air, will cause
it to jump a gap of only 1/32 inch under normal running compression.
Failure to appreciate this often results in a man thinking his ignition
is all right when there may be a crack in the insulation so small that
it scarcely can be seen with the naked eye when cleaning the plug, or
there may be a porous spot in the porcelain, either of which would
leak current under compression. Many manufacturers test the porcelains
to see if they leak, using a spark gap of 3/4 inch under a very high
voltage.
If there is carbon on the porcelain, the current very often will travel
through this under high compression, because there is less resistance
offered by the carbon than by the spark gap. It short-circuits the
plug and there is no ignition. Yet, when this same plug is removed and
tested in the open air, there will be a good spark; the resistance of
the gap in the open air would be less than the resistance offered by a
path through the carbon coating. If on testing a plug in this way there
seems to be a good spark, yet the cylinder will not fire, clean the
plug thoroughly and try it out in the cylinder. If it still does not
fire, try a plug which you know is all right and see if it fires. If it
does, you may know that the old plug leaks current somehow and probably
needs a new porcelain.
It is a common error with motorists when cleaning a spark plug to
polish up the points and call it a job. What really is required is to
remove all of the carbon, which is a fair conductor of electricity,
from the porcelain or mica and get these back into the insulator class
again.
The conditions which allow the adjustment of the interrupter points
in magneto ignition to accommodate certain speeds, do not arise when
battery ignition is used. Battery interrupter points are usually
adjusted for an opening of 1/64 inch and the gap at the spark-plug
points must be greater usually than when magneto ignition is used,
because the spark given by battery systems is naturally of less volume
than that given by the magneto. Therefore the gap must be greater to
intensify the current as before described.
The interrupter points sometimes become coated with an insulating
glaze, composed of oxides and dirt hammered by the action of the
movable interrupter point, or hammer. The points will have the
appearance of being all right; there will be no pits or points;
nevertheless there may be enough insulating material there to keep the
current from flowing and no spark will occur in consequence. The points
may be cleaned by inserting a very thin file between the points when
they are open and then allowing them to close on the file, being held
together with no more pressure than the spring exerts, and working the
file back and forth until the glaze is removed. It is necessary to
have a flat surface, so that care in using the file is needed, and, as
platinum is more valuable than gold at present, it will be apparent
that as little filing as possible should be done.
The condenser is another point of possible trouble. The condenser
increases the volume of the spark about twenty-five times. We often
find that a very weak spark may be caused by a partial breaking down
of the condenser, or occasionally to the fact that it is disconnected
at the terminal. Condensers are usually hidden away so that the novice
cannot locate troubles of this nature, but an excessive sparking at
the interrupter or vibrator points, with a very weak spark at the plug
points, indicates that the condenser is not working properly. When this
sparking occurs have an expert examine the condenser for trouble.