nicely explained ignition. sys. definition Chuck.
...I was getting tired of all the coil talk....
it was scaring me a little that explanations I was
seeing
were so mis defined. I almost stepped in my self
.... didn't need to, you did. Great Job Chuck
-----Original Message-----
From: Chuck Schaefer +ADw-crschaef+AEA-mc.net+AD4-
To: mgs+AEA-autox.team.net +ADw-mgs+AEA-autox.team.net+AD4-
Date: Wednesday, August 19, 1998 8:06 PM
Subject: Re: new coil (longish)
After following this for a loooong time, I feel
the need to add my +ACQ-0.02
worth. partially because nobody has really
explained the functions of
the components. Not being a designer of ignition
systems, but of power
supplies, let me try my efforts to explain in some
simple terms, whats
going on here. Its not unlike a flyback switchmode
power supply.
Using the schematic and waveforms on Barneys
site, follow along with
my version of sparks and other interesting
phenomenon. do not follow the
MGA schematic.
To start with, for these purposes, Ill use current
flow (from to -)
theory. Also please understand that the purpose of
the coil is to store
energy, not the capacitor. In order fo the
waveforms to be proper, the
windings are wired with starts ( or finishes) at
the same point, the
+ACI-points+ACI- terminal of the coil. Therefore it is not
used as an
auto-transformer, even though it may look like
one.
Starting out just after the first plug firing, the
points close. Voltage
across the points becomes zero. This causes the
voltage to be imposed
across the primary of the coil. The current begins
to flow from the
terminal of the battery, thru the ignition switch,
thru the primary of
the coil and thru the closed points to ground. And
then to the - term of
the battery. The current (and magnetic field)
builds throughout the
dwell time until the points are opened. The reason
that you see the coil
current taper off at about 5 Amps is due to the
ballast resistor and
primary resistance. These resistances limit the
current in the circuit
to the voltage divided by resistance or 12/2.5 or
4.8 Amps. In our LBCs
the coil has a ballast resistor built internally.
(Perhaps it is just
wound with higher resistance wire.) During this
charge time, there is a
voltage at the HV terminal of the coil which is
approximately the turns
ratio times the applied voltage: 100+ACo-12v or 1200
volts. Dont chastise
me for the use of 12 v here. Its only for
explanation. This 1200 volts
is not enough to arc over the gap between the
rotor and distributor cap
terminal AND the spark plug gap. Assuming 0.010
gap at the cap and
0.025+ACI- at the plug, arcing would take somewhere
around 70,000 volts per
inch in free air or 2400 Volts ( higher under
cylinder pressures).
When the points open, with no more potential
applied to the primary,
the field begins to collapse. This collapsing
field induces a voltage in
BOTH the primary and the secondary windings. The
voltage rises at the
open points and falls at the HV terminal of the
coil. The voltages
continue to rise (fall) until something fails. In
this case, it is the
insulating gap at the points and rotor. They
+ACI-break down+ACI- or spark over
at about 30,000 volts. This causes the voltage at
the points to be 300
volts due to the 100:1 turns ratio of the coil.
Energy is depleted
during this arc-over until the magnetic field is
collapsed
The purpose of the capacitor is limit the rate of
rise and fall of the
voltages on the coil and consequently the points,
NOT to provide the
energy to fire the plug. This reduces the sparking
across the gap as
they open and thus extend the useful life of same.
A properly designed
capacitor / coil will be designed for a specific
time constant do do
some wave shaping. Too low a value of capacitance
will eventually create
a metal spike on one terminal while too high a
value will create a spike
on the other.
I do not claim to be a physics expert on the
dynamics of the spark and
how long, or how much energy it contains, however
the energy definitely
decreases as engine speed increases.
Please note that changing the plug gap wider
without any other changes
will provide a higher voltage spark but also a
higher secondary voltage
on the coil, perhaps exceeding the design limits
of its insulation
system. Also it provides a higher voltage on the
capacitor possibly
causing early failure. I have known some early CD
ignition systems (Mark
III I think) to have so high a voltage as to be
un-useable during damp
weather conditions. The distributor cap does not
support the higher
voltages and shorts out. The CD ignition systems,
when wired out of the
circuit, started the cars just fine+ACE-( they used
the existing points.)
Sorry if this was too long an explaination but I
put it in as simple of
terms that I can to describe what is happening. If
I misspelled a few
words, then so be it. If I stepped on a few toes,
it was not my
intention.
This will be my one and only discussion on this
subject. Next lets talk
about why the points in the fuel pump burn out and
why the capacitor
fixed it somewhat but the anti-parallel diode
fixes it better. And how
about adding a transistor switch as a final
solution.
Of course, this is just my opinion. yours may be
different.
Chuck Scahefer
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