[BOUNCE triumphs@Autox.Team.Net: Message too long (>10000 chars)]
Date: Wed, 16 Apr 1997 22:24:14 -0400 (EDT)
From: DANMAS@aol.com
Subject: Ignition Stuff (very long)
Awhile back there was a thread on this list concerning igniton systems -
coils, capacitors, etc. Bob Sykes and I offered to take our discussion off
line for fear of boring too many people, but we had several requests not to
do so. Unfortunately, our discussion soon involved the transmittal of
diagrams, schematics, etc, which would not work to well on the list. However,
as promised, we have compiled the results of our research and discussions and
have included them below.
We would like to state at the outset that we are not experts. The following
is merely the results of our studies and research. As stated before, we stand
ready, willing, and eager to be corrected, and welcome any and all comments.
Dan Masters and Bob Sykes
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Basic Points Type Inductive Discharge Ignition Systems
Most LBCs use this type of ignition system. Later cars benefit from the
miracles of Electronic Ignition systems which are not described here, but
most of the same principles apply. One can think of electronic ignitions as
"improved points".
The basic ignition system consists of; the Ignition Coil, Points, Capacitor
(aka Condenser), Distributor and Sparking Plugs. A ballast resistor may also
be included in this system. Various bits of wire connect all these parts
together and move the electrons to the right place at the right time
hopefully. Without the aid of diagrams, the scope of this is limited but the
function of each component is described briefly below, For simplicity sake,
no formulas will be used, only descriptions of the various aspects.
Ignition Coil - This is the part that makes high voltage ( approx. 20KV for a
stock coil, and up to 40KV for a high performance coil) for the spark plugs
from the low voltage (12V) that is supplied to it by the car. It is
basically a simple transformer operating on the principle of "mutual
inductance". The coil stores up energy over a relatively long (for ignition
systems) period of time and then releases it suddenly to the spark plugs via
the distributor and HT wiring.
Coil operation - when the points close, current through the coil primary
increases from zero to a maximum value (determined by circuit resistance) in
an exponential manner, rapidly at first, then slowing as the current reaches
it’s maximum value. The rate at which the current rises is determined by the
coil inductance and the circuit resistance. At low engine speeds, the points
are closed long enough to allow the current to reach a level limited only by
the total circuit resistance, ie, a DC value. At higher speeds, the points
open before the current has time to reach this maximum value. In fact, at
very high speeds, the current may not reach a value high enough to provide
sufficient spark, and the engine will begin to miss. This current through the
coil builds a magnetic field around the coil. When the points open, The
current through the coil is disrupted, and the field collapses. The
collapsing field tries to maintain the current through the coil. Without the
capacitor, the voltage will rise to a very high value at the points, and
arcing will occur. The time for the field to collapse will also increase.
With the capacitor, the current provided by the collapsing field will
discharge through it, limiting the voltage at the points, and the
current/field will collapse very rapidly, having a discharge path to ground
through the capacitor.
The coil, capacitor, and resister form a tuned, oscillater circuit. When the
coil is completely discharged, the capacitor is completely charged. Now, the
capacitor will try to discharge to the coil. Without resistance, there is
nothing to limit the coil or capacitor discharge current, and the cycle will
repeat, ie, the coil will charge, then discharge to the capacitor, which will
charge, then discharge to the coil, etc. With the resistance, however, the
current is "dampened," and the amplitude of the oscillating current is
reduced rapidly, dropping to negligible within 3-4 cycles.
When the magnetic field of the primary coil collapses, it cuts through the
windings of the secondary coil, producing an out put voltage. The magnitude
of the secondary output voltage is determined primarily by the windings ratio
and by the speed at which the primary field collapses. A slow collapse will
produce a lower output than a rapid collapse. Until the arc occurs at the
plugs, the output of the secondary is nearly an open circuit, allowing the
voltage to reach a peak before current is produced. As soon as the spark
occurs, the resistance is reduced, and current flows through the plug gap,
maintaining the arc. The primary and secondary windings are isolated from
each other, so that no current in one flows through the other. However, the
secondary is connected to the primary at the point where the primary connects
to the points and capacitor, and there is no direct path for the return of
the secondary current other than through the capacitor. As a result, the
capacitor is part of the secondary as well as the primary. There is an
oscillation in the secondary, just as there is in the primary, for the same
reasons. By properly selecting the coil/capacitor parameters, the designer
can "tune" the circuit to provide the most effective output voltage, as
described below.
Typical Ignition-Coil Parameters
Turns Ratio 100:1
Secondary 25,000 turns #41
Primary 250 turns #22
Primary Inductance 6 to 10 mH
Primary Resistance about 1.5 ohms
Secondary Inductance 40 H
Secondary Resistance 10 kilohms
Points - Ignition points are a set of electrical contacts to switch the coil
off and on at the appropriate time. The points are opened and closed by the
mechanical action of the distributor shaft lobes pushing on them. The
maximum amount of (coil primary) current that can be switched by points is
about 4 amps. Above this level points burnout may occur.
Capacitor (Condenser) - The capacitor performs several functions. It
prevents the points from arcing and prevents coil insulation breakdown by
limiting the rate of voltage rise at the points. It’s primary function is to
provid for a rapid decay of the primary coil current. The capacitor also
"third-harmonic" tunes the coil, raising the peak output voltage and
increasing the secondary voltage rise time. This increases the efficiency
and the amount of energy transferred to the spark plugs. If the coil
secondary voltage rises too quickly, excessive high frequency energy is
produced. This energy is then lost into the air-waves by electro-magnetic
radiation from the ignition wiring instead of going to the spark plugs where
we would like it to go. Voltage rise time should be more than 10
microseconds; a 50-microsecond rise time is OK. Conventional systems have a
typical rise time of about 100 microseconds.
Distributor - The electrical "traffic cop" which directs the voltages to
their proper places at the correct time. It routes the high voltage
generated by the coil to the intended spark plug via the HT wiring. The
distributor houses and operates the points, and capacitor (described above).
The standard points type distributor can produce ignition timing errors in
three ways: 1) wear of the rubbing block, 2) variations in the cam profile,
3) shaft eccentricity.
Sparking Plug(s) - These are the business end of the ignition system. The
sparkplugs take the electrical energy provided to them by the rest of the
ignition system and turn this into the (hopefully) optimum spark event which
ignites the fuel. Proper polarization of the coil is necessary to provide a
negatively charged high voltage to the center electrode of the spark plug,
which is hotter than the outside electrode. This enables us take advantage
of thermionic emission* which reduces the voltage required by 20 to 50% for a
given spark magnitude. The plug gap affects both the voltage and the energy
required. As the plug gap is increased, the required voltage increases, but
the required energy decreases.
*Thermionic emission - (aka Edison effect) The propensity of some metals to
give up their free electrons more easily when heated, actually boiling off of
the metal. This is the fundamental operating principleof vacuum tubes, once
called thermionic tubes.
Ballast Resistor - This is an electrical resistor which is switched in and
out of the supply voltage to the ignition coil. It makes the engine much
easier to start by effectively doubling** the voltage provided to the
ignition coil when the engine is being cranked, compensating for the reduced
battery voltage. This provides a much better spark just when the car needs
it most. When starting a cold engine, the plugs and the air are cold, the
cylinder pressure is up, and the fuel / air mixture is poorly controlled. The
oil is thick, the battery is cold and its voltage drops as much as 60%
because of the high current drained by the starter motor. It’s a wonder the
car starts at all.
**Actually is cuts the voltage to the coil in half when the car is already
running, but it’s easier to understand the first way. A nominally 6 Volt
coil is used in a ballasted ignition system.
Internal coil resistance - Resistance built into the coil, in addition to the
inherent resistance in the copper windings, to limit current through the
points at idle and low rpm operation. This resistance is not to be confused
with the ballast resister mentioned above. It serves a completely different
function. As stated above, at low engine speeds, the primary coil current can
reach a higher value than at high speeds. If a coil is designed to provide
sufficient output at high speeds, the primary coil current can reach
excessive values at low speed. Conversely, if the coil is designed to limit
low speed primary current, it may lack sufficient power at high speeds. One
way to provide for both low and high speed operation is to provide a
"ballast" resister in series with the primary winding. This resistance
consists of an iron wire coil. Iron has the property of increasing resistance
with temperature. At low speed, the high current heats up the iron wire,
increasing its resistance, and reducing current. At high speed, the current,
as described above, is less, so the iron wire resistance does not increase,
thus the current is not limited. There are other design technique available
to provide for wide speed variations, so not all coils will use an internal
resistance.
Dwell angle - the degrees of rotation of the cam/distributor during which the
points are closed. During each rotation of the cam/distributor, the points
must open and close once for each cylinder. For a 4 cylinder engine, this
allows 90 degrees of rotation for each cylinder, (360/4,) for a 6 cylinder,
60 degrees, and for an 8 cylinder, 45 degrees. For reasons stated above, the
points must stay closed long enough to allow the coil primary current to
reach an acceptable value, and open long enough to discharge and produce a
spark. Typically, the ratio of closed to open is on the order of 3 to 1, ie
closed for 45 degrees and open for 15 in a 6 cylnder engine. The time at
which the points open is set by rotating the distributer, the time at which
they re-close is set by adjusting the gap. The larger the gap, the longer the
points are open, and the smaller the dwell angle. Thus, the dwell angle is
determined by the points gap.
This really only scratches the surface of ignition systems. You have been
spared topics like Cylinder pressure, Ignition-voltage waveshape, Timing,
Capacitance, Inductance, formulas with greek letters and other technical
mumbo-jumbo. But then there’s always part 2.....
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