EAA493 Mailing List: Fwd: Re: [CorvAircraft] Distributor wiring

[Norm Beauchamp (nebchmp]

Interesting discussion.  Norm


No, you do NOT want to use a condenser on an electronic ignition system.

There are a number of popular misconceptions about ignition points
and coils, a few of which came up in an earlier post, so I'll address
them in case there's any confusion:

At 10:11 AM 1/5/2014, you wrote:
> The points and the electronic gizmo in the distributor are switches
> that are operated by the engine.

        TRUE.
        As the distributor shaft spins, points open and close or a
hall-effect or optical sensor causes some form of semiconductor
"switch" in an electronic ignition system to conduct or not conduct.

> Those switches are open most of the time.

        NOT TRUE.
        Because it takes time to build up a significant magnetic
field in a coil, and an ignition coil needs a large magnetic field in
it to generate a spark, the switches are closed most of the time in
most ignition systems to assure that the coil has time to build up
the magnetic field it needs.

> When the engine needs a surge of electricity to make a spark with a
> spark plug, the switch closes momentarily.

        NOT TRUE.
        The spark is actually created when the points *open*.  To
prove this to yourself, rotate your distributor shaft to a place
where the points are closed and apply power to the ignition system
with the engine not running.  Remove the high-tension wire from the
coil to the distributor cap at the distributor cap end, and hold the
metal part about 0.020" inches (about the width of a spark gap) from
a metal surface of the engine.  Then gently pry the points open with
a small screwdriver.  You'll see a spark jump when the points *open*,
not close.
        The reason for this is that it's the collapse of the
magnetic field in the coil that causes the spark to occur.  When the
points open, there's no longer any electricity conducting on the low
voltage side of the ignition system, so the magnetic field inside the
coil starts to collapse.  As it does, that energy has to find some
other path out of the coil.  In response, it makes its way out via
the high-voltage side of the coil, first by ionizing the fuel air
mixture inside the cylinder to the point where the dielectric of the
gaseous mixture starts to break down.  Once it's broken down far
enough, it's possible for the ionized gas to conduct enough for a
spark to jump.  This is similar to the phenomenon that is "arc start"
in an electric welder.
        With "hotter" ignition systems, essentially you use a coil
that generates higher voltage at the spark plug, so it can ionize
more gas between the spark plug tip and the core.  The result is that
you can then open up the plug gap further and create a bigger "flame
front" with a longer spark.  Therein lies the power of high voltage
ignition systems: Being able to open spark plug gaps further.  Just
adding a "performance" ignition coil to any engine without regapping
the plugs doesn't do a think to help engine performance.  Once the
plugs are opened for a larger spark and better flame front, however,
you can adjust timing to take advantage of the fuel-air mixture being
ignited more quickly.  And this is where any increased power or lower
propensity to detonate comes from.

> But the spark plug needs a real whopping surge of electricity to
> make a hot, reliable spark.

        TRUE.
        Typically 12V ignition systems on internal combustion
engines pull between four and six amperes of current, once you're
beyond the ballast resistor.  And typically the ballast has about the
same resistance as the DC equivalent series resistance (ESR) of the
coil windings.  So typically you're putting about half of the total
power of the ignition system's lower current loop into the coil.
        In a system pulling 6A of current (averaged over time for
"points open" and "points closed") on a 12V electrical system, you're
dumping 72 watts of power through the low-side current loop.  With
half of that going to the ballast, the coil is probably sucking down
on the order of 36W, which is a considerable amount of power.

> The "at rest" electrical system just doesn't have the juice to
> provide that surge.

        NOT TRUE.
        "At rest" is a meaningless term.  There's really only
"circuit complete" (points closed) and "circuit open" (points
open).  When the points are closed, the coil is looking back through
a low-resistance ballast into the battery itself.  Consider that a
battery can put out 750 cold-cranking amperes to start a car if it
has to.  750A x 12V = 9 Kilowatts.  That's a little over 12
horsepower.  And I could push a small skiff to about 30mph on the
water with a 12HP outboard engine, even after all the inefficiencies
of a propeller and drive shaft.
        There's *plenty* of power there.

> The condenser is an electrical storage device that can store a LOT
> of electricity.

        NOT TRUE.
        The purpose of a condenser to store a very small amount of
electricity so that the voltage across the points doesn't change
instantaneously when they open.  If it did, the air between the
points would ionize and dielectric would break down causing the
points to throw a spark across them -- and burn.  And this is why
when your condenser goes bad, your points burn out quickly.
        In practice, there actually *is* always a small spark at the
points, even when there's a good condenser there, but the point is to
limit that energy so the points don't burn prematurely.  Over time,
they do finally degrade to where they have to be filed flat again or
simply replaced.
        And coming back to the original question, you don't need a
condenser with a solid state ignition system because there are no
mechanical points to burn.  In practice, with most electronic
ignition systems there's a diode somewhere on the low-side circuit to
provide a path for any stored energy back to ground so that the
transistor (or other semiconductor device) that's switching doesn't
see an overcurrent and burn out.
        It's beyond the scope of this discussion, but the large
field effect transistors used in electronic ignition systems often
have a residual back-biased substrate diode built into them by
default as a result of the manufacturing process that makes
them.  (The silicon substrate is usually implanted with a dopant
species from the opposite III-I V column in the periodic table as the
dopant species used in the conduction channel of the FET where the
space-charge layer is open when transistor Vgs gets to where there
enough free electrons / holes that conduction is possible.)

> It can also dump all its contents instantaneously

        NOT TRUE.
        The formula for capacitor (condenser) behavior is (in
Laplace transform terms): i = Csv  (or i = C dV/dt, expressed in the
terms of standard differential calculus).
        The bit about "s" (or "dv/dt") tells you that there's a time
factor involved in changes of voltage on or current flowing into or
out of a capacitor.  Typically in a first order (Laplace Transform)
system capacitor voltages and currents follow the path of
"exponential increases or decays over time."  The reasons for this
are also a bit beyond the scope of this discussion, but if you'd a
quick tour of the calculus of complex numbers, starting with Euler's
identity (which breaks the world into real and imaginary parts and
relates them through sine and cosine functions), drop me a note offline.

> which it does when the points close and complete the electrical circuit.

        NOPE.
        Go back and read the bit about "points open."

> That gives the spark plug that whopping surge it needs.

        NOPE.
        The whopping surge comes from the collapse of the magnetic
field in the coil when the points *open*.

> So, yes, condensers are needed for both the mechanical and the
> electronic points.

        Again coming full circle...
        Condensers in point type (Kettering) ignitions.
        NO condensers in electronic ignitions.

-- Eric Overton

p.s.  Yes, I do this for a living.


Eric D. Overton
CEO, Focus Embedded
Visionary Electronic Design
PO Box 270096
Austin, TX  78727
(512) 246-9012 (direct)
(888) FOC-7924 (toll free)
eric.overton [at] focusembedded.com
http://www.linkedin.com/in/ericoverton
www.focusembedded.com


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