Times Have Changed: Cold Starting

One point that is often left out of these "bigger gap in the circuit"
discussions is that most of the *energy* delivered by the ignition
system gets released in the spark that is jumping the biggest gap. So...
while an extra gap may force the coil secondary voltage high enough to
fire a terribly fouled spark plug, the *energy* (and thus the ability to
reliably fire a the mixture) delivered to the plug is far less than a
CLEAN plug with with absolutely NO other gap in the secondary circuit,
and most of the energy is being wasted in that extra gap. IOW- adding
spark gaps to the secondary side is a half-assed band-aid for a problem
that should be fixed instead of patched. That is also why DIS systems
are superior to distributors, even though distributor systems
(artificially) force a higher peak voltage in the secondary, all other
things being equal.

 

Using an air gap to measure voltage isn't the same thing at all as
claiming that an air gap will CREATE a higher voltage. I agree
completely that an adjustable air gap can be used to measure the
potential available between two points, in this case the secondary coil
and ground. That is pretty basic physics. What I don't believe is that
making a gap larger will INCREASE the voltage available from the coil.
I maintain that the coil has a certain voltage capability and the size
of the gap you put in series with the secondary doesn't change that one
whit. If the gap is small enough so that the voltage available will
ionize the air in the gap, then a spark will occur. If the gap is
larger than what the available voltage can span, then no spark will
occur. But making the gap wider doesn't make the voltage higher. At
last I've seen no theory or evidence that this is the case.


Matt

 

If the secondary voltage is raised by a gap upstream of the plug gap, it's
higher *through the whole secondary path*, including at the plug gap.

It didn't do anything useful on engines in anything approaching proper
repair. It only propped-up neglected or worn-out systems.

DS

 

Yep. It's a neat trick for getting a recalcitrant, neglected engine
started, or wringing every last possible tenth of a mile out of a
thoroughly whipped engine, but that's about it.

 

As I understand it, once the field in the coil has collapsed and the HT
voltage has been generated, that voltage is present until grounded.

The bigger the gap to jump, the longer the coil windings have to saturate,
and thus the bigger the jolt generated once the field does collapse.

Again, as I understand it, the more more volts, the more likely it will be
that some of the current will manage to jump the gap on a sub-optimal spark
plug.

As always, corrections welcome.

 

It should be the voltage on the primary (probably 12-14 volts) times the
turn ratio between the primary and secondary. That is basic transformer
theory.

This link explains it quite simply. Notice that the voltage on the
secondary is a function of the voltage used to "charge" the coil and the
turns ratio, and not a function of any air gap on the secondary circuit.

http://rds.yahoo.com/S=2766679/K=au...=1106533064/*-http://www.jlctech.net/ign.html


Matt

 

It won't increase the total voltage available from the coil, if
the coil has a maximum capability of 40 KV, then that is the most
you will get.
But looking at the spark tester, if a smaller gap represented
where the number 20 is equals a coil output of 20KV and a larger
gap where the number 40 is equals a coil output of 40KV, what
other conclusion could be reached other than a larger gap will
cause the circuit voltage to increase?

I'm not talking about the total voltage capability, ignition
coils do not normally run nor would you want them to run at there
maximum total capability. If making the gap wider doesn't make
the voltage higher, then please explain what value does need to
change in a case such as a worn spark plug where the gap has
increased because it sure as hell shows up as an increased
voltage every time I've ever encountered it.

 

It should be the voltage on the primary (probably 12-14 volts) times the
turn ratio between the primary and secondary. That is basic transformer
theory.[/QUOTE]

You haven't answered the question. Either you know it and have
seen (measured) it or you haven't.
I'm patient, I'll wait. Take your time.
But I'll tell you this, a little practical application will go a
whole lot farther than what you learned in college.
Take your volt meter, set it on min-max and connect it to the
coil of your minivan.
Aww hell, let's quit screwing around, go here:

http://www.interro.com/wav.htm

They give some great examples of exactly what I've been saying.
Compare the distributor ignition waveforms to the DI waveforms,
notice on the conventional waveforms that the firing voltage AND
the spark line voltage are measurably higher than those on a DI
system.
Look at the waveform labeled DIS waste parade, it shows a very
high firing voltage due to there being a gap greater than the
spark plug gap (an open in a wire).
Of course this can all be dismissed I suppose by simply claiming
that Interro is some bogus fly by night outfit because you've
never heard of them before.

That link is about as basic as it gets and in NO way delves into
what we're talking about here, and just out of curiosity, what
are the authors qualifications?

 

Not quite clear what higher "through the whole secondary path" would
mean...

However - first, I misremembered. The coil does indeed have an
induced voltage, not the induced current I was remembering.

And, it turns out (isn't the web wonderful?) that "The Subsidiary Gap
as a Means for Improving Ignition", NACA report 57 (1920), is
available at

http://naca.larc.nasa.gov/reports/1920/naca-report-57/naca-report-57.pdf

The weird thing is that it turns out that the capacitances in the
circuit are crucial. Basically, when the voltage is building up to
fire across the series gap, a charge is also building up. When it
gets to breakdown, this discharges, and a charge builds up across the
fouled plug. The voltage across the plug is determined by the charge
and the capacitance, and will be much higher than you'd expect from
the current and resistance.

 

That is my premise, but that isn't what has been claimed by a couple of
others here. I'm not ruling out that this may even be possible, I just
don't see how and haven't seen any theory that is supported by
well-known laws of physics.

No, the larger gap just shows that more voltage was available. It
didn't change it. That is the same as looking at your outdoor
thermometer when it is 80 out and then looking again and seeing that it
is now down to 40 and claiming that your thermometer made it get colder
out! Measuring something and changing something are two different
concepts.

How does a coil change its voltage capability? This is determined by
the coil ratio and the voltage on the primary side. Are you saying that
old cars with points ignition systems had a way to adaptively change the
voltage being applied to the primary side of the coil? I don't think
so. And you can't change the coil ratio so what else are you changing
to affect the voltage at the secondary?


Matt

 

Excellent find! I scanned this report briefly, but will have to study
it a little later. Looks like that the key issue is controlling the
rate of discharge of the coil so that the energy peak will be higher
than would be the case discharging through a fouled plug where the
leakage would allow broadening of the energy pulse and thus lower the
peak. Very interesting and finally some good theory and practice discussed.

How did you come across this?

I wonder if this still applies to ignitions where the coil is switched
via a transistor rather than a contact opening...


Matt

 

That is my premise, but that isn't what has been claimed by a couple of
others here. I'm not ruling out that this may even be possible, I just
don't see how and haven't seen any theory that is supported by
well-known laws of physics.[/QUOTE]

Certainly not by me.

You either missed or are ignoring my previous comments where I
stated that I had used this device while connected to my
secondary ignition scope, when set to the 20KV gap, the coil
output was 20KV, when set to the 40KV gap, the coil output was
40KV , that amounts to changing something and measuring the
results. That is the result when you open the gap.

I never said it could. But they certainly do not output the same
voltage under all operating conditions. Just like an engine does
not produce the same horsepower under all conditions even though
it may have a horsepower capability of 350 H.P.

That would be one of the things one might observe a ballast
resistor doing under various operating conditions although I'd
maintain that it's actual purpose is to control current.

The gap. It doesn't change the maximum voltage that an ignition
coil can output but it (the gap) will change the voltage output
up to the point where maximum voltage is achieved.
Open the gap, it takes more voltage to arc across, cram more
oxygen molecules in between the gap, and it will take more
voltage to arc across the gap.
Observe firing voltage at idle, let's say it's 12 KV, snap the
throttle, the firing voltage increases to 22 KV, the reason it
increased is because opening the throttle allowed more air
molecules into the combustion chamber/in between the plug gap.
When the engine decelerates, the firing voltage may drop to 4 KV.
Secondary ignition voltage does not remain constant under various
operating conditions. Observable fact, easily observable.

 

The problem arises not when the plug resistance is too high, but when
its too LOW (fouling providing an alternate conductive path around the
gap). The slight conductivity of the fouled plug can prevent the coil
from fully saturating, even though its still a realtively high
resistance path- ignitiion coils don't generate much current at all, so
it doesn't take much "leakage" to prevent it from saturating. Inserting
another gap "upstream" of the plug will prevent current from "leaking"
through the plug, allow the coil to saturate, and then when the upstream
gap breaks over the full voltage suddenly appears at the plug gap and
jumps it despite the "leakage" path around the gap.