Ignition only

[Chris Conlon]

At 06:41 PM 11/10/00 -0500, Bruce Plecan wrote:

>>It's
>> inductance and capacitance are low by comparison. The trick is it's
>> a variable resistance, also it has what's called a "negative
>> resistance" characteristic.
>
>Negative resistance?.
>So it generates a Current?

No, negative resistance is a common term for a type of circuit
element. It might be an unfortunate term, and I'm sorry if it
caused any confusion. Ignoring the term, a spark plug acts a hell
of a lot like a neon lamp, as I described below.

>>It's like a little neon lamp. It's an
>> open circuit (roughly) when not lit, but put enough V across it
>> and current through it, and it becomes a low resistance device.
>> If you could put an ohmmeter to it when firing (somehow), you'd
>> see that the R has become pretty low.



>Geesh the spark voltage itself is AC, very high freguency.

Disagree strongly, at least for the systems I had on my bench.


>> Further, I put inductive pickups (attached to a scope) on the
>> plug wires of a couple different systems. These pickups are
>> sensitive to the direction of current flow in the plug wire as
>> well as rate of change. They showed the same thing, one single
>> spark in the expected direction.
>
>Then too heavily dampened.

There was no damping at all.

>The A/C is on the firing line

No. Even if the coil somehow put out 100+ MHz HV (which it can't,
at any large power level, due to it's very high secondary
inductance), if someone were to swap in spiral core or other
inductive suppression type wires, that'd filter and absorb most
of the spark energy. (This type of wire absorbs more and more of
the signal as frequency increases.) While most people may say
there is some slight difference in performance between solid
core wires, carbon resistor type, and inductive plug wires, I
don't know many people who say their ignitions work great with
solid and carbon wires but fail utterly with inductive wires.
Yet this is exactly what would happen if the spark really was VHF
or UHF A/C. The wires would sop up damn near all of the spark.


> , not that is passes thru *ground*

You lost me here.


>> >The spark is A/C so nothing is backwards
>> Disagree strongly. I even heard lately of some OEM DIS system that
>> required platinum plugs on 1 of 2 cylinders fired by each coil. Wish
>> I could remember now which it was... late model Ford v-6 IIRC.
>
>They burn just fine on my GN, a poorly designed system that needs a crutch
>is hardly an example of the norm...

That they work fine says nothing about whether they're firing backwards
or not. All it says is there's enough spark voltage for the gap and
cylinder pressure.


>
>How do you fire a plug on 1,400 volts then?.

I don't, but I haven't tried either. Do you? Though I did read an
SAE paper that had some information on minimum breakdown voltages
(1.1 mm plug gap), and some were as low as 1.5 kV. This was only
under conditions of low MAP before the compression stroke, or
during the exhaust stroke.

So I guess one *could* fire a plug with 1.4 kV... but I don't see
how this relates.


>Why do CDs with a primary of 400v have a faster rise time.

Very simple. In an inductive discharge system, the primary is charged
to battery voltage, say 14V, and suddenly opened. (This 14V charge
translates to some level of magnetic field in the coil, the value
is not important, say it's X.)  After the primary is opened, the
magnetic field starts collapsing. Before the spark has jumped, the
field is decaying at it's natural undamped rate, which has to do
mainly with the design of the coil. This free discharge from 14V
to 0v determines the rise time leading to the spark formation.
A CD system rapidly charges the coil from 0V to say 400V. In this
case the magnetic field is being forcibly driven, and to a much
higher level than it ever would have been at 14V. (400/14 = 28.5
times as high. It may reach 28.5X, though probably not.) This
results in the magnetic field in the coil changing much faster
than if it's just allowed to freely decay from 14V. The much
faster changing field gives you a sharper rise.

Also at least some CD coils are designed differently to work
better with CD systems, and part of what results from this is an
even faster rise time.


>How come on a waste fire coil you can short one side together and it still
>fires the other plug?.

I'm not 100% clear on what you mean here; I'm guessing you're asking
about shorting out one of the 2 plugs, say jamming a gap gauge in
there or something. The other plug still fires because now you have
the rough equivalent of a coil per plug system. If you treat the
shorted plug as just a wire, which is roughly what it's reduced to,
you have both HV terminals going to opposite sides of a plug. Works
fine. In fact I've been looking into waste spark type coils (with
2 isolated HV terminals) for use in the coil per plug system I had
on the bench a while back. (That's not necessary, but supports some
other design goals.)


I do agree that it would be nice to have scope traces on line for
a common point of reference. Unfortunately I have no way to make
those, and none to scan, or I'd put them up myself.


   Chris C.

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