>Date: Sun, 30 Mar 1997 16:54:45
>To: S1500@aol.com
>From: neil drake <ndrake@direct.ca>
>Subject: Re: ignition coil
>
>At 09:13 PM 4/1/97 -0500, you wrote:
>>In a message dated 97-03-31, Bob Sykes wrote:
>>[...]
>>> The capacitor is primarily to
>>> suppress arcing at the points (and RFI of course). It's not really a
>>resonant
>>> LC circuit at all.
>>
>>In a message dated 97-03-31, Dan Masters wrote:
>>>Actually, no. I had been told all my life that the capacitor was for the
>>purpose you
>>>suggested, but not too long ago, I tried to run my TR6 without one, and it
>>flat would not
>>>run. Would not even start! Never fired once. Replaced the capacitor, and it
>>kicked right
>>>off [...] -snip-
>>
>>I tried this today and my car would fire intermittenly but would not start
>>without
>>the capacitor connected either. I stand (sit) corrected and a little wiser,
>>but this
>>still puzzles me.
>>
>>>When the points open, the magnetic field in the primary collapses to produce
>>the HV
>>>output from the secondary. This collapsing field produces a current in the
>>primary, in
>>>the same direction as when the points were closed. Without the capacitor,
>>there would
>>>be no place for this current to go. An arc is an ineffective current path.
>>The instant the
>>>points open, the capacitor, which has been discharged, looks to the coil
>>like a short
>>>circuit, as it charges to the 12 volts.
>>
>>Agreed so far.
>>
>>> Without a good current path, the field collapses too slowly, and produces a
>>weak
>>>spark.
>>
>>I'm not so sure about this. With a good current path (points closed) the
>>field
>>does not collapse at all. If we suddenly interrupt the primary current path
>>(points open, no cap) the field collapses at the maximum rate, producing the
>>highest voltage. The capacitor slows the rate of change in current and
>>consequently
>>the rate of magnetic field collapse, reducing the voltage.
>>
>>[we are in agreement about the primary and secondary sides being
>>linked only by mutual inductance, no DC path]
>>
>>>I agree that resonance is not of concern here (at least I don't think it
>>is),
>>> but any time you have a capacitor and an inductor wired together, either
>>series or parallel,
>>> there is a resonant circuit. In this case, I think the resonant frequency
>>is
>>> outside the range of concern.
>>
>>Agreed, but I'm beginning to wonder if resonance doesn't play an active part
>>here.
>>Initially I didn't regard this as a resonant circuit because the the
>>capacitor is effectively
>>shorted out by the points ~90% of the time.
>>
>>[interesting information about books snipped for brevity & bw]
>>
>>...> ( have a reasonal grasp of electrical engineering principles, but the
>>peculiar aspects
>>> of automotive ignition systems is new to me). Now that I am retired, I have
>>time to
>>> persue such stuff as this.
>>
>>The more I learn about automotive ignition systems, the more peculiar they
>>appear to me.
>>My understanding of inductive circuit principles comes mainly from the 'laws'
>>of Kirchhoff &
>>Lenz but these guys obviously weren't exposed to automotive electrical
>>systems ;-)
>>
>>>As an example of my lack of understanding, according to the books, there is
>>some
>>> resonance in the output of the secondary, which has no discrete capacitor.
>>> This puzzles me. Can anyone explain that?
>>
>>I'm afraid I can't, but would love to hear someone's theory!
>>
>>>I appreciate your comments, and look forward to an enjoyable discussion of
>>this
>>> subject with the members of this list
>>
>>I have enjoyed this and learned something that I never would have thought
>>possible. I guess I'll have to add a condenser to my 'road trip kit' ;-)
>>I fear that this subject may be getting a little esoteric for the "Triumphs
>>List"
>>though. Especially in light of some recent posts.. I would be glad
>>to pursue this further 'off line' ( I can get a little long-winded). Posting
>>schematics and equations describing series resonant circuits might elicit
>>some complaints<g>.
>>
>>Bob Sykes - - - __,@_\____ s1500@worldnet.att.net
>>'78 Spitfires - -- }-0-----0-> s1500@aol.com
>>"Every great and commanding movement in the annals of the world is the
>>-triumph- of enthusiasm. Nothing great was ever achieved without it."
>>--Ralph Waldo Emerson
>>
>>
>>
Gooday Chaps,
If I may add something here, there are sources of capacitance in the
secondary side of the ingnition coil. Firstly, the coil itself has some
capacitance itself in its secondary albeit minimal. Also, the spark plug
provides a source of capacitance. Governered by the equation C=eA/d where
e(epsilon) is the relative permittivity(dielectric constant), A is the cross
sectional area of the spark plug, and d being the spark gap. e for air is
about 1, thus with a spark plug with a cross sectional area of 7mm and a
spark plug gap of 3mm, the resulting capacitance is about
2.4microfarads(relatively large).
However, assuming a secondary inductance of around 45 milliHenries(I am
assuming here), the resonant frequency calculated to about 500Hz. In a
4-cylinder, this means at about 15,000 RPM the spark will have the most
energy. Any TR3 owners care to give this a try?
Hope I haven't bored you,
N Drake
Biomedical Engineer
Vancouver B.C.
1959 Berkeley
ex-TR7.5 owner
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