Re: [Phys-l] cars and physics

[Michael Edmiston <edmiston@bluffton.edu>]

The link Brian Whatcott provided nicely presents my understanding of the 
ignition systems I worked on prior to "electronic (point-less) ignition" 
systems.

I spent a few minutes with Google to see if I could find any explanation of 
why the change was made from 6 volts to 12 volts.  The only thing I found 
was that the change was made "to improve engine performance."  This made 
some sense to me after noticing something that was stated in the link Brian 
provided.  That author mentioned the pressure in the cylinder (along with 
the falling voltage) rapidly quenches the spark.  As engines moved to higher 
compression it probably required a higher-voltage to the spark plug in order 
to get a reliable spark across a sufficiently-long gap at the higher 
cylinder pressure.  The coil could have been redesigned with more secondary 
turns to provide the higher voltage, but that would reduce the current and 
hence the intensity of the spark.  The spark intensity could be increased by 
more primary current but that would require a larger capacitor and it also 
would wear out the points faster.  A change to higher battery voltage would 
be the obvious way to fix all this.  So I am going to hazard a guess that 
engine designers needed a higher voltage spark with good current in order 
get reliable ignition at higher compression ratios.

As an aside, here is fun story about a failed ignition capacitor...

When I was in graduate school, my wife and I had two cars: a Fiat 124 and a 
Fiat 128.  Don't laugh.  They were purchased new for well under $2000 each, 
and they were a lot of fun to drive.  But the 4-cylinder engines were fairly 
high-compression and ran at fairly high rpm.  The composite fiberboard 
portion of the "points" that rubbed on the distributor cam would wear down 
quickly, and this required a timing adjustment.  The electrical contacts on 
the points also wore quickly, and this required replacement of the points. 
I used to adjust the timing at each 3000-mile oil change, and I replaced the 
points at alternate oil changes.  I did not have a timing light, so I pushed 
the car in gear to get the timing mark to the right place, then I used an 
ohmmeter to show me when the points opened as I twisted the distributor back 
and forth.

Points and condenser (capacitor) were inexpensive and didn't take long to 
replace, but I was too cheap and too lazy to replace the capacitor.  I guess 
I figured a capacitor would have infinite life.  I guessed wrong.  My wife 
got stranded at the one-room country school where she was the teacher.  (I'm 
not kidding about the one-room school in order to imply this is ancient 
history.  From 1974 to 1976 my wife was the only teacher in a one-room 
school west of Lansing, Michigan where she had about 20 students from 
kindergarten to seventh grade.)  She got stranded after dark on a winter 
night and had to call me to rescue her.  (The school did have electricity, 
running water, and a telephone.)  I drove the other Fiat to the school, but 
couldn't figure out the problem and couldn't get her car started, so a 
parent of one of her school kids towed the car to the nearest service 
station with his tractor.

The next day the mechanic diagnosed the problem as a failed condenser, and 
he asked me how come the points looked brand new but the condenser looked 
like it had been there forever.  I explained that the condenser had been 
there for a long time because I only ever changed the points.  He said, "Why 
in the world would you replace the points and not replace the condenser at 
the same time.  Don't you know the car won't run if the condenser fails?" 
Well, I hadn't thought about it until then, so this country mechanic gave me 
a little physics lesson about how the condenser and coil work as an LC 
oscillator set off by the collapsing magnetic field in the coil when the 
points open.  I felt pretty silly for not knowing that, and I felt pretty 
silly for not paying about $1 for a new condenser every time I changed the 
points.  Since the distributor cap and rotor were already removed to replace 
the points, I could have replaced the condenser in less than 30 seconds.  I 
definitely paid the mechanic more than I ever saved by not replacing the 
condenser every 6000 miles.


Michael D. Edmiston, Ph.D.
Professor of Chemistry and Physics
Chair, Division of Natural and Applied Sciences
Bluffton University
1 University Drive
Bluffton, OH 45817

419.358.3270 (office)
edmiston@bluffton.edu


--------------------------------------------------
From: "brian whatcott" <betwys1@sbcglobal.net>
Sent: Monday, February 21, 2011 11:42 PM
To: <phys-l@carnot.physics.buffalo.edu>
Subject: Re: [Phys-l] cars and physics

> Oren's question triggered a train of posts which focused on the
> capacitor, rather than on the
> time constant of the L/R path of the old-time ignition coil.
> The following folksy  enthusiast note carries a couple of scope traces,
> and allows one to visualize the time constant of a normal 12 volt coil
> at < 1.5 millisec, and a typical winding resistance of 3 ohm at the
> primary and a turns ratio of 1:100 which transforms the primary voltage
> across the capacitor of 300 volts to 30 kv for the initial strike of the
> plug. Elsewhere, the writer notes an intially negative voltage at the
> hot electrode  provides a somewhat easier strike.
>
> http://mgaguru.com/mgtech/ignition/ig108.htm
>
> The coil schematic shown was all but universal to coil ignitions without
> ballast resistor.
>
> Brian W