Re: [Phys-l] cars and physics

[brian whatcott <betwys1@sbcglobal.net>]

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

On 2/21/2011 2:46 PM, Quist, Oren wrote:
> I was told this, but never verified it.
>
> One reason cars switched from 6-volt to 12-volt (when I started driving, most cars were 6-volt) was that the time constant to charge up the condenser (what we call the capacitor) was too long to effectively fire eight spark plugs at normal rpm speeds.  As V-8's became popular, the 12-volts became necessary.  Of course the time constant would not change as the voltage went up, but double the voltage would lead to double the charge which was then enough to fire the plugs.
>
> True or false???   -- whatever, it makes for an interesting calculation.