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Re: [Phys-l] cars and physics



On 02/22/2011 02:47 AM, Michael Edmiston wrote:

.... 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.

That is indubitably true.

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.

Doubling the operating voltage also requires a larger capacitor,
generally speaking. Not larger in terms of capacitance, but
larger in terms of physical size, cost, et cetera.

Current (by itself) is not what wears out points, especially if you
adhere to what I call the two laws of non-dissipative switching:
-- Never close a switch in parallel with a capacitor
unless you know the voltage is zero.
-- Never open a switch in series with an inductor
unless you know the current is zero.

The circuit in question can be arranged to uphold these laws, to
a good approximation.

The coil could have been redesigned with more secondary turns to
provide the higher voltage,

Indeed it could.

but that would reduce the current and hence the intensity of the
spark.

That's not the necessary (or even likely) outcome. The turns
ratio tells us the ratio of primary current to secondary current
... but there is no reason to think the primary current remains
constant, unless somebody designed it to be so, which they would
never do; see above.

A change to higher battery voltage would be the obvious way to fix
all this.

That's not at all obvious to 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.

Indeed. This is a nifty example of real-world physics. It
is somewhat out-of-date, but it is still directly relevant to
antique-car enthusiasts.

The circuit is complicated enough to solve a real-world problem,
but still simple enough to be analyzed using introductory-level
physics ideas.

On 02/22/2011 04:47 AM, Bennett wrote:

About the same time 12v came on the market, I noticed some cars had
a ballast resistor with the coil.

There are various reasons why you might want some resistance
in the circuit, mostly having to do with the grossly nonlinear
IV characteristic of a spark.

The most obvious reason can be understood by considering what
happens when the spark is extinguished. That will, in general,
happen while there is still nonzero energy in the LC oscillator.
Where is that energy going to go? You would like to get rid of
it before the points reclose.

Similarly you would like to have a well-behaved upper bound on
the voltage if somebody operates the circuit with the spark plug
not connected.

On 02/21/2011 09:41 PM, Bernard Cleyet wrote:
And I always thought the cap. was too protect the points (switch on
opening), and to collapse the mag. field in the "coil" faster.

You can't have it both ways.

If you want to make the magnetic field collapse faster, you don't
want a capacitor in there, since it provides a path for the current
to keep flowing. And making the field collapse faster provably
means having a higher voltage across the open points, which is the
opposite of what you want if you are trying to protect the points.

It would be closer to the truth to say that the capacitor protects
the points by allowing the magnetic field to collapse more slowly
... but that is an overly simplistic view of the situation. It is
better to analyze the circuit as a whole. The capacitor doesn't do
only one thing; there is a choreographed interplay between the R,
the L, the C, the battery, and the switch.

======

More generally: Guessing and hand-waving are not required. If we
ignore the nonlinearity of the spark, this can be modeled as a simple
RLC circuit, plus a battery and a switch. It can be analyzed using
elementary techniques,

You should think twice before letting students observe the system
as a lab exercise, because of the possibility of blowing the front
end off the oscilloscope. There are lots of ways of protecting
against this, but I don't at the moment see how to prevent students
from bypassing the protections.

======================

If you still want a hand-waving answer to the original question:
Think about the power and current needed by the starter motor, in
contrast to the power and current needed by the ignition circuit.

Given that even in a 6V system it was possible to provide enough
juice to the starter motor, do you really think they were motivated
to switch to 12V because that was the only way to get enough juice
to the ignition circuit? That seems implausible, with several
orders of magnitude to spare.