Ignition question

[Ron & Cathy Webb]

OK - I'll take a shot at this - and happily sit down if someone
who has designed one wishes to interject.

There are a million ways to do almost anything electronic... the
challenge is to do the required job cheaply and efficiently.
Asking why something was done this way instead of that sometimes
gets one farther into the details than one needs to go...

For a spark, the desired action is to deliver a certain amount of
energy to the plugs. Energy is measured in Joules (or
Watt*seconds - same thing). Other things are important too -
enough voltage to reliably jump the gap, and of course precise
control of timing.

The existing CD design solves all these problems neatly by
generating a moderate voltage AC from an oscillator at a fairly
high frequency, rectifying it, charging a capacitor of known
capacitance to a known voltage (thereby nailing down the energy
in the spark) then firing it at exactly the right time by
triggering an SCR.

Other designs are possible. Your idea may produce a slightly less
complex device, at the expense of less well defined spark timing
and energy. It would also produce current spikes instead of a
nice smooth current drain. These problems are probably solvable.

Someone somewhere made a design decision to do it this way
instead of that. If you think he made a big mistake...well that's
called an opportunity - have at it!



bruce plecan wrote:

> This may sound really dumb, but I gotta ask.  In a CD ignition
> system
> they "dump" a 1 MFD. cap (at 200-300v) thru the primary side of
> the coil.
> The reason I was told DC is to begin the arc at the plug on a
> specific
> electrode.  In a distributorless ignition it looks like it
> doesn't
> matter.  So why not just pulse the CD charge transformer thru
> the ign.
> coil for say a little over a millisecond.  If the answer causes
> smoke
> just e-mail me, and I'll put my hand down.
>   TIA Bruce  nacelp at bright.net