[urq] ignition system electrical question

[Phil Payne]

>   Okay, the coil generates a spark that's like 50 billion volts or
> something (or at least a lot more than 12) and sends that out, through
> the distributor, through the plug wires, through the plug.  I kind of
> understand where this voltage comes from...the sudden collapse of a
> magnetic field in the coil which creates something that probably has a
> much more technical term but is essentially an electrical whiplash...the
> same sort of thing that relays without diodes can do that destroys
> switches over time.  Now, what I don't get is where this voltage goes
> after the spark plug.  It seems like it would just ground to the engine
> and then eventually the battery.  But how can this kind of voltage
> ground to a 12v negative?  BTW, anyone know exactly how much voltage and
> current is actually taking place here?

I think around 50kV is pretty normal.

The ground isn't -12V - it is by definition _ZERO_ volts.  On an ur-quattro, there are
actually quite a few different voltages in use, mainly within the digital dash (30V) but also
within the radio (anything up to 50V, but not much of it).  Both of these voltages, being
higher than the battery voltage, are created by voltage converters (static inverters, to be
accurate).

The actual charge - the current - that an ignition system delivers is pretty tiny.  The spark
grounds to the engine block via the plug's body and the return path, in most cases, is one of
the earthing straps to the front and rear of the engine.  Because the current involved is so
tiny, it can actually flow back through all the various engine and transmission attachments.
Sensor grounds are much more important.

-- 
  Phil Payne
  http://www.isham-research.com
  +44 7785 302 803