[s-cars] Re: Coils and PSO's

[Paul Gailus]

The dwell is the interval when the battery voltage (minus POS
voltage drop) is applied across the coil primary winding.
In the old days this was the duration when the points were closed
in some fixed number of crankshaft degrees.
So the dwell time is when the coil is "charging up" and energy is
being stored in its inductance. Electronic control is not limited
to a fixed number of degrees like with points, and the dwell
interval can be set over a wide range.

The endpont of the dwell interval is established by the precise time
when the plug must be fired based on the ignition maps, etc.
The start of the dwell interval has to be early enough that the
coil has sufficient time to charge, but not so long that the
primary current and/or the power dissipation get too high.

On the AAN the dwell is completely determined within the
ECU, and the POS is just a "dumb" amplifier.
I've heard that some very recent designs may have at least part
of the dwell control within the coil unit. There are certainly
some units around that provide feedback signals back to the ECU
for current sensing, misfire detection, etc.

Coils with lower primary inductance will need a shorter dwell
time and vise-versa. This is because the current ramps up
with a slope of dI/dt=V/L (where V  is the voltage across the
primary, and L is its inductance).


----- Original Message -----
From: Dave Forgie <forgied at ae.ca>
To: <s-car-list at audifans.com>; <keepitclean at spamarrest.com>
Sent: Friday, June 03, 2005 3:59 PM
Subject: RE: [s-cars] Re: Coils and PSO's


Dwell Question:  I think of dwell as the length of time that the spark is
generated (this goes back to my old points, capacitor and single coil
youth).  Under our current ignition system, i.e. Crank Position Sensor, ECU,
POSs and individual coils, what controls the spark dwell?  It seems to me
that the ECU just sends a trigger signal to the POS and something in the
design of the POS circuitry (sp?) creates the "dwell", i.e. a time lag  But
then again, I'm a Civil Engineer.

Dave F.