Dwell time as a %

[Roy]

Sean Connelly wrote:
> 
> Can someone help me... I am trying to work outthe ignition coil dwell time
> expressed as a % for various engine combinations but I am getting confused.
> 
> I know the time for one engine rev (in ms), coil dwell time (in ms) and
> number of cylinders (1, 2, 3 or 4) so how do I get dwell time ?
> 
> Is it
> 
>     Dwell(%) = (Dwell Time / (Time for 1 engine Rev / # Cylinders)) * 100
> 
> Sorry its such a trivial problem, but the easy ones are sometimes too
> easy and you miss them completely.
> 
> Cheers
> 
> Sean Connelly.


Dwell time is simply the time that current flows through the ignition
coil.  With contact breakers the dwell time is controlled only by the
profile of the distributor points cam, hence why the more cylinders you
have the shorter was the dwell period.  

The dwell period was an issue because the coil required a certain time
for the maximum magnetic flux to build up, remember points could only
switch a few amperes.  

With electronic systems dwell time as such is no longer important
because you use low impedance coils which allow very rapid build up of
magnetic flux  but need current control, which is often dwell period
control.  Say your coil needs 10us to saturate, at max engine Rpm this
may be all the time you have got, but at idle you have plenty of time so
you only switch the coil on for 10us before turn off which generates
your high voltage spark in the secondary.

I hope this helps ROY.