Dwell time as a %

[Frank Mallory]

Sean Connelly wrote:
> 
> >From: Frank Mallory <frankm at erols.com>
> >Subject: Re: Dwell time as a %
> 
> >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.
> 
> >I think you have to divide by 2 (or use time for 2 engine revs) to
> >account for the fact that in the Otto-cycle gas engine, the engine
> >revolves twice for every revolution of the camshaft.
> 
> >To convert from percent to degrees dwell, multiply by:
> >       90 for a 4-cyl. engine
> >       60  "  " 6-cyl.    "
> >       45  "  " 8-cyl.    ".
> 
> This might be the case for conventional ignition, but what happens for
> a wasted spark system (distributor less) ? The ignition coil will charge
> up twice in the one cycle. Should this affect the dwell measurement
> (degrees or percent) ?
> 
> I need to do this calculation on both types of system. I know expressing
> it as a time value would be much easier, bu they (customer) want it as
> a percentage.....

If your total dwell in ms. includes the time for the wasted spark cycle,
I think you have to divide it in half before calculating the %dwell.

If you put a scope on your running engine, you could measure the % dwell
directly. Using the conventional definition of "dwell", you would
measure the period from the time the plug ceased firing (equivalent to
"points close" in a conventional system) until the plug fired again
("points open"). The fact that the plug firing is wasted is irrelevant. 

This gives you an indication of the degree of saturation of the coil,
which I assume is what you are interested in.