Multi Coil Setups

Bernd Felsche bernie at innovative.iinet.net.au
Mon Jun 26 17:41:35 GMT 2000


Nicholas Parker wrote:

>      I want to use 4 ignition coils for my 4 cylinder application.  I have
> designed an ECU that uses a single 'output compare channel'  to produce four
> negative going pulses (one every 180 crank degrees + advance retard offset
> etc) that I am de-multiplexing to one of four coils (via driver etc) .  At
> present all coils are always on except the one that is making a spark, which
> is off for only 2ms then back on until it's cyclinders next spark - another
> 720degs approx

Can you remind/tell us what sort of driver? Anything like the efi332
drivers?

> Is it possible to leave the coils on permanently (except for a spark 'dwell'
> / coil off period of say 2 ms for every spark  ) ?  Obviously the highest
> rpm is limited with a fixed dwell like this to speeds where engine
> revoultion takes more than  4ms a revolution.

Ideally (and this may not be practical) I believe you should only
"charge up" the coil until it saturates. (Please don't take that as
an answer - I'm also fishing for more details.) Anything more than
that and you're just throwing away energy. If that is the case, then
the required time would be fairly predictable (probably even
measurable and feeding back to your ECU) and could then be
incorporated into your spark timing fairly easily.

I vaguely recollect circuitry to effect the measurement, etc in the
efi332 drivers... but my brain wants to go to sleep now...

> Is using a Ballast resistor to limit coil current o.k? Or will this
> compromised the coils 'spark ability' given that it's 1/2LI^2 energy will
> not be able to be at the coils maximum...after all the point is for me to
> build a GOOD system.

This bit has me puzzled. Doesn't the driver circuit limit the
primary current anyway? Or isn't the driver that sophisticated?

Can anybody share the secondary functions (if any) of the ballast resistor?

-- 
Bernd Felsche - Innovative Reckoning
Perth, Western Australia
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