EST Systems

[robert dingli]

On the subject of dwell control..

> >When  you  say  "limit  dwell  time  internally", do you mean that the
> >module automatically turns the coil on again at some fixed delay after
> >EST has been triggered, regardless of the current state of EST? I'd be
> >dissapointed if this was the case, since  I  had  planned  to  have  a
> >variable  dwell based on RPM to keep the power dissipation in the coil
> >to a minimum. (still referring to the standard GM 7 pin module)
> 

J.W. Harris replied,

> I think I saw somewhere that some modules will limit dwell if it exceeds a 
> maximum value.  It might just limit current or both.  I'm not sure.
> 

There are ign controller chips available which do both the current limiting
and dwell control functions.  For example, the MC3334, will accept an 
input from a reluctor pickup and automatically determine when to commence
coil charging.  When a pre-set primary current is reached, the current
is maintained until firing.  The chip adds a small amount of extra charge
time to allow for engine acceleration and also has a minimum discharge
time in the order of one millisecond to allow the spark to ignite the 
mixture.

The individual Bosch ign amps also have a dwell control and minimum
discharge feature.  From memory, they will accept a digital input but 
will commence charging sooner than instructed if the input is of too 
short a duration.

Ideally, the charge time would be constant in the time domain since the
coil charging characteristics don't vary with engine speed.  The charge
time would be chosen so that the desired current limit would be reached
just before firing.  Since the charging must commence before we know 
when to fire the plug, we also have to add some extra time so that the 
coil is fully charged in the worst case of rapid engine acceleration.

I personally use a fixed charge time in the range 1-5 mS with a minimum
discharge time of 1 mS.  Modern, low impedance epoxy filled coils can
be charged very quickly, but tend to destroy themselves if the 
secondaries are left open circuit or if proper current limiting isn't
used.

regards,

Robert Dingli

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             Robert Dingli           r.dingli at ee.mu.oz.au

Power and Control Systems                 Thermodynamics Research Lab
Electrical Engineering                    Mechanical Engineering
   (+613) 9344 7966                          (+613) 9344 6728
  University of Melbourne, Parkville, 3052, Victoria, AUSTRALIA
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