[Diy_efi] Re: Waveforms

[Mark Bowers & Family]

> You think that perhaps this is an issue related to
> component heating and heat sinking?  The ECU in
> question has no external metal heat sink...  I suppose
> it might make sense to use P&H to control idle more
> precisely, and then switch to saturation driving under
> such circumstances...  Allowing less heat sinking and
> air circulation, but providing the benefits of P&H
> with idle control.

With my still very limited understanding of the whole injection world, I
can't see why one would switch back and forth between P&H and sat driving
during a praticular steady state mode, like you seem to be seeing in idle
mode, unless the 'P&H or sat mode?' decision was being made on some sensor
input that was fluctuating duing idle. Again, working only from circuit
theory, one would expect the injector driver to get a lot hotter during P&H
mode. But could a heat sensor be the input for the driving mode decision?
Probably not anywhere near fast enough to react to transistor junction temp
changes. So, maybe they kept blowing up the drivers in pure P&H mode duing
idle, and found they had to compromise to keep them cool?? Now that's some
real speculation....but engineers do some interesting things at times,
either from interest or if production deadlines are looming! (Been there,
done that...)

>
> Which raises an ancillary question for me; what
> component ratings (particularly on the zener ahd
> driver transistors) would differ when driving
> high-impedance injectors via P&H, instead of
> low-impedance injectors?  How would it affect
> component expense and heat production?
>
> I really want to take a further look at the behavior
> of this machine now.  It's starting to sound like they
> might be using a novel method of solving some
> problems, and if so, it might be worthy of further
> investigation for the use of list members...

Again, with limited injection systems practical knowledge, and working just
from circuit design theory (and some circuit experience), one would expect
all the ratings on a P&H drive to a high Z injector to be less stressful.
The drive currents should be lower, so the current ratings and heat
generated in the drive devices should be less. The zeners may be stressed
less, as there may be less magnetic charge in the injector's coil to be
dissapated at turn-off based on the lower currents, but one could not give a
definite answer on that without knowing the typical injector coil
inductances of  high Z versus low Z injectors. The high Z injector coils may
have a much higher inductance which would increase the magnetic charge in a
way that would counteract the decrease in magentic charge due to lower drive
current...Again, my practical knowledge is still limited in this area.

The heat generated in the drive devices should also be lower, as the drive
currents are lower for high Z injectors.

As far as expense, I would not think that the zener costs would go down
enough to be of concern, if at all, but the drive device prices might be
lower. On a hobbyist's basis, I would wonder if this cost reduction was
worth the effort; on a production basis, maybe.

Mark B.


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