(fwd) Injectors: P&H and SAT - Part 1

[Garfield Willis]

Ahem, I posted this over on the gmecm list, since we've been discussing
over there an interface module to allow those ECMs originally designed
for only Saturated injectors, especially batch-fired, to also drive
multiples of the Peak&Hold types. I thot it might also be useful for DIY
guys; I bet pardon for the unavoidable duplication for those on both
lists.

***************

Ya know, I'm just wondering if it might not be good at this point to
mention a few pertinents, so everybody's on the same page, regarding
injectors.

Basically they're just solenoids. They're often spec'd by their DC
resistance (roughly what you'd see if you meter'd them), but that figure
alone is misleading. Maybe we should try to pull the cat a little
further outta the bag?

Peak & Hold:
	Port Injector Size/Style 	(aka TPI/PI) 	=	2.4ohms & 4.0mH
	Throttle Body Size/Style	(aka TBI)		=	1.2ohms & 2.0mH
Saturated:									=	12-16ohms & 18-25mH


Now we're all use to dividing ohms into volts to get a current,
I = V/R, so-called Ohm's law, but when you apply pulses to an inductor,
the inductance also comes into play; that's what the "mH" means, it's
"milliHenry" (not a small guy, but a measure of inductance named after
Mr. Joseph Henry, a self-educated American scientist who later taught at
Princeton!, and who bribed the judges into giving him credit for the
idea of inductance :).

So ya know how Farad is the measure of how big a capacitor is ? (as in
microFarad, named after another lil guy named Faraday :) So Henry is the
measure of how big an inductor is. Ya know how you have to consider the
"RC time-constant" because charging up a capacitor takes time, just so
you have to consider the "L/R time-constant" when charging up an
inductor. Of course, we don't usually call it "charging up an inductor";
usually we know the current in the winding has to reach a certain peak
value for the solenoid's inductor to do the mechanical deed, but it's
the same thing. Because of this "inductance" phenom in coils, it takes
them some time to "charge up" to their eventual current level, which
also relates to how strong a magnet they are.

I know it seems spookie to someone who hasn't had a course or two in
electrical dynamics, but since most people are familiar sooner with
capacitors and the idea of a tank taking time to fill up, just think
about an inductor in a similar way; in a capacitor, it takes time for
voltage to build up. In an inductor, it takes time for current to build
up. The higher the charging current, the faster a capacitor charges up
(voltage rises); the higher the charging voltage, the faster an inductor
charges up (current rises).

In the case we've just previously seen, two P&H injectors were put in
series, and we all remember (right?) that voltage divides across things
in series. OK, well, can you see it now? Putting two inductors in series
divides their "driving voltage", and we said above that "the higher the
voltage charging an inductor, the faster the current builds up", and we
also said "the current in an injector winding has to reach a certain
peak value" in order to pull in. Or to turn it around to fit this case,
"the lower the voltage charging an inductor, the SLOWER the current
builds up".

Generally, while it's expected that multiple SAT injectors will be
paralleled up, it's NOT usually planned that P&H, especially TBI
injectors, would be wired in series. They're intended for more
intelligent, high initial current drivers.

I realize this will sound like talking down to many, and fly over the
heads of others; if the former, I apologize, if the latter, ask further
questions.

[Next go-around on this, we'll work thru the L/R time-constants from the
numbers above for the different sizes; there's an interesting quirk in
the numbers you might wanna observe].

HTH,
Gar


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