Injectors: P&H and SAT - Part 2

[Garfield Willis]

OK, here's somemore of the cat...

[continued from Part 1]

Remember the typical specs on injector types:

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

Recall also that the "time constant" for an inductor is L/R, so if we
apply that to the above numbers, we get:

Peak & Hold:
	Port Injector Size/Style 	= 	4.0mH/2.4ohms 	= 	1.7mS
	Throttle Body Size/Style	=	2.0mH/1.2ohms =	1.7mS
Saturated:						18mH/12ohms	=	1.5mS

HEY, you're saying, what gives? How can the time constants for the P&Hs
be longer than the SATs? The P&Hs are known to be faster, there must be
somethin goofed up somewhere!? Not exactly. :)

What the term "time constant" refers to is how long the inductor takes
to charge to about 63% of it's FINAL current value. In each case, the
final value is determined by the *resistance* and driving voltage, so
let's look at those:

Peak & Hold:
	Port Injector Size/Style 	= 	12V/2.4ohms 	= 	5A
	Throttle Body Size/Style	=	12V/1.2ohms 	=	10A (woohee!)
Saturated:						12V/12ohms	=	1A

Of course, the currents in the P&H case are never *allowed* to reach
their "final value"; the drivers for the P&H devices have the following
nominal current limits that kick in once they're reached:

Peak & Hold: (hold current in parens)
	Port Injector Size/Style 	= 	2A (0.5A)
	Throttle Body Size/Style	=	4A (1.0A)

Soooo, altho the time constants for the P&H refer to how long they would
take to rise to 63% of those final values above (the 5A & 10A figures),
they don't NEED to reach that current level before pull-in. In fact,
looking at the nominal peak current limits above of 2A & 4A, these also
contain a goodly margin of overdrive to insure that the injectors ALWAYS
will be opened quickly at these values. Let's do a for instance. Suppose
conservatively that a TPI P&H injector reliably pulls in at 1.5A. If the
final charging current is 5A, that's less than 1/3 the final value, say
30% of it. So the effective "delay" for this injector will be about 25%
of the time constant, or around 0.4mS. Of course, significant fuel is
getting thru even during it's opening and closing times, so it's not
quite as bad as it looks. These delays don't really get shaved directly
off the pulse width, because what's lost in slow opening is also largely
gained in slow closing.

Now let's look at the SAT case. In constrast, saturated injectors are
simply connected to 12V and left to charge; no high currents initially
followed by lower holding currents. If the final current value is 1A,
they probly need at least 0.5A to reliably pull in, and this is maybe
around 50% of their time constant, or 0.75mS.

That explains why the P&Hs are indeed faster, even if their "time
constants" don't work out to be significantly lower than the SATs. The
final current value those time-constants apply to are much higher in the
P&H, hence the accelerated opening times (and closing time, BTW: we'll
touch on that last).

One more thing. You'll notice that the time-constants are damn near on
the order of the well-known ruleOthumb on minimum pulse-widths for these
injectors: namely, 1.5mS for P&H and 2.0mS for SAT. You can see with the
estimates we've made of pull-in (and corresponding drop-out) times, that
they're becoming a significant part of the overall pulse-width, and
altho the drop-out delay somewhat compensates for the pull-in delay,
they're never equal. That's the reason for the corrections to PW in some
ECMs as you get down toward minimum PW.

Lastly, what about those drop-out times? You can see how the much higher
peaking currents can shorten the pull-in time on the P&Hs, but how is it
they're ALSO able to close faster? Ahh, well that's because of the
hold-current being kept so low. There are a number of equivalent ways of
looking at this; some may be more intuitive than others. The thing that
largely controls the speed of drop-out is how fast the magnetic field in
the coil can collapse. In both types of injectors, because of the
so-called "free-wheeling diode" (aka flyback diode) across the driver to
protect it from the transient caused whenever you shut off the driver,
current continues to circulate in the injector winding after the driver
shuts down. Basically the diode provides a shunt path for the injector
coil to discharge into. Hence, once again, the time constant of the
injector from it's R & L come into play. And altho the time constants of
the SATs might be slightly lower in appearance, remember once again it's
also the percentage of the final value that they have to discharge that
determines how much of the time constant applies. When you shut off a
SAT injector, it starts discharging from it's full saturation value. The
P&H's are already being held at less than 1/4 of their peak current
value (a hold current of 25% of the *peak* value is typical for P&H
drivers), so they have much less discharging time required, relative to
saturated injectors. In effect, when the injector driver transitioned
from "peak" to "hold", the P&H injector coil started discharging to just
above what's necessary to hold it in. Thus when the driver is eventually
turned off, the coil has already had time to discharge a goodly portion
of the way already, during it's hold time.

The peak & hold currents are designed so that the effect of pull-in and
drop-out are roughly symmetrical (or as symmetrical as possible). The
SAT injectors are by their nature (charge and discharge are very nearly
identical) fairly symmetrical. That's because the current they charge TO
(1A) is also the current they discharge FROM. And differences between
the driving transistor during charge, versus the free-wheeling diode
during discharge, are very small compared to the 12-16ohms internal
resistance.

OK, that's probly way more than enough on injectors for the time being.

Gar

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