Non Destructive testing (letting the smoke out)

[Walter Sherwin]

>What would be the effect of placing 3 saturated type
>injectors in parallel on one peak-n-hold driver?





Three parallel injectors (@15 ohms & 3mH each) will give you an equivalent
circuit of 5 ohms & 1mH.   The "RL" combination of the equivalent circuit
versus the electrical characteristics of the injector current driver is what
is most important to you.  A peak&hold driver, GM 4/1 amp for instance,
must
"sense" 4 amps before transitioning to 1 amp of holding current.   Other
manufacturer's drivers have other current ratings, and may include
transition
timers which override the current considerations.  Two questions must
usually
be answered before deciding whether things will work/last.............1)can
the
current driver ever sense "4" amps through your new equivalent circuit,
and.................2)how "long" will it take for said circuit to build up
to "4" amps?

The first question is governed by I=E/R, or in our case 13.8volts/5ohms.
So, in our example, even with a good battery  we will never reach the 4 amp
transition and chances are pretty good that the driver will burn out
prematurely because it is not rated for significant use at levels over 1
amp.
In this example, the attainable current will be 2.76 amps or less.

The second question is governed by the "RL" time constant of our equivalent
circuit.  The current buildup of an "RL" circuit (which injectors are)  is
based on I=[E/R -(E/R * exp ((-Rt/L)))].  Basically,  saturated injectors
require
opening currents of 3+ amps for about 1ms, in order to ensure that the
injector
opens against mechanical inertia and hydraulic pressure.  Our example
parallel circuit will never transition from 4 to 1 amp (see above), so this
is
irrelevant.  But, you should run the numbers for your case, if you plan to
adhere
to peak&hold control.  If the peak&hold transition happens well before 1ms
(for saturated injectors) then chances are that your saturated
injectors will not open, especially in the presence of increased fuel
pressure.




>
>Will the peak function of the peak-n-hold driver be
>of benefit to opening the saturated injectors?



Depends upon what flavour of peak&hold driver that you have (see above).


>
>Do three 15 ohm injectors in parallel look like one
>5 ohm injector to the driver?


Sort of.  See above.  You should also include a series resistance of about
.5ohms, to account for line voltage drop and a "sensing" resistor.



If you want to mix/match injectors and drivers then it is usually best to
use
an external injector current driver box.  With such a setup, the OE driver
"triggers" an input signal for the external driver, and the external driver
(presumably peak&hold) drives the injector.  Accel have such a product,
under the name "VIC", or Variable Injector Controller.  Or, you could build
your own external driver circuit from industrial components.




Bye;
Walt.