Min. injector off time

[John Dammeyer]

At 03:35 PM 06/02/1998 PST, you wrote:
>ref: John Dammeyer:
>Then what about appling a pre current after the injector
>turns off so that at the next on signal the is some current
>and then there is less time needed for the full flow to 
>take place??..

Then you are dissipating power and might even lock up the injector due to
excess heat.  The 85% rule isn't all that complicated and doesn't fall under
the examle of 85 days on 15 days on.  8-)

Figure out how many lbs/hr fuel you expect to use give density of air at
your altitude and the RPM you intend to run.  Perhaps you'll end up with a
value like 36 lbs/hr at 7000RPM as an example.  Now that value is 85% of the
selected injector rating.  
ie:  36    85
     -- = ---  Therefore X ~ 42lbs/hr.
      X   100 

Now you are sure to not exceed the spec of 85% on, 15% off because at 7000
RPM the injector will be on for the length of time it takes to deliver
36lbs/hr.  No overheated injector (read stuck) and therefore a working engine.

[snip]

>  Or at high speed not enough time is available
>for doing this, but by the same token wouldn't
>it help to more accurattely control large
>injectors at idle.

If the above example injector is too large and does cause idle problems use
one half the size and trigger it on both power and intake stroke.  At high
RPMs it shouldn't matter that you're squirting fuel at a closed valve and at
low RPMs you can always just use one injector and bring the second one on
just before an RPM where you require more than 18lbs/hr.

Back to the energizing of an injector.  There are three ways of handling
this lower voltage style injector.  One way is called the LR method also
used for driving stepper motors.  A resistor is placed in series with the
injector coil and is chosen to limit the current to the injectors maximum
based on 12 volts (13.8V actually) applied across the resistor and injector.
Don't forget power rating of the resistor.  This is very wasteful of energy
but does increase the speed that the current builds up.

The second technique called chopper limited uses a very small value series
resistor to produce a voltage drop that is measured by the logic circuitry
and compared against a reference.  The reference is chosen to produce a
disable signal to the injector drive when the current exceeds the injector
value.  This happens automatically at a rate dependent on the injector
inductance and spurious capacitance.  Might be 5KHz, might be 20KHz.  In
either case it will cause a whopping 13.9V to be placed across the injector
causing a rapid current buildup forcing the injector open in a hurry.  Then
the current stays at the desired value because the voltage is 'chopped' on
and off holding the current at the refernce point.

The third technique uses a time delay and actually allows a higher current
(open current) to flow through the injector for a brief period of time; to
get it to open up against a sticking seat perhaps.  It then drops the
current back to the holding current for the rest of the duration of the
injector on time.

The injector manufacturer should be able to tell you how to run a particular
injector.

Regards,

John

BTW,  if you are an old timer you will recall that many of the ignition
coils had a ballast resistor in series with the coil.  This was there
because the coil was also a lower voltage coil and the rate the current
built up in the coil could be controlled by this resistor.  Often they were
temperature dependent so when the resistor and the engine was cold the coil
was in effect over driven to produce a hotter spark.  Some ignition systems
also shorted out the coil while the start motor was engaged and applied the
12V directly to the 8V coil.
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