Volume of fuel injected

[Stephen Dubovsky]

At 07:03 PM 10/8/96 +0100, you wrote:
>>Here's my problem.  I'm trying to calculate the volume per burst for a
>>typical fuel injector.  I've downloaded some specifications, but I'm at
>>a loss to break them down.


  Ok, here's my understanding of injectors - the flow rate (cc/min or
lb/min) is rated at 100% duty cycle.  As for other duty cycles you need to
do a little testing, but it helps to understand how the injector works.
  FI are little solenoids.  When you first apply the voltage, it takes some
time to built up enough current (and magnetic feild -> force) to start the
valve opening.  This is dead time and can be corrected for easily (add it to
the required 'on' time).  The valve then starts to open and assume the flow
increases linearly until it is wide open (and has reached its 'rated' flow).
At this point you can reduce the voltage on the injector to save power and
off time - more on this later.  You leave it on for some desired time at
rated flow - this is about the only linear part of the curve.  When you go
to turn off the injector, you do something to try and stop the magnetic
field holding the valve open.  Disconnect it.  The problen is that the
injector is a coil of wire around magnetic stuff and has inducance so the
current (and the magnetic field) wont go to zero instantaneously (would take
infinite negative voltage).  Dont know if your an EE, but there is this rule
that goes V=di/dt...  Anyway, thats why you usually need some sort of clamp
to keep from killing your drive circuit.  So, after some time delay the
magnetic field drops below that required to hold the valve open and the
valve starts to close (due to that little spring and maybe fuel pressure).
Again assume some linear curve (which I again doubt - but hey, these are
supposed to be negligible times right;) And finally the valve is closed.  
  So the things to acct for are:
        1) delay until beginning of turn on
        2) time required to turn on the injector (some fixed amt of fuel
will be squirted during this period.  If you assume linear flow increase,
this amt is quadratically (or worse) related to this time.)
        3) actual time the FI delivers 'rated' flow
        4) delay from when you remove power until valve starts closing
(again some twisted relationship of flow vs time)
        5) the valve actually closes.

At high duty cycles, period 3 should swamp all others and you can assume
'linear'.  Time 1 is easy to figure out.  See how small of an injector pulse
delivers NO flow (it never gets to open).  The other times might be taken
into account (although I dont know of anyone who does this, but its
possible) if you plot flow vs. on time.  This is probably only interesting
at very low flow rates.  

  Hope this helps.  If anything here is too complicated, just ask.  We build
things that drive inductors and actuators all the time and sometimes I
confuse myself.  If you want more detail, ask and Ill scare you w/ math;)
(albeit simple calculus)

SMD