Using a vacuum referenced pressure regulator isn't manditory, but should
help you tune the system. If you run a constant fuel pressure you must do
a complete re-mapping of the needed pulse width (lets just say rich or
lean for the non-techno types among us) The system would be rich at idle and
lean at WOT if it functions as we would expect. Mapping the other
situations where the throttle opening doesn't match exactly could be much
harder. Like WOT at idle (sudden acelleration) or closed throttle at high
RPM (decent or deceleration) and what I consider the toughest for most people
transitions through tuning zones. An explaination is most engines
rotaries included have RPMs where the combination of overlap, intake tuning,
and exhaust tuning cause reversions in the intake or lean mixtures. I had one
engine that I tuned that had a weird cam/overlap tuning situation where
closing the throttle and then opening it at a medium RPM could cause the
engine to just stop! This engine would just scream straight to red line if you
opened it up at idle, go figure. Point is there are enough problems to handle,
using a MP referenced FP regulator will help you get through the problems. But
you will probably still have some problem areas.
Blake Lewis
wrote:
> I am just learning this stuff. This is my line
of thoughts on the > subject.
>
> The amount of
fuel that will flow through the injector during a
> pulse width,
is proportional to the pressure differential across the
>
injector.
> (Delta Pressure) * (Pulse Time) = constant *
Volume
> Without the "Vacuum Regulator", at Idle when you have a
vacuum
> (less pressure) the Delta Pressure would be bigger,
flowing more fuel
> per pulse time.
> I guess the
computer wants a constant flow rate.
Not really. The
computer has a lookup table that matches injector open time to manifold
pressure. The issue is, "How much adjustment range does the computer have for
modifying the open time?"
Pulling some numbers from the
air:
You need 1GPH to get a good idle. But not just any
1GPH, you need a nice steady 1GPH. The computer has to do this by commanding a
10% duty cycle on the injectors. But in the real world, there is no way to
perfectly maintain a steady 10% cycle with all the electromechanical stuff
going on. Sometimes it wanders to 11 and sometimes it drops to 9. That 1 point
change is a tenth of the whole duty cycle. What you get is a rough idle. The
problem is even worse if you're turboed, and injectors are sized to flow
enough fuel at full boost. By the time you get to idle, those injectors may
have to drop to a 5% duty cycle to get down to 1GPH.
Now,
if the computer could command a 15 or even 20% duty cycle to get that 1GPH,
then the wandering would be much less of the total open time. How could we
make the computer have to use a longer duty cycle? How about we drop the delta
pressure across the injector opening? Good solution. Enter the vacuum
referenced pressure regulator. As manifold pressure goes down, so does the
fuel pressure. The injectors have to stay open longer.
Is
that the only solution? We could accept the rough idle as the cost of a
simpler solution. We could use a much smaller injector for the primary which
will be used at idle. The primaries could be sized to need a 30% cycle to get
that 1GPH, and monster secondaries would insure that there is enough flow for
WOT. That complicates the software, but the hardware remains essentially the
same. There is the option of higher quality injectors that will wander less
(Oops! That's sounding expensive. Sorry I mentioned that
one.)
There may be other solutions, but given Ed's
experience with nothing more than a rough idle, I think staged injectors and
the acceptance of a little roughness at idle will let you use any pressure
regulator you choose.
-- ,|"|"|, Ernest Christley
|
----===<{{(oQo)}}>===---- Dyke Delta Builder
|
o| d |o
http://ernest.isa-geek.org
|
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