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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