RE return to fuel tank

[Walter Sherwin]

Hey Greg, can you tell me any more about the Bosch "Air Shroud" injectors
mentioned in your post?  Thanks.


-----Original Message-----
From: Greg Hermann <bearbvd at sni.net>
To: diy_efi at efi332.eng.ohio-state.edu <diy_efi at efi332.eng.ohio-state.edu>
Date: Thursday, October 22, 1998 4:11 PM
Subject: Re: RE return to fuel tank


>>Hi all
>>
>>Been reading this thread and can say that several european auto
>>manufacturers are working on a returnless fuel system, where the fuel
>>pump is controlled by the ECU using PW modulation so that only enough
>>fuel is pumped into the fuel rail to meet the engines needs.
>>
>>The biggest reason behind this is emissions, passing warm fuel back to
>>the tank causes fuel vapour to build up & pressurise the tank.  It
>>cannot just be dumped to atmosphere anymore so the vapour is passed
>>through a charcoal filter to sepperate any fuel before venting to
>>atmosphere through a valve controlled by the ECU.
>
>But--for really high performance--without quite so much worry about evap
>emissions, I believe that it would be fairly easy to show the following
>(with a bunch of differential equations):
>
>The most stable fuel pressure control will be possible if you use a
>variable drive fuel pump,  a pressure regulator,  feedback on what the
>regulator is doing from a fuel pressure transducer, and a controlled
>reference pressure on the back side of the fuel pressure regulator
>diaphram, as well as a fuel temperature transducer to allow proper
>correction for fuel density. Use the fuel pressure regulator as the primary
>control with control over pump speed following what is going on so as to
>minimize the amount that the regulator armature has to move in order to
>keep the fuel pressure where the computer wants it. This also allows you to
>vary fuel pressure to wherever you want it at different points on the
>engine MAP/speed matrix so as to maximize the dynamic range of the
>injectors.
>
>With proper PID control over what the regulator and pump are doing, one
>could get extremely stable, but variable fuel pressure control, and thus
>much closer f/a ratio control over a wider dynamic range. Yes, there is
>room here for a code guy to figure out how to put some learning blocks into
>this loop so that the control loop could anticipate things a bit, and hold
>things even closer.
>
>Also, with the fuel pressure transducer input, the computer could catch any
>pressure glitches on the fly, and correct pulse widths accordingly.
>Injector pulse widths could also be corrected for fuel temperature in the
>rails on the fly.
>
>Including a float vent at the high point of each rail (piped back to the
>low pressure side of the system) and mounting the fuel pressure regulator
>at a lower point, with its outlet pointed downward, would (1) assure that
>no bubbles ever got into the injectors to cause random lean mixture
>anomalies, (2) make any hot soak vapor in the fuel rails situations and
>problems into history, and (3) avoid any possibility of fuel pressure
>anomalies (and resulting a/f mixture anomalies) due to the regulator
>randomly passing fuel vapor instead of liquid fuel.
>>
>>To achieve this they fit a pressure sensor in the fuel line and by
>>knowing pressure and injector duty cycle the pump duty cycle is
>>calculated.
>>
>>>From experience on dynos I can say that for power you want hot fuel in
>>the fuel rail
>
>I strongly suspect that this finding would change if you were getting
>better (finer) atomization of the fuel! Have any of your tests been run
>using Bosch's "air shrouded" injectors and at part throttle (which is where
>said injectors improve atomization)??
>
> and cold air because of its increased density.
>
>No question about this, but let us not forget that fuel VAPORIZED (as
>opposed to fuel ATOMIZED) before the intake valve closes displaces air and
>affects VE negatively through this mechanism! And the lighter the molecular
>weight of the fuel being used, the stronger this effect becomes.
>>
>>               Regards, Greg
>
>I understand that I am discussing issues here that go WAY beyond normal
>needs for street performance , although bits and pieces of it go straight
>at some fairly common street problems. But HIGH performance is what
>interests me most, and I would hazard a guess that a bunch of others on
>list have interests in the same kind of stuff, so don't bother telling me
>about the economics of it!!
>
>Further, if you have doubts about the validity of the control theory I am
>applying to fuel pressure control, I have a text on a shelf somewhere which
>thoroughly describes the theory of accurate water level control in large
>utility steam boilers (100 megawatts and on up in size) in more detail than
>you could want. Fuel rail pressure control for an EFI system is a
>reasonably analogous situation. Name and author of the book escapes me
>right now, but if anyone wants to spend some money and wade through it in
>an effort to learn, it IS a really good book, and I would post title and
>author name later if there is interest.
>
>-G.
>