[Diy_efi] Donegan ECU

[Carter Shore]

On 2007-01-06 John Gross <jogross3 at hotmail.com> wrote:

> ... However, you
> touched on an issue 
> which could cause potential problems under idle,
> when you desire precise, 
> repeatable control of your total injector flow, and
> that is that you can 
> lower the pressure too much for that level of
> control of fuel for a given 
> injector. ...
 
> ... if you are dealing with an injector
> that is designed to run at 
> 45 psi, and you then subject it to 90 psi rail
> pressures, your opening and 
> closing events will be much different than at the
> design pressure.  This 
> will obviously effect the total fuel delivery at a
> given "design" pulse 
> width for the injector.
....
> The most difficult part of the whole strategy of
> lowering your pressure at 
> light load and idle conditions is finding that
> minimum allowable fuel 
> pressure that still gives good, repeatable fuel
> flows while still 
> maintaining a good enough spray pattern for a high
> quality combustion event.

There is some info out there on injector behavior vs
pressure. The 'pressure' is the difference between the
fuel supply and the manifold. That's why mechanical
fuel regulators are referenced to the manifold, and
why fuel pumps on boosted motors must be capable
delivering required volume of fuel at fuel supply
pressure *plus* max boost.

Contemporary automotive injectors have to be operable
under a wide range of conditions, from low voltage
during cranking, to extremes of temperature, and of
fuel pressure.

So tweaking pressure by 10%, or even 20%, is not
pushing too far. There are lots of folks running in
that band without any issues.

I cannot think of any reason why an injector would not
open at very low pressures, although as you state,
atomization could suffer.

But many injectors do have a max pressure, above which
they *will not* open at all.

The extra cost of multiple injectors (and control
electronics) is not the big deal to specialist and
hobbyist folks like us, it's the performance that we
want. 
So I believe that multiple injectors, either staged,
or of different flow rates, are a more straightforward
method than fuel pressure control via ECU.

Carter

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