[Diy_efi] Donegan ECU

Sun Jan 7 05:06:30 UTC 2007

Original Message:
-----------------
From: John Gross jogross3 at hotmail.com
Date: Sun, 07 Jan 2007 00:03:52 +0000
To: diy_efi at diy-efi.org
Subject: Re: [Diy_efi] Donegan ECU

All of which changes if you use air atomization--- :-)

Greg

Carter,
  Your points are well taken, and I admit I was thinking more along the 
lines of a race car than a street car.  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.  This is one of the main reasons why some of the higher
horsepower 
injected engines out there have a tendency to use multilple injectors per 
cylinder.  Sometimes those injectors are of the same size and sometimes 
there is one that is significantly smaller for low rpm, light load 
operation.  I have seen and dealt with both.  IMHO, if you want to run a 
single injector per cylinder, and want to control the fuel pressure as a 
function of load, I think you will find better reliability and overall 
control of the system if you maintain a minimum pressure in the range of 
40-45 psi, and increase the pressure under load from that point.  However, 
even in such a situation, if you start with a set of 36 lb-hr injectors at 
45 psi, if you are able to increase the pressure under load to 90 psi
(which 
would mean a large amount of fuel bypass at idle, both due to the low flow 
requirements at or near idle, and because of the reduced pressure, your max 
effective flow rate will be 51 lb-hr.  While this provides much better 
dynamic range of the injector, there are two things to consider.  One is 
that this strategy, while allowing good precise delivery at or near idle, 
doesn't do anything to help out the life of your fuel pump, which I've 
inferred is one reason behind looking into this control method.  The second 
is that you mentioned that the main variables which affect the opening and 
closing rate of the injector is the design of the injector itself.  While 
this is true, 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 strategy of increasing the fuel pressure above the design pressure as a 
function of load for street cars is far from a new one, however.  This was 
the strategy used by Ford on the Mustang SVOs and T-bird Turbo Coupes. 
They 
ran a line from the intake manifold to the spring side of the mechanical 
regulator.

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.

John

>From: Carter Shore <clshore at yahoo.com>
>Reply-To: diy_efi at diy-efi.org
>To: diy_efi at diy-efi.org
>Subject: Re: [Diy_efi] Donegan ECU
>Date: Sat, 6 Jan 2007 11:19:36 -0800 (PST)
>
>I think the most important justification for ECU
>control of the fuel pressure is to extend the dynamic
>range of the injectors on the low side, not the high
>side.
>The time required to open and close elctromagnetic
>injectors is determined mainly by the injector design,
>and the supply voltage.
>At very low duty cycles, even P&H injectors have
>difficulty delivering a consistent amount of fuel.
>But if you lower the pressure, then you can use a
>longer pulsewidth to deliver the required amount of
>fuel, mitigating the issue.
>The same square-root relationship between pressure and
>flow that makes it harder to increase the flow by
>increasing the pressure, works to our advantage when
>reducing the pressure to deliver accurate fuel on the
>low end.
>Of course, there are limits to how far the pressure
>should be reduced, that depend on the specific
>injector.
>But reducing the pressure by just 10% yields nearly
>20% reduction in flow, corresponding to a 20% increase
>in injector 'open-time', ie a longer pulsewidth.
>
>This isn't that much of an issue for racecars, but
>streetcars must idle, and pass emissions tests.
>
>Carter Shore
>
>
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