[Diy_efi] Fixed frequency PWM batch mode FI

Wed Aug 21 20:13:28 GMT 2002

Hi Les,

Would not agree with the comment on carbs. But that's another discussion.
Let's just say I'm negotiating on a HUGE twin SU ( off a 1650 CI engine with
25PSI boost) as a mantle piece.

With EFI all you are worried about in the first instance is getting the
right amount of fuel in, so that each time the cylinder takes a lungful
there is a nice mix. Injecting once per induction cycle will do this, batch
or sequential.

Continuous injection will also work. Think of the millions of CIS systems
out there. The problem with CIS is getting the dynamic range, which will be
around 10:1 at a minimum Per induction. Add in a 10:1 dynamic range on the
revs side and you have a 100:1 ratio to work with on constant flow. Turbos
will up this even higher, so say 200:1 between idle and red line WOT. Flow
is proportional to root of pressure, so you would need 4000:1 range if just
varying pressure.

Can be done if you employ tricks (after all a lot of serious racers still
use hilborn injection) and I have put in an extreme case, but you can see
why squirt per RPM became the solution of choice.

Bill
----- Original Message -----
From: Les Dittert <lesd at earthlink.net>
To: <diy_efi at diy-efi.org>
Sent: Wednesday, August 21, 2002 8:13 PM
Subject: RE: [Diy_efi] Fixed frequency PWM batch mode FI

> Bruce, I'm using four injectors fairly close to the ports.
> That's why I'm more concerned about fuel starvation in cylinders,
> as opposed to a throttle body setup and where things have a chance to
> even out.
> I keep thinking of the General Motors in batch fire system where
> everything fires at the same time.
> But they're firing upon close valves open valves and valve somewhere in
> between.
> So I'm thinking, doesn't really matter when it fires, if the valves are
> in random positions?
> I'm keeping in mind that this is a crude air cooled Volkswagen engine.
> I just want something better than the hard to synchronize carburetors,
> and something that
> allows me finer mixture control and adjustment ability without changing
> jets!
> Carburetors when using blow through Turbo are really pathetic situation,
> because carburetors don't know how-to richen and under boost when blow
> through happens,
> i.e. they don't understand pressurized air.
> -- Les
>
>
> -----Original Message-----
> From: diy_efi-admin at diy-efi.org [mailto:diy_efi-admin at diy-efi.org] On
> Behalf Of Bruce
> Sent: Monday, August 21, 1995 12:06 PM
> To: diy_efi at diy-efi.org
> Subject: Re: [Diy_efi] Fixed frequency PWM batch mode FI
>
>
>
> Are you going to use 1-2-4 injectors?.
> If more then one, where exactly do you want to place them?. Bruce
>
>
> ----- Original Message -----
> From: "Les Dittert" <lesd at earthlink.net>
> > Mine is also air cooled  Volkswagen, turbocharged.
> > My only concern is having a beat frequency between the engine RPM and
> > the pulse rate.  In thinking about it more I just need to guarantee
> > the pulse rate is higher than the RPM. I want to use a small
> > microcontroller for this engine.  While it is possible to control the
> > rate of the PWM, it would then
> > perhaps require a crank sensor which is added complexity.
> > I figured since some General Motors engines do batch fire of all
> > injectors
> > simultaneously, they would run into the same problems of uneven fuel
> > distribution
> > if I'm not mistaken.  The other limitation might be getting low enough
> > fuel rate
> > for idle, because of multiple injections happening with very short
> pulse
> > width.
> > I want to keep the system very simple, so I am using a mass airflow
> > sensor to
> > avoid having fancy multidimensional lookups. A couple of lookup tables
> > should do it.
> > I'm not delegating a mission to this either. I'm using a crane hi 6
> with
> > boost retard
> > and that should work fine for this simple engine. Anything is better
> > than the two carburetors that are on there now!
> >
> > -Les
> > -----Original Message-----
> > From: diy_efi-admin at diy-efi.org [mailto:diy_efi-admin at diy-efi.org] On
> > Behalf Of Bruce
> > Sent: Sunday, August 20, 1995 5:26 AM
> > To: diy_efi at diy-efi.org
> > Subject: Re: [Diy_efi] Fixed frequency PWM batch mode FI
> >
> >
> >
> > There was a VW guy that did an ecm that way.  I think he was firing
> > them every 12.5msec. For racing/off road why not?.
> > So long as it's clean enough to idle, who cares?.   I think the other
> > guy
> > took some heat about it not being able to work, but he said it did.
> > Some GMs default to that under some conditions Bruce
> >
> >
> > ----- Original Message -----
> > From: "Les Dittert" <lesd at earthlink.net>
> > To: <diy_efi at diy-efi.org>
> > Sent: Monday, August 19, 2002 8:42 PM
> > Subject: [Diy_efi] Fixed frequency PWM batch mode FI
> >
> >
> > > I have searched high and low and haven't found the answer to this
> > > question:
> > > are there any disadvantages to running the injectors at a fixed
> > > frequency of injection that isn't related to RPM? I realize that
> doing
> >
> > > this in batch mode will result in some injections hitting a close
> > > valve sometimes and then sometimes hitting an open valve.
> > > Since some OEM fuel systems run batch mode where they fire all
> > injectors
> > > at the same time
> > > I figured they have the same situation anyway.
> > > Another question:
> > > If using a fixed frequency you what should that frequency be? I'm
> > > assuming it should be something equivalent to running a sequential
> > > injection system at about 4000 RPM.
> > >
> > > -Les
>
>
>
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