Fuel injection questions - -

[thomas walter x5955]

> From: Andre Roy <ANDRE at einstein.unipissing.ca>

> MPI ( Multi Port Injection) has one (or more) injector for each cylinder
>     The injectors fire in unison. The disadvantage is that there can be
>     a bit of fuel puddling (like on a carb) because air only flows while
>     the intake valv is open. The rest of the (admittadly short) time, the
>     air in the intake runner is not moving.
> 
> SPFI Sequential Point Fiel Injection (there are other acronyms, no doubt)
>    Same as above, but the injector for a particular cylinder only fires
>    when the intake valve is open (or just before) the rest of the time
>    there is just stationary air with no fuel in the intake runner.
>    Advantage, no fuel puddling. Disadvantage, slightly more complicated
>    control strategies.
> 

Andre,

Minor item. Ideally SPFI will turn off the fuel flow BEFORE the valve
closes. Keep in mind the intake valve is open, maybe 35%, of the time
where as the fuel injector will have a maximum duty cycle of 80-90%
at WOT. i.e. it will be INJECTING fuel longer than the valve can be
open.  [Ok, Ok, doesn't apply to a RX-7! ;-)].

Still ideal to turn off fuel flow before the intake valve closes, and
then to turn it back on (assuming 90% duty cycle) for the fuel to hit
the back of the valve allowing better vaporization.

So if you are after maximum part thottle economy, and low emissions,
SPFI is best. For maximum power, either approach will do!

REVERSIONS: While we all realize the the closing of the intake valve
will cause a "back pulse" of air through the intake track, I was not
really aware of how critical it could be.  An engine builder could not
get enough fuel into an injected engine... at WOT the engine was running
LEAN, despite "having more than enough fuel to flood it". Hmm, problem
was a unique reversion pattern. Rather than the usual area around the
intake walls, there was ALSO an effected area in the middle. Dead center
of the intake area of the FUEL injector jet!!! The reversion pulse of this
radically cammed motor was enough to blow fuel back out the short intake 
track! [Haltech throttle bodies... look like Weber DCOE's with an injector 
port]. Result was interesting .  So when done with all the calculations, 
and theories, it is great fun to get it all blown back in your face! ;-)

Andre has a good point on fuel puddling.
For those of you adapting "thottle bodies" to your home brew
system. While the phsical hardware may be easier to bolt up, the
software will have to correct for "pooling" the fuel in the manifold
under some conditions... a little more difficult task for home brewed
systems.  Hmm, seems like the port injection would be a simplier SOFTWARE
task to start with (even with the system allowing for SPFI control of
the injectors, just do rev 1.0 software for batch mode... then tweak it).

Odd thought... with a SENSITIVE air pressure switch in the intake
manifold, I wonder if it would be possible to detect the closing of
the intake valve... i.e. for sequential timing WITHOUT using a camshaft
sensor. Any comments?

Tom "How could the motor be too lean?" Walter
Austin, TX.