[Diy_efi] Timing Calculations

[Adam Wade]

--- Amd CPU <amdcpu at hot.ee> wrote:

> i've heard that the best angle to start injection is
> after closing of intake valve, opposite to popular
> opinion - to warm up the mixture. With that theory,
> the closing point doesn't matter. Is that so?

At higher engine speeds, assuming there is not a
ridiculous amount of valve overlap, there seems to be
something to the idea that more time to take heat from
the intake valve means more fuel vapor and better
mixing of the mixture.  Note that this is on
port-injection systems in particular; on TBI systems,
you've got a long manifold and all cylidners being fed
from a single location, and on direct injection, you
inject during the compression stroke.

> Also have heard that could win when not injecting
> during overlap.

Especially at lower engine speeds, this tends to be a
factor, and most especially where there is a lot of
overlap.  While you can certainly tune the vehicle to
get the same AFR in the combusted mixture, you'll lose
some of your fuel out the exhaust valve, and thus lose
efficiency.  for those interested in power alone, I
don't know if they would consider it worthwhile to
limit their system in that manner just for an increase
in fuel economy.

> Which one is correct? How big is the difference, can
> we see +5% of efficiency?

Probably not that much.  And a lot will depend on the
tuning of the intake, exhaust, and cam
lift/duration/timing.  Those are the things that seem
to impact port injection the most.

> In theory you can use 1 timer and 4x16bit of memory
> to time 4injectors. You must not hold them open with
> timer pin. Haven't seen it, but should be possible.

I'm not sure of the hardware end of it, but if you can
predict acceleration/deceleration, why can't you just
calculate an "offset trend", how much the system is
expected to change between this injection firing event
and the next, and apply that to each injector as its
turn comes around?  The value would be constantly
adjusted based on delta n.  Seems like it should be
simple enough to implement...

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