transients

[Clare Snyder]

> Date: Thu, 2 Dec 1999 10:59:46 -0600
> From: "G. Scott Ponton" <gscottp at ix.netcom.com>
> Subject: Re:Transients
>

A few things to consider.

Stoich is the point where all of the fuel inhaled by the engine is combined
with all of the available oxygen in the cyl. Any leaner means less than
optimum power. Any richer means less than optimum power per lb of fuel used,
as some is not burned, as well as less power overall. There is a fine line
between too lean for maximum power and too rich. The transition from Idle to
full throttle throws more than one monkey-wrench into the works. Many have
been discussed here already.
The ones I see are, in no particular order:
1] Reversion -at low rpm when the throttle is cracked open, cam design can
cause/alow some compression back into the intake. This dilutes the intake
charge, as well as weakening the venturi signal and pushing the fuel back
the runner, delaying it's ingress to the cyl. All the way around this means
less A/F mixture in the engine, and at a reduced strength.
2] Inertia - OK, I've got my Nomex on - I got flamed for this once before.
The density of gasoline as compared to the density of air? well let's say
there is no comparison. The gasoline is MUCH heavier than the air, so has
much more inertia. Crack the throttle open and you "instantly" get more air.
The fuel follows - quite a bit behind. This causes the mixture to weaken
again.
3]Gravity - OK, not gravity in the strictest sense of the word, but hey,
I've got my Nomex on. The fuel tends to "settle out" of the mixture in the
runners at low speeds. This wets the walls of the runners, weakening the
mixture during transition - then when the airspeed catches up and peels the
fuel off the walls it causes a rich event. Hard to compensate for.

The solution?
In carbureted engine practice, an accellerator pump is used to squirt extra
fuel into the airflow during rapid transitions. The amount squirted is
determined by both the speed, depth, and duration of the acceleration with
the help of a clever "Computer" consisting of a lever, spring, check-valve
and calibrated bleeds. On a CV carb this is replaced by a "damper" which
acts to slow down the increase in air flow so the fuel can catch up.
At best the accellerator pump or damper are a compromise - a "best guess"
arrived at through lots of dyno testing, road testing, slide-rule slipping,
and other Voodoo.
IR systems get away from SOME of these problems - but not all.

Fuel injection can aproach the problem's solution in several ways. The
inertia and gravity can be delt with the same way IR systems attempt to deal
with it. Shorten the path the fuel must travel with the air by employing
port injectors. TBI buys you less advantage, reducing the inertial effects
prior to the venturi by providing fuel under pressure, but not addressing
the "gravity" or the inertia in the intake tract.

To mimic the action of the fuel pump EFI can take several tacks. On the
L-Jet based Nippondenso systems employed by Toyota in the eighties there was
an extra pulse of fuel provided every time the throttle opened past so many
degrees - basically off-idle transition. If a regular pulse at idle was 6 ms
long, you got an extra 6 ms worth of fuel. Mixture was also controlled by
rate of change at the airflow sensor, stretching the pulse under high
airspeed acceleration. The Throttle position sensor also told the computer
the engine was in one of, if I remember correctly, 4 operating modes - idle,
off idle or low power, cruise or medium power, and full power  At idle
mixture was tailored for smooth idle and reasonable off-idle transition -
read that as just rich of  "stoich" -( untill emission  reduction dictated
otherwise - late, hot, and lean, withthe accompanying driveability problems)
At off idle and low power, mixture was tailored for emissions and economy -
read that as weak mixture. At cruise/medium power slightly richer for better
power production - and at full power "pig rich" - as muich fuel as could be
pumped in without causing a decrease in power.

Like the carb, this was a "best guess" aproxemation.

With todays EFI, we have MAP, or load sensors, artificial inteligence in the
CPU, and closer control of other variables like  ignition timing, and even
intake runner sise and length, valve timing , lift, and duration, etc.

A DIY system can meet many of your rquirements, but will tend to be closer
to a carb in it's precision.You can have power, economy, simplicity,and
driveability - pick any two. With a lot of work you can pick 3. By modifying
an existing system, a dedicated, inteligent experimenter may be able to get
all 4 by  tuning out the compromises  required to "satisfy the majority of
people most of the time". Read that as give up something that does not
matter to you.

The application I am thinking of does not suffer much from transitions, so
dirt simple and rock solid are my goals. Economy and power are important
too, but driveability - the hardest to acheive- is secondary at best.

Today's EFI
> <unlurk>
>
>     First the disclaimer. I tend to speak in a way that some people have a
> problem with. I can come across as being condescending and the like. This
> isn't my intention so I hope you will all take this for what it is worth.
I
> am putting on my flame retardant suit and placing my CSH nearby just in
case
> they are needed.
Snippp.
>         I have used up enough bandwdth for now. Obviously this is a very
> simplistic view of all the variables and how they interact. Hope it will
> help some.
> Back to lurk mode.
>
> Scott
>


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> End of DIY_EFI Digest V4 #671
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>
Ditto