> > Attached is the Input-Output block diagram for the
simple EFI
> system.
> > This diagram is intended to discover any missing
sensors or
> engine
> > outputs ( i.e., other than for pilot information).
> >
> > Please comment, especially omissions. I would rather
provide
> for a
> > sensor up front rather than have to patch or edit the
code
> later on. I
> > realize that not everyone will want all the sensors.
> >
> > Fuel flow can be calculated from the fuel pressure given a
> calibrated
> > hole in the slide throttle. The two pumps on each side
> (tank?) can be
> > operated in parallel, i.e., both on all the time; in the
> event one
> > fails, the uC can notify the pilot and increase the duty
> cycle of the
> > remaining good pump in order to maintain proper fuel
> pressure. Or, a
> > switch could be provided for the pilot to switch to
the other
> pump.
> > Builders choice.
> >
> > The uC system will have total redundancy a la Tracy,
i.e., an
> A uC and
> > a
> > B uC. If one fails, switch to the other and land.
> >
> > Doc Custer
> > Retired Software Engineer and sometime Audiologist
> >
> > -----------------------------------------------------
> > Looks OK to me Doc. I am glad you did that diagram.
> >
> > I am thinking out loud now.
> > You can get fuel flow from the pulse width you are feeding
> the pumps.
> > It must be calibrated because all pumps do not in general
> > pump an amount of fuel directly proportional (in a linear
> manner)
> > as a function of the the pulse width. Even tho the
average
> voltage
> > applied to the pump is directly proportional to the pulse
> width.
> >
> > In other words applying a 50% wide pulse to a 12 volt
> > pump is the same thing as applying 6 volts DC. In any
event
> > you may need a voltage regulator on the 12V supply. If the
> battery
> > is 14 volts the pump gets 7 volts.
> >
> > Another way of doing it is feeding the battery voltage
to a 1/3
> > divider
> > and feed the output of the divider to one of the A to
Ds in the
> > chip so the computer knows the battery voltage at all
times.
> > It then uses a look up table to adjust the fuel pump
pulse
> width
> > according to the available battery voltage.
> >
> > This is the cheap
> > way to do it as you don't need a high power voltage
regulator
> > on the 12 volt supply. If the battery voltage is low
you widen
> > the pulse to compensate. Pumps can take up to 7 or 10 amps
> and as far
> > as I know there are no single chips regulators that will
> handle
> > that. Well at least the last time I looked. In any
event you
> can
> > display the battery voltage on the instrument panel as a
> trouble
> > shooting aid.
> >
> > The other possibility is to ignore the voltage level
as most
> > alternators have a regulator that is pretty much set
to 13.5
> or 14
> > volts or so. Assume that will always put you in the
mixture
> ball
> > park and the pilot in the loop will fine tune with the
> leaning knob.
> >
> > The EGT is a bit hard to use as a feed back sensor as the
> response
> > time is very slow and
> > it changes direction as you pass through peak EGT. You are
> better off
> > with the O2 sensor IMHO if you are looking for a
closed loop
> system.
> > I would forget the closed loop and keep the pilot in the
> leaning
> > loop. The engine will run over a wide mixture range.
10:1 to
> 20:1.
> > Air/fuel.
> >
> > EGT rises as you lean the mixture and then drops as
the mixture
> > gets leaner and leaner past peak. A tough nut to crack
as far
> as
> > programming is concerned. You can lean the mixture and
see if
> it
> > rises. If it rises you are on the rich side of peak
EGT. If
> it lowers
> > you are on the lean side of peak EGT. This takes a lot
of time
> > in addition to the slow response of the TC. .
> > the only way to speed up the response of a thermal
couple is
> > use a smaller one. That has life limit implications.
> > --
> > Paul Lamar ...No rotor no motor.
> >
> >
> > Doc,
> >
> > Fuel pressure alone won't give you fuel flow. You will
also need
> to use
> > the MAP value. You need the pressure differential across
the hole
> to get
> > your flow rate.
> >
> > Do you plan to use any barometric compensation?
> >
> > Ken Bauman
> >
>
> Good point Ken Bauman about the pressure differential,
especially in
> a low
> to moderate pressure system.
>
> I hadn't considered barometric compensation as I assumed manifold
> pressure
> would suffice. Is this wrong?
>
> Doc Custer
> Retired Software Engineer and sometime Audiologist
>
> We can simulate it by pulling a vacuum on a glass jar and see
> how long it takes to inject X amount of fuel at Y pulse width.
> I have an electric vacuum pump.
>
> Paul Lamar
>
>
> Took this from an old MegaSquirt posting.
>
>
> > > I am totally confused here. My experience with reciprocating
> aircraft
> > > engines climbing to altitude is that it gets leaned out, both
> carb and FI.
> >
> > (Yeah, I know. A bunch of you are thinking "here we go again," but
> I've done
> > this often enough that I think I can answer the question
> satisfactorily in a
> > single e-mail.)
> >
> > Imagine an engine with constant VE, it's 100% across the tables.
> We only
> > run this engine at 70f, so the only variable is manifold absolute
> pressure.
> >
> > Throttle the engine to 50 kPa at sea level with an ambient
pressure
> of 100 kPa,
> > and pulse with is, for example, 5.0 ms. Now fly up to 5000 feet,
> ambient
> > pressure drops to 84 kPa and make sure we are still throttled
to 50
> kPa in
> > the manifold. What is our pulsewidth now? Correct, it is
_still_
> 5.0 ms.
> > Change to any altitude you want, but as long as you have a MAP of
> 50 kPa,
> > the PW is 5.0.
> >
> > Well, this is only taking into account the intake side of the
> engine! In
> > a real engine, the volumetric efficiency is not dictated
solely by
> the intake
> > tract, but also by the exhaust system (headers, cam timing,
exhaust
> valve
> > size and all that can change the power output/VE, right?).
> >
> > Imagine we are running the engine in a dyno cell ambient 100 kPa,
> 50 kPa of
> > MAP, resulting VE = 100% and we get that 5.0 ms PW. Now put a
> vacuum cleaner
> > on the end of the exhaust pipe, and pull it down so that the
> exhaust system
> > is under a perfect vacuum (extreme case, but gets the point
> across). The
> > VE magically goes way up, because we've got a super extractor
> exhaust now.
> > Suddenly the engine goes lean, because the VE went up, but we
> didn't know
> > it inside MS because we only measure MAP, not the ambient pressure
> at the
> > exhaust outlet!
> >
> > This is
_exactly_ what happens as you gain altitude. The MAP
> sensor corrects
> > the fuel pulse based on MAP, so altitude changes are of no
> consequence here,
> > but MS has no way of knowing what is going on in the exhaust
system.
> >
> > Here's the bottom line:
> >
> > Barometric correction ENRICHS the mixture as you gain altitude to
> correct
> > for the increasing efficiency of the exhaust system.
>
> Ken
>
> Interesting. Looks like we need another pressure sensor.
>
> Paul Lamar ...No rotor no motor.
I agree. Thanks Ken.
Attached is the latest diagram.
Doc Custer
Retired Software Engineer and sometime Audiologist
What about a coolant temp sensor? Would be handy for cold start (choke)
enrichment. Does not need to be dual as it is not critical.
I'm looking at this from the expectation that the engine would start
just like a car. No matter what the weather conditions are.
Ken
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