>> > 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
why 2 manifold temps? would there really be enough difference to
need 2?
how could fast moving intake air change a significant amount during the
brief period it passes thru the intake? kevin
Redundancy.
Doc, that will be a very nice setup. Are you talking 4 fuel pumps? Or 2
pumps with redundant PWM? How will the torque sensor work?
Ken
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