>
> 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.
>
>
> I thought of a way to handle the EGT issue a while back.. If
running
> auto fuel or 100LL+TCP, include a cheap narrow band O2 sensor to know
> which side of stoich you are on and then use the EGT to set the
mixture
> from there. The programming gets much simpler then. The only
remaining
> problem I can think of is that as you change manifold pressure
peak EGT
> changes - at lower mp, there's less total energy in the exhaust
stream,
> at max boost there's more.
>
> I think the computer would have to either use a lookup table to guess
> what peak EGT is at the current m.p.-rpm operating point (probably
> close enough) or periodically cycle the mixture to stoich so that it
> knows what the peak temp actually is. I'd prefer the lookup
table method.
>
> I'd still have a mixture control.. But instead of it directly
> controlling fuel flow it would control how far from peak EGT you
are,
> lean or rich. Set the mixture knob straight up gets you max power
> (50deg ROP), left is even more ROP, slightly to the right of
straigth up
> is peak, further to the right is LOP. As you climb and descend the
> computer keeps the fuel flow where it needs to be to keep that
> relationship.
>
> Using the computer to accomplish this is nice because on the LOP side
> the engine response curve is a much different shape (temp/power
vs fuel
> flow) than on the ROP side. The control is very sensitive on the LOP
> side - temp/power drop quickly for a small change in fuel
flow. On the
> ROP side the curve is much flatter. A mechanical mixture control
while
> LOP is always so twitchy that managing it precisely is
annoying. A good
> job for a computer.
>
> The mixture feature is less important on a turbocharged engine
which can
> maintain constant MP over most altitudes...
>
>
> Matt-
>
Thanks Paul & Matt for the valuable suggestions!
Attached is an updated diagram with the changes you suggested.
Notice the
voltage sensor and the mixture control.
Anybody have a recommendation for "a cheap narrow band O2 sensor?"
Experience from the field is much more valuable than a catalog.
Parts recommendations for all the sensors and actuators is greatly
appreciated! Saves me a lot of time and results in superior choices.
Doc Custer
Retired Software Engineer and sometime Audiologist
I have not played with an O2 sensor but have read a fair amount about
them. The narrow band flavor have a sharp change in voltage when
transitioning through stoichiometric mixture. I think something like
this one would work properly:
http://store.summitracing.com/partdetail.asp?autofilter=1&part=CTO%2DO2%2D13&N=700+115&autoview=sku
<http://store.summitracing.com/partdetail.asp?autofilter=1&part=CTO%2DO2%2D13&N=700+115&autoview=sku>
That's summit racing
http://www.summitracing.com part number CTO-O2-13 $25.
The input circuit will need to be fairly high impedance, to an A/D
probably would be best. Or a simple comparator would work pretty well too.
Matt-
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