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
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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
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