I am very pleased to have discovered this group, and have the utmost
respect for the participants and their obvious expertise. One of the
first things I saw was the 555 injector control.

I have been planning for several weeks to use the 555 injector
system on
my project, so I was entering the schematic in preparation for laying
out the circuit board. I was considering how to make provision for the
initial setup when I realized the MAF sensor output is related to air
mass per unit of time, whereas the 555 controls fuel mass per firing
cycle. There is not a defined correspondence between the two except
for
a fixed engine speed..

Assume an engine running wide-open-throttle (WOT) at 4000 rpm, and the
555 delivering the proper pulse width for proper fuel delivery. Lower
the nose and allow the engine to advance to 6000 rpm. The airflow
increases by 50%, and the MAF sensor output increases 50%, lengthening
the pulse by 50%. Because of the increase in rpm, there are 50% more
pulses, each of which injects 50% more fuel, for an increase to 225%
of
the original flow instead of the desired 150%. In fact, absent a
change
in volumetric efficiency between the two rpm settings, the pulse width
should not change, but deliver the same amount of fuel per
combustion cycle.

Fortunately there is a simple solution. The integral of the 555 output
is proportional to fuel flow per unit of time. The mass of fuel flow
per
unit time (instead of per revolution) is easily obtained by
multiplying
the flow capacity of the injector by the duty cycle of the 555. Duty
cycle is determined by the duration of the 555 pulse and the frequency
of the trigger (a direct function of rpm), and is represented by the
average voltage on the 555 output divided by the 555 output "HIGH"
voltage.

By connecting a resistor from the output of the 555 to the inverting
input of the TLC279 op amp and a capacitor from that input to ground,
the average voltage is presented to that input. The op amp now
compares
the (mass airflow)/time to the (mass fuel flow)/time, and generates an
output to control the pulse width. It then becomes a matter of
determining the correct component values to set the 555 timer
characteristics and the op amp gain for the injectors and MAF sensor
used.

A variable resistor would still be used to allow  the pilot to alter
the
mixture would still be desired, but the system would automatically
compensate for volumetric efficiency variations with rpm, because it
would always maintain a fixed relationship between the air and fuel
volumes (aside from injector latency, fuel pressure variations,
etc.) We
are obviously not measuring fuel flow directly, but are measuring
directly the command to the injector. Installation-specific
adjustments
would be required for idle and full power, but everything in between
would be automatic.

Victor Roberts

With all due respect I don't think you are right.

This is just a rough approx. to keep the engine running over the
rev range.

The VE also  changes with RPM. It can go from 100% to 120% over a
2000 RPM
change in a organ pipe tuned system.

I would not use a 555 system in a car
as it will not tune the engine properly over a wide range of HP, RPM
and load combinations under transient conditions. You also might not
want to
use the 555 system in an aerobatic airplane.

I Think you have to go from a high angle climb to a vertical dive to
get
the engine to rev from 4000 full load to 6000 full load. I think
that is an
unrealistic scenario for most aircraft. The prop load  changes a lot
over such a wide range during normal operation. The fixed pitch prop
load
is the square or cube of RPM. The engine RPM might change
several hundread RPM in a cruise situation but then you must adjust
the leaning just like any other aircraft engine.

BTW The rotary unlike an aircraft engine  will run over an air fuel
mixture ratio of 10:1 to 20:1.

Paul Lamar

Paul, Victor -

Victor is right that with an Inject-Per-Revolution scheme (IPR - I
made it up), a simple MAF sensor/555 timer would need some RPM
feedback and compensation, making it ... not simple.

Paul is wrong that it should be designed/built as "just a rough approx
just keep the engine running".  It should be designed to match the
fuel injected to the incoming air.

Paul is right that VE can change.  However it can change for the worse
over a given RPM range as easily as it can change for the better.  IPR
schemes need crank angle and RPM feedback to work effectively,
otherwise the air/fuel ratio will be all over the map (no pun intended).

I had always assumed that the 555 timer would inject independent of
RPM or crank angle.  The injectors would sit upstream enough in the
manifold, happily injecting away in fine grained pulses, so the fuel
ingested per intake stroke would average out and only slightly vary.
This would be an Inject-Over-Time scheme (IOT - I made that up, too).

NOTE: That is the beauty of MAF sensors - they tell you the amount of
air going in - you simply inject a matching ratio of fuel, and it'll
burn at 350 RPM, 4000 RPM, or 6500 RPM - hence the simplicity of the
555 timer for controlling fuel injector pulse width.

BUT... If you're going to start varying the injection RATE (tied to
crank angle/RPM), then you need to COMPENSATE the fuel pulse width
with the same variable (RPM), in a feedback to the FORMERLY simple
system.

Paul - just have the 555 Timer system inject in a relatively fast
pulse stream independent of the RPM/crank angle, using the IOT scheme.

In other words, keep it simple.  (Just my 2 cents worth.)

-Vince Orton

I you think it will work that scheme  solves a lot of problems with high speed
injection. There is not a lot of time  to inject fuel once per rev at 7500 RPM
anyway. All we need is an oscillator to trigger the 555's.

It does make it  a lot  simpler as we don't need injection triggers and the associated
circuits. I am getting close  to trying the 555 system as the RX8 engine slide throttle
and intake tubes are near done. The motor mount is done and the prop dyno is near done.

Paul Lamar