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


I realize I'm a newbie on this forum,  but I'm an electrical engineer and I designed a mass flow controlled continuous fuel injection system about 1974.  Think of it as an electronic carburetor.

Anyway, Victor Roberts is correct.  I simulated the 555 timer circuit and looked at the waveforms.  I then added an RC network to smooth out the injector drive pulse stream to produce an analog to the fuel flow.  Not a problem.

 So, my view of this EFI concept is that the optimum system would compare the air mass flow/time to the fuel mass flow/time and only adjust the injector drive pulse width if the two don't track.  Scaling one of the two signals and then subtracting one from the other would create an error voltage that would be zero as long as the two inputs maintained a constant fuel/air ratio.

 Lets see how this would actually work.  I will assume some technique for calibrating the system for the proper mixture at some constant altitude and RPM (engine loading).

 First, assume an altitude increase.  For a constant RPM and throttle position, the air mass flow will decrease as the altitude increases (air density drops while volume flow stays almost constant).  To maintain a constant fuel/air mass ratio to the engine you would like to decrease the fuel flow until it once again matches the air mass flow.  So the system envisioned would lower the 555 control voltage until the average injector drive ON time matched the new, lower air mass flow.  That is, slowly lower the control voltage until the difference of the air and fuel flow again meets whatever F/A ratio is dialed in during calibration (learning).

Likewise, if the air temp decreased for constant RPM and throttle, the air mass flow would increase the error voltage (to the 555 control input) and the fuel flow would be increased until the difference is again zero ( which means the ratio is again at the preset constant).

 Note I'm not doing anything to compensate for the lower pressure in the engine possibly requiring a different F/A ratio.  That's much too sophisticated for this design.  Although it could be added as a tweak at a later time.

 To summarize,  this approach would try and maintain a constant F/A ratio over a relatively wide range of
throttle settings and RPM.
  It would not simultaneously increase fuel flow because of engine RPM increase AND then again as air mass flow increase.  The current realized system will always produce a fairly large F/A error when conditions vary about the initially learned setting.  As Victor Roberts pointed out.

Craig Taylor