Hi all.
This is a quick pass at the design using an average of the injector voltage as an analog of the fuel volume.  It compares the air mass flow to the fuel volume flow and adjusts the fuel flow (via Control on the 555) to force the fuel to match the air flow. The signals are summed in the integrator made up of the opamp IOP1 and C5, R8.
   I ran a simulation to make sure it was stable.  The injector pulse smoothing filter values ( R7 and C7) are approximate, but they shouldn't be critical.  Since the constant pulse width, one pulse per rev approach gives a first approximation of the fuel requirements of the engine, allowing 1 second or more  for the feedback loop to adjust to the new MAF sensor voltage shouldn't be an issue.
 For an actual system, some component values need to be optimized.  I used 15k and 200nF for the one shot RC time constant.  For a 555, the RC product of the components is very close to the actual pulse width; here it's 1.5E4 Ohms x 2E-7 Farads= 3E-3 seconds= 3ms.

 That means that when the Control voltage is 2/3 of the supply voltage (6.66V in my case) the output pulse is 3ms.  This base value can be easily adjusted by varying R1.

 You should be aware that increasing Control voltage above about 0.95xVsupply  will cause strange things to happen.  So  shouldn't make that voltage part of the normal operating range.  Therefore I would pick RC to be the middle of the expected injector pulse width range.  That allows Control to adjust the pulse width over a reasonable range.

 Also note that I haven't make any attempt to "calibrate" the system.  I have no idea, for instance, what full scale ( 5V? ) on the MAF sensor output is equal to expected full speed-full throttle air flow.  I don't even know if the MAF sensor output is LINEAR with air mass flow.  I sure hope so!

Craig Taylor

Thanks for doing this Craig.

Let me get this straight in my feeble mind. VG1 is a fixed frequency square wave generator correct?

VS23 is the mass airflow sensor correct?

Why not just connect the MAF sensor directly to the control pin?

Paul Lamar