basic fuel metering equations

John S Gwynne jsg
Fri Jul 1 18:53:49 GMT 1994


   Before the discussion on lookup tables really gets going, I would like to
make sure that I fully understand the basic concepts and equations needed to
control the injector portion of this project.  The following is a start on
what I believe to be the relevant equations for a speed density system. If
I'm not on track (check my math), please let me know and feel free to 


1 --- Injector duration

Description of the control algorithm for injector duration.

1.1 --- Calculation of air flow: lb/hr of air 

The following describes the calculation of air flow (lbs/hr) as a function of
the input parameters MAP, Ta, RPM

1.1.1 --- Constants and variables

A_lb/hr: air flow in lb/hr
MAP: Manifold Absolute Pressure (atm) (1 atm = 29.29 in (760 mm) of Hg)
RPM: revolutions per minute (1/min)
Sa: specific gravity of air at 0 deg C and 1 atm
      Sa = 0.0012929 gr/cc = 0.080713 lb/ft^3
Ta: air temperature (K) (T_in Kelvin = T_in Celsius + 273.15)
Vd: engine displacement (ft^3) (i.e., 0.2025 ft^3 for 350 cu. in.)
%VE: the volumetric efficiency

1.1.2 --- Equation

A_lb/hr = (Sa) X (temp & pressure correction factor, assuming ideal gas) X 
    (engine displacement) X (RPM/2) X (%VE)

A_lb/hr (lb/hr) = Sa (lb/ft^3) * MAP/1 () * 273.15/Ta () * Vd/2 (ft^3) * 
    RPM (1/min) * 60 (min/hr) * %VE ()

A_lb/hr = (661.4 * Vd) * MAP / Ta * RPM * %VE

- the 2 in Vd/2 is for a 4-stroke engine.
- %VE is primarily a function of MAP and RPM.
- (661.4 * Vd) is a constant for a given engine.

1.2 --- Calculation of fuel rate: gal/hr of fuel

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