Required Fuel

Required fuel is a constant that directly affects an ECU’s tuning characteristics. Stoichiometric air (sea level) to fuel ratio as we know is 14.7:1 by mass. Density of air is 1.225 grams, and density of gasoline is 740 grams per liter.

Take for an example a 4 cylinder 1 liter engine. Let’s calculate the fuel required to burn at stoichiometry for one cycle.

\frac{1.225g}{Lair}\cdot\frac{1}{14.7}=\frac{0.0833g}{Lair}

\frac{1000cc}{740g}\cdot0.0833g=0.112567cc

0.112567cc of fuel is required for 1 liter of air. To calculate the required fuel, the constant required by the ECU to characterize the injectors, we must find the pulse width of fuel required per cylinder per cycle. Assume 630CC/min injectors.

\frac{0.112567cc}{4cyl}=0.02814175\frac{cc}{cyl}

Let’s convert 630cc/min into cc/s

\frac{630cc}{60\frac{s}{min}}=10.5\frac{cc}{s}

Finally to compute the pulse width required for one cycle at stoich

\frac{0.02814175cc}{10.5\frac{cc}{s}}=0.0026802s

2.68ms

Rewriting the RF calculation with variables to represent injector size in cc, number of cylinders, and displacement:

\frac{\frac{0.112567cc\cdot displ}{nCyl}}{\frac{injCC}{60s}}=reqFuel

The ECU will take this number, modify it based on MAP or MAF reading, along with other modifiers such as warm up enrichment, acceleration enrichment etc.