10

In another post (A most intriguing cold-start misfire mystery) I found this quote:

Mass air flow = 3.66 g/s Fuel flow = 1.27 l/h, Fuel rail pressure = 380 kPa

This indicates to me that AFR is around 14.88, close to stoich.

How is AFR calculated from these numbers? I know it's the ratio of air mass to fuel mass, but it's making the units work that I'm getting a little lost. In this case the air mass flow rate is in grams per second, and the fuel flow rate is liters per hour. Grams is a measure of mass, and liters a measure of liquid volume. Does this require knowing the density of the fuel to make the calculation?

As always thank you to @Zaid for getting me thinking.

Community
  • 1
cdunn
  • 9,326
  • 8
  • 44
  • 76

1 Answers1

5

Assuming that liquid fuel is incompressible, the missing piece of the puzzle here is fuel density, which is around 700 kg/m³.

So the fuel mass flow rate is as follows:

Mass flow rate = Density * Volumetric flow rate
               = 700 kg/m³ * 1.27 l/h
               = 0.2469 g/s ¹

Dividing the air mass flow rate by mass flow rate yields the AFR number:

AFR = air mass flow rate / fuel mass flow rate
    = 3.66 g/s / 0.2469 g/s
    = 14.82

(I may have mistyped 0.2469 as 0.2459 when performing the original calculation, hence the discrepancy in the result)

¹ - Calculated here

Zaid
  • 39,276
  • 50
  • 151
  • 294