Assuming Eq. (2) as the combustion equation, relative air–fuel ratios could be approximated by Eq. (5), which is derived from element conservation expressions. In Eq. (5), HCR is the hydrogen to carbon ratio of fuel; M is a constant calculated based on fuel molar weight and stoichiometric air–fuel ratio; and XCO2, XCO, XTHC, and XO2 are the molar fractions of the prod- ucts, which can be assumed equal to their volumetric concentrations.