Theory

Burning CH $_y$ completely gives one CO₂ and $y/2$ H₂O per carbon atom and consumes $1 + y/4$ mol of O₂. A larger $y$ therefore puts more water vapour into the exhaust: water has a lower molar mass (18 kg/kmol) and a higher heat capacity than nitrogen, so the exhaust gets lighter and its heat capacity rises. The full reaction is given in Fuel.

Equations

n_{\mathrm{CO_2}} = 1 + 0.00172\,a, \quad n_{\mathrm{H_2O}} = \frac{y}{2}, \quad n_{\mathrm{O_2}} = (\lambda - 1)\left(1 + \frac{y}{4}\right), \quad n_{\mathrm{N_2}} = 3.7279\,a, \quad n_{\mathrm{Ar}} = 0.04459\,a

(1)

a = \lambda\left(1 + \frac{y}{4}\right), \qquad x_i = \frac{n_i}{\sum_j n_j}

(2)

where	meaning	unit
$y$	H/C atom ratio (this setting)	-
$\lambda$	Excess-air ratio	-
$n_i$	Moles of product $i$ per mole of carbon	mol
$x_i$	Mole fraction of product $i$	-

Default and rule

Default: 1.8 (set by the fuel; typed only for a custom fuel)
Rule: The fuel's preset: 1.8 for diesel, 1.87 for gasoline, 4.0 for natural gas. For a custom fuel the last typed value (1.8 at first).
Source: Stoichiometry of complete combustion, equation (1); the air ratios are those of the program's dry-air composition.

Valid range 0 to 4

Values

0 (pure carbon) to 4 (methane). Locked to the preset unless the fuel is "custom".

Fuel oxygen and sulphur

The surrogate contains no oxygen, nitrogen or sulphur; oxygenated fuels (alcohol blends, biodiesel) are approximated by their H/C ratio alone.

Theory

Equations

Default and rule

Valid range 0 to 4

Related 3