13.12 Octane (C8 H18) burns completely with 120 % of...

$13.12$ Octane $\left(\mathrm{C}_{8} \mathrm{H}_{18}\right)$ burns completely with $120 \%$ of theoretical air. Determine (a) the air-fuel ratio on a molar and mass basis. (b) the dew point temperature of the combustion products, in ${ }^{\circ} \mathrm{C}$, when cooled at $1 \mathrm{~atm}$.

$13.12$ Octane $\left(\mathrm{C}_{8} \mathrm{H}_{18}\right)$ burns completely with $120 \%$ of theoretical air. Determine
(a) the air-fuel ratio on a molar and mass basis.
(b) the dew point temperature of the combustion products, in ${ }^{\circ} \mathrm{C}$, when cooled at $1 \mathrm{~atm}$.

Key Concepts

Dew Point Temperature

The dew point temperature in a combustion context is the temperature at which the water vapor present in the combustion products begins to condense into liquid water. It is determined by the partial pressure of water vapor in the gas mixture and is an important parameter in assessing heat recovery and potential condensation issues in exhaust systems.

Excess Air

Excess air refers to the additional air provided beyond the theoretical or stoichiometric requirement. In a combustion process, supplying excess air (e.g., 120% of the theoretical air) ensures more complete combustion and can affect the temperature of the combustion products as well as the composition, including the amount of unreacted oxygen in the exhaust gases.

Air?Fuel Ratio

The air?fuel ratio is a measure of the amount of air used in the combustion process compared to the amount of fuel. It can be expressed on a molar basis (moles of air per mole of fuel) or a mass basis (mass of air per mass of fuel). This concept is crucial for determining optimal combustion, controlling emissions, and ensuring that the combustion process is effective and efficient.

Combustion Stoichiometry

Combustion stoichiometry involves balancing a combustion equation to determine the precise theoretical amounts of fuel and oxidizer (air) needed for complete combustion, where all fuel is converted to products such as CO? and H?O. Understanding stoichiometry helps in determining the necessary proportions for ideal combustion and forms the basis for calculating excess air ratios.

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