Gaseous octane is burned with 40 percent excess air in an...

Name: Problem 95, Chapter 16: Thermodynamics: An Engineering Approach
Uploaded: 2022-01-31T09:45:37-08:00
Duration: 6 min 31 s
Channel: Mohammad Mehran
Description: Gaseous octane is burned with 40 percent excess air in an automobile engine. During combustion, the pressure is 600 psia and the temperature reaches 3600 R. Determine the equilibrium composition of the products of combustion.

Gaseous octane is burned with 40 percent excess air in an automobile engine. During combustion, the pressure is 600 psia and the temperature reaches 3600 R. Determine the equilibrium composition of the products of combustion.

Key Concepts

Chemical Equilibrium

This concept involves the state when a chemical reaction proceeds at the same rate in both forward and reverse directions, resulting in constant concentrations of reactants and products. In combustion, especially at high temperature, chemical equilibrium analysis is used to predict the final composition of the products by minimizing the Gibbs free energy of the system under given conditions of temperature and pressure.

Combustion Stoichiometry

Combustion stoichiometry is the calculation of the exact proportions of reactants (fuel and oxygen) needed to achieve complete combustion. It involves writing a balanced chemical equation that accounts for all atoms in the fuel and is fundamental to determining both the theoretical air required and the resulting products when the fuel is burned.

Excess Air

Excess air refers to the additional amount of air supplied over the stoichiometric requirement necessary for complete combustion. It is expressed as a percentage over the stoichiometric amount, and it affects the composition and temperature of the combustion products, often resulting in residual oxygen and influencing equilibrium conditions.

Air Composition

Understanding the composition of air is crucial since it is the source of oxygen generally used in combustion. Air is primarily a mixture of oxygen and nitrogen (approximately 21% oxygen and 79% nitrogen by volume), and the inclusion of inert nitrogen affects the thermal properties and equilibrium calculations of the combustion process.

Equilibrium Constant

The equilibrium constant is a temperature-dependent value that quantifies the ratio of the concentrations (or partial pressures) of products to reactants at equilibrium. In high-temperature combustion, equilibrium constants are used to relate the partial pressures of combustion products and to determine the extent of reactions, especially where dissociation or side reactions occur.

Thermodynamic Conditions

Thermodynamic conditions such as high pressure and temperature significantly influence the equilibrium composition of combustion products. For detailed combustion analysis, parameters like 600 psia pressure and 3600 R temperature are used to evaluate equilibrium constants, reaction extents, and the final mixture composition by affecting both reaction kinetics and equilibrium states.

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