In an analysis of the products of combustion of isooctane (c8h18), it is found that the exhaust gas mixture contains 6% of co2 (by volume) and 1% of co (by volume). Assume that all the carbon in the fuel is converted to co2 and co, while all the hydrogen in the fuel is converted to water. Assuming lean combustion, find the equivalence ratio of the air-fuel mixture.
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Answer:
Atmospheric Air contains approximately 21% oxygen (O2) by volume. The other 79% of "other gases" is mostly nitrogen (N2), so we will assume air to be composed of 21% oxygen and 79% nitrogen by volume. Thus each mole of oxygen needed to oxidize the hydrocarbon is accompanied by 79/21 = 3.76 moles of nitrogen. Using this combination the molecular mass of air becomes 29 [kg/kmol]. Note that it is assumed that the nitrogen will not normally undergo any chemical reaction.
The Combustion Process - The basic combustion process can be described by the fuel (the hydrocarbon) plus oxydizer (air or oxygen) called the Reactants, which undergo a chemical process while releasing heat to form the Products of combustion such that mass is conserved. In the simplest combustion process, known as Stoichiometric Combustion, all the carbon in the fuel forms carbon dioxide (CO2) and all the hydrogen forms water (H2O) in the products, thus we can write the chemical reaction as follows:
where z is known as the stoichiometric coefficient for the oxidizer (air)
