# Thermochemistry of fuel air mixtures

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Thermochemistry of fuel air mixtures
Dr. Primal Fernando Ph: (081)

Combustion process Thermodynamic aspects of particular type process involving chemical reactions, is called combustion Usually occurs between fuel and an oxygen carrier (air) Energy stored in the bonds between constituent atoms of fuel and air (form of internal energy) and in the combustion process it will transformed to new molecules of lower energy level combustion products plus release heat (exothermic reaction)

Combustion process Controls the engine power Efficiency
Controls the emissions Different for SI and CI engines

Flames A flame is a combustion reaction propagate subsonically through space; motion of gas relative to unburn gas is important. The existence of flame motion implies that the reaction is confined to a zone which is small in thickness compared to the engine combustion chamber. The reaction zone is usually called the flame front Flames can be categorized as premixed and diffusion flame (mixed together at same place where the reaction takes place) Flames also categorized as laminar (mixing and transport done by molecular process) and turbulent (enhanced by eddies and lumps) Flames also categorized by whether the flow is steady or unsteady

Fuel - fossil fuels mainly consists of H and C
By mass By mass By volume

Chemical equation and conservation of mass
Consider a simple equation Note: all gasses occupy equal volume for kmol when they are at same pressure and temperature (exactly true for perfect gases, but for other gasses substantially true). Volume occupied by liquid and solids are negligibly small compared to gasses.

Chemical equation and conservation of mass
If insufficient O2 presents

Necessary Oxygen is mainly obtained by mixing fuel with are
Note: Molar mass of N2 is 28 kg/kmol (28.16), and that for air 29 kg/kmol (28.962)

Example Determine the stoichiometric air/fuel ratio for a petrol approximating to hexane C6H14. Hence deduce the chemical equation if the petrol is burnt in 20 percent excess air, and the wet volumetric analysis of the products If all the water vapor is present If products are cooled to an atmospheric pressure and temperature of 1 bar and 15 °C. Determine also the dry volumetric analysis. Estimate the chemical equation if only 80% of the air required for stoichiometric combustion is provided The partial pressure of saturated water vapor at 15 °C is bar

Solution Products

Solution Wet volumetric analysis of the Products including N2
Amount-of-substance in the product

Solution Amount-of-substance in the product
The wet volumetric analysis

Volume fraction=Mole fraction=partial pressure/total pressure
Solution If products are cooled to an atmospheric pressure and temperature of 1 bar and 15 °C. Part of water will be condensed, if new amount of water is y, total substance is = y Volume fraction=Mole fraction=partial pressure/total pressure Above volumetric analysis repeats base on the total volume of 51.67

Solution Determine also the dry volumetric analysis
Analysis is done by assuming no water present, then the amount of substance becomes = 50.79 Above volumetric analysis repeat base on total volume of 50.79

Solution Chemical reaction with insufficient air (80%)
When insufficient O2 is given there will be unburned C and H2. H2, However, has a greater affinity for O2. If mixture is not too rich in fuel, it is reasonable to assume that all the H2 will be burnt. Some of C will be burnt to CO and other to CO2.

General combustion stoichiometry

General combustion stoichiometry
Take, y=b/a = ratio of H2 to C

Energy balance Systems changes from reactants to products (since mass constant, can apply first law for a close system)