Question

What is the relationship between the percent of oxygen in a fuel and the amount of energy avaliable per mole of fuel?

Answer 1

Ans- Combustion- In order for combustion to occur you need two things, fuel and oxygen. In a typical spudgun, air is the source of the oxygen and the fuel is a gaseous hydrocarbon such as propane. To get the maximum amount of power out of spudgun the ratio of fuel to oxygen must be correct.

Combustion Equation- The complete combustion of hydrocarbon fuels using oxygen (O₂) produces carbon dioxide (CO₂), water and heat. The balanced chemical equation for the complete combustion of propane (C₃H₈) in oxygen is;

C₃H₈ + 5O₂ = 3CO₂ + 4H₂0 + Heat

The equation above tells us that for each molecule of propane we need five molecules of oxygen. If we have too much or too little fuel, the reaction does not go to completion and the energy released by the combustion is decreased.

Optimal Fuel Ratio- For every fuel the maximum energy will be produced if the ratio of fuel to oxygen is correct. However, most spudguns use air as the oxygen source instead of pure oxygen. Air is about 21% oxygen (by volume) and 88% nitrogen and about 1% other gases. We have to take this into account when calculating the amount of fuel to use in a gun. The chemical equation for the combustion of propane indicates that we need 5 volumes of oxygen per volume of propane. Since air is 21% oxygen then we need about (1/5*21%)=4% by volume propane in the chamber with the air.