Stoichiometry
Stoichiometry is an important concept in combustion and hybrid launchers, since they use the energy from combustible fuels. A stoichiometric mixture is one where the ratio of fuel and oxygen molecules is balanced, so all the fuel and oxygen in the chamber is (in optimal conditions) consumed in the reaction. A fuel meter system is usually designed to ahcieve this mixture, which will yield the most power out of the combustion reaction.
Calculations
The stoichiometric ratio for gaseous fuels can be calculated as follows.
Find the chemical formula for the fuel, and count how many hydrogen, carbon and oxygen (if any) atoms it contains. Hydrogen in the fuel will combine with oxygen to form H2O, so each four hydrogen atoms will consume one O2 molecule. Carbon atoms will combine with oxygen to form CO2, so each carbon atom will consume one O2 molecule. Any oxygen in the fuel will lead to one less oxygen atom being consumed form the air. To find how many oxygen molecules each fuel molecule will consume, calculate as follows:
- for each hydrogen atom, add 0.25 to the amount of oxygen molecules
- for each carbon atom, add 1
- for each oxygen atom, subtract 0.5
The resulting number is the oxygen to fuel ratio, divide 1 by this result to find the fuel to oxygen ratio. The amount of oxygen in air is 21%, so to find the fractional ratio of fuel to air, multiply the fuel to oxygen ratio with 0.21. To find the percentage, multiply this by 100.
To find the volume of fuel needed for a particular chamber, multiply the chamber volume with the fractional ratio.
Concrete example
Formula of propane: C3H8
- number of carbon molecules: 3
- number of hydrogen molecules: 8
- number of oxygen molecules: 0
number of oxygen molecules needed = 3*1 + 8*0.25 = 5 molecules
fuel/oxygen ratio = 1/5 = 0.2
fuel/air ratio = 0.2*0.21 = 0.042, or 4.2%
fuel volume in a 3000 cc chamber = 3000*0.042 = 126 cc
Single formula: (1/(C+(H/4)+(O/2)))*0.21
