What is the best stoichiometric air-fuel mixture?

• A. 12:1
• B. 14:1
• C. 15:1
• D. 16:1

Answer: (C)

Stoichiometric air-fuel ratio is the exact mix needed to burn all the fuel with all the oxygen in the air, leaving no leftover fuel or oxygen. For gasoline, the practical stoichiometric value is about 15 parts air to 1 part fuel by mass. This comes from estimating gasoline as a hydrocarbon (like C8H18) and calculating how much oxygen is required for complete combustion, then dividing by the oxygen content of air. With this approach, the mass of air needed per mass of fuel comes out to roughly 15:1. The other options don’t hit that balance: too little air (a very rich mix) leaves unburned fuel, while too much air (a lean mix) can prevent complete combustion. So, 15:1 represents the best rounded stoichiometric ratio commonly used.