What is the ideal stoichiometric ratio for gasoline engines?

• A. 13:1
• B. 15:1
• C. 14.7:1
• D. 14:1

Answer: (C)

Gasoline engines achieve the right balance when the air and fuel are mixed in just the right proportions for complete combustion. That exact balance, called the stoichiometric air–fuel ratio, for gasoline is about 14.7 parts of air to 1 part of fuel by mass. At this ratio, all the fuel can be burned using the available oxygen, producing mostly carbon dioxide and water with minimal leftover fuel or oxygen.

This level is ideal because it also lets the three-way catalytic converter work most effectively to reduce emissions, while maintaining good power and fuel economy. If the mixture is richer (less air, more fuel), you get more unburned fuel and carbon monoxide, and engine efficiency drops. If the mixture is leaner (more air, less fuel), you can run into misfires, rough operation, and higher nitrogen oxides. The 14.7:1 ratio is the standard target for gasoline engines, making it the best choice among the options.