In a 1:15 fuel-air mixture, what does the 15 represent?

• A. the amount of fuel per unit weight in the mixture
• B. the amount of air per unit weight in the mixture
• C. the ratio of ignition timing
• D. the engine displacement

Answer: (B)

This expresses how much air is present for each unit of fuel by weight. In a 1:15 fuel-air mixture, there are fifteen units of air for every one unit of fuel, so the 15 represents the air portion. This is an air-to-fuel ratio of 15:1, which is near the ideal balance for gasoline combustion (stoichiometric is about 14.7:1). It’s not about ignition timing or engine displacement.