What is a typical air/fuel ratio for normal engine operation?

The typical air/fuel ratio for normal engine operation, especially in spark ignition engines, is around 14.7 parts of air to one part of fuel by weight. This is known as the stoichiometric ratio for gasoline engines and indicates the ideal mix for complete combustion.

Among the options provided, the one suggesting 12 parts of air to one part of fuel by weight indicates a richer mixture. While this is still within a reasonable operating range, especially under heavy load conditions or when tuning for performance, it is not the normal operating point for fuel efficiency and emissions control. Such a mixture can lead to increased power output but may also result in higher emissions and decreased fuel efficiency.

The other options presenting different ratios either specify a mixture based on volume or are further from the typical operational standards. Overall, selecting the air/fuel ratio that accurately reflects common operational practices is essential for optimizing engine performance and efficiency.