What is meant by a 'lean air/fuel mixture' in an engine?

A 'lean air/fuel mixture' in an engine refers to a situation where there is too much air present relative to the amount of fuel. This concept is fundamental in internal combustion engines, as it impacts efficiency, emissions, and engine performance. In a lean mixture, the ratio of air to fuel is higher than the optimal stoichiometric ratio, meaning more air is available for combustion compared to the fuel.

When the air in the mixture increases relative to the fuel, it can lead to more complete combustion, which often improves fuel efficiency and reduces certain emissions. However, if the mixture becomes too lean, it can hinder the combustion process, leading to issues like engine knocking or misfiring. Therefore, understanding and maintaining the right balance of air and fuel is crucial for optimal engine operation.