What can be a consequence of using a lean fuel mixture in an engine?

Using a lean fuel mixture in an engine can lead to higher combustion temperatures, which poses a risk of engine damage. A lean mixture contains more air and less fuel than the stoichiometric ratio, resulting in more complete combustion of the fuel. This can increase the peak temperatures within the combustion chamber, potentially leading to issues such as pre-ignition, detonation, or even damage to engine components, including valves and piston parts. Maintaining an appropriate air-fuel mixture is crucial to engine health, as excessive temperatures can adversely affect the materials used in engine construction, leading to premature wear or failure.

The other options describe scenarios that do not align with the outcomes of a lean fuel mixture. For example, lower fuel efficiency and increased fuel consumption generally are associated with a rich mixture rather than a lean one, while increased emissions of certain pollutants can occur with different mixtures but are typically more pronounced when the mixture is rich. Thus, the notable consequence of a lean fuel mixture is indeed the higher combustion temperatures and the potential harm they can cause to the engine.