What occurs to engine efficiency when air temperature decreases?

When air temperature decreases, engine efficiency generally increases due to the principles of thermodynamics and fluid dynamics. Colder air is denser, which means that it contains more oxygen molecules per unit volume compared to warmer air. As a result, an engine can achieve better combustion because more oxygen allows for a better fuel-to-air mixture. This leads to a more complete combustion process, increasing the engine's power output for the same amount of fuel burned.

Additionally, lower temperatures can improve the performance of turbine engines. The increased air density at lower temperatures results in higher mass flow rates through the engine, which contributes to increased thrust and better performance overall. This effect is particularly leveraged in high-performance aviation engines.

Under these circumstances, the increased efficiency manifests as either increased power output or reduced fuel consumption for a given level of performance. Thus, the correct choice reflects the positive correlation between decreasing air temperature and engine efficiency.