What causes a four-stroke gasoline engine to operate in a lean condition?

A four-stroke gasoline engine operates in a lean condition primarily due to an excess of air in the air-fuel mixture. This situation occurs when there is more air being drawn into the combustion chamber compared to the amount of fuel that is supplied.

In a lean mixture, the air-fuel ratio increases, meaning there is less fuel relative to air. This condition can lead to incomplete combustion because the available fuel may not be sufficient to effectively utilize all the air present, which can result in lower power output, higher combustion temperatures, and potentially detrimental effects on engine performance and longevity.

Factors such as defective fuel pumps, blocked air filters, and insufficient compression can affect engine performance, but they do not directly cause a lean condition through an imbalance of air and fuel in the same manner. A faulty fuel pump may reduce fuel delivery, leading to a lean condition, but it fundamentally operates by failing to provide the necessary fuel rather than creating excess air. Similarly, a blocked air filter would typically restrict air intake, potentially resulting in a richer mixture rather than a lean one. Insufficient compression primarily affects the engine's ability to ignite the air-fuel mixture properly, leading to poor engine performance rather than an imbalance in the air-fuel ratio itself.