What does the bottom ring on a four-cycle engine primarily control?

The bottom ring on a four-cycle engine primarily plays a crucial role in controlling oil application to the cylinder. This ring is often referred to as the oil control ring and is designed to manage the amount of oil that reaches the combustion chamber. Its primary function is to scrape excess oil from the cylinder walls and prevent it from being burned during the combustion process, which can lead to increased emissions and decreased efficiency.

By effectively controlling the oil film thickness on the cylinder walls, the bottom ring ensures that there is adequate lubrication for the piston and cylinder wall interaction while minimizing oil consumption. This balance is vital for maintaining engine performance and longevity. A well-functioning oil control ring helps in reducing the chances of oil fouling the spark plugs and contributing to engine knock or pre-ignition issues, thus optimizing engine operation.

The other options are related to different functions or aspects of engine mechanics, like controlling compression or dealing with piston fit. However, the specific function of managing oil flow aligns with the role of the bottom ring, making it the correct choice.