What happens to the compression ratio of an engine with a large volume of combustion deposits?

When an engine accumulates a large volume of combustion deposits, the effective volume of the combustion chamber decreases. This is because the deposits take up space that would otherwise be occupied by the air-fuel mixture at the point of combustion. Consequently, when the volume of the combustion chamber is reduced while the swept volume of the engine remains constant, the compression ratio is effectively decreased.

The compression ratio is calculated by dividing the total cylinder volume when the piston is at the bottom of its stroke (BDC) by the volume when the piston is at the top of its stroke (TDC). With deposits in the combustion chamber, the volume at TDC becomes smaller than it would be without those deposits, leading to a lower compression ratio overall.

This reduction in the compression ratio can negatively impact engine performance, efficiency, and emissions, as the engine is designed to operate optimally within a specific compression range.