What is the purpose of a compression ring?

• A. To seal the combustion chamber by forcing against the cylinder wall.
• B. To lubricate the piston rings.
• C. To cool the engine.
• D. To regulate fuel flow.

Answer: (A)

Sealing the combustion chamber to keep high-pressure gases from leaking past the piston is the primary job of a compression ring. By pressing against the cylinder wall with the right tension, it creates a gas-tight barrier during the compression and power strokes, which maintains engine compression, improves efficiency, and minimizes blow-by into the crankcase. The ring does help transfer some heat from the piston to the cylinder wall, aiding cooling, but its main purpose is sealing. It does not lubricate the piston rings (that comes from the engine’s oil system), it does not cool the engine directly (cooling is done by the cooling system), and it does not regulate fuel flow (that’s handled by the fuel system).