Which piston ring prevents combustion pressure from entering the crankcase?

Which piston ring prevents combustion pressure from entering the crankcase?

• A. Wiper Ring
• B. Sealing Ring
• C. Compression Rings
• D. Oil Ring

Answer: (C)

Piston rings create the seal that keeps combustion pressure inside the cylinder. The compression rings sit high on the piston and press against the bore as the piston moves, forming a tight gas seal. When the engine fires, the pressure pushes on these rings and, with their elastic tension and precise shape, they expand slightly to block the tiny gaps between piston and wall. This prevents most of the combustion gases from leaking into the crankcase, so the crankcase pressure stays low and engine power is preserved. The oil ring assembly at the bottom is responsible for oil management—scraping excess oil off the cylinder wall and returning it to the sump—so it doesn’t foul the combustion chamber. Wiper rings help with oil control as well, but they aren’t meant to seal combustion pressure. That sealing job belongs to the compression rings.

Piston rings create the seal that keeps combustion pressure inside the cylinder. The compression rings sit high on the piston and press against the bore as the piston moves, forming a tight gas seal. When the engine fires, the pressure pushes on these rings and, with their elastic tension and precise shape, they expand slightly to block the tiny gaps between piston and wall. This prevents most of the combustion gases from leaking into the crankcase, so the crankcase pressure stays low and engine power is preserved.

The oil ring assembly at the bottom is responsible for oil management—scraping excess oil off the cylinder wall and returning it to the sump—so it doesn’t foul the combustion chamber. Wiper rings help with oil control as well, but they aren’t meant to seal combustion pressure. That sealing job belongs to the compression rings.