What is the function of the upper piston rings?

• A. Oil control or scraper type designed to prevent lubricating oil from entering and being burnt in the combustion space.
• B. Top of each cylinder to enclose the combustion space.
• C. Prevent combustion gases from escaping between the piston and cylinder walls.
• D. Ignite the compressed fuel-air mixture.

Answer: (C)

Sealing the combustion chamber and preventing blow-by is what the upper piston rings do. These are the compression rings near the top of the piston. When the fuel-air mixture ignites, the high pressure pushes the rings against the cylinder wall, forming a tight gas seal that keeps most of the combustion gases from slipping past the piston into the crankcase. This maintains engine compression and efficiency, and helps control heat transfer from the piston to the cylinder wall. Below them are the oil control rings, whose job is to manage the oil film and prevent excess oil from entering the combustion space. The cylinder head contains the combustion chamber itself and houses the ignition system, not the rings. Ignite the mixture is done by the spark plug or injector, not by the rings.