Piston Rings: Function and Design

A piston ring is a metallic split ring attached to the outer diameter of a piston within an internal combustion engine. Located between the cylinder and the piston, the piston ring is a necessary component that allows the engine to operate efficiently. In this blog, we will discuss the functions and design of piston rings in detail.

Piston rings have four primary functions. The first is to maintain gas compression between the piston and cylinder wall. Piston rings seal the cylinder, preventing the combustion gas generated during ignition from leaking into the opening between the piston and cylinder. If the combustion gas was allowed to leak, fuel consumption would increase and the engine would not put out sufficient power, neither of which are ideal. The second function of piston rings is to ensure the proper amount of lubricant is distributed throughout the system to prevent scuffing. Because the process of gas ignition is repeated over and over, the piston goes up and down within the engine thousands of times per minute. A small amount of lubricating oil is poured over the pistons to ensure they move smoothly with little friction. The piston rings control the amount of oil.

The third function of piston rings is to transfer heat from the piston crown to the cylinder. During the gas ignition process, the temperature within a piston reaches approximately 300 degrees Celsius. If too much heat is allowed to accumulate within the piston, the engine can become damaged. For this reason, it is necessary to release the heat build-up, which piston rings help do. The final function of piston rings is to support the piston in the cylinder and prevent it from knocking on the cylinder wall. If the piston inclines inside the cylinder, it could touch critical interior parts and lead to engine failure. Piston rings support the pistons to allow for smooth up and down motion.

Next, let’s look at the design of piston rings. As stated, they are designed to seal the gap between the piston and cylinder wall. If this gap was too small, thermal expansion of the piston could cause it to seize in the cylinder, resulting in serious damage to the engine. However, too large a gap would make for insufficient sealing and result in combustion gases entering the crankcase and less pressure on the cylinder, both of which reduce the power output of the engine. The sliding motion of the piston ring inside the cylinder wall results in friction losses for the engine, accounting for nearly one fourth of the engine’s total mechanical friction losses. As such, the goal in the design of piston rings is to minimize friction while achieving good sealing and an acceptable lifespan.

Sealing is often done by multiple rings using a metal-on-metal sliding contact. Most pistons have at least two piston rings per cylinder, each one having their own function. Automotive piston engines, for example, generally feature three rings per cylinder. The top two rings, known as compression rings, are primarily for sealing the combustion chamber. The bottom ring, also called the oil control ring, is mainly used to control the supply of oil to the cylinder wall in order to lubricate the piston skirt and oil control rings.

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