While you probably don't spend much time thinking about your car's piston rings, this relatively small part is a crucial engine component. Without piston rings, it's likely that the internal combustion engine with never have come to fruition.
What follows is a comprehensive explanation of piston rings, how they work, what they're made from, and more.
Primary Functions of a Piston Ring
For just about as long as the internal combustion engine has been in existence, the average piston ring has always served three essential functions within the confines of the average engine:
- Piston rings help prevent blow-by, where small amounts of unburned fuel and exhaust gases escape blow right by the piston and instead enter the crankcase. Blow-by kills engine power, as it's the expansion of these gases that drives the crankshaft and powers the vehicle.
- Piston rings also help dissipate heat from the combustion chamber. The rings transfer heat to the cylinder walls, where the excess heat is transferred once more into motor oil flowing through coolant passages inside the engine block.
- The piston rings hold a thin film of lubricant along the cylinder wall, allowing the piston to move inside the cylinder freely. Additionally, the rings prevent lubricant from entering the combustion chamber.
As the typical piston moves up and down inside the cylinder, three rings work in concert to hold lubricant between the cylinder and the piston, seal off the combustion chamber, and ensure that any excess oil in the cylinder is kept out of the combustion chamber.
- A thin top ring that easily conforms to irregularities in the cylinder wall
- A secondary ring creates a seal against the cylinder wall
- A third large oil ring with a special two-rail design collects and sends excess motor oil back to the source
Piston Ring Design and Construction
In the early days of automotive engine design, piston rings were often made out of cast iron. Despite a penchant for becoming brittle under certain circumstances, cast-iron piston rings worked well with iron cylinder walls, and they could be coated to prevent corrosion. They were also heavy and prone to breakage because the casting process results in a somewhat inconsistent composition.
Today, steel alloys and ductile iron have replaced cast-iron piston rings. These new materials allow for tighter clearances between the piston and the cylinder wall, a better seal on the combustion chamber, improved lubrication and reduced wear, and greater resistance to heat. Modern piston rings are even designed to flex a bit as the piston moves up and down without weakening.
Like just about any other automotive part, piston rings can fail if they?re subjected to severe conditions. Excess heat can cause the rings to become warped, thus rendering them ineffective. High combustion chamber pressures or an improperly installed cylinder head can cause the cylinder to lose its natural shape and also cause the rings to fail. Piston rings can also suffer damage from scuffing, where small amounts of metal come into contact with one another (usually due to a lack of lubrication). Oil contaminants can have an abrasive effect, reducing cylinder ring performance over time. Finally, unexpected pressures resulting from detonation can cause piston rings to break suddenly.
However, it must be noted that piston rings are a wear item. Even the best maintenance and most gentle use will not prevent gradual wear, as the rings are in constant contact with the cylinder wall. As rings begin to wear, the most common symptom is oil use and a slight reduction in engine power. If you find that your vehicle is losing oil between regular scheduled maintenance intervals, that's a sure sign that your vehicle is burning oil due to ring wear. Black smoke emanating for the tail pipe under wide open throttle (WOT) is another sure sign of ring wear.