Some of the most common misconceptions when it comes to pistons and piston ring operation, are the function of the bevel found in many different style piston rings and the various responsibilities each ring has. Many assume that the purpose of a beveled edge is to allow gas pressure to slip behind the ring – which is not true, and is caused by another factor entirely.
Keith Jones and the team at the piston ring manufacturer, Total Seal, created the short video above to help explain these beveled edges and its relationship with positive twist.
Which Ring Is Responsible For What Function?
Before diving into positive twist and ring bevels, it’s important to understand the basic function of each piston ring.
The main responsibility for the top piston ring (compression ring) is to be the first line of defense against gases sneaking past into the crankcase on the exhaust stroke and losing cylinder pressure on the compression stroke. This is achieved by a combination of different factors creating a strong dynamic seal against the cylinder wall. This seal is influenced by ring groove side clearance, cylinder pressure, ring shape, and positive twist.
Beyond its primary role of maintaining compression, the top ring also acts as the last line of defense against any oil that may have made its way past the second ring on the intake or power stroke. Last but not least, the top ring also takes on the role of bodyguard by absorbing most of the thermal energy and pressure from the combustion process to protect the rings below the second ring land.
The second ring’s primary purpose can simply be explained as the secondary compression ring, making sure any gases or pressure that was able to make its way past the top ring on the exhaust or compression stroke are stopped before they can enter the crankcase.
The rings in the bottom piston groove, which are commonly seen as a three piece design, are responsible for scraping the oil from the cylinder walls, helping to regulate oil consumption. This ring assembly itself commonly includes two thin outer steel “rails” and an inner expansion ring/spacer, tasked with the job of holding the outer rails at the correct axial distance while also pressing them outwards against the cylinder walls to scrape the oil away.
What Is The Purpose Of The Bevel?
At the beginning of this article we mentioned the misconception that the main purpose of the beveled edge is to allow compression gases to sneak behind the ring – this is actually the job of the combustion pressure and ring groove side clearance, which is the rings’ specific amount of space between the top face of the ring and the ring land. This gap is what allows the gases behind the ring to push it out against the cylinder walls.
Which piston ring edge is beveled will vary depending on the rings function and location in the piston.
The top compression ring is most commonly cut on the top inside edge, which then creates positive (upward) twist as compression pushes down on the piston, giving it a look similar to a belleville washer. This torsion then pushes the lower inside edge of the ring down into the ring groove to create a tight seal.
The second compression ring bevel is commonly found on the bottom inside edge to create negative (downward) twist. Unless a Napier style ring is used, which would then have a bevel in the same position as the top ring or on the bottom outside edge, creating positive twist.
Since the top compression ring is pretty effective at preventing most of the cylinder pressure from reaching the second ring, the integrity of the seal created is left to the rings natural twist since it doesn’t have the assistance of compression like the top ring. The outer tapered edge is designed to assist with residual oil removal. As the piston travels downward the tapered bottom edge is dragged against the cylinder wall, removing the excess oil pulled up by the vacuum created on the intake stroke.
The bevel found in many different style piston rings is integral to separating the radically different environments of the combustion chamber and crankcase during each step of the four-stroke combustion process. The cut allows the ring to pivot further and create twist, sealing the ring groove against combustion pressure, exhaust gases, and excess oil contamination better than ever thought previously possible.