Pistons rings are items that we are perhaps not accustomed to having to worry too much about, especially if our race engine is not of bespoke design. Once ‘bedded-in’, they normally form a reliable seal between the piston and bore.
However, there is a particular case where piston rings can cause a problem owing to a vibration condition which is due to a combination of factors centred around the fundamental geometry of the engine, the mass of the piston rings, engine speed and the pressure differential across the piston ring.
As the piston approaches bottom dead centre on the exhaust stroke, the combination of inertia forces acting on the piston ring and forces acting on the ring owing to the pressure differential across it act to push the piston ring up against the top side of the ring groove. This effectively causes a seal between the ring and the top of the groove. For the piston ring to work effectively it is energised against the cylinder wall by a combination of its own inherent radial forces when compressed into the bore (often referred to as piston ring ‘tension’) and a force owing to the radial force exerted as a result of the gas pressure acting on the inside of the piston ring.
Let us take the example of a piston ring with an 80 mm inside diameter, and which is 1 mm thick. The cross-sectional area is (80 x π x 1) = 251mm2, and if the cylinder pressure is 5 bar then the force on the inside of the ring is 5 x 0.101325 x 251 = 127 N. Robbed of this force, the radial force of the piston ring can become insufficient to allow the piston to seal effectively. This leads to a vibration condition known as ring flutter.
As the speed of the engine increases, so the piston acceleration increases as the square of engine speed, and even a small increase in maximum engine speed cause the onset of ring flutter. If we say that the fundamental geometry of the cranktrain and the new maximum engine speed is fixed (this defines the piston acceleration), and that the pressure differential across the ring is also fixed (this can be a dangerous assumption), then if we want to eliminate ring flutter we can do so using a few different methods, but the aim is to increase the speed at which flutter will occur so that it is above the operating speed of the engine.
We could consider an increase in ring tension, so that the ring still seals even when it is forced against the top of the ring groove. This can lead to significantly higher engine friction though, and adds to the forces which push the ring toward the top of the ring groove.
The level of piston acceleration at which ring flutter begins can be raised by decreasing the mass of the ring. For a given cranktrain geometry, piston acceleration increases in proportion to the square of engine speed, so by increasing the piston acceleration at high flutter starts we can take it out of the engine running range. Engineers tend to like reducing the mass of reciprocating engine components, so this is a solution that will find favour among designers and developers. The problem though is that it means having to change the design of the piston ring and piston. The elimination of flutter is one reason why we have seen a constant decrease in top ring widths in recent years. As engine speeds increase, a thin low-mass ring is important for eliminating flutter as well as reducing the inertia forces acting on the cranktrain.
Another option is to modify the piston so that the cylinder pressure is not ‘denied access’ to the inside diameter of the piston ring. There are machined design features that can be added to pistons to allow the cylinder pressure to act on the inside diameter of the top ring, even when the ring is forced against the top of the ring groove.