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/ Categories: Archive, pistons-rings

All of a Flutter

ringsA phenomenon rarely heard of these days, is that of ring flutter. Manifesting itself in the form of unusual wear characteristics and possibly high exhaust gas blow-by, particularly at light load conditions, ring flutter if left unchecked, can even cause pre-ignition/detonation of the incoming fuel-air charge, eventually leading to piston failure. The solution however, is not always that obvious.

You may always hear that the root of the problem is all to do with the acceleration and deceleration of the piston and is therefore a phenomenon much more likely to happen at high engine speeds. That isn't always the case. If the conditions are right, ring flutter can occur at any speed and is therefore considered now to be more of a dynamic condition as a result of the piston ring moving up and down in the ring groove, twisting as it goes, as well as moving in and out and losing contact with the cylinder wall.

To understand the phenomenon even in part, we need to look at the forces acting on the top ring of the piston. These are the combustion pressure forcing the ring down and the crankcase pressure underneath, counterbalancing it to a limited extent. This ring will also have a slight clearance in the retaining groove in the piston and to assist sealing, a tangential force acting on the cylinder wall. When the engine is operating at full load, the combustion gas will flow between the top of the piston ring and the ring itself and from behind the ring, forcing it outwards. A gas tight seal will therefore result as the ring is seated against the lower face of the piston groove and forced against the cylinder wall.

However under the conditions of flutter when lightly loaded, as the piston slows down, the ring may wish to twist or move and part company with the lower groove face, cutting off the flow of gas to its rear. The ring will then effectively collapse inward, perhaps even breaking contact with the wall and the gas pressure eventually overcoming its now impaired ability to seal, finding its way down towards the crankcase. Passing either side of the piston ring, pressure may build up between the top and second ring grooves, which will then further upset the top ring at certain stages in the cycle and if it hasn't already done so, lift it momentarily off the bore thus making matters even worse. Sometimes accompanied by high oil consumption, the motion is irregular and in a way quite light, like the fluttering of a butterfly. But unlike that (unless you are an advocate of chaos theory) the consequences can be more devastating.

rings-flutter

Lest you forget, the piston ring is a major route for heat transfer between the piston crown and cylinder bore, and eventually into the coolant system and anything that interrupts this flow can have serious consequences to the local temperatures in the piston. During the periods when the ring is not in contact with the surface of the bore, no heat can flow and the piston therefore risks overheating leading to pre-ignition in the combustion chamber.

But rather than just the inertia of the ring in the groove or its design, modern thinking suggests that the phenomenon of ring flutter is more down to the incompatibility between the piston ring and the piston itself, than one factor alone. The design of the ring, barrel or otherwise, if it has a bevel on the inside or not and also the quality of the ring groove in the piston are all issues. Incompatibilities in thermal expansion as well as that in thermal conductivity can both influence the condition and if left uncorrected will steadily destroy the geometry of the area and give the wear characteristics so evidently identifiable as ring flutter.

Although heard of comparatively rarely these days, the issue most certainly hasn't gone away.

Fig. 1 - Flutter: (verb) to move or fall with light irregular motion

Written by John Coxon

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