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lt was only in the early 1930s that any semblance of good ring design had evolved. Prior to this, aircraft engines and any other engine for that matter, tended to suffer from excessive oil consumption. This was a result of the piston ring’s steam engine ancestry. The two primary functions of piston rings are (1) to seal the piston against the enormous gas pressures generated during the power stroke and (2) to reduce the flow of oil into the combustion chamber to a minimum. Always a difficult design challenge, ring development tended to progress on an empirical basis. The spring tension in a ring is not very important, since the major component of the radial pressure for sealing is provided by gas pressure behind the ring exerting pressure against the cylinder wall. Spring tension, however, does play a more important role in the case of a badly wom cylinder. Side clearance in the piston ring land is a key dimension. Sufficient clearance needs to be provided for to allow gases to flow over the top face of the ring and pressurize it against the cylinder wall. On the other hand excessive clearance will result in hammering of the ring against the lands, resulting in premature failure. This is especially true of the vulnerable top ring land. Early engine designers failed to realize the importance of sufficient land clearance. This resulted in collapsed rings, i.e., the gas pressure would tend to force the ring inwards thus destroying the seal. Sir Harry Ricardo was one of the pioneers to realize this anomaly with a tank engine he developed during World War I. As a quick expedient he had grooves machined into the top faces of the rings to allow the gas pressure to get to the back ofthe ring. Ring face width was another area of controversy. The proponents of a wide ring face argued that there was less chance of the oil film being squeezed out, particularly towards the top of the stroke where boundary layer lubrication exists. This, of course, resulted in less cylinder barrel wear. On the other hand, a wider ring carries more inertia, resulting in a greater tendency to ring flutter. Ring flutter is the phenomenon caused at high piston speeds, typically 2500 feet per minute and higher, when the ring will float off the lower ring land consequently destroying the gas seal. This results in violent blow-by and significant loss of power. For many years this phenomena was thought to be caused by radial vibration of the ring. Paul Dykes, for whom his piston ring is named, demonstrated what really occurred. Under normal circumstances, as the piston rises on the compression stroke, the ring is held, at first by inertia, and later by gas pressure, against the lower ring land. In this way, the full clearance above the ring is available for gas pressure to seal the ring against the cylinder wall. And at the same time the lower land is completely sealed. At some critical piston speed, however, the ring’s inertia will exceed both friction and gas pressure during the compression stroke and allow the ring to float off the lower land. Under these running conditions the ring will lose the gas seal and collapse resulting in the classic case of "ring flutter." The foregoing gives an idea of the idiosyncrasies involved in ring design. Most of it resulted from trial and error. Rolls-Royce ran afoul of ring problems developing the 1931 Schneider trophy "R" racing engine. At one time during this engines development, oil consumption reached an unheard of rate of 112 gallons per hour!

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