The oil control ring
If the top compression ring has the most difficult of tasks in the engine then, at the other end of the piston ring pack, the oil control ring doesn’t have it much easier. Travelling at an average speed approaching 4000 feet per second or so, the component has to strip away any excess oil from the cylinder bore on the downward stroke and ensure just sufficient passes to lubricate the upper rings. The excess oil is then forced through the ring and drains back to the crankcase via a series of slots or holes in the piston.
Unlike the compression ring/rings above, there is little or no combustion gas pressure to find its way into the void behind the ring and force it against the cylinder wall and so these rings have to use a totally different method of maintaining some kind of seal. The single piece oil ring, for example, with its two outer lands parallel to each other relies totally on the inherent tension in the ring itself. These rings can have chamfered edges on either the outer edges of the lands – to increase the contact pressure – or chamfered edges on that part of the land facing the combustion chamber, which will also assist oil consumption through improved oil scraping from the bore. With slots to allow the oil to flow back through the piston, whatever the detail design, these are somewhat boxy affairs and find it difficult to contort and conform to the shape of the bore, which is often far from round during cylinder firing. Rings of this design are very rarely used today having been superseded by the most part by two and three piece designs.
Similar in some ways to the single piece ring, the two-piece version applies its pressure onto the cylinder bore using a coil spring in compression. Cylindrical and often made from heat-treated spring steel, this coil locates into a semi-circular groove or V- shape at the back of the ring and acts uniformly around the whole of the circumference. While not only helping to positively locate the spring this also reduces the cross-section enabling it to twist and conform more to the shape of the bore. At one time slotted holes were popular but now drilled holes have taken over because of their more uniform strength and even contour. Made from high chromium nitrided steels, with a variety of chamfers or more pronounced bevelled edges, these rings are more frequently found in diesel applications.
In high performance engines and especially those with thinner wall cylinder liners, three-piece oil control rings are a popular solution. Consisting of two thin steel rings (the rails), held apart by an expanding spacer, which also presses them against the cylinder wall, these three ring systems are designed to be fully flexible and ‘hug’ the cylinder wall as closely as possible. Having low inertia around their cross-section, these separate elements can also twist and squirm in response to the dynamic shape of the bore allowing minimum oil leakage to the compression rings above and at the same time helping to stabilise the piston in the bore. This conformability of the rails can be improved by increasing the tangential load or by decreasing its moment of inertia. However, this is not always desirable since the higher tangential loads increase engine friction and reducing the section can in turn introduce other durability issues.
In racing engines where oil control is not quite so important, friction is the enemy. The trade-off between oil control and friction will therefore inevitably fall in favour of low-tension rings and high oil consumption.
Some things just never change.
Written by John Coxon.