Only the distinctly higher temperatures of the internal combustion engine and above all the temperature dependent tolerance between piston and cylinder made a movable sealing to the cylinder wall obligatory. Furthermore, heat is dissipated and the piston is stabilised in its operation, e.g., possible piston-tipping when the engine is cold. Because the cylinder is never quite cylinder-shaped, the piston ring material should be accordingly flexible. The sealing effect depends on the adaptation to the cylinder, the primary tension and the pressure distribution on the piston ring. Later this sealing is complemented by the oil scraper rings which take care of the oil rest remaining on the cylinder wall.
Compression rings have a slightly larger diameter than the caliber of the cylinder-sleeve. Due to this, a certain primary tension is developed. Particularly the upper one has a considerably wider inside diameter than the corresponding piston ring groove. The pressure from the larger ring-groove can thereby, press the ring against the cylinder wall. To allow this, a small amount of vertical play within the ring-groove is necessary. The amount of play should be kept low because otherwise, pumping loss will develop. With the assembly of used pistons/piston rings the vertical clearance is to be measured with a feeler gauge. To prevent the ends of the piston rings from getting jammed in the slots, they are protected, in two-stroke engines by pins, against twisting.
The compression rings also differ in their shape. The rectangular cross section provides the highest pressure force. With a slightly convex surface to the cylinder wall, a counterforce can be generated by the compression- and working pressure even if the upper-bevelled ring only has contact to the bottom edge of the ring-groove. In addition, the efficiency of the oil-scraper is increased, thus shortening the running-in time. Quite often there is a combination of advantages by using only partly level fitting, beveled rings.
An engine has one, or at the most two oil scraper rings which can be recognised by the openings through which the scraped off oil runs, through the inside of the piston and back to the oil sump. Should the piston have slots behind the oil scraper rings, it is known as an auto-thermic piston. With drillings in the oil scraper ring slots it is called autothermatic piston. Oil-scraper rings often have an inside mounted coil spring (flexible tubing spring) which increases the primary tension, more steadily. This is an important measure of the oil consumption, the friction and the wear and tear. Modern oil scraper rings can also consist of thin metal rings above and below with a corrugated spring between them.
Piston rings are still made of special cast iron, which has good gliding properties and elasticity with a specific, always consistent pressure against the cylinder wall without permanently warping. It harmonises with the piston material as well as with the cylinder material and provides for good heat transmission. In the case of frequent dry running, it can even store a certain amount of oil.
Modern compression rings - even if e.g. only about one millimeter thick - are made of nitrided or CrSi alloyed steel or ductile graphite iron. The surface can be coated with plasma, chromium nitride or ceramic. Under difficult operating conditions, they are hardened or chromed. As coating materials come into consideration further molybdenum, nickel, titanium, silicon, and ceramic materials.
Modern oil scraper rings are often threepart and coated with chromium nitride PVD. But they can also be made of nitrided steel with a suitable coating. 09/12