The piston ring should glide as smoothly as possible along the cylinder wall. Purely liquid friction is the target. In practice however, mixed friction mostly takes place, even if it glides on an oil film with lubricating pockets. The portion of liquid friction can be achieved if the piston ring (piston) moves at a certain speed. This is why.e.g., the TDC and the BDC are unfavourable for the efficiency of the lubricating film.
At first glance, today's piston rings appear to consist of straight surfaces. There is no bevelling, e.g., as we would imagine in a keystone-ring. Indeed, such a keystone shape may exist, only with much less slant than one would generally expect. There are also slightly arch-shaped examples, even though the piston ring has a height of only one millimeter.
Oddly enough, the narrowing gap is good for the full-lubrication. If then sufficient storage space exists, the depth of which is precisely defined, i.e. if enough lubricant is present, the friction between the piston ring and the cylinder wall can be efficiently avoided. In addition, this is also an advantage for the sealing.
The creating of defined storage space is sometimes called 'honing'. Honing takes place at the end of the cylinder manufacturing process. At the same time it represents a compromise between the roughness, which is necessary for the oil-retention, sufficient carrying structure for the piston- skirt and rings and the low friction which is necessary to comply with the required CO2-emission regulations.
In this context, three procedure steps must still also be mentioned. The honing into shape assumes a knowledge of possible distortion of the cylinder in operation and thus, in the manufacturing process, deviates from the cylinder shape, so that it can be better achieved during operation. Nano-honing, before the laser-honing takes place, provides a chipping treatment, which gives the sprayed particles improved adhesion. Finally, after the laser-honing one further operation is still necessary, one which smoothes and removes the parts which have been melted-off. 09/12