In previous cylinder processing, honing was the last processing step. Rotating, up-and-down moving brushes with honing stones at the tips roughen the cylinder surface. The resulting cross slots are quite pronounced and ensure that enough oil remains on the wall for good lubrication and low friction as the piston descends.

Traditionally, the spring preload of the oil scraper rings is adjusted as a compromise between sufficient lubrication and not too high oil consumption. Incidentally, it tends to be higher with gray cast iron than with coated aluminum cylinders. However, for this form of honing, the rougher the surface, the longer the running-in period.

Soft materials do not slide well on each other. As long as only the pistons were made from aluminum alloy, there were no problems in cast iron cylinder blocks once a solution was found to the differential thermal expansion.

Ever since cylinder blocks have been made of AlSi alloy to save weight, gray cast iron liners have often been build in. However, two materials with very different thermal expansion properties come together here. That creates tension. It is much better to etch away the aluminum or to coat the piston/cylinder.

One of the methods for this is plasma coating. The word 'plasma' appears as early as 1920 as a designation for a fourth state of aggregation. The can only be achieved at unbelievably high temperatures. In this process, the proportion of electrons in atoms changes. An electrically charged gas is produced. With enough energy, this is possible with almost all substances.

The plasma gas flowing into the water-cooled mixing nozzle is drawn to the anode with high energy and exits there. An arc is formed with more than 10,000°C, which entrains the coating powder (alloyed steel and molybdenum) and conveying gas from an additional supply line and allows it to hit the cylinder wall still plastically at very high speed.

Ceramic particles are also possible instead of ferrous material. After solidification, a layer of less than 1/100 millimeter is positively connected to the starting material. In the meantime, the torch with the supply pipe has long since turned further. The entire cylinder is coated in no time at all and, unlike with a dry liner, has hardly become any heavier afterwards.

With plasma-coated cylinders, the preload on the piston rings can be reduced, which also benefits internal engine friction during the running-in phase. During the final honing, only small, non-contiguous gaps are created that store oil. Overall, the surface is less rough and there is hardly any mixed friction during the running-in period.

A method that can only be used after honing is UV photon honing. Here, the surface melts at a depth of 2 microns, nitrogen for hardening penetrates and a structure is formed in which can collect oil through more and smaller gaps and is safe from oil scraping. Consequence: Oil consumption, wear and tear and friction are reduced, and the exhaust gas is less polluted.

Instead of honing, a gray cast iron cylinder is processed with laser light in the running area of the piston rings. The laser pulses specifically vaporize material and also create a plasma. Nitrogen is added and a wear-resistant layer with precisely defined roughness is created.

Alternative processes to classic honing have long been used, because the very rough surface with its peaks ensures mixed friction with the piston rings and a longer running-in period. Also, more small, unconnected micro- pressure chambers would be better for the engine oil, because then it would always stay in the same place.

A precisely structured surface would originate, which might offer less but much better distributed lubricant. There would almost only be liquid friction with corresponding advantages in terms of service life, consumption and emissions. This structure, which varies according to the strain, is created, for example, by honing with a laser. Of course, this technique of micro-pressure chambers can be used on many plain bearings, e.g. on crankshafts and/or piston pins.