Experience with CKS 36 in the field so far shows that run-ning times of at least 20,000 hours are achievable in heavy fuel operation and 40,000 hours with gas oil.
3.2.3 GDC 50
To fullfill the demands of higher scuff resistance in very high loaded or critical engines a further development of the CKS 36 plating leaded to diamant particles instead of the aluminiumoxide particles. Tests have shown higher scuff resistance in combina-tion with a lower ring wear and similiar liner wear compared to the CKS 36 plating.
Fig. 19 MKP 81 A Plasma Coating
3.3 Thermal Sprayed Coatings
3.3.1 Plasma Coatings
Although the CKS 36 coating can satisfy virtually all dura-bility and reliability requirements, thermal sprayed coatings will continue to be used in some engines in order to meet the tough scuffing resistance required. The MKP 81 A coating shown in (Fig 19) is used in several series production engines; the powder is composed of molybdenum and hard material, and the coating has very high wear and breakout resistance in medium speed engines. The chemical composition is:
Mo:67-77% Ni: 16-24%
Cr: 3-7% C:0.5-2.0%
Fe, Co, O and N together: 5% maximum
3.3.2 HVOF coatings
A further refinement, especially in terms of inherent wear resistance are HVOF coatings which are undergoing basic testing in various engines on field trial in marine engines ameter. The wear results measured so far are very good, so further applica-tions of the coating are expected.
4. OIL CONSUMPTION AND RING DYNAMICS
The oil consumption in todays engines is formed out of the cooperation between the antipolishring, the piston and the pis-ton ring pack. The guideline values depend on fuel type as fol-lows:
0.4 g/kWh for gas oil/distillate fuel
0.6 g/kWh for heavy fuel operation
<0.3 g/kWh for gas engines
It must be self-evident that a larger amount of oil can act positively on the wear rates of the ring and cylinder and provide a greater safety margin against scuffing.
Another important factor influencing oil consumption is the correct harmonization of the whole pack of compression rings. It is essential that the build up of gas pressure between the rings should be such as to ensure stable contact conditions, particu-larly for the top ring.
The measurement of inter-ring gas pressure is now performed more regularly to ascertain that the pressure between the top and second ring is not significantly greater than the gas pressure in the cylinder, ie above the top ring. This is necessary in order to ensure that the balance of forces will not cause the ring to become dynamically unstable and disturb the entire sealing system.
The attainment of ring stability may require for the top ring a very small closed gap, sharp edges at the gap ends and on the bottom edge of the running surface and for the second ring an enlarged gap clearance. Also modifications on the pistons to in-crease the inter-ring volume can become necessary. Only a dy-namically stable ring pack can perform a satisfactory sealing function against gas and oil; indeed, the knowledge of ring dy-namics has also contributed to a reduction in the number of compression rings per piston. An important factor is also the axial and radial clearance of the rings. (Fig 20) shows some common figures related to the diameter.
