Piston ring grooves
The top and bottom surfaces in the ring grooves are chrome plated, and in order to increase the lifetime of the ring grooves before reconditioning, the chrome layer on the bottom surface has been increased by 0.2 mm.
Piston cleaning ring (PC-ring)
In some cases, and especially on highly rated large bore engines, a heavy build-up of coke on the piston topland has been experienced in recent years. In some of these cases the consequence has been bore polish (the coke rubs the liner surface and removes the cylinder lube oil from the liner), leading to high wear rates or even scuffing.
To eliminate this risk, investigations of ways to remove the coke layer were carried out, and one of the results was to introduce a ring with a smaller diameter than the liner and with a diameter larger than that of the piston crown, as shown in Fig.6.
Service tests were initiated some years ago, and the hypothesis that 'bore-polish' can be a decisive factor in the deterioration of the cylinder condition was verified. The PC-ring was therefore introduced as standard on large bore engines, and the overall results have been very good.
Cylinder liner
The cylinder liner of the large bore engine types is bore cooled, and the cooling intensity is adjusted to maintain an optimum temperature level, and provide optimum tribological conditions for the cylinder lube oil.
MAN B&W Diesel has for many years used a wave cut liner surface. This was modified only a few years ago to a semi-honed surface to facilitate running-in of the high-loaded engines. The original wave-cut had a depth of approx. 0.02 mm. Experience has shown that a deeper wave-cut is an advantage, so as to increase the lifetime of the oil pockets, and also lead to better running-in. We have consequently introduced the deep wave cut shown in Fig.7.
3.3 Water Mist Catcher
It is well known that water droplets entering the cylinder may have a negative effect on the cylinder condition by washing the lube oil off the surface.
