F

Figure 10.16 New Oros-shaped combustion chamber geometry (right) has contributed to reduced temperature measurements

Upper piston ring with double-lap S-seal and Controlled Pressure Relief (CPR) gaps Even heat distribution on 2nd piston ring

2nd, 3rd and 4th piston rings with oblique cut ring gaps New piston ring material: RVK-C for 50-26 and RVK with plasma coating on 98-60

Upper piston ring with double-lap S-seal and Controlled Pressure Relief (CPR) gaps Even heat distribution on 2nd piston ring

2nd, 3rd and 4th piston rings with oblique cut ring gaps New piston ring material: RVK-C for 50-26 and RVK with plasma coating on 98-60

Figure 10.17 Configuration of Controlled Pressure Relief (CPR) top piston ring surfaces during the running-in period. Alu-coated rings make it possible to load up a completely new engine on the testbed within only five hours, fostering time and fuel savings; running-in can also be performed with reduced cylinder lubrication. The lifetime of the coating is 10002000 hours, depending on such factors as the cylinder oil feed rate and surface roughness of the liner. The technical explanation is that no scuffing occurs with the combination of aluminium bronze/grey cast iron as the wear components in the cylinder. Alu-coated rings allow the normal increase in cylinder oil feed rate after changing rings and/or liners to be dispensed with.

• Piston ring groove. In order to extend the interval before reconditioning of the piston ring grooves is required the chromium layer has been increased to 0.5 mm and induction hardening of the grooves before chrome plating introduced. The chromium layer is thus better supported by the base material and the risk of cracking of the brittle chrome reduced, fostering a longer service life.

• Piston cleaning (PC) ring. Incorporated in the top of the cylinder liner, the PC-ring has a slightly smaller inner diameter than the liner and hence scrapes off ash and carbon deposits built up on the piston topland (Figure 10.18). Without such a ring, contact between the topland and the liner wall could wipe off the injected cylinder oil, preventing the lubricant from performing its optimized role. In some cases, deposit formation on the topland could cause bore polishing of the liner wall, contributing to deterioration of the cylinder condition. Introducing the PC-ring

Lacquering Condition For Cylinder Liner

eliminates contact between deposits on the topland and the liner, promoting an enhanced cylinder condition and lube oil performance.

Cylinder liner

The liners of the large bore engines are bore cooled; the cooling intensity is adjusted to maintain an optimum temperature level and to ensure optimum tribological conditions for the cylinder lube oil. For years MAN B&W Diesel used a wave-cut liner surface, which was modified a few years ago to a semi-honed surface to facilitate running-in of the highly-loaded engines. The original wave-cut had a depth of approximately 0.02 mm. Experience showed that a deeper cut is advantageous in increasing the lifetime of the oil pockets, which, together with the Alu-coated piston rings, leads to very safe running-in. An improved semi-honed wave-cut liner surface was therefore introduced.

Fuel valve

The fuel valve design was changed a few years ago from a conventional type to the mini-sac type. The aim was to reduce the sac volume in the fuel nozzle and curb dripping, thus improving combustion; introducing the mini-sac valves reduced the sac volume to approximately one-

third of the original (see the chapter on Fuel Injection). To improve combustion even further, however, a new slide-type valve was introduced for all large bore engines, completely eliminating the sac volume (see the chapter on Exhaust Emissions and Control). A significant improvement in combustion is accompanied by reduced NOx, smoke and particulate emissions. The reduced particulates also improve the cylinder condition, and the wear rates of cylinder liner, piston rings and ring grooves are also generally lower with slide-type fuel valves.

Slide-type valves were introduced on the K98MC engine from the beginning, their positive effect confirmed on the testbed. With fuel nozzles optimized with respect to heat distribution and specific fuel consumption, the NOx emission values associated with this engine are described as very satisfactory.

Electronic high pressure cylinder lubricator

Cutting cylinder lube oil consumption—which represents a significant annual cost, especially for large bore engines—has been an important R&D target, resulting in the development of a computer-controlled high pressure cylinder lubricator. Development of the electronic Alpha Lubricator started in 1997 and the prototype was installed on a 7S35MC engine in the following year; refinements were carried out on MAN B&W's 4T50MX research engine.

System flexibility makes it possible to choose any number of engine revolutions (four, five, six, etc) between injections of a specific amount of oil into the cylinder. For example, lubrication can be effected every fifth revolution in the compression stroke and every tenth in the expansion stroke if that turns out to be the optimal. The aim is to inject the cylinder oil exactly where and when it is needed: in the piston ring pack as it passes the lube oil quills. Thanks to the high pressure, it is possible to establish an injection period that starts just when the uppermost piston ring is passing the quills and ends exactly when the lowermost ring is passing. Furthermore, injection is in the tangential direction, ensuring optimal distribution of the oil in the complete ring pack and ring grooves.

The Alpha Lubricator features a small piston for each lubrication quill in the cylinder liner. Power for injecting the oil is derived from the system pressure, supplied by a pump station. A conventional common rail system is used on the driving side and a high pressure positive displacement system on the injection side. Equal amounts of oil are supplied to each quill and the highest possible safety margin against clogging of individual quills is secured. The basic oil feed rate can be set by a screw which limits the stroke of the main lubricator piston.

All MC and MC-C engines (new and in service) can benefit from the system; large bore engines are provided with two Alpha Lubricators per cylinder and small bore engines with one. The actual savings from the reduced cylinder oil feed rates achieved depend on the engine size but MAN B&W Diesel has suggested 20 per cent-plus cost reductions as possible. Further reductions are promised by applying the 'Sulphur Handle' to dose oil in proportion to the amount of sulphur entering the cylinder in the fuel; this Alpha ACC (Adaptive Cylinder oil Control) is now standard on Alpha Lubricators. (See the chapter on Fuels and Lubes for more details of the system.)

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