All two-stroke engines are therefore fitted with a water mist catcher but, with the increasing pressures and air amounts, the efficiency has become more difficult to obtain, and significant efforts have been invested in ways of improving this situation.
To find the optimum method of separating the droplets from the scavenge air, MAN B&W Diesel has performed advanced CFD (Computational Fluid Dynamics) calculations to find the trajectories of the droplets as they leave the cooler. An analysis of the influence of different guide vanes on the air flow made it possible, by calculation, to predict that all droplets larger than 0.1 mm would be separated before the air entered the traditional water mist catcher element.
The surprising result during full scale tests was that more than 80% (mass) of the water droplets leaving the air cooler were separated before reaching the traditional water mist catcher, confirming that a large part of the water is present in the form of droplets.
In order to further verify the calculated results, MAN B&W has built a pressurised recirculating wind tunnel in which the separation of droplets can be studied.
The measurements on the test set-up have confirmed our observations from tests on actual engines, with the following conclusions:
・Satisfactory separation of maximum condensate from the cooler cannot be achieved with the WMC element alone.
・The tests have confirmed that our latest design of profile for the water mist catcher has the highest efficiency of the types tested to date.
・The design and location of the drain lines and drain system are of great importance, especially at high air flow energy levels, as already separated water may be re-entrained in the air flow if the detail design is not satisfactory, Fig.8.
3.4 Cylinder Lube Oil
The cylinder lube oil, and the way the oil is fed to the liner, generally play an important role for a good cylinder condition. The development of the new MAN B&W cylinder lubricator is described in Ref. [3].
It goes without saying that the behaviour of the cylinder lube oil also depends on the temperature of liner, piston crown and piston rings. The introduction of the before-mentioned design modifications leads to a wider margin against differences in the properties of the cylinder lube oils, and the lube oil companies have also taken up the challenge and are developing new cylinder oils.
Investigations on many plants in service have indicated differences in the behaviour of the cylinder oils, and that this influences the cylinder condition significantly, as seen in Fig.9, which shows an extract from our comprehensive database.
In many of today's cylinder lube oils, the detergency and the TBN number are closely connected. This has an adverse effect, especially during testbed trials, where the detergency is needed during running-in, and where the TBN additives are not neutralised because of the use of light fuels with low sulphur content. Increased coke deposit is provoked, and most probably accelerated when humidity is high. In service this could happen if there is a change from fuel oil to diesel or gas oil with low sulphur content.
There is no doubt that the maximum permissible sulphur content in the fuel will be reduced in the future, and author's company is working closely together with the oil companies in the development of the cylinder oil for the future.
3.5 Bearings
All the latest MAN B&W two-stroke engines are provided with the thin-shell type of main, crankpin, and crosshead bearings. For the large bore engines, the bearing shells are lined with white metal and for the remaining engine types with Sn-Al. These bearings have generally operated trouble-free.
