2. ENGINE DEVELOPMENT
During the last 20 years, the development of two-stroke engines has resulted in a large increase in power output per cylinder unit. As illustrated in Figure 1, the mean effective pressure has increased from 13 to 19.5 bar in this period, while the firing pressure has increased alongside up to 150 bar in order to keep, or even to lower, the fuel oil consumption. These developments are driven by the demands from the maritime market requiring ship engines to deliver the same, or more, power with a lower number of cylinders. As a typical example, the pressure rise for an MAN B&W 90MC engine, launched for the first time in 1982, represents an increase in the maximum output per cylinder from 4680 to 6650 BHP, i. e. an increase of 42%. No immediate technical limitations exist that prevent this development from continuing.
Figure 1. Development of Some Key MAN B&W Engine Parameters
When increasing the specific power output, the challenge to the engine designer is to ensure that no increase occurs in the maximum temperatures of the combustion chamber components (piston, liner, rings and cover). As the gas temperatures and pressures in the combustion chamber increase, the cylinder lube oil is directly exposed - in a very small time frame - to more intense stresses compared with previous engine ratings with lower outputs.
This means that the engine designer needs reliable methods to verify component temperatures. The common practice is to measure such temperatures during testbed trials at a given load. Such data from all engines can be readily compared because the new engines are still "clean", the diesel fuel used is very similar in performance all over the world, and the load can be kept steady. When subsequently comparing the component mean temperature of different designs, the engine designer will have been able to ensure that a constant level can be kept.
The engine design department of MAN B&W Diesel has been careful in ensuring to keep main component temperatures constant and at a level where good lube oil performance can be expected. This was achieved by re-design of cooling techniques, new component designs and choosing alternative component materials to overcome local high temperatures. The result of this development process (and standard in the latest engine designs) are the high-topland OROS piston, the piston cleaning ring and the controlled pressure relief (CPR) top ting.
The new OROS piston design has proved to be superior to older piston designs, in regard to gas side surface temperatures, coolant side surface temperatures, mechanical integrity and ease of overhaul. On the high topland OROS pistons, the ring pack is moved further away from the combustion chamber and thus from the high combustion temperature. This reduces the tendency to deposit formation on ring lands and in ring grooves, which need to be kept clean. Furtherore, the maximum temperature of the liner surface actually contacted by the ring pack is lower (Fig. 2).
A piston cleaning ring scrapes off deposits that are continuously being built up on the piston top land and actually prevents the accumulation of such deposits above a certain thickness. Heavy deposits form a risk of wiping off the cylinder oil from the liner and this could initiate scuffing.
Figure 2. MAN B&W's Combustion Chamber Design
Figure 3. Controlled Pressure Relief Top Ring
