3.2 Combustion Chamber Design
Particularly for the large bore diesel engine design, the combustion chamber requires special attention, and the continuous development has enabled us to introduce a number of new features:
Piston crown with high topland
The high topland, or low-positioned piston rings, is a feature which has been found to have a very positive influence on the cylinder condition, probably because of the larger air buffer volume trapped between the topland and the cylinder liner running surface, and the positioning of the rings in a colder region of the piston and cylinder liner, see Fig.6. This results in reduced heat load on the rings and improved working conditions for the cylinder lube oil, and thus better performance of the ring pack.
Tests with high-topland pistons started about five years ago on an S80MC engine, and demonstrated a significant improvement in the wear condition and increased times between overhauls. This design was subsequently introduced as standard on all engines and since then a large number of high-topland pistons in service have confirmed the test results.
Piston crown with "Oros" design
A challenge in the development of the combustion chamber is to avoid excessive heat load on both the cylinder cover and piston crown to eliminate the risk of burning. The short-stroke large bore engines have a narrow combustion chamber due to the small compression volume. A narrow combustion space means limited space for the fuel oil sprays and thus limited distances to the walls, The component temperatures can be reduced by applying a sophisticated cooling system, but a better solution is to reduce the heat load, which has been obtained by providing more space around the fuel sprays, and a new piston crown design, as seen in Fig.6 right side, with the nickname "Oros" (Greek for small mountain) was introduced.
This new combustion chamber design is actually the first with a scientific background. Since the early days of uniflow-scavenged two-stroke marine engines, the combustion chambers have been based on the idea of an "ideal spherical chamber", but the calculation tools to confirm the validity of that claim were not available. However, our new combustion chamber is designed on the basis of comprehensive CFD calculations of air and fuel spray motion in the chamber.
The calculation indicated that by changing the shape from the previous "spherical" design to the new design, a substantial reduction in heat load could be obtained - a drop in piston crown (gas side) temperature of 70℃ was predicted.
Comprehensive development tests on K90MC engines confirmed this - actually we obtained around 100℃ lower piston crown temperature by further optimising the fuel nozzle spray pattern.
Piston rings
The present standard for piston rings is one high CPR (Controlled Pressure Relief) ring and three oblique-cut lower rings. Originally, the CPR ring was plasma-coated, but the introduction of a high topland and the PC-ring (Piston Cleaning) as mentioned later, made it possible to apply a soft coating to all rings.
The author's company has applied for a patent for a running-in coating which is either a soft nickel-graphite coating or an aluminium-bronze coating applied to the running surface of the piston rings. The soft coating ensures low friction, good adaptability and, at the same time, good wear resistance, which allows the breaking-in of the liner running surface to be speeded up. A good gas seal between rings and liners, formed earlier than usual, is probably the reason why the running-in time can be reduced with no adverse effects.
When 'soft coated' piston rings are fitted in connection with a normal in-service overhaul, no special load reduction for running-in is required. The gas seal is ensured from the very beginning by the soft coating, and the coating prevents the tendency towards micro seizure. The crew therefore does not need to take special precautions.
