Low NOx Vasa

The Vasa 32 engine in 1994 became the first in the Wartsila Diesel programme to be released with low NOx combustion technology, a key measure calling for the compression ratio to be increased from 12 to 14:1 to secure a sufficiently high compression temperature. The smaller combustion space dictated reshaping of the piston crown and the cylinder cover flame plate to allow for the fuel jets and good air/ fuel mixing.

A new piston was developed for the higher maximum firing pressure (raised by 10 bar to 165 bar). Efficient cooling of the alloyed steel crown of the composite piston was secured by applying the cocktail shaker principle. The piston pin diameter was increased by 10 per cent to match the higher cylinder pressure. The three-ring concept

(as opposed to the original four-ring set) was also adopted following good experience on the Wärtsilä 46 engine, the two compression rings benefiting from a special wear-resistant chrome ceramic coating (Figure 27.5).

Figure 27.5 Composite piston of Vasa 32 Low NOx engine designed for increased combustion pressure and low friction

The stiff bore-cooled cylinder liner was equipped with an anti-polishing ring to keep carbon and ash build-up on the piston top land thin enough to prevent contact between the liner inner wall and the deposits, regardless of the piston's position. The liner lifetime is reportedly doubled and the lubricating oil consumption halved to 0.6 g/kWh or less.

The increased cylinder pressure also required the connecting rod to be changed from the original diagonally-stepped two-piece component to a fully machined three-piece design with a horizontally split big end bearing. The change yields greater safety at increased load and eliminates the need to interfere with the big end bearing assembly when overhauling the piston; a disadvantage is the increased number of parts. The load-carrying capability of the big end bearing was raised by switching from an SnSb or SnAl to a BiAl material type.

Only a minor modification to the cylinder head was necessary: a slight reshaping of the flame plate to provide more space for the fuel jets.

The fuel injection system was redesigned to handle demands for an increased injection rate and improved atomization. As a result, the fuel cam is faster and provides higher lift to compensate for the reduced plunger bore/stroke ratio. The injection valve opening pressure was raised from 350 to 600 bar, mainly to improve atomization at the start and stop of injection, and to maintain the higher injection pressure needed for a shorter injection period and reduced ignition delay. Improved part- and low-load performance was apparent, along with reduced NOx formation and fuel consumption. Test results showed it was possible to reach an NOx level of 11 g/kWh, a 30 per cent reduction compared with a standard Vasa 32 engine (IMO's limit is 12 g/kWh NOx at 720/750 rev/min). At the same time, the specific fuel consumption was lowered from 187 to 180 g/kWh while maintaining a reasonable combustion pressure of 165 bar, only 10 bar higher than that of a standard Vasa 32 engine.

Wartsila Diesel also developed a planetary gear with controls to replace the standard intermediate gear of the camshaft transmission. The arrangement allows a retarded fuel injection mode to be temporarily engaged while the engine is in operation and meet modest local NOx emission limits (up to 30 per cent reduction on normal emission levels at the expense of a rise in fuel consumption).

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