Environmental restrictions on fuel emissions from ships under The International Maritime Organization’s Tier II and Tier III revisions to Marpol Annex VI demand new thinking on key engine components to deal with operations at higher pressures and temperatures. Specialised Swedish supplier Daros Piston Rings says that, as well as concentrating on component geometry and materials quality, it has been testing new types of resilient coatings as a means of prolonging piston ring life.
Tier II limits, due to come into force in 2011, roughly demand a 20% cut in NOx emissions from shipboard engines. The more exacting Tier III limits, set to enter into force in 2016, mean that NOx emissions will not be permitted to exceed 20% of the level found acceptable today
Engine makers intend to meet Tier III requirements through a fundamental advance in the concept of combustion, optimising injection time and incorporating multiple-mode fuel injection. At the same time, reducing the amount of particulate matter emitted will rely on improving electronically-controlled common rail high pressure injection systems, using short injection time, micro fuel atomization via high pressure injection and improved combustion during the final stage via multi-stage injection.
All of these steps have consequences for components, such as tightened manufacturing tolerances, and require new materials and coatings for emissions-critical engine parts, including turbochargers, pistons, piston rings, cylinder liners and fuel injectors.
Two-stroke engines emit more particulate matter than their four stroke counterparts because piston ring lubrication film tends to burn when passing over the exhaust ports in the combustion zone of the cylinder. Technology developed to reduce NOx can actually increase PM emissions, and vice-versa.
As engine sizes have increased, and in the run up to Tier II regulation, engine manufacturers have made piston ring coatings a central part of their research and development work. However, meeting Tier III has required further research into the coatings used. According to Daros Piston Rings, it has been a severe challenge to find the ideal coating material that will not cause excessive wear on the liner but at the same time is resistant to normal wear, scuffing, heat and corrosion. Tests are underway on board operational vessels, with coated piston rings working in two-stroke engines from leading suppliers MAN Diesel and Wärtsilä Corp. The company expects to release full results on the preferred coatings by the end of 2009.