Finished Lubricants

A New Course for Marine Cylinder Oils


Demand for fuel efficient, low emissions marine engines will increase significantly over the next two decades, according to ExxonMobils Energy Outlook for 2016-2040. With performance demands for marine cylinder oil closely tied to the type of fuel these engines burn, such changes will have a significant impact on the marine lubricants market.
Current and upcoming global emissions standards are encouraging changes to vessel and engine design. Ships that use steaming drive will decrease dramatically in the 2030s and become almost nonexistent in the 2040s, the report predicts. Natural gas demand could account for close to 10 percent of total marine fuels, up from less than 1 percent now. The industry will also see increasing use of low-sulfur diesel fuels in place of fuel oil.
The International Maritime Organ­ization has restricted air pollution from ships since 1997. Updated limits on nitrogen oxide emissions have been in place since January 2000 and sulfur oxide emissions since July 2010, for marine diesel engines with over 130 kilowatts of output power. In emission control areas, which include waters within 20 nautical miles of the continental shelf of the United States as well as the Baltic and North Seas in Europe, sulfur content in fuels has been limited to 0.1 percent mass/mass since January of last year. Outside those areas, sulfur is currently limited to 3.5 percent m/m.
At an October meeting of its Marine Environment Protection Committee, the IMO adopted mandatory reporting requirements for ships of 5,000 gross tonnage or higher. These vessels account for 85 percent of carbon dioxide emissions from international shipping, the IMO stated in a press briefing, and must now collect consumption data for each type of fuel they use. The information will provide a framework for future measures to reduce air pollution. The data collection system will equip IMO with concrete data to help it make the right decisions, the organizations secretary-general Kitack Lim said in the release.
Also during the October MEPC meeting, the IMO confirmed it would require ship operators to apply scrubbers to neutralize nitrogen oxide gases and would further clamp down on sulfur content in fuels outside emission control areas. Both regulations will go into effect on Jan. 1, 2020, but could be deferred to 2025, depending on the outcome of a review that should be complete by 2018.
An NOx scrubber is a very expensive filtering unit, but operators only have to use it if they sail in the U.S. after 2020 and in the European Union after 2025, Jesper M. Pedersen, superintendent engineer for diesel and turbo at Man, told the ACI European Base Oils and Lubricants conference. The regulation first targets a 0.5 percent sulfur cap, but that could go down to 0.1 percent somewhere in the late 2020s when all world seas will be regulated for low emissions.
Oil and Water
German engine builder Man makes marine diesel engines in several versions-four-stroke engines range in power from 450 kilowatts to 21,600 kw, while two-stroke engines range from 3,200 kw to 82,400 kw. The OEMs least powerful engine is the size of four Volkswagen Golf TDI models, while the largest is the size of 747 such cars.
Two-stroke crosshead marine engines that run on distillate fuels such as diesel and gasoline, or fuel oil, provide the largest percentage of propulsive power for ships globally, reported Pedersen, who is based in Copenhagen, Denmark. Industry insiders say that a major portion of marine lubricant consumption is cylinder oil, representing up to 60 percent of a ships technical budget.
A typical cylinder oil is SAE 50 viscosity and comprises base oil blended with a few additives, including surface active compounds like detergents and dispersants, and calcium carbonate, an alkaline metal compound also called overbased detergent. Alkalinity in cylinder oils is measured as base number (BN). Higher BN oils contain more calcium carbonate molecules.
In general, the required BN for a cylinder oil is tied to the type of fuel a vessel is burning. Low-sulfur fuel operators must use low-BN cylinder oil, while high-sulfuric fuels must use high-BN cylinder oils, said Pedersen during the September gathering in Warsaw, Poland. Acceptable base number ranges are wider now than they have been in the past, and will likely extend further, he projected. Present BNs range from 16 to 100, whereas in the future it should go to up to 140 BN.
Man believes that lower BN oils will be the main industry target in four years time. What we are looking for is BN 40 or BN 30 cylinder oils for more fuel efficient engines. More changes are expected, and changes are good, Pedersen stated.
The purpose of cylinder oil is to lubricate the compression stroke, as well as provide protection between the cylinder liner and piston rings. The lubricant is injected into the cylinder every 1 to 20 revolutions, depending on the basic feed rate and engine load, according to Man. Some is burned with the fuel, and the rest is scraped into storage for later disposal.
During combustion, sulfur oxides combine with condensation to form sulfuric acid. When an acid droplet enters into the oil film, surfactants stabilize the droplet. When that droplet collides with the overbased detergent, neutralization starts. Once neutralized, the surfactants keep the product afloat in the oil, Pedersen explained. When the cylinder oil is depleted and the base is no longer able to contain the acid, the acid begins to attack the surface of the cylinder liner.
Loss of BN in cylinder oil can also lead to deposits on the piston rings, which begin to move sluggishly and eventually collapse. It means engine failure, and we dont want that, Pedersen reminded. We aim for these engines to operate five years continuously, or 30,000 running hours between docking and pulling the pistons for servicing.
Detergent levels must be kept in balance. In addition to the threat of cylinder liner wear, because detergents themselves are slightly basic, cylinder oils cannot be depleted to zero BN without degrading the lubricants performance. However, BN that is too high can cause bore polish and deposit buildup. A certain BN safe margin has to be kept, Pedersen observed, in order to keep the neutralized droplets of acid in suspension so they can be scraped down to the drains. This flushing decreases the risk of deposit buildup on the piston crown.
To address this challenge, Man will soon begin installing automated cylinder oil mixing units on its engines, which mix high BN and low BN oils. The units will ensure that ships are operating with optimal BN cylinder oils for all fuels and loads. It is not blending per se, but a mix of two types of oils that should be compatible to each other and have equal amounts of detergents and dispersants that can make a 50-50 or 20-80 split, or whatever combination is needed, Pedersen elaborated. The advantages of using such a mixing unit is that it requires storing only two oils and it produces a constant viscosity and level of detergent for any fuel.
The other lubricant used in marine diesel engines is system oil. Typically, we have SAE 30 viscosity system oil, which stands in the crankcase and is intended to stay there for the vessels lifetime, at least 20 or 30 years, explained Pedersen.
Man doesnt recommend changing the system oil unless there is a problem with the engine. It could be contamination, or it could be interaction with higher viscosity cylinder oil impacting the system oil. In those cases, it should either be topped up with very thin oil, or be replaced, Pedersen instructed. The sheer volume of system oils would prevent ship owners from changing them very often, he added. For the initial fill, we are talking about 10,000 liters of oil just to fill up.
On the Horizon
In the ongoing effort to reduce shipping emissions, the IMO mandates vessels constructed from this month forward must meet total weighted cycle emission limits for nitrogen oxide.
Modern engine designers have to find ways to restrict NOx emissions without increasing fuel consumption, Pedersen pointed out. The moment that Tier II regulations were enforced, we had to reduce emissions while at the same time the engines would have to operate in higher pressures. He continued, We have temperatures of up to 280 degrees Celsius and extremely high pressures of up to 170 bars, in addition to a corrosive environment.
These increasingly severe operating conditions also present challenges for marine lubricant formulators. Future oils will need to prevent abrasive wear in the piston ring, ring grooves and the cylinder liner, as well as corrosive wear in the cylinder liner and surface fatigue, cavitation and erosion wear, Pedersen concluded.