Fuel treatment and additives companies are advocating a focus on cylinder lubrication to counter concerns over the quality of very low-sulphur fuel oils
According to Rathesan Ravendran, a postdoctoral researcher at Aalborg University in Denmark, changing from heavy fuel oil to low-sulphur alternatives has been shown to cause bore polishing and scuffing problems in cylinders.
Experimental studies have shown that fuel with lower sulphur content has a lower lubricity and therefore a lower scuffing resistance. In some studies, fuels containing sulphur have about 20% higher scuffing resistance than fuels without sulphur. The general belief is that the sulphur in the fuel has a positive effect on cylinder condition, due to build-up of a solid lubricating oil film and by promoting a beneficial mild corrosive wear.
After running in an engine for a short time, a cylinder liner will exhibit a surface profile honed to a very smooth level. The initial wear of normally honed cylinder liners is high, after which the surface is relatively stable. During this second period, the surface contains large and smooth plateaus. These provide a large bearing area and deep valleys which retain cylinder oil and provide cover for wear particles that could otherwise damage the lining. A smooth and stable plateau honed liner offers a substantial improvement in oil consumption, piston ring wear and cylinder liner wear at the ring reversal zone.
In the final phase of the wear rate curve the cylinder liner wear increases dramatically, due to the wear down of the plateaus and valleys on the surface. The surface will not be able to maintain the lubrication oil pockets or pockets for wear particles; this increases the risk of bore polishing and scuffing.
Dr Ravendran explains that sulphur-rich fuels prolong the middle, stable period of wear. The fuels’ lubricating properties will reduce the wear down of the plateaus on the surface, and mild corrosive wear from sulphuric acids produced during combustion will maintain an open cylinder liner material surface. Low-sulphur fuels, on the other hand, could make the period of steady state wear shorter. The lost lubricity of the low-sulphur fuel must be replaced by conventional cylinder lubrication oil to provide a continuous, stable, hydrodynamic oil film between surfaces in relative motion. This reduces friction and prevents wear, thereby preventing seizure of the mating parts. Ensuring a stable hydrodynamic oil film between the surfaces will prevent the wear down of the surface plateaus and thereby increase the period of steady state wear.
To overcome the challenges of operating with fuel with lower sulphur, cylinder lubrication oil must be distributed correctly on the cylinder liner. This has been a priority for Hans Jensen Lubricators. Its SIP spray injection valves, placed at the liner circumference, direct the lubricating oil spray upwards and into the engine’s scavenging air swirl. This distributes lubrication oil evenly at the upper part of the cylinder liner, where the oil film is exposed to potentially damaging high pressures and temperatures.
Ensuring a stable oil film
According to Hans Jensen Lubricators, the company’s SIP valves and Lubtronic lubricator ensure good cylinder liner condition, minimise wear and reduce the risk of bore polishing and scuffing. The HJ Lubtronic lubricator operates with automated stepless stroke adjustment and timed lubrication. Cylinder lubrication oil injection at each piston stroke will refresh the oil film at each piston stroke, which ensures a stable oil film as well as minimising the stress level of the oil film on the cylinder liner.
Given the potential for fuel challenges, Lubrizol Corp decided to run a preliminary study on the properties of very low sulphur fuel oils (VLSFO). The additive supplier sourced 0.5% sulphur test fuels from China, the market where they are currently most widely available, given their use in the Pearl River Delta, Yangtze River Delta and Bohai-rim waters since they became emission control areas in January 2016.
The fuel was tested in a modern two-stroke marine engine with conventional 25 BN and 40 BN cylinder oils. Residual BN in scrape down oil samples taken every 25 hours was monitored and the average residual BN for the test lubricants recorded. The 25 BN oil was depleted to 12.5 BN and the 40 BN oil to 24.2 BN. As enginebuilders advise that residual BN should be above 15 BN to maintain corrosion protection, Lubrizol concluded that 40 BN was the more appropriate choice for cylinder oils to be used with VLSFOs.
Maintaining engine cleanliness is the most important part of cylinder condition, and Lubrizol’s tests also highlighted potential challenges caused by VLSFOs in this area. One of three batches tested showed increased deposit formation, reflected in a lower groove cleanliness rating. This was attributed to a variation in the fuel, as engine running conditions and cylinder oil were the same.
According to Lubrizol’s Harriet Brice: “This shows that variation within the same small sourcing area today can be seen to affect the amount of deposit formed in the engine. Thus, with the almost overnight global expansion of 0.5% sulphur fuels the variation could be even greater, as more atypical blend constituents are used to meet demand. Using a more robust lubricant will help to reduce the impact to the engine of this variability.”
Conventional lubricants may not do the job. Existing commercial 40 BN lubricants were formulated for current residual fuels, not those that those will come on the market from 2020. Nor are conventional cylinder lubricants formulated to address the challenges that these new blends may pose, including varnish and asphaltene deposition. To ensure deposit control at 40 BN across the anticipated range of variability for VLSFOs, Lubrizol recommends reformulation with robust deposit control.
The company tested a product it believes could be suitable. Dispersants are good for piston cleanliness and have been used in heavy-duty diesel engines for several years, but are not common in marine cylinder oils for deposit control. A Lubrizol advanced dispersant known to be effective at addressing piston groove deposits and varnish was used to formulate a 25 BN cylinder oil. This was tested with VLSFO alongside conventionally formulated 25 BN and 40 BN oils in a two-stroke marine engine to see its effect on deposit control. The lubricant containing the advanced dispersant was the best performer.
Ms Brice concludes: “To reduce the impact of VLSFO variability on engine durability, the right lubricant selection will be key. Our research concludes that a 40 BN cylinder lubricant with boosted chemistry will be required to manage the characteristics of the new VLSFOs for use after 2020.”
But sulphur is not the only emissions battle that could require additive intervention. IMO’s Tier III NOx limits – or rather the use of after-treatment to comply with them – may also be a challenge for conventional fuels and lubricants. Established detergents in cylinder oils can bring metal ash that may not be acceptable for after-treatments, such as exhaust gas recirculation.
An ashless soap source can, when formulated as part of a lubricant solution, provide supplementary cleanliness without metal deposits. This allows for aftertreatment compatibility. Additive company Infineum compared its ashless soap chemistry with a fully formulated commercial lubricant using an engine test screener (designed to simulate EGR operations). This highlighted the benefits of the approach in terms of piston groove cleanliness.
There are further challenges afoot, says Infineum’s Marco Corradi: “While the large engine industry is rightly focusing on IMO 2020, we are at the threshold of further changes that may even lead to the decarbonisation of our industry,” he says.
“That may yet be a long way off, but the principles required for the lubricant to enable the solutions will be identical; the lubricant and the additive chemistry within will need to support the changes in hardware, combustion and fuels, driven by change in legislation and customer needs.”
Lubricant suppliers switch focus to fuel economy
Lubricant suppliers are developing products that could help reduce greenhouse gas emissions from shipping
Lubricant suppliers are now developing fuel-economy products that could help mitigate greenhouse gas (GHG) emissions from shipping. According to papers presented at CIMAC World Congress last month, several companies are working on new formulations that could reduce fuel consumption by cutting friction losses.
The development follows similar progress in the automotive market and represents a notable shift in focus for marine lubricant suppliers, which have traditionally focused on engine reliability rather than fuel consumption. New formulations will need to balance those demands.
Total Lubmarine has developed a fuel-economy lubricant for medium-speed, four-stroke engines that aims to reduce fuel consumption – and therefore GHG – by 1%
The company used viscosity and friction modifiers to create two experimental formulations of its Aurelia range, with base numbers of 30 and 40. Engine bench test results validated the products’ fuel economy performance at between 1-3% while confirming that the oils retained other important lubricant factors, including engine cleanliness and wear resistance.
Total Lubmarine senior research engineer Catherine Amblard confirmed that the fuel economy tests will contribute to the development of the company’s future range. The new fuel economy lubricant is under test in the field.
Formulating fuel-economy lubricants for two-stroke engines could be a more challenging task, according to research presented by JXTG Nippon Oil & Energy Corp. The company recorded an improvement of around 3% on fuel efficiency in a Daihatsu 3DK-20 four-stroke engine with a fuel-economy trunk piston oil. But its efforts with fuel-efficient cylinder oil were less successful, with an engine test on a Mitsui-MAN 6S50MC two-stroke engine recording only a 0.5% improvement in fuel consumption (compared to simulation results of around 1.5%).
The company noted that the effect of a friction modifier additive was evident in the four-stroke oil but barely in the two-stroke formulation’s results. It attributed this to differences in engine speed and lubricating conditions.
Elsewhere, in a new paper, ExxonMobil and Chevron Oronite have confirmed their interest in developing fuel-economy lubricants, revealing the results of a collaborative study into the accurate measurement of fuel efficiency performance for engine oils.
The authors of a joint paper concluded: “The results of this work will enable the development of high-performance lubricants to address key economic and sustainability issues, such as costs and emissions, within the marine industry through reduced fuel consumption.”