The industry is bracing itself for what many consider to be the inevitable compatibility problems associated with low-sulphur fuels, including those with the same ISO grading
Imminent emissions regulations are opening up a whole range of possibilities in cylinder lubrication that will lead, among other benefits, to longer periods between overhauls, according to engine-builders Winterthur Gas & Diesel. However, WinGD’s experts also foresee compatibility problems between low-sulphur fuels.
“Moving away from burning high-sulphur fuels in marine propulsion engines yields an opportunity to develop novel technologies for the combustion chamber and the design of lubricating systems,” says WinGD’s director of research and development Konrad Räss.
One such benefit involves improved corrosion resistance. Already the Switzerland-based group is working on new piston-related technologies that boost lubricity. “This applies in particular – but is not limited to – new piston ring base materials, coatings to achieve higher durability and wear resistance as well as piston cooling, piston skirt centering techniques, a cylinder liner cooling system for flexible temperature control and optimised lubricant supply systems,” explained Mr Räss in a study delivered at the Cimac conference in Vancouver in June.
Entitled Impact of sulphur cap 2020 on two-stroke engine tribology aspects, the study also forecasts some challenging and even alarming developments in the fuels themselves, when the low-sulphur emissions regulations come into force from January next year. As well as different kinds of low-emission fuels coming to market, WinGD expects a divergence in the distribution of heavy fuel oils: “[There will be] a split between IMO-compliant very low-sulphur fuel oil (VLSFO) ranging between 0.10-0.50% sulphur content and high-sulphur fuel oil (HSFO) for scrubber operation, with a sulphur content of up to 3.50% or higher.”
Early on after the introduction of the sulphur cap, VLSFO will be the fuel of choice, according WinGD’s research. “Only a small percentage of vessels will be equipped with scrubbers and therefore able to operate on HSFO,” Mr Räss predicts. And that may pose compatibility problems for VLSFO-users because the fuel may contain more paraffinic fractions compared to today’s residual fuels. “Different levels of paraffinic and aromatic fractions with fuels of the same ISO 8217:2017 grade could lead to incompatibility between different fuel batches,” he warns. “This problem will demand a higher level of fuel preparation and awareness about fuel compatibility. Additionally, due to different compositions, the fuels might have very different parameters – for example, viscosity – within the same ISO grade.”
But there is a silver lining; with many years’ experience in low- and high-sulphur fuels, WinGD believes the imminent regulations represent a window of opportunity that will lead to new technologies that should deliver significant bottom-line gains in terms of capital and operating expenditure for its two-stroke engines. One promising solution is the two piston-ring concept, recently introduced on the WinGD X52 engine in place of the long-established three-ring configuration.
“Due to the reduced number of piston rings and corresponding ring grooves, life-cycle cost savings of the two-piston ring concept in terms of capital operating expenditure [provide] a reduction in maintenance costs for the remanufacturing of piston heads,” Mr Räss reports. “Furthermore, the amount of lubricant loss is reduced due to the piston ring pack optimisation. The trouble-free operation of the first X52 and X52F engines confirms this optimised tribology concept and the success of the development strategy of WinGD.” [Tribology being the science and engineering of interactive surfaces in relative motion, encompassing the study and application of the principles of friction, lubrication and wear.]
In further extensive tests with these new piston rings that are designed to accumulate lubricating oil – a technology known as ‘lubrication cell design’ – the result was an impressive total average reduction in lubricating oil in exhaust gases of around 36% at an engine load of 75%.
In another advance linked to emissions regulations, WinGD has worked on making its engines more multi-functional in fuelling terms. “While the development of the X-DF engines was progressing, it became apparent that, depending on the fuel in use, different and even contradictory functions needed to be fulfilled by the cylinder cooling system,” says Mr Räss.
This was because when the engine is in fuel mode, sulphur-containing fuels require a high temperature in the liner wall to prevent cold corrosion, whereas in gas fuel mode the pre-mixed combustion regime yields best performance when the surfaces of the combustion chamber are maintained at a lower temperature. The solution, which will be introduced on the X92DF engines, is a newly designed cylinder liner cooling system with a modified recirculation design.
Nothing if not versatile, the system can deliver coolant temperatures of up to 130°C, which is suitable for the liner wall when burning sulphur-containing fuels, but falling to as low as 55°C for LNG or other non-sulphur fuels. The latter is achieved with a single constant-flow coolant stream.
The new emissions regulations will also lead to a redefinition of the base number (BN) that historically describes cylinder lubricants in terms of acid neutralisation capacity, predicts Mr Räss. “Due to the Marpol regulations the base number of cylinder lubricant oils in use will become bimodal, split between very low BN and high BN lubricant oils.” He foresees the low BN numbers being used for globally complaint 0.50% fuels, for Emission Control Area-compliant 0.10% fuels and for LNG and gas operation, while the high numbers will mainly be reserved for scrubber-equipped engines using high-sulphur fuels up to 3.50%.
Simultaneously, cylinder oils will have to be reconfigured for the new era to prevent and remove deposits, says Mr Räss. “The most important properties for post-2020 lubricants, such as detergency and dispersancy, need to be adjusted for the requirements of these novel, low-sulphur residual fuels,” he says. “Previously, due to the chemical nature of the added alkaline components, detergency has correlated well with BN numbers. However, this may not [in future] always be the case, as certain oils may require additional detergency in the absence of the acid neutralisation components.”
In another post-2020 response that also promises to yield useful bottom-line benefits in engine design, the Swiss manufacturer has been working with Japan’s IHI Corporation and others on a project to increase the compression ratio in two-stroke engines in a way that reduces greenhouse gas emissions. It is all about improving thermal efficiency, explains IHI director Yutaka Masuda, in a paper entitled Key device technologies to realise the unprecedented variable compression ratio system of a two-stroke engine.
The secret, he told delegates at Cimac, is a new technology known as ‘variable compression ratio’ (VCR), developed for crosshead-type two-stroke engines by IHI and Diesel United, working with WinGD engines. The heart of the system is a special hydraulic cylinder that is installed between the piston rod and the crosshead pin which serves to change the projection length of the piston rod. The result obtained by VCR was maximum hydraulic pressure of 100 mega pascals (mpa, a measure of pressure), necessitating the design of tougher new seal rings with a previously unmatched durability of four years: “That is equivalent to a dock-to-dock maintenance interval,” says Mr Masuda.
The technology remains under development with WinGD, the licensor of crosshead-type two-strokes, but preliminary results indicate better compression and reduced emissions. Exactly what shipowners and the IMO want.