Less than a year before the introduction of a global low-sulphur regime, there is still no consensus about the damage compliant fuels may do to engine cylinders
Leading two-stroke engine developers MAN Energy Solutions and WinGD have contrasting views on the impact that low-sulphur fuels, including gas fuels, will have on cylinder condition. According to independent experts in engine condition, the wrong decision could leave shipowners facing emergency cylinder repairs or replacement. Both outcomes would require vessels to be taken off hire.
The confusion focuses on the role of cylinder corrosion in helping a lubricant film to stick to the cylinder liner wall, reducing scuffing damage. According to MAN, a controlled level of wear is desirable, while WinGD is focusing on eliminating wear from its engines.
Fuel with a lower sulphur content is less corrosive, leading MAN to suggest that there will not be enough wear once engines switch permanently to using 0.5% sulphur fuels. Head of marine and offshore sales Kjeld Aabo explained that, as a result, the company had recommended owners should install cermet-coated piston rings if they switch to low-sulphur fuels. The ceramic-metal composite creates a harder surface than conventional cast-iron rings, leading to controlled wear.
“We are facing a large-scale experiment. No one knows what will happen when ships run on low-sulphur oil for two or more months”
In a service letter circulated in March last year, MAN wrote: “As 2020 is approaching, we recommend that introduction of this new ring configuration is done at the next overhaul.”
MAN also suggests that operators increase the basicity of their lubricant “from time to time” to ensure there are no cylinder deposits that might cause scuffing. The company is working with lubricant providers to find a permanent solution, involving oils with low basicity but greater detergency.
Winterthur-based WinGD denies that any wear is needed, or that low-sulphur fuels can lead to cylinder scuffing. It reports that it is already monitoring multiple engines operating long term or permanently on low-sulphur fuel (and LNG) within emission control areas without any issues.
In a statement, WinGD said: “The liner surface structure we specify is a well-proven concept and should not be destroyed by any kind of wear. We measure hardly any wear on our liners in operation. We are still working hard to eliminate corrosion in order to keep the ring and liner surfaces as specified and machined for as long as possible.”
Drawing the guidelines
Although the looming 2020 sulphur cap is bringing the issues of corrosion and cylinder condition into sharp relief, it is an argument that goes back several years. CIMAC’s 2017 guidelines on cold corrosion in marine two-stroke engines, to which both MAN and WinGD contributed, is written in a diplomatic tone that belies the opposing views held by the two companies.
“Different strategies are implemented by the different engine designers to reach a good tribology situation in the combustion chamber,” the document notes. “One strategy [proposed by WinGD] is to have components with pre-defined geometries. Differences in hardness and metallurgy prevent contact and seizures during normal operation. [MAN’s] is a ‘controlled-wear’ strategy, where the tribology situation is controlled by adequate surface roughness and wear-shaped geometries, which enable a good lube oil film to form.”
Cylinder honing and cleaning specialist Chris-Marine believes the impact will fall between the two main developers’ positions, having the worst impact on older engines that are less corrosive than newer models featuring longer stroke and higher firing pressures.
“We are facing a large-scale experiment,” said Chris-Marine chief technology officer Daniel Grunditz. “Until now, most ships have been using low-sulphur fuels for maybe two weeks at a time. No one knows what will happen when they run on low-sulphur oil for two or more months.”
A more cynical observer suggests that fear or the 2020 sulphur cap could be a good way of selling components or cylinder cleaning. He notes that dual-fuelled engines have been running predominantly on LNG for more than three years, with no negative consequences for cylinder condition. This, he says, indicates that the wear provided by high-sulphur fuels is not essential to piston running.
“Bunker providers will want to blend the biggest amount of low-cost, high-sulphur fuel with the smallest amount of high-cost, low-sulphur fuel. Inevitably, some will cross the line”
The lack of consensus will concern shipowners that already face several fuel condition challenges under the sulphur rule due to take effect on 1 January 2020. Condition monitoring specialist Parker Kittiwake has several tests geared towards the fuel challenges that will face shipowners under the low-sulphur regime. According to principal chemist Dr David Atkinson, recent sales suggest that the market is taking a keener interest in these issues as the sulphur cap looms.
The most immediate worry for shipowners is that fuel should comply with the sulphur cap. This will be particularly challenging given the tight line suppliers will be walking between cost and sulphur content. Dr Atkinson explains that bunker providers will want to blend “the biggest possible amount of low-cost, high-sulphur fuel with the smallest amount of high-cost, low-sulphur fuel”. Inevitably, some will cross the line.
For this, the company’s X-ray fluorescence (XRF) spectrometer is useful. Developed over the past 18 months, the portable device provides analysis of fuel sulphur content at an accuracy that could previously only be attained in the laboratory, within minutes. While Parker Kittiwake sells the device to port state control and enforcement agencies that will monitor compliance, the XRF Analyser will also be useful for shipowners wanting to ensure that they are bunkering fuel in compliance with the new sulphur rule.
Dr Atkinson reports that despite a lot of interest, the company recorded low sales of the XRF Analyser initially. But now, as the low-sulphur regime nears, some of that early interest is turning into solid orders, he says.
Fuel compatibility is another area of concern. Parker Kittiwake has recently seen a spike in ‘comp ovens’ – heaters in which two samples of fuel are warmed and then interpreted via ISO guidelines to give an indication of potential compatibility issues. The ovens, which take around an hour to produce a result, will mean that shipowners can test fuel from a different port or supplier for compatibility while it is quarantined in a separate fuel tank.
Cat fines add further to the dilemma. These are residues of the catalytic cracking that removes sulphur from crude oil. They are present in many of the cheaper, low-sulphur fuel streams – exactly those that blenders are likely to choose to mix with high-sulphur streams in order to comply with the 2020 limits. Dr Atkinson concurs with the warnings of other fuel analysts that the low-sulphur regime is likely to be accompanied by an increase in incidences of cat fines damage to engines.
The challenges of a low-sulphur regime will be intense even before the impact of fuel on cylinder wear and lubrication is considered. The uncertainty around cylinder condition, and the lack of agreement between the two major engine designers, will make that switch even more tense for shipowners’ technical teams.