X-DF engines: a ‘no-brainer’ solution for LNG carriers

One of the bugbears cited by critics of Otto-cycle low-pressure technology is methane slip. Methane – a potent greenhouse gas – is some 28 times more powerful than CO₂ on a 100-year timescale. Researchers at Winterthur Gas & Diesel have been hard at work coming up with a solution to mitigate the engine’s methane slip. At a virtual press conference in June, WinGD unveiled exhaust recycling technology, which it anticipates will reduce methane slip by half while also incrementally improving fuel consumption.

Introduced with its second-generation X-DF 2.0 engine, WinGD’s Intelligent Control Exhaust Recycling (iCER) technology captures the unburnt fuel (methane) before it is ejected through the exhaust, providing the engine a second chance to combust it.

As a result, WinGD is confident its X-DF 2.0 engines will have methane slip levels of about 1.0-1.2 g/kwh, as compared to 2.0-2.5 g/kWh in its first-generation engines. Fuel consumption is also improved by 3% in gas mode and 5% in diesel mode.

“We have reached a level in terms of engine performance and emission that was unthinkable only five years ago,” says Mr Galke. “It was essential to us to offer shipowners a solution for reducing methane slip which didn’t compromise efficiency.”

“With the X-DF 2.0, we have reduced the methane slip by about 50% compared with the previous version,” says Mr Galke. iCER recycles the exhaust gas to reintroduce it into the combustion chamber. “Such gas recirculation then acts as an ‘inert’ gas that slows the combustion, making it more efficient,” he says.

Since the first X-DF engine came into service in 2016, WinGD has gained considerable insight to enable improvements to the engine design. “We pioneered the lean engine combustion applied to low-speed engines,” says Mr Galke. “Today, we can say that we have a solid knowledge of the thermodynamic and the cause-effect of the several parameters that influence the Otto process applied to a low-speed engine.

“The engine performance has been regularly improved until reaching the efficiency of 52%, which is very close to the traditional diesel low-speed engine,” he adds.

Maintenance considerations

Besides capex, when it comes to engines, reliability and lifecycle maintenance are hot-button issues for shipowners. WinGD has made several design improvements to the engine, especially the piston running aspects, to make its reliability comparable to a diesel engine.

Results from the overhaul of the first X-DF-engines in operation have been encouraging.

“The benefits of the low-pressure gas solution: low capex, low emissions”

CSSC Marine Service (CMS), a subsidiary of China State Shipbuilding Corp (CSSC), completed the first major overhaul on two 6X62DF engines on LNG carrier SK Audace while at anchor in Sabine Pass, Texas.

Owned and operated by South Korea’s SK Shipping Co, SK Audace was the first-ever X-DF-powered vessel in operation. Since SK Audace’s delivery in 2017, its X-DF engines have reached 18,000 operating hours, more than 99% of which was in gas operation. The LNG carrier operated on diesel less than 1% during manoeuvring.

During the overhauls, carried out over nine days, no abnormalities were found. The CMS Korea team also observed that the 6X62DF engine parts were in excellent condition compared to engines historically overhauled by CMS that operate on heavy fuel oil (HFO).

Preparing for X-fuels

As shipping transitions towards decarbonisation in the decades ahead, HFO and even fossil LNG will become fuels of the past. Existing LNG-fuelled vessels will need to transition to burning lower carbon-intensive ‘drop-in’ fuels, such as bioLNG or biomethane.

But WinGD does not see these future fuels as a problem for the X-DF technology. “We do not expect any technical or operational issues,” says Mr Galke. “There are already some customers that successfully use biomethane in their X-DF engine without any relevant performance difference compared with fossil methane.”

Adds Mr Galke: “Actually, the use of bioLNG (and synthetic methane from renewable energy) is a scenario that we believe to be one of the most sustainable from an environmental, technical and economical point of view.”

Researchers at WinGD have been conducting testing to validate the application of several alternative fuels – so called ‘X-Fuels’ – examining their injection behaviour and combustibility. “That is then followed with a pilot application to prove the reliability of the fuel before considering introducing it to our engine portfolio,” explains Mr Galke.

Thinking about the future fuel landscape, Mr Galke says: “As an industry, we are still a long way from identifying which fuel or fuel combination will follow or compliment LNG to further pave the way to decarbonisation.”

He concludes: “Our job is not to determine which will be the fuel of the future, but to provide the technology required to ‘digest’ the fuel of choice. That remains our core competence in Winterthur.”