Maritime engine technology company WinGD of Winterthur, Switzerland, has launched a new design for a dual fuel (diesel / gas) engine, X-DF 2.0.
The engine design shows improvements in the two most important environmental aspects of engines. The methane slip [methane which is unburned in the engine and so exits into the atmosphere via the exhaust gas flow], and overall control of the combustion process, which is the pathway to improved fuel efficiency.
The methane slip is improved using the “intelligent Control by Exhaust Recycling” (iCER) technology – basically taking up to half of the exhaust gas and putting it back through the engine a second time.
It means that the overall methane emission released into the atmosphere is halved.
The engine has also been improved to gain better control over parameters like fuel admission and ignition timing, giving scope to improve efficiency in many ways.
These benefits can add up to a 3 per cent efficiency improvement in gas mode and 5 per cent in diesel mode, the company says.
The iCER is available for all X-DF engines as part of the new X-DF 2.0 design upgrade.
The X-DF engines counted over 550,000 hours operating time as of June 2020.
The company claims it has the “bestselling dual-fuel low speed engine in the maritime market since the second half of 2017” – with a 60per cent market share today in the whole DF market, based on its own data. There are currently 60 engines in operation, and more than 260 on order.
Its first low speed dual fuel engine was type approved in 2015. In May 2020, the biggest engine in the portfolio, X92DF, for a large container vessel, was type approved. The smaller sister engine the X72DF based on the same concept is now “WinGD’s best seller and the standard choice for LNG carriers”, says Volkmar Galke, global director, sales.
The engine is “best in class” for the lowest CO2 emissions, lowest toxic emissions (NOx)as well as best in class on particle emissions, and best methane slip compared to other Otto cycle engines, says Dominik Schneiter, VP research and development with WinGD.
Many shipping companies are expecting further regulatory requirements or incentives around fuel efficiency, reducing methane slip or limiting black carbon, he says.
WinGD does not manufacture the engines itself, but licenses the designs to manufacturing companies, including in China, South Korea and Japan.
Since 2016 the company has been 100 per cent owned by China State Shipbuilding Corporation (CSSC). It was formerly the low speed, two stroke engine division of Wärtsilä but spun out as a separate company in 2015.
The company claims to have 90 per cent market share of engines for vessels in the LNG carrier sector, 100 per cent of all LNG vessels being built in 2020 up to the time of writing (August 2020), and 60 per cent market share for all dual fuel vessels. These are the vessels most likely to want to use gas as fuel. It expects to see a number of LNG newbuilding contracts coming shortly.
Another part of the market is shipowners with other types of vessels who are looking at LNG as a fuel, including container vessel operators, and now the first tankers. It has developed ways to incorporate VOC gas (from the cargo) mixed with the LNG and burned in the engine.
Path to low carbon
The company sees the engine as a way for companies to maintain flexibility as new fuels come into operation. Due to the actual bunker market they may use diesel now, but can move to bio diesel or LNG fuel when it becomes available, and move to synthetic LNG (produced from renewable fuel) or bio-derived LNG (from rotting biomatter) as it becomes available.
But while LNG may be a bridging fuel to zero carbon LNG, we may see zero carbon liquid fuels being introduced meanwhile. The company cites DNV GL data predicting that use of ammonia might begin in 2037, forming a sizeable chunk of maritime fuel by 2050, as a liquid fuel replacing diesel.
As an engine designer, WinGD sees its role having technology ready for whatever fuels shipowners may be using and making sure the technology is ready in time.
Each fuel has different burning principles, which the engine will need to be tuned to.
The combustion process of the engines has been tested out with new fuels with a lower flash point (temperature at which they form an ignitable mixture) like ethanol and methanol. “Ones available low flashpoint fuel injection systems and respective engine upgrades would be applicable for newbuildings first but should be made available for retrofitting too,” said Volkmar Galke, global director, sales.
The company does not anticipate any change to the 2-stroke engine (such as a move to electric motors to drive propellers) as the main ship engine “for a long time”. But it anticipates an energy mix to move into the engine room making the set up more complex.
“The journey to 2050 requires experts to come together. WinGD is open to embrace this journey and collaborates with as many stakeholders as possible,” he says.
Adjustable combustion
Despite not implemented into the serial design yet, the engine has the capability to vary the ratio between combustion chamber and piston stroke, known as the ‘compression ratio’, while the engine is in operation. This makes it possible to run the engine at the best fuel efficiency while the operating mode between Otto and Diesel as well the load on it changes. The method is known as Variable Compression Ratio (VCR).
For instance, in Otto mode, higher engine load benefits from a lower compression ratio allowing to reach more power, but lower engine loads benefit from higher ratios because there is lower fuel consumption.
iCER also has capability to adjust the gas / air mixture. If Otto combustion moves away from the ‘rich limit’ – so has less fuel that might ignite, there is a slightly slower combustion speed, and less tendency for early ignition.
If diesel combustion is slowed down, you have a reduced peak pressure (from the combustion) and temperature, and form less NOx, but also a little less efficiency.
“Depending on your gas / air amount – you can nicely control combustion speed – and also avoid early ignition,” says Mr Schneiter. “So, you are much more in control of your combustion.”
CO2
The engine reduces the reactivity of the gas-air mixture by replacing oxygen in the suction air by CO2.
This provides additional options for optimising engine performance.
The engine is normally operated at an air fuel ratio of 2.5. If you have more fuel, you have risks of self-ignition, but if you have more air, you risk combustion instability, so you need to stick in this operating window. But by using CO2, you can use more air (less fuel) in the mix if you want to. This gives you freedom to optimise the engine for lower consumption, when the load requirement is lower, without any compromise on the maximum power output you can get (the maximum fuel concentration).
The iCER system is replacing oxygen in the combustion with CO2 by default.
Exhaust gas recycling
As mentioned above, the exhaust gas recycling takes half of the exhaust gas and puts it back through the engine another time. It means that the overall methane slip is halved.
It is not possible to completely eliminate methane slip with an Otto cycle engine, so recirculating the exhaust gas back through the engine may be the best idea, “giving the methane a second chance to burn,” Mr Schneiter says.
The exhaust gas needs to be cooled before it can be recycled – a cascade cooler is used, with no direct contact between the gas and the cooling water.
Any ‘bleed water’ can be discharged without any extra water treatment – because the system is only operated when in gas mode. There is no soot or sulphur from gas. This keeps the water treatment system very simple.
The amount of exhaust gas is controlled by a back-pressure valve, which releases exhaust gas into the cooler – so there is no need for an exhaust gas blower, he said. This also reduces power consumption.
You can use an exhaust gas economizer additionally (such as one made by Alfa Laval) which take otherwise wasted heat from the exhaust and use it to make steam (if steam is required).
Diesel and Otto cycle
The company claims to be the only 2 stroke engine design which can operate on both the diffusion (diesel) and pre-mixed (Otto /gas) cycle.
With the Otto cycle, the fuel is ignited by pilot fuel. Pilot fuel is pre-mixed with air before entering the engine.
In diesel engines, fuel is injected into the engine cylinder near the end of the compression stroke. During a phase known as ignition delay, the fuel spray atomizes into small droplets, vaporizes, and mixes with air, before being ignited.
Whether fuel is burned in a diesel cycle or the Otto cycle can depend on the situation and load required.
WinGD is also developing a combustion cycle which is a mixture of both, using LNG mixed with diesel, which it calls fuel sharing – providing a new degree of engine optimisation.
Research
At the time of writing (June 2020), the X-DF 2.0 engine technology was in its final stages of 2 years of trials at a test engine facility operated by WinGD.
WinGD has test infrastructure with five test engines, and test rigs to test combustion of individual fuels. It also runs computer simulation models alongside them.
The company has been putting most of its research energies into developing better lean burning combustion on a 2-stroke engine but taking a conservative approach to ensure reliable operation.
All the new engine technologies need to be carefully checked, to avoid the risk that they impact the reliability of the engine.
It is trying one method after another and plans to continue doing so for the decades to come, says Mr Schneiter.