The first technology unveiled as ‘supporting act’ for WinGD’s recently released X-DF 2.0 platform promises to open up the engine’s operating window, writes Stevie Knight
The grand-sounding Intelligent Control by Exhaust Recycling (iCER) is actually rather a clever addition to WinGD’s portfolio.
Starting with the basics, the X-DF engine has a very lean burning gas combustion cycle. During LNG operation it already meets toxic emission requirements with very low particulates and NOx down to “one-tenth of Tier III levels” says WinGD’s Dominik Schneiter.
However, it has a small but dirty secret: “Not quite all of the fuel gets burned,” he admits. While the X-DF’s gas mode gets close to complete combustion, a lean-burn Otto cycle always leaves a little unoxidised LNG to be released into the air. In short, it’s methane slip.
The iCER goes some way to address the problem, “by taking exhaust gas out after the turbine, and bringing it back into the air intake channel,” he explains.
Fundamentally, it’s a version of an Exhaust Gas Recirculation system. But while EGRs have been typically utilised on a diesel cycle to lower NOx emissions by bringing down the combustion temperature, here it has a slightly different focus.
Firstly it helps reduce wasted fuel, “because if you can pull it back for a second burn we can make the whole system more efficient,” says Schneiter, resulting in 3% reduced energy consumption in gas mode. Interestingly, the hard numbers from testbed trials measuring methane slip go much further, demonstrating that it can halve emissions of this particular greenhouse gas.
But it also has another use. Schneiter outlines the issue: “The problem is that while a diesel cycle only injects the fuel when the crankshaft is in the right position, in an [Otto cycle] engine the premixed fuel-air is already in compression in the combustion chamber, so there’s not as much control over the ignition.” It is especially problematic for lean-burn LNG operation, making higher loads somewhat tricky to handle despite tweaks to fuel admission and ignition timing. Unfortunately, staying out of the ‘rich-mixture danger zone’ on one side can push engines into combustion instability on the other.
However, the iCER “gives us another degree of freedom”, says Schneiter. Replacing just 1% or 2% of the charge air’s oxygen with inert, largely oxidised gas reins in the tendency toward uncontrolled, early ignition and knocking. It also slows overly fast combustion, damping down the pressure peaks and reducing the sudden torque on the crankshaft, plus, it makes life a little easier for the other mechanical components.
It’s rather a neat system: for example, it avoids using a blower to overcome the pressure difference between the exhaust gas and the scavenging air receiver. Just closing a flap reduces the volume of exhaust gas flowing to the economiser, pushing the air back to the compressor side of the turbocharger, which in turn creates a suction effect. “We can force the excess gas to recirculate by physics alone, so there’s no parasitic load,” Schneiter says. “That’s one of the advantages.”
That doesn’t mean there haven’t been developmental challenges: “The iCER recirculates about 40% to 50% of the exhaust gas, but to take it straight off the turbine would constantly increase the temperature in the combustion chamber – until the engine finally destroyed itself,” he explains.
Therefore, the exhaust gas needs to drop from post-turbine temperatures averaging around 200C to around 40C, or just within ‘hand-hot’ wash temperatures.
It’s a big step down. “First of all we started with a plate-cooler approach, but we found the volumes were too large, so we decided to partner with Alfa Laval as they’re the experts in this field,” he explains. The resulting Cascade Exhaust Cooler (CEC) is close to an off-the-shelf tower of the type Alfa Laval commonly uses with scrubber equipment. This uses a freshwater spray on the hot gas, but the clean gas fuel and combustion results in negligible contamination of the cooling water, so it can be captured returned without depositing noxious substances into the sea.
Interestingly, the iCER has an optional extra economiser as despite somewhat variable temperatures from the exhaust, capturing enough thermal energy for steam production remains a necessity for LNG fuelled ships. Therefore, a second, smaller economiser sits in parallel with the CEC, creating an alternative, bypassing heat stream (while also doing its bit to cool the flow).
So, how does it play with the energy carriers of future? Schneiter points out “as long as these are gaseous fuels such as LNG, synthetic or bio-methane, we can use the technology as it is, since the fuel’s characteristics will be much the same”. He adds: “This gives us an immediate, sustainable solution with a zero-carbon footprint based on the X-DF platform.”
However, that’s not the end of the story: the iCER also opens up the potential for a second modification to WinGD’s 2.0 platform in future. As it effectively gives the X-DF engines enough elbow room for an increase in the maximum brake mean effective pressure (BMEP), it could raise the power density of the X-DF from its current 17.3 bar: “Some of the fuel alternatives will be limited by lower BMEP, so the iCER could help to raise their BMEP maximum – although the exact potential needs to be investigated,” he says.
THE RIGHT MOMENT
According to Schneiter, the iCER has been waiting in the wings for the right moment. While it’s already been applied to high-speed gas engines, when it came to these low-speed two-strokes “we wanted to ensure reliable operation of the X-DF engines in service first – before adding technical complexity”, he explains.
However, he adds that this is not the only innovation about to make use of the X-DF platform – others are just around the corner.
This includes Variable Compression Ratio (VCR) technology. It effectively changes the length of the conrod by introducing a pressurised chamber into its ‘eye’. As a result, it can raise the piston’s top position to burn a reduced volume of fuel or lower it, creating a responsive compression ratio.
“Using VCR, we can optimise the fuel performance of the engine, whether it’s in gas or diesel mode,” says Schneiter: “Additionally, it allows us the possibility of operating on new fuels.”
“Although we are currently running LNG, alternatives include ethanol, methanol and other alcohols – and even ammonia is becoming a popular topic for discussion,” he adds. While these fuels may require somewhat adapted supply systems, “VCR technology gives us a huge advantage as we can create a range of combustion settings”, he says. That includes “low or high compression, or two fuels running at different mean effective pressures”. Admittedly, the technology is still in the R&D phase, but test engine trials are not that far off.
Certainly, given the X-DF’s ability to move between Otto and Diesel cycles, the additional scope of the VCR “makes me believe that even if the picture is not yet so clear, we are pretty safe in claiming we’re prepared for the future”, concludes Schneiter.