Dual fuel’s high pressure, four-stroke future
Diesel engines have been optimised for a narrow band of efficiency, but shipping is now facing a future that relies on multiple energy sources. Given this, dual-fuel operations stand to play an increasingly significant role.
But, there’s an issue with the current medium to high-speed offerings.
While these low-pressure engines have the advantage of fuel flexibility, they need to be kept on a tight rein when it comes to ignition. The trouble stems from the necessary mixing of gas and air outside the combustion chamber: too much air creates flame propagation issues, but too rich a mixture can ignite early, causing knock.
By contrast, a high-pressure dual-fuel engine utilising a pure diesel cycle may look similar as both rely on compression ignition paired with pilot oil, but it “operates on a quite different principle... as it only mixes the gas very close to the point of ignition”, says Manuel Boog of MTU. This avoids knock, enables higher power operation and a fast response comparable with its diesel counterpart. Most importantly, it stands to lower methane emissions.
As a result, there’s a lot to play for. The system investigated firstly by the publicly sponsored FlexDi project and now by Germany’s overarching MethQuest programme fits the bill as it pairs liquid pilot fuel with high-pressure direct gas injection.
However, as Ingmar Berger of Woodward L’Orange explains, this type of high-pressure gas engine involves more expensive hardware and greater technical complexity.
Firstly, high-pressure dual-fuel engines require cryogenic pumps to ramp up the gas pressure from onboard LNG. The issues have generally centred on market availability: process pumps lack the necessary optimisation for mobile applications and according to Boog, there’s currently a problem with units able to supply very transient high-speed engines. Despite this, Berger predicts the market will respond to requirements “with growing demand”.
But the real challenge has been the injector: this has three gas needles which have to be able to withstand far greater stresses than the typical, low-pressure DF systems. The central diesel needle requires 1,000 bar fuel for pilot operation, rising to the full 2,500 bar pressure for liquid fuel in diesel mode, quite a range to cover. However, it’s the direct injection of gas which really creates the issues: this has to take place around top dead centre (TDC) and to deliver it, the system has to be capable of delivering gas pressures more than twice that of the cylinder: the necessary 500 bar results in “a supersonic flow through the nozzles” explains Berger.
Further, pressure needs to vary with the load “so at reduced speed, overall you have a longer combustion cycle and operation at a lower pressure delivers the right amount of fuel”, says Boog. “At higher speeds – and we are still talking microseconds – you need to be very much faster to get the right amount of fuel into the combustion chamber in the required time.”
It wouldn’t be so hard to pack all this in if the power train could be reinvented, but the idea is to retain a similar volume and footprint to an ordinary diesel installation, allowing the engines to be switched over with minimal fuss. Although the newly developed hydraulic control valve sits outside the most crowded area, “integrating four, not just one needle inside the cylinder head means space is an issue”, says Berger.
It’s especially challenging as there are several flow lines to allow fuel delivery under varying pressures: in fact, he describes the topography as having “so many holes inside it looks more like Swiss cheese”. Moreover, although those holes are straight when the gas is off, the high working pressures tend to push the sections outward, deforming the channels to different degrees. Therefore, ensuring that the gas needles remain exactly synchronised throughout the entire operating range required painstaking, section-by-section analysis. As Berger points out: “You need tolerances narrow enough to stop gas and oil leakages, but wide enough the needle doesn’t stick.”
SEALING OIL SYSTEM
There were also challenges for the sealing oil, as this too has to run at a higher pressure than the gas: “If any sealing surface ‘opens’, the oil has to push into the gas and not the other way around,” underlines Berger.
While its distant cousin (the original Wärtsilä 32GD engine from the 90s), used servo oil as a sealant, using diesel is far simpler. It doesn’t just avoid introducing yet another fluid into an already dense system; it also yields other useful characteristics. The tiny - but constant - flow lubricates the gas needle and seat and its combustion characteristics don’t create extra challenges when being burned in the gas flow.
There is still some work to be done on the way to commerciality as the extremely compact nozzle and consequent demand for precision engineering “pose a big challenge for manufacturing”, he concludes.
Usefully, while the composition of the fuel mix can be controlled by injection pressure, the gas combustion process can also be tweaked by the relative timing of the events. In fact, investigations indicate that the particular form the heat release takes is more influenced by the period between pilot and gas injection than almost any other parameter, including nozzle angle.
Interestingly, tests show a relatively early pilot injection results in a longer burn out and lower intensity combustion. But, as Berger explains: “Change that for a later pilot ignition and the gas has time to mix more, making for a higher peak, and faster burn... and the rapid combustion helps raise engine efficiency.”
However, you don’t want to run unchecked in this direction says Boog: “Go too far and you’ll have unburned gas reaching the chamber wall, with a consequent slight rise in CH4 emissions.” Further, there is another issue: very short combustion periods can result in steep pressure increases with higher peaks, “so you want to be a little careful about that”, he remarks. Therefore, the ability to moderate the delay between the two is extremely useful.
There are other considerations: while the later pilot allows for a level of premixing that lowers soot formation, the extra heat does stand to increase NOx emissions and, at its latest, tends to slightly heighten hydrocarbon pollution.
However, Berger underlines “the resulting emissions are still far, far below the amount released by current, low-pressure dual-fuel engines”, adding that tests demonstrate methane emissions drop by “a whole order of magnitude”.
It all has to fit into the bigger picture. Boog points to the ‘see-saw’ effect between the issues: roughly speaking, higher temperatures and increased NOx creation stand on one end, while lean-burn, lower temperatures a consequent rise in CH4 – methane slip - stands on the other.
Therefore the industry faces a choice between engine optimisation pathways, both being investigated by the MethQuest programme. Certainly, it is possible that low-pressure lean-burn four strokes will find a way to deal with methane slip, catching the molecules and oxidising them in a new type of catalytic converter – though the general feeling is that this is still some way off commercial viability.
Which leaves the high-pressure development route, explains Boog: the HP DF principle takes you toward greater engine efficiency as it allows throttling back at part load. On balance it may prove easier to deal with the resulting NOx with an SCR system at the exhaust than chasing methane molecules.
Importantly, while LNG is the first step, Berger firmly believes the next will be power-to-fuel combinations.
This is a big deal, as methane itself is one of the potential game-changers that could be key to the future. The issue is that its greenhouse gas effect is catastrophic: around 28 times that of good ol’ CO2. But, as Boog points out, even high-pressure engines with their natural advantages still have to be paired with effective containment and supply systems that will mitigate slip.
However, other fuels are waiting in the wings, ready to play their part in this new arena. Berger explains some of these candidates “are similar in that they need a diesel-like combustion process with a pilot to assist ignition” and these, he says, could be successfully matched with high-pressure dual fuel developments. Further, a few have comparable requirements: “For example, ammonia, like methane, needs complete combustion... you certainly don’t want to create ammonia slip”, he adds.
It may also be that ships will need a level of flexibility. “I think we’ll see several fuels or maybe more, availability depending on the region and the production of renewable fuel from wind, solar and so on,” says Boog. Once again, high-pressure technology could deliver a distinct advantage, being able to burn a range of alternatives more fully while also delivering greater efficiency per tonne.
Still, Berger has his favourites amongst the potential offerings: “Methanol and ammonia are the easiest to store, you don’t need cryogenics,” he says, adding that a diesel/methanol dual-fuel engine “would probably require additional accumulator volumes to achieve stable injection pressures...but all the basic parts are the same.” He concludes: “This injector offers dual-fuel engine developers a platform for utilising these new synthetic and regeneratively produced fuels.”
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