Japanese two-stroke tailored to MGO
The first development project for Japan’s two-stroke engine designer following its recent restructure is a range of engines optimised for low-sulphur distillate fuels, writes Gavin Lipsith.
While most engine makers are focusing their efforts on increasing fuel flexibility, Japan Engine Corp – newly created from the two-stroke businesses of Mitsubishi Heavy Industries and former licensee Kobe Diesel – believes that, in one case at least, a tighter focus on single-fuel engines may hold an answer to emissions challenges.
The global sulphur cap in 2020 gives ship owners various alternatives for compliance. For those hoping not to install scrubbers or equip ships to run on alternative fuels such as LNG, compliance will depend on the use of low-sulphur fuel. But so-called ‘hybrid fuels’ are likely to present challenges in terms of compatibility – one brand may not mix well with another – as well as ensuring consistent, global supply. For many, distillate oil or marine gas oil (MGO) may be the way to go.
For bigger vessels, higher fuel consumption will make other compliance options more financially attractive, with cheaper fuel costs quickly outweighing initial outlays. But for small and medium-sized vessels, Japan Engine Corp believes that MGO offers benefits for operator, owner and shipyard.
From the yard and owner’s perspective, the absence of a scrubber means a simplified engine room and no issues regarding production or installation capacity. For the owner, this simplicity translates into less maintenance cost. And for operators, Japan Engine Corp believes its new range of MGO-focused, ‘mono-fuel’ two-stroke engines will offer highly competitive fuel consumption.
The company will launch the first two models of the new range next year. The UEC50LSH-Eco-C2-MGO will feature Japan Engine Corp’s full suite of solutions for MGO optimisation – two-stage turbocharging, combustion tuning, water injection and NOx after-treatment (here, exhaust gas recirculation). Typical vessels include supramax and handysize bulk carriers and MR tankers.
A second model, the UEC35LSE-Eco-B2-MGO, will feature what the company calls its ‘light mono-fuel concept’ – essentially everything except two-stage turbocharging, and with the more size-appropriate selective catalytic reduction replacing EGR. Suitable vessels include small chemical tankers, car carriers, containerships and LPG carriers. Both engines will be available in Tier II and Tier III NOx versions.
The new engines do not rely solely on after-treatment to reduce NOx production. One of the most innovative elements of the range is a water injection system which Japan Engine Corp says both reduces NOx (by lowering combustion temperature) and - as a result of the momentum added by water injection – provides a better mix of fuel and air, reducing fuel consumption. The technology therefore considerably improves the inevitable trade-off between fuel consumption and NOx production.
The technology itself is simple: Water is pulled into the fuel valve during the standstill period of each injection cycle. Fuel and water can then be injected into the combustion chamber in ‘layers’, depending on the actuation of the fuel pump. It is also a well-tested technique. Although the use of water injection is noteworthy for a commercialised two-stroke engine, the technology has been tested thoroughly over four trials totalling nearly 50,000 hours. Of these just one trial, on Japanese training ship Ginga Maru as far back as 1994, was on a two-stroke engine.
The engine is also relatively unusual as one of the few so far to feature two-stage turbocharging. This process, which enables greater compression ratios (and ultimately cylinder pressures) thanks to an intercooling stage between two compressions, means that Miller timings can be used to full effect to improve fuel consumption. Miller timings, where the closing of the exhaust valve is delayed to reduce the compression stroke, result in higher cycle efficiency and lower fuel consumption. In order to maintain the compressed air pressure of the combustion chamber, higher scavenging air pressure is required, of the levels only realistically achievable by two-stage turbocharging.
Fuel economy on the new MGO range is enhanced by what Japan Engine Corp describes as ‘perfect combustion tuning’. Higher combustion efficiency is achieved by optimising the mixture of fuel oil and scavenging air. This is achieved in two principle ways: by encouraging the appropriate swirl flow thanks (among other factors) to the scavenging air inlet arrangement; and an optimised atomizer design that moderates fuel injection pressure and direction.
As mentioned previously, when designing engines there is always a trade-off between fuel efficiency and NOx production. Describing how the new MGO range moderates these factors, Japan Engine Corp notes that its combustion tuning plus Miller timing reduce fuel consumption by around 7%, but increase NOx production by 40%. Water injection is then used to bring NOx down to original levels – within Tier II limits - with just a marginal increase in fuel consumption. The engine achieves Tier III limits with the use of further NOx abatements – SCR or EGR – and another minimal increase in fuel consumption. The MGO engine’s final fuel consumption is around 5% below that of Japan Engine Corp’s corresponding, existing UE engine, which the maker claims is already 2-3% more fuel efficient than engines from other designers.
While most engine makers are striving to make sure their engines work across a range of preferences, Japan Engine Corp has spotted a niche for an engine optimised for one specific solution. In doing so, it has employed a range of technologies that are worth considering whatever compliance option owners ultimately select.
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