Game changer in marine propulsion?

That may sound a bit optimistic, seeing that MAN Diesel & Turbo has already released its equivalent, the ME-GI, which has received its first orders*. So why should the second to market be so different? The answer would appear to be in the gas system. Wärtsilä has succeeded in applying its low pressure, lean-burn, Otto cycle dual fuel principle from the company’s four-stroke technology to a two-stroke engine, something that many though would be difficult, if not impossible. The result is, without doubt, a simpler, and thus less expensive, solution to the LNG question. And it works – we have seen it.

So what might be the disadvantages? Ships – other than LNG tankers – will still need to store the LNG fuel onboard. Although the low-pressure solution avoids the costs of the high-pressure gas handling equipment, gas tanks are still large, complex and expensive so the cost advantage of the low-pressure system may be less than first apparent. And although the LNG fuel solution provides a neat answer to known current and forthcoming emissions limits, which deal with SOx, NOx and PM, there is lower benefit in greenhouse gas emissions. How the shipping world will mandate GHG reduction is still being debated, and the debate has centred on carbon dioxide. But methane emissions from an LNG-fuelled ship exhaust are a much more powerful greenhouse gas. Although when looking at the equivalent GHG effect of LNG and diesel fuels the gas fuelled engine enjoys a slight advantage, in the case of the low pressure two stroke it is very slight**, and further work will be needed to bring this down to more acceptable levels, as has been achieved with dual-fuel four-strokes.

MAN’s high pressure ME-GI engine, however, claims no methane slip. *** Although GHG emissions will not concern this technology, it is acknowledged that it will need after-treatment in the form of EGR or SCR to meet IMO Tier III NOx limits. And although the low pressure engine is Tier III compliant, when starting on diesel fuel – which will happen in ECAs where Tier III applies – it may not comply. ****

So there is no winner as yet – at least not until the GHG debate is resolved, and there is a clearer picture as to how Tier III will be implemented, monitored and enforced.

* Editorial note – this comment piece was written for publication in the December issue of The Motorship, and timed to go live online at the same time as the printed issue was published. Wärtsilä has asked us to point out that the first order for its dual-fuel RT-flex50DF was announced on 2 December 2013, which was between writing and publication of the article. The contract was the subject of a separate news item.

**In addition, the company says that methane (CH4) emissions from a DF engine account for 15% of total greenhouse gases, classed as CO2 equivalent, so CO2 is still a more important gas (in fact the DF engine, according to Wärtsilä, has lower CO2 emissions than a GD engine). A diagram shown to us at the launch of the low pressure dual fuel two stroke suggested that for the low pressure DF two-stroke running on gas, total CO2 equivalent emissions were 90% of those for a Diesel engine on HFO. However, Wärtsilä now points out that total GHG emissions are “more than 10% lower with DF compared to traditional fossil fuels.” The company also states that “already today the 2 –S DF THC emissions are at the same level as the best four-stroke DF engines”.

*** As the high pressure two-stroke claims negligible methane emissions we saw that as one area where the high pressure technology scores over the low pressure two-stroke. But Wärtsilä tells us that this is not true, because although CH4 is minimal, the engine still has higher total GHG emissions than the Wärtsilä technology.

**** We were told at the Wärtsilä launch that compliance with Tier III when starting or running on diesel fuel had not been resolved, but the company now points out that it believes that its engine, as with DF four-strokes, will be able to start and stop in gas mode and so will be compliant when Tier III comes in.

We thank Wärtsilä for these extra comments and clarifications. On the other hand, the EU-backed Helios project, a presentation on which we attended shortly after the launch of the Wärtsilä DF two-stroke engine, and which will be reported in our January 2014 issue, was equally emphatic about the benefits of the ME-GI high pressure gas injection system, not surprisingly, as the project was led by MAN Diesel & Turbo. So we stand by our comment that there is no clear winner as yet. No doubt the high pressure technology will suit some, and the low pressure engine will be favoured by others.