Two-stroke engine design is advancing rapidly as developers aim to improve environmental performance across several fronts
Choosing a priority for an engine designer is not easy. Improving environmental efficiency is one area of focus, but within that lofty goal come several other aspects, many of which contradict each other. If you want to meet Tier III NOx emission limits you may well have to resort to an engine tuning that sacrifices fuel efficiency and bumps up greenhouse gas output. If you look to minimise sulphur by exhaust gas cleaning, there will be a CO2 penalty every time you turn on your scrubber.
The picture is complicated further by the range of solutions on the market. From exhaust aftertreatment to alternative fuels and hybrid electric power, engine designers must balance complexity as never before. So it is not surprising that the makers of the biggest engines are embarking on very different development programmes.
Japan Engine Company has demonstrated the first of a new series of mono-fuelled engines that offer an unusual solution to the impending IMO sulphur limits – burning marine gas oil (MGO), one of the most expensive fuels available. The company believes that optimising engine design for this fuel will enable some operators to achieve cost-efficient compliance while avoiding some of the uncertainties around other options.
Although future fuel prices and the impact of the sulphur rule are hotly debated, MGO currently sells at a premium to heavy fuel oil and LNG; it is also expected to be pricier than the new low-sulphur fuel blends which are set to reach the bunker market by 2020. But there are some clear advantages.
“From exhaust aftertreatment to alternative fuels and hybrid electric power, engine designers must balance complexity as never before”
The viscosity of MGO is higher than fuel oils, so the fuel requires no heating. This means a simpler fuel system as well as less wear on fuel train components such as purifiers, translating to lower maintenance, higher reliability and greater ease for crew. The availability of MGO is already assured, while one of the recognised challenges of new low-sulphur blends is that – even if availability is not a problem – fuel compatibility across products (even products from the same supplier at different ports) is far from certain.
Japan Engine Corp reports that the new UEC-LSJ two-stroke engine met that challenge, reducing fuel consumption by 5% compared to the corresponding conventional engine model. For the Tier III mono-fuel version, NOx optimisation marginally compromises fuel efficiency, with the saving reduced to 4%. According to a spokesman, the fuel consumption saving reaches nearly 10% if the difference in calorific value between MGO and heavy fuel oil is taken into account.
The efficiency is achieved partly by fine-tuning combustion, including atomiser design, fuel injection pressure and scavenge air port arrangement. Peculiar to J-Eng designs is that water injection is used to reduce NOx formation (and therefore the need to compromise fuel consumption as a countermeasure) by lowering combustion temperature. This system, which the company also claims reduces fuel consumption directly by improving the fuel-air mix, has been in use on one of the company’s engines on Japanese training ship Ginga Maru since 1994, accumulating 2,000 hours of operation.
The company believes that its novel, mono-fuelled engines will be best suited to vessels below 60,000 dwt. Its first engine in the series – the five-cylinder, 500 mm bore 5UEC50LSJ – is best suited to handysize bulk carriers. It will be available for delivery in late 2019, with a six-cylinder variant expected the following year.
“The inherently better efficiency of two-stroke engines means that using the main engine to generate electricity while sailing is more efficient than using the auxiliaries”
A spokesman for J-Eng said: “We expect to acquire some market share for medium-sized and small vessels which operate mainly in Emission Control Areas, and to increase production to 12 engines a year from 2022. Chinese licensees might also manufacture the engines in the near future. The Chinese market is quite promising for mono-fuel engines, due to local sulphur regulations.”
While J-Eng is looking to optimise propulsion driven by oil, other developers are tweaking their gas-fuelled designs. Winterthur-based WinGD is introducing a new system that brings the function of the gas valve unit (GVU) within its X-DF dual-fuel engines, easing installation and improving integration.
Integrated gas pressure regulation will be rolled out across the X-DF range (credit: WinGD)
The role of the GVU in the LNG propulsion configuration is to regulate the pressure of gas from the fuel gas supply system to the engine and to shut down supply quickly and safely. GVUs usually sit in the engineroom, although they can also be housed in a dedicated room. The new integrated gas pressure regulator (iGPR) incorporates that functionality within the X-DF engine.
The new concept results in a space saving in the engineroom, as well as a weight saving (of around 1,100 kg compared to the GVU-equipped alternative design in the debut case), easing installation. But according to WinGD manager, marketing and application, Daniel Strödecke, the impact on control and communication is more significant than the weight and space savings.
“Rather than communicating between the GVU and engine via an external system, now there is just internal communication within the engine,” he said.
Integration is improved in other areas too. Interfaces with other equipment are simplified and more of the engine set-up can be completed by the engine builder rather than the shipyards. Mr Strödecke noted that the iGPR design enables installation of significantly smaller ventilation air fans, for example, and gives WinGD more control over the design and construction in general.
“If we want to optimise the pressure control valve it can be done without liaising with a third party,” he explained.
The system has been introduced on the first X52DF engines, destined for two 125,000 dwt shuttle tankers being built for AET by Samsung Heavy Industries. The engines are also notable for their ability to burn volatile organic compounds – the gasses emitted by crude oil under transfer – without the need for a reformer.
The iGPR will also be introduced on the X92DF engines that are set to power CMA CGM’s nine, gas-fuelled ultra-large container ships. It will eventually be rolled out across WinGD’s X-DF portfolio.
Elsewhere, a recent focus has been on making sure that engines work with other power sources to contribute to improved, system-wide efficiency. MAN Energy Solutions has acknowledged the growing role of hybridisation in deepsea shipping by establishing a default set-up for using power take-off (PTO) with its dual-fuelled two-stroke engines.
" Japan Engine Corp concludes that an engine optimised for burning MGO could be a viable option for some ships"
The company used the configuration to show how an 11,000 TEU, gas-fuelled container ship can be made more efficient by harnessing PTO to generate electricity from its main engine. MAN cited a recent application as the inspiration for a study which found that connecting a PTO to an ME-GI engine could cut fuel consumption by 6.4%, or 165 tonnes of gas fuel per 18,000 nm trip; that equates to 1,238 tonnes less fuel a year.
“The new fuels, regulations and the challenging environment in the marine business over the past years have created a complicated equation in terms of optimising a future vessel,” the authors of the study explained. “In order to address the increasing needs from vessel operators, MAN has gathered data and operating experience and created a default set-up.”
MAN’s solution is to connect a 2.4 MW power take-off to an eight-cylinder G95ME-GI engine (with a maximum continuous rating of 42.31MW at 76.9 rpm). The company argues that the inherently better efficiency of two-stroke engines means that using the main engine to generate electricity while sailing is more efficient than using the auxiliaries.
To optimise fuel efficiency, the company deploys the latest version of both its ultra-long stroke G95 engine and the ‘Mark 2’ update of its ME-GI gas injection concept. It also includes the recently developed pump vaporiser unit; this combines the cryogenic pumps and vaporiser in one unit, simplifying the fuel-gas supply system.
The efficiency gain can be used to either improve sea endurance, allowing for longer time between bunkering, or to reduce the size of LNG tanks needed to manage the same voyage.