WinGD’s dual-fuel X-DF engine has been years in the making. Here, the company outlines the background to this innovation
When the development of two-stroke low pressure dual-fuel (DF) lean-burn technology started, the extensive experience that Wärtsilä had gained from its four-stroke development work could be utilised as a starting point in a number of technology areas.
These included engine automation and control, pilot injection, the gas supply system and engine testing, to mention just a few. Nevertheless, while some of the technologies were in place, the characteristics of the two-stroke engine required additional development work, some of which opened up new opportunities.
One example is the differences that are introduced by a low engine speed. This gives more time for gas mixing but also requires better control of the speed of combustion. Another difference is the uniflow scavenging process of the two-stroke engine. On one hand this enables a certain stratification of the air and gas mixture, the fresh air and the exhaust gas. On the other hand, it presents a challenge to minimise the quantity of rest gas in the large cylinders. This is achieved more effectively by a four-stroke engine, which has a separate scavenging stroke.
One of the benefits of the two-stroke engine is the fully variable exhaust valve timing. This can be utilised to flexibly adjust the effective compression ratio and thereby control combustion by means of the air excess ratio and compression temperature.
Understanding the boundaries while further developing the features has required some iterations, but has been one of the keys to achieving today’s results with X-DF technology. The most encouraging achievement has been the successful transfer of the technology from the 50cm bore research engine to the X72DF engine tested at Diesel United, where the expected results were confirmed and, in some cases, even exceeded.
Development of the low pressure, dual-fuel technology for low speed two-stroke engines began in 2011, following an assessment of market requirements and available technologies. A designated test engine was installed in the Wärtsilä engine laboratory in Trieste, Italy. As a first step, one cylinder of the RT-flex50 based diesel engine was converted to enable it to operate on gas, while the other five cylinders remained running on diesel with the original equipment. This set-up was used to develop and validate the basic concepts for gas admission and ignition, and to define the control principles.
In 2013, the full engine was equipped with more heavy-duty equipment for gas operation, upgrading it into an excellent development tool for full scale engine operation tests. The main focus was on the development of engine components, performance and control system features, which eventually resulted in the X-DF solution which is available commercially today.
Wärtsilä identified an imminent market need for larger DF engines, and so the technology was transferred to the larger bore engine types X62DF and X72DF. At the same time, a full scale 6X72DF technology demonstrator engine was set up at Wärtsilä’s licensee Diesel United in Aioi, Japan. Diesel United has been the co-operation partner in this development since the beginning of the project.
The engine went into operation in early 2015 and was demonstrated to numerous representatives of the shipping world in April 2015. By this time, Wärtsilä and China State Shipbuilding Corp had established joint venture Winterthur Gas & Diesel (WinGD).
The fact that the low pressure technology has proved to be suited to larger bore engines has triggered enormous interest in the X-DF solution in the market.
The low pressure, dual-fuel technology developed by WinGD for its X-DF engine series builds on Wärtsilä’s long experience with what has become a proven industry standard in medium speed dual-fuel engines. In contrast to high pressure gas injection engines, which operate on the Diesel cycle, WinGD’s low pressure X-DF engines work on the lean-burn Otto cycle – that is to say, the ignition of a compressed lean air and gas mixture by means of the injection of a very small amount of liquid pilot fuel – when operated in gas mode.
As demonstrated on test and customer engines, WinGD’s X-DF engines are characterised by stable combustion, inherently low NOx emissions and high overall system efficiencies as well as safe gas operation. With regard to NOx, WinGD X-DF engines undercut IMO Tier III limits for emission control areas by considerable margins, without any additional measures such as exhaust gas recirculation or selective catalytic reduction, while running on gas.
Moreover, with the low-pressure gas admission the gas fuelling system does not require any high pressure compressors, considerably reducing equipment costs, onboard energy consumption and maintenance during operation. A large supplier base is available for gas supply systems, as the components are similar to systems installed on numerous four stroke DF engines, proven in thousands of hours of field operation.
Because of the possibility of fast combustion or pre-ignition, the maximum brake mean effective pressure (BMEP) of the X-DF engines has been limited to 17.3 bar. This results in a reduced power density of the DF engines compared to the 21 bar maximum achievable BMEP on modern two-stroke diesel engines. However, the market response shows that the engines are commercially attractive, thanks to their low investment cost and the superior overall system efficiency.
While the X-DF technology has been released for commercial engines, development work is continuing with the objective of evaluating its potential for increased fuel efficiency in gas and diesel mode, as well as for higher maximum power output. Additionally, new features will be introduced, providing further benefits to operators.
One such new feature is a fuel sharing mode. This development will allow liquefied natural gas carriers to burn gas and liquid fuel simultaneously. This will enable them to achieve their desired speed even with a limited amount of natural boil-off gas, so eliminating the need to force boil-off. Because a considerable part of the fuel is combusted in a diffusive regime, the fuel sharing mode will be tuned to be IMO Tier II compliant.
Doosan Engine passes 100 million bhp milestone
MAN Diesel & Turbo’s licensee Doosan Engine Co held a ceremony in Changwon, South Korea on 20 January to celebrate the production of 100 million two-stroke bhp. The milestone was achieved with the successful test operation of an MAN B&W 11G95ME-C (103,000hp) engine, the world’s most powerful engine type per cylinder and the most powerful engine in MAN Diesel & Turbo’s engine portfolio.
MAN Diesel & Turbo drafted its first licence agreement with Doosan in 1983.
Thomas Knudsen, MAN Diesel & Turbo head of low speed, led a company delegation to South Korea and spoke at the event, noting that Doosan Engine had celebrated the cumulative production of 50 million bhp just nine years previously. He said that Doosan Engine had mastered the production of diesel technology and developed into a valued partner in the continuous development of this type of technology.
He continued: “There have been a lot of major milestones to celebrate at Doosan Engine, and there will undoubtedly be many more. But for now, I would just say congratulations on the 100 million bhp – and we are looking forward to the 100 million kW mark!”
Dr Knudsen concluded his speech by mentioning the current, turbulent marine market. He said: “Analysts say there is a crisis in shipping and in shipbuilding. However, we have to keep in mind that world trade will grow as more regions attain a higher level of economic development. Therefore, there is no doubt but that the shipbuilding industry will eventually gain pace again.”