Introducing Two-Stroke Propulsion for LNG Carriers
For nearly four decades, steam turbines was the only propulsion system available for LNG ships even though its efficiency was considerably lower than diesel engines. The main advantage was the simplicity of utilising the boil-off gas in the steam boilers to produce steam for the steam turbines. Dual-fuel diesel-electric (DFDE) systems came to the fore about 10 years ago and quickly replaced steam engines as the undisputed LNGC propulsion solution of choice. However, with the introduction of the reliquefaction plant and gas-driven diesel engines, it is today possible to install high-efficiency diesel engines as prime movers, referred to as two-stroke propulsion, which offer significant savings in fuel consumption and reductions in environmental emissions from ship engines.
Owners currently have the choice of two different engine solutions for two-stroke, low speed propulsion for LNGC vessels: MEGI, by MAN Diesel and Turbo, and X-DF, by WinGD (previously Wärtsilä).
The MEGI Engine – High Pressure Gas Injection
The MEGI diesel engine, developed and marketed by MAN Marine Engines and Systems, applies the principle of non-premixed combustion (the Diesel principle). The first LNG MEGI vessel was delivered in 2016.
The dual-fuel two-stroke engine is based on the combustion principle of operating on Heavy Fuel Oil (HFO) or Marine Diesel Oil (MDO) together with high-pressure natural gas, where the fuel is injected and burned directly as opposed to the premixed or Otto-cycle combustion. In brief, two or three gas fuel valves inject high-pressure natural gas to the combustion chamber and to ensure an optimally controlled combustion, a small amount of pilot oil is injected simultaneous with the natural gas via two or three conventional fuel oil injectors.
The MEGI engine is equipped with additional safety systems that ensure a safe operation on gas without requiring rupture discs in the scavenge air receiver, the exhaust gas receiver, and in the exhaust gas piping.
Owners and operators are provided with maximum fuel flexibility and, depending on the relative price and availability of gas and fuel oil, are free to choose the most competitive fuel as the engine operates with the same efficiency on both gas and fuel. The Diesel-cycle ensures stable gas combustion under all weather conditions, such as heavy weather and high ambient temperatures, without any risk of misfiring or knocking.
All qualities of LNG can be burned with the same high efficiency, and the engine has no specific requirement to the methane number. The dual fuel engine can operate on natural gas in the load range from 10% to 100% load. Furthermore, depending on the fuel availability on board, the engine can combust any ratio of natural gas and HFO/MDO. The MEGI engine is ignited on diesel, and changeover to gas operation can take place at 10% engine load. Both HFO and MDO can be used as pilot fuel.
Another advantage of gas-fuelled tonnage is the ability to adjust operation according to the changing fuel prices and exhaust-emission limits. Service experience shows that the MEGI engine delivers significant reductions in CO2, NOx and SOx emissions.
MEGI vessels generate negligible methane slip during gas operation making it the most environmentally-friendly technology available. The reduction of greenhouse gas emissions, including methane slip, has been found to be 22% lower compared to fuel oil.
The X-DF Technology: Low Pressure Gas Injection
Inview of the increasing demand for low-speed, dual-fuel engines, WinGD developed the lean burn Otto combustion process with low-pressure gas admission and micro-pilot ignition for its two-stroke engine portfolio. The first LNG X-DF vessel was delivered in 2017.
The low-pressure dual-fuel technology, known as the X engine series, is a further development of the Wärtsilä’s well-proven 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 Otto cycle when operated in gas mode – i.e. ignition of a compressed lean air/gas mixture by injection of a very small amount of liquid pilot fuel.
The gas distribution and admission system have been specifically designed for the requirements of the two-stroke engine. The gas is supplied and distributed in gas manifolds, along both sides of the engine, to feed gas to each cylinder. Two hydraulically actuated Gas Admission Valves (GAV) inject gas directly into each cylinder through the cylinder’s liner wall. Since the gas is injected at the beginning of compression, depending on the selected rating point, a low gas pressure feed of 10-13 barg is sufficient to achieve a homogenous air/gas mixture, even at full engine load.
In the latest generation of X-DF engines, the gas feed is regulated by the built-on engine integrated gas pressure regulating unit (iGPR) which gives greater freedom to optimise the engine room design, enabling significant savings on space, steel construction, ventilation, cabling – effectively driving down the vessel price further. This is achievable thanks to the introduction of the iGPR which replaces the need for a dedicated compartment for an open type gas valve unit (GVU) or enclosed type GVU.
The WinGD X-DF engines meets the regulations of IMO’s Tier III NOx limits in gas mode in ECA by considerable margins without any additional exhaust gas abatement measures such as EGR or SCR.
With liquid fuel consumption for pilot ignition below 1% of total heat release and with practically no sulphur content in LNG, X-DF technology is believed to be a reliable solution to achieve the 0.5% global cap on sulphur in marine fuels proposed to become effective January 2020.
The particulate matter emissions on X-DF engines is reduced to almost zero, and the CO2 emissions, inherent to burning natural gas, are further reduced. The total hydrocarbon content of X-DF is considerably lower, compared to four-stroke low-pressure DF engines, which employ the same technology and are used as auxiliaries in every vessel.