New MAN Diesel Four-Stroke for LNG Carriers

(PresseBox) ( Augsburg, )
Gaseous fuels are set to play a vastly increased role in the energy economies of many countries. For example, for the European Union member states one forecast calls for an increase in usage from 130 to 300 million tons per year for all purposes up to 2030, while for power generation natural and non-natural gases will become the single largest fuel contingent in the EU electrical power market.

In addition to domestic production and gas transported by pipeline from Russia and other supplying countries on the Eurasian landmass, an increasingly vital factor will be liquefied natural gas (LNG) transported in tankers at cryogenic temperatures from producing countries on other continents.

Building on several decades of experience with dual fuel engine technology, and targeted mainly at LNG sea routes up to 8000 nautical miles port-to-port and LNG carriers in the 150,000 to 270,000 cubic metres capacity class, MAN Diesel in Augsburg, Germany, is currently developing a new dual-fuel engine. Its type 51/60DF dual-fuel engine (bore 510 x stroke 600 mm) is capable of operating on LNG in the gaseous fuel mode and marine diesel oil (MDO) or heavy fuel oil (HFO) in the liquid fuel mode. At the time of writing the engine is undergoing testing at MAN Diesel's Augsburg works in Bavaria, Germany.

Already, the new engine is causing considerable interest among potential buyers. "With a maximum design output of 1000 kW per cylinder at rated speed 514 r/min in both diesel and diesel-gas modes, the 51/60DF engine is designed to drive the generator sets which power both the electric propulsion systems favoured on LNG carriers and their onboard electrical networks," notes Sokrates Tolgos, sales director, cruise vessels and LNG propulsion. "This level of output gives the new engines the potential to propel the aforementioned classes of LNG carriers at speeds of 19 to 20 knots while consuming about 50% less fuel than the steam turbines which have predominated in LNG carriers for the past 30 years."

The 51/60DF engine will be offered in inline versions with 6, 7, 8 and 9 cylinders and in vee configuration versions with 12, 14, 16, and 18 cylinders and, in gaseous fuel operation, employs a common-rail concept for pilot injection. As its designation suggests, the new engine is based on MAN Diesel's well proven 48/60B HFO engine: In a typical application, four to five 51/60DF engines would be installed in a 50 or 60 Hz diesel-electric propulsion and onboard electrical power system operating at rated speeds of 500 or 514 r/min.

In a well-established but nonetheless ingenious concept, in normal operation, loaded LNG tankers provide their own fuel in the form of the "boil-off gas" which necessarily gathers over the liquefied gas in the tanks. For many years the boil-off gas was used to fire boilers to feed steam turbine generator sets. A change to dual-fuel engines is currently underway, driven by a desire amongst tanker charterers and operators for the combination of higher engine efficiencies and multi-fuel operation.

"An electric propulsion system based on the 51/60DF dual-fuel engine can achieve overall propulsion system efficiencies of 42%, compared with around 28% for a typical system based on a steam turbine," notes product line manager Dr. Axel Hanenkamp. "The two alternative operating modes - gaseous fuel with micro-pilot or 100% diesel charge - offer charterers and owners of LNG carriers fuel flexibility as well as fulfilling a Classification Society requirement for redundant operation." Project leader Dipl.-Ing. Nicolaus Böckhoff outlines further development targets. "As well as high thermal efficiency and high power output in both gas and liquid fuel modes, the engine has been designed for low NOx emissions in the gas mode (500 mg /mn3 at 5% O2) and to comply with the latest IMO limits for NOx in the liquid fuel mode. To reach these targets we employ sophisticated, state-of-the art development tools like 3D-Computer Aided Design and 3D-Computational Fluid Dynamics in the development of gas-related components.

Otherwise, we draw extensively on well-proven componentry from the 1200 kW/cyl., 48/60B HFO engine - it has an excellent record of high availability and has gained wide acceptance since its 2002 introduction, with over 150 engines sold and over 330,000 total running hours. The increase in bore dimension from the 48/60B's 480 mm to 510 mm in the 51/60DF is designed to enable high output at a reliable level of brake mean effective pressure (bmep).

In line with the state-of-the-art for dual-fuel engines, the 51/60DF employs a liquid fuel "micro pilot" in gaseous fuel operation, Hanenkamp reports. "Using less than 1% of the normal liquid fuel quantity ensures very low NOx emissions from the 51/60DF engine. Injection of the pilot fuel relies on a common rail pump and a solenoid actuated pilot injector. In gaseous fuel mode the 51/60DF engine operates according to the lean-burn Otto combustion process, with the lean, premixed gas and air ignited by the compression ignition of a small quantity of MDO.

In the liquid fuel mode, oil is injected via conventional main injection pumps adopted from the 48/60B HFO engine. The thermodynamic working process in this mode is the diesel combustion process and the range of usable fuel qualities extends from MDO to HFO."

Liquid fuel injection and admission of air and the gaseous fuel are controlled by a variant of MAN Diesel's proven, Classification Society approved "SaCoS" Safety and Control System. "The architecture of the SaCoS systems is characterised by two independently operating governors," Hanenkamp explains. "One governor controls the fuel rack actuator of the main injection pumps or the individual gas admission valves for each cylinder, depending on fuelling mode. The second governor controls the common rail pilot fuel injectors."

In "SaCoS DF", as the version for dual-fuel engines is known, the system controls pilot injection and gas admission individually at each cylinder, plus a compressor by-pass flap on the turbocharger to adjust the air:fuel ratio. "A phase pick-up delivers the necessary signals for crank angle in both modes," Böckhoff observes. "In a closed loop system, we achieve precise balancing of output across the cylinders of the 51/60DF engine using exhaust gas temperature at the exhaust valve as the feedback value for cylinder output."

Among other major input data to the control system for gaseous fuel operation are combustion knock values acquired by a knock detection unit. "Avoiding pre-ignition or 'combustion knock' is central to reliable operation in any gas engine," Böckhoff notes. "On the 51/60DF engine, knock conditions are detected for each cylinder by acoustic sensors mounted on the cylinder heads. The individual signals are fed into the main sensor unit where the level of knock is determined. The control system responds to the combustion knock signal by making appropriate changes to engine settings. For safety reasons, pressure relief valves are mounted on the charge air manifolds and exhaust gas manifolds.

In this way the SaCoS DF systems ensures stable operation in gas mode with a combination of high efficiency and low emissions while maintaining a sufficient margin to the knock and misfire boundaries." MAN Diesel notes that installation of a diesel-electric marine propulsion system based on a dual-fuel engine is subject to various safety regulations formulated by IMO and the Classification Societies etc. For this purpose, the company has collaborated with the Societies to develop an overall concept covering all issues relevant to the safe and reliable operation of this form of propulsion. In determining the safety concept modern investigation methods such HAZID/HAZOP and FMEA are in use, the company reports. To date, the concept has been approved in principle by Lloyds Register, one of the main Classification Societies covering LNG tankers. Approval from other Societies is expected in due course. "As a result of our efforts in engine design, control system architecture and the new safety concept, we feel we have significantly advanced dual-fuel technology as a reliable propulsion and power generation option in both marine and stationary applications," Hanenkamp concludes.
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