Lean and clean marine gas engines launched

Rolls-Royce?s two new lean, clean and mean marine gas engines provide a power range from 1MW to 8.5MW

Designed and manufactured at Rolls-Royce?s Bergen facility in Norway, the K and B series of gas fuelled engines were recently introduced to the marine market in order to meet the increasingly tough low emission targets. The starting point for the development of these engines was the exhaust emissions issue which has become one of the most important controlling factors for all future engine developments.

Since 40% of NOx emissions in Norway comes from the domestic shipping industry (coastal and offshore vessels) and fishing, political pressure in that country has set a target of NOx emissions which are 80% lower than those proposed by the IMO. The obvious solution is a gas driven lean burn engine which gives 85% reduction in NOx and around 35% reduction in carbon dioxide (CO2), while the absence of sulphur eliminates SOx emissions. A number of manufacturers offer dual-fuel engines but, while these appear attractive, they always need to burn diesel fuel with the optimum ratio being 95% gas and 5% diesel. In addition, the dual-fuel units rely on compression ignition creating high pressure and temperature in the cylinder. This can cause the gas to self-ignite (knocking) so a compromise must be sought to avoid knocking at high load, miss-firing at low load and problematic transient engine operation.

Gas engine development

Bergen began to carry out its research and development of gas engines back in 1984. After evaluating a number of alternatives, the spark-ignited pre-chamber lean burn gas concept was chosen as the most promising for high specific power, with the prospect of achieving high efficiency and low emissions. Bergen introduced the KV-G type gas engine in 1991 for the land power industry and has built up a good reputation by steadily developing the engine range to the current 200kW per cylinder at 43% efficiency. This gas fuelled Otto ?cycle? engine is now available to the marine industry for direct mechanical drive, as a gas electric drive or genset. The K series range is produced with cylinder configurations of 6, 8 or 9 cylinders in line and V12, V14 or V18 to give a power range from 1MW to 4MW.

The success of the K engine led RR to introduce a significantly larger and more powerful engine, the new B35:40V gas fired engine which will be available in 12V, 16V and 20V cylinder versions, with power ratings from 4.5 to 8.5MW. Originally designed as a medium speed diesel engine, it was decided in spring 2002 that it should also be developed as a gas engine as part of its basic design concept. Learning from their experience with the K engines, RR developed a more robust means of igniting the pre-chamber charge which resulted in the ‘micro-pilot’ ignition principle being adopted.

RR also took the opportunity of increasing the bore to 350mm and, in spite of this, the total weight of the running gear (complete piston and connecting rod) is still 10kg lighter than that of the 320mm bore diesel version.

High-tech features

By suitable development of a strong ignition source, the pre-chamber and the gas-air mixture in the cylinder can be leaned out, giving much improved engine performance. Efficiency is increased, emissions are reduced, particularly NOx, and the specific power of the engine can be significantly increased because the knock limit is extended.

Fast and complete combustion is achieved by means of two ABB turbochargers, mounted back-to-back with one exhaust outlet. A mechanically operated gas valve comprising admission and flow-valves, using the ?Miller? type valve timing, is set into the inlet port of each cylinder. The valves inject gas into the inlet stream while the special inlet port design, flame deck layout, piston bowl and the engine control system ensures a homogenous and lean mixture of air and gas for quick and complete combustion under all operating conditions.

The advanced electronic engine control system ensures that operating parameters of the engine are adjusted and optimised in relation to each other. The engine has been optimised with regard to process parameters, turbocharger type (VTG) and excess ratio in such a way as to maintain a high exhaust temperature thus maximizing waste heat recovery.

Pros and cons

The manufaturer claims that the gas-only engine delivers optimum performance throughout the whole operating range without having to compromise between efficiency, performance and low emissions. It also simplifies the design, provides a wide operating range with no low-load limitations and enables crash-stopping without ?choking? the engine.

Due to the simple design of the gas engines and their relatively low running speed, the intervals between overhauls can be longer giving lower service costs. The robust construction of the engine is based on a number of advanced features such as nodular cast iron structure, six studs per cylinder head and latest bearing and valve materials. In addition, there are fewer parts subject to wear while the generous sizing of parts offers larger wear margins.

The disadvantage, at the moment, is that these engines are reliant on a readily available source of natural gas. While this is not a problem where there is an existing or planned LNG infrastructure, such as in Norway, it is a major constraint in most other countries. The situation could arise where a vessel built for trading in an LNG area is subsequently moved to an area where the gas supply is non-existent. In such cases, RR?s gas engines can be converted to burn diesel fuel should a vessel’s trading pattern change.

With the growing availability of LNG world-wide however, the opportunities for gas and dual-fuel engines is expanding particularly with certain ship types including FPSO vessels, shuttle tankers, offshore support vessels, LNG carriers and ferries.

Gas fuelled

double-enders for Norway

An example of the resolve with which the Norwegian government is pursuing its campaign to lower emissions is the tender it has put out to a number of domestic and overseas ferry operators. One of the companies competing for the tender is HSD Sj? AS, a subsidiary of Hardanger Sunnhordlandske Dampskipsselskap based in Hardanger. The tender calls for five gas fuelled ferries, three for the Bergen to Stavanger route and two for the Halhjem to Sandvikvag service. The Norwegian ministry of road administration will be announcing the outcome of the tender process on 20 September after which construction can begin with the first ferry scheduled to enter service in January 2007.

Rolls-Royce Bergen gas engines will be one of the options under consideration for the power plant of these newbuildings which will have a speed of 22 knots. Other contenders for the propulsion engines are likely to be Wärtsilä with its dual fuel engines and Mitsubishi with its gas engines.