Feature 2 | MEDIUM-SPEED ENGINES
Designers rise to economic, enviro challenges
Advances in medium-speed, four-stroke engine technology and performance, extensions to the product range at both ends of the power spectrum, and improved delivery competitiveness, have widened the market reach and uptake of such machinery, writes David Tinsley.
D
esigners’ endeavours to enhance engine economics and environmental performance in
parallel, while increasing power density factors, have been vindicated by the level of receptivity to new engine models and new concepts, against a backcloth of growing regulatory controls and intensifying competitive pressures on shipowners. Fuel consumption and exhaust emissions
are ever more inextricably linked. The drive to optimise combustion quality has intensified in a bid cut atmospheric pollutant release without penalising fuel consumption or, ideally, with an attendant reduction in specific fuel consumption. In looking towards the so-called ‘Tier 2’ of international environmental compliance, which is set to be implemented in two years’ time, the focus has been on reducing emissions by engine-internal measures, within the combustion chamber itself, rather than through secondary, add-on methods. Over recent years, considerable
attention has been given at the design stage to enhancing the reliability, compactness and durability of medium- speed engines. Te further development of monitoring systems and diagnostics as tools of in-service support reflects the demand for heightened availability and dependability. Spurred by the boom in global fleet
investment that immediately preceded the current global financial crisis, all the main suppliers of medium-speed engines have undertaken or implemented large-scale investments in production plant, capacity and organisation, conferring increased unit cost competitiveness and reducing lead times.
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Clean Design Trough continuous product development, coupled with investments to increase manufacturing efficiency, Rolls-Royce has consolidated the position of its Bergen medium-speed engines in the offshore support vessel market and attracted new business in the small ferry and coastal vessel sectors. One of the company’s latest initiatives has seen the unveiling of a CD version of the modern C-type medium- speed unit, to afford ship operators greater environmental advantage. In CD guise, the enhanced C-engine
meets the international ‘Clean Design’ standard by achieving a 20% reduction in NOx (oxides of nitrogen) exhaust emissions relative to the present NOx limits mandated by the International Maritime Organization (IMO). Te revised series from the Bergen stable will accordingly comply with IMO Tier II NOx emission criteria, due to be implemented in 2011. Significantly, it is claimed that the cut in pollutant emissions has been achieved without penalising specific fuel consumption rates. The improvement is the result of
a careful combination of increased compression ratio with the manner in
To meet tougher emissions criteria, Rolls-Royce has developed a Clean Design (CD) version of the Bergen C25:33 engine, shown here in a marine genset (credit: Rolls-Royce).
which the Miller cycle (the modification of the engine’s valve timing to achieve cooler combustion) has been applied. The engine has been equipped with a variable valve timing mechanism in order to avoid low-load smoke and poor transient load behaviour, which are negative consequences of the Miller cycle. The Miller cycle air intake can thereby be automatically switched-off for low-load running. The conferring of Clean Design
credentials on the C-type follows the design’s power uprating. The compact C25:33 series is now offered at 330kW per cylinder, at the same running speed as before, so that the largest, nine-cylinder model takes the unit output scope to nearly 3000kW. Te larger, more powerful B32:40 series
in the Rolls-Royce Bergen portfolio has an established CD version, which has found particular favour in the offshore support vessel sector. Such has been the uptake of the environmentally enhanced models in the Bergen range that the company anticipates that some 90% of the engines sold into the offshore market by 2009 will meet the demands of Clean Design classification.
Ship & Boat International March/April 2009
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