enginebuilder profile
variants, the series covers a power range from 5,500kW to 17.55MW.
In gas mode, with fuel supplied at a low pressure (less than 5 bar at the engine inlet), the engine operates according to the lean-burn Otto process. The mixture of air and gas in the cylinder contains more air than is needed for complete combustion – typically a 2.2:1 ratio – which lowers peak temperatures and hence NOx emissions. A higher compression ratio is also facilitated, raising engine efficiency and further reducing emissions. The fuel system is divided into three elements: for gas, back-up fuel oil and pilot fuel oil. Gas is admitted into the air inlet channels of the individual cylinders during the intake stroke to create a lean, premixed air-gas mixture in the combustion chambers. Reliable ignition of the mixture is secured by injecting a small quantity of diesel oil directly into the combustion chamber as pilot fuel which ignites by compression ignition as in a conventional diesel engine.
Micro-pilot ignition injection is exploited, such that less than 1 per cent of the overall fuel energy is required as liquid fuel at nominal load. Pilot fuel oil is delivered via a common rail system based on an engine-mounted high pressure pump supplying the fuel
to every
injection valve at around 900 bar. The injection valves are of twin-needle design, with the pilot fuel needle
electronically controlled by the engine control system.
Electronic control closely regulates the pilot injection system and air-gas ratio to keep each cylinder at its correct operating point between the ‘knock’ and misfiring limits; this, Wärtsilä explained, is the key factor in achieving reliable operation in gas mode: automatically tuning the engine to match varying conditions. Securing the highest efficiency and lowest emissions, each cylinder is individually controlled to ensure operation at the correct air- fuel ratio, with the correct amount and timing of pilot fuel injection. Both gas admission and pilot fuel injection are electronically controlled and engine functions are
controlled by an
advanced automation system allowing optimum running conditions to be set independently of the ambient conditions or fuel type. The global air-fuel ratio is controlled by a wastegate valve which allows some of the exhaust gases to bypass the turbine of the turbocharger, ensuring that the ratio is correct regardless of changing ambient conditions, such as temperature. Starting is normally executed in diesel mode, using both main diesel and pilot fuel. Gas admission is activated when combustion is stable in all cylinders.
When running in gas mode, the engine automatically switches over to diesel fuel operation if the gas feed is interrupted or component failure occurs. The switchover takes less than a second and has no effect on engine
18 I Marine Propulsion I April/May 2014
speed and load during the process. In diesel mode the engine works according to the normal diesel concept using a jerk pump fuel injection system. Diesel fuel is injected at high pressure into the combustion chamber just before top dead centre. Gas admission is deactivated but the pilot fuel remains activated to ensure reliable pilot fuel injection when the engine is transferred back to gas operation. Transfer from diesel to gas running is a more gradual process than gas to diesel mode; the diesel fuel supply is slowly reduced while the amount of gas admitted is increased. The effect on engine speed and load fluctuation during transfer to gas is reportedly minimal. Lean combustion enables a high compression ratio, which in turn increases engine efficiency
and reduces peak temperatures, thereby fostering lower NOx emissions. The environmental merits of LNG-fuelled engines are particularly valued for operations in NOx and SOx emissions- sensitive regions.
In gas mode, the 50DF engine’s NOx emissions are said to be at least 85 per cent below those specified in IMO Tier II regulations, while carbon dioxide emissions are some 25 per cent less than those of a conventional marine engine running on diesel fuel. Furthermore, SOx and particulate matter emissions are negligible at almost zero per cent.
• Wärtsilä has now extended its choice of dual- fuel medium speed engines with the introduction of a 46DF model, a gas-burning derivative of the successful 460mm-bore diesel design. MP
www.mpropulsion.com
Arrangement of the four V12-cylinder 50DF-driven gensets specified to convert Totem Ocean’s Orca-class roro ships to gas burning; Wärtsilä is also supplying the associated LNGPac fuel handling systems
A V18-cylinder version of the 50DF engine, used in electric power stations of LNG carriers
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