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Feature 1 | DIESEL TECHNOLOGY Wärtsilä’s medium speed 46CR engine.

The ferry will have a capacity of 3200 passengers and 1060 vehicles, and a top speed of 27.5knots. Delivery of the vessel is due in the first half of 2012. The 48/60CR injection system uses the

Finnish manufacturer Wärtsilä advocates

the employment of the common rail system which can maintain high injection pressures independently of engine load, thus ensuring good spray atomization even at very low loads. In mechanical fuel injection systems, the injection pressure is a function of engine speed and load, therefore, at low loads the pressure drops and the result is very large fuel droplets which remain as such until they contact the combustion space surfaces. Common rail fuel injection technology on the other hand, offers the option of maintaining a high injection pressure which means smaller droplets at all loads down to idling. Te risk of over-injection of fuel is avoided by utilising a computer to monitor air temperature, pressure and rotational speed and using the resulting figures to calculate the amount of air trapped in the cylinder. Based on empirical maps, the computer then decides how much fuel can be injected. A further advantage of the common rail system is that injection timing can be varied during running of the engine to optimise engine performance, and provide further emissions control. In Wärtsilä’s 46CR engine, the common

rail is built using one accumulator which feeds the injectors of two cylinders. Te accumulators, mounted on the engine block, are connected to each other with small-bore pipes to eliminate pressure waves in the rail. By splitting up the fuel volumes in several accumulators the risk of pressure waves is avoided. Te flow to each fuel pump feeding an


accumulator is controlled by a flow control valve driven by a double cam. Te function of each accumulator is to take the fuel from the high pressure pump, store it under high pressure and distribute it to the injectors. Te accumulator itself is an empty space. Flow fuses located at outlets from the accumulator to the injector, limit the quantity of fuel that can be transferred from the accumulator to the injector if the latter malfunctions. Te fuel injection valve is perhaps the most

important part of the common rail injection system. It is designed to prevent fuel entering the combustion space when it should not. Te main components are the nozzle seat, the shuttle valve, solenoid valve and the piston acting on the nozzle needle. Te shuttle valve controls injections and de-pressurises the injector needle. In its closed position the valve isolates the injector passages and nozzle from the pressure source to ensure no pressure is applied to the nozzle seat between injections. Pressure is normally only applied to the piston at the start and end of injections, this promotes fuelling consistency. Te automation system for the Wärtsilä CR engines controls the injection and rail pressure, which is incorporated within the standard engine alarm and monitoring system. Rival manufacturer MAN Diesel & Turbo

produces common rail engines as well. Daewoo Shipbuilding & Marine Engineering Co. Ltd. (DSME) of Korea, has recently ordered four common-rail 12V48/60CR Tier-II propulsion engines from the company to power a new RoPax ferry for Tunisian ship owner Compagnie Tunisienne de Navigation.

company’s latest common-rail technology allowing independent setting of injection timing, duration and pressure for each cylinder allowing for the optimisation of fuel consumption, NOx and smoke emissions at any point on its operating profile, especially at part-load. Retarded injection timing delays the combustion-heat release, thus lowering combustion chamber temperature peaks and NOx emissions. Te new piston design provides a higher compression ratio and faster temperature reduction after ignition and variable valve timing. Te injection system is totally integrated

with the engine management system. The result, says the company, is extremely quick and accurate reaction times. Te common rail system has been also designed for operation with heavy fuel oil (HFO) in accordance with specification DIN ISO 8217. Te fuel oil system allows the engine to be started and stopped with HFO. This provides the advantage of allowing the ship’s engines to use both high-grade fuel during periods alongside and when transiting coastal waters, while allowing for the switch back to heavy fuel oil when out of territorial waters. Te design of the MAN Diesel & Turbo

common rail system includes safety features such as ensuring all high pressure lines, rails and high pressure connections are double wall insulated to ensure there is no danger of fuel spray through leaking or broken pipes. To protect against uncontrolled injections, flow limitation valves are placed at each cylinder to prevent unintended injection. Redundancy is provided both in high pressure pumps that guarantee engine operation in the event of pump malfunction as well as in the rail pressure and the speed/top dead centre sensor to ensure continued operation in the case of a sensor or pick up failure. Despite the reservations of ships, engineers

and the costs of parts, the order books for common rail engines have not waned. Until the new generation of dual fuel engines using LNG enters the marketplace, the common rail engine is here to stay. NA

The Naval Architect March 2012

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