The source for power is most often a generator set driven by a combustion engine which is fueled with diesel or heavy fuel oil.
Occasionally one can find gas engines, and also gas turbines, steam turbines or combined cycle turbines, especially for higher power levels, in light high-speed vessels, or where gas is a cheap alternative (e.g. waste product in oil production, boil-off in LNG carriers, etc.).
Diesel-electric Propulsion System Engines
In a diesel-electric propulsion system, the diesel engines are normally medium to high-speed engines, with lower weight and costs than similar rated low speed engines that are used for direct mechanical propulsion.
Availability to the power plant is of high concern, and in a diesel electric system with a number of diesel engines in a redundant network; this means high reliability but also sophisticated diagnostics and short repair times.
Efficiency of Ship’s Diesel Generators
The combustion engines are continuously being developed for higher efficiency and reduced emissions, and at present, a medium speed diesel engine has a fuel consumption of less than 200g per produced kWh at the optimum operation point.
Even though this is regarded to be a high utilization factor of fuel, it represents only about 40% of the energy in the fuel, the rest of the energy being removed by the exhaust or heat
Moreover, the efficiency drops fast as the load becomes lower than 50% of MCR (Max Continuous Rating). At this working condition, the combustion is inefficient, with high NOx and SOx content, and with a high degree of soothing which increases the need for maintenance.
In a diesel electric system with several diesel engines it is hence an aim to keep the diesel engines loaded at their optimum operating conditions by starting and stopping
generator sets dependent on the load, with an aim to keep the average loading of each running diesel engine closest possible to its optimum load point.
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