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De-Rating of Main Engine

An option available to reduce the specific fuel consumption of diesel engines is derated or so-called "economy" ratings. This means an operation of an engine at its normal maximum cylinder pressure for the design continuous service rating, but at lower mean effective pressure and shaft speed.

By alternating the fuel injection timing to adjust mean pressure/ maximum pressure relationship the result is a worthwhile saving in fuel consumption. The horsepower required for a particular speed by a given ship is calculated by the naval architect and once the chosen engine is coupled to a fixed pitch propeller, the relationship between engine horsepower propeller revolutions and ship speed is set according to the fixed propeller curve. A move from one point on the curve to another is simply a matter of giving more or less fuel to the engine. 

Derating is the setting of engine performance to maximum cylinder pressure at lower than normal shaft speeds, at a point lower down the propeller curve. For an existing ship and without changing the propeller this will result in a lower ship speed, but in practice when it is applied to a new building, the derated engine horsepower is that which will drive the ship at a given speed with the propeller optimized to absorb this horsepower at a lower than normal shaft speed.

Saving in specific fuel consumption by fitting a derated engine can be as much as 5g/bhp. However, should it be required to operate the engine at its full output potential (normally about the derated value) the ship would require a new propeller to suit both higher revolutions per minute and greater absorbed horsepower. The injection timing would also have to be reset.

It involves:
  1. Reducing Pmean at MCR point.
  2. Matching of T/C again to new NCR (0.9 MCRnew)                    It involves adjusting height, width, angle of blades ...Turbine side                                                                                    Number of vanes, angle of bent, diffuser ...... Compressor side  Each T/C is matched to a particular engine. Surge line of T/C depends on internal characteristics (blades, diffuser) of T/C & can be altered
  3. Adjusting the clearance volume of piston & cylinder head.


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