Skip to main content

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.


Popular posts from this blog

Load Line & Why it is Important

Merchant ships have a marking on their hull know as the Plimsoll line or the Plimsoll mark, which indicates the limit until which ships can be loaded with enough cargo, internationally, the Plimsoll line on a ship is officially referred to as the international load line. Every type of ship has a different level of floating and the Plimsoll line on a ship generally varies from one vessel to another.  All vessels of 24 meters and more are required to have this Load line marking at the centre position of the length of summer load water line. There are two types of Load line markings:- Standard Load Line marking – This is applicable to all types of vessels. Timber Load Line Markings – This is applicable to vessels carrying timber cargo. These marks shall be punched on the surface of the hull making it visible even if the ship side paint fades out. The marks shall again be painted with white or yellow colour on a dark background/black on a light background.  The comp

Difference Between A, B & C-Class Divisions?

IMO Symbol A Class Division  IMO Symbol B Class Division  SOLAS has tables for structural fire protection requirement of bulkheads and decks. The requirements depend on the spaces in question and are different for passenger ships and cargo ships. The Administration has required a test of a prototype bulkhead or deck in accordance with the Fire Test Procedures Code to ensure that it meets the above requirements for integrity and temperature rise. Types of Divisions: "A" Class "B" Class "C" Class "A" Class: "A" class divisions are those divisions formed by bulkheads and decks which comply with the following criteria: They are constructed of steel or equivalent material They are suitably stiffened They are constructed as to be capable of preventing the passage of smoke and flame to the end of the one-hour standard fire test. they are insulated with approved non-combustible materials such that the average tempera

Pump Shaft Alignment Procedure

Types of shaft alignment methods: Visual Line-Up Straightedge/Feeler Gauge Rim and Face Cross Dial Reverse Dial Laser Visual Line-Up The visual line-up method is the most common method of alignment. Used in initial installations, visual line-up allows technicians to analyze the working conditions and feasibility of installation. Straightedge/Feeler Gauge Straightedges are used to determine the offset between coupling halves. Corrections are made under all four of the machines feet. Feeler gauges or taper gauges measure the gap between coupling halves at the bottom and top of the coupling. Rim and Face This method is similar in principle to using a straightedge and feeler gauge, but more accurate since dial indicators are used. The rim reading measures the offset between the coupling halves. The face reading measures the angular difference between the faces of the coupling. Changes are calculated with the same formula as the straightedge/feeler gauge met