Engine builders supply an engine load diagram with their engine to allow the operator to establish whether the engine is being operated within limits.
The nominal propeller characteristic curve is plotted from information gained from running on the testbed where a load is put on the engine to simulate the propeller. This is sometimes referred to as the heavy running curve, which the engine would follow with a badly fouled hull or in heavy seas.
The propeller curve for a fully laden ship represents the engine load with a clean hull in calm weather. This is sometimes referred to as the light running curve. In normal operation the engine should be operating between these two propeller lines. If the ship is in ballast (i.e. light ship), then the operating point will move to the right of the propeller line.
Sulzer RTA96 Engine Load Diagram
There is a defined relationship between the propeller speed and the absorbed power in ships equipped with fixed pitch propellers.
With a given propeller this relationship mainly depends on its rotational speed.
The following formula provides us with an approximation which is adequate for the general consideration of conventional vessels:
Its graph is called the propeller characteristic.
If an engine is in good condition and properly supplied with air (i.e. turbocharger(s) in good order and the air and exhaust lines have low additional resistance) and the effective strokes of the fuel pump plungers are properly adjusted, the mean effective pressure developed under service condition according to the specific reading of the load indicator corresponds approximately with the mean effective pressure established for this particular position on the test bed.
In the diagram, the propeller characteristic line through the point of CMCR (Contract Maximum Continuous Rating), i.e. nominal power at nominal engine speed (100% power at 100% engine speed) is called the nominal propeller characteristic. Engines which are to be employed for the propulsion of vessels with fixed propellers are loaded on the test bed according to this propeller characteristic. However, the power requirement of a new ship with a smooth and clean hull should be less and correspond to the range D.
With increasing resistance, changes in wake flow conditions, due to marine growth and ageing of the vessel’s hull, a rough or mechanically damaged propeller, unfavourable sea and weather conditions or operation in shallow water, the propeller will require a higher torque to maintain its speed than it did at the time of sea trial. The mean effective pressure of the engine (and thus the position of the load indicator) will increase accordingly. In such a case, the operating point will then be located to the left of the original propeller curve which was established during sea trials.
Although cleaning and re-painting will help to reduce the increased resistance of the ship’s hull, the new condition can no longer be attained.
Whereas the thermal loading of an engine depends chiefly on the mean effective pressure, the position of the operating point is also important; the farther left it is situated from the propeller curve in the diagram (page 2), the poorer the air supply to the engine and the more unfavourable the engine’s operating conditions will become.
In order to attain optimum working conditions, the operating point of the engine for continuous service should lie in range A on the right side of the nominal propeller characteristic.
The portion on the right of the nominal propeller characteristic is the service range without continuous operating restrictions related to the selected CMCR point.
The portion on the left of the nominal propeller characteristic is the service range for transient operating conditions (acceleration) and should be avoided for continuous operation.
Maximum permissible engine power 40% CMCR from approx. 50% up to 67% of CMCR speed.
Service range with operational time limit, follows a characteristic: P ≈ n 2.45
This characteristic originates from the reference point 95% CMCR power and 95% CMCR speed. With longer operating time in this range, thermal overloading and possible resulting engine damage may be expected.
Service range with overspeed of 104 to 108% of CMCR speed, only permissible during sea trials to demonstrate the CMCR power in presence of authorized representatives of engine builder.
Recommended layout range for fixed pitch propeller, valid for the maximum draught, clean hull under contractual
weather and sea conditions.
Overload range permissible only for maximum one hour during seatrials in presence of authorized representatives of engine builder.
Load Diagram for MAN B&W MC 60 Engine