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Engine Assessment

Other Methods of Determining Engine Power

 

 

Engines Driving Generators

 

The power output of an engine driving a generator can be measured fairly accurately by measuring the generator power output (Pw) and dividing it by the generator efficiency. (the losses in the generator will be frictional losses in the bearings and heat losses). This efficiency does vary slightly over the operating range of the engine, but a figure of 0.92 will not be to far out.

 

The generator efficiency is defined as  measured generator power / effective power or Pw / Pe

 

From Manufacturers Data

 

 Engine manufacturers sometimes produce a graph from which the power output of the engine can be estimated. It must be said, however, that for the results to have any validity, that the fuel injection equipment and cylinder condition must be in optimum condition, the hull must not be fouled and the turbocharger clean and operating efficiently. When operating with a variable pitch propeller, the pitch must be the same for all readings (normally 100% ahead)

 

The type of fuel will have an effect and this must be taken into account.

 

The graph on the left is for a MAN B&W medium speed engine.

 

To use the graph the engines rated speed and power output must be known.

e.g 6200kW at 450rpm.

 

Next take the engines actual speed and fuel pump settings. In this example 432rpm at fuel pump setting of 59

 

Convert the speed into a percentage of the rated speed. In this case 432rpm is 96% of 450.

 

Project the 96% of rated speed vertically upwards on the x axis, until it intersects with a line drawn parallel to the fuel pump settings for a setting of 59 (Point A)

 

From point A, project a horizontal line to the y axis and read the % power output, in this case 86%

 

Power = 0.86 6200 = 5330kW

 

 

Torsionmeters
 

A torsion meter measures the twist (in radians) in a shaft over a given length as it is rotating. Experimental data will have established the shaft constant which when multiplied by the rpm and the angle of twist will give the shaft power.

 

The shaft twist is measured electrically by using the twist in the shaft to vary the air gap in a transformer. This will vary the resistance and thus the current.

 

ELECTRICAL TYPE TORSIONMETER

 

Two sleeves are rigidly fixed to the shaft having flanges at 90 to the shaft axis. Twist causes relative displacement between the flanges. Two cores are attached to one flange and the iron piece to the other so that relative movement between flange faces, due to shaft twist, alters the air gap of the differential transformer.

 

The primary circuit is wound to give the same polarity and the secondary circuits are in opposition. With no torque the air gaps are equal and the two secondary circuits are equal and opposite, but when torque is applied air gaps become unequal and a current flows in the secondary circuit which can be read on the galvanometer. An identical unit is fitted in the indicator box and by rotation of the handle the iron piece can be moved until the air gaps in the indicating unit are identical with those of the shaft unit. This restores the electrical equilibrium in the secondary circuit, as opposed equal currents the galvanometer reads zero, and the amount of movement at the indicating box dial is indicative of the angle of twist restoration required and hence gives the angle of twist for the length of shaft between the two flange faces in the shaft unit.

 

By application of the meter constant and rev /s the shaft MW is thus determined.

 

Another method is to use two position encoders between two clamp rings in 500mm apart on the propeller shaft. Using this method it is possible to measure the torque absolutely contact free and with quite high accuracy. Together with the speed, which is also measured contact free, the power output of the engine is calculated and displayed on a touch screen panel mounted in the control room. On the display the actual data can be compared with the original, pre-set load diagram.

 


 

 

 

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