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Indicated Power is defined as the power developed by combustion of fuel inside the engine cylinder.

 

Effective power is the output of the engine.

 

Calculation of the indicated, effective and finally shaft power of marine engines in practical case consists of the following steps:

 

Calculate:

• The mean indicated pressure, pi
• The mean effective pressure, pe
• The cylinder constant, k2
• The indicated engine power, Pi
• The effective engine power, Pe

 

The mean indicated pressure, pi

pi = A / (L × Cs)        bar

where:

A (mm2) = area of the indicator diagram, as found by planimeter.

L (mm) = length of the indicator diagram.

Cs (mm/bar) = spring constant (= vertical movement of the indicator stylus (mm) for a 1 bar pressure rise in the cylinder).

 

The mean effective pressure, pe

pe = pi – k1 (bar)

where,

k1 = the mean friction loss (The mean friction loss has proved to be practically independent of the engine load. By experience, k1 has been found to be approx. 1 bar)

k2 = the cylinder constant (k2 is determined by the dimensions of the engine, and the units in which the power is wanted.)

For power in kW : k2 = 1,30900 × D2 ×S
For power in BHP : k2 = 1,77968 × D2 × S

where:
D (m) = cylinder diameter
S (m) = piston stroke

Value of k2 for different MAN B&W engines types is given below.

 

The indicated engine power, Pi

Pi = k2 × n × pi 

where,

n (rpm) = engine speed.

The effective engine power, Pe

Pe = k2 × n × pe 

where,

 

n (rpm) = engine speed.

Due to the friction in the thrust bearing, the shaft power is approx. 1% less than the effective engine power. From this calculation of shaft power in marine engines can be carried out on ships.

 

SFOC

Specific Fuel Oil Consumption (SFOC):

• It is the mass of fuel consumed per unit time to produce per KW
• In order to achieve accuracy, the fuel consumption and power developed is always measured over a suitable time period on a good weather. 
• The formula used for calculating SFOC is:

SFOC (g/kwh) = Mass of fuel consumed per hour / Power developed in KW 

SFOC = ( volume x corrected density ) / hour x PE

Volume is obtain from flowmeter 

Corrected density = specific gravity @ 15 degreeC x ( 1 – (T – 15) x 0.00064 )

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