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.

Place the planimeter and indicator card on a piece of plane cardboard (not too smooth), and trace the diagram. Only consider the result satisfactory, when two readings are obtained which do not differ more than ‘1’ on the planimeter vernier scale.

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).

pi corresponds to the height of a rectangle with the same area and length as the indicator diagram. i.e., if pi was acting on the piston during the complete downwards stroke, the cylinder would produce the same total work as actually produced in one complete revolution.

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 (ikW or ihp)

where,

n (rpm) = engine speed.

The effective engine power, Pe

Pe = k2 × n × pe (kW or bhp)

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.

Reference:

Instruction Book ‘Operation’ for 50-108MC/MC-C Engines