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09 November 2018

57.Power calculation in Main engine

November 09, 2018 Posted by AK No comments
The indicator diagram (pv diagram: work diagram), illustrates the pressure variations in the engine cylinder as a function of the main piston position.

The diagram area can be integrated by means of a planimeter or mid ordinate rule(simson), and
the mean indicated pressure calculated.

The power developed in the particular cylinder can then be found by multiplication by the engine speed and the cylinder constant.

Calculation of the indicated and effective engine power 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 = A / (L X Cs)

where:
A (mm2) = area of the indicator diagram, as found by planimeter
L (mm) = length of the indicator diagram (= atmospheric line).
Cs (mm/bar) = spring constant (= vertical movement of the indicator stylus (mm) for a 1 bar pressure rise in the cylinder).

How to draw Atmospheric line:
– Keep the indicator valve closed.
– Press the stylus against the paper.
– Release the stylus when the indicator drive
has turned the drum one or two times.

The mean effective pressure

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.

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

The indicated engine power

Pi = k2 × n × pi (ikW or ihp)

where
n (rpm) = engine speed.

The effective engine power

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.

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