Page 386 - Introduction to Marine Engineering

356 Appendix

indicated mean area length number of number
power of = effective x of x of x power x of
engine pressure piston stroke strokes/sec cylinders
(Pm) (A) (L) (N)

= Pm L A N x number of cylinders

838.5 X x 410 X x 6
" " x
= 6
10* 4 x 10 60
= 1461.28 kW
Shaft power
A torsionmeter is usually used to measure the torque on the engine shaft
(see Chapter 15). This torque, together with the rotational speed, will
give the shaft power of the engine.
shaft power = torque in shaft
x rotational speed of shaft in radians per second

Example

The torque on an engine shaft is found to be 320 kNm when it is rotating
at 1 lOrev/min. Determine the shaft power of the engine.
shaft power = shaft torque
x 2n
x revolutions per second

= 320 x 2*r x—
60
= 3686.1 4 kW

Mechanical efficiency

The power lost as a result of friction between the moving parts of the
engine results in the difference between shaft and indicated power. The
ratio of shaft power to indicated power for an engine is known as the
'mechanical efficiency'.

, - t Cf • shaft power
mechanical efficiency = - *- -
indicated power

Power utilisation
The engine shaft power is transmitted to the propeller with only minor
transmission losses. The operation of the propeller results in a forward