DieselShip

Indicator Diagram for a Slow Speed Marine Diesel Engine

Indicator Diagram

The term indicated power refers to the theoretical power that develops inside the cylinder of an engine though in actual practice the power available at the outside (brake power) is lesser but for the time being we will not consider the latter but only talk about indicated power. The best way to find out the indicated power is from calculating the area under indicator diagram. But how is an indicated diagram drawn for an actual engine. Just take a look at the left hand side of this text to find out how an indicator diagram looks. Actually you will notice that it is the same P-V diagram which we studied in the previous articles relating to various types of marine diesel engine cycles such as diesel cycle, dual cycle and otto cycle.

Engine Indicator

Just take a look at the figure to see what an engine indicator looks like. As the name suggests it is used to draw the indicator diagram for an actual engine while it is firing (working). The black coloured handle you see at the bottom of the instrument helps to fit this instrument in the appropriate slot in the cylinder head known as the indicator cock. Once the arrangement is in place the piston inside the instrument is exposed to the pressure variations happening inside the cylinder and these are them transferred on to a paper through the stylus which moves in proportion to the movement of the piston and the spring on top which opposes the instrument piston movement.

Limitations

Although it is a good instrument to measure power basically its use is limited to measuring indicated power of large slow speed engines only, which in the marine context means the main propulsion plant of the ship. Several factors such as spring inertia and stylus movement limit the diagrams in case of high speed engines for which separate indirect methods are used to measure power.

In the next article we will learn the procedure of using an engine indicator to actually draw the diagram for a large slow speed marine diesel engine.

FLOW LINES TO DIAGRAM

There are certain points which need to be kept in mind before plotting an indicator diagram in order to ensure that it gives a fairly true indication of the power generated. These points are listed in the bullet format as follows.

• The propeller of the ship should be fully immersed in water
• The ship should be at even keel and preferably fully laden
• There are no strong winds present either against or in favour of the direction of motion of the ship. If winds are present the engine will either produce more power or will produce lesser power for the given speed because of the wind resistance or aid, as the case may be
• The underside of the ship is pretty clean and is not fouled with underwater growth, otherwise the engine would develop more power for the given speed
• The engine indicator is clean and properly lubricated so that friction does not cause an error in the reading
• The writing pressure of the stylus against the inserted sheet of paper should neither be too low nor too strong. If it is strong it might tear the paper, while if it is too low it will not leave a strong impression
• The cock should be clean so that all the pressure is made available to the piston of the engine indicator

Recording the Diagram

• The engine indicator should be fastened to the cock and the cord on the instrument should be kept taut
• While the cock is still in the shut position, the atmospheric pressure line should be traced
• The indicator cock should be opened and the stylus should be pressed against the paper till the cycle gets imprinted after which the cock should be safely shut
• The cord now needs to be removed and the indicator cock opened again and to be pulled with hand, the timing being such that the piston is at the top dead center at that time